Characterization of nanomaterials

Characterization of NanomaterialsEstareja, Rogel Lindolf B.

How did it all start?Professor Richard Feynman presented the idea of manipulating and controlling things on an extremely small scale by building and shaping matter one atom at a time.Feynman described how the 24 volumes of the Encyclopedia Brittanica can be written on the head of a pin. He said that letters could be represented by six to seven bits of information for each letter. He also suggested using the inside as well as the surface of a metal to store information. Feynman allowed that if each bit was equal to 100 atoms, all the information of all the books of the world could be written in a cube of material 1/200 inch wide, about the size of a tiny speck of dust. THERE IS PLENTY OF ROOM AT THE BOTTOM!

NanotechnologyIn 1989, Don Eigler of IBM formed the letters of the company from 35 xenon atoms by STM.

Optical Microscopes are not for nanoscale analysis?“The smallest resolvable distance between objects is about half the wavelength of the light used”So, for visible light (500nm), resolving power is about 250 nm only! Nanomaterials (<100 nm) would appear blurred or undistinguishable.Only wavelengths from UV, X-ray, up to Gamma are applicable for visual characterization.

Using Electron beamBy de Broglie wave equation

Scanning Electron MicroscopySEMLow voltage SEMThe extreme surface sensitivity of this technique is a result of the reduced interaction volume.This allows the measurement of images with nanometer scale resolution with under 1 kV acceleration.At these low energies, charging is not an issue and it is possible to measure images without conductive coatings.Analytical Electron MicroscopySEMs’ can be equipped with other analytical tools such as X-ray, Electron, and Ion detectors which could be used for qualitative and quantitative informations.CharacterizationTopographyCompositionSecondaryElectrons – produced by Inelastic collision with a Core electron, having energy lower than the Primary electrons. Responsible for IMAGING

BackscatteredElectrons –produced by Elastic scattering of Core electrons and having the same energy as the primary electrons. Responsible for CONTRAST.

AugerElectrons – Instead of photon emission when electron-hole decay, higher orbital electrons would instead be emitted.

Transmission Electron MicroscopyTEM“The high magnification or resolution of all TEM is a result of the small effective electron wavelengths”Higher electron energy causes smaller electron wavelengths.CharacterizationImaging ModeTopographyScattering ModeMorphologyCompositionFocused ModeThermal PropertiesMechanical PropertiesElastic scattering – no energy loss and gives rise to diffraction patterns.

Inelasticinteractions - energy loss due to grain boundaries, dislocations, second-phase particles, defects, density variations leading to spatial variation in intensity of transmitted electrons.

SEM vs TEMScanning Electron MicroscopeSamples must be conductive or else it will BURN!Im in 3D!Over 300k times magnificationTransmission Electron MicroscopeSamples must be thin enough or else it wont project a recognizable image!Just 2D.Up to 1million times magnification

X-ray DiffractionXRD“Smaller crystals produce broader XRD peaks”Scherrer’s Formulat- thickness of crystalliteK- shape constantλ- wavelengthB- FWHMϴ- Bragg AngleCharacterizationsLattice constantd-spacingCrystallinityThickness (films)Particle Size (grains)X-ray interactions depends on the number of atom on a plane. This is why only specific planes would cause diffraction.

Small Angle X-ray ScatteringSAXS“SAXS is the scattering due to the existence of inhomogeneity regions of sizes of several nanometers to several tens nanometers.”CharacterizationParticle SizeSpecific Surface AreaMorphologyPorosityFluctuations in electron density over lengths on the order of 10nm or larger can be sufficient to produce an appreciable scattered X-ray densities at angles 2ϴ < 50

It is capable of delivering structural information of molecules between 5 and 25 nm. Of repeated distances in partially ordered systems of up to 150 nm.

How Scanning Probe Microscopy Works?

Scanning Tunneling MicroscopySTM uses the a quantum physics phenomenon called “Tunneling” to provide detailed images of substances that can conduct electricity.The probe is brought to within a few angstroms of the surface of the material, and a small voltage is applied between the surface and the probe. Because the probe is so close to the surface, electron leaks, or tunnel, across the gap between the probe and surface , generating current.

The strength of the tunneling current depends on the distance between the surface and the probe.If the probe moves closer o the surface, the tunneling current increases, and if the probe moves away from the surface, the tunneling current decreases.As the scanning mechanism moves along the surface, the mechanism constantly adjusts the height of the probe to keep the tunneling current constant.By tracking these minute changes, a computer can create a 3-D representation of the surface.

Atomic Force MicroscopyAs the metal probe in an AFM moves along the surface of a sample, the electrons in the probe are repelled by the electrons of the atoms in the sample and the AFM adjusts to the height of the probe to keep the force on it constant.By the up-and-down movements of the probe, a computer can track and make a 3-D image of the surface sample

Scanning Probe MicroscopySPMTypes of SPMScanning Thermal MicroscopyMagnetic Force MicroscopyDynamic Force MicroscopyElectrostatic Force MicroscopyChemical Force MicroscopyFriction Force MicroscopyMagnetic Resonance MicroscopyKelvin Force MicroscopyScanning Capacitance MicroscopyElasticity Modulus MicroscopyUltrasonic Force MicroscopyForce Distance MicroscopyCharacterizationTopographyMagneticThermalElectricalMechanicalChemicalBy modifying the tip chemically allows various properties of the sample surface to be measured.

Characterization depends on the type of the interactions between the tip and the sample surface.

Resolving power depends on the geometry of the tip, a sharper tip would be able to scan a deeper surfaceScanning Probe MicroscopyThe up-down movement of the tip of the probe are measured by a laser beam that reflects off the top of the cantilever, and its jiggling as the beam vibrates is measured by a optical detector that maps the surface.

The Tiniest Wires An image from a STM reveals wires on eight to ten atoms wide. Researchers at Hewlett-Packard Company in Palo Alto, California, developed the nanowires, the tiniest wires yet created.

Gas AdsorptionKELVIN EQUATION:P – Equilibrium vapor pressureP0 – Equilibrium pressurer – radius of poreγ – surface tensionV – molar volumeRg – Gas constantT – Absolute Temperature CharacterizationParticle SizeSpecific Surface AreaPore volumeAdsorption maybe physical or chemical

Physical Adsorption is particularly useful in the determination of specific surface area and pore volume in mesopores (2~50nm) or micropores (<2nm).

Chemical Adsorption is also for the determination of surface area; however it takes place via specific chemical forces and is thus unique to the gas and solid in question.Gas Adsorption

Energy Dispersive X-ray SpectrometryEDS/EDXHOW DOES EDS WORKS?When an X-ray photon falls on an intrinsic semiconductor, charge carriers are created, the electrons and holes.The X-ray photon is slowed down by colliding inelastically with other electrons in the semiconductor, and thus creating many charge carriers.The number of charges created will count as pulses, this pulses then are converted to voltage, representing the energy of the X-ray photons.Different atoms would emit different energies of X-ray since every atom has different atomic sizes and electron configuration.CharacterizationCompositionThe primary beam of an SEM/TEM collides with “Core” Electrons and ejects them.

The core hole thus created may get filled by electron de-excitation resulting in X-ray.

All photons (X-ray) emitted by the sample are collected and measured simultaneously by a solid-state X-ray detector

The common EDS detector is lithium drifted silicon (Si/Li)Energy Dispersive X-ray Spectrometry

Characterization of nanomaterials

Photoluminescence(XPS, UPS)How does PL works?A material gains energy by absorbing photon at some wavelength by promoting an electron from a low to a higher energy level.The system then undergoes a non-radiative internal relaxation involving interactions with crystalline or molecular vibration and rotational modes, and the excited electron moves to a more stable excited level such as the bottom of the conduction band or the lowest vibrational molecular state.After a characteristic lifetime in the excited state, electron will return to the ground state. Some or all of the energy is released during this final transition.Characterization:CompositionalConcentrationThe emission of light can result from a variety of stimulations.

When the emission is resulted from electronic stimulation, it is referred as cathodoluminescence.

When a high-energy photon such as X-ray is used to excite the sample, it is called to as X-ray fluorescence.Infrared SpectroscopyFT-IR(vibrational)What is the principle behind IR spectroscopy?Firstly, molecules and crystals can be thought of as systems of balls (atoms or ions) connected by springs (chemical bonds).These systems can be set into vibration, and vibrate with frequencies determined by the mass of the balls (atomic weight) and by the stiffness of the springs (bond strength).With these oscillations of the system, a impinging beam of infrared EMR could couple with it and be absorbed.These absorption frequencies represent excitations of vibrations of the chemical bonds and, thus, are specific to the type of bond and the group of atoms involved in the vibration.In an infrared experiment, the intensity of a beam of IR is measured before and after it interacts with the sample as a function of light frequency.Characterization:CompositionalConcentrationAtomic StructureThe mechanical molecular and crystal vibrations are at very high frequencies ranging from 1012 to 1014 Hz (3-300μm wavelength), which is in the infrared (IR) regions of the electromagnetic spectrum.

Characterization of nanomaterials

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Characterization of nanomaterials