Characterization Tools

Transcription

1 Lectures in Nanoscience & Technology Characterization Tools K. Sakkaravarthi Department of Physics National Institute of Technology Tiruchirappalli Tamil Nadu India ksakkaravarthi.weebly.com

2 Objectives Characterization of the prepared/synthesized nanomaterials to understand their structural, chemical & physical properties with specialized instrumentation techniques. Normal/Bulk materials Nanomaterials K. Sakkaravarthi Lectures in Nanoscience & Technology 2/45

3 My sincere acknowledgments to Nanotechnology and Nanoelectronics: Materials, Devices, Measurement Techniques, W. R. Fahrner (Editor), Springer, Germany (2005). Introduction to Nanotechnology, C.P. Poole and F.J. Ownes, Wiley India, New Delhi (2007). Many other free & copyright internet resources. K. Sakkaravarthi Lectures in Nanoscience & Technology 3/45

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5 Need for Electron Microscope The main difference in much better resolution of an electron microscope. Imaging of a sample by scanning the surface with a focused beam K. Sakkaravarthi of electrons. Lectures in Nanoscience & Technology 5/45

6 Scanning Electron Microscope Imaging of a sample by scanning the surface with a focused beam of electrons. Electrons interact with atoms in the sample, produce various signals (secondary electrons, X-rays, etc.) that can give information about the surface topography & composition. The most common SEM mode: Detection of secondary electrons emitted by atoms excited by the electron beam. SEM can achieve resolution better than 1 nm. Specimens should be in high vacuum for conventional SEM. Also, possible for low vacuum or wet conditions in variable pressure/environmental SEM. K. Sakkaravarthi Lectures in Nanoscience & Technology 6/45

7 Scanning Electron Microscope... Image can be obtained from the detection of secondary electrons results due to the interactions of electron beam with atoms at various depths within the sample. Secondary electron detectors are essential in all SEMs. Back-scattered electrons (BSE): Electrons that are reflected from the deeper locations of sample by elastic scattering. Intensity of the BSE signal is strongly related to atomic number (Z) of the specimen. So, BSE images provide information about the distribution. K. Sakkaravarthi Lectures in Nanoscience & Technology 7/45

8 SEM: Electron Interaction Scanning Electron Microscope... Characteristic X-rays: Emitted when the electron beam removes an inner-shell electron from the sample and causes a higher-energy electron to fill the shell & release energy. X-ray image: To identify & measure the abundance of elements in the sample and map their distribution. K. Sakkaravarthi Lectures in Nanoscience & Technology 8/45

9 SEM: Block Diagram K. Sakkaravarthi Lectures in Nanoscience & Technology 9/45

10 SEM or FE-SEM? SEM: Electrons are produced by thermionic process. The electron emitter is made of with V-shaped tungsten wire that emits electron in the presence of high dc current. FE-SEM (Field Emission SEM): Production of electrons without thermal energy => Low-Temp. SEM. Single-crystal tungsten wire with sharp point. Significance of the small tip radius (about 100 nm or less) is that an electric field can be concentrated to an extreme level. When the tip is held at negative 3 5 kv relative to the anode, the applied electric field at the tip is so strong that the potential barrier for electrons becomes narrow in width. These narrow barriers allow electrons to tunnel directly through the barrier and leave the cathode. K. Sakkaravarthi Lectures in Nanoscience & Technology 10/45

11 Scanning Electron Microscope: Block Diagram K. Sakkaravarthi Lectures in Nanoscience & Technology 11/45

12 Scanning Electron Microscope... Courtesy: Prof. Ing. Rainer Schwab, Karlsruhe University of Applied Sciences, Germany. K. Sakkaravarthi Lectures in Nanoscience & Technology 12/45

13 Scanning Electron Microscope... Understanding the surface structure of a sample: Very narrow electron beam enables SEM micrographs to have a large depth of field => High-resolution image. A wider range of magnifications is possible (from about 10 times to more than 500,000 times), better than optical microscopes in about 250 times. Requirements: Sample status, K. Sakkaravarthi Lectures in Nanoscience & Technology 13/45

14 Transmission Electron Microscope Imaging by transmission of electron beam through the specimen. The specimen is most often an ultrathin section less than 100 nm thick. An image is formed from the interaction of the electrons with the sample as the beam is transmitted through the specimen. smaller de-broglie wavelength of electrons enables the ihigh resolving power even as small as a single column of atoms & thousands of times better than optical microscope. K. Sakkaravarthi Lectures in Nanoscience & Technology 14/45

15 SEM-TEM: Similarities K. Sakkaravarthi Lectures in Nanoscience & Technology 15/45

16 Differences b/w SEM & TEM K. Sakkaravarthi Lectures in Nanoscience & Technology 16/45

17 TEM: Block Diagram K. Sakkaravarthi Lectures in Nanoscience & Technology 17/45

18 Tunneling Electron Microscope... Courtesy: Physics Reimagined group (LPS, CNRS Universite Paris-Sud. K. Sakkaravarthi Lectures in Nanoscience & Technology 18/45

19 Atomic Force Microscope/Scanning Probe Microscope Imaging of the surfaces at by feeling" or touching". Resolution on the order of fractions of a nanometer. The information is gathered from the surface with a mechanical probe: Piezoelectric elements (precise & accurate scanning). Force measurement, imaging, and manipulation. K. Sakkaravarthi Lectures in Nanoscience & Technology 19/45

20 AFM/SPM... Force measurement: To measure forces between the probe and the sample (based on the mutual separation) => Force spectroscopy. Helps to measure the mechanical properties of the sample (Ex. sample s Young s modulus, a measure of stiffness) Imaging: Reaction of the probe to the forces to form a 3D image (topography) of a sample surface at a high resolution. Imaging by raster scanning the position of the sample with respect to the tip and recording the height of the probe that corresponds to a constant probe-sample interaction. Manipulation: Using the forces between tip and sample the properties of the sample can be changed. Ex. atomic manipulation, scanning probe lithography & local stimulation of cells. Mechanical properties (stiffness or adhesion strength) and electrical properties (conductivity or surface potential). K. Sakkaravarthi Lectures in Nanoscience & Technology 20/45

21 AFM/SPM: Principle (1) Cantilever, (2) Support for cantilever, (3) Piezoelectric element(to oscillate cantilever at its resonance frequency.), (4) Tip (Fixed to open end of a cantilever, acts as the probe), (5): Detector of deflection and motion of the cantilever, (6): Sample to be measured by AFM, (7): xyz drive, (moves sample & stage) (8): Stage. K. Sakkaravarthi Lectures in Nanoscience & Technology 21/45

22 AFM/SPM: Principle... Hooke s law: Contains a cantilever (silicon or silicon nitride with a tip radius in nanometers) with a sharp tip/probe. When the tip is brought into proximity of a sample surface, forces between the tip and the sample lead to a deflection of the cantilever. Forces that are measured in AFM include mechanical contact force, van der Waals forces, capillary forces, chemical bonding, electrostatic forces, magnetic forces, etc. Types: According to the nature of the tip motion (1) Imaging modes - static (or contact) (2) Dynamic & non-contact (or "tapping") modes: the cantilever is vibrated or oscillated at resonance frequency. K. Sakkaravarthi Lectures in Nanoscience & Technology 22/45

23 AFM/SPM: Advantage SEM gives s a 2D projection/image. but, AFM provides a 3D surface image. Do not require any special treatments (such as metal/carbon coatings). While an electron microscope needs an expensive vacuum environment for proper operation. But, AFM modes can work perfectly well in ambient air or even a liquid environment. Good for biological macromolecules and even living organisms. Atomic resolution in ultra-high vacuum (UHV), also in liquid environments. High resolution AFM is comparable in resolution to STM & TEM. AFM can also be combined with a variety of optical microscopy and spectroscopy techniques such as fluorescent microscopy or infrared spectroscopy. Combined AFM-optical instruments in the biological sciences. Recent K. Sakkaravarthi interest in photovoltaics, Lectures in Nanoscience energy-storage & Technology 23/45

24 AFM/SPM: Disadvantage Single scan image size: SEM can image an area and depth in the order of square millimeters. But, AFM can only image a maximum scanning area of about micrometers and a maximum height on the order of micrometers. Scanning speed. AFM images can be affected by nonlinearity, hysteresis, and creep of the piezoelectric material and cross-talk between the x, y, z movements. Advanced AFM with software corrections. Image damage: Unsuitable tip, a poor operating environment, poor sample itself, fast scanning, unreasonable rough sample surface damages the tip. AFM probes cannot normally measure steep walls. Modified AFM - highly expensive cantilever. K. Sakkaravarthi Lectures in Nanoscience & Technology 24/45

25 Atomic Force Microscope K. Sakkaravarthi Lectures in Nanoscience & Technology 25/45

26 Atomic Force Microscope K. Sakkaravarthi Lectures in Nanoscience & Technology 26/45

27 Scanning Tunnelling Microscope Imaging surfaces at the atomic level. Inventors (1981): Gerd Binnig and Heinrich Rohrer got 1986 Nobel Prize in Physics. Good resolution: 0.1 nm lateral and 0.01 nm (10 pm) depth. STM can be used in ultra-high vacuum. Also, in air, water, and various other liquid/gas, and at temperatures ranging from near zero kelvin to over 1000 C. Concept of quantum tunneling. When a conducting tip is brought very near to the surface, a bias(voltage difference) applied between the two can allow electrons to tunnel through the vacuum. Tunneling current: Depends on the tip position, applied voltage, & the local density of states (LDOS) of the sample. Requires extremely clean & stable surfaces, sharp tips, excellent vibration control & sophisticated electronics. K. Sakkaravarthi Lectures in Nanoscience & Technology 27/45

28 Scanning Tunnelling Microscope... Scanning tip, piezoelectric controlled height and x,y scanner, sample-to-tip control, vibration isolation system. Single or double-tip imaging. Tips with CNTs. Tips are tungsten or platinum-iridium and rarely gold. Vibration insulation needed due to the extreme sensitivity. Image processing is computer controlled. K. Sakkaravarthi Lectures in Nanoscience & Technology 28/45

29 Scanning Tunneling Microscope... Courtesy: Physics Reimagined group (LPS, CNRS Universite Paris-Sud. K. Sakkaravarthi Lectures in Nanoscience & Technology 29/45

30 Fluorescence microscope An optical microscope that uses fluorescence in addition to reflection and absorption to study properties of organic or inorganic substances. The specimen is illuminated with light of a specific wavelengt, absorbed by the fluorophores, emission of light with longer wavelength. K. Sakkaravarthi Lectures in Nanoscience & Technology 30/45

31 Conventional microscope uses light to illuminate the sample and produce a magnified image of the sample. Fluorescence microscope uses higher intensity light to illuminate the sample. and also produces magnified image of the sample based on the second light source. K. Sakkaravarthi Lectures in Nanoscience & Technology 31/45

32 Light source * Excitation filter Dichoric mirror * Emission filter K. Sakkaravarthi Lectures in Nanoscience & Technology 32/45

33 The microscope has a high power lamp source, usually a mercury or xenon arc lamp. An excitation filter transmits the band of the excitation radiation. The excitation radiation is reflected by the dichroic mirror towards the condenser/objective lens that focuses the light on the specimen. The illumination of the specimen as well as the collection of the fluorescence light is achieved by a the single lens. This has become possible due to the incorporation of dichroic mirror in the optics. A dichroic mirror is largely reflective for the light below a threshold wavelength and transmissive for the light above that wavelength. Light emitted by the fluorescent molecules (higher wavelength due to Stokes shift) is collected by the same lens and is transmitted by the dichroic mirror towards the ocular lens. K. Sakkaravarthi Lectures in Nanoscience & Technology 33/45

34 Used for the study of biological molecules. Imaging structural components of small specimens. Conducting viability studies on cell populations to find whether the cells are alive or dead. Imaging the genetic material within a cell (DNA and RNA) also possible. Dead & living cancer cells: cellular components K. Sakkaravarthi Lectures in Nanoscience & Technology 34/45

35 Scattering in Molecules Interaction of photons ( 0 ) with molecules: (i) Elastic (Rayleigh) scattering: Most molecules. (ii) Inelastic (Raman) scattering: Very few molecules Frequency/Energy shift (1928: CV Raman & KS Krishnan) Raman Scattering Lower freq. ( = ) ) Stoke s lines. Higher freq. ( = + ) ) Anti-Stoke s lines. Raman shift (around 10 4, 000 cm 1 ). Original freq. ) Rayleigh lines. K. Sakkaravarthi Lectures in Nanoscience & Technology 35/45

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37 Hyper-Raman scattering (HRS) effect A modified version of Raman scattering! Inelastic second harmonic scattering of photons. Sum frequency generation: A coherent process involving two incident fields (with frequencies 1 & 2 and wave vectors k 1 & k 2 ) produce a single field with frequency and wave vector k 1 + k 2. Hyper-Rayleigh scat.: Elastic scat. for 1 = 2 & k 1 = k 2. Hyper-Raman scattering: Inelastic form of Hyper-Rayleigh scattering!! K. Sakkaravarthi Lectures in Nanoscience & Technology 37/45

38 Hyper-Raman scattering (HRS) effect... Hyper-Raman scattering: Inelastic scattering of incident photons (with frequency 1 = 2 = ) intophotonsoftwo different frequencies 2 ±!! HRS Stokes lines: S = HRS anti-stokes lines: AS = depends on the usual scattering of molecular vibrations. HRS effect is usually very weak, but has aspects which make it interesting for Raman spectroscopy. HRS can provide vibrational information on molecules where ordinary Raman Scattering is suppressed due to symmetry issues. K. Sakkaravarthi Lectures in Nanoscience & Technology 38/45

39 Basic Raman spectrometer K. Sakkaravarthi Lectures in Nanoscience & Technology 39/45

40 Schematic: Basic Raman spectrometer K. Sakkaravarthi Lectures in Nanoscience & Technology 40/45

41 Schematic: Basic Raman spectrometer K. Sakkaravarthi Lectures in Nanoscience & Technology 41/45

42 Basic Raman spectrometer K. Sakkaravarthi Lectures in Nanoscience & Technology 42/45

43 Several variants of Raman spectrometers... To enhance the sensitivity (ex. surface-enhanced Raman), improve the spatial resolution (Raman microscopy), very specific information (resonance Raman), etc. Spontaneous Raman spectroscopy Surface-enhanced Raman spectroscopy (SERS) Resonance Raman spectroscopy Surface-enhanced resonance Raman spectroscopy (SERRS) Angle-resolved Raman spectroscopy Transmission Raman spectroscopy Tip-enhanced Raman spectroscopy (TERS) Stand-off remote Raman Many hand-held Raman spectrometers! K. Sakkaravarthi Lectures in Nanoscience & Technology 43/45

44 Few Applications of Raman spectra K. Sakkaravarthi Lectures in Nanoscience & Technology 44/45

45 Summary We have studied different optical/electronic imaging techniques to understand the structural & optical properties of synthesized nanomaterials! Thank You! K. Sakkaravarthi Lectures in Nanoscience & Technology 45/45