Electron Microscopy (continued) PDF

Electron Microscopy (continued) 23/10/2024 Dr. Anna Klinkova, Associate Professor Department of Chemistry and Waterloo Institute for Nanotechnology Electron Energy-Loss Spectroscopy (EELS) PAGE 2  EELS: beyond compositional analysis E...

Electron Microscopy (continued) 23/10/2024 Dr. Anna Klinkova, Associate Professor Department of Chemistry and Waterloo Institute for Nanotechnology Electron Energy-Loss Spectroscopy (EELS) PAGE 2  EELS: beyond compositional analysis EELS spectra and EELS intensity maps for the peaks indicated in (a). ACS Photonics ArticleDOI: 10.1021/acsphotonics.9b01179ACS Photonics 2019, 6, 2974−29842980 DOI: 10.1039/C6CP05262B PAGE 3 In situ electron microscopy imaging In situ electron microscopy enables to watch a sample's response to a stimulus in real time. In scanning electron microscopy (SEM), the electron beam scans the surface and backscattered or secondary electrons SEM and TEM require different sample are detected, affording topographical information. In holders for non-volatile and volatile samples. transmission electron microscopy (TEM), the electrons are transmitted and one can obtain a higher resolution (albeit purely 2D) image depicting the inner structure of the sample. PAGE 4 Observations of chemical processes in liquid medium by EM PAGE 5 Beam damage considerations PAGE 6 In situ electron microscopy imaging E-chem TEM holder (Protochips) Typical window material: silicon nitride: Nanoscience Instruments: in situ TEM liquid + heating + biasing holder PAGE 7  In situ electron microscopy imaging: cheap DIY version Graphene liquid cell in standard single tilt TEM holder: The colloidal solution and reagents have to be premixed, and no external stimulus other than Gold nanorod etching in a graphene liquid cell pocket. Frames of a representative TEM video of a gold the beam of electrons is possible in this setup. nanorod etching under dose rate of 800 e-/Å2s. PAGE 8 DOI: 10.3791/57665  Metal nanoparticle synthesis and dissolution in situ imaging Radiolytic conversion of Pt(acac)2 into a growing Pt nanoparticle Radiolysis of H2O affords radicals that can reduce H[AuCl4] producing Au particles. Ag nanocube undergoes an aqueous-phase galvanic replacement reaction, in which oxidation with [AuCl4]− affords a Au shell (orange arrows) on the Ag surface, with concomitant formation of AgCl. A cavity (green arrows) develops between the shell and nanocube, and a second galvanic replacement reaction was also observed (blue arrow). Metal nanoparticle synthesis and dissolution in situ imaging Fe3+ solutions oxidatively etch Pd nanoparticles. The dissolution of Ni(SR)2 particles as they transfer thiolates to a Pd catalyst for cross-coupling to PhI to give PhSR PAGE 10 Liquid-phase EM for observing e-chem processes in Li-ion batteries LiPF6 can undergo reduction to deposit Li metal (dark regions) on a Pt electrode (light region). This process is partially reversed in the dissolution process. PAGE 11 EM revealing dynamics in soft-material solutions 2,3,6,7 ,10,11-hexahydroxytriphenylene (a trivalent node) and 1,4- phenylenebis(boronic acid) (a divalent connector) condense in liquid MeCN to give the covalent organic framework COF-5, the crystallization of which was observed by transmission electron microscopy. PAGE 12 A singular molecule-to-molecule transformation on video 1 nm ACS Nano 2021, 15, 8, 12804 PAGE 13 Scanning Probe Microscopy 23/10/2024 Dr. Anna Klinkova, Associate Professor Department of Chemistry and Waterloo Institute for Nanotechnology Scanning Probe Microscopy SPM is a collection of near‐field characterization techniques, which eliminates the resolution limit by far‐field interactions in optical and electron microscopy; the image of a surface is formed using a physical probe that scans the specimen. 10.1016/j.orgel.2017.01.038 PAGE 15 Scanning Tunneling Microscopy Gerd Binning and Heinrich Rohrer at IBM in 1981 Nobel prize in 1986 STM is based on quantum tunneling of electrons through a thin potential barrier separating two electrodes. By applying voltage between the tip and the metallic or semiconducting sample, a current can flow between these electrodes when their distance is reduced to a few atomic nanometers. PAGE 16 QUANTUM TUNNELING How does it work? PAGE 17 Quantum tunneling Potential-energy function: Schrödinger equation: Tunneling is a quantum mechanical phenomenon when a particle is able to penetrate through a potential energy barrier that is higher in energy than the particle's kinetic energy. PAGE 18 Quantum tunneling An electron wavepacket directed at a potential barrier: Quantum tunneling occurs because there exists a nontrivial solution to the Schrödinger equation in a classically forbidden region, which corresponds to the exponential decay of the magnitude of the wavefunction. PAGE 19  As long as both the tip and the sample are held at the same electrical potential, their Fermi levels line up exactly. There are no empty states on Tunneling current in STM either side available for tunneling into! This is why we apply a bias the sample by a negative voltage -V with respect to the tip. In a metal, the energy levels of When the specimen and the If the distance d between specimen and the electrons are filled up to a tip are brought close to each tip is small enough, electrons can particular energy, known as other, there is only a narrow ‘tunnel’ through the vacuum barrier. the ‘Fermi energy’ EF. In order region of empty space left When a voltage V is applied between for an electron to leave the between them. On either side, the specimen and tip, the tunneling effect metal, it needs an additional electrons are present up to the results in a net electron current. In this amount of energy Φ, the so- Fermi energy. They need to example from specimen to tip. This is called ‘work function’. overcome a barrier Φ to travel the tunneling current. from tip to specimen or vice versa. PAGE 20 Scanning Tunneling Microscopy We pick a tip material which has a flat density of states within the energy range of the Fermi surface that we wish to study. For example, if we wish to study the sample density of states within 200 meV of the Fermi surface, then the measured tunneling current will be a convolution of the density of states of the tip and sample in this energy range. So we pick a tip material which has a flat density of states in this range In addition to topography, STM can be used to measure the DOS curve of the sample. PAGE 21  Scanning Tunneling Microscopy For tunneling of metal tips, say tungsten, the tunneling gap is vacuum and the electron energy is 4.5 eV below the vacuum level (the work function) About 10× change in tunnel current per Å change in distance (i.e., for 0.3 nm atomic diameter the tunneling current changes by a factor of 1000) that’s what makes STM so sensitive. PAGE 22 Scanning Tunneling Microscopy: basic components Five basic components: 1. Metal tip, 2. Piezoelectric scanner, 3. Current amplifier (nA), 4. Bipotentiostat (bias), 5. Feedback loop (current). PAGE 23  Scanning Tunneling Microscopy: piezoelectric scanner SPM scanners are made from piezoelectric material, which expands and contracts proportionally to an applied voltage. Whether they elongate or contract depends upon the polarity of the voltage applied. With a positive voltage, the material will expand in one axis and contract in the other. With a negative voltage, the opposite occurs: The scanner can manipulate samples and probes with extreme precision in 3 dimensions. PAGE 24 Preparation of STM tips Common tip material: Platinum‐Iridium (Pt‐Ir), it is oxidation resistant. The Ir addition makes the alloy harder Prepared by cutting Pt‐Ir wire while pulling, the wire breaks at the weak point and is atomically sharp Tungsten (W) tip, readily electro‐chemically etch into a fine tip, but it slowly oxidizes in air Etching in KOH or NaOH solutions, use the W wire as anode Cathode: 6H2O+6e‐ ‐> 3H2+6OH Anode: W+8OH‐ ‐> WO42‐+4H2O+6e‐ (W undergoes oxidation dissolution) PAGE 25 Tip calibration The calibration lattice made of sharp pins allows to image the tip apex while the rectangular lattice helps to restore the form of the tip lateral surface. Special test structures with known topography Combining the results obtained by scanning these are used for calibration and determination of lattices, it is possible to restore completely the form of the form of the working part of the tip. the working part of tips. Calibration lattice made of sharp pins: PAGE 26  Surface restoration using a known tip shape Actual topography Restored image Schematic drawing of an image acquisition process and restoration Left: sample surface R(x) and initial image I(x) Right: partially restored image R’(x), accounting for the tip form P(x) PAGE 27 Operation modes of STM Constant current mode: Constant height mode: PAGE 28 Operation modes of STM Imaging the surface topography at atomic resolution if the surface is composed of the same atoms, i.e., the only Constant current mode: factor affecting the tunneling current is the distance. most commonly used STM imaging mode PAGE 29  STM examples: constant current mode High-pressure STM images of Pt(557) (a) in UHV at 10−10 Torr base pressure, (b) under ∼5 Å~ 10−8 Torr of CO which produces a coverage to cause step-doubling of the surface, and (c) at 1 Torr the coverage is close to 1 ML and causes restructuring and formation of clusters of Pt atoms inside the double-width terraces. The images are 40 Å~ 50 nm2 in size. PAGE 30 10.1126/science.1182122 Operation modes of STM Imaging the different surface atoms (due to their different work functions), revealing the surface composition or defects. Constant height mode: In this case the tip moves above the surface at a distance of several Angstrom, and the tunneling current changes are recorded as STM image (Fig. 42 (b)). Scanning may be done either with the feedback system switched off (no topographic information is recorded), or at a speed exceeding the feedback reaction speed (adjusting of the surface height is not needed and only smooth changes of the surface topography are recorded). This way implements very high scanning rate and fast STM images acquisition, allowing to observe the changes occurring on a surface practically in real time. risk of crashing tips; useful for dynamic processes PAGE 31  STM examples: constant height mode Constant-height image over CoPc on Ag(110). CO-terminated tip cobalt phthalocyanine PAGE 32 10.1126/science.1253405 Artefacts in STM perspective transformations Drift distortion (shear/stretch) (distortions appear along the scan direction, when the sample is moved continuously with respect to the tip, or the other way around; can be described as a linear affine transformation of the underlying data, resulting in shear and scale transform; are routinely corrected in STM) original linear Non-linear scanning distortion (position sensors are not sensitive enough to facilitate closed- loop control for the STM on the atomic scale; can be compensated by post-processing of images, especially when nonlinear+ the surface has a known periodic structure, e.g., substrate jump discontinuity random noise atomic lattice or adsorbate overlayer) Tip distortion (abrupt image contrast changes due to structural tip change) PAGE 33  Artefacts in STM 2012 Lévy vs Stellacci “striped nanoparticle controversy” https://www.chemistryworld.com/news/striped-nanoparticle-controversy-blows-up/5715.article 1. A M Jackson, J W Myerson and F Stellacci, Nature Materials, 2004, DOI: 10.1038/nmat1116 2. Y Cesbron et al, Small, 2012, DOI: 10.1002/smll.201001465 PAGE 34 3. M Yu and F Stellacci, Small, 2012, DOI: 10.1002/smll.201202322 Operation modes of STM: manipulation mode A Boy and His Atom 2013 stop-motion animated short film released on YouTube by IBM Research. It was made by moving carbon monoxide molecules with an STM (Cu tip and substrate). It was made out of 242 still images with 65 CO molecules. Each frame is 45x25 nm. It took 4 researchers x 2 weeks x 18-hour workdays to produce this. 35  Operation modes of STM: manipulation mode Physical mechanisms generating tip-atom interaction: b) lateral manipulation, c) vertical manipulation, d) extraction of adatoms with bias pulses, e) field-induced manipulation of adsorbates or subsurface defects. PAGE 36  STM limitations — Conductive tip / conductive material surface required; — Very sensitive to mechanical noise (vibrations) and electrical noise; — Very clean sample surfaces required (we are looking at the first atomic layer on the sample) in situ growth and transfer (deposition and STM systems clustered together) in situ surface preparation (ion sputter cleaning; high temperature annealing) in situ cleaving (mechanically cleave single crystal samples) STMs can be difficult to use effectively. There is a very specific technique that requires a lot of skill and precision. PAGE 37  Atomic Force Microscopy 1986: Binnig, Quate and Herber, the idea of AFM, which used an ultra-small probe tip at the end of a cantilever Phys. Rev. Letters, 1986, 56, 930 1987: Wickramsinghe et al. developed an AFM setup with a vibrating cantilever technique, which used the light-lever mechanism J. Appl. Phys. 1987, 61, 4723 STM: AFM: measures tunneling current between a sharp tip and measures the force between a sharp tip and the sample the sample surface; surface; An order of magnitude per Å height change Less sensitive, the force interactions are much (exponential dependence, 0.1 Å vertical resolution); longer‐ranged (power law dependence), but still enough The tunneling mechanism determines that only one for vertical atomic resolution; atom at the tip apex dominates the whole process Tip size limits the lateral resolution; (atomic lateral resolution); Probing mechanical properties (elasticity, friction, Probing at electrical properties; adhesion, etc.); Highly sensitive to surface conditions, needs high Works in ambient/liquid condition and in vacuum; vacuum for performance; PAGE 38 Atomic Force Microscope PAGE 39 Atomic Force Microscopy: cantilever Operation principle: Photodiode — Cantilever deflected when the tip scans across different heights — Laser beam reflected from the back of the cantilever deviates from the center, and read on the detector Extreme conditions: very soft, powder‐like materials, chance of picking up materials with the tip; very dramatic height variation or too fast scans, chance of crashing tip with sample; Based on a surface profiler invented by Schmalz in 1929 (light lever used to amplify the distance of movement) PAGE 40 Atomic Force Microscopy: cantilever typical cantilever length ~ 100μm, screen distance ~ 100mm ‐> gain of 103 PAGE 41 AFM: atomic interactions at different tip-sample distances Typical strengths: ~ 10‐9N 1) Repulsion: At very small tip-sample distances (a few angstroms) a very strong repulsive force appears between the tip and sample atoms. Its origin is the so-called exchange interactions due to the overlap of the electronic orbitals at atomic distances. When this repulsive force is predominant, the tip and sample are considered to be in “contact”. 2) Attraction (Van der Waals): A polarization interaction between atoms: An instantaneous polarization of an atom induces a polarization in nearby atoms – and therefore an attractive interaction. PAGE 42 AFM: atomic interactions at different tip-sample distances 3) Friction: The cantilever bends laterally due to a friction force between the tip and the sample surfaces - lateral force microscope (LFM). 4) Adhesion: Adhesion can be defined as “the free energy change to separate unit areas of two media from contact to infinity in vacuum or in a third medium”. The snap-in distance The pull-off force is considered as the adhesion force, which increases with the relative is in the range of a few nanonewton to tens of nanonewton. humidity, up to 10-15 nm. Significantly larger than the other forces, 10-8-10-7 N, and therefore would dominate the process if present, i.e., this is interaction to avoid (e.g., use hydrophobic material coating, use AFM in fluid mode as there is no surface tension, or use vacuum mode where the sample is dehydrated). PAGE 43  Preparation of AFM tips: examples Random: Commercial AFM tips: Si pyramids for AFM: Normally made from Silicon or Si3N4, beam length and width chosen to satisfy the desired operation frequency and force sensitivity; Typical tip radius 10‐30 Pre-patterned: nm; super sharp tips attractive force results; surface with an adjustable, small force PAGE 48 Common AFM operation modes: static contact mode Static contact mode: Lateral resolution in contact mode: ~ 1 nm, limited by contact diameter (Reduce tip diameter, use more sensitive force detection system) Vertical resolution: 1 Å resolution PAGE 49 Common AFM operation modes: lateral force microscopy Detecting “sideway” deflection: Lateral friction detection: image contrast results from both geometric features and chemical inhomogeneity (Interaction forces perpendicular to the sample surface, such as those used in the normal contact mode, will not generate any lateral deflections) PAGE 50 Common AFM operation modes: dynamic tapping mode — Typical cantilever resonant frequency ~ 100 kHz (probe tuning needed before operation for efficient oscillation driving) — Oscillation amplitude changes as the tip scans across the sample surface We define a pre‐set amplitude in the process of tip approaching the sample; As the tip scans across hills and valleys, its oscillation amplitude changes; The controller feedback try to keep the amplitude constant; Forces on the sample is much gentler than in the contact mode Tapping mode has significant No sideway shear forces advantages over other modes for soft samples PAGE 51 Common AFM operation modes: dynamic tapping mode Force modulation: — Can also be performed in dynamic tapping mode, with constant (average) force applied — Cantilever deflection under constant force ‐> sample elasticity — Topographical information collected simultaneously Carbon fiber in epoxy: PAGE 52 Common AFM operation modes: dynamic non-contact mode — Sensing the attractive forces between tip and sample (at Hydrogen‐terminated silicon separation ~ 10 nm) monohydride surface: — Both frequency and amplitude information can be used in the feedback mechanisms — Much weaker forces, non‐destructive, atomic resolution possible The attractive force between tip and sample leads to frequency shift towards lower frequencies PAGE 53 Examples of other AFM operation modes: MFM Magnetic force microscopy (MFM): Magnetic materials tend to break into magnetic “domains” to reduce their magnetic static energy A magnetic tip (AFM tip coated with magnetic layers) can pick up information from stray magnetic fields Topography MFM Topography and MFM images of a commercial hard disk PAGE 54 Common AFM imaging artefacts Tip shape artifacts: Tip sharpness: — Sharp tip preferred, in order to better follow the topography — When we say “sharp” tip, we mean it is sharper than the features. They do not have to be overly sharp Tip distortion: — Partially damaged tip, with non‐ideal apex on the top; or tips that picked up debris Dull of dirty tip Contamination from sample surface PAGE 55 Common AFM imaging artefacts Nonlinearity from piezoelectricity: Scanner artifacts: Nonlinearity: — More pronounced in large lateral scan range, say 100 μm. — Also has a component associated to the bending of piezoelectric tube Hysteresis: — Intrinsic to piezoelectric. Trace and retrace could differ by as much as 15% — Image is normally formed with trace (or retrace) only PAGE 56 Scanning thermal microscopy Developed by Wickramasinghe, 1986 Scanning electron microscope image of a 200 nm wide Pt line; (b) 3D topographic of a 2.5 μm 2.5 μm region where the 200 nm wide Pt line lies on top of a 1 μm wide Pt line; (c) thermal image of the same region shown in panel b. The temperature rise of the 200 nm line is seen to be lower in the region where it intersects the 1 μm wide line because the 1 μm wide Pt line acts as a fin. SThM maps the local temperature and thermal conductivity of an interface. The probe in a scanning thermal microscope is sensitive to local temperatures – providing a nano-scale thermometer. PAGE 57 Scanning probe electrochemistry (SPE) Developed by A. Bard in 1989 (= scanning electrochemical microscopy) In SPE, spatially resolved electrochemical signals is acquired by measuring the current at an ultramicroelectrode tip as a function of precise tip position over a substrate region of interest. Interpretation of the signal is based on assuming diffusion-limited current. Two- dimensional raster scan information can be compiled to generate images of surface reactivity and chemical kinetics. PAGE 58 Scanning probe electrochemistry (SPE): operation and tips Pt disk Pt nanoelectrode SPE operational modes: (A) Steady-state behavior (diffusion- limited) in bulk solution. (B) Feedback mode over an inert substrate (negative feedback). (C) Feedback mode over a conducting Hg hemispherical substrate (positive feedback). (D) microelectrode Substrate-Generation/ Tip- Collection mode. (E) Tip- Generation/Substrate-Collection Au conical mode. (F) Redox competition microelectrode mode. (G) Direct mode, where “M” is a metal precursor in solution with charge n (n = integer) and M is a solid metal. (H) Potentiometric mode with an ion-selective electrode, where “X” Au ring is an ion in solution with charge n microelectrode AFM tip with Au layer (n = integer). PAGE 59 Summary Related fundamental concepts: Techniques covered: Quantum tunneling STM constant current Piezoelectric tube STM constant height STM and AFM tip fabrication AFM static contact mode STM and AFM imaging artefacts AFM lateral force AFM dynamic tapping mode Recommended reading for this lecture material: AFM force modulation 1) Fundamentals of Scanning Probe Microscopy (V. AFM dynamic non-contact L. Mironov, 2004) AFM MFM 2) High-Pressure Scanning Tunneling Microscopy Chem. Rev. 2021, 121, 962 SThM 3) Scanning Electrochemical Microscopy: A SPE Comprehensive Review of Experimental Parameters from 1989 to 2015. Chem. Rev. 2016, 116, 13234 PAGE 60