--------- #1428 - The History leading to the Scanning Tunneling Microscope
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- Attachments : atoms
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- If you turn your astronomy telescope around and look into the wrong end it becomes a microscope. If you suspend a water drop on your smart phone camera lens it becomes a microscope. This review covers the history of this invention and the lecture by Dr. Ho at Sonoma State University, yesterday, about the Scanning Tunneling Microscope used to create 3-D pictures of atoms.
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- The very first microscope was built by a Dutchman in 1590. It had a magnification of 9 using 2 lenses.
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- In 1609 Galileo invented the first compound microscope using both a convex and a concave lenes.
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- In 1665 Robert Hook was using a microscope to examine cork. He called the tiny pores that he saw “ cells” . It was not until 1838 that science realized that they were plant cells.
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-In 1674 a microscope was invented that had 275 times magnification. This was the first time bacteria were seen
- In 1860 German physicist developed the math to help microscopes to produce a sharper image. The math was called Abbe Sine Condition.
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- A good optical microscope can reveal detail down to 200 nanometers. In 1903 Austria and Germany physicists invented the “ ultramicroscope” that could show detail down to 2 nanometers using shorter light wavelengths. Optical microscopes are limited by the wavelength of light that is 700 to 900 nanometers. We need smaller wavelengths to see smaller things. Electrons are about the same wavelength as X-rays. Electrons are much easier to manipulate because they have a charge. X-rays are very difficult to lens.
- In 1931 the electron microscope was invented and it could resolve individual atoms of 10 nanometer diameter. In order to work using electrons for magnification the samples had to be coated in a thin carbon or a thin metal- alloy.
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- In 1934 the phase-contrast microscope was invented that could study transparent materials.
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- In 1981 the scanning tunneling microscope (STM) was invented that could produce a 3-D image at the atomic level. It could resolve detail down to 0.01 nanometers.
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- In 2012 the newest technology is in bioimaging. This technology can produce moving images of what goes on inside a human cell, in real time.
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- To appreciate these magnifications lets start with the human hair at 50,000 nanometers. The distance between the human cell and its capillary is 10,000 nanometers. The pores in a hens egg is 17,000 nanometers. The egg has a film coating to prevent bacteria from entering the egg shell and spoiling the egg. Never wash an egg. Bacteria range from 1,000 to 10,000 nanometers and could easily penetrate a shell.. The smallest virus is 17 nanometers. The pipette that the human sperm uses to fertilize the female egg is 10 nanometers diameter.
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- Dr. Ho’s students at Irvine University physics class built their Scanning Tunneling Microscope. He showed dozens of pictures of atoms and molecules that the students produced. The pictures are already published in Chemistry and Physics textbooks.
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- To get the pictures the microscope probe does a raster scan over the surface of a substrate. An “x-y” plot is created as the microscope probe travels across the surface. The gap between the probe and the substrate is the “z” distance in the 3-D plot. The resolution is so small that the “z” distance actually traces over each and every atom.
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- The needle probe is a tungsten rod that is acid etched to come to a very sharp point where a single tungsten atom can sit. The probe is connected to an electric circuit with an ammeter, a DC voltage, and a voltmeter that complete the circuit to the metal substrate.
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- The way the image is created is with the z-distance (the gap) of the probe which is raised and lowered in order to keep the electron current flow at a constant level. In turn the gap is kept at a constant distance above the atoms. The scanning probe has to move up and down in order to trace over each individual atom. The gap is very small, 0.3 to 0.8 nanometers. The tip of the probe is 20 nanometers but with a single atom sitting on top of the needle. The electrons flow from the needle point to the substrate. The vacuum maintained in the gap is 10^-11 torr. A torr is a unit of pressure used in vacuum technology. It is equal to 133 pascals. A pascal is the pressure of one Newton of force and acting over one meter of area. The temperature is reduced to 10 Kelvin to slow down the vibrations of the atoms. The highest resolution uses temperature of 200 milliKelvin.
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- The DC voltage is increased from 48.8 millivolts to 488 millivolts, a factor of 10, and that results in the probe moving 10 nanometers. But, the trick in 3-D imaging is to keep the gap constant over the scanned surface.
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- A constant gap means a constant current flowing from the probe. As the probe moves up an down passing over each individual atom the “z” distance the probe is moved is recorded. It represents the height and depth of each atom and in between atoms. The resolution obtained by this technique can be as small as 0.01 nanometers. For an atom that is 10 nanometers in diameter it is like having1,000 pixels across the surface of one atom. That is a high resolution picture!
- To get an idea of this resolution of 10^-13 meters think of a space ship 3,900,000 miles from Earth where the Earth appears the same size as the Moon. The microscope aimed at the Earth could see the city, the building, and even the size of a window in the building from that distance. Amazing!
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- The probe is moved up and down by piezoelectric crystals. These crystals expand and contract when a varying voltage is applied perpendicular to the crystal. Computer software controls the voltage to raise and lower the probe keeping the gap and the current flow at a constant level.
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- The STM microscope does much more than just make pictures of atoms. It can move and relocate atoms. It can combine atoms and make molecules. For example a Carbon Monoxide molecule is on the metal substrate. The static voltage on the probe approaches the molecule and picks it up. The probe carries it to another location where an iron atom resides. It lowers the molecule over the atom and reverses the voltage. The Carbon Monoxide attaches itself to the iron atom creating a new molecule Fe (CO)2.
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- This is not unlike Carbon Monoxide poisoning. The hemoglobin in the blood carries iron. The Carbon Monoxide gas gets absorbed into the blood stream and attaches itself to these iron atoms. Fe(CO)2 is poisonous to the body.
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- The STM microscope can measure the oscillations of individual atoms creating a spectroscopy that is unique to each element atom. The probe stationary over the atom measuring the slight variations in voltage and current due to the vibrating atom and calculating the second derivative, the rate of change of the rate of change of the ratio delta current / delta voltage and a unique spectroscopy signature can be developed. Each atom oscillation is unique to each element. In that way the STM can tell which atom is which.
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- Another application is to measure Brownian Motion of atoms. A hydrogen atom on a copper substrate will vibrate and take a “random walk” through the copper matrix. The probe placed over the hydrogen atom can measure 8 points that circle the atom. Each point carrying the same current. This defines the atoms position. When the atom moves the 8 points that had equal current levels will experience less current on one side and more current on the other side. In this way the direction the atom is moving can be determined. Computer software will then direct the probe to follow the atom and reposition over it at the new location.
- If the random walk is followed for 200 positions in 69 seconds, the distance traveled from the original location will be proportional to the square root of time. If the atom moved in a straight line distance would be directly proportional to time. But, mathematically it can be proven that a random walk will have a distance proportional to the square root of time. Amazing!
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- The STM can also study the “lock and key “ of forming molecules. First gold atoms are attached to each other in a straight line, a wire. Two gold wires are formed. A precise gap is created between the two wires. A Nickel-Aluminum molecule is positioned to fit the gap connecting to the two gold wires and completing the circuit. If the molecule is just slightly off center, or in the wrong orientation, the connection will not be made properly. This basic science is revealing the “lock and key” characteristic that allow molecules to attach to each other. Once we learn this basic science, technology could begin building circuits at the atomic level.
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- If an electron is deposited inside a matrix formed by certain molecules it can become a “particle in a box”. Then if a voltage is applied to excite the electron the energy levels of the electron can be stepped from a single node of probability of position to 2 nodes, then 3 nodes , up to 6 nodes of energy levels. In addition the spin levels of the electron can be determined and even switched between + ½ spin and - ½ spin.
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- All of these experiments and others were performed by Dr. Ho’s students who built the STM microscope. The funding came from the Department of Energy and the National Foundation of Science. It is basic research performed by students. The engineering was broad covering designs that suppress vibrations and acoustic isolation by floating the microscope on air and using eddy current damping on springs. The computer software written by students in C++ and assembly language to control scanning the probe, the controls for the piezoelectric crystals to map the surface of the atoms, the color imaging that has shown up in Chemistry and Physics text books are a credit to Dr. Ho’s miraculous opportunities in education. Congratulation!!!
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