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Optical Properties of Monolayer-Protected Aluminum Clusters: TimeDependent Density Functional Theory Study Johan Lindgren, Andre Clayborne, and Lauri Lehtovaara* Department of Chemistry, Nanoscience Center, University of Jyväskylä, FI-40014 Jyväskylä, Finland

Downloaded by TEXAS A&M INTL UNIV on August 23, 2015 | http://pubs.acs.org Publication Date (Web): August 11, 2015 | doi: 10.1021/acs.jpcc.5b05894

S Supporting Information *

ABSTRACT: We examine the electronic and optical properties of experimentally known monolayer-protected aluminum clusters Al4(C5H5)4, Al50(C5Me5)12, and Al69(N(SiMe3)2)183− using time-dependent density functional theory. By comparing Al4(C5H5)4 and the theoretical Al4(N(SiMe3)2)4 cluster, we observe significant changes in the optical absorption spectra caused by different hybridization between metal core and ligands. Using these initial observations, we explain the calculated spectra of Al50(C5Me5)12 and Al69(N(SiMe3)2)183−. Al50(C5Me5)12 shows a structured spectrum with clear regions of low-intensity core-to-core transitions followed by high-intensity ligand-to-core transitions due to its high symmetry and π-bonding to the Cp ligands. The spectrum of Al69(N(SiMe3)2)183− is rather featureless as the core-to-core and ligand-to-core regions partially merge because of the lower symmetry found in the metal core and differences in the ligand−core hybridization. Though there are minimal features in the spectra, the most intense features are identified as excitations from ligand states to metal core states. We show that our calculated absorption spectrum for Al69(N(SiMe3)2)183− agrees with recent experimental results for N(SiMe3)2-protected Al nanoclusters with an average diameter of 1.5 nm.

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with other metals (i.e., Ag,15,16 Cu,17,18 Pd,19−21 Pt,22 etc.) or changing the ligand composition.23 Though monolayer-protected noble metal clusters have received a great deal of attention, MP main group metal clusters are becoming increasingly important because of their low cost and similar applications toward the design of promising materials as compared to their noble metal counterparts. Schnöckel, Schnepf, and others have focused on group 13 and 14 elements such as aluminum and gallium,24−29 germanium,30 tin,31,32 and indium.33,34 The Schnöckel group has successfully synthesized various monolayer-protected aluminum clusters, which he termed “metalloid” clusters because of the number of metal−metal bonds being greater than the number of metal−ligand bonds. Using ligands such as cyclopentadienyl/pentamethylcyclopentadienyl (Cp/Cp*) and N(SiMe3)2 [NR2, bis(trimethylsilyl)amide], his group synthesized more than 20+ aluminum metalloid systems with a range of sizes and compositions, including Al4Cp4*,35 Al50Cp12*,36 SiAl14Cp6*,37 Al69(NR2)183−,38 and Al77(NR2)202−.39 Of the aforementioned aluminum systems, Al4 Cp 4*, Al50Cp12*, and SiAl14Cp6* have been reported to contain electronic structures that adhere to the superatom complex model similar to monolayer-protected gold clusters. Schnöckel and co-workers proposed the jellium model could explain the stable nature of SiAl14Cp6*.37 Later, one of the authors showed that both Al4Cp4*11 and Al50Cp12*40 could be considered superatom complexes along with SiAl14Cp6*. In a comparative

INTRODUCTION Atomically precise, monolayer-protected (MP) noble metal clusters with diameters of 99 65 18 35 54

a

In the case of Al4Cp4, the whole D manifold is considered, whereas for Al4(NR2)4, only the two lowest D states are taken into consideration. “Ligand” refers to the manifold below the 1P-HOMO manifold.

ligand hybridizes with the 2S state of the Al core, creating a smooth superatom-like state spanning the whole cluster and leading to a large transition dipole moment with the 1P states. The shoulder at 3.7 eV is due to transitions from the 1P states to the split D manifold. Next, we examine how electronic and optical properties of the ligand-protected Al4 cluster change when the Cp ligands are replaced by N(SiMe3)2 ligands. We would like to point out that though Al4(NR2)4 has not been synthesized, it is important to understand the effect on the electronic structure, and ultimately the optical spectra, caused by changing the ligand on both small

Figure 1. Relaxed structures of (A) Al4Cp4, (B) Al4(NR2)4, (C) Al50Cp12*, and (D) Al69R183−. The aluminum cores Al50 and Al69 are shown below their respective clusters. Colors: dark brown for Al, cyan for C, white for H, blue for N, and yellow for Si.

states that are split because of tetrahedral symmetry (Figure S1A of the in Supporting Information and Figure 2A). A large HOMO−LUMO gap of 3.38 eV reflects high electronic stability originating from the shell closing.

Figure 2. (A) Projection of KS eigenstates onto local atomic orbitals of Al (core, blue) and ligand atoms (C and H, red) of Al4Cp4. The black line is the total density of states (TDOS). The KS eigenenergy at zero is the Fermi energy. (B) Calculated absorption spectrum of Al4Cp4 from the visible region (2.5 eV) to the deep UV region (4 eV). The spectrum was folded with a Gaussian with a width of 0.04 eV. As in panels A and B, the projected densities of states (N, Si, C, and H, red) and calculated absorption spectrum of Al4(NR2)4 are shown in panels C and D, respectively. 19541

DOI: 10.1021/acs.jpcc.5b05894 J. Phys. Chem. C 2015, 119, 19539−19547

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The Journal of Physical Chemistry C

Figure 3. Total density of states (black) and the density of states projected onto the local atomic orbitals of the metal core (blue) and the ligands (red) for (A) Al50Cp12* and (C) Al69(NR2)183−. Calculated absorption spectrum for (B) Al50Cp12* and (D) Al69(NR2)183−.

Al4Cp4 and Al4(NR2)4 clusters share some characteristics of the Au25(SR)18− cluster; most notably, all of them have eight superatomic electrons with 1P states as HOMOs and split 1D states as LUMOs. However, the optical properties of Al4Cp4 are clearly different from those of Au25(SR)18−. Although, both clusters have a 1D ← 1P transition, the spectrum of Al4Cp4 is dominated by the 2S ← 1P transition, which is not seen in the spectrum of Au25(SR)18− as an individual intense peak. Al50Cp12*. The structure of the monolayer-protected cluster Al50Cp12* (see Figure 1C) is onion-like. The innermost layer consists of eight Al atoms, the second shell (middle layer) of 30 atoms, and the outermost layer of 12 AlCp* units that take an icosahedral symmetry with respect to the Al38 core.36,42 In a previous study, Clayborne et al. concentrated on the groundstate properties and included a TDDFT absorption spectrum.40 There, they reported the cluster could be considered as a superatom with ne = 138 core electrons (with a 2F143P61I26 electronic shell configuration), a large HOMO−LUMO gap of 0.9 eV (see also Figure S1C), a high ionization potential, and a high electron affinity.40 However, they did not provide a detailed analysis of their calculated absorption spectrum. Therefore, we have extended their calculation by including more KS electron−hole pairs and analysis of the observed spectral features using the TD-DFPT approach. We find our calculated ground-state results agree with the aforementioned observations. The total density of states and projection of KS states onto atomic orbitals of the metal core and the ligands are shown in of Figure 3A. The KS states on both sides of the Fermi level are dominant states localized in the metal core. The first set of KS states localized dominantly on the ligands is shown around −2 eV. As we will see later, these ligand states have a major contribution to the absorption spectrum. The optical absorption of Al50Cp12*, shown in Figure 3B, begins around 2 eV with a nearly flat, low-intensity feature extending up to 2.8 eV. Next, the absorption intensity begins to rise quickly, reaching a maximum at 3.5 eV followed by a second maximum at 3.8 eV with a high-intensity tail. To understand the origin of the observed features, we performed a

and larger systems. Later we will address electronic and optical properties of Al50Cp12* and Al69(NR2)183−. The structure of Al4(NR2)4 was optimized starting from the relaxed structure of Al4Cp4 in three steps. (1) The metal core was frozen to the geometry of the Al4Cp4 core, and the NR2 ligands had hydrogens instead of methyl groups. (2) Methyl groups were added with the core still frozen. (3) All contraints were removed. The relaxed structure is shown in Figure 1B. When the ligand is changed to NR2, a directional bond between an aluminum atom and a nitrogen atom is formed. The geometry of the core remains tetrahedral, but the Al−Al bond length is reduced to 2.66 Å. Whereas the Al4Cp4 cluster is a highly symmetric tetrahedral cluster with highly symmetric delocalized electronic states, the “stick-like” bonding of nitrogen ligands in Al4(NR2)4 alters the cluster in three ways. (1) The geometry of the ligand shell becomes less symmetric, reducing the symmetry of the cluster from D2h to C2. (2) Delocalization and symmetry of the electronic states are greatly reduced in the ligand shell. (3) New ligand states appear in the optically active region just below the HOMO states. The new states are shown in the projected density of states (Figure 2C) between −2.1 and −2.5 eV with a large ligand contribution. The effects of reduced symmetry and delocalization are best seen from the changes in the molecular orbitals (Figures S2 and S3). Also, these effects lead to larger splitting of the 1D manifold (Figure S1B) and, thus, to a decreased HOMO− LUMO gap of 2.9 eV. The changes in ground-state electronic structure result in a more complex absorption spectrum shown in Figure 2D (see also Table 1). Three features below 4 eV can be seen. The lowest-energy peak at 3.0 eV arises from transitions from the 1P-HOMO states to the lowest D states at 1.5 eV. The peak at 3.5 eV does not have the same origin as in the case of Al4Cp4, but it is a combination of transitions from 1P-HOMO and the manifold below the HOMO (between −2.1 and −2.5 eV) to the lowest 1D states. The most intense feature, an asymmetric peak at 3.8 eV, can be assigned to transitions from the 1PHOMO states to the 2S state surrounded by transitions from the manifold below HOMO to the 1D states (Figure 2C). 19542


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Figure 4. Transition contribution map (TCM) for excitation at (A) 2.17 and (B) 3.50 eV for Al50Cp12*. Note that the TCM intensities are not on the same scale. The intensity maximum is actually 3 times higher in panel B. The Fermi energy was shifted to zero in DOS. The absorption spectrum is given in the bottom right corner of the panels with a dashed line indicating the excitation energy.

detailed analysis of the excitations at five different energies: 2.17 and 2.35 eV in the low-intensity regime, a shoulder at 3.17 eV, and 3.50 and 3.79 eV in the high-intensity regime. Excitations from the metal core to the metal core are dominant for the low-intensity regime, that is for peaks at 2.17 and 2.35 eV, as seen from TCM in Figure 4A and Figure S4, respectively. The excitation at 2.17 eV is mainly from the occupied KS state at −0.55 eV to the unoccupied KS state at 1.60 eV. These states are both localized in the metal core as seen from PDOS and the approximate electron−hole density, which is localized in the core. The shoulder at 3.17 eV shows some core-to-ligand character as seen from Figure S5. However, the high-intensity region above the shoulder is dominated by transitions from ligand states to core states. The absorption maximum at 3.50 eV is a prime example of this behavior. The TCM in Figure 4B shows that the electron is excited from the occupied KS states at −2.07, −2.24, and −2.42 eV to the unoccupied states at 1.31, 1.18, and 1.01 eV, respectively. The occupied states are predominantly ligand states, whereas the unoccupied states are practically pure core states. The approximate electron−hole density shows clearly how excitation creates a hole in the ligand shell and an electron to the core, i.e., a charge transfer from

ligand to core. This also leads to a significant ligand contribution in the induced density. A similar behavior is observed for an excitation energy of 3.79 eV (Figure S6); however, unbound states above 2.5 eV have a larger contribution as expected. Previously, it was suggested that in Al50Cp12*, the highintensity features of the calculated spectra may originate from transitions between high-angular momentum superatom-like core states (i.e., 1I to 2G). Our analysis illustrates this may not be the case. Although the excitations are to high-angular momentum superatom-like core states, the origin states are predominantly ligand states. Al69(NR2)183−. The X-ray structure of one of the largest aluminum clusters, Al69(N(SiMe3)2)183−, was determined in 2001,38 but to the best of our knowledge, there has been no ab initio study of the system. The structure of the Al69(N(SiMe3)2)183− cluster, shown in Figure 1D, can formally be written as [Al51(AlNR2)18]3−, where a central Al atom is surrounded by 12 Al atoms forming the first shell, followed by a second shell of 38 Al atoms, which is covered by an outermost layer of 18 Al(NR2) units.38,42 The average distance between the center atom and the first shell is 2.81 Å, between the first and second shells 2.85 Å, and between the second and third 19543


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Figure 5. Transition contribution maps and induced densitites for transitions at (A) 2.05 and (B) 2.94 eV for Al69(NR2)183−. The maximal value in the TCM in panel B is ∼1.5 times higher than in panel A. See Figure 4 for more details.

broad hybridization of the Cp ligands to the Al50 core and the directional hybridization observed in Al69. The core electronic states of Al69(NR2)183− near the Fermi level show intercrossing of two or more angular momenta (Figure S1D and Figure 3C). This is in contrast to Al50Cp12* in which angular momenta remain nearly pure. Because of the intercrossing of angular momentum states, core−ligand hybridization, the reduced symmetry of the metal core, and the small HOMO−LUMO gap, it is unclear if Al69(NR2)183− can be considered as a superatom complex of 192 delocalized electrons. Also, because the electronic structure indicates the cluster is approaching a metal-like continuum (Figure S1D and Figure 3C), it should be considered as a true metalloid cluster, i.e., a stable intermediate toward the metallic bulk phase. As mentioned previously, the NR2 ligands should introduce more ligand states into the optically active region as was seen in the case of Al 4 (NR 2 ) 4 . The absorption spectrum of Al69(NR2)183− (Figure 3D) shows a continuous, almost featureless rise of intensity starting from 1.7 eV. A small peak can be distinguished from the background at 3 eV and then a more rapid rise around 4 eV. For analysis, we focused on the following excitations: the beginning of the spectrum at 2.05 eV, the peak at 2.94 eV, a shoulder just before the rapid rise at 3.79 eV, and a shoulder in the rapid rise at 4.22 eV. For the first

shells 2.72 Å. The distances within the shells are almost identical in the first (2.80 Å) and second (2.81 Å) shells but in the outermost layer of 18 Al atoms have an average Al−Al distance of 5.02 Å. Our computational results agree well with the experimental Al−Al distances obtained from the X-ray structure.38 The Bader analysis suggests ionic binding as the outermost Al atoms lose approximately one electron to the ligands. Ionic bonding has been observed in previous studies of Al4Cp4,11,41 Al50Cp12*,40 and Al77R20.70 Interestingly, the geometry of Al69(N(SiMe3)2)183− is not as symmetric as that of Al50Cp12* and has a small HOMO−LUMO gap of 0.2 eV (Figure 3C and Figure S1D). The gap is smaller than those of the previous aluminum clusters presented in this work. The calculated gap is an indication that the cluster is approaching the bulk limit, i.e., being nearly metallic, similar to the Al77 cluster.70,71 Though the gap is small, the electronic structure of Al69(NR2)183− has similarities to the Al4(NR2)4 cluster. As in the case of Al4(NR2)4, there are ligand states introduced within the electronic structure below the HOMO manifold of Al69(NR2)183−, which may be in the optically active region and will be addressed below. Also, core−ligand hybridization is induced becaue of the directional bond from the NR2 ligands. Compared to Al50Cp12*, there is a clear difference between the 19544


The Journal of Physical Chemistry C excitation, at 2.05 eV, the transitions are predominantly from core to core (Figure 5A). Notice the initial peak has a relatively low intensity compared to that of the entire spectra. The first clear increase in intensity corresponds to the peak at 2.94 eV, where the ligands begin to contribute substantially (Figure 5B). The excitation is almost a pure transition from the occupied states at −2.5 eV to the unoccupied core states at 0.4 eV. The occupied states between −2.5 and −2.3 eV are the first states, which have a significant ligand contribution. The rapid rise that begins approximately at 4 eV is due to a new manifold of nearly pure ligand states between −4.0 and −3.3 eV that contribute to the absorption process together with the states between −2.5 and −2.3 eV, as seen in Figures S7 and S8.

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CONCLUSION

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ASSOCIATED CONTENT

We have investigated the electronic and optical properties of ligand-protected aluminum clusters Al 4 Cp 4 , Al 4 (NR 2 ) 4 , Al50Cp12*, and Al69(NR2)183− using TDDFT. Al4Cp4 and Al4(NR2)4 served as minimal model systems for studying hybridization between ligand states and superatomic metal core states. In the Al4Cp4 cluster, the aromatic π-states of Cp ligands hybridize with the core states, leading to delocalized, highly symmetric states in the optically active region. These superatomic states span the entire cluster, leading to a UV− vis spectrum with a single intense peak originating dominantly from the 2S ← 1P transition, and a shoulder originating from the 1D ← 1P transition. Replacing the Cp ligands with NR2 ligands leads to more localized core−ligand bonding, which not only introduces new states into the optically active region but also provides a complex optical absorption spectrum. We find similar effects in Al50Cp12* and Al69(NR2)183−. Both Al50Cp12* and Al69(NR2)183− have spectra with regions of lowintensity core-to-core transitions and high-intensity ligand-tocore transitions. However, the NR2 ligand of the Al69(NR2)183− cluster reduces the symmetry within the core, presents localized (directional) bonding, and introduces more ligand states in the optically active region compared to Al50Cp12*. Further, the small HOMO−LUMO gap and intercrossing of superatom-like states to create continuous (metallic) bands present evidence that Al69(NR2)183− is more metalloid than superatomic. Overall, the UV−vis spectrum for Al69(NR2)183− is rather featureless compared to the spectrum of Al50Cp12*. Nevertheless, the few apparent features, namely, the peak at 3 eV and the rapid rise around 4 eV, can be attributed to ligand-to-core transitions. Though we cannot find evidence of plasmon behavior for the systems presented here, the optical spectrum of Al69(NR2)183− is very similar to the optical spectrum of NR2-protected Al clusters with an average diameter of 1.5 nm.64 We hope these results encourage experimental and theoretical research of optical properties of ligand-protected aluminum and other main group metalloid clusters, as this aspect has not been tackled extensively. We believe that our work can offer insight into the interpretation of future experimental findings.

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COMPARISON AND DISCUSSION The Al50Cp12* and Al69(NR2)183− clusters share similar characteristics of larger gold MPCs such as Au144(SR)60 and Au102(p-MBA)44. For example, the electronic states near the Fermi level begin to experience intercrossing with other higherangular momentum states. In both monolayer-protected gold and the aluminum systems presented here, this is indicative of the systems approaching the metal continuum. Similarities within their respective optical properties also exist. The lowenergy excitations are mainly core-to-core and are followed by higher-energy transitions that involve both ligand and core states. Nevertheless, there are also clear differences. For example, in gold MPCs, both ligand-to-core and core-to-ligand transitions make a significant contribution to the spectrum, whereas in aluminum MPCs, ligand-to-core transitions are dominant. Moreover, the Al50Cp12* and Al69(NR2)183− clusters do not have any d electron contribution, which is in contrast to the case in gold MPCs, where interband Au(6sp) ← Au(5d) transitions have a major contribution to the spectrum at higher energies. Although we are not aware of any experimental optical absorption spectrum for Al50Cp12* or Al69(NR2)183−, Sen et al. recently presented an experimental spectrum for polydisperse NR2 ligand-protected aluminum nanoclusters with an average diameter of 1.5 nm.64 They observed a maximum at 350 nm (∼3.5 eV) and a rapid rise after 320 nm (∼3.9 eV). If we take into account the polydispersity of the experimental sample and the fact that the adiabatic PBE functional typically underestimates excitation energies in MPCs45 (as well as charge transfer energies and ionization thresholds), remarkably, their experimental spectrum is similar to the simulated spectrum of Al69(NR2)183−. Further, the core diameter of Al69(NR2)183− is 1.3 nm, which resides slightly below the average diameter of the nanoclusters reported by Sen et al. Sen et al. also applied a phenomenological multilayer Mie model to describe the observed maximum at 350 nm as a plasmon, where ligand shell and quantum size effects cause significant deviation from the bulk behavior. On the basis of our results, we cannot judge whether the 350 nm excitation is a plasmon. Visual inspection does not show a clear localized surface plasmon resonance, which has been observed in gold MPCs53 and silver nanoparticles.72 Nevertheless, this does not exclude the possibility of plasmonic behavior hidden in the complexity of the metal−ligand interface. Furthermore, our conclusions agree with theirs about the importance of the ligand effect as well as on quantum size (and shape) effects. It would be interesting to examine larger aluminum metalloid systems, such as Al77(NR2)202−,70,71 using ab initio methods to better understand plasmons in protected aluminum systems.

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b05894. Angular momentum-projected density of states for all clusters (Figure S1), molecular orbital figures for Al4Cp4 (Figure S2) and Al4(NR2)4 (Figure S3), TCM plots of selected excitations for Al50Cp12* (Figures S4−S6), TCM plots of selected excitations for Al69(NR2)183− (Figures S7 and S8), and complete refs 61 and 66 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: lauri.lehtovaara@jyu.fi. Notes

The authors declare no competing financial interest. 19545


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ACKNOWLEDGMENTS This work was financially supported by the Academy of Finland through Projects 273499, 269402, and 258500. The electronic structure calculations were conducted using supercomputing resources provided by CSC-The Finnish IT Center for Science and local FGI cluster. We thank Prof. Hansgeorg Schnöckel, Prof. Hannu Häkkinen, and Dr. Sami Malola for helpful discussions.


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