How do we see the Nano-World? Microscopic Techniques

Transcription

1 Lecture 2 How do we see the Nano-World? Microscopic Techniques

2 Comparison of Microscopies 10 6 Vertical scale (A) TEM SPM SEM OM Lateral scale (A)

3 Comparison of Microscopies Characteristics OM EM (SEM/TEM) SPM Operation air, liquid Vacuum* air, liquid, UHV Depth of field small large medium Lateral resolution 1 mm Vertical resolution N/A N/A Magnification 1 ~ nm: SEM 0.1 nm: TEM 10 ~ 10 6 : SEM 10 ~ 10 8 : TEM 2-10 nm: AFM 0.1 nm: STM 0.1 nm: AFM 0.01 nm: STM ~ 10 8 Sample not completely transparent un-chargeable vacuum compatible thin film: TEM surface height < 10 mm *not necessary for environmental EM

4 EM

5 Interactions of Electrons with Solids Incident e Backscattered e X-rays (XRF) Auger e Cathodoluminescence Secondary e (SEM) Inelastically Scattered e (EELS) Unscattered e (TEM) Elastically Scattered e (Diffraction)

6 Scanning Electron Microscopy (SEM) Beam size: a few 30 Å Beam Voltage: kv Resolution: Å Magnification: 20 ~ 650,000 Imaging radiations: Secondary electrons, backscattering electrons Topographic ( 표면묘사 ) contrast: Inclination effect, shadowing, edge contrast Lens Secondary electron Sample E-gun Detectors Composition contrast: backscattering yield ~ bulk composition Detections: - Secondary electrons: topography - Backscattering electrons: atomic # and topography - X-ray fluorescence: composition E-SEM (environmental SEM) FE-SEM (field-emission SEM)

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8 Secondary Electron Microscopy (SEM) Scanning Auger Microscopy (SAM) Focused electron gun Detector Secondary electrons SiC grain size = 0.04 mm SEM topograph of Au-SiC codeposits Energy Analyser Auger electrons SAM image of Ag particles (d=1nm)

9 SEM Images of nanostructures

10 FEI Verios 460 XHR SEM

11 Environmental SEM A grain of tricalcium silicate when dry (left) and after 2 minutes of hydration (right). Exposure to water permits the formation of a semi-permeable membrane around the particle, which slows down subsequent reactions.

12 Transmission Electron Microscopy (TEM) Beam size: a few 30 Å Beam Voltage: 40kV- 1MV Resolution: 1-2Å Imaging radiations: transmitted electrons, Imaging contrast: scattering effect Magnification: 60 ~ 15,000,000 Image Contrast: 1) Amplitude (scattering) contrast - transmitted beam only (bright field image) - diffraction beam only (dark field image) 2) Phase (interference) contrast - combination of transmitted and diffraction beam - multi-beam lattice image: atomic resolution (HRTEM)

13 Atomic Resolution TEM image of a CdSe Nanocrystal TEM Images of nanostructures

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15 Aberration-corrected scanning TEM (C s -corrected STEM) High-angle annular dark-field (HAADF)-STEM Schematic showing the main components of a highresolution dedicated STEM Uncorrected Cs-corrected

16 HAADF-STEM Images of nanostructures nanocrystals nanocrystals

17 In Situ TEM High-Resolution TEM of Colloidal Nanocrystal Growth Pt nanocrystal dynamics before (A and B) and after (C) coalescence. Still snapshots of Pt nanocrystal growth via coalescence and crystal-structure evolution Alivisatos group, Science 2012, 336, 61.

18 Scanning Probe Microscopy (SPM)

19 Schematic of generalized SPM

20 SPM Environments 1. UHV: necessary for STM 2. Ambient: easiest, popular environment for SPM 3. Liquid: useful for studies of biology, geologic systems

21 SPM Techniques Scanning Tunneling Microscopy (STM): topography, local DOS Atomic Force Microscopy (AFM): topography, force measurement Lateral Force Microscopy (LFM): friction Magnetic Force Microscopy (MFM): magnetism Electric Force Microscopy (EFM): local charge Scanning Electro-Chemical Microscopy (SECM): electrode reaction Force Modulation Microscopy (FMM): elastic properties Phase Detection Microscopy (PDM): mechanical properties Near-field Scanning Optical Microscopy (NSOM): optical properties Scanning Capacitance Microscopy (SCM): dielectric constant, doping Scanning Thermal Microscopy (SThM): temperature Ballistic Electron Emission Microscopy (BEEM): interface structure Spin-polarized STM: spin structure Scanning Tunneling Potentiometry (STP): potential surface Photon emission STM: photochemistry

22 SPM in Materials and Surface Science Patterned SAMs Disc surface Polymer blend 3M Scotch tape Polystyrene microspheres CVD diamond film

23 SPM in Biology Red blood cells Living Xenopus glial cell Y-shaped IgG antibody dsdna molecules Tobacco Mosaic Virus Housefly eye

24 Scanning Tunneling Microscopy (STM)

25 Tunneling

26 Schematic Diagram of STM Coarse positioning device Tip Piezo tube scanner X,Y,Z Sample Current Feedback Computer amplifier controller Sample bias voltage 1. Electrons tunnel through the ~10 Å gap 2. Tunneling current maps the topography 3. Sample & tip must be conductors or semiconductors 4. A sub-å vertical precision and an atomic resolution laterally Only the closest atom on the tip interacts with the closest atom on the sample: Real space imaging tool with atomic resolution!!

27 STM Mode Constant height mode Constant current mode (feedback, slow)

28 Applications of STM Surface geometry Molecular structure Local electronic structure Local spin structure Single molecular vibration Electronic transport Nano-fabrication Atom manipulation Nano-chemical reaction

29 Atom Resolved Surface Structure p(2x2) Buckled 2x1 2x1 p(2x2) c(4x2) Ge(100) c(4x2)

30 E f Molecular Orbitals - + sample tip Occupied state (HOMO) E f d + - Unoccupied state (LUMO) E f E f d J.J.Boland, Adv. Phys. 42, 129(1993)

31 Atomic Manipulation Fe/Cu(111) at 4K Quantum-mechanical interference patterns Don Eigler, IBM

32 Atomic Force Microscopy (AFM) Sample: conductor, nonconductor, etc. Force sensor: cantilever Deflection detection: laser + photodiode interferometry (10-4 A)

33 AFM Tip: A sharp tip (a few mm long, less than 10 nm in diameter) is located at the end of a cantilever ( mm long) 3 mm Silicon nitride tip contact point: ~ 10 nm diameter cantilever Length (l): mm Width (w): mm Thickness (t): mm 180 mm The V-shape provides low mechanical resistance to vertical deflection, and high resistance to lateral torsion.

34 Resolution of SPM In STM, only the closest atom on the tip interacts with the closest atom on the sample; providing atomic resolution In AFM, several atoms on the tip interacts with several atoms on the sample

35 Tip shape & resolution The sharpest commercial tip has a radius ~5 nm lateral resolution of 1-2 nm. Tip size = 40 nm Tip size = 5 nm [AFM images of perovskite surface]

36 AFM Detection 1. Forces between the tip & the sample surface cause the cantilever to bend or deflect. 2. A detector measures the cantilever deflection. F = kx F = force k = spring constant x = distance

37 Interactions between Sample and AFM Tip vdw force (Attractive) and Ionic repulsion Magnetic, Electrostatic Forces Adhesion Bonding friction forces elastic and plastic properties of the surface

38 Force vs. Distance Curve Van der Waals Force R D F = ar/6d 2 r: tip radius, D: distance between tip and sample a: Hamaker constant ~10-19 J F vdw ~2nN for D = 0.5 nm, r = 30 nm

39 Force Force vs. Distance Curve Vacuum untouch touching repulsive attractive Pull off Repulsive regime Tip approach Tip retraction Air Capillary force Air with a contamination layer Pull-out-of-contact force Attractive regime Distance

40 Scanning Modes Contact Noncontact Vibrating (tapping) Cantilever soft hard hard Force 1-10 nn nn Friction large small small Distance < 0.2 nm ~ 1 nm >10 nm Damage large small small Polymer latex particle on mica

41 Application: Protein Substructure From wikipedia ATP synthases are enzymes that can work in two directions to catalyze either the synthesis or break down of ATP : the smallest rotary motors in biology. ATP is shown in red, ADP and phosphate in pink and the rotating γ subunit in black. This proton-powered turbine is predicted to consist of 12 subunits, based on data for Escherichia coli. AFM images of the ATP synthase from leaf chloroplasts atomic force microscopy Surprisingly, its turbine has 14 subunits, arranged in a cylindrical ring. H. Seelert et al, Nature 405, 418 (2000)

42 Conformational changes in surface structures of isolated connexin 26 gap junctions Connexon surface as a function of Ca 2+ concentration. Extracellular surfaces of connexons displayed in an idealized 2D lattice were recorded in the absence (A) and presence (B) of Ca 2+. Daniel J. Müller, Galen M. Hand, Andreas Engel, and Gina E. Sosinsky, EMBO J July 15; 21(14):

43 Direct Imaging of Covalent Bond Structure in Single-Molecule Chemical Reactions STM images of a reactant-decorated Ag(100) surface before and after thermally induced cyclization reactions Noncontact AFM: reaction-induced changes in the detailed internal bond structure of individual oligo-(phenylene-1,2-ethynylenes) on a Ag(100) as they underwent a series of cyclization processes Dimas G. de Oteyza et al. Science 2013, 340,

44 Lateral Force Microscopy (LFM) Measures lateral deflections (twisting) of the cantilever A C B D Images of octadecanethiol patterned on Au via microcontact printing Lateral Force: sideways tilting (A+C)-(B+D) 20 mm LFM Dark areas are low friction: methyl groups Topography: up-down deflection (A+B)-(C+D) 20 mm Topography Light areas are high topo: C 18 chain vs Au LFM is a very useful tool to image monolayer patterns

45 Magnetic Force Microscopy (MFM) At close distances, works like an AFM At large distances, magnetic effect dominant C-AFM MFM MFM image of a hard disk

46 Electric Force Microscopy (EFM) 1. Applies a voltage between the tip and the sample 2. The cantilever deflects when it scans over static charges: thus EFM plots the locally charged domains of the sample surface EFM images (reversed tip bias from a to b) of polarized ferroelectric domains in an epitaxial, single crystalline Pb(Zr 0.2 Ti 0.8 )O 3 thin film. Scanning Electro-Chemical Microscopy (SECM)

47 Force Modulation Microscopy (FMM) 1. A periodic signal (hundreds of khz) is applied to the tip or sample 2. The amplitude of cantilever oscillation varies according to the elastic properties of the sample Carbon black deposit in automobile tire rubber. 15mm scan.

48 Phase Detection Microscopy (PDM) 1. Monitors the phase lag between the signal driving the cantilever to oscillate and the cantilever oscillation output signal 2. The phase lag varies in response to the mechanical properties of the sample surface AFM PDM TappingMode (left) and phase (right) images of a composite polymer embedded in a uniform matrix. The high resolution of the phase contrast image highlights the two component structure of the composite regions.

49 SPM in Nanolithography & Nanomanipulation Anodic oxidation of Si Nanolithographic poetry Nanomanipulation of CNT Nanomanipulation of DNA

50 Near Field Scanning Optical Microscopy (NSOM) Single Rhodamine B Molecules in Silicate Glass