Hanoi Tower-like Multilayered Ultrathin Palladium Nanosheets

Letter pubs.acs.org/NanoLett

Hanoi Tower-like Multilayered Ultrathin Palladium Nanosheets Xi Yin,† Xinhong Liu,† Yung-Tin Pan,† Kathleen A. Walsh,‡ and Hong Yang*,† †

Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana−Champaign, MC-712, 114 Roger Adams Laboratory, 600 S. Matthews Avenue, Urbana, Illinois 61801, United States ‡ Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana−Champaign, 104 South Goodwin Ave, Urbana, Illinois 61801, United States S Supporting Information *

ABSTRACT: This paper describes the synthesis, formation mechanism, and mechanical property of multilayered ultrathin Pd nanosheets. An anisotropic, Hanoi Tower-like assembly of Pd nanosheets was identified by transmission electron microscopy and atomic force microscopy (AFM). These nanosheets may contain ultrathin Pd layers, down to single unit cell thickness. Selected area electron diffraction and scanning transmission electron microscopy data show the interconnected atomically thick layers stacking vertically with rotational mismatches, resulting in unique diffractions and Moiré patterns. Density functional theory (DFT) calculation with van der Waals correction (DFT+vdW) shows the adsorption of Pd4(CO)4(OAc)4 on Pd(110) surface (Ead = −5.68 eV) is much stronger than that on Pd(100) (Ead = −4.72 eV) or on Pd(111) (Ead = −3.80 eV). The adsorption strength of this Pd complex is significantly stronger than that of CO on the same Pd surfaces. The DFT+vdW calculation results suggest a new mechanism for the observed anisotropic growth of nanosheets with unusually high aspect ratio, in which the competitive adsorptions between Pd4(CO)4(OAc)4 complex and CO on various surfaces result in a favored growth along the ⟨110⟩ directions and inhibition along ⟨111⟩ directions. The mechanical property of these multilayered Pd nanosheets was studied using AFM and nanoindentation techniques, which indicate multilayered nanosheets show more plastic deformation than the bulk in response to an applied force. KEYWORDS: Palladium, 2D material, nanosheet, DFT, nanoindentation

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metal nanostructures includes (a) surface energy-controlled growth on metal surfaces without28,29 or with30−34 surfactant modification; (b) kinetic-controlled growth;35 (c) template/ substrate-directed growth;25,36−39 (d) twin plane-promoted anisotropic growth;40−42 and (e) oriented attachment of 2D building blocks.43 In some cases, the anisotropic growth is considered as the result of several of these factors.31,33 On the surface-energy-controlled growth, CO has been demonstrated as a powerful ligand in the shape-controlled growth of metal nanoparticles, due to its large binding selectivity on low-index metal surfaces.44−47 Strong binding of CO on Pd(111) was shown to facilitate the formation of Pd nanosheets.46,47 However, the selective binding of ligands, such as CO, only acts as an energy barrier in the thermodynamic landscape, inhibiting the crystal growth on certain surfaces. The energetic driving forces for the nucleation and growth of metal nanoparticles are rarely discussed at the molecular level,48 simply because of the complexity of the existing synthetic

he multilayered architectures of freestanding two-dimensional (2D) materials were actively studied recently for their electronic,1−7 quantum mechanical,8 optical,9−12 and catalytic properties13−15 due to interlayer interactions. Both layer-by-layer and self-assembly have been used to build higherorder structures from freestanding 2D materials.1,16−22 However, the bottom-up approach, or the direct growth of freestanding layered 2D structures, is still rare.23−25 Specifically, the growth of multilayered ultrathin metal nanosheets with large aspect ratios has not widely been demonstrated. Compared with self-assembled nanostructures, direct-grown multilayered metal sheets may show a stronger coupling effect between the adjacent layers and yield unconventional electronic and quantum mechanical properties. Such structures may also serve as scaffolds for 2D heterogeneous nanostructures or quantum wells, by incorporating other materials into the interlayer spaces. To achieve a controllable, direct growth of multiple-layered structures of 2D metal sheets, a better understanding of their anisotropic growth mechanism is essential. The mechanisms controlling the anisotropic growth of face-centered cubic (fcc) metal nanosheets, however, are still not fully understood.26,27 The current understanding on the anisotropic growth of 2D © XXXX American Chemical Society

Received: October 9, 2014 Revised: November 2, 2014

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dx.doi.org/10.1021/nl503879a | Nano Lett. XXXX, XXX, XXX−XXX

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systems. Understanding the reaction mechanism and thermodynamic factors contributing to the anisotropic growth becomes necessary for the development of 2D metal nanomaterials. In this paper, we present the ensemble structures of multilayered ultrathin Pd nanosheets that have shared growth centers. This structure can be viewed as an analogy of the “Tower of Hanoi” structure in which the stacked disks (or layers) are connected via a shared central pole (Scheme 1). The Scheme 1. Illustration of the Formation of Multilayered Pd Nanosheets from Pd4(CO)4(OAc)4a

Figure 1. Multilayered Pd nanosheets with different rotational mismatches of (a,b) 2° and (c,d) 8°, respectively. (a,c) TEM micrographs and (b,d) SAED patterns.

electron diffraction (SAED) pattern. Figure 1a,c shows the nanosheets are hexagonal with the diagonal distance of ∼2 μm. These hexagonal nanosheets are rotationally mismatched in their in-plane orientations. Figure 1b,d shows the corresponding SAED patterns of these nanosheets. The diffraction pattern indicates that the sheets lie within the (111) plane, with the corners pointing to the ⟨110⟩ directions. The diffraction spots in Figure 1b,d extend into concentric arcs with rotational mismatch of 2° and 8°, respectively, which suggests that these Pd nanosheets are composed of layers with different orientations in the (111) plane.50 This misalignment of the lattice between different layers was also confirmed by the TEM dark-field contrast (Supporting Information Figure S3). The observed kinematically forbidden 1/3 {422} diffraction spots can be explained either by the existence of (111) twinning within the layers or by atomically thin layers.51 A recent work on atomically thin Rh nanosheets points out that this spot pattern and the correlated spot intensity could also be the evidence of single layer atom structures.34 Figure 2a shows annular dark-field scanning transmission electron microscopy (ADF-STEM) micrographs of a multilayered Pd nanosheet with overlapped hexagonal layers clearly shown by different brightness. We also observed multilayered Pd nanosheets with other degrees of rotational mismatch, in some cases larger than 20° (Supporting Information Figure S4). These mismatches clearly show that the nanosheet structure is an ensemble of individual sheets centered at the same core. In STEM micrographs, a shared bright center could be observed in all multilayered Pd nanosheets, corresponding to the thickest part of the nanosheets. This central axis connects all the individual layers, even when some layers are folded, and could contain the initial nuclei for growth of individual layers (Supporting Information Figure S5). The central core, a bright region with triangle-like pattern, was clearly visible in the STEM images (Supporting Information Figure S6). The core is a part of the Pd multilayers and

a

Photograph of a Hanoi Tower is included to show the similarity between these two structures.

thickness of the layers within each Hanoi Tower-like structure can range from single to multiple unit cells in height with ultrathin layers having the thickness less than 1 nm, close to the unit cell thickness of Pd. The simplicity of this system and our recent progress in identifying the Pd4(CO)4(OAc)4 intermediate49 enables us to explore the growth mechanism of this unique structure. A molecular level model, which takes into consideration competitive binding of the Pd4(CO)4(OAc)4 complex on Pd(110)/(100)/(111) surfaces, is developed to explain the anisotropic growth of such well-defined, extremely thin 2D Pd nanosheets. Palladium(II) acetylacetonate (Pd(acac)2), acetic acid (HOAc), and CO were used to make the Hanoi Tower-like Pd nanostructures. The synthesis procedure is as the following: Pd(acac)2 was dissolved in glacial acetic acid, followed by bubbling CO gas into the solution at room temperature for 30 min (see the Supporting Information for detail). The solution changes color from dark yellow to light yellow during this process indicating the formation of Pd4(CO)4(OAc)4 complex.49 The reaction was kept at room temperature for additional 24 h until the color of the solution gradually turned black, indicating the formation of Pd metal, as confirmed by powder X-ray diffraction (XRD, Supporting Information Figure S1) and X-ray photoelectron spectroscopy (XPS, Supporting Information Figure S2). This synthetic process only uses three chemicals, namely Pd(acac)2, acetic acid, and CO, thus the effects of other surfactants and ions can be eliminated.46 Figure 1 shows TEM micrographs of a typical multilayered ultrathin Pd nanosheet and the corresponding selected area B


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Figure 2. ADF-STEM micrographs of a multilayered Pd nanosheet at (a) low magnification; (b) high magnification showing the Moiré pattern; (c) enlarged image of the boxed region in (b) demonstrating a 6-fold symmetric Moiré pattern. (d) Simulation of overlapping lattices of two Pd sheets rotated at a 2° angle along the ⟨111⟩ axis; (e) enlarged region of (d) focusing on Pd lattices with a small angle rotational mismatch; and (f) simulated ADF-STEM micrograph corresponding to this degree of lattice rotation.

several layers. The multilayered Pd nanosheets are approximately 10 nm in overall thickness (Supporting Information Figure S7). Figure 3b shows that the thickness of some Pd layers is as thin as 0.5 to 1.1 nm with an extreme case of approximately 0.26 nm, which is the height corresponding to the thickness of individual Pd layer. Figure 3c show a representative ADF-STEM image of the Pd nanosheets. By analyzing the brightness along the lines indicated in Figure 3c, we obtained the estimated thickness profiles, which further confirm the subnanometer thick layers as indicated by the arrows (Figure 3d, see the Supporting Information for detailed analysis). The measured thicknesses are close to the length of a Pd unit cell along the ⟨111⟩ direction, which is around 0.67 nm, and the 0.26 nm step is close to a single layer atomic height on the Pd(111) surface (Figure 3e). It is worthwhile to point out that these layers are not from the terraces in the layer-by-layer growth mode that produces an epitaxial film with singlecrystalline domains. In the multilayered Pd nanosheets, the hydrocarbon contaminants were excluded out from the interlayer spaces due to the van der Waals interactions.4 Such thin Pd sheet layers represent the thinnest structure that contains a planar ABA-type twinning defect structure.34 These sheets are made of individual layers connected at a shared center, as illustrated in Scheme 1. Although these sheets resemble the overall structure that might be generated from screw dislocation-driven growth,23 we did not observe any evidence of screw dislocation growth. Besides its roles as reductant and surface ligand, CO is known to react with the Pd organometallic precursors as a bridge ligand. In this reaction system, Pd4(CO)4(OAc)4 is the intermediate formed during the reaction, as illustrated in Scheme 2.49 This intermediate can decomposes into Pd metal

inseparable from the whole structure. With the inter-layer rotational mismatches, it is practically impossible to obtain clear lattice fringes of the central region using TEM. Figure 2b,c shows the Moiré superlattice pattern observed in Pd nanosheets under STEM. The periodicity of the Moiré pattern varies between different regions, and it correlates to the local lattice mismatch and tilting in orientation. We simulated the observed Moiré pattern with QSTEM software using the multislice method.52 Figure 2d shows the ⟨111⟩ view of a simplified model for the simulation of Moiré pattern. The model system is composed of two three-atomic-layered Pd (111) nanosheets with a 2° rotational mismatch angle in the (111) plane and a 1° tilt in both X and Y axes. Figure 2e,f shows the scanning window and the simulated Moiré pattern with a 4.7 nm period, respectively. Further simulation results (not shown here) indicate a strong correlation of the Moiré pattern with the rotational mismatch angle and tilting of the sample, which accounts for the different periodicities of Moiré patterns in Figure 2b. Moiré patterns in low-dimensional materials have previously been observed by TEM in stacked graphene,53 ultrathin CuS,54 Ag nanoplates,55 and PbS nanosheets.56 They are commonly observed in scanning tunneling microscopy (STM), where the pattern correlates with the electronic structures of 2D materials, which can be tuned by the rotational mismatch.57−60 It was also reported recently that the electronic structure of MoS2 bilayer is greatly affected by the twist angle.7 Thus, the convergent evident indicates the electronic and catalytic properties of multilayered Pd nanosheets can be controlled by the twist angle. Thickness of the Pd nanosheets were characterized by atomic force microscopy (AFM). Figure 3a,b shows an AFM topography of a Pd nanosheet and its height profile across C


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Figure 3. Thickness characterization of multilayered Pd nanosheets: (a) AFM 3D topography, (b) height profile in a selected region with 30 points line width, (c) STEM micrograph, (d) line brightness profiles, and (e) schematic of a mono- and multilayered steps on a Pd(111) surface.

precursors on the Pd low-index surfaces. To be specific, we calculated the Ead of Pd4(CO)4(OAc)4 complex, which accounts for a partial driving force during the growth, and compared it with the Ead of CO on Pd surfaces, which is used to determine the thermodynamic barrier. DFT calculations were performed using plane-wave basis code CASTEP63 with ultrasoft pseudopotentials.64 The generalized gradient approximation (GGA) with the Perdew−Burke−Ernzerhof (PBE)65 functional was used to treat the electronic exchange and correlation. Dispersion interaction correction66 to account for van der Waals interactions (denoted as PBE+vdW) and the self-consistent dipole correction67 were also applied (details of the DFT calculation are described in the Supporting Information). Figure 4 shows the Ead of Pd4(CO)4(OAc)4 complex on different Pd low-index planes and the corresponding configurations, calculated by DFT using the PBE+vdW method. The Ead of Pd4(CO)4(OAc)4 on Pd(110) was found to be −5.68 eV, much stronger than those on Pd(100) (−4.72 eV) and Pd(111) (−3.8 eV). This result suggests the adsorption of the Pd4(CO)4(OAc)4 complex is preferred on Pd(110) over Pd(100) or Pd(111) surface with the sequence of Ead(110) < Ead(100) < Ead(111), where the adsorption is strongest on Pd(110). The adsorption energies of CO on Pd low-index surfaces with low coverage were also calculated and the results agree with the previously reported values, ranging from −1.4 to −2.2 eV (Supporting Information Table S1).68−70 The slightly larger Ead values are due to the use of the PBE functional and the dispersion interaction correction; but the PBE+vdW

Scheme 2. Proposed Formation Pathways of Pd(0) Nanostructures

upon exposure to β-diketone (acetylacetone, Hacac) via two possible pathways.61 One pathway is that Pd4(CO)4(OAc)4 adsorbs onto existing Pd surface after the initial nucleation stage and then decomposes to form Pd adatoms. The other pathway is Pd4(CO)4(OAc)4 reacts with Hacac in solution and turns into free Pdn(CO)x clusters,62 which then react and form nanostructures. To understand the anisotropic growth mechanism, we performed the density functional theory (DFT) calculation and compared the adsorption energy terms (Ead) of Pd D


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Stacked 2D materials can often show interesting mechanical properties due to the twinning,72 ultrathin,73,74 or layered structures.50,75 Figure 5a shows AFM force curves of stacks of

Figure 4. DFT calculated adsorption sites and Ead of Pd4(CO)4(OAc)4 complex on various Pd surfaces: (a) Pd(110); (b) Pd(100) hollow site; (c) Pd(111) fcc−fcc site; and (d) Pd(111) hcp−hcp site; and (e) ΔE(= −Ead) of Pd4(CO)4(OAc)4 complex on Pd surfaces with ΔE of CO on Pd(111) surface as the reference.

Figure 5. (a) AFM force curves on Pd nanosheets deposited on Pd foil and the corresponding spots marked in the inset. Solid lines are the approach curves; dotted lines are the retraction curves. The black curve is on the Pd foil substrate. (b) Nanoindentation partial unloading curves on Pd nanosheets deposited on a silicon wafer. Color code: black, Si wafer; red-thin region of Pd layers; green, thick region of Pd layers. Inset image shows the topography of the nanosheet (scan size 2.5 μm × 2.5 μm).

method is considered to be an improved approach in Ead for including the long-range dispersion interactions. The calculated adsorption strength (ΔE = −Ead, in the unit of eV/molecule) of CO is higher than that reported in the literature, which may be related to the low coverage model used in our calculation.70 The adsorption of carboxylic acid on Pd surfaces (ca. −0.4 eV on Pd(111))71 is much weaker than that of CO, thus is negligible. Figure 4e compares the adsorption strength of Pd4(CO)4(OAc)4 complex on Pd low-index surfaces with the strongest adsorption of CO on the Pd(111) hcp site (Ead = −2.37 eV). Because the Pd4(CO)4(OAc)4 complex occupies at least two atomic sites on Pd surfaces, we compare its adsorption strength with twice that of CO, which only adsorbs on one atomic site. On the basis of this assumption, the adsorption of Pd4(CO)4(OAc)4 complex on Pd(111) should be inhibited by CO, as ΔEPdcomplex‑Pd111 is less than 2ΔECO‑Pd111. However, the adsorption of Pd4(CO)4(OAc)4 complex is much favored over CO on Pd(110), as ΔEPdcomplex‑Pd110 is substantially larger than 2ΔE CO‑Pd110 . Similarly, the adsorption of Pd4(CO)4(OAc)4 complex on Pd(100) should be slightly favored over CO. This theoretical result agrees very well with the observed anisotropic shape of Pd nanosheets as shown in the SAED (Figure 1). Thus, the preferred adsorption of the Pd4(CO)4(OAc)4 complex on to Pd(110) surface should be the determining factor for the CO-mediated anisotropic growth of Pd nanosheets.

Pd 2D nanosheets on a Pd foil substrate. The bare surface of Pd foil substrate was used for calibrating the optical lever sensitivity of the cantilever and should appear as a vertical line (black spot and lines in Figure 5a). As the layer thickness or number increases, the curves show increased compliance of the system, that is, more deformation in response to the applied force, as evident by the decreased absolute value of the slope of the approaching curves. The retraction curves are dominated by the response of the substrate, which accounts for the similarity shown for the different layer thicknesses. The trend indicates that Pd nanosheet on Pd foil is a more compliant system than Pd foil alone. This compliance increases with nanosheet thickness corresponding to an increased number of layers. We also measured AFM force curves on Pd nanosheets on Si⟨111⟩ surface, showing the same increase of compliance with the thickness and number of layers of the nanosheets (Supporting Information Figure S9). A series of nanoindentation measurements were also performed on Pd nanosheets on silicon wafer (Figure 5b). A Berkovich tip was used to obtain the various partial unloading curves. Reduced modulus rather than Young’s modulus was reported for the Pd nanosheets because an appropriate value for the Poisson’s ratio E


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(3) Britnell, L.; Gorbachev, R. V.; Jalil, R.; Belle, B. D.; Schedin, F.; Mishchenko, A.; Georgiou, T.; Katsnelson, M. I.; Eaves, L.; Morozov, S. V.; Peres, N. M. R.; Leist, J.; Geim, A. K.; Novoselov, K. S.; Ponomarenko, L. A. Science 2012, 335, 947−950. (4) Haigh, S. J.; Gholinia, A.; Jalil, R.; Romani, S.; Britnell, L.; Elias, D. C.; Novoselov, K. S.; Ponomarenko, L. A.; Geim, A. K.; Gorbachev, R. Nat. Mater. 2012, 11, 764−767. (5) Dean, C.; Young, A. F.; Wang, L.; Meric, I.; Lee, G. H.; Watanabe, K.; Taniguchi, T.; Shepard, K.; Kim, P.; Hone, J. Solid State Commun. 2012, 152, 1275−1282. (6) Georgiou, T.; Jalil, R.; Belle, B. D.; Britnell, L.; Gorbachev, R. V.; Morozov, S. V.; Kim, Y.-J.; Gholinia, A.; Haigh, S. J.; Makarovsky, O.; Eaves, L.; Ponomarenko, L. A.; Geim, A. K.; Novoselov, K. S.; Mishchenko, A. Nat. Nanotechnol. 2013, 8, 100−103. (7) Liu, K.; Zhang, L.; Cao, T.; Jin, C.; Qiu, D.; Zhou, Q.; Zettl, A.; Yang, P.; Louie, S. G.; Wang, F. Nat. Commun. 2014, 5, 4966. (8) Gorbachev, R. V.; Geim, A. K.; Katsnelson, M. I.; Novoselov, K. S.; Tudorovskiy, T.; Grigorieva, I. V.; MacDonald, A. H.; Morozov, S. V.; Watanabe, K.; Taniguchi, T.; Ponomarenko, L. A. Nat. Phys. 2012, 8, 896−901. (9) Huang, S.; Ling, X.; Liang, L.; Kong, J.; Terrones, H.; Meunier, V.; Dresselhaus, M. S. Nano Lett. 2014, 14, 5500−5508. (10) Jorio, A.; Kasperczyk, M.; Clark, N.; Neu, E.; Maletinsky, P.; Vijayaraghavan, A.; Novotny, L. Nano Lett. 2014, 14, 5687−5692. (11) Di Florio, G.; Bründermann, E.; Yadavalli, N. S.; Santer, S.; Havenith, M. Nano Lett. 2014, 14, 5754−5760. (12) Joo, P.; Jo, K.; Ahn, G.; Voiry, D.; Jeong, H. Y.; Ryu, S.; Chhowalla, M.; Kim, B.-S. Nano Lett. 2014, DOI: 10.1021/nl502883a. (13) Zhuang, X.; Zhang, F.; Wu, D.; Forler, N.; Liang, H.; Wagner, M.; Gehrig, D.; Hansen, M. R.; Laquai, F.; Feng, X. Angew. Chem., Int. Ed. 2013, 52, 9668−9672. (14) Kong, D.; Wang, H.; Cha, J. J.; Pasta, M.; Koski, K. J.; Yao, J.; Cui, Y. Nano Lett. 2013, 13, 1341−1347. (15) Kong, D.; Dang, W.; Cha, J. J.; Li, H.; Meister, S.; Peng, H.; Liu, Z.; Cui, Y. Nano Lett. 2010, 10, 2245−2250. (16) Butler, S. Z.; Hollen, S. M.; Cao, L.; Cui, Y.; Gupta, J. A.; Gutiérrez, H. R.; Heinz, T. F.; Hong, S. S.; Huang, J.; Ismach, A. F.; Johnston-Halperin, E.; Kuno, M.; Plashnitsa, V. V.; Robinson, R. D.; Ruoff, R. S.; Salahuddin, S.; Shan, J.; Shi, L.; Spencer, M. G.; Terrones, M.; Windl, W.; Goldberger, J. E. ACS Nano 2013, 7, 2898−2926. (17) Hu, C.; Lin, K.; Wang, X.; Liu, S.; Yi, J.; Tian, Y.; Wu, B.; Chen, G.; Yang, H.; Dai, Y.; Li, H.; Zheng, N. J. Am. Chem. Soc. 2014, 136, 12856−12859. (18) van der Kooij, F. M.; Kassapidou, K.; Lekkerkerker, H. N. W. Nature 2000, 406, 868−871. (19) Paik, T.; Ko, D.-K.; Gordon, T. R.; Doan-Nguyen, V.; Murray, C. B. ACS Nano 2011, 5, 8322−8330. (20) Ye, X.; Chen, J.; Engel, M.; Millan, J. A.; Li, W.; Qi, L.; Xing, G.; Collins, J. E.; Kagan, C. R.; Li, J.; Glotzer, S. C.; Murray, C. B. Nat. Chem. 2013, 5, 466−473. (21) Saunders, A. E.; Ghezelbash, A.; Smilgies, D.-M.; Sigman, M. B.; Korgel, B. A. Nano Lett. 2006, 6, 2959−2963. (22) Jones, M. R.; Macfarlane, R. J.; Lee, B.; Zhang, J.; Young, K. L.; Senesi, A. J.; Mirkin, C. A. Nat. Mater. 2010, 9, 913−917. (23) Morin, S. A.; Forticaux, A.; Bierman, M. J.; Jin, S. Nano Lett. 2011, 11, 4449−4455. (24) Wang, F.; Seo, J.-H.; Ma, Z.; Wang, X. ACS Nano 2012, 6, 2602−2609. (25) Huang, X.; Li, S.; Huang, Y.; Wu, S.; Zhou, X.; Li, S.; Gan, C. L.; Boey, F.; Mirkin, C. A.; Zhang, H. Nat. Commun. 2011, 2, 292. (26) Millstone, J. E.; Hurst, S. J.; Métraux, G. S.; Cutler, J. I.; Mirkin, C. A. Small 2009, 5, 646−664. (27) Wang, F.; Wang, X. Nanoscale 2014, 6, 6398−6414. (28) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153−1175. (29) Guan, J.; Liu, L.; Xu, L.; Sun, Z.; Zhang, Y. CrystEngComm 2011, 13, 2636−2643. (30) Zhang, Q.; Hu, Y.; Guo, S.; Goebl, J.; Yin, Y. Nano Lett. 2010, 10, 5037−5042.

for Pd in such ultrathin nanosheet geometry is lacking. A reduced modulus of Ere = 154.1 ± 20 GPa was obtained for Pd nanosheets deposited on the silicon wafer, close to that reported for 1 μm thick polycrystalline Pd film on Si⟨111⟩ Ere = 154.51 ± 10.53 GPa.74 Despite the shallowness of the indents, the tip did induce plastic deformation in the Pd nanosheets, as can be seen by the nonzero area under the curves for the Pd samples in Figure 5b; similar behavior is clearly visible in AFM force curves shown in Figure 5a. The appearance of horizontal bars in the load versus displacement curves (Figure 5b) is due to creep of the sample during the hold segment of each partial unloading cycle. Nanoindentation experiments show the same qualitative behavior as AFM force curves on this system. In summary, we report Hanoi Tower-like structure of stacked Pd nanosheets, which contains rotational mismatched layers, are interconnected via a shared center. This multilayered Pd nanosheet shows more deformation in response to an applied force than Pd bulk in AFM force curve measurement and nanoindentation study. The formation of Pd4(CO)4(OAc)4 complex is important for the formation of Pd(111) nanosheets. DFT+vdW calculations suggest that the competitive adsorption of Pd4(CO)4(OAc)4 complex and CO on Pd surfaces are the key factors controlling the anisotropic growth mode of 2D Pd nanosheets. The Pd(111) surface-dominant hexagonal shape are the results of preferential growth in ⟨110⟩ directions in plane. We envision when the layer thickness is around unit cell length, the multilayered Hanoi tower of Pd nanosheets may be treated as periodic quantum wells due to the confinement along the vertical direction, if the interlayer separations can be realized.76,77

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ASSOCIATED CONTENT

S Supporting Information *

Details of sample preparation and characterization, STEM simulation, DFT calculation, and supplementary data. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by NSF (CHE1213926) and a start-up fund from UIUC. The authors acknowledge helpful discussions with Dr. Scott MacLaren and Dr. Qiang Wang. The authors thank Kai-Chieh Tsao and Kam Sang Kwok for assistance and Steven A. Warren for bringing up the analogy of Hanoi Tower. Y.T.P. is a recipient of the Government Scholarship for Overseas Study (GSOS) from the Ministry of Education of Taiwan. Sample characterization was carried out in part at the Frederick Seitz Materials Research Laboratory Central Research Facilities, University of Illinois.

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REFERENCES

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