From Single Pt Atoms to Pt Nanocrystals: Photoreduction of Pt2+

Letter pubs.acs.org/JPCL

From Single Pt Atoms to Pt Nanocrystals: Photoreduction of Pt2+ Inside of a PAMAM Dendrimer Yuri Borodko,† Peter Ercius,§ Vladimir Pushkarev,†,‡ Chris Thompson,†,‡ and Gabor Somorjai*,†,‡ †

Materials Sciences Division and §National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ‡ Department of Chemistry, University of California, Berkeley, California 94720, United States S Supporting Information *

ABSTRACT: The dynamics of structural transformations of Pt aggregates in the “quantum size” range where their molecular structure transforms into crystalline nanoparticles with metallic properties is an important issue in nanoscience. Here, we show high-resolution transmission electron microscopy (HRTEM) and spectroscopic observations of the polyamidoamine (PAMAM) dendrimer-mediated system, Pt− PAMAM, after UV irradiation that reveal the formation of small Ptδ+n clusters (n = 2− 8) with linear chains of −Pt−Pt−, which are the building blocks of stable nanocrystals. Dynamic imaging in an aberration corrected TEM at atomic resolution shows intermediate molecular and crystalline states and coalescence of Pt clusters into stable nanocrystals via an oriented attachment assembly process. We propose that the structural transformation from Pt aggregates to nanocrystals occurs between 1.5 and 2 nm and that a phase transition of the type “disordered-to-crystalline” exists depending on the number of atoms in the cluster. SECTION: Nanoparticles and Nanostructures

P

Scheme 1. Pt−PAMAM Reduction (a−c) and the Structure of the Core Framework of Tetranuclear Pt-Blue (d)a

t nanoparticles are a widely used material for many practical applications in catalysis, biomedicine, and electronics.1 In catalytic chemistry, transition metals are dominant as catalysts for both homogeneous and heterogeneous reactions. The size range of metal active sites extends from mononuclear compounds as catalysts for homogeneous reactions to metal nanoparticles and bulk metal single crystals as catalysts for heterogeneous reactions. Currently, the development of synthesis methods for nanocatalysts has proved to be a promising niche for discovering new selective catalysts with narrow size distributions in the range of 2−10 nm.2 The least understood size range for catalyst activity is small clusters of metal atoms intermediate between those clusters with molecular structure and those with metallic structure.3 In this context, great efforts have now been made to study these Ptn clusters in the elusive range of 5−30 atoms. The naked massselected cationic and anionic Pt clusters, Pt+n, Pt−n, and Pt0n (n = 2−30), could be highly reactive in gas-phase reactions.4 Theoretical calculations showed that Ptn clusters with n = 4−6 are planar; however for 6−9 Pt atoms, planar two-dimensional (2D) structures and 3D isomers are very close energetically.5 Numerous Pt clusters with an amazing variety of structures and oxidation states of Pt atoms have been synthesized.6 In particular, it has been found that so-called Pt-blue clusters (which are related to the main subject of this paper) have a linear structure with a chain of −Pt−Pt− cations (Scheme 1d). Pt-blue complexes with Pt2, Pt4, and Pt8 linear cores are the product of the reaction between mononuclear chlorides of Pt2+, Pt4+, and amidate bridging ligands.7−9 However, the complexity © 2012 American Chemical Society

a

Three electron-donor groups are shown in (*a), 1-amide, 2-carbonyl, and 3-amine (see also the inset in Figure 1a).

of studying the structure of Ptn clusters stabilized by polymeric ligands may explain the lack of data regarding the structural transformations of such subnanometer Pt clusters to date. The use of a PAMAM polymeric macroligand with amidate groups can facilitate the detection of small Pt clusters10 because amidate-rich media will stabilize Ptn clusters by a chelating Received: December 5, 2011 Accepted: January 4, 2012 Published: January 4, 2012 236

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of the amide groups and the d orbital of the Pt cations (inset molecular model #1 of Figure 1a). Raman spectra for the initial sample show bands at 350 (Pt−Cl stretch), 1085 (νs(NC3, weak), 1434 (bend H−N−C, strong), and 1565 cm−1 (amide II). An increased irradiation dose causes a decrease of intensity for the Pt−Cl and νs(NC3) bands relative to that of the amide II band, which indicates that the chelate amide groups are relatively resistant to irradiation. The UV−vis spectroscopic test for the presence of Pt-blue structures is the absorption band near 480 nm in the UV−vis spectrum, which is assigned to the dipole-allowed transition of dπ(Pt) → dσ*(Pt).17 UV−vis diffuse reflectance spectra for Pt−PAMAM in the irradiation time range of 0−360 min (shown in Figure 1b) have two bands at 450 and 490 nm (see the inset) that can be attributed to Pt linear chains. The intense bands at 260 and 290 nm have been assigned to a ligand-to-metal electron-transition n → d* from N into a Pt cation in mono- and polynuclear Pt complexes. In Pt−PAMAM, the ratio of intensities for 290 and 490 nm bands is 1/(2 − 4) × 10−2, which is in a good agreement with the data for charge-transfer (274 nm) and d−d* (480 nm) bands for individual Pt-blue tetranuclear complexes.18 XPS measurements show a change in the Pt electronic state after photoreduction of Pt−PAMAM. The typical values of binding energies Pt4f7/219 for a wide range of Pt inorganic and coordination compounds in different oxidation states is presented in Table 1.

effect. The first coordination spheres of Pt-blue and Pt− PAMAM are quite similar. It has been shown that subnanometer Pt aggregates that have been heterogenized in the PAMAM dendrimer are active catalysts in liquid-phase reactions.11,12 Recently, we proposed that Pt-blue compounds might be precursors in the synthesis of Pt nanoparticles stabilized by the polyvinylpyrrolidone (PVP) and PAMAM macroligands.13,14 Here, we have studied the assembly of Pt mononuclear compounds into Pt nanocrystals induced by UV irradiation at 254 nm (Scheme 1). Supramolecular PAMAM is a microreactor with a strong “cage” effect, which can hinder diffusion of photoreduced encapsulated Pt atoms out from the dendrimer.15 Scheme 1 depicts the synthesis of Pt nanoparticles inside of a starburst PAMAM dendrimer, where (a) is an initial dendrimer, (b) is a dendrimer with Pt ions ligated to a heteroatom (shown in *a), and (c) is the result of photoreduction of Pt2+ inside of the dendrimer; (d) is a cartoon of the core framework of tetranuclear Pt-blue determined by crystal X-ray diffraction.7,8 Here, we show HRTEM and spectroscopic evidence for intermediate Ptn structures on the way from Pt mononuclear compounds to Pt nanocrystals. Resonance UV Raman (244 nm) can detect what electrondonor group of PAMAM is bound to Pt cations. The spectra in Figure 1a show selective enhancement for the two coupled

Table 1. Binding Energy of Pt4f7/2

The spectra of Pt4f7/2 for (d) initial Pt−PAMAM (0 min of irradiation) and (e) after 360 min of irradiation is shown in Figure 1c. The dotted line is the subtracted difference between the initial spectrum and the irradiated sample, which highlights the new XPS lines acquired from the photoreduced sample. After UV reduction, there are two new Pt4f spin-doublets, which can be assigned to Pt0 (71.8 eV) and Pt2+ aggregates (72.9 eV), as seen in the dotted line spectrum. It is difficult to accurately attribute the line with Eb = 73.4 eV. This value is characteristic of [Pt(NH3)4]Cl2, Pt(NH3)2Cl2,19 and Pt− PAMAM reduced by [BH4]− 20 and close to the range of 73.1−73.5 eV for Pt-blue compounds.9 After photoreduction of Pt−PAMAM, the system has Pt atoms in four electronic states, and a strong line Pt4f7/2 at 73.4 eV is close to the Eb (Pt4f7/2) for Pt-blue compounds. Aberration corrected TEM images provide sufficient resolution and contrast to image single Pt atoms on graphene (white dots), as seen in Figure 2. Electrons interact with the Pt atoms on the graphene surface and can cause changes in the sample; therefore, we test for changes in the sample with a large dose. Figure 2a shows Pt atoms with no exposure to UV light deposited on a graphene surface, which were then irradiated by

Figure 1. (a) Resonance Raman spectra of PtG4OH after irradiation by UV light. The insets show possible bonds between Pt2+ and the heteroatoms. (b) Normalized UV−vis diffuse reflectance spectra of PtG4OH with exposure times of 0 (black), 15 (blue), 60 (green), and 360 min (red). Characteristic d−d* bands are shown in the inset. Intensity (absorptance (100−T−R)100%) in arbitrary units. (c) XPS spectra of Pt4f7/2: (d) initial PtG4OH, (e) after 360 min of irradiation, and (f) substraction of (e) − (d).

vibrational motions, amide II (1565 cm−1) and H−N−C (1440 cm−1),16 which is evidence that the charge-transfer band at 260 nm is due to an n → d electron transition between the N 237

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has a flexible linear chain structure, whereas fourth-generation OH-terminated PAMAM-G4OH has a starburst rigid structure in aqueous solution at neutral pH (7.0) with effective diameter d ≈ 4.2 nm.24 We assume that one PAMAM dendrimer is in a position to stabilize a Pt nanoparticle inside of the dendrimer with a size close to half of the diameter of the PAMAM dendrimer. The presented data are direct evidence for the growth mechanism of the Pt nanocrystals inside of the PAMAM dendrimers. HRTEM images show that at the initial stage of irradiation, small clusters are formed with linear −Pt−Pt− chains, which then become the building blocks of the precrystalline disordered aggregates (Figure 3). The mechanism by which disordered aggregates transform into wellordered Pt nanocrystals is a matter of debate, but we believe that the number of atoms contained within the aggregate plays a critical role in this transformation. Our study shows that during crystal growth, Pt atoms may self-assemble into kinetically metastable intermediates, referred to as mesocrystals, superstructures with crystal-like appearance.25 Such mesocrystals consist of Pt-blue building blocks, which are oriented and interspaced by amidate ligands. Under the microscope electron beam, these mesocrystals may transform reversibly into disordered clusters. Figure 3 shows time series (arranged in columns) acquired by HRTEM of the dynamics of the nanoparticles under electron beam irradiation. The series consist of a number of images each acquired over 1 s with a 0.5 s readout (dead) time between each frame, and we show particular frames from each series with indicated relative acquisition times (movies are in the Supporting Information). The restructuring of Pt aggregates with sizes below 1.5 nm is induced by the incident electron beam; however, our observations of the time-dependent restructuring of Pt aggregates elucidates a pathway for the growth of nanocrystals from single Pt atoms under UV reduction. Figure 3a shows the restructuring of a Pt aggregate (just below the disorder-to-crystalline transition size) from an initial disordered state into a metastable crystalline-like structure 12 s later. The metastable state persists for less than 3.5 s before the structure returns to a disordered state. This restructuring is reversible, which means that these mesocrystals consist of aligned Pt compounds with periodic orientation and weak bonding between linear Pt building blocks. Figure 3b shows two small aggregates initially separated that mutually attract and combine over 30 s of observation. The resulting Pt nanocrystal thus contains sufficient atoms to maintain a fully crystalline form. Figure 3c shows crystallization of a disordered Pt aggregate induced by coalescence with a larger crystalline particle. The smaller Pt cluster suddenly undergoes a structural transformation of the type disordered-to-crystalline between image frames at 15 s, and the resulting larger, ordered crystal is clearly visible in the final frame. This demonstrates sudden transformation of a disordered Pt aggregate to a nanocrystal (movies given as Supporting Information). We believe that the assembly mechanism of Pt nanocrystals inside of the PAMAM dendrimer is relatively similar for different types of reductants, [BH4]−, H2, UV light, X-ray and e-beam, up until the reducing agent destroys the molecular structure of PAMAM, which determines the structure of reoxidized Pt clusters. From the dynamics observed for Pt cluster formation and growth, we can speculate about the likely steps of the transformation of Pt2+ ions into clusters under UV irradiation. First, the photoreduction of Pt2+ results from 254 nm UV irradiation, which overlaps with a ligand-to-metal (charge

Figure 2. HRTEM images of unreduced Pt atoms (white dots) (a) before and (b) after 30 s of electron beam irradiation. (c) Pt-blue-like linear clusters and (d) ordered Pt nanocrystals on a graphene substrate. All scale bars are 2 nm.

the electron beam for 30 s at a high dose (∼103 e−/Å2). Figure 2b shows the final state of the particles after such irradiation, and the overall distribution of Pt atoms is similar to that of the initial image. Pt−PAMAM nanocrystals were also shown to be stable under electron beam irradiation in ref 21. HRTEM images of a Pt sample irradiated for 15−30 min with UV light show the formation of increasing numbers of Ptn clusters (n = 4, 6, 8) with a linear structure −Pt−Pt−Pt− and an interatomic distance of r(Pt−Pt) ≅ 0.28 nm (Figure 2c), which is close to values of r(Pt−Pt) = 0.2779 and 0.2885 nm in tetranuclear Ptblue.8 UV exposure beyond 30 min leads to coalescence of small Pt clusters to form disordered aggregates with size ≤ 2 nm and ordered Pt nanocrystals (Figure 2d). Pt nanostructures above ∼2 nm in diameter are cuboctahedral Pt nanocrystals with interplanar spacings of d[111] = 0.220 nm and d[200] = 0.200 nm, which are in good agreement with the values for bulk FCC Pt, d[111] = 0.227 nm and d[200] = 0.196 nm. Pt nanoparticles with a diameter below 1.5 nm always show a noncrystalline structure of Ptδ+n with rounded blurry edges due to atomic motion under the electron beam irradiation. With the PAMAM stabilizer, the end size of the nanoparticles remains quite constant despite the wide variation of the synthetic parameters. This is evident from the comparison of growth of Pt-PVP, Pt-cetyltrimetilammonium bromide (PtCTAB), and Pt−PAMAM nanocrystals. If PVP is used as a stabilizer, the average size of the nanocrystals can be gradually tuned by alcohol reduction methods in the size range of 1.7−7 nm,22 and if CTAB is used as a stabilizer, the size of the Pt nanocrystals can reach up to 30 nm under UV reduction of [PtCl4]2−.23 However, when PAMAM is applied as a stabilizer, the size of the Pt nanocrystals does not surpass 2−3 nm even after irradiation of up to 20 h. This indicates that the molecular structure of the capping agents has a profound effect on the end size of the colloids. Despite the fact that both PVP (55 000 amu) and PAMAM (14 200 amu) are polyamide molecules with amidate functional groups ligated to Pt cations, the former 238

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Figure 3. The dynamics of Pt cluster transformations imaged with HRTEM arranged as columns with indicated relative acquisition times. The columns from left to right show a metastable mesocrystal, a mesocrystal combination to form a full crystal, and oriented attachment. Scale bars are 2 nm.

transfer) CT absorption band n → d* at 260 nm for the chelate Pt complex and therefore could induce electron transfer from the amidate ligand to the Pt2+. As a result, short-lived reduced Pt0 atoms arise. These are surrounded by loosely bound oxidized amidate ligands (Pt0 + Nδ+)*. Second, the mobile Pt0 atoms can either be reoxidized and return to their initial state or alternatively be coupled with a neighbor and form Pt2 before then being reoxidized via metal-to-ligand electron transition, returning back to their ground state as a Pt-blue complex. The reaction is described in Scheme 2.

(300 K, 0.025 eV) and with a Fermi energy for metallic Pt of ∼5.5 eV, the number of Pt atoms in the nanoparticle should be more than ∼300 atoms to possess metallic properties.27 This number of atoms corresponds to a size of the Pt nanocrystal (with FCC structure) of about 2 nm in diameter. This can therefore be considered as a lower threshold in size at room temperature for the formation of metallic Pt nanoparticles from disordered aggregates of (Ptδ+)n−PAMAM. For particles as small as 2 nm, half of all atoms are surface atoms and therefore will interact with the N of amidate ligands. Thus, the actual critical number of atoms required will depend substantially on the extent of CT between the capping agent and the metal. In conclusion, our spectroscopic and HRTEM results identify small Pt clusters (of 4−8 Pt atoms) with a linear structure as the building blocks for Pt nanoparticles in the Pt− PAMAM system. Dynamic imaging of the intermediates, motion, and coalescence of Pt atoms and Pt aggregates also shows the occurrence of an oriented attachment mechanism for the assembly of the Pt nanocrystals. The HRTEM observations of a transition from disordered Pt aggregates, an agglomeration of weakly bound Pt clusters, into Pt nanocrystals at a critical particle size of 1.5−2.0 nm provides evidence of the steps between single Pt atoms and Pt nanocrystals, which are specific to this Pt−PAMAM system. The 30-atom size range of metal clusters should be a chemically rich regime for catalysis and diversity of chemical bonding. Our studies provide a direct view of the Pt subnanometer cluster transformations that occur in this exciting but elusive area of cluster science.

Scheme 2.

Finally, because the UV CT absorption bands of Pt2+− PAMAM and (Pt2+)2−PAMAM are very close, the UV light generation of reduced Pt can be repeated and lead to the formation of bigger linear Pt clusters (Pt2+)n with n = 3−8. This mechanism does not involve the existence of stable free Pt0 because in protic media, they should always rapidly hydrolyzed once formed. It should be noted that the TEM images do not show any particles >2 nm with a “hybrid” structure involving simultaneously crystalline and disordered parts. This suggests that the transition from disordered aggregates to nanocrystals does not occur in a continuous manner and is a sudden transition between phases of different symmetry.26 At room temperature 239

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(3) Calhorda, M. Braga, D. Grepioni, F. Transition Metal ClustersThe Relationship between Molecular and Crystal Structure. In Metal Clusters in Chemistry; Braunstein, P., Oro, L. A., Raithby, P. R., Eds.; Wiley-VCH Verlag: New York, 1999; , Chapter 5.2, pp 1491−1508. (4) Kaldor, A.; Cox, D. M.; Zakin, M. R. In Evolution of Size Effects in Chemical Dynamics; Advances in Chemical Physics; Prigogin, I., Rice, S. A., Eds.; Wiley: New York, 1988; Part 2, Vol. LXX. (5) Nie, A.; Wu, J.; Zhou, C.; Yao, S.; Luo, C.; Forrey, R. C.; Cheng, H. Structural Evolution of Subnano Platinum Clusters. Int. J. Quantum Chem. 2007, 107, 219−225. (6) Mingos, D. M.; Wardle, R. W. Homonuclear Cluster Compounds of Platinum. Trans. Met. Chem. 1985, 10, 441−459. (7) Lippard, S. J. New Chemistry of an Old Molecule: cis[Pt(NH3)2C12]. Science 1982, 218, 1075−1082. (8) Barton, J. K.; Rabinowitz, H. N.; Szalda, D. J.; Lippard, S. J. Synthesis and Crystal Structure of cis-Diammineplatinum α-Pyridone Blue. J. Am. Chem. Soc. 1977, 99, 2827−2829. (9) Matsumoto, K.; Sakai, K.; Nishio, K.; Tokisue, Y.; Ito, R.; Nishide, T.; Shichi, Y. Syntheses, Crystal Structures, and Electronic, ESR, and X-ray Photoelectron Spectra of Acetamidate- and 2Fluoroacetamidate-Bridged Mixed-Valent Octanuclear Platinum Blues. J. Am. Chem. Soc. 1992, 114, 8110−8118. (10) Zhao, M.; Sun, L.; Crooks, R. M. Preparation of Cu Nanoclusters within Dendrimer Templates. J. Am. Chem. Soc. 1998, 120, 4877−4878. (11) Witham, C. A.; Huang, W.; Tsung, C.-K.; Kuhn, J. N.; Somorjai, G. A.; Toste, F. D. Converting Homogeneous to Heterogeneous in Electrophilic Catalysis Using Monodisperse Metal Nanoparticles. Nat. Chem. 2010, 2, 36−41. (12) Li, Y.; Liu, J.; Witham, C. A.; Huang, W.; Marcus, M. A.; Fakra, S. C.; Alayoglu, P.; Zhu, Z.; Thompson, C. M.; Arjun, A.; et al. A PtCluster-Based Heterogeneous Catalyst for Homogeneous Catalytic Reactions: X-ray Absorption Spectroscopy and Reaction Kinetic Studies of Their Activity and Stability against Leaching. J. Am. Chem. Soc. 2011, 133, 13527−13533. (13) Borodko, Y.; Habas, S. E.; Koebel, M.; Yang, P.; Frei, H.; Somorjai, G. A. Probing the Interaction of Poly(vinylpyrrolidone) with Platinum Nanocrystals by UV−Raman and FTIR. J. Phys. Chem. B 2006, 110, 23052−23059. (14) Borodko, Y.; Thompson, C. M.; Huang, W.; Yildiz, H. B.; Frei, H.; Somorjai, G. A. Spectroscopic Study of Platinum and Rhodium Dendrimer (PAMAM G4OH) Compounds: Structure and Stability. J. Phys. Chem. C 2011, 115, 4757−4767. (15) Hecht, S.; Frechet, J. M. J. Light-Driven Catalysis within Dendrimers: Designing Amphiphilic Singlet Oxygen Sensitizers. J. Am. Chem. Soc. 2001, 123, 6959−6960. (16) Lee, S.-H.; Krimm, S. Ab Initio-Based Vibrational Analysis of αPoly(L-alanine). Biopolymers 1998, 46, 283−317. (17) Ginsberg, A. P.; O’Halloran, T. V.; Fanwick, P. E.; Hollis, L. S.; Lippard, S. J. Electronic Structure and Optical Spectrum of cisDiammineplatinum α-Pyridone Blue: Metal−Metal Bonding and Charge Transfer in a Four-Atom Pt(2.25) Chain. J. Am. Chem. Soc. 1984, 106, 5430−5439. (18) Fedotova, T. N.; Aleksandrov, G. G.; Kuznetsova, G. N. Synthesis and Crystal Structure of Platinum Blue [(1,10-phen) Pt(μNHCOCH3)2Pt(1,10-phen)]2(NO3)4. Russ. J. Inorg. Chem. 2008, 53/ 3, 372−377. (19) Nefedov, V. I. X-Ray Photoelectron Spectroscopy of Solid Surfaces; VSP Books: Alexandria, VA, 1988. (20) Huang, W.; Kuhn, J. N.; Tsung, C.-K.; Zhang, Y.; Habas, S. E.; Yang, P.; Somorjai, G. A. Dendrimer Templated Synthesis of One Nanometer Rh and Pt Particles Supported on Mesoporous Silica: Catalytic Activity for Ethylene and Pyrrole Hydrogenation. Nano Lett. 2008, 8, 2027−2034. (21) Knecht, M. R.; Weir, M. G.; Myers, V. S.; Pyrz, W. D.; Ye, H.; Petkov, V.; Buttrey, D. J.; Frenkel, A. I.; Crooks, R. M. Synthesis and Characterization of Pt Dendrimer-Encapsulated Nanoparticles: Effect of the Template on Nanoparticle Formation. Chem. Mater. 2008, 20, 5218−5228.

EXPERIMENTAL METHODS Pt nanoparticles stabilized with the PAMAM fourth-generation hydroxyl-terminated dendrimer were synthesized under UV irradiation (254 nm) in aqueous solution from a Pt−PAMAM molecular complex (Scheme 1). The Pt−PAMAM complex was prepared by mixing a 100 M dendrimer solution in water with 20 mol equiv (20 mol Pt2+ per mole of dendrimer) of an aqueous solution of 0.002 M K2PtCl4 and then allowing them to complex for 10 days at 4 °C. After dialysis and degassing, the solution was placed in a 1 cm diameter fused silica tube and exposed to 254 nm radiation at a 25 W/m2 energy density while the solution temperature was maintained at 25 °C. The processes of [PtCl4]2− hydrolysis and ligand exchange during the complexation stage were recently presented elsewhere.21 The deep UV−Raman (244 nm) and UV−vis techniques used were also described previously.13,14 X-ray photoelectron spectra (XPS) were acquired with a Perkin-Elmer PHI 5300 XPS spectrometer.20 The Pt aggregates were imaged at atomic resolution at 80 keV using the transmission electron aberration corrected (TEAM I) instrument, which is an FEI Titan 80-300 TEM aberration corrected for spherical and chromatic aberrations. The TEAM I was operated with third-order spherical aberration Cs = −10 μm for negative Cs imaging conditions, and thus, atoms appear bright in positive defocus (overfocus) in each HRTEM image shown. Images and time series were acquired with a Gatan US1000 camera with a readout rate of approximately 0.5 s. Suspended graphene membranes prepared by direct polymer-free transfer onto typical lacey carbon TEM grids were used as a support for TEM sample preparation to minimize background noise.28

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ASSOCIATED CONTENT

S Supporting Information *

Three movies containing all of the frames of the time series associated with Figure 3 in this article. The movies show the dynamics of Pt cluster transformations at atomic resolution. Each frame was acquired with 1 s of acquisition time and 0.5 s of dead time for a total of 1.5 s between frames. The movies are shown faster at 5 frames-per-second. 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]. Phone/Fax: (510) 642-4053.

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ACKNOWLEDGMENTS The authors would like to thank William Regan for his help with synthesis of the graphene layer on the TEM grid. This project is supported by the Director, Office of Science, and Office of Basic Energy Sciences of the U.S. Department of Energy under Contract DE-AC02-05CH11231. The NCEM (Contract No. DE-AC02-05CH11231) and the TEAM project are supported by the Office of Science, Office of Basic Energy Sciences of the U. S. Department of Energy.

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REFERENCES

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