Cage-Like Nanoclusters of ZnO Probed by Time-Resolved

Letter pubs.acs.org/JPCL

Cage-Like Nanoclusters of ZnO Probed by Time-Resolved Photoelectron Spectroscopy and Theory Julian Heinzelmann,‡ Alexander Koop,‡ Sebastian Proch,*,‡ Gerd F. Ganteför,‡ Roman Łazarski,† and Marek Sierka*,† ‡

Fachbereich für Physik, Universität Konstanz, Universitätsstrasse 10, 78464 Konstanz, Germany Otto-Schott-Institut für Materialforschung, Friedrich-Schiller Universität Jena, Löbdergraben 32, 07743 Jena, Germany

†

S Supporting Information *

ABSTRACT: Zinc oxide nanoclusters have been predicted as promising building blocks for cluster-assembled materials with unprecedented properties. Here, for the first time these clusters are probed by time-resolved photoelectron spectroscopy and characterized in detail by density functional theory. Their validity as building blocks for clusterassembled materials is confirmed via rigid cage-like structures facilitating threedimensional aggregation in combination with large band gaps that are nevertheless significantly lower than any known ZnO polymorph. In addition, electron−hole pair localization in the excited state of the cluster anions combined with their structural rigidity leads to extraordinary long-lived states above the band gap virtually independent of the cluster size, defying the rule “every atom counts”. SECTION: Spectroscopy, Photochemistry, and Excited States

Z

phase (ZnO)n clusters have been so far limited to ZnO and ZnO−,18 anionic clusters containing up to six Zn atoms,19 and (ZnO)+n cluster cations with n = 3−16 showing the smallest tube-like structure for (ZnO)+6 .20 Small clusters have been found to show a strong sizedependence (“every atom counts”)2 of their properties such as the gas phase stability,21 reactivity toward oxygen,22−28 and catalytic activity.29,30 Depositing clusters on surfaces does not change this nonscalable behavior much, but it adds another crucial parameter by introducing different substrates, for example, on titania, only Au6 and Au7 are CO oxidation active,31−33 whereas on magnesia, gold clusters with eight atoms and above do the trick.34−36 Electronic properties of clusters in the gas phase are so far no exception to this general trend, and their nature strongly depends on size and structure.37−39 An unusually long-lived excited state has been observed in Au−6 ; its lifetime is owed to a planar structure and excitation into an orbital outside the cluster plane thus resulting in low de-excitation probability. Such an extraordinary long lifetime was not found in other Au−n clusters.40 Long excitedstate lifetimes are usually associated with special cluster geometries as found for Au−n ,37 Hg−n ,38 or Pb−n 39 but can also be influenced by the degree of solvation41,42 or oxidation.43 Studies of mixed clusters like CdxTey44 or VxCy45 also disclosed size-dependent trends in the pump−probe transients; however, general indicators that always apply have not been identified.

inc oxide (ZnO) is a current hot topic material and a wideband gap semiconductor with a gap of 3.3 eV at 300 K. Its optoelectronic applications are close to those of GaN with uses in light-emitting devices, photodiodes, metal−insulator−semiconductor diodes, and transparent thin film transistors.1 The large forbidden zone makes ZnO also an ideal material for spintronics (dilute magnetic semiconductors), where ferromagnetism is usually introduced by doping with various transition metals (V, Co, Cr, Mn, Fe).2 Furthermore, zinc oxide thin films are applied in dye-sensitized solar cells as electron transporting medium because its wide gap prevents efficient sunlight capture.3 On top of these favorable properties it is readily available, cheap, and nontoxic.1 In recent years, an increasing scientific and technological interest has been attracted to nanostructured metal oxides in form of clusters, nanoparticles, and nanofilms.4 At the nanoand subnanoscale, some properties of these systems can be very different from those of their atomic and bulk counterparts. The extreme confinement due to the proximity of the interfaces stabilizes new structures and phases that otherwise cannot be obtained as bulk materials.5 This opens new possibilities for the development of highly functional tailor-made structures by a bottom up approach with clusters serving as building blocks, leading to the so-called cluster-assembled materials (CAMs).6,7 There are special prerequisites for clusters to serve as such building blocks, namely, a very rigid structure that also favors three-dimensional assembly and large HOMO−LUMO gaps to prevent fusion of clusters to form larger entities.6 Small (ZnO)n clusters have been predicted by theory to exhibit alternant cagelike structures that fulfill these requirements,8−17 possibly leading to thermally stable, nanoporous zinc oxide CAMs with unprecedented properties.18−22 Experimental studies of gas© 2014 American Chemical Society

Received: June 10, 2014 Accepted: July 18, 2014 Published: July 18, 2014 2642

dx.doi.org/10.1021/jz501181c | J. Phys. Chem. Lett. 2014, 5, 2642−2648

The Journal of Physical Chemistry Letters

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In this work, (ZnO)−n (n = 5, 8, 12, 13, 14, 16) have been investigated by conventional and time-resolved photoelectron spectroscopy (TR−PES) accompanied by detailed analysis via time-dependent density functional theory (TD−DFT). Our studies not only provide an evidence for the alternant cage structures predicted for (ZnO)n nanoclusters but also demonstrate their so far unprecedented electronic properties. Laser excitation (1.55 eV) invariably results in exceptionally long-lived excited states independent of the number of ZnO monomers in the investigated size regime. Computational studies find spin density localization on a subset of Zn atoms upon structural relaxation of excited states (electron−hole pair localization) accompanied by radiative lifetimes on the order of 100 ns in all investigated (ZnO)−n (n = 5, 8, 12, 13, 14, 16) cluster ions. This behavior is independent of the symmetry, structure, and size of the clusters. Thus, electron−hole pair localization following the excitation along with structural rigidity of their cage-like structures can be regarded as a prerequisite for extended excited state lifetimes in ZnO cluster anions. In addition, the same behavior could be observed for n = 18 and 23 also supporting the interpretation, albeit no theoretical assessment has been carried out. The mass spectrum of zinc oxide cluster anions produced in a pulsed-arc discharge source from zinc metal and oxygen presented in Figure 1 shows high intensities of oxygen-rich

within the LUMO of the neutral cluster. Results of the natural population analysis46 (natural electron configurations of Zn and O atoms) presented in Supporting Information Tables S7 to S12 show that in all investigated clusters this fully symmetric singly occupied molecular orbital (SOMO) consists mainly of 4s and 2p states of Zn and O atoms, respectively. In principle, HOMO−LUMO gaps can directly be extracted from the respective PE spectra when electrons are ejected from orbitals corresponding to the LUMO and the HOMO of neutral clusters. However, spectra recorded at 3.1 eV (Supporting Information Figure S3) show only the LUMO of the neutral cluster and cannot trigger electron ejection from the HOMO. Additional measurements with a laser energy of 4.66 eV (Supporting Information Figure S4) might be complicated by a low photoionization cross section of the cluster anion. In turn, high laser intensities are required and, therefore, hot species are probed. This could lead to thermionic electron emission at low kinetic energies, but this is not assumed to be the case because the number of atoms in the clusters is relatively high, leading to a considerable number of degrees of freedom and, thus, low cluster temperatures.47−49 Therefore, it is possible to find rough estimates for the HOMO−LUMO gap values using PE spectra from Supporting Information Figures S3 and S4. The procedure uses the slopes of the peaks to determine adiabatic detachment energies (ADEs) of the first and second feature in the PE spectra and then takes their difference as a measure for the gap size. The corresponding values presented in Table 1 Table 1. Experimental and Calculated Electronic Properties of Zinc Oxide Clustersa ΔEg

VDE n (structure)

exp

calc

exp

calc

5 (5A) 8 12 13 14 16

1.7 1.9−2.0 2.0 2.1 2.2 2.1

1.82 1.98, 2.13b 2.22 2.37 2.41 2.44

2.0 ± 0.3 not possiblec 1.9 ± 0.3 1.9 ± 0.3 1.8 ± 0.3 1.9 ± 0.3

2.57 2.29, 2.06b 2.43 1.94 1.98 2.14

Vertical detachment energies (VDEs) of (ZnO)−n (n = 5, 8, 12, 13, 14, 16) as well as the HOMO−LUMO gap (ΔEg) of (ZnO)n clusters (eV). For corresponding calculated electron detachment energies used to calculate ΔEg, see Table S13. bValues calculated for the two most stable isomers 8A and 8B, respectively. cThe PE spectra of (ZnO)−8 show an additional unassigned peak which is observed for the 3.1 and the 4.66 eV excitation energies, respectively, which is not the case for all other clusters. a

Figure 1. Mass spectrum of anionic ZnnO−n; n+1; n+2 clusters. The arrows (red) indicate positions of the (ZnO)−n series from n = 5−12.

clusters and almost no oxygen-deficient clusters. The series of clusters containing stoichiometric (ZnO)−n (red arrows) are found as the first peak in the series ZnnO−n; n+1; n+2. This structure is similar to results obtained by Gunaratne et al. from laser-ablated zinc oxide. Here, also oxygen-rich clusters are observed, albeit no additional oxygen was used.19 Photoelectron spectra with a laser energy of 3.1 eV (4.66 eV) obtained for (ZnO)−n (n = 5, 8, 12, 13, 14, 16) are presented in Supporting Information Figure S3 (S4). Other clusters including n = 6, 8, 9, 10, 11, 15, 17, 18, 23, and 25 had photoionization cross sections that were too low for obtaining reasonable spectra with photon energies of 4.66 eV. However, these clusters also showed the long-lived excited state, but only for n = 18 and 23 was it possible to obtain good quality TR−PE spectra. Because all neutral clusters of the type (ZnO)n are closed-shell entities, the additional electron of the anion resides

show somewhat larger errors due to this crude approach. However, they support results of our calculations, which yield band gaps of approximately 2 eV, well below bulk ZnO. The congruence of theory and experiment is further confirmed by a good agreement of the measured and calculated vertical detachment energies (VDEs), as shown in Table 1. Calculated structures of (ZnO)−n are shown in Figure 2. It is noteworthy that for each cluster size, the neutral as well as the ground and excited state of the anion virtually share the same structure. A detailed discussion of each separate cluster is presented in the Supporting Information (Section 3). Electronic properties of (ZnO)−n (n = 5, 8, 12, 13, 14, 16, 18, 23) cluster anions were probed by TR−PES. The intensity decay of the excited state obtained by 1.55 eV excitation was followed by electron detachment with a 3.1 eV probe pulse. 2643



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Figure 2. Calculated structures of the ground (D0) and excited (D1 and D2) states of (ZnO)−n (n = 5, 8, 12, 13, 14, 16), along with related symmetry point groups. Selected Zn−O bond lengths in angstrom and those for neutral clusters in parentheses.

studied cluster anions are excited vertically from the ground state D0 into D1* which relaxes, oftentimes via Jahn−Tellertype distortion, to D1 from which electron detachment is observed. This causes the time-variant photodetachment feature to be found at lower energies than 1.55 eV above the ground state. Similar behavior is exhibited by C−60.50 D1 is vertically de-excited to D0*, which finally relaxes back to the ground state D0. The general excitation−de-excitation scheme for (ZnO)−n is presented in Figure 4, with (ZnO)−12 serving as an example, and additionally for (ZnO) 5− in Supporting Information Figure S7, but here it is complicated by another possible mechanism. A detailed description of all investigated species is presented in the Supporting Information (Section 4). The good agreement between experimental and calculated VDEs (Table 1) mostly confirms structures reported previously and shown in Figure 2.8−16,51−54 However, our calculations predict HOMO−LUMO gaps of the clusters in the range of 1.94−2.57 eV, well below the band gap of bulk zinc oxide (hexagonal and cubic) of 3.3 eV. As already mentioned, the

The temporal evolution of the transient pump−probe feature (A) and the waterfall plot of (ZnO)−5 is shown in Figure 3, whereas the corresponding waterfall plots of the other cluster anions are displayed in Supporting Information Figure S9. The complete row of clusters investigated by red-blue pump−probe spectroscopy shows lifetimes far exceeding the 80 ps limit of our experiment (see Supporting Information, Section 1). Moreover, all sizes but n = 18 and 23 have been under scrutiny by TD−DFT and low-lying electronic transitions are presented in Supporting Information Figure S6. Conventional UV PES with a photon energy of 4.66 eV could not be obtained for n = 18 and 23, again most probably because of their low photoionization cross sections. Due to high computational demands of global structure optimizations at the DFT level these clusters were also not investigated by theory. Nevertheless, we show time-dependent experimental data for n = 18 and 23 to corroborate the generality of the long-lived excited state. The labeling of the excited states investigated in this work is shown in Supporting Information Table S14. Basically, all 2644



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Figure 3. Temporal evolution of the transient pump−probe feature (A) following excitation of (ZnO)−n (n = 5, 8, 12, 13, 14, 16, 18, 23) with a 1.55 eV laser pulse. In case of (ZnO)−5 , the waterfall plot showing the actual photoelectron spectra at different time delays is presented in addition. Feature A corresponds to the photodetachment from the D1 state via a 3.1 eV (blue) pulse. Feature B probably stems from photodetachment out of D1 via a 1.55 eV (red) pulse (two-photon process, red), and finally X is related to the detachment of an electron from D0 with a blue pulse (Dn states are defined in Figure 4 and in Supporting Information Figures S5, S7, and S8). The remaining waterfall plots are shown in Supporting Information Figure S9.

actual gap of the clusters can roughly be estimated from the experimental PE spectra and indeed turns out to be approximately 2 eV (cf. Table 1). This means that the general principle for band gaps to grow with decreasing particle size due to the quantum confinement is broken here, that is, (ZnO)n clusters show an “inverse quantum confinement effect”. Such behavior is also known from small TiO2 cluster anions; however, in this case, the bulk value is reached fast at n = 7.55 Clusters investigated here retain their reduced band gap up to at least 16 ZnO units. These properties might offer a unique possibility to create oxide semiconductors with lower band gaps, a much sought after material, because sulfides and selenides admittedly provide smaller gaps but are less stable and oftentimes undergo photocorrosion.56 The allotropes of carbon suggest a route to materials with lower band gaps, whereas graphite has no gap at all, C60-fullerite exhibits ΔEg = 1.7 eV,57 and diamond 5.5 eV. The gap in C−60 from the photoelectron spectrum is 1.6 eV,58 which is very close to the bulk value. Diamond and zinc oxide share the same structure in bulk; therefore, the approach to lower the forbidden zone in ZnO by creating a fullerite-like phase seems viable. Moreover, our investigations demonstrate that ZnO clusters meet perfectly all prerequisites for good CAM building blocks, very rigid

Figure 4. Spin density isosurfaces of the ground and excited states of (ZnO)−12 (general excitation−de-excitation scheme). Changes of the spin density during excitation and de-excitation are shown next to the vertical arrows (positive values in red, negative values in blue).

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prerequisite for hot carrier extraction (hot carrier chromophores).62 The time-dependent photoelectron spectroscopic investigation of zinc oxide nanostructures shows that the excited state with its long lifetime prevails even if the number of ZnO units is changed. This type of behavior is rather uncommon in small clusters but is not undesired because it makes structures and their respective properties more predictable. This is very advantageous for experiments on surfaces because a targeted feature can then be conserved even if mass resolution is too low to distinguish between adjacent homologues. In addition, cluster-assembled materials are more easily realized because small deviations in the building block would not carry so much weight. Time-dependent functional calculations suggest an electron−hole pair localization that combined with rigid structures of the clusters leads to long-lived excited states, independent of the cluster size and geometrical structure.

structures, large HOMO−LUMO gaps in the range of 2 eV, and high symmetry favoring three-dimensional assembly − − (especially (ZnO)12 and (ZnO)16 ; see Figure 2). Such thermally stable, low-band gap and low-density CAMs from dodeca- and hexadecamers of zinc oxide have been predicted by theory.18−22 In addition, dilute semiconductor materials have not lived up to their full potential as spintronics materials, yet due to problems with sample preparation,2 that is, doping patterns are very sensitive to generation conditions. Clusters might serve as a solution here as well because they could be doped in specific patterns and then assembled in a material where dopants are on fixed positions within the building blocks. For example, beaded nanorods of (ZnO)12 have been theoretically described59 and a cobalt-doped version of these rods60 on surfaces may create a great way of facilitating transport measurements to investigate their spintronics properties. As a remarkable feature, all (ZnO)−n (n = 5, 8, 12, 13, 14, 16, 18, 23) clusters exhibit very long excited state lifetimes defying the common rule in cluster science “every atom counts”. Calculated radiative decay times are approximately 100 ns (see Supporting Information Table S16) and very well explain the observed ones far in excess of 80 ps for all species under investigation. Nonradiative decay pathways seem to be absent in most cases (Figure 3 and Supporting Information Figure S9). This absence is explained in terms of the rigid structures of these clusters since calculations find virtually the same structure for neutral, anion, and also the excited state of the anion (Figure 2). The unusually long-lived excited states are not related to cluster symmetry (Figure 2) but rather to the localization of electron density to a subset of Zn atoms (electron−hole pair localization, Figure 5). These properties

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ASSOCIATED CONTENT

S Supporting Information *

Experimental and computational methods. Experimental photoelectron spectra with 3.1 and 4.66 eV laser energy and timeresolved waterfall plots are found here. Moreover, vertical electron detachment energies and natural electron configurations for all clusters investigated by TD−DFT are presented as well as spin density analysis for all clusters in their respective ground and excited states. Detailed discussion of individual clusters. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

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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Figure 5. Spin density isosurfaces for the lowest relaxed (adiabatic) excited states of (ZnO)−n . In all structures, spin density is localized on a subset of the Zn atoms.

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Cage-Like Nanoclusters of ZnO Probed by Time-Resolved

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