A scanning tunneling microscope (STM) is an instrument for imaging surfaces at the atomic level. Its development in 1981 earned its inventors, Gerd Binnig and Heinrich Rohrer (at IBM Zürich), the Nobel Prize in Physics in 1986. For an STM, good resolution is considered to be 0.1 nm lateral resolution and 0.01 nm (10 pm) depth resolution. With this resolution, individual atoms within materials are routinely imaged and manipulated. The STM can be used not only in ultra-high vacuum but also in air, water, and various other liquid or gas ambients, and at temperatures ranging from near zero kelvin to over 1000 °C.
STM is based on the concept of quantum tunneling. When a conducting tip is brought very near to the surface to be examined, a bias (voltage difference) applied between the two can allow electrons to tunnel through the vacuum between them. The resulting tunneling current is a function of tip position, applied voltage, and the local density of states (LDOS) of the sample. Information is acquired by monitoring the current as the tip's position scans across the surface, and is usually displayed in image form. STM can be a challenging technique, as it requires extremely clean and stable surfaces, sharp tips, excellent vibration control, and sophisticated electronics, but nonetheless many hobbyists have built their own.
First, a voltage bias is applied and the tip is brought close to the sample by coarse sample-to-tip control, which is turned off when the tip and sample are sufficiently close. At close range, fine control of the tip in all three dimensions when near the sample is typically piezoelectric, maintaining tip-sample separation W typically in the 4-7 Å (0.4-0.7 nm) range, which is the equilibrium position between attractive (3<W<10Å) and repulsive (W<3Å) interactions. In this situation, the voltage bias will cause electrons to tunnel between the tip and sample, creating a current that can be measured. Once tunneling is established, the tip's bias and position with respect to the sample can be varied (with the details of this variation depending on the experiment) and data are obtained from the resulting changes in current.
If the tip is moved across the sample in the x-y plane, the changes in surface height and density of states causes changes in current. These changes are mapped in images. This change in current with respect to position can be measured itself, or the height, z, of the tip corresponding to a constant current can be measured. These two modes are called constant height mode and constant current mode, respectively. In constant current mode, feedback electronics adjust the height by a voltage to the piezoelectric height control mechanism. This leads to a height variation and thus the image comes from the tip topography across the sample and gives a constant charge density surface; this means contrast on the image is due to variations in charge density. In constant height mode, the voltage and height are both held constant while the current changes to keep the voltage from changing; this leads to an image made of current changes over the surface, which can be related to charge density. The benefit to using a constant height mode is that it is faster, as the piezoelectric movements require more time to register the height change in constant current mode than the current change in constant height mode. All images produced by STM are grayscale, with color optionally added in post-processing in order to visually emphasize important features.
In addition to scanning across the sample, information on the electronic structure at a given location in the sample can be obtained by sweeping voltage and measuring current at a specific location. This type of measurement is called scanning tunneling spectroscopy (STS) and typically results in a plot of the local density of states as a function of energy within the sample. The advantage of STM over other measurements of the density of states lies in its ability to make extremely local measurements: for example, the density of states at an impurity site can be compared to the density of states far from impurities.
Framerates of at least 25 Hz enable so called video-rate STM. Framerates up to 80 Hz are possible with fully working feedback that adjusts the height of the tip. Due to the line-by-line scanning motion, a proper comparison on the speed requires not only the framerate, but also the number of pixels in an image: with a framerate of 10 Hz and 100x100 pixels the tip moves with a line frequency of 1 kHz, whereas it moves with only with 500 Hz, when measuring with a faster framerate of 50 Hz but only 10x10 pixels. Video-rate STM can be used to scan surface diffusion.
The components of an STM include scanning tip, piezoelectric controlled height and x,y scanner, coarse sample-to-tip control, vibration isolation system, and computer.
The resolution of an image is limited by the radius of curvature of the scanning tip of the STM. Additionally, image artifacts can occur if the tip has two tips at the end rather than a single atom; this leads to “double-tip imaging,” a situation in which both tips contribute to the tunneling. Therefore, it has been essential to develop processes for consistently obtaining sharp, usable tips. Recently, carbon nanotubes have been used in this instance.
Due to the extreme sensitivity of tunnel current to height, proper vibration insulation or an extremely rigid STM body is imperative for obtaining usable results. In the first STM by Binnig and Rohrer, magnetic levitation was used to keep the STM free from vibrations; now mechanical spring or gas spring systems are often used. Additionally, mechanisms for reducing eddy currents are sometimes implemented.
Maintaining the tip position with respect to the sample, scanning the sample and acquiring the data is computer controlled. The computer may also be used for enhancing the image with the help of image processing as well as performing quantitative measurements.
Many other microscopy techniques have been developed based upon STM. These include photon scanning microscopy (PSTM), which uses an optical tip to tunnel photons; scanning tunneling potentiometry (STP), which measures electric potential across a surface; spin polarized scanning tunneling microscopy (SPSTM), which uses a ferromagnetic tip to tunnel spin-polarized electrons into a magnetic sample, multi-tip scanning tunneling microscopy which enables electrical measurements to be performed at the nanoscale, and atomic force microscopy (AFM), in which the force caused by interaction between the tip and sample is measured.
Other STM methods involve manipulating the tip in order to change the topography of the sample. This is attractive for several reasons. Firstly the STM has an atomically precise positioning system which allows very accurate atomic scale manipulation. Furthermore, after the surface is modified by the tip, it is a simple matter to then image with the same tip, without changing the instrument. IBM researchers developed a way to manipulate xenon atoms adsorbed on a nickel surface. This technique has been used to create electron "corrals" with a small number of adsorbed atoms, which allows the STM to be used to observe electron Friedel oscillations on the surface of the material. Aside from modifying the actual sample surface, one can also use the STM to tunnel electrons into a layer of electron beam photoresist on a sample, in order to do lithography. This has the advantage of offering more control of the exposure than traditional electron beam lithography. Another practical application of STM is atomic deposition of metals (gold, silver, tungsten, etc.) with any desired (pre-programmed) pattern, which can be used as contacts to nanodevices or as nanodevices themselves.
Variable temperature STM was used to investigate temperature dependency of molecular rotations on single crystalline surfaces. Rotating molecules appear blurred compared to non-rotating ones.
Recently groups have found they can use the STM tip to rotate individual bonds within single molecules. The electrical resistance of the molecule depends on the orientation of the bond, so the molecule effectively becomes a molecular switch.
Tunneling is a functioning concept that arises from quantum mechanics. Classically, an object hitting an impenetrable barrier will not pass through. In contrast, objects with a very small mass, such as the electron, have wavelike characteristics which permit such an event, referred to as tunneling.
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. 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,
if E > U(z), which is true for a wave function inside the tip or inside the sample. Inside a barrier, E < U(z) so the wave functions which satisfy this are decaying waves,
quantifies the decay of the wave inside the barrier, with the barrier in the +z direction for .
|Scanning tunneling microscopy|
Knowing the wave function allows one to calculate the probability density for that electron to be found at some location. In the case of tunneling, the tip and sample wave functions overlap such that when under a bias, there is some finite probability to find the electron in the barrier region and even on the other side of the barrier. Let us assume the bias is V and the barrier width is W. This probability, P, that an electron at z=0 (left edge of barrier) can be found at z=W (right edge of barrier) is proportional to the wave function squared,
If the bias is small, we can let U − E ≈ φM in the expression for κ, where φM, the work function, gives the minimum energy needed to bring an electron from an occupied level, the highest of which is at the Fermi level (for metals at T=0 kelvins), to vacuum level. When a small bias V is applied to the system, only electronic states very near the Fermi level, within eV (a product of electron charge and voltage, not to be confused here with electronvolt unit), are excited. These excited electrons can tunnel across the barrier. In other words, tunneling occurs mainly with electrons of energies near the Fermi level.
However, tunneling does require that there be an empty level of the same energy as the electron for the electron to tunnel into on the other side of the barrier. It is because of this restriction that the tunneling current can be related to the density of available or filled states in the sample. The current due to an applied voltage V (assume tunneling occurs sample to tip) depends on two factors: 1) the number of electrons between Ef and eV in the sample, and 2) the number among them which have corresponding free states to tunnel into on the other side of the barrier at the tip. The higher the density of available states the greater the tunneling current. When V is positive, electrons in the tip tunnel into empty states in the sample; for a negative bias, electrons tunnel out of occupied states in the sample into the tip.
Mathematically, this tunneling current is given by
One can sum the probability over energies between Ef − eV and Ef to get the number of states available in this energy range per unit volume, thereby finding the local density of states (LDOS) near the Fermi level. The LDOS near some energy E in an interval ε is given by
and the tunnel current at a small bias V is proportional to the LDOS near the Fermi level, which gives important information about the sample. It is desirable to use LDOS to express the current because this value does not change as the volume changes, while probability density does. Thus the tunneling current is given by
where ρs(0,Ef) is the LDOS near the Fermi level of the sample at the sample surface. This current can also be expressed in terms of the LDOS near the Fermi level of the sample at the tip surface,
The exponential term in the above equations means that small variations in W greatly influence the tunnel current. If the separation is decreased by 1 Å, the current increases by an order of magnitude, and vice versa.
This approach fails to account for the rate at which electrons can pass the barrier. This rate should affect the tunnel current, so it can be treated using the Fermi's golden rule with the appropriate tunneling matrix element. John Bardeen solved this problem in his study of the metal-insulator-metal junction. He found that if he solved Schrödinger’s equation for each side of the junction separately to obtain the wave functions ψ and χ for each electrode, he could obtain the tunnel matrix, M, from the overlap of these two wave functions. This can be applied to STM by making the electrodes the tip and sample, assigning ψ and χ as sample and tip wave functions, respectively, and evaluating M at some surface S between the metal electrodes, where z=0 at the sample surface and z=W at the tip surface.
Now, Fermi’s Golden Rule gives the rate for electron transfer across the barrier, and is written
where δ(Eψ–Eχ) restricts tunneling to occur only between electron levels with the same energy. The tunnel matrix element, given by
is a description of the lower energy associated with the interaction of wave functions at the overlap, also called the resonance energy.
Summing over all the states gives the tunneling current as
where f is the Fermi function, ρs and ρT are the density of states in the sample and tip, respectively. The Fermi distribution function describes the filling of electron levels at a given temperature T.
An earlier, similar invention, the Topografiner of R. Young, J. Ward, and F. Scire from the NIST, relied on field emission. However, Young is credited by the Nobel Committee as the person who realized that it should be possible to achieve better resolution by using the tunnel effect. It was later discovered that an even higher resolution could be achieved by calculating the Doppler effect.
A Boy and His Atom is a 2013 stop-motion animated short film released on YouTube by IBM Research. The movie tells the story of a boy and a wayward atom who meet and become friends. It depicts a boy playing with an atom that takes various forms. One minute in length, it was made by moving carbon monoxide molecules with a scanning tunneling microscope, a device that magnifies them 100 million times. These two-atom molecules were moved to create images, which were then saved as individual frames to make the film. The movie has been recognized by the Guinness Book of World Records as the World's Smallest Stop-Motion Film.
The scientists at IBM Research – Almaden who made the film are moving atoms to explore the limits of data storage because, as data creation and consumption gets bigger, data storage needs to get smaller, all the way down to the atomic level. Traditional silicon transistor technology has become cheaper, denser and more efficient, but fundamental physical limitations suggest that scaling down is an unsustainable path to solving the growing Big Data dilemma. This team of scientists is particularly interested in starting on the smallest scale, single atoms, and building structures up from there. Using this method, IBM announced it can now store a single bit of information in just 12 atoms (current technology takes roughly one million atoms to store a single bit).Atomically precise manufacturing
Atomically precise manufacturing (APM) is the production of materials, structures, devices, and finished goods in a manner such that every atom has a specified location relative to the other atoms, and in which there are no defects, missing atoms, extra atoms, or incorrect (impurity) atoms.
Molecules are atomically precise objects and, as such, are essential building blocks in atomically precise manufacturing. Novel molecular designs can, themselves, be considered atomically precise products; for example, enzyme-like catalysts can be crafted to accelerate chemical reactions. Beyond synthesis techniques to create single molecules, the key challenge of atomically precise manufacturing is in the assembly of molecular building blocks into larger and more complex objects that are also atomically precise. The two known methods for doing this are self-assembly and positional assembly. Molecules that have been designed or have evolved to bind together, typically along conformal surfaces, will self-assemble under the right conditions. In the production of atomically precise membranes, molecules can arrange themselves on the surface of a liquid and then be chemically bound to each other . Complex atomically precise self-assembled objects are also possible: striking examples include the robot-like Enterobacteria phage T4 and the bacterial flagellar motor . In these cases, free-floating "parts" (proteins) in solution self-assemble into three-dimensional objects.
In non-biological systems, the positional assembly (that is, the chemical binding) of a single atom to a single molecule was first demonstrated by Ho and Lee at Cornell University in 1999 using a scanning tunneling microscope (STM) . In this seminal work, a single carbon monoxide molecule on the tip of an STM was moved to a single iron atom sitting on the surface of a crystal and chemically bound by applying electric current.
In August 2015, the United States Department of Energy (DOE) Advanced Manufacturing Office (AMO) invited researchers to their Workshop on Integrated Nanosystems for Atomically Precise Manufacturing (INFAPM) to gather information for accelerating the development of APM. "A fundamentally new approach to INFAPM structures and applications, tools, and demonstration is needed to realize the enormous savings potential of atomic-scale, defect-free manufacturing." There are two assembly approaches for achieving an atomic precision. The first approach is tip-based positional assembly using scanning probe microscopes, which would also include Joseph W. Lyding's selective deprotection and atomic layer epitaxial deposition. The second approach is an integrated nanosystems using molecular machine components. "Both approaches have considerable challenges to implementation, including positional accuracy (which is influenced by factors such as component stiffness and thermal vibration), repeatability, working tip design and synthesis, suitable building block design, transport of molecules to the working tip, and scalability."Binnig and Rohrer Nanotechnology Center
Binnig and Rohrer Nanotechnology Center in Rüschlikon/ZH is a Research Facility for Nanotechnology owned by IBM.
This building was named after the two Nanotech-Pioneers and Nobelaureates Gerd Binnig and Heinrich Rohrer, who invented the Scanning Tunneling Microscope in the year 1986 in the Zurich Research Laboratory next to the Nanotechnology Center.
The Binnig and Rohrer Nanotechnology Center is specialized on basic research but also in photonics and nanotechnology. The facility is based on the premise of open collaboration with IBM scientists and external partners. One of these partners is ETH Zurich, the renowned Swiss university that agreed to rent part of the Center for 10 years. EMPA is a third partner.Christoph Gerber
Christoph Gerber is a titular professor at the Department of Physics, University of Basel, Switzerland.
Christoph Gerber is the co-inventor of the atomic force microscope. He was among the 250 most cited living physicists in the world in the year 2000.Christoph Gerber is a titular professor at the Department of Physics, University of Basel, Switzerland. He was a founding member and Director for Scientific Communication of the NCCR (National Center of Competence in Research Nanoscale Science). He was formerly a Research Staff Member in Nanoscale Science at the IBM Research Laboratory in Rueschlikon, Switzerland, and has served as a project leader in various programs of the Swiss National Science Foundation.
For the past 35 years, his research has been focused on nanoscale science. He is a pioneer in scanning probe microscopy, who made major contributions to the invention of the scanning tunneling microscope, the atomic force microscope (AFM), and AFM techniques in high vacuum and at low temperatures.He is the author and co-author of more than 165 scientific papers that have appeared in peer-reviewed journals and has been cited approximately 29'000 times in cross-disciplinary fields. He belongs to the one hundred worldwide most cited researchers in Physical Sciences. He has given numerous plenary and invited talks at international conferences.
His work has been recognized with multiple honorary degrees and various awards and appeared in numerous articles in daily press and TV coverage. 2016 he has been awarded the Kavli Prize in Nanoscience together with Gerd Binnig and Calvin Quate for the Scanning Force Microscope. He became a fellow of the Norwegian Academy of Science and Letters. He is a Fellow of the American Physical Society and a Fellow of the Institute of Physics UK. His IP portfolio contains 37 patents and patent publications.
His current interests include
Biochemical sensors based on AFM Technology
Chemical surface identification on the nanometer scale with AFM
Nanomechanics, nanorobotics, molecular devices at the ultimate limits of measurement and fabrication
Atomic force microscopy research on insulators
Self-organization and self-assembly at the nanometer scaleDon Eigler
Donald M. "Don" Eigler is an American physicist associated with the IBM Almaden Research Center, who is noted for his achievements in nanotechnology.Electrochemical AFM
Electrochemical AFM (EC-AFM) is a particular type of Scanning probe microscopy (SPM), which combines the classical Atomic force microscopy (AFM) together with electrochemical measurements. EC-AFM allows to perform in-situ AFM measurements in an electrochemical cell, in order to investigate the actual changes in the electrode surface morphology during electrochemical reactions. The solid-liquid interface is thus investigated.
This technique was developed for the first time in 1996 by Kouzeki et al ,who studied amorphous and policristallines thin films of Naphthalocyanine on Indium tin oxide in a solution of 0.1 M Potassium chloride (KCl). Unlike the Electrochemical scanning tunneling microscope, previously developed by Itaya and Tomita in 1988 , the tip is non-conductive and it is easily steered in a liquid environment.Electrochemical scanning tunneling microscope
The electrochemical scanning tunneling microscope (ESTM) is a scanning tunneling microscope that measures the structures of surfaces and electrochemical reactions in solid-liquid interfaces at atomic or molecular scales.Gerd Binnig
Gerd Binnig (born 20 July 1947) is a German physicist, who won the Nobel Prize in Physics in 1986 for the invention of the scanning tunneling microscope.He was born in Frankfurt am Main and played in the ruins of the city during his childhood. His family lived partly in Frankfurt and partly in Offenbach am Main, and he attended school in both cities. At the age of 10, he decided to become a physicist, but he soon wondered whether he had made the right choice. He concentrated more on music, playing in a band. He also started playing the violin at 15 and played in his school orchestra.Binnig studied physics at the J.W. Goethe University in Frankfurt, gaining a bachelor's degree in 1973 and remaining there do a PhD with in Werner Martienssen's group, supervised by Eckhardt Hoenig.In 1969, he married Lore Wagler, a psychologist, and they have a daughter born in Switzerland and a son born in California. His hobbies are reading, swimming and golf.
In 1978, he accepted an offer from IBM to join their Zürich research group, where he worked with Heinrich Rohrer, Christoph Gerber and Edmund Weibel. There they developed the scanning tunneling microscope (STM), an instrument for imaging surfaces at the atomic level.
The Nobel committee described the effect that the invention of the STM had on science, saying that "entirely new fields are opening up for the study of the structure of matter." The physical principles on which the STM was based were already known before the IBM team developed the STM, but Binnig and his colleagues were the first to solve the significant experimental challenges involved in putting it into effect.The IBM Zürich team were soon recognized with a number of prizes: the German Physics Prize, the Otto Klung Prize, the Hewlett Packard Prize and the King Faisal Prize.
In 1986, Binnig and Rohrer shared half of the Nobel Prize in Physics, the other half of the Prize was awarded to Ernst Ruska.
From 1985-1988, he worked in California. He was at IBM in Almaden Valley, and was visiting professor at Stanford University.In 1985, Binnig invented the atomic force microscope (AFM) and Binnig, Christoph Gerber and Calvin Quate went on to develop a working version of this new microscope for insulating surfaces.In 1987 Binnig was appointed IBM Fellow. In the same year, he started the IBM Physics group Munich, working on creativity and atomic force microscopy In 1994 Professor Gerd Binnig founded Definiens which turned in the year 2000 into a commercial enterprise. The company developed Cognition Network Technology to analyze images just like the human eye and brain are capable of doing.in 2016, Binnig won the Kavli Prize in Nanoscience. He became a fellow of the Norwegian Academy of Science and Letters.The Binnig and Rohrer Nanotechnology Center, an IBM-owned research facility in Rüschlikon, Zürich is named after Gerd Binnig and Heinrich Rohrer.Heinrich Rohrer
Heinrich Rohrer (6 June 1933 – 16 May 2013) was a Swiss physicist who shared half of the 1986 Nobel Prize in Physics with Gerd Binnig for the design of the scanning tunneling microscope (STM). The other half of the Prize was awarded to Ernst Ruska.History of nanotechnology
The history of nanotechnology traces the development of the concepts and experimental work falling under the broad category of nanotechnology. Although nanotechnology is a relatively recent development in scientific research, the development of its central concepts happened over a longer period of time. The emergence of nanotechnology in the 1980s was caused by the convergence of experimental advances such as the invention of the scanning tunneling microscope in 1981 and the discovery of fullerenes in 1985, with the elucidation and popularization of a conceptual framework for the goals of nanotechnology beginning with the 1986 publication of the book Engines of Creation. The field was subject to growing public awareness and controversy in the early 2000s, with prominent debates about both its potential implications as well as the feasibility of the applications envisioned by advocates of molecular nanotechnology, and with governments moving to promote and fund research into nanotechnology. The early 2000s also saw the beginnings of commercial applications of nanotechnology, although these were limited to bulk applications of nanomaterials rather than the transformative applications envisioned by the field. .IBM (atoms)
IBM in atoms was a demonstration by IBM scientists in 1989 of a technology capable of manipulating individual atoms. A scanning tunneling microscope was used to arrange 35 individual xenon atoms on a substrate of chilled crystal of nickel to spell out the three letter company initialism. It was the first time atoms had been precisely positioned on a flat surface.On Apr 30, 2013 IBM published an article on its website and a video on YouTube called "A Boy And His Atom: The World's Smallest Movie".IBM Research
IBM Research is IBM's research and development division. It is the largest industrial research organization in the world, with twelve labs on six continents.IBM employees have garnered six Nobel Prizes, six Turing Awards, 20 inductees into the U.S. National Inventors Hall of Fame, 19 National Medals of Technology, five National Medals of Science and three Kavli Prizes.As of 2018, the company has generated more patents than any other business in each of 25 consecutive years, which is a record.Nanoart
NanoArt is a novel art discipline related to science and technology. It depicts natural or synthetic structures with features sized at the nanometer scale, which are observed by electron or scanning probe microscopy techniques in scientific laboratories. The recorded two or three dimensional images and movies are processed for artistic appeal and presented to the general audience.
One of the aims of NanoArt is to familiarize people with nanoscale objects and advances in their synthesis and manipulation. NanoArt has been presented at traditional art exhibitions around the world. Besides, online competitions have been launched in the 2000s such as the “NANO” 2003 show at Los Angeles County Museum of Art and “Nanomandala”, the 2004 and 2005 installations in New York and Rome by Victoria Vesna and James Gimzewski, and the regular "Science as Art" section launched at the 2006 Materials Research Society Meeting.A characteristic example of nanoart is A Boy and His Atom, a one-minute stop-motion animated film created in 2012 by IBM Research from 242 images sized by 45×25 nm, which were recorded with a scanning tunneling microscope. The movie tells the story of a boy and a wayward atom who meet and become friends. The film was accepted into the Tribeca Online Film Festival and shown at the New York Tech Meet-up and the World Science Festival.
Earlier in 2007 a book Teeny Ted from Turnip Town was created at the Simon Fraser University in Canada using a gallium-ion beam with a diameter of ~7 nanometers. The book contains 30 silicon-based pages sized by 0.07×0.10 mm; it was published in 100 copies and has an ISBN number.
In 2015, Jonty Hurwitz pioneered a new technique for creating nanosculpture using multiphoton lithography and photogrammetry. His work Trust was prepared in collaboration with Karlsruhe Institute of Technology and set a Guinness World Record as the "Smallest Sculpture of a Human Form".Nanoengineering
Nanoengineering is the practice of engineering on the nanoscale. It derives its name from the nanometre, a unit of measurement equalling one billionth of a meter.
Nanoengineering is largely a synonym for nanotechnology, but emphasizes the engineering rather than the pure science aspects of the field.Outline of nanotechnology
The following outline is provided as an overview of and topical guide to nanotechnology:
Nanotechnology is science, engineering, and technology conducted at the nanoscale, which is about 1 to 100 nanometers.Photon scanning microscopy
The operation of a photon scanning tunneling microscope (PSTM) is analogous to the operation of an electron scanning tunneling microscope (ESTM), with the primary distinction being that PSTM involves tunneling of photons instead of electrons from the sample surface to the probe tip. A beam of light is focused on a prism at an angle greater than the critical angle of the refractive medium in order to induce total internal reflection (TIR) within the prism. Although the beam of light is not propagated through the surface of the refractive prism under TIR, an evanescent field of light is still present at the surface.
The evanescent field is a standing wave which propagates along the surface of the medium and decays exponentially with increasing distance from the surface. The surface wave is modified by the topography of the sample, which is placed on the surface of the prism. By placing a sharpened, optically conducting probe tip very close to the surface (at a distance <λ), photons are able to propagate through the space between the surface and the probe (a space which they would otherwise be unable to occupy) through tunneling, allowing detection of variations in the evanescent field and thus, variations in surface topography of the sample. In this manner, PSTM is able to map the surface topography of a sample in much the same way as in ESTM.
One major advantage of PSTM is that an electrically conductive surface is no longer necessary. This makes imaging of biological samples much simpler and eliminates the need to coat samples in gold or another conductive metal. Furthermore, PSTM can be used to measure the optical properties of a sample and can be coupled with techniques such as photoluminescence, absorption, and Raman spectroscopy.Scanning probe microscopy
Scanning probe microscope (SPM) is a branch of microscopy that forms images of surfaces using a physical probe that scans the specimen. SPM was founded in 1981, with the invention of the scanning tunneling microscope, an instrument for imaging surfaces at the atomic level. The first successful scanning tunneling microscope experiment was done by Binnig and Rohrer. The key to their success was using a feedback loop to regulate gap distance between the sample and the probe.Many scanning probe microscopes can image several interactions simultaneously. The manner of using these interactions to obtain an image is generally called a mode.
The resolution varies somewhat from technique to technique, but some probe techniques reach a rather impressive atomic resolution. This is due largely because piezoelectric actuators can execute motions with a precision and accuracy at the atomic level or better on electronic command. This family of techniques can be called "piezoelectric techniques". The other common denominator is that the data are typically obtained as a two-dimensional grid of data points, visualized in false color as a computer image.Scanning tunneling spectroscopy
Scanning tunneling spectroscopy (STS), an extension of scanning tunneling microscopy (STM), is used to provide information about the density of electrons in a sample as a function of their energy.
In scanning tunneling microscopy, a metal tip is moved over a conducting sample without making physical contact. A bias voltage applied between the sample and tip allows a current to flow between the two. This is as a result of quantum tunneling across a barrier; in this instance, the physical distance between the tip and the sample
The scanning tunneling microscope is used to obtain "topographs" - topographic maps - of surfaces. The tip is rastered across a surface and (in constant current mode), a constant current is maintained between the tip and the sample by adjusting the height of the tip. A plot of the tip height at all measurement positions provides the topograph. These topographic images can obtain atomically resolved information on metallic and semi-conducting surfaces
However, the scanning tunneling microscope does not measure the physical height of surface features. One such example of this limitation is an atom adsorbed onto a surface. The image will result in some perturbation of the height at this point. A detailed analysis of the way in which an image is formed shows that the transmission of the electric current between the tip and the sample depends on two factors: (1) the geometry of the sample and (2) the arrangement of the electrons in the sample. The arrangement of the electrons in the sample is described quantum mechanically by an "electron density". The electron density is a function of both position and energy, and is formally described as the local density of electron states, abbreviated as local density of states (LDOS), which is a function of energy.
Spectroscopy, in its most general sense, refers to a measurement of the number of something as a function of energy. For scanning tunneling spectroscopy the scanning tunneling microscope is used to measure the number of electrons (the LDOS) as a function of the electron energy. The electron energy is set by the electrical potential difference (voltage) between the sample and the tip. The location is set by the position of the tip.
At its simplest, a "scanning tunneling spectrum" is obtained by placing a scanning tunneling microscope tip above a particular place on the sample. With the height of the tip fixed, the electron tunneling current is then measured as a function of electron energy by varying the voltage between the tip and the sample (the tip to sample voltage sets the electron energy). The change of the current with the energy of the electrons is the simplest spectrum that can be obtained, it is often referred to as an I-V curve. As is shown below, it is the slope of the I-V curve at each voltage (often called the dI/dV-curve) which is more fundamental because dI/dV corresponds to the electron density of states at the local position of the tip, the LDOS.Timeline of microscope technology
Timeline of microscope technology
c. 700 BCE - The "Nimrud lens" of Assyrians manufacture, a rock crystal disk with a convex shape believed to be a burning or magnifying lens.
167 BCE - The Chinese use simple microscopes made of a lens and a water-filled tube to visualize the unseen.
13th century - The increase in use of lenses in eyeglasses probably led to the wide spread use of simple microscopes (single lens magnifying glasses) with limited magnification.
1590 - earliest date of a claimed Hans Martens/Zacharias Janssen invention of the compound microscope (claim made in 1655).
After 1609 - Galileo Galilei is described as being able to close focus his telescope to view small objects close up and/or looking through the wrong end in reverse to magnify small objects. A telescope used in this fashion is the same as a compound microscope but historians debate whether Galileo was magnifying small objects or viewing near by objects with his terrestrial telescope (convex objective/concave eyepiece) reversed.
1619 - Earliest recorded description of a compound microscope, Dutch Ambassador Willem Boreel sees one in London in the possession of Dutch inventor Cornelius Drebbel, an instrument about eighteen inches long, two inches in diameter, and supported on 3 brass dolphins.
1621 - Cornelius Drebbel presents, in London, a compound microscope with a convex objective and a convex eyepiece (a "Keplerian" microscope).
c.1622 - Drebbel presents his invention in Rome.
1624 - Galileo improves on a compound microscope he sees in Rome and presents his occhiolino to Prince Federico Cesi, founder of the Accademia dei Lincei (in English, The Linceans).
1625 - Francesco Stelluti and Federico Cesi publish Apiarium, the first account of observations using a compound microscope
1625 - Giovanni Faber of Bamberg (1574 - 1629) of the Linceans, after seeing Galileo's occhiolino, coins the word microscope by analogy with telescope.
1655 - In an investigation by Willem Boreel, Dutch spectacle-maker Johannes Zachariassen claims his father, Zacharias Jansen, invented the compound microscope in 1590. Zachariassen's claimed dates are so early it is sometimes assumed, for the claim to be true, that his grandfather, Hans Martens, must have invented it. Findings are published by writer Pierre Borel. Discrepancies in Boreel's investigation and Zachariassen's testimony (including misrepresenting his date of birth and role in the invention) has led some historians to consider this claim dubious.
1665 - Robert Hooke publishes Micrographia, a collection of biological micrographs. He coins the word cell for the structures he discovers in cork bark.
1674 - Anton van Leeuwenhoek improves on a simple microscope for viewing biological specimens.
1850s - John Leonard Riddell, Professor of Chemistry at Tulane University, invents the first practical binocular microscope.
1863 - Henry Clifton Sorby develops a metallurgical microscope to observe structure of meteorites.
1860s - Ernst Abbe discovers the Abbe sine condition, a breakthrough in microscope design, which until then was largely based on trial and error. The company of Carl Zeiss exploited this discovery and becomes the dominant microscope manufacturer of its era.
1928 - Edward Hutchinson Synge publishes theory underlying the near-field scanning optical microscope
1931 - Ernst Ruska starts to build the first electron microscope. It is a Transmission electron microscope (TEM)
1936 - Erwin Wilhelm Müller invents the field emission microscope.
1938 - James Hillier builds another TEM
1951 - Erwin Wilhelm Müller invents the field ion microscope and is the first to see atoms.
1953 - Frits Zernike, professor of theoretical physics, receives the Nobel Prize in Physics for his invention of the phase contrast microscope.
1955 - George Nomarski, professor of microscopy, published the theoretical basis of Differential interference contrast microscopy.
1957 - Marvin Minsky, a professor at MIT, invents the confocal microscope, an optical imaging technique for increasing optical resolution and contrast of a micrograph by means of using a spatial pinhole to block out-of-focus light in image formation. This technology is a predecessor to today's widely used confocal laser scanning microscope.
1967 - Erwin Wilhelm Müller adds time-of-flight spectroscopy to the field ion microscope, making the first atom probe and allowing the chemical identification of each individual atom.
1981 - Gerd Binnig and Heinrich Rohrer develop the scanning tunneling microscope (STM).
1986 - Gerd Binnig, Quate, and Gerber invent the Atomic force microscope (AFM)
1988 - Alfred Cerezo, Terence Godfrey, and George D. W. Smith applied a position-sensitive detector to the atom probe, making it able to resolve materials in 3-dimensions with near-atomic resolution.
1988 - Kingo Itaya invents the Electrochemical scanning tunneling microscope
1991 - Kelvin probe force microscope invented.
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