Sem & Tem
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LOVELY PROFESSIONAL UNIVERSITY Term Paper synopsis of Physics Topic ~ construction working and uses of S.E.M. and T.E.M. Submitted to~ Ms. Anita Thakur
Submitted by~ Name ~Gagandeep Singh Class~ B.Tech C.S.E Reg.No~10810557 Section~B1802 Roll No. 29
ACKNOWLEDGEMENT First and foremost I thank my teacher who has assigned me this term paper to bring out my creative capabilities.
I express my gratitude to my parents for being a continuous source of encouragement and for all their financial aids given to me. I would like to acknowledge the assistance provided to me by the library staff of LPU Phagwara. My heartful gratitude to my friends, roommate, for helping me a lot to complete my work in time.
The scanning electron microscope (SEM):It is a type of electron microscope that images the sample surface by scanning it with a high-energy beam of electrons in a raster scan pattern. The electrons interact with the atoms that make up the sample producing signals that contain information about the sample's surface topography, composition and other properties such as electrical conductivity. It produces many types of signals include secondary electrons, back scattered electrons (BSE), characteristic x-rays , light , specimen current and transmitted electrons. These types of signal all require specialized detectors for their detection that are not usually all present on a single machine. Due to the way these images are created, SEM micrographs have a very large depth of field yielding a characteristic three-dimensional appearance useful for understanding the surface structure of a sample. This is exemplified by the micrograph of pollen shown to the right. A wide range of magnifications is
possible, from about x 25 to about 250,000, about 250 times the magnification limit of the best light microscopes. For the same reason BSE imaging can image colloidal gold immuno-labels of 5 or 10 nm diameter, that would otherwise be difficult or impossible to detect in secondary electron images in biological specimens. Characteristic X-rays are emitted when the electron beam removes an inner shell electron from the sample, causing a higher energy electron to fill the shell and release energy. These characteristic x-rays are used to identify the composition and measure the abundance of elements in the sample. Scanning tunneling microscope (STM):It is also a powerful technique for viewing surfaces at the atomic level. STM probes the density of states of a material using tunneling current. For STM, good resolution is considered to be 0.1 nm lateral resolution and 0.01 nm depth resolution. The STM can be used not only in ultra high vacuum but also in air and various other liquid or gas ambients, and at temperatures ranging from near zero kelvin to a few 100˚c. The STM is based on the concept of quantum tunnelling. When a conducting tip is brought very near to a metallic or semiconducting surface, a bias between the two can allow electrons to tunnel through the vacuum between them.
What is Tunnelling???? Tunnelling is a functioning concept that arises from quantum mechanics. For objects of very small mass, as is the electron, wavelike nature has a more pronounced effect, so such an event, referred to as tunneling, has a measurable probability. Electrons behave as beams of energy, and in the presence of a potential U(z), assuming 1-dimensional case, the energy levels ψn(z) of the electrons are given by solutions to Schrödinger’s equation,
, where ħ is the reduced Planck’s constant, z is the position, and m is the mass of an electron[4]. If an electron of energy E is incident upon an energy barrier of height U(z), the electron wave function is a traveling wave solution,
, where
if E > U(z), which is true for a wave function inside the tip or inside the sample[4]. Inside a barrier, such as between tip and sample, E < U(z) so the wave functions which satisfy this are decaying waves, , where
quantifies the decay of the wave inside the barrier.
Submitted by~ Name ~Gagandeep Singh Class~ B.Tech C.S.E Reg.No~10810557 Section~B1802 Roll No. 29
ACKNOWLEDGEMENT First and foremost I thank my teacher who has assigned me this term paper to bring out my creative capabilities.
I express my gratitude to my parents for being a continuous source of encouragement and for all their financial aids given to me. I would like to acknowledge the assistance provided to me by the library staff of LPU Phagwara. My heartful gratitude to my friends, roommate, for helping me a lot to complete my work in time.
The scanning electron microscope (SEM):It is a type of electron microscope that images the sample surface by scanning it with a high-energy beam of electrons in a raster scan pattern. The electrons interact with the atoms that make up the sample producing signals that contain information about the sample's surface topography, composition and other properties such as electrical conductivity. It produces many types of signals include secondary electrons, back scattered electrons (BSE), characteristic x-rays , light , specimen current and transmitted electrons. These types of signal all require specialized detectors for their detection that are not usually all present on a single machine. Due to the way these images are created, SEM micrographs have a very large depth of field yielding a characteristic three-dimensional appearance useful for understanding the surface structure of a sample. This is exemplified by the micrograph of pollen shown to the right. A wide range of magnifications is
possible, from about x 25 to about 250,000, about 250 times the magnification limit of the best light microscopes. For the same reason BSE imaging can image colloidal gold immuno-labels of 5 or 10 nm diameter, that would otherwise be difficult or impossible to detect in secondary electron images in biological specimens. Characteristic X-rays are emitted when the electron beam removes an inner shell electron from the sample, causing a higher energy electron to fill the shell and release energy. These characteristic x-rays are used to identify the composition and measure the abundance of elements in the sample. Scanning tunneling microscope (STM):It is also a powerful technique for viewing surfaces at the atomic level. STM probes the density of states of a material using tunneling current. For STM, good resolution is considered to be 0.1 nm lateral resolution and 0.01 nm depth resolution. The STM can be used not only in ultra high vacuum but also in air and various other liquid or gas ambients, and at temperatures ranging from near zero kelvin to a few 100˚c. The STM is based on the concept of quantum tunnelling. When a conducting tip is brought very near to a metallic or semiconducting surface, a bias between the two can allow electrons to tunnel through the vacuum between them.
What is Tunnelling???? Tunnelling is a functioning concept that arises from quantum mechanics. For objects of very small mass, as is the electron, wavelike nature has a more pronounced effect, so such an event, referred to as tunneling, has a measurable probability. Electrons behave as beams of energy, and in the presence of a potential U(z), assuming 1-dimensional case, the energy levels ψn(z) of the electrons are given by solutions to Schrödinger’s equation,
, where ħ is the reduced Planck’s constant, z is the position, and m is the mass of an electron[4]. If an electron of energy E is incident upon an energy barrier of height U(z), the electron wave function is a traveling wave solution,
, where
if E > U(z), which is true for a wave function inside the tip or inside the sample[4]. Inside a barrier, such as between tip and sample, E < U(z) so the wave functions which satisfy this are decaying waves, , where
quantifies the decay of the wave inside the barrier.
