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Encyclopedia > Scanning Tunneling Microscope
Image of reconstruction on a clean Gold(100) surface.

Scanning tunneling microscope (STM) is a powerful technique for viewing surfaces at the atomic level. Its development in 1981 won its inventors, Gerd Binnig and Heinrich Rohrer (at IBM Zürich), the Nobel Prize in Physics in 1986[1]. 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[2]. 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 0 Kelvin to a few hundred degrees Celsius[3]. Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... GOLD refers to one of the following: GOLD (IEEE) is an IEEE program designed to garner more student members at the university level (Graduates of the Last Decade). ... Examples of directions Miller indices are a notation used to describe lattice planes and directions in a crystal. ... Self-assembly is the fundamental principle which generates structural organization on all scales from molecules to galaxies. ... Supramolecular chemistry refers to the area of chemistry which focuses on the noncovalent bonding interactions of molecules. ... Semiconductors are compounds whose electrical conductivity is midway between that of typical metals and that of insulating compounds. ... Quinacridones are a family of synthetic pigments used to make high performance paints. ... For other uses, see Graphite (disambiguation). ... Gerd Binnig (born July 20, 1947) is a German-born physicist who shared with Heinrich Rohrer half of the 1986 Nobel Prize for Physics for their invention of the scanning tunneling microscope (STM). ... Heinrich Rohrer (born June 6, 1933) is a Swiss physicist who, with Gerd Binnig, received half of the 1986 Nobel Prize for Physics for their joint invention of the scanning tunneling microscope (STM). ... Hannes AlfvÃ©n (1908â€“1995) accepting the Nobel Prize for his work on magnetohydrodynamics [1]. List of Nobel Prize laureates in Physics from 1901 to the present day. ... Density of states (DOS) is a property in statistical and condensed matter physics that quantifies how closely packed energy levels are in some physical system. ...

The STM is based on the concept of quantum tunneling. 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. For low voltages, this tunneling current is a function of the local density of states (LDOS) at the Fermi level, Ef, of the sample[3]. Variations in current as the probe passes over the surface are translated into an image. STM can be a challenging technique, as it requires extremely clean surfaces and sharp tips. Quantum tunneling is the quantum-mechanical effect of transitioning through a classically-forbidden energy state. ...

## Contents

Tunnelling is a functioning concept that arises from quantum mechanics. Classically, an object hitting an impenetrable wall will bounce back. Imagine throwing a baseball to a friend on the other side of a mile high brick wall, directly at the wall. One would be rightfully astonished if, rather than bouncing back upon impact, the ball were to simply pass through to your friend on the other side of the wall. 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 much greater probability[3]. For a generally accessible and less technical introduction to the topic, see Introduction to quantum mechanics. ...

Electrons behave as waves 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, In physics, the Schrödinger equation, proposed by the Austrian physicist Erwin Schrödinger in 1925, describes the time-dependence of quantum mechanical systems. ...

$- frac{hbar^2}{2m} frac{partial^2psi_n (z)}{partial z^2} + U(z) psi_n (z) = Epsi_n (z)$,

where ħ is Planck’s constant, z is the position, and m is the mass of an electron[3]. 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, A wave function is a mathematical tool that quantum mechanics uses to describe any physical system. ... A wave is a disturbance that propagates through space or spacetime, often transferring energy. ...

$psi_n (z) = psi_n (0)e^{pm ikz}$,

where

$k=frac{sqrt{2m(E-U)}}{hbar}$

if E > U(z), which is true for a wave function inside the tip or inside the sample[3]. Inside a barrier, such as between tip and sample, E < U(z) so the wave functions which satisfy this are decaying waves,

$psi_n (z) = psi_n (0)e^{pm kappa z}$,

where

$kappa = frac{sqrt{2m(U-E)}}{hbar}$

quantifies the decay of the wave inside the barrier, with the barrier in the +z direction for − κ [3].

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[3]. Let us assume the bias is V and the barrier width is W, as illustrated in Figure 1. 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,

$P propto |psi_n (0)|^2 e^{-2 kappa W}$ [3].

If the bias is small, we can let UEφM 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, are excited[3]. 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 is 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[3]. The higher 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[3].

Mathematically, this tunneling current is given by

$I propto sum_{E_f-eV}^{Ef} |psi_n (0)|^2 e^{-2 kappa W}$.

One can sum the probability over energies between EfeV and eV 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[3]. The LDOS near some energy E in an interval ε is given by

$rho_s (z,E) = frac{1}{epsilon} sum_{E- epsilon}^{E} | psi_n (z)|^2$,

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[3]. It is desirable to use LDOS to express the current because this value does not change as the volume changes, while probability density does[3]. Thus the tunneling current is given by

$I propto V rho_s (0, E_f) e^{-2 kappa W}$

where ρs(0,Ef) is the LDOS near the Fermi level of the sample at the sample surface[3]. By using equation (6), this current can also be expressed in terms of the LDOS near the Fermi level of the sample at the tip surface,

$I propto V rho_s (W, E_f) V$

The exponential term in (9) is very significant in that small variations in W greatly influence the tunnel current. If the separation is decreased by 1 Ǻ, the current increases by an order of magnitude, and vice versa[4].

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 accounted for by using Fermi’s Golden Rule with the appropriate tunneling matrix element. John Bardeen solved this problem in his study of the metal-insulator-metal junction, MIM[5]. 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[3]. 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 at z=zo, where z=0 at the sample surface and z=W at the tip surface[3]. John Bardeen (May 23, 1908 â€“ January 30, 1991) was an American physicist and electrical engineer, who won the Nobel Prize in Physics twice: first in 1956 with William Shockley and Walter Brattain for the invention of the transistor; and again in 1972 with Leon Neil Cooper and John Robert Schrieffer...

Now, Fermi’s Golden Rule gives the rate for electron transfer across the barrier, and is written

$w = frac{2 pi}{hbar} |M|^2 delta (E_{psi} - E_{chi} )$,

where δ(Eψ-Eχ) restricts tunneling to occur only between electron levels with the same energy[3]. The tunnel matrix element, given by

$M= frac{hbar}{2 pi} int_{z=z_0} ( chi* frac {partial psi}{partial z}-psi frac{partial chi*}{partial z}) dS$,

is a description of the lower energy associated with the interaction of wave functions at the overlap, also called the resonance energy[3].

Summing over all the states gives the tunneling current as

$I = frac{4 pi e}{hbar}int_{-infty}^{+infty} [f(E_f -eV) - f(E_f + epsilon)] rho_s (E_f - eV + epsilon) rho_T (E_f + epsilon)|M|^2 d epsilon$,

where f is the Fermi function, ρs and ρT are the density of states in the sample and tip, respectively[3]. The Fermi distribution function describes the filling of electron levels at a given temperature T. Fermi-Dirac distribution as a function of Îµ/Î¼ plotted for 4 different temperatures. ...

## Procedure

First the tip is brought into close proximity of the sample by some coarse sample-to-tip control. The values for common sample-to-tip distance, W, range from about 4-7 Ǻ, which is the equilibrium position between attractive (3<W<10Ǻ) and repulsive (W<3Ǻ) interactions[3]. Once tunneling is established, piezoelectric transducers are implemented to move the tip in three directions. As the tip is rastered across the sample in the x-y plane, the density of states and therefore the tunnel current changes. 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[3]. These two modes are called constant height mode and constant current mode, respectively. An angstrom, angstrÃ¶m, or Ã¥ngstrÃ¶m (symbol Ã…) is a unit of length. ... Piezoelectricity is the ability of certain crystals to produce a voltage when subjected to mechanical stress. ...

In constant current mode, feedback electronics adjust the height by a voltage to the piezoelectric height control mechanism[6]. 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[4].

In constant height, 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[4]. The benefit to using a constant height mode is that it is faster, as the piezoelectric movements require more time to register the change in constant current mode than the voltage response in constant height mode[4].

In addition to scanning across the sample, information on the electronic structure of the sample can be obtained by sweeping voltage and measuring current at a specific location[2]. This type of measurement is called scanning tunneling spectroscopy (STS). Scanning tunneling spectroscopy (STS) performed with a scanning tunneling microscope (STM) is a technique which gives information about the local density of electronic states on surfaces at atomic or molecular scale. ...

Framerates of at least 1 Hz enable so called Video-STM (up to 50 Hz possible). This can be used to scan surface diffusion. diffusion (disambiguation). ...

## Instrumentation

Schematic view of an STM

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[6]. Image File history File links Download high resolution version (1114x912, 33 KB) Description: Schematic diagram of a scanning tunneling microscope Source: Michael Schmid, TU Wien; adapted from the IAP/TU Wien STM Gallery Date: 2005-Jun-07 Author: Michael Schmid Permission: Michael Schmid put it under Creative Commons Attribution ShareAlike... Image File history File links Download high resolution version (1114x912, 33 KB) Description: Schematic diagram of a scanning tunneling microscope Source: Michael Schmid, TU Wien; adapted from the IAP/TU Wien STM Gallery Date: 2005-Jun-07 Author: Michael Schmid Permission: Michael Schmid put it under Creative Commons Attribution ShareAlike...

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[2]. Therefore it has been essential to develop processes for consistently obtaining sharp, usable tips. Recently, carbon nanotubes have been used in this instance. Image resolution describes the detail an image holds. ...

A closeup of a simple scanning tunneling microscope head at the University of St Andrews scanning MoS2 using a Platinum-Iridium stylus.

The tip is often made of tungsten or platinum-iridium, though gold is also used[2]. Tungsten tips are usually made by electrochemical etching, and platinum-iridium tips by mechanical shearing[2]. Both processes are outlined in C. Bai’s book, reference[2] below. Image File history File links Metadata Size of this preview: 800 Ã— 600 pixelsFull resolution (2048 Ã— 1536 pixel, file size: 231 KB, MIME type: image/jpeg) File historyClick on a date/time to view the file as it appeared at that time. ... Image File history File links Metadata Size of this preview: 800 Ã— 600 pixelsFull resolution (2048 Ã— 1536 pixel, file size: 231 KB, MIME type: image/jpeg) File historyClick on a date/time to view the file as it appeared at that time. ... St Marys College Bute Medical School St Leonards College[5][6] Affiliations 1994 Group Website http://www. ... For other uses, see Tungsten (disambiguation). ... GOLD refers to one of the following: GOLD (IEEE) is an IEEE program designed to garner more student members at the university level (Graduates of the Last Decade). ...

Due to the extreme sensitivity of tunnel current to height, proper vibration isolation 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 spring systems are often used[3]. Additionally, mechanisms for reducing eddy currents are implemented. This article is about magnetic levitation. ... An eddy current is a phenomenon caused by a moving magnetic field intersecting a conductor or vice-versa. ...

Maintaining the tip position with respect to the sample, scanning the sample in raster fashion and acquiring the data is computer controlled[6]. The computer is also used for enhancing the image with the help of image processing as well as performing quantitative morphological measurements. UPIICSA IPN - Binary image Image processing is any form of information processing for which the input is an image, such as photographs or frames of video; the output is not necessarily an image, but can be for instance a set of features of the image. ...

## Other STM Related Studies

Many other microscopy techniques have been developed based upon STM. These include Photon Scanning Tunneling Microscopy (PSTM), which uses an optical tip to tunnel photons[2]; Scanning Tunneling Potentiometry (STP), which measures electric potential across a surface[2]; and spin polarized scanning tunneling microscopy (SPSTM), which uses a ferromagnetic tip to tunnel spin-polarized electrons into a magnetic sample[7]. Spin polarized scanning tunneling microscopy (SP-STM) is a specialized application of scanning tunneling microscopy (STM) that can provide detailed information of the the surface magnetization distribution of a sample. ... Ferromagnetism is a phenomenon by which a material can exhibit a spontaneous magnetization, and is one of the strongest forms of magnetism. ...

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 absorbed on a nickel surface[2] 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 E-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. For other uses, see IBM (disambiguation) and Big Blue. ... General Name, Symbol, Number xenon, Xe, 54 Chemical series noble gases Group, Period, Block 18, 5, p Appearance colorless Standard atomic weight 131. ... For other uses, see Nickel (disambiguation). ... For other uses, see Electron (disambiguation). ... It has been suggested that this article or section be merged with resist. ... Lithography is a method for printing on a smooth surface. ... // Conventional electron-beam lithography The practice of using a beam of electrons to generate patterns on a surface is known as Electron beam lithography. ...

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. Electrical resistance is a measure of the degree to which an electrical component opposes the passage of current. ...

## Early Invention

An early, patented invention, based on the above-mentioned principles, and later acknowledged by the Nobel committee itself, was the Topografiner of R. Young, J. Ward, and F. Scire from the NIST ("National Institute of Science and Technolology" of the USA)[8]. As a non-regulatory agency of the United States Department of Commerce’s Technology Administration, the National Institute of Standards (NIST) develops and promotes measurement, standards, and technology to enhance productivity, facilitate trade, and improve the quality of life. ...

## References

1. ^ G. Binnig, H. Rohrer “Scanning tunneling microscopy” IBM Journal of Research and Development 30,4 (1986) reprinted 44,½ Jan/Mar (2000)
2. ^ a b c d e f g h i C. Bai Scanning tunneling microscopy and its applications Springer Verlag, 2nd edition, New York (1999)
3. ^ a b c d e f g h i j k l m n o p q r s t u v w C. Julian Chen Introduction to Scanning Tunneling Micro scopy(1993)
4. ^ a b c d D. A. Bonnell and B. D. Huey “Basic principles of scanning probe microscopy” from Scanning probe microscopy and spectroscopy: Theory, techniques, and applications 2nd edition Ed. By D. A. Bonnell Wiley-VCH, Inc. New York (2001)
5. ^ J. Bardeen “Tunneling from a many particle point of view” Phys. Rev. Lett. 6,2 57-59 (1961)
6. ^ a b c K. Oura, V. G. Lifshits, A. A. Saranin, A. V. Zotov, and M. Katayama Surface science: an introduction Springer-Verlag Berlin (2003)
7. ^ R. Wiesendanger, I. V. Shvets, D. Bürgler, G. Tarrach, H.-J. Güntherodt, and J.M.D. Coey “Recent advances in spin-polarized scanning tunneling microscopy” Ultramicroscopy 42-44 (1992)
8. ^ R. Young, J. Ward, F. Scire, The Topografiner: An Instrument for Measuring Surface Topography, Rev. Sci. Instrum. 43, 999 (1972)

The Opensource Handbook of Nanoscience and Nanotechnology
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Subfields and related fields Part of a series of articles on Nanotechnology Image File history File links Wikibooks-logo-en. ... Nanotechnology refers to a field of applied science and technology whose theme is the control of matter on the atomic and molecular scale, generally 100 nanometers or smaller, and the fabrication of devices that lie within that size range. ... History Implications Applications Organizations In fiction and popular culture List of topics Although nanotechnology is a relatively recent development in scientific research, the development of its central concepts happened over a longer period of time. ... Potential risks of nanotechnology can broadly be grouped into four areas: the risk of environmental damage from nanoparticles and nanomaterials the risk posed by molecular manufacturing (or advanced nanotechnology) societal risks health risks Nanoethics concerns the ethical and social issues associated with developments in nanotechnology, a science which encompass several... This article or section does not cite its references or sources. ... This is a list of organizations involved in nanotechnology. ... This is a list of references and appearances of Nanotechnology in works of fiction. ... This page aims to list all topics related to the field of nanotechnology. ... Nanomaterials Fullerenes Carbon nanotubes Nanoparticles Nanomaterials is the study of how materials behave when their dimensions are reduced to the nanoscale. ... The Icosahedral Fullerene C540 C60 and C-60 redirect here. ... // 3D model of three types of single-walled carbon nanotubes. ... The term nanoparticle is generally used to refer to a small particle with all three dimensions less than 100 nanometres [1]. The term also includes subcategories such as nanopowders, nanoclusters and nanocrystals. ... Nanomedicine Nanotoxicology Nanosensor Nanomedicine is the medical application of nanotechnology. ... Research on ultrafine particles has laid the foundation for the emerging field of nanotoxicology, with the goal of studying the biokinetics of engineered nanomaterials and their potential for causing adverse effects. ... Nanosensors are a technology that may exist in the future. ... Molecular self-assembly Self-assembled monolayer Supramolecular assembly DNA nanotechnology An example of a molecular self-assembly through hydrogen bonds reported by Meijer and coworkers in Angew. ... Self assembled monolayers are surfaces consisting of a single layer of molecules on a substrate. ... A supramolecular assembly is an assembly of molecules held together by noncovalent bonds. ... DNA nanotechnology is an area of scientific research which seeks to use the unique molecular recognition properties of DNA and other nucleic acids to create novel, controllable structures out of DNA. The DNA is thus used as a structural material rather than as a carrier of biological information. ... Nanoelectronics Molecular electronics Nanocircuitry Nanolithography [[[Image: --203. ... Molecular electronics (sometimes called moletronics) is a branch of applied physics which aims at using molecules as passive (e. ... Nanocircuits are electrical circuits on the scale of nanometers. ... Nanolithography â€” or lithography at the nanometer scale â€” refers to the fabrication of nanometer-scale structures, meaning patterns with at least one lateral dimension between the size of an individual atom and approximately 100 nm. ... Scanning probe microscopy Atomic force microscope Scanning tunneling microscope Scanning probe microscopy (SPM) is a branch of microscopy that forms images of surfaces using a physical probe that scans the specimen. ... Topographic scan of a glass surface The atomic force microscope (AFM) is a very high-resolution type of scanning probe microscope, with demonstrated resolution of fractions of a nanometer, more than 1000 times better than the optical diffraction limit. ... Molecular nanotechnology Molecular assembler Nanorobotics Mechanosynthesis Molecular nanotechnology (MNT) is the concept of engineering functional mechanical systems at the molecular scale. ... A molecular assembler is a molecular machine capable of assembling other molecules given instructions, energy, and a supply of smaller building block molecules to work from. ... Nanorobotics is the technology of creating machines or robots at or close to the microscopic scale of a nanometres (10-9 metres). ... It has been suggested that this article or section be merged with mechanochemistry. ... This box: view • talk • edit

Microscopy is any technique for producing visible images of structures or details too small to otherwise be seen by the human eye, using a microscope or other magnification tool. ... Scanning probe microscopy (SPM) is a branch of microscopy that forms images of surfaces using a physical probe that scans the specimen. ... Scanning tunneling spectroscopy (STS) performed with a scanning tunneling microscope (STM) is a technique which gives information about the local density of electronic states on surfaces at atomic or molecular scale. ... The electrochemical scanning tunneling microscope, or ESTM, was invented in 1988 by Kingo Itaya in Japan. ... Topographic scan of a glass surface The atomic force microscope (AFM) is a very high-resolution type of scanning probe microscope, with demonstrated resolution of fractions of a nanometer, more than 1000 times better than the optical diffraction limit. ... An electron microscope is a type of microscope that uses electrons as a way to illuminate and create an image of a specimen. ... Spin polarized scanning tunneling microscopy (SP-STM) is a specialized application of scanning tunneling microscopy (STM) that can provide detailed information of the the surface magnetization distribution of a sample. ...

Results from FactBites:

 Scanning tunneling microscope - Wikipedia, the free encyclopedia (510 words) The scanning tunneling microscope (not to be confused with scanning electron microscopes), STM, was invented in 1981 by Gerd Binnig and Heinrich Rohrer of IBM's Zurich Lab in Zurich, Switzerland. The STM is a non-optical microscope which employs principles of quantum mechanics. By scanning the tip over the surface and measuring the height (which is directly related to the voltage applied to the piezo element), one can thus reconstruct the surface structure of the material under study.
 Scanning tunneling microscope - definition of Scanning tunneling microscope in Encyclopedia (785 words) The STM is not the first machine to incorporate the concept of scanning a sample with a stylus. The first researchers to succeed in building a scanning tunneling microscope were Gerd Binnig and Heinrich Rohrer at the IBM Research Laboratories in Zürich, Switzerland. One example of this is that disturbing vibrations from the environment were eliminated by building the microscope upon a heavy permanent magnet floating freely in a dish of superconducting lead.
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