Challenges and Approaches to Nanoscale Materials Characteriza5on at Atomic Resolu5on

Transcription

1 Challenges and Approaches to Nanoscale Materials Characteriza5on at Atomic Resolu5on Ralph G. Nuzzo G. L. Clark Professor of Chemistry and Professor of Materials Science and Engineering University of Illinois at Urbana Champaign National Academy of Sciences 2010

2 J. Phys. D. 2010, 43, Nano is both ubiquitous and diverse in areas of impact

3 Chem. Commun. 2009, mrl.uiuc.edu A big toolbox for characteriza1on at atomic resolu1on! Top. Catal. 2000, 10, 143. Phys. Rev. B, 2008, 77,

4 Advanced AnalyEcal Electron While rapidly advancing there remain important challenges and opportuniies for improvement AberraIons generally limit the resoluion of common microscopes Over the last decade many limitaions have been addressed successfully Microscopy Anal. Chem. 2010, 82, 2599.

5 Electron Microscopy: Spherical Aberra5on Arises from the curvature of lens Although magne5c lenses are not made of glass, the op5cs behave as if a glass lens system is in use Electron beams not all at focal plane Distorts the resolu5on of the image C s corrector compensates for effects of aberra5on Supplies a separate magne5c field to focus electrons farthest from the op5c axis to one focal point improves resolu5on Posi5oning the corrector Before the sample Results in a finely focused electron probe capable of atomic resolu5on Anal. Chem. 2010, 82, 2599.

6 Electron Atomic Resolu5on JEOL 2200FS Microscope C s corrected subangstrom probe enables atomic resolu5on in STEM imaging. The intensity from HAADF detector is propor5onal to the: 1. number of stacked atoms 2. atomic number, Z Single atom cross sec5on for sca@ering Microsc. Microanal. 2010, 16, 183. (a) STEM image of single crystal Si along the <110> with an intensity profile (b) and the accompanying FT (c) demonstra5ng ability to resolve atomic columns of Si nm apart.

7 HexachloroplaEnic Acid Polyvinylpyrrolidone (PVP) PVP protected Pt NanoparEcle Models: Synthesis and Characteriza1on of Pt and Pd Par1cles X ray Diffrac5on Profiles Palladium Acetate Polyvinylpyrrolidone (PVP) PVP protected Pd NanoparEcle Micrograph of PVP capped Pt Nanopar5cle Micrograph of PVP capped Pd Nanopar5cle Atom Quan5fica5on and Crystal Morphological Approxima5on J. Am. Chem. Soc. 2009, 131, 8683.

8 Models II: Synthesis and Characteriza1on of Bimetallic Pt Pd Nanostructures X ray Diffrac5on Profiles J. Am. Chem. Soc. 2009, 131, 8683.

9 Modeling Nanopar5cle Pd Nanopar5cle Oriented Along a 2 fold Axis Morphologies Modeled Icosahedron Along a 2 fold Axis Pd par5cle modeled aier icosahedron Electron microscopy simula5ons used to replicate experimental pa@erns Theore5cal core shell structures also modeled Simulated scans adequately replicate intensity pa@erns that were found experimentally J. Am. Chem. Soc , 131 (24), Microsc. Microanal. 2008, 14, 920.

10 X Ray Absorp1on Spectroscopy (XAS): What Spectroscopy Teaches About Atomic Structure Photoelectric Effect Photoelectron Interac5on XAS equa5on: N: Average coordina5on of probed element R: Real space distance traveled by photoelectron, combina5on of single and paths σ 2 : Informa5on about bond disorder Top. Catal , 10, 143.

11 Mesoscopic Characteris1cs at the Nanoscale: Nega1ve Thermal Expansion of Pt/γ Al 2 O 3 Electron Microscopy Data (STEM) Different sized clusters of Pt on γ Al 2 O 3 were examined by XAS. EXAFS Data XANES Data Temperature dependent measurements reveal a contrac5on of the first nearest neighbors bond distances with increasing temperature nega5ve thermal expansion (NTE). J. Am. Chem. Soc. 2009, 131, This behavior varies with the size of the supported par5cle, the ambient gas, and the choice of support. XANES measurements show that NTE is correlated with changes in electronic structure.

12 Theory and Simula1on (a fron1er challenge) XANES Simula5on Compared to Experimental Data Molecular Dynamics Simula5on of a 10 atom Pt cluster on [110] γ Al 2 O 3 at 573 K Comparison of theore5cal and experimental XANES data for a 10 atom Pt cluster supported on γ Al 2 O 3. The theore5cal spectra are obtained from a configura5onal average of 32 random conforma5ons extracted from a 5.5 ps MD simula5on. Molecular Dynamics simula5on of the cluster determined that the bonding between the cluster and the support was dynamic in nature. In the simula5on the gold atoms represent Pt atoms bonded directly to the γ Al 2 O 3 whereas purple atoms denote Pt atoms free from the interface. Phys. Rev. B 2008, 78, (R).

13 Anomalous/Mesoscopic Bond Disorder: Quan1fica1on Using XAS Disorder can be characterized for these clusters via Debye Waller factor (σ 2 ) measurements obtained using XAS. The level of disorder is highly sensi5ve to nanopar5cle size, the presence of a reac5ve gas, and the choice of the support. The nature of the disorder requires analy5cal models. To defini5vely determine whether the enhanced disorder is strictly coordina5on dependent or more randomized requires direct observa5ons. J. Am. Chem. Soc. 2009, 131, 7040.

14 Nano Area Coherent Electron Diffrac1on and Modeling of Surface Relaxa1on in Au Clusters HRTEM image of Au nanopar5cle Bragg Peak Selected Diffrac5on Peak Simula5ons Based on Coordinates From Simulated Structure Model par5cle Mul5slice simula5on Contrac5on model based on distance to cluster center and coordina5on Dashed white arrow in (a) is the <002> direc5on in reciprocal space and the solid white arrow points toward the central beam. r=distance to center, r 0 =average cluster diameter, n=coordina5on, α=scaling parameter, κ(h,k,l)=ra5o describing facet dependent contrac5on Nature Mater. 2008, 7, 308. Vectors represen5ng surface atom displacements. The magnitudes of the displacements are rendered using colors.

15 Metal Support Interac1ons and Atomic Strains Impregna1on of and reduc1on of [Ir 3 Pt 3 (μ CO) 3 (CO) 3 (η C 5 Me5) 3 ] on γ Al 2 O 3 Stoichiometry of precursor retained Hemispherical cubeoctahedral structures LaZce of support observable and iden1fiable near clusters with aligned zone axes To be submi@ed.

16 Characteriza1on of Interfacial Electron energy loss spectroscopy (EELS) Can be used to characterize the electronic structure of a material Elucidate oxida5on states of atomic columns when using an aberra5on corrected microscope Charge Transfer Arrows A and C are represented by the EELS spectra Outside Nanocrystal. The shi_s in the red and black spectra correlate with Ti +3 and Ti +4, popula1ons for surface and bulk TiO 2 sites, respec1vely. Arrow B represents the EELS spectrum collected At Nanocrystal. Au on TiO 2 Microsc. Microanal. Proceed., Richmond, VA, 2009.

17 Coherent Diffrac5ve Imaging Diffrac5ve imaging provides atomic resolu5on structural determina5ons even when an atomic resolu5on imaging lens is not available radia5on damage lessened Synergis5c informa5on from electron diffrac5on and low resolu5on images Micrograph of CdS quantum dot (with inset electron diffrac5on prior to refinement by diffrac5ve imaging Only shows lavce planes but no atomic detail Aier integra5ng with the electron diffrac5on data atomic details are revealed Resolu5on improved to subangstrom resolu5on 0.84 Å separa5on between Cd and S atoms This technique has also been implemented to provide the structure of carbon nanotubes Chirality and presence of double walls clearly visible CdS Before Diffrac5ve Imaging 0.84 Å Carbon Nanotube Nature Phys , 129. Science 2003, 300, CdS Aier Diffrac5ve Imaging

18 Some Important Limita1ons for Analysis and Future Direc1ons Current methods can reveal Atomic structure SpeciaEon of elements at the nanoscale Electronic structure Challenges Structural dynamics Limited (or no) temporal resolueon to monitor processes Atomic level characterizaeon of adsorbate interface bonding In situ studies Theory vs. experiment ( too many atoms, excited electronic states, chemistry..)