Palladium in the Gap: Cluster Assemblies with Band Edges Localized

Article pubs.acs.org/JPCC

Palladium in the Gap: Cluster Assemblies with Band Edges Localized on Linkers Arthur C. Reber,†,⊥ Sukhendu Mandal,‡,⊥ Meichun Qian,† Hector M. Saavedra,‡ Paul S. Weiss,*,‡,§,∥ Shiv N. Khanna,*,† and Ayusman Sen*,‡ †

Department of Physics, Virginia Commonwealth University, Richmond, Virginia 23284, United States Department of Chemistry, and §Department of Physics, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ∥ California NanoSystems Institute, Departments of Chemistry & Biochemistry and Materials Science & Engineering, University of California, Los Angeles, Los Angeles, California 90095, United States ‡

S Supporting Information *

ABSTRACT: Cluster-assembled materials composed of [Pd2As14]4− polyvalent anions, where a palladium dimer links two As7 clusters, have been synthesized in zero- and two-dimensional architectures. Their dimensionality was controlled through the selection of the counterions, and their structures were characterized by single-crystal X-ray diffraction. Reflectance spectroscopy measurements indicate that the zero- and twodimensional assemblies have band gaps of 1.35 and 1.15 eV, respectively, and are smaller than the 1.80 eV gap of the As73− cluster. Theoretical investigations reveal that the frontier orbitals of the cluster building blocks are located primarily on the Pd dimer. Further, these orbitals are embedded inside the intrinsic band gap of the As73− clusters, reducing the band gap energy with edge states that are localized on the Pd sites. It is suggested that such a periodic array of narrow localized states may lead to interesting transport and optical properties as the carriers may become localized at these sites.

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INTRODUCTION Nanoscale materials in which size-specific clusters serve as the building blocks provide a strategy for producing solids with tunable characteristcs.1−9 The properties of the clusterassembled materials can then be altered by choice of cluster building blocks and the linking units, as well as the architecture of the resulting solids.10−16 An important class of clusters, amenable to such assemblies, are the polyvalent cluster anions known generally as Zintl ions.17−19 These ions are highly stable and are found to form stable salts/solids when combined with alkali ions/alkali-derived countercations. Depending on the size of the building motifs, the solvent, and the countercations, such assemblies can assemble in a variety of architectures including linear and spiral chains,20 stacked two-dimensional (2D) planes, or three-dimensional (3D) solids with linked or isolated polyvalent anions.7 We recently demonstrated the potential offered by such assemblies by considering the band gaps of the resulting materials as the property of choice.6,7,21−23 Our studies, based on As73− and As113− combined with a variety of countercations, showed that the resulting solids are semiconductors where the band gap can be altered from 1.09 eV for the Cs3As7 to 2.08 eV for the 2D As73−‑ linked by Cs.6 The band gap energies of cluster assemblies may be controlled through several mechanisms because the valence band edge is primarily composed of states derived from the © 2012 American Chemical Society

polyvalent anion, while the conduction band edge is derived primarily from the countercations. Changing the countercation leads to changes in the energy of the conduction band edge.6 The valence band edge may be varied through a different mechanism in the assemblies synthesized from the same polyvalent anion by varying the dimensionality of the cluster assemblies. The studies indicated that the well-positioned countercations can lead to intense internal electric fields that can lower the occupied band and hence the valence band edge. Additional variations in the assemblies can be induced by using assemblies based on [As7]2−, which has an unpaired electron21 and hence results in magnetic solids. A great deal of research has gone into linking23−30 and replacing atoms in the Zintl clusters.31−41 These linked clusters may produce new motifs and have a different intrinsic band gap energy than the clusters that are linked together. The properties of the resulting material can then be quite different from those of the building cluster motif.42−47 For example, the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in a free Li3As7 cluster is 2.86 eV, while the band gap of the corresponding solid is only 0.52 eV. Received: February 1, 2012 Revised: April 3, 2012 Published: April 13, 2012 10207

dx.doi.org/10.1021/jp301049e | J. Phys. Chem. C 2012, 116, 10207−10214

The Journal of Physical Chemistry C

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crystallization. After 3−4 days, black bricklike crystals were formed at the bottom of the test tube. Compound 2. In a scintillation vial, a 1 mL en solution of Pd(PPh3)4 (0.15 g, 0.13 mmol) was added dropwise to a 1.5 mL en solution of Cs3As7 (0.075 g, 0.08 mmol) and 1.5 mL of K3As7 (0.05 g, 0.08 mmol) at room temperature and stirred for 30 min. Crypt (0.095 g, 0.25 mmol) was added to this solution and stirred for another 1 h at room temperature. The red suspension was filtered through a syringe filter and the resulting red solution layered with toluene to allow crystallization. After 3−4 days, platelike black crystals were formed at the bottom of the test tube. Single-Crystal Structure Determination. X-ray diffraction data were collected at 123 K on a Bruker APEX diffractometer with a CCD area detector equipped with an X-stream 2000 Cryo-system low-temperature device. A suitable crystal for each compound was carefully selected under a polarizing microscope and mounted on a loop using Paratone-N oil and quickly placed under the nitrogen flow of the cryostream. The X-ray generator was operated at 50 kV and 32 mA using Mo Kα (λ = 0.710 73 Å) radiation. Data were collected with a ω scan width of 0.3°. A total of 600, 430, 235, and 50 frames were collected in four different settings of φ (0°, 90°, 180°, and 270°, respectively) keeping the sample-to-detector distance fixed at 5.8 cm and the detector position (2θ) fixed at −25°. Pertinent experimental details of the structure determination of 1 and 2 are presented in Table 1. The data were reduced using SAINTPLUS, and an empirical absorption correction was applied using the SADABS program.48 The crystal structure was solved by direct methods using SHELXS97 and refined using SHELXL97 present in the

In this paper, we offer a new class of such assemblies composed of linked cluster anions and where the valence and conduction band edges are both determined by the states derived from the linking atoms. We demonstrate these intriguing findings via ionic cluster assemblies built of covalently linked multicenter building blocks of [As7−Pd2− As7]4− composed of two As7 units linked by a Pd dimer. The covalent linking not only changes the oxidation state of the composite cluster to −4 but also allows assemblies of higher dimensionality through alkali metal counterions that link the multiply charged clusters and thereby increase the dimensionality. Theoretical studies find that the frontier orbitals in the [Pd2As14]4− are primarily localized on the Pd dimer offering a different mechanism that determines the band gap energy. The intrinsic band gap energy of the oxidatively coupled As7 clusters is reduced because the electronic states of the Pd dimer are embedded within the band gap. The resulting solid can thus be visualized as a superlattice with the band edges localized at the Pd sites. Such solids can be expected to exhibit a new class of conducting behaviors. In particular, one can envision excitations, e.g., an array of electron−hole pairs, located at the Pd sites.

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EXPERIMENTAL SECTION Synthesis. All of the reagents used were commercially available. 4,7,13,16,21,24-Hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane (Crypt) and anhydrous ethylenediamine (en) (99.5%, purified by redistillation, packed under Ar) were purchased from Aldrich. Pd(PPh3)4 and As powder (−70 mesh, 99.99%), K (99.95%, ampouled under argon), and Cs (99.98%, vacuum-sealed in break-sealed ampules) were purchased from Alfa Aesar. Toluene was dried by passing through an activated alumina column followed by deoxygenating by passing over a copper catalyst. All glassware (oven-dried), reactants, and solvents were stored in a glovebox filled with argon. All manipulations were performed in an argon-filled glovebox. The detailed synthetic procedures of the precursors As7A3 (where, A = K and Cs) and compounds 1 and 2 are summarized below. Precursor As7A3 (A = K and Cs). The precursors, As7A3, were directly synthesized from the corresponding elements in en in scintillation vials and used for further synthetic and crystallization manipulations. For example, As7K3 was synthesized by mixing of As (∼700 mg) with a preheated mixture of K (∼120 mg) and en (3 mL) in a scintillation vial (K and en mixture was stirred at 50 °C for 1 h to dissolve K partially in en as it produced a blue-colored solution). This mixture was continuously stirred for 30 min at 50 °C followed by the addition of 3 mL of en, and the mixture was stirred overnight at room temperature. The red suspension was filtered through an Acrodisc premium 25 mm syringe filter with GxF/0.2 μm pores (syringe filter), and the resulting dark red solution was used for further reactions. Note: As and K metals did not completely dissolve. The preparation of other precursors (As7A3) with A = Cs employed a similar procedure except the preheating steps were not necessary as these metals readily dissolve in en. Compound 1. In a scintillation vial, a 1 mL en solution of Pd(PPh3)4 (0.192 g, 0.16 mmol) was added dropwise to a 3 mL en solution of K3As7 (0.110 g, 0.17 mmol) at room temperature and stirred for 30 min. Crypt (0.2 g, 0.53 mmol) was added to this solution and stirred for another 1 h at room temperature. The red-colored suspension was filtered through a syringe filter and the resulting red solution layered with toluene to allow

Table 1. Crystal Data and Structure Refinement Parameters for Compounds 1 and 2a compound

1

empirical formula formula weight crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z size ρcalc (g cm−3) θ range (deg) reflections collected unique reflections no. of parameters goodness of fit (S) final R indices (all data) largest diff. peak and hole (e·Å−3)

C42H72As7K2N9O12Pd 1592.12 monoclinic C2/c (no. 15) 15.833(2) 23.595(3) 34.004(5) 90.000 90.709(3) 90.000 12702(3) 8 0.14 × 0.10 × 0.08 1.665 1.20−28.26 41842 15211 659 1.047 R1 = 0.0669, wR2 = 0.1146 1.102 and −0.827

2 C20H44As7CsKN4O6Pd 1239.44 orthorhombic Pbca (no. 61) 14.507(3) 18.533(4) 27.263(6) 90.000 90.000 90.000 7330(3) 8 0.22 × 0.14 × 0.08 2.246 1.93−28.31 46292 8886 361 1.034 R1 = 0.0417, wR2 = 0.0692 1.090 and −0.777

R1 = Σ||Fo| − |Fc ||/Σ |Fo|; wR2 = {Σ [w(Fo2 − Fc2)]/Σ [w(Fo2) 2]}1/2 . w = 1/[ρ2(Fo)2 + (aP)2 + bP]. P = [max (Fo, O) + 2(Fc) 2]/3, where a = 0.0536 and b = 18.1889 for compound 1 and a = 0.0281 and b = 7.6490 for compound 2, respectively. a

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SHELXTL V6.14 package.49,50 For compounds 1 and 2, all non-hydrogen atoms were easily found from the different Fourier maps and refined anisotropically. In compound 1, the C and N atoms of the ethylenediamine molecule are disordered so we were not able to fix the hydrogen on this molecule. The last cycles of the refinement included atomic positions, anisotropic thermal parameters for all the non-hydrogen atoms, and isotropic thermal parameters for all the hydrogen atoms. Full-matrix-least-squares structure refinement against F2 was carried out using the SHELXTL V6.14 package of programs. CCDC 864116 and 864117 contain the crystallographic data for compounds 1 and 2. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre (CCDC) via www.ccdc.cam.ac.uk/data_request/cif. Yields were 75% and 80% for 1 and 2, respectively, based on metal; crystals were monophasic in nature. The crystals were washed with ethylenediamine and dried in vacuum in a glovebox and ground finely into powder form prior to the measurements. The crystals were air and moisture sensitive. Solid-State Band Gap Measurements. Diffuse reflectance spectra were collected at room temperature using a PerkinElmer Lambda 950 UV−vis−NIR Spectrophotometer, equipped with a Harrick Praying Mantis diffuse reflectance accessory. A photomultiplier tube was used for detection in the 320− 860.5 nm range, while a lead sulfide detector was used for the 860.5−2300 nm range. The spectra were collected from 320 to 2300 nm, with 1.0 nm resolution and integration times of 0.64 s. A dry and finely ground magnesium oxide (MgO) powder was used as a reflectance reference material. Prior to any measurements, all the compounds were finely ground and diluted with MgO to 30% by weight. We measured the optical band gap energy of 1 and 2 using diffuse reflectance spectroscopy and the Kubelka−Munk model.51−53 This two-flux model, which considers only diffuse light, is generally used to determine the absorption coefficients from a surface that both scatters and absorbs incident radiation. For a crystalline solid with a nonzero band gap (Ebg), the frequency dependence (ν) of the absorption coefficient (κ) can be approximated as κ (ν ) =

Figure 1. Tauc plots showing the band gap energies determined from the optical absorption spectra (see text) of (a) [Kcrypt]4[Pd2(As7)2]·3en, 1, and (b) [K-crypt]2[Pd2(As7)2Cs2]·2en, 2.

BT (hν − E bg )n

Table 2. Experimentally Measured and Theoretically Calculated Band Gaps for 1 and 2

hν

where BT is a constant derived from the square of the averaged dipolar momentum matrix element and n is equal to 0.5 and 2 for direct and indirect band gap transitions, respectively.51−53 Using the above equation, the band gap of a material can be obtained by extrapolating to the x-axis intercept with a linear fit to a plot of (κhν))1/n vs hν. Figure 1 shows these Tauc plots for the measured cluster assemblies, while the values of the band gap energies are listed in Table 2. The formula for indirect band gap energies was used in all cases as we have found this to be most appropriate in our previous work on these systems with rather low coupling and nearly flat bands.6,7 Theoretical Methods. First-principles electronic structure studies within a gradient-corrected density functional framework54 were carried out to probe the nature of electronic bonding and of the electronic bands and to understand the experimental findings on the band gaps. The investigations included studies on individual clusters as well as their assemblies. Gradient-corrected electronic structure calculations were performed on periodic solids using the experimentally

compound

formula

exptl band gap (eV)

theor. band gap (eV)

1 2

[K-crypt]4[Pd2(As7)2]·3en [K-crypt]2[Pd2(As7)2Cs2]·2en

1.35 ± 0.003 1.15 ± 0.004

1.39 1.14

determined crystal structures to understand the nature of the electronic bands and the origins of the band gap energy. These calculations were performed using the Vienna Ab-Initio Simulation Package (VASP).55 The projector augmented wave (PAW) pseudopotentials were used to describe the electron−ion interaction. The exchange interactions and correlations effects were incorporated using the generalized gradient-corrected functional proposed by Perdew et al. Brillouin zone integrations were carried out on Monkhorst− Pack grid k-points, using the tetrahedral method. The kinetic energy cutoff of 300 eV was found to give converged results and was used for the plane wave basis. The geometries in these studies used the experimentally determined crystal structures; geometry optimization was found to have a small effect on the 10209



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2b). There are no bonding interactions between the [Pd2As14]4− clusters and the cations. The distances between the centers of gravity of the clusters are in the range from 18.10 to 19.65 Å. The en solvent molecules occupy the space between the [Pd2As14]4− clusters showing no bonding interactions to the chains or to the cations. The X-ray crystal structure of 2 reveals that it crystallizes in an orthorhombic system [Pbca] and the unit cell of the structure contains two Pd atoms, two As7 units, two Cs ions, four [K(2,2,2-crypt)] units, and two molecules of ethylenediamine. The structural analysis reveals that the [Pd2As14]4− units are linked by Cs+ ions to form the 2-dimensional 12-membered honeycomb-like structure (Figure 3a). The layers are arranged

band gap energy, less than 0.1 eV. Second, gradient-corrected calculations were performed on free clusters to understand the nature of bonding, using the Amsterdam Density Functional Package (ADF).56 Geometries were optimized fully without constraint. Relativistic effects were taken in to account using the Zeroth Order Regular Approximation,57 and the TZVP basis set was used. There is surprisingly close agreement between the measured and calculated band gap energies. As the materials are made of weakly interacting clusters, the good agreement of the bulk cluster material and experiment is a reflection of the situation in free clusters.58

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RESULTS AND DISCUSSION The X-ray crystal structure of 1 reveals that the cluster building block consists of two As7 clusters linked by a Pd-dimer center, and the structure can be viewed as two norbornadiene-like As7 groups bound to a Pd−Pd dimer center in a μ, η2, η2 fashion, as shown in Figure 2a. The compound crystallizes in a monoclinic

Figure 2. (a) Cluster unit of 1, [K-crypt]4[Pd2(As7)2]·3en. [K-crypt] and ethylendiamine molecules are not shown for clarity. (b) Crystal structure of 1 with projection along the c-axis. The cluster units are ordered analogously to hexagonal close-packing. The lines joining the K+ ions and [Pd2(As7)2]4− clusters are a guide to the eye to illustrate the hexagonal network but do not represent bonds.

Figure 3. (a) Two-dimensional layer arrangement of [Kcrypt]2[Pd2(As7)2Cs2]·2en, 2, in the ab plane and (b) in the ac plane. Crypt and solvent molecules are not shown for clarity.

in an ABAB fashion in the ac plane (Figure 3b). The [KCrypt]+ ions are situated in the interlayer spaces. The en solvent molecules bonded with Cs+ ion are protruding into the interlayer spaces. We measured the optical band gap energies of 1 and 2 using diffuse reflectance spectroscopy and analysis from the Kubelka−Munk model. Figure 1 shows these Tauc plots for the measured cluster assemblies. The band gap energies were calculated using the formula for indirect transitions and are listed in Table 2. Note that the band gap energy of the zerodimensional (0D) assembly 1 is 1.35 ± 0.003 eV; however, the energy gap decreased to 1.15 ± 0.004 eV for 2D assemblies, 2.

system [C2/c], and the unit cell of the structure contains two Pd atoms, two As7 units, four [K(2,2,2-crypt)] units, and three molecules of ethylenediamine. In the structure of 1, the Pd ions adopt a distorted square-planar coordination geometry and are linked by an axial Pd−Pd bond of 2.7092(7) Å. Dinuclear Pt complexes with Pt−Pt59,60 bonds are common whereas those with Pd dimers are not. The Pd−As bond distances average 2.465 Å and are consistent with previous reports of this cluster in the literature.61 The [Pd2As14]4− units in 1 are separated from each other by [K(crypt)]+ ions and are arranged parallel to the c -axis with distorted hexagonal close-packing (Figure 10210



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In both cases, band gap energies are relatively low compared to the As-based cluster assemblies we studied previously; only Cs3As7 and As72− cluster assemblies6,21 had lower band gap energies out of the 20 assemblies studied to date. Second, we note that the band gap energy is larger for the 0D cluster assembly than for the 2D cluster assembly. Ionic cluster assemblies constructed from As clusters generally have larger band gap energies in 2D assemblies than 0D due to internal electric fields,7 unless the HOMO is shielded from the electric field generated by the counterions.22 We have performed theoretical investigations on the periodic solid using the geometry and unit cell determined from the Xray structure. The band gap energy was calculated to be 1.39 eV (exptl = 1.35 eV) for 1 and 1.14 eV (exptl = 1.15 eV) for 2. Excellent agreement between theory and experiment has previously been observed for these cluster assemblies and is primarily a result of the reduced delocalization error found in density functional calculations of molecules and clusters. The projected density of states for 2 is plotted in Figure 4 and

Figure 5. (a) Geometry of the cluster model for [Pd2As14]4−. (b) HOMO−LUMO gap energy of [As7]3− and [Pd2As14]4− as a function of point charges. (c) Projected density of states on [Pd2As14]4− as a function of point charges and Pd2(As7)2Cs4.

modulated by a significant local gradient in the electrostatic potential, which generally stabilizes the lone pair orbitals in arsenic-based clusters, resulting in an increase in the band gap energy. Only adjacent counterions may create an inhomogeneous electric field that adjusts the relative positions of states in the clusters. Ionic charges positioned at larger distances do not produce a significant gradient in the electric field because the gradient falls off rapidly with distance and screening.7 So, we find that the gap energy decreases with increasing internal electric fields for the Pd-linked clusters, which is unexpected, as they usually increase for arsenic-based clusters such as [As7]3−. To understand the decrease in gap energy with increasing internal electric fields, we show the projected density of states for the clusters with varying Z, as well as the projected density of states for Cs4Pd2(As7)2 for reference in Figure 5C. We use a fragment analysis to identify the features that control the band gap energy of the clusters.58 This method uses the electronic structure of the fragments [(As7)2]6− and Pd2 as the basis set to analyze the electronic structure of the resulting unit and enables direct analysis of the cluster orbitals at the frontier. For As7, we split the cluster orbitals into four shells, each with closely spaced groupings of orbitals. Shell 1 and shell 2 are bonding orbitals between the arsenic atoms, and shell 3 forms the lone pair localized on the three equatorial arsenic atoms on each cluster. Shell 4 is antibonding in nature and is unfilled in the As73− cluster. The Pd2 orbitals are assigned by the symmetry of the molecular orbital in the dimer and are plotted in the negative direction to clarify their positions. First, we note that the HOMO is dominated by the Pd2 Σu orbital at all plotted values of Z. This explains why the gap energy does not increase in the Pd-linked clusters while it does in the [As7]3−, because the HOMO is localized on the Pd dimer and is shielded from the internal electric field. The position of shell 3 orbitals decreases from nearly degenerate with the HOMO energy at Z = 0 to −0.5 eV below the HOMO for Z = 0.8 because the lone pair orbitals are pointed at the counterions and are lowered in energy as the charge increases. This lowering of energy of the lone pair is the expected result; however, because they are not the HOMO of the cluster, it does not affect the gap energy. Because the Pd dimer sees no gradient of the electric field, the HOMO localized on Pd2 is unaffected by the internal electric field, and hence it is unaffected by the change in dimensionality. The internal electric field does effect the LUMO and LUMO+1 of [Pd2(As7)2]4− which are marked by shell 4, shell 3, and Delta

Figure 4. Calculated density of states for 2.

shows that the band gap is controlled by the Pd and As7 states, with a sharp peak in the Pd density of states at the Fermi energy. The K-Crypt has negligible density at the Fermi energy, and note that the scale for the Cs plot is magnified. Theoretical investigations were performed on cluster models to identify the nature of the frontier orbitals of the cluster motif that will control the band gap energy. The first question we would like to address is why the band gap energy decreases from 1.35 eV in the 0D structure to 1.19 eV in the 2D structure. Figure 5A shows the cluster model that we used to understand the band gap energies: a [Pd2As14]4− cluster with four point charges to serve as a model for the counterions in the positions found after optimizing the locations of the Cs atoms. Figure 5B shows the change in the HOMO−LUMO gap as the value of the point charges is varied, with a similar calculation for Z3As7 for comparison. As shown in Figure 5b, we find that the gap energy for the [Pd2(As7)2]4− model system decreases from 1.20 eV where Z = 0.0 to 0.87 eV where Z = +0.8, while the band gap of [As7]3− increases from 1.80 to 2.78 eV. We use Z = +0.8 because this approximates the Bader charge found on the Cs counterions in periodic calculations. In a previous study, we found that the band gap energy may be 10211



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contributions in the DOS, and these orbitals cross at Z = 0.7. Consequently, there is an acceleration in the decrease in the gap at Z = +0.7 at which point the band gap energy decreases more significantly, as the shell 3 delta orbital becomes the LUMO. These MOs are plotted in Figure 6 for the

Figure 7. Local density of states at different cross sections of the Cs4Pd2As14 cluster.

of the Cs4Pd2As14 cluster motif. On the left, the density of states is color coded to match the atoms in the adjacent ball and stick plot. The top of the valence band is located almost exclusively on the Pd dimer, and the bottom of the conduction band is located on the Pd dimer and adjacent As atoms. While in most As clusters the frontier orbitals are delocalized around the entire cluster motif, in this case the frontier orbitals are precisely located on the Pd atoms in real space. These frontier orbitals, localized on Pd sites, transform into band edges in the cluster solid and are likely to control the electronic transport in the solid. Such a feature may also play an important role in optical excitations, as such solids may offer the possibility of generating electron−hole pairs localized at Pd sites that could significantly affect the conductivity. The collective behavior of such series of pairs requires further investigation.

3−

Figure 6. Molecular orbital diagram from [As7] and Pd2(As7)2Cs4. The HOMO−2, HOMO−1, and HOMO for [As7]3− are plotted, along with the HOMO−2, HOMO−1, HOMO, LUMO, and LUMO +1.

Cs4Pd2(As7)2 case. This explanation is confirmed by the striking similarity between the DOS of Cs4Pd2(As7)2 and [Pd2(As7)2]4− when the counterions are Z = 0.8. The HOMO− LUMO of the Cs bound cluster is 0.93 eV, and model cluster with Z = +0.8 is 0.87 eV. The calculated HOMO−LUMO gap of [Pd2(As7)2]4− where Z = 0.0 is 1.20 eV, so the Cs addition corresponds to a decrease in the gap energy of 0.27 eV, demonstrating that the small decrease in band gap energy when going from 0-D to 2-D assemblies is due to internal electric fields having no effect on the HOMO orbital which is localized on the Pd dimer, but the internal electric fields stabilize the LUMO and LUMO+1 orbitals lowering the gap energy with the addition of the Cs counterions. To understand why the band gap energy of the [Pd2As14]4− motif is smaller than that of nearly all previously studied Asbased cluster assemblies, we examine the molecular orbital diagram of the As73− and Cs4[Pd2As14] cluster in Figure 6. The intrinsic band gap energy of As73− is 1.80 eV so we have aligned the two nearly degenerate lone pair orbitals from [Pd2As14]4− with the HOMO of As73−. The lone pair orbitals in As73− and Cs4[Pd2As14] are found on the equatorial atoms and have similar character as shown in Figure 6. The Pd2 Σu orbital lies 0.36 eV higher than these lone pair orbitals. The molecular orbital of the HOMO is constructed from the 4dz2 orbitals of the Pd dimer and is localized almost exclusively on the Pd. The LUMO as shown in Figure 6 is an orbital which mixes with both As7 motifs through the Pd atoms resulting in a new and slightly lower LUMO than As73−, and the LUMO+1 has significant charge density on the Pd atom. The result is that we can think of the Pd dimer as creating a set of states that are embedded in the band gap of the As73− cluster, which decrease the band gap energy. The presence of these states indicates a new means by which the band gap energy of cluster assemblies may be tuned. To visualize the location of the frontier orbitals, in Figure 7 we have plotted the density of states at different cross sections

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CONCLUSIONS AND PROSPECTS We have synthesized two cluster assemblies with the motif [Pd2As14]4− and measured the band gap energies of the 0D and 2D assemblies. The band gap energy of the 0D assembly is found to be 1.35 eV, and the 2D assembly linked by Cs atoms has a band gap energy of 1.15 eV. The band gap energy of the [Pd2As14]4− is lower than the 1.80 eV found for the As73− cluster because orbitals localized on Pd sites form pairs of unoccupied/occupied states embedded in the band gap of the As73− cluster, reducing the band gap energy. This also explains why the band gap energy is only mildly decreased when linked by Cs into a 2D sheet, because the internal electric fields generated by the Cs counterions do not affect the Pd-based states that form the top of the valence band. We believe that such transition-metal-linked assemblies offer a new means to control the band gap energy of nanomaterials by changing the transition metal, but could also lead to novel electronic behavior. In particular, such assemblies offer the possibility of generating a series of electron−hole pairs localized at the Pd sites via absorption of photons at selected frequencies. We are in the process of evaluating the full potential offered by such periodic arrays of localized electron−hole pairs.

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ASSOCIATED CONTENT

S Supporting Information *

Selected bond distances in compounds 1 and 2. This material is available free of charge via the Internet at http://pubs.acs.org. 10212



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (P.S.W.), [email protected] (S.N.K.), [email protected] (A.S.). Author Contributions ⊥

A.C.R. and S.M. made equal contributions to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge funding by the U.S. Department of the Army Research Office through a MURI grant W911NF-061-0280.

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