Structural Study of Organic/Inorganic Structural Study of Organic/Inorganic Nanocomposites Nanocomposites

Radu Andreea

STMA1

“There is plenty of room at the bottom”

- Richard Feynman, December 29th 1959, APS Meeting -

Structural Study implies…Structural Study implies…

……methods for methods for crystallographycrystallography

……methods for methods for morphologymorphology

……methods for methods for chemical compositionchemical composition

• Light Microscopes which are limited by the physics of light to 500x or 1000x magnification and a resolution of 0.2 micrometers.

• In the early 1930's this theoretical limit had been reached and there was a scientific desire to see the fine details of the interior structures of organic cells (nucleus, mitochondria...etc.). This required 10,000x plus magnification which was just not possible using Light Microscopes.

• Electron Microscopes (EMs) function exactly as their optical counterparts except that they use a focused beam of electrons instead of light to "image" the specimen and gain information as to its structure and composition. This examination can yield the following information:

• Topography - the surface features of an object or "how it looks“;

• Morphology - the shape and size of the particles making up the object;

• Composition - the elements and compounds that the object is composed of and the relative amounts of them;

• Crystallographic Information - how the atoms are arranged in the object.

Electron MicroscopyElectron Microscopy

The basic steps involved in all Electron Microscopes :

• A stream of electrons is formed (by the Electron Source) and accelerated toward the specimen using a positive electrical potential

• This stream is confined and focused using metal apertures and magnetic lenses into a thin, focused, monochromatic beam.

• This beam is focused onto the sample using a magnetic lens

• Interactions occur inside the irradiated sample, affecting the electron beam. These interactions and effects are detected and transformed into an image

Types of Electron Microscopes:

• Transmission electron microscope (TEM)

• Scanning electron microscope (SEM)

• Reflection electron microscope (REM)

• Scanning transmission electron microscope (STEM)

• Low-voltage electron microscope (LVEM)

Electron MicroscopyElectron Microscopy

• It was developed by Max Knoll and Ernst Ruska in Germany in 1931

• It studies the primary electrons transmitted through the sample

• It allows for magnification of up to 100,000x

• It allows for resolutions in the nanometer range – it can easily resolve details of 0.2nm.

• A maximum thickness of 60 nm of the sample is required

It generates..

• 2D images

• 3D images by succession of images whilst tilting the specimens through increasing angles.

Transmission electron microscope (TEM)Transmission electron microscope (TEM)

• Emission source - a tungsten filament or a lanthanum hexaboride (LaB6) source – connected to a high voltage source (typically ~100-300 kV) emission of electrons either by thermionic or field electron emission into the vacuum

• Two condenser lenses focus a small, thin, coherent beam

• The first lens (usually controlled by the "spot size knob") largely determines the "spot size";

• The second lens (usually controlled by the "intensity or brightness knob“) actually changes the size of the spot on the sample.

• The beam is restricted by the condenser aperture (usually user selectable), knocking out high angle electrons

Transmission electron microscope (TEM)Transmission electron microscope (TEM)

• The beam strikes the specimen and parts of it are transmitted

• The transmitted portion is focused by the objective lens into an image

• Optional Objective and Selected Area metal apertures can restrict the beam;

• the objective aperture enhances contrast by blocking out high-angle diffracted electrons,

• the selected area aperture enables the user to examine the periodic diffraction of electrons by ordered arrangements of atoms in the sample

• The image is passed down the column through the intermediate and projector lenses, being enlarged all the way

• The image strikes the phosphor image screen and light is generated, allowing the user to see the image.

Transmission electron microscope (TEM)Transmission electron microscope (TEM)

Energy-Dispersive X-Ray Spectroscopy (EDS)

• EDS systems include

• a sensitive x-ray detector made of Si (Li) crystals that operate at low voltages to improve sensitivity. The detector is mounted in the sample chamber of the main instrument at the end of a long arm, which is it self cooled by liquid nitrogen.

• a liquid nitrogen dewar for cooling

• a software to collect and analyze energy spectra.

• The crystal in the detector absorbs the energy of incoming x-rays by ionization free conductive electrons an electrical charge bias.

• The electrical pulses correspond to the characteristic x-rays of the element a plot of x-ray counts vs. energy (in keV)

Transmission electron microscope (TEM)Transmission electron microscope (TEM)

• 1938 ( Von Ardenne)

• It studies the secondary electrons emitted from the surface due to excitation by the primary electron beam.• Data are collected over a selected area (ranging from approximately 1 cm to 5 microns in width) of the surface of the bulk samples

• magnification ranges from 20X to approximately 30,000X• spatial resolution ranges from of 50 to 100 nm

• It is also capable of performing analyses of selected point locations on the sample..• qualitatively or semi-quantitatively determining chemical compositions (using EDS -Energy-Dispersive X-Ray Spectroscopy - and backscattered (or reflected) electrons)• crystalline structure• crystal orientations (using EBSD – electronprobe micro-analyzer - and diffractedbackscattered electrons)

• It generates 3D topographical images.

Scanning electronScanning electron microscope (SEM)microscope (SEM)

Instrumentation

• an electron gun

• condenser lens stream is condensed

• condenser aperture (usually not user

selectable)  elimination of some high-angle

electrons

• second condenser lens  a thin, tight,

coherent beam

• a user selectable objective aperture eliminates high-angle electrons from the beam

• a set of coils "scan" or "sweep" the beam in a grid fashion

• the objective - focuses the scanning beam onto the part of the specimen desired

• when the beam strikes the sample (and dwells for a few microseconds) interactions occur inside the sample and are detected with various instruments

• before the beam moves to its next dwell point these instruments count the number of interactions and display a pixel on a CRT whose intensity is determined by this number (the more reactions the brighter the pixel).

• this process is repeated until the grid scan is finished and then repeated, the entire pattern can be scanned 30 times per second.

Scanning electron microscope (SEM)Scanning electron microscope (SEM)

Reflection electron microscope (REM)

An electron beam is incident on a surface, but instead of using the transmission (TEM) or secondary electrons (SEM), the reflected beam of elastically scattered electrons is detected

Scanning transmission electron microscope (STEM)Scanning transmission electron microscope (STEM)It asters a focused incident probe across a specimen that (as with the

TEM) has been thinned to facilitate detection of electrons scattered through the specimen. the high resolution of the TEM is thus possible in STEM.

The focusing action (and aberrations) occur before the electrons hit the specimen in the STEM, but afterward in the TEM.

Low-voltage electron microscope (LVEM)Low-voltage electron microscope (LVEM)It is a combination of SEM, TEM and STEM in one instrument, which

operates at relatively low electron accelerating voltage of 5 kV increases image contrast which is especially important for biological specimens.

Sectioned samples generally need to be thinner than they would be for conventional TEM (20-65nm).

• It is a non-optical technique 

• A sharp (1-10 nm) probe that is electrically conductive is scanned just above the surface of an electrically conductive sample It generates 3D topographic images at atomic scale level (2 angstroms - 0.2 nanometer).

• The dependency on tunneling current and probe-sample distance allows for precise control of probe-sample separation high vertical resolution (<1 Å).

• Tunneling is only carried out by the outermost single atom of the probe high lateral resolution (<1 Å).

Scanning Tunneling Microscope Scanning Tunneling Microscope (S(STMTM))

There are two methods of imaging in STM:There are two methods of imaging in STM:

1. Constant Current - a constant tunneling current is maintained during scanning (typically 1 nA). This is done by vertically (z) moving the probe at each (x,y) data point until a “setpoint” current is reached. The vertical position of the probe at each (x,y) data point is stored by the computer to form the topographic image of the sample surface. This method is most common in STM.

2. Constant Height - the probe-sample distance is fixed. A variation in tunneling current forms the image. This approaching allows for faster imaging, but only works for flat samples.

LIMITATIONS:LIMITATIONS:

1. Complex and expensive instrumentation

2. Subject to noise (electrical, vibration)

3. Must fabricate probes - dull probes or multiple tips at the end of probe can create serious artifacts

4. Only works for conductive samples: metals, semiconductors - samples can be “altered” to be conductive by coating with Au, but this coating can mask/hide certain features or degrade imaging resolution

• It provides a 3D profile of the surface on a nanoscale topographic, frictional and adhesion information

• It measures forces between a sharp probe (<10 nm) and surface at very short distance (0.2-10 nm probe - sample separation) by “gently” touching the surface with a tip and recording the small force between the probe and the surface.

• The force is calculated by measuring the deflection of the lever, and knowing the stiffness of the cantilever. Hook’s law gives: F = - kz

• Probes are typically made from Si3N4, or Si

•  Samples do not require any special treatments (such as metal/ carbon coatings)

• It can work perfectly well in ambient air or even a liquid environment

• It makes it possible to study biological macromolecules and even living organisms

Atomic Force MicroscopyAtomic Force Microscopy (AFM) (AFM)

AFM Modes of operationAFM Modes of operation

1. Contact Mode (< 0.5 nm probe-surface separation - VdW repulsive) – when the spring constant of the cantilever is less than surface, the cantilever bends. By maintaining a constant cantilever deflection (using the feedback loops) the force between the probe and the sample remains constant and an image of the surface is obtained.

Advantages: fast scanning, good for rough samples, used in friction analysis Disadvantages: at time forces can damage/deform soft

1. Intermittent Mode (Tapping: 0.5-2 nm probe-surface separation ) -The cantilever is oscillated at its resonant frequency. The probe lightly “taps” on the sample surface during scanning, contacting the surface at the bottom of its swing. By maintaining a constant oscillation amplitude a constant tip-sample interaction is maintained and an image of the surface is obtained.

Advantages: allows high resolution of samples that are easily damaged and/or loosely held to a surface; Good for biological samples

Disadvantages: more challenging to image in liquids, slower scan speeds needed

1. Non-contact Mode (0.1-10 nm probe-surface separation) - The probe oscillates above the adsorbed fluid layer on the surface during scanning. Using a

feedback loop to monitor changes in the amplitude due to attractive VdW forces the surface topography can be measured.

Advantages: VERY low force exerted on the sample(10-12 N), extended probe lifetime

Disadvantages: generally lower resolution; contaminant layer on surface can interfere with oscillation; usually need ultra high vacuum (UHV) to have best imaging

X-ray X-ray CrystallographyCrystallography• It is an extremely precise, but also difficult and expensive means of determining the arangement of atoms within a crystal, the exact structure of a given molecule or macromolecule in a crystal lattice.

• It is not an imaging technique

• From the angles and intensities of the diffracted X-ray beams, a crystallographer can produce a 3D picture of the density of electrons within the crystal

the mean positions of the atoms in the crystal can be determined

their chemical bonds

their disorder and various other information.

• It was the tool first used to discover the structure of DNA

X-ray photoelectron spectroscopyX-ray photoelectron spectroscopy (XPS)(XPS)• It is also known as Electron Spectroscopy for Chemical Analysis (ESCA)

• It detects all elements with an atomic number (Z) of 3 (lithium) and above using soft x-rays (with a photon energy of 200-2000 eV) to examine core-levels.

• Detection limits for most of the elements are in the parts per thousand range.

• XPS is used to measure:• elemental composition of the surface (top 1–10 nm usually)• empirical formula of pure materials• elements that contaminate a surface• chemical or electronic state of each element in the surface• uniformity of elemental composition across the top surface • uniformity of elemental composition as a function of ion beam etching (or depth profiling)• the binding energy of one or more electronic states• the thickness of one or more thin layers (1–8 nm) of different materials within the top 12 nm of the surface• the density of electronic states

• It is routinely used to analyze inorganic compounds, metal alloys, semiconductors, polymers, elements, catalysts, glasses, ceramics, paints, papers, inks, woods, plant parts, make-up, teeth, bones, medical implants, bio-materials, viscous oils, glues, ion modified materials and many others.

Auger Electron Spectroscopy Auger Electron Spectroscopy (AES)(AES)

• Auger and X-ray photoelectron spectroscopy give similar information

• The Auger spot size is much smaller than the XPS and has the capability of identifying fine features on the surface

• Auger lines also exhibit chemical shifts, these are not generally as large or as well-documented as those obtained by XPS

• The spatial analysis and imaging capabilities of the scanning Auger microprobe make it complementary technique to XPS

• In AES the sample of interest is irradiated with a high energy (2 - 10 keV) electron beam

• The Auger analysis can include..• Survey Scans the presence of contaminants on the sample surface• Mapping display the presence and the distribution of elements of interest within the area analyzed • Depth Profiles the relative concentrations of elements of interest as a function of depth• Imaging• Point Analysis• High-Resolution spectra

Example• Silica (Si) and aluminosilicate (AlSi), with molar ratio Al2O3/SiO2=20/80, microspheres of less than 20 m in diameter were prepared by the sol-gel and spray drying methods

Characterization

• NMR - the local structure

• X – ray diffraction (DRX)

• electron microscopy (SEM and TEM).

1. A. Nabok, Organic and Inorganic Nanostructures, Artech House, Inc., Norwood, 2005

2. D. A. Bonnell, Scanning Probe Microscopy and Spectroscopy: Theory, Techniques, and Applications, Wiley-VCH: New York, 2001

3. R. A. Wilson and H. A. Bullen, Introduction to Scanning Probe Microscopy (SPM): http://asdlib.org/onlineArticles/ecourseware/Bullen/SPMModule_BasicTheorySTM.pdf

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7. http://serc.carleton.edu/research_education/geochemsheets/techniques/SEM.html

8. Frenţiu Bogdan, Magnetic resonance study on biomedical systems, PhD Thesis, UBB, Cluj-Napoca, 2010

Works Cited