CdO and ZnO Clusters as Potential Building Blocks for Cluster

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CdO and ZnO Clusters as Potential Building Blocks for ClusterAssembled Materials: A Combined Experimental and Theoretical Study Roman Lazarski, Marek Sierka, Julian Heinzelmann, Alexander Koop, Rene Sedlak, Sebastian Proch, and Gerd F. Gantefoer J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b00333 • Publication Date (Web): 05 Mar 2015 Downloaded from http://pubs.acs.org on March 10, 2015

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The Journal of Physical Chemistry

CdO and ZnO Clusters as Potential Building Blocks for Cluster-Assembled Materials: A Combined Experimental and Theoretical Study Roman Łazarski,‡ Marek Sierka,‡* Julian Heinzelmann,† Alexander Koop,† René Sedlak,† Sebastian Proch,†* Gerd F. Ganteför†

†

Fachbereich für Physik, Universität Konstanz, Universitätsstrasse 10, 78464 Konstanz,

Germany ‡

Otto-Schott-Institut für Materialforschung, Friedrich-Schiller-Universität Jena,

Löbdergraben 32, 07743 Jena, Germany

ABSTRACT: Small semiconductor nanoclusters exhibit unique properties often very different from those of their atomic and bulk counterparts. Their better understanding and characterization is expected to be useful in the development of highly functional clusterassembled materials (CAMs) with tunable properties. In this work, the structural and electronic properties of size-selected (CdO)n clusters were examined by conventional and time-resolved spectroscopy (TR-PES) combined with density functional theory (DFT) calculations. The observed highly symmetric alternant-cage structures and large band gaps confirm validity of those species as CAM building blocks. Moreover our results demonstrate a striking similarity between (CdO) and (ZnO)n clusters of the same size that is not n restricted to the ground state but also comprises properties of excited states. We suggest that in general valence isoelectronic small binary (XY)n clusters might exhibit similar structures and comparable properties. The long lifetimes of excited states observed in our experiment are proposed as a general probe helpful in identifying the suitable structures for CAMs and hence simplifying their design.

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INTRODUCTION Size-selected clusters have been shown to exhibit properties beyond those of standard elements, adding a third dimension to the periodic table. This opens the possibility to compose highly functional, tailor-made cluster-assembled materials (CAMs) via a bottom-up approach.1 However, the periodic table itself offers a very large number of elemental combinations so that some basic rules for the aptitude as CAM building blocks are indispensable. Fundamental prerequisites have been derived by Castleman et al.:2 rigid structures to avoid elemental and geometrical changes, high symmetry to favor threedimensional assembly and large HOMO-LUMO gaps to prevent fusion of the clusters. To this date, fullerites with carbon fullerenes as building blocks that meet all these requirements present the only CAM obtained from gas-phase entities.3-4 However, theoretical predictions indicate that new crystalline phases may be accessible via coalescence of size-selected clusters with ZnO, CdO and MgO as prominent examples.5-9 For these clusters highly symmetric alternant cage structures have been predicted.10-22 Unlike carbon fullerenes comprising five- and six-membered rings these cages are constructed from rings containing four and six atoms. This tendency arises from the need to avoid homobonding in these heterofullerenes. In particular, (MO)12 and (MO) 16 clusters (M = Zn, Cd, Mg) are potential

building blocks for exceptionally stable, low-band gap nanoporous phases very different from any known polymorphs.5-9 Similar highly symmetric alternant cage structures have been predicted for numerous binary semiconductors (XY)n including group II-VI23-29 and III-V30-47 compounds. As all the “XY” fragments bear eight valence electrons and are thus valence isoelectronic, similar structures would ensure comparable electronic properties of cages. This leads to the idea of alternant cage cluster assembled materials (ACCAMs) construction kit. In this kit the structures of the clusters serving as building blocks are fixed but the electronic properties may be tuned by varying the XY fragments. 2 ACS Paragon Plus Environment

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Zinc oxide nanoclusters have been under intense scrutiny by theoretical methods since zinc oxide is a hot topic material with wide-ranging technological applications including optoelectronics,48 gas sensors,48 catalysis49 and solar cells.50 Recently, favorable features of (ZnO)n clusters confirming their aptitude as CAM building blocks, such as rigid alternant cage-like structures and large band gaps that are nonetheless far below 3.3 eV of bulk ZnO, were confirmed by our combined experimental and computational study.51 These features go along with another unusual one – size-independent, exceptionally long-lived excited states associated with rigid structures and strong electron localization in the excited state.51 Cadmium oxide is a wide-bandgap direct semiconductors with the band-gap of 2.2 eV and applications similar to that of ZnO.52-55 Despite significant scientific interest in nanoparticulate CdO based materials to our best knowledge no experimental studies on sizeselected CdO clusters have been reported and computational studies are scarce. Calculations on neutral (CdO) n species with n up to 15 predicted alternant cage structures as global minima for n ≥ 8 and planar rings for smaller n.19 In contrast, recent calculations of Srinivasaraghavan et al.56 place this crossover point already at n = 6. This study reports the first combined experimental and theoretical study of gas-phase (CdO)n clusters with n = 8, 9, 12 and 16. Ground and excited states of these clusters are probed by conventional and time-resolved photoelectron spectroscopy (TR−PES). The corresponding lowest energy structures are determined by global structure optimizations at the density functional theory (DFT) level and analyzed via time-dependent DFT (TD−DFT). Finally, the results are compared to those obtained for (ZnO)n clusters.51

METHODS Experiment. The experimental setup is described in details elsewhere.51,57-58 In short, cadmium metal was vaporized in a pulsed-arc discharge source (PACIS), reacted with oxygen, and mass selected via time-of-flight mass spectroscopy. Conventional photoelectron 3 ACS Paragon Plus Environment


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spectroscopy (PES) and its time-resolved analogue (TR-PES) were carried out for (CdO) n clusters with n = 8, 9, 12 and 16. A selected bunch was irradiated with a blue laser pulse of energy 3.1 eV, which yields the photoelectron spectrum (PES) of a particular cluster species. Time-dependent behavior of these species following excitation (1.55 eV, red) was probed by detachment of the excited electron via a second laser pulse (3.1 eV, blue; pump-probe spectroscopy). In contrast to (ZnO)n 51 the additional measurements with a laser energy of 4.66 eV do not produce any usable spectra due to the low photoionization cross section. Calculations. The methodology employed is analogous to our earlier work on (ZnO)n clusters.51 All DFT and TD-DFT calculations were performed using the TURBOMOLE44 program package. In order to speed up the calculations, the multipole accelerated resolution of identity method for Coulomb term (MARI-J)59 along with appropriate auxiliary basis sets60 was used. Structures of neutral and anionic CdO clusters were obtained by global optimizations employing genetic algorithm (GA)61 at the density functional theory (DFT) level using the PBE62-63 exchange-correlation functional and the polarized split-valence SVP64-65 basis sets, along with appropriate66 effective core potential (ECP) for Cd atoms. The most stable structures were subsequently refined using the quadruple zeta valence plus a double set of polarization functions (QZVPP)65,67 basis sets and the ecp-28-mwb66 ECP for Cd. To check basis set convergence additional calculations were performed for the most stable (CdO)8 isomer using the aug-cc-pV5Z68 basis sets for oxygen and aug-cc-PV5Z-PP69 basis sets along with the ecp-28-mdf70 ECP for cadmium. Results summarized in Table 1 show that the effects of increasing the basis set size beyond QZVPP are negligible. Therefore, QZVPP basis sets along with the ecp-28-mwb66 ECP for Cd atoms were used throughout this work. For neutral (CdO) n clusters the average binding energy b was calculated according to


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b =

− 1 ,

(1)

where n is the number of CdO units in the cluster, is the corresponding total (CdO) n energy and 1 is the energy of one CdO unit. Excitation and recombination (deexcitation from relaxed excited state) pathways for anionic species along with the corresponding changes of electron density were examined using time-dependent DFT (TD-DFT).71-73 All energy minima were verified by analytical71,74 and numerical harmonic frequency calculations for the ground and excited states, respectively. Calculations of anionic clusters employed spin-unrestricted formalism. In all cases the values of ⟨S2⟩ deviated less than 0.003 from ⟨S2⟩ of the pure spin state (0.75), indicating negligible spin contamination. Vertical detachment energies (VDEs) were calculated as differences between energies of anionic and neutral species at the structure of the anion. Calculations of adiabatic detachment energies (ADEs) used energies of fully relaxed structures of neutral and anionic clusters. During the PES experiment an electron can be ejected from an orbital below the SOMO of the anion (LUMO of the neutral) leaving an uncharged cluster in its excited singlet or triplet state. Related energies were calculated as the energy differences between the (CdO)n ground state and vertical excited states of (CdO)n at the corresponding structure of the anion. The lowest energy combined with the corresponding VDE yields the value of the HOMO-LUMO gap for neutral clusters. Radiative decay times were calculated as the inverse of the Einstein coefficient75 for a transition from the state 2 to the state 1 of the same spin using =

, 2 ℏ

(2)

where and are degeneracy factors of the states 1 and 2, respectively, is the elementary charge, is the vacuum permittivity, is the speed of light, is the electron mass, ℏ is the reduced Planck constant, is the oscillator strength and is the energy of the transition between the states 2 and 1. 5 ACS Paragon Plus Environment


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RESULTS AND DISCUSSION Mass spectroscopy. The mass spectrum of reacted Cd depicted in Figure 1 shows the stoichiometric compounds (CdO)n and also high intensities of oxygen-rich clusters Cd n O n; n+1; n+2 but no oxygen-deficient ones. This is very similar to the mass spectra for 76 Znn O n+x obtained by Gunaratne et al. from laser-ablated zinc oxide and the recent spectra

obtained from reacted zinc generated within the PACIS.51 Photoelectron spectroscopy and ground states. Calculated global minimum structures of (CdO)n and (CdO) n are shown in Figure 2. The computed VDEs and ADEs along with corresponding values extracted from PES (Figure 3) are summarized in Table 2. In case of VDEs the agreement between theory and experiment is very good, confirming our structure predictions. For ADEs the discrepancies are somewhat larger, likely due to peak broadening arising from probing hot species. With exception of n = 16 the VDEs of (CdO)n are roughly 0.5 eV above the VDEs determined previously for (ZnO)n clusters.51 In all examined structures the unpaired electron occupies the fully symmetric SOMO that consists mostly of cadmium 5s and oxygen 2p states (cf. Tables S1-S4). The global minimum 8 of both anionic and neutral CdO octamer is the D4d symmetric double four-membered ring shown in Figure 2. This result is consistent with the computational study of Matxain et al.19 and further confirmed by excellent agreement between calculated and experimental VDE of 2.43 and 2.4 eV, respectively (Table 2). This structure is similar to one of the two lowest energy structures found for ZnO octamer.51 The ground state of neutral and anionic cluster is 1A1 and 2A1, respectively. The calculated HOMO-LUMO gap of the neutral cluster is 1.65 eV (Table 2). Both the structure of the most stable cluster and the spin density distribution in the anion (see Figures 2, 4 and 5) are virtually identical to (ZnO)8 .51


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For the CdO nonamer, the most stable structure is the D3h symmetric tube-like assembly composed of three hexagonal rings (9A, Figure 2). This structure has been reported as the global minimum by Matxain et al.19 and also proposed as the global minimum of ZnO nonamer.77 However, in other studies 9A was reported only as a low energy local minimum and the C3h symmetric cage-like 9B (Figure 2) as the global minimum.10,14 According to our calculations both structures are low-lying minima for (CdO)9 and (CdO) with 9A only 0.17 9 and 0.30 eV more stable than 9B for the anionic and neutral cluster, respectively. In order to compare these results with ZnO we performed additional calculations for the ZnO nonamer. Both the tube- and the cage-like structures were found to be local minima for (ZnO)9 and (ZnO)-9 . However, the stability order of both structures is reversed with respect to CdO. The C3h symmetric cage is 0.27 and 0.15 eV more stable than the D3h symmetric tube for (ZnO)9 and (ZnO)9 , respectively. Calculated VDE for the global minimum 9A of (CdO)9 is 2.59 eV, in good agreement with the experimental value of 2.4 eV. The ground state of (CdO) 9 and (CdO)9 is 1 A1 and 2 A1 , respectively. The calculated HOMO-LUMO gap of (CdO)9 is 1.21 eV (Table 2). The most stable structure of (CdO) 12 and (CdO) is the Th symmetric cage 12 shown in 12 Figure 2. This structure has been reported as the global minimum for both CdO19 and ZnO51 dodecamer. Calculated VDE of 2.85 eV is close to the experimental value of 2.6 eV (Table 2). and (CdO) is 1Ag and 2Ag, respectively. The calculated The ground state of (CdO)12 12

HOMO-LUMO gap of (CdO)12 is 1.54 eV (Table 2). The spin density of the anion (cf. 51 Figures 4 and 5) closely resembles that of (ZnO)12 .

The Cs symmetric 16A (Figure 2) composed of a fused cubic fragment and hexagonal tube reminiscent of (CdO) is the global minimum structure of both (CdO) 16 and (CdO) . To our 9 16 best knowledge this is the first report of the global minimum for this cluster. 16A differs from the Td symmetric cage 16B found as the global minimum of ZnO hexadecamer (Figure 2).51 7 ACS Paragon Plus Environment


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However, 16B is a low energy minimum that is 0.43 and 0.30 eV less stable than the global , respectively. In case of ZnO 16B is 2.0 and 1.9 eV minimum 16A for (CdO) 16 and (CdO) 16 more stable than 16A for (ZnO)16 and (ZnO)16 , respectively. It is noteworthy, that the global

minimum of CdO hexadecamer partially adopts the cubic rock salt structure of bulk CdO. This may indicate a faster structural convergence of CdO clusters towards the bulk limit compared to ZnO. Calculated VDE for the most stable structure of (CdO)16 is 3.0 eV, in good

agreement with the experimental value of 2.9 eV (Table 2). The ground state of (CdO) 16 and (CdO)16 is 1 A and 2A , respectively. In contrast to smaller clusters the unpaired electron of is strongly localized already in the ground state as shown in Figures 4 and 5. The (CdO)16 calculated HOMO-LUMO gap of (CdO)16 is 0.79 eV, significantly lower than for smaller

cluster sizes (Table 2). The CdO clusters show similar structural rigidity as their ZnO counterparts51 – in all cases the neutral and anionic species share virtually the same structure. Since all (CdO) n are closed shell entities with rigid structures the observed VDE corresponds to electron detachment from the LUMO of the neutral cluster. Figure 5 demonstrates that shapes of the frontier orbitals (SOMO in anionic and LUMO in neutral clusters) in CdO)n are virtually identical to those in (ZnO)n . If present, the second feature in PES due to electron ejection from the HOMO of the neutral species can be used to provide a rough estimate of the HOMO-LUMO gap, ∆Eg, of the neutral cluster. Unfortunately, the photoionization cross section with a UV photon of 4.66 eV used in our experiments is too low to produce utilizable spectra and a 3.1 eV photon does not trigger electron detachment from the HOMO of the neutral clusters. However, in our previous study of (ZnO)n anions we were able to estimate the experimental ∆Eg from the PES, which were in the reasonable agreement with calculated values. It revealed an “inverse quantum confinement effect” with band gaps of ZnO)n clusters well below that of bulk ZnO.51 The 8 ACS Paragon Plus Environment

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calculated HOMO-LUMO gaps for (CdO)n clusters in the range of 0.79 - 1.65 eV (Table 2) are also well below the bad gap of bulk cadmium oxide (2.2 eV) and indicate a similar effect. Time-resolved

photoelectron spectroscopy

and

excited

states.

Time-resolved

photoelectron spectroscopy measurements were carried out for (CdO)n clusters with n = 8, 9, 12 and 16. Figure 6 shows the waterfall plots obtained for (CdO) . The temporal evolution of n the transient pump-probe feature (A) is presented in Figure 7. Calculated radiative decay times are between 186 and 471 ns (Table 2) explaining the observed ones far in excess of the 50 ps limit of our experiment. Nonradiative decay pathways seem to be nearly absent in all cases. This is the same type of behavior as found in (ZnO)n clusters.51 Energy diagrams of the lowest energy electronic transitions in (CdO) species are presented in Figure 8. In all n examined structures the lowest (SOMO → LUMO) excitation is accompanied by spin density reorganization leading to its depletion from the inner to the outer part of the cluster and localization of the unpaired electron. The subsequent structure relaxation leads to further electron localization within 5s states of a subset of Cd atoms. The corresponding spin densities are shown in the Figure 4, and the natural electron configurations are included in clusters Tables S1 to S4. The general excitation and recombination mechanism for all (CdO) n is virtually identical to that of (ZnO) .51 Figure 9 shows an example for (MO)12 (M = Zn, Cd) n

serving also as a general excitation-recombination scheme for (CdO)n . In all investigated cluster anions the ground state D0 is excited vertically into D1* which relaxes, in case of (CdO)12 via a cluster analog of the Jahn-Teller type distortion, to D1 from which electron

detachment is observed. D1 recombines vertically to D0* which relaxes back to the ground state D0. These results are validated by a good agreement between experimental and calculated VDEs of the D1 state of (CdO)n (Table 2). The calculated stability difference between ground states before (D0*) and after relaxation (D0) is small (up to 0.082 eV, see Table 4), further confirming the rigidity of the clusters. 9 ACS Paragon Plus Environment


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For (CdO)8 there are only a few low-lying excited states. The lowest ones are the dipole allowed 8 D0 → 8 D*1 (SOMO → LUMO) and a1 → e1 (SOMO → LUMO+1) transitions at 1.24 and 1.47 eV, respectively. Higher energy excitations are the dipole forbidden: e3 → a1 (SOMO-1 → SOMO) and e2 → a1 (SOMO-2 → SOMO) transitions at 1.65 and 1.68 eV, respectively (Figure 8). The estimated radiative decay time of 379 ns (Table 2) confirms experimentally observed long lifetime of the excited state of (CdO)8. The calculated VDE of the D1 state is 1.34 eV, somewhat lower than the experimental value of 1.8 eV (Table 2). Vertical excitation results in a spin density reorganization that is virtually the same (cf. Figure 4) as in case of (ZnO)8 .51 However, in contrast to (ZnO)8 relaxation of the lowest excited state structure does not lead to symmetry lowering. The lowest electronic excitations in (CdO)9 are the dipole allowed 9A D0 → 9A D*1 (SOMO → LUMO) transition at 1.08 eV along with two dipole forbidden e → a1 (SOMO-1 → SOMO) and e → a1 (SOMO-2 → SOMO) excitations at 1.21 and 1.44 eV, respectively (Figure 8). The estimated decay time for 9A D1 is 258 ns (Table 2). The calculated VDE is 1.57 eV, in very good agreement with the experimental value of 1.6 eV. For (CdO)12 there are only four low-lying excited states (Figure 8). The lowest one is the

12 D0 → 12 D*1 (SOMO → LUMO) transition at 1.33 eV. Higher energy excitations are the tg → ag (SOMO-1 → SOMO), tu → ag (SOMO-2 → SOMO) and eu → ag (SOMO-3 → SOMO) at 1.53, 1.62 and 1.81 eV, respectively. The SOMO-1 → SOMO and SOMO-3 → SOMO transitions are dipole forbidden. The triple degeneracy of the 12 D*1 state leads to an excited state Jahn-Teller type distortion upon structure relaxation resulting in symmetry reduction from Th to C2h. In the final D1 state the unpaired electron is located within the bu symmetric SOMO, localized mainly outside the cluster and consisting of 5s states of two Cd atoms (Figure 4). The calculated VDE of the excited state of 1.69 eV is in good agreement with the experimental value of 1.6 eV. The estimated decay time for the 12 D1 → 12 D*0 10 ACS Paragon Plus Environment

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transition is 186 ns (Table 2). The spin density changes during vertical excitation and 51 geometry relaxation are virtually the same as in the case of (ZnO)12 (cf. Figures 4 and 9). are denser than in case of smaller clusters The electronic excited states of (CdO)16

resulting in numerous low energy transitions (Figure 8). The lowest one is the 16A D0 → 16A D*1 (SOMO → LUMO) transition at 0.98 eV. The calculated VDE of the D1 state is 2.13 eV, in good agreement with the experimental value of 1.9 eV (Table 2). The estimated decay time is 471 ns. Neutral CdO clusters. Table 5 summarizes calculated excitation energies, oscillator strengths and decay times for the lowest dipole allowed singlet excitations of the neutral (CdO)n clusters. In case of (CdO)8 the two lowest transitions at 1.87 and 1.92 eV are dipole forbidden. For the dodecamer there is a dark excitation at 1.67 eV. It is noteworthy that for all but (CdO) 8 clusters the excitation energies are significantly lower than for the bulk phase (2.2 eV). The estimated radiative decay times vary between 161 and 19382 ns for octamer and hexadecamer, respectively.

CONCLUSIONS The most obvious conclusion to be drawn from our combined experimental and theoretical study is a striking similarity between (CdO)n and (ZnO)n fragments at least up to n = 12. Although in some cases the order of stability of low energy structures is different both systems show very similar structures and electronic properties in their both ground and excited states. For larger clusters the structural differences can be attributed to a different convergence rate towards different bulk limits - wurtzite or zinc blende for ZnO and rock salt structure for CdO. Due to similar frontier orbitals the excitation-recombination pathways in (CdO)n are virtually identical to their (ZnO)n counterparts.51 They involve electron localization within the relaxed excited state, which together with rigid cluster structures leads to the observed exceptionally long-lived excited states approximately 1 eV above the 11 ACS Paragon Plus Environment


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conduction band edge (LUMO of the neutral). These results render both rows of cluster homologs suitable building blocks for ACCAMs and, in addition, excited state lifetimes a valuable probe for the aptitude to serve as such. Despite the similar structures, electronic properties of the clusters are slightly different for both materials. The VDE basically determines the location of the conduction band (LUMO) of the neutral cluster and it can be tuned from 2.0 eV for (ZnO)12 51 up to 2.6 eV for (CdO)12 . Also the band gap is tunable between 2 eV for (ZnO)n and 1.2 eV for (CdO)n . This has practical implications since the former as light-harvesting chromophore would not capture the complete spectrum of sunlight while the latter with its low band gap is absolutely capable of doing just that. These results demonstrate, that gas phase studies of potential ACCAMs building blocks could facilitate band engineering and rational materials design. This approach seems especially viable since the HOMO-LUMO gap of C60 in the gas phase is 1.6 eV,78 very close to bulk fullerite with ∆Eg = 1.5 eV79 Finally, our study suggests the existence of more general rule for EIIO fragments (EII = divalent element), forming (EIIO)n clusters with similar properties. In order to expand the ACCAM construction kit it will be necessary to investigate more general rows of cluster homologs, including group II-VI and III-V semiconductors of the type (EIICh)n (Ch = chalcogen, e.g., ZnS, ZnSe) and (EIIIPn)n (EIII = trivalent element, Pn = pnictogen, e.g., AlN, AlP), respectively.

ASSOCIATED CONTENT Natural electron configuration of Cd and O atoms in the ground and excited states as well as Cartesian atomic coordinates of (CdO)n and (CdO)n clusters. This material is available free of charge via the Internet at http://pubs.acs.

AUTHOR INFORMATION 12 ACS Paragon Plus Environment

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Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS Financial support from the German Research Foundation (DFG grant no. GA-389/12-2) and the Fonds der Chemischen Industrie is gratefully acknowledged.

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(79) Wen, C.; Aida, T.; Honma, I.; Komiyama, H.; Yamada, K. The Optical-Absorption and Photoluminescence Spectra of C60 Single-Crystals. J. Phys.: Condens. Matter 1994, 6, 1603– 1610.



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Figure 1. Mass spectrum of Cd n O n; n+1; n+2 clusters. Arrows indicate positions of the (CdO)n series with n = 6–16.


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Figure 2. Global minimum structures of the ground (D0) and excited (D1) states of (CdO)n (n = 8, 9, 12, 16) along with the corresponding symmetry point group of D0. Selected Cd-O bond lengths in Å, values for the ground state of neutral clusters in parentheses. For comparison the global minimum structures of (ZnO)9 and (ZnO)16 are shown.



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Figure 3. Conventional photoelectron spectra of cadmium oxide cluster anions (3.1 eV, blue). The observed feature corresponds to electron ejection from the LUMO of neutral clusters.


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Figure 4. Spin density isosurfaces for (CdO)n clusters. The ground state (D0 ) and the lowest excited vertical (D*1 ) and adiabatic (D1 ) states are shown in the columns 1, 3 and 4, respectively. Labels of the excited states are given in Table 3. Columns 2 and 5 show the differential spin density for vertical excitation and decay leading to D∗1 and D1 states, respectively. Positive values in red, negative values in blue.



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Figure 5. Comparison of SOMO isosurfaces for (CdO)n and (ZnO) . n


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Figure 6. Waterfall plots of the transient pump-probe features of (CdO) . Feature A n corresponds to the photodetachment from the D1 state via a 3.1 eV (blue) pulse. Feature X is related to the detachment of an electron from D0 with a blue pulse (3.1 eV). (CdO)16 does

not show feature X since the VDE is at 2.9 eV and in turn X cannot be seen on the kinetic energy scale.



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Figure 7. Temporal evolution of the transient pump-probe feature following excitation of (CdO)n (n = 8, 9, 12, 16) with a 1.55 eV laser pulse.


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Figure 8. Energy diagrams of lowest energy electronic transitions in (CdO) . Excited states n accessible via a dipole allowed transition from the ground state and dark states are drawn in black and grey, respectively.



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Figure 9. General excitation-recombination scheme (ZnO)n and (CdO) . Spin density n isosurfaces of the ground and excited states of (ZnO)12 and (CdO) . Changes of the spin 12

density during excitation and recombination are shown next to the vertical arrows (positive values in red, negative values in blue).


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Table 1. Basis set dependence of the vertical detachment energies (VDE), excitation energies (∆E) and oscillator strengths (f) of the lowest excitations in (CdO)8 . The transitions e3 → a1 and e2 → a1 are dipole forbidden ( = 0). Energies in eV.

basis

VDE

D0 → D*1

a1 → e1

e3 → a1

e2 → a1

∆E

∆E

∆E

∆E

QZVPP

2.43

1.24

0.063

1.47

0.235

1.65

1.68

aug-cc-pV5Z(-PP)

2.41

1.22

0.067

1.46

0.249

1.66

1.68



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Table 2. Experimental and calculated vertical detachment energies (VDEs) of (CdO)n in the ground and excited states as well as adiabatic detachment energies (ADEs) in the ground state, vertical excitation energies (∆ ∆E) and radiative decay times τ (ns), calculated HOMO-LUMO gaps (∆ ∆Eg) and binding energies (Eb) of (CdO)n clusters. Energies in eV.

structure

VDE (D0) a

VDE (D1) b

ADE (D0)

∆E

τ

∆Eg

Eb

2.41

1.24

379

1.65

-3.34

2.2

2.57

1.08

258

1.21

-3.38

1.69

2.4

2.83

1.33

186

1.54

-3.56

2.13

-c

2.93

0.98

471

0.79

-3.65

exp

calc

exp

calc

expa

calc

8

2.4

2.43

1.8

1.34

2.2

9A

2.4

2.59

1.6

1.57

12

2.6

2.85

1.6

16A

2.9

3.00

1.9

a

±0.1 eV

b

±0.2 eV

c

not available due to a high noise level in PES

32

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Table 3. Labels of the lowest energy excited states of (CdO)n clusters along with the corresponding orbitals involved in the electronic transitions. Vertical excitation states are denoted with an asterisk. vertical transitions structure

adiabatic states

label

transitiona

label

transitionb

8

8 D*1

a 1 → b2

8 D1

b2 → a 1

9A

9A D*1

a1 → a2

9A D1

a 2 → a1

12

12 D*1

ag → tu

12 D1

bu → a g

16A

16A D*1

a → a

16A D1

a → a

a

ground state → excited state

b

excited state → ground state



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Table 4. Calculated fluorescence energies ∆Ef (eV), relative stabilities of the ground state structures before and after relaxation ∆Er (eV) and oscillator strengths

(length representation) of the lowest dipole

allowed transitions in (CdO)n . structure

∆Ef

∆Er

8

1.08

0.082

0.052

9A

1.03

0.023

0.084

12

1.18

0.076

0.088

16A

0.86

0.059

0.066


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Table 5. Calculated vertical transition energies ∆E (eV), oscillator strengths

(length representation) and corresponding radiative decay

time (ns) of the lowest dipole allowed excitations in (CdO)n (singlet excitations). structure

transition

∆E

8

b2 → a 1

2.30

0.0269

161

9A

e → a1

1.42

0.0077

12

tu → ag

1.79

16A

a1 → a1

1.12

a

a

2945

0.0133

a

1625

0.0009

19382

Including degeneracy factor.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of Content:


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


49x38mm (600 x 600 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

88x95mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


72x65mm (600 x 600 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

160x119mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60x151mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

78x75mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


63x49mm (600 x 600 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

75x70mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


81x161mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

55x43mm (300 x 300 DPI)


Page 46 of 46

CdO and ZnO Clusters as Potential Building Blocks for Cluster

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