Characterization of Nanomaterials Estareja, 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!
Nanotechnology In 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 beam By de Broglie wave equation
Scanning Electron MicroscopySEM Low voltage SEM The 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 Microscopy SEMs’ can be equipped with other analytical tools such as X-ray, Electron, and Ion detectors which could be used for qualitative and quantitative informations. Characterization Topography Composition
SecondaryElectrons – 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. Characterization Imaging Mode Topography Scattering Mode Morphology Composition Focused Mode Thermal Properties Mechanical Properties
Elastic 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 TEM Scanning Electron Microscope Samples must be conductive or else it will BURN! Im in 3D! Over 300k times magnification Transmission Electron Microscope Samples must be thin enough or else it wont project a recognizable image! Just 2D. Up to 1million times magnification
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.” Characterization Particle Size Specific Surface Area Morphology Porosity
Fluctuations 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.
Scanning Tunneling Microscopy STM 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 Microscopy As 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 MicroscopySPM Types of SPM Scanning Thermal Microscopy Magnetic Force Microscopy Dynamic Force Microscopy Electrostatic Force Microscopy Chemical Force Microscopy Friction Force Microscopy Magnetic Resonance Microscopy Kelvin Force Microscopy Scanning Capacitance Microscopy Elasticity Modulus Microscopy Ultrasonic Force Microscopy Force Distance Microscopy Characterization Topography Magnetic Thermal Electrical Mechanical Chemical
By 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 surface
Scanning Probe Microscopy The 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 Adsorption KELVIN EQUATION: P – Equilibrium vapor pressure P0 – Equilibrium pressure r – radius of pore γ – surface tension V – molar volume Rg – Gas constant T – Absolute Temperature Characterization Particle Size Specific Surface Area Pore volume
Adsorption 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.
Energy Dispersive X-ray SpectrometryEDS/EDX HOW 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. Characterization Composition
The 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)
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: Compositional Concentration
The 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: Compositional Concentration Atomic Structure
The 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.
The oscillations induced by certain vibrational frequencies provide a means for matter to couple with an impinging beam of infrared electromagnetic radiation and to exchange energy with it when frequencies are in resonance.
Raman Spectroscopy(vibrational) How Raman Spectroscopy works? When an incident photon interacts with the chemical bond, the chemical bond is excited to a higher energy state. Most of the energy would be re-radiated at the same frequency as that of the incident exciting light, which is known as Rayleigh scattering. A small portion of the energy is transferred and results in exciting the vibrational modes, and this Raman process is called Stokes scattering. The subsequent re-radiation has a frequency lower than that of the incident exciting light. The vibrational energy is deducted by measuring the difference between the frequency of the Raman Line and the Rayleigh Line. Characterization: Compositional Concentration Atomic Structure
It differs from the IR spectroscopy by an indirect coupling of high-frequency radiation, such as visible light, with vibrations of chemical bonds.
Raman Spectroscopy is very sensitive to the lengths, strengths and arrangements of chemical bonds, but less sensitive to the chemical compositions
References Nanotechnology Demystified -Linda Williams & Wade Adams NANO: The Essentials (Understanding Nanoscience and Nanotechnology) - T. Pradeep Nanostructures and Nanomaterials (Synthesis, properties & applications) - Guozhong Cao