|
|
| Line 1: |
Line 1: |
| [[File:Atomic resolution Au100.JPG|thumb|Image of reconstruction on a clean [[Gold]][[Miller index|(100)]] surface]]
| |
| [[File:Chiraltube.gif|thumb|An STM image of a single-walled [[carbon nanotube]]]]
| |
| <!--[[File:Selfassembly Organic Semiconductor Trixler LMU.jpg|250px|thumb|STM image of [[self-assembly|self-assembled]] [[supramolecular]] chains of the [[organic semiconductor]] [[quinacridone]] on [[graphite]].]]-->
| |
|
| |
|
| 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.<ref name="Binnig">{{Cite journal|author=G. Binnig, H. Rohrer|title=Scanning tunneling microscopy|journal=IBM Journal of Research and Development|volume=30|page=4|year=1986}}</ref><ref>[http://nobelprize.org/nobel_prizes/physics/laureates/1986/press.html Press release for the 1986 Nobel Prize in physics]</ref> For an STM, good resolution is considered to be 0.1 [[nanometre|nm]] lateral resolution and 0.01 nm depth resolution.<ref name="Bai">{{Cite book|author=C. Bai|title=Scanning tunneling microscopy and its applications|publisher=Springer Verlag|place=New York|year=2000|url=http://books.google.com/?id=3Q08jRmmtrkC&pg=PA345|isbn=3-540-65715-0}}</ref> 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 [[absolute zero|zero kelvin]] to a few hundred degrees Celsius.<ref name="Chen">{{Cite book |author=C. Julian Chen |title=Introduction to Scanning Tunneling Microscopy |year=1993 |url=http://www.columbia.edu/~jcc2161/documents/STM_2ed.pdf |isbn=0-19-507150-6 |publisher=Oxford University Press}}</ref>
| |
|
| |
|
| The STM is based on the concept of [[quantum tunneling]]. When a conducting tip is brought very near to the surface to be examined, a [[Biasing (electronics)|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.<ref name="Chen"/> 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 isolation|vibration control]], and sophisticated electronics, but nonetheless many hobbyists have built their own.<ref>{{Cite web|url=http://www.e-basteln.de/index_r.htm|accessdate=July 13, 2012|title=STM References - Annotated Links for Scanning Tunneling Microscope Amateurs}}</ref>
| | As the earth economies merge more and more folks are seeking at owning homes and investments in distinct nations around the world.<br><br>If you beloved this post in addition to you would like to obtain more details with regards to [http://nova3hackapk.getresponsepages.com nova 3 mod] i implore you to go to our own web page. The current planet vast recession has significant lighted how steady Canada is as a nation and Canada's banking procedure stood out as just one of the greatest in the environment.<br>As the planet financial state recovers I believe that it will get time for traders to have confidence in a still unstable stock sector. Also numerous shaken neighborhood economies are not steady and several community governments now have serious deficit positions. Ice Land would be an instance of what could occur.<br><br>I would like to make a case for investing in Nova Scotia true estate.<br>Nova Scotia is a single of Canada's smallest provinces with only about a million individuals in the total province. They say at any point in Nova Scotia you are by no means farther than 30 miles from the ocean. The whole province is entirely surrounded by water except for a smaller part connecting it to the province of New Brunswick where by you can travel throughout the Tantramar salt water marsh on the trans Canada 102 freeway to obtain the rest of Canada.<br><br>From Nova Scotia you can tour all of Canada (by paved highway in most instances) by vehicle or RV. Even the Island of Prince Edward Island is only a couple hrs crossing the Confederation Bridge in the vicinity of the New Brunswick border. Canada is a massive state with breath having landscapes a correct delight with a lot of Canadians shelling out their retirement years just touring in their RV's.<br><br><br>Nova Scotia actual estate can be amazingly economical when in contrast to lots of other markets. There is no growth bust cycle like you see in other Canadian provinces and important metropolitan areas. Practically no prospect of obtaining on the upside and getting rid of on the draw back. Most of the actual estate is privately owned and there is a great deal of coast line.<br>The Money metropolis is Halifax Regional Municipality which was developed with the amalgamation of the towns of Halifax and Dartmouth some a long time again. Pretty much 50 % of the individuals in the province reside and function with-in forty miles of this Municipality. This is one of the best deep water harbours in the earth.<br><br>As Nova Scotia adjusts from a resource base (fishing, lumbering, farming) to a far more and more know-how, monetary and know-how primarily based financial system. Rural Nova Scotia is bit by bit but certainly remaining depopulated as much more and extra people today move to Halifax Regional Municipality for task options.<br>Research in Movement just situated and crafted an business constructing there (blackberry fame) Several Investment decision companies and banking companies have positioned there and many others. This entire instances opens up two terrific actual estate investment decision scenario's based on your situations.<br><br>Halifax will proceed to mature and that growth will be alongside the 102 highway between Halifax and Truro. The federal governing administration has committed funding to what is identified as the Atlantic Gateway. Options are in the operates and research underway to Build Halifax as a super port.<br>Halifax is a single of the several harbours on the East Coastline that is deep sufficient to accommodate the new tremendous container ships. In my belief authentic estate among Halifax and Truro will appreciate substantially as development and need proceed. The simplest philosophy in investing "invest in minimal and promote significant".<br><br>The smartest go in actual estate "get before the desire comes and wait around for the expansion and desire to come to you."<br>The second option is outside the house Halifax Regional Municipality in the more compact communities much too considerably for commuting to the city. For example the neighborhood of Sheet Harbour on the japanese shore is about sixty miles from the town. This is fairly a great deal a self contained group with a Scotia Bank, House Hardware, Credit rating Union, Grocery, Healthcare facility, RCMP, 2 motels, Deep water harbour and wonderful persons.<br><br>Here you can buy a rather good retirement or seasonal household in all probability with a perspective of the drinking water. (frontage is more costly) for amongst $eighty,000 to $one hundred,000 Canadian. There are communities inland where by you could get a home or cottage between $60,000 and $80,000 or less dependent on your expectations.<br><br>Right now in Upper Stewiacke there is a cottage with one.five acres outlined for $15,900 Canadian.<br><br>In closing I would like to say the good quality of lifetime in Nova Scotia is next to none. Lots of people today in rural Nova Scotia do not lock their doors. If you are seeking for a fantastic location with fantastic people you will in no way uncover anything at all to review. There is no limits on foreign ownership in Nova Scotia and in this article is our authorities web-site for facts relating to citizenship and doing work in Canada.<br><br>website I really don't believe you can get the job done with out a visa but you can remain below 6 months out of the calendar year with out any complications. Nova Scotia accommodations are economical especially in the off year and Nova Scotia journey is straightforward by auto with most significant streets paved.<br>Why not choose a journey to this attractive province and get pleasure from our downeast hospitality.<br><br>Many thanks for looking through my posting.<br>Yours in true estate.<br>Larry Matthews |
| | |
| US4,343,993,<ref>US4,343,993 [http://www.google.com/patents/US4343993] Priority number(s): CH19790008486 19790920, family patents are also published as: EP0027517 (A1) EP0027517 (B1), and CH643397 (A5) [http://worldwide.espacenet.com/publicationDetails/biblio?DB=EPODOC&II=0&ND=3&adjacent=true&locale=en_EP&FT=D&date=19820810&CC=US&NR=4343993A&KC=A].</ref> written by [[Gerd Binnig]] and [[Heinrich Rohrer]] is the basic patent of STM.
| |
| | |
| [[File:Quantum tunnel effect and its application to the scanning tunneling microscope.ogv|thumb|right|upright=1.5|Animation showing the tunnel effect and its application to Scanning Tunneling Microscope]]
| |
| | |
| ==Procedure==
| |
| [[File:Stmsample.jpg|thumb|250px|A close-up of a simple scanning tunneling microscope head using a platinum–iridium tip.]]
| |
| | |
| 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 [[Piezoelectricity|piezoelectric]], maintaining tip-sample separation W typically in the 4-7 [[Angstrom|Å]] (0.4-0.7 [[Nanometer|nm]]) range, which is the equilibrium position between attractive (3<W<10Å) and repulsive (W<3Å) interactions.<ref name="Chen"/> In this situation, the voltage bias will cause electrons to [[Quantum tunnelling|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 cause 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.<ref name="Chen"/> 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.<ref name="Oura">{{Cite book|author=K. Oura, V. G. Lifshits, A. A. Saranin, A. V. Zotov, and M. Katayama|title=Surface science: an introduction|publisher=Springer-Verlag|place=Berlin|year=2003|url=http://books.google.com/?id=TTPMbOGqF-YC&pg=PP1|isbn=3-540-00545-5}}</ref> 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.<ref name="Bonnell"/> 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.<ref name="Bonnell"/> 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.<ref name="Bonnell"/> 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.<ref name="Bai"/> 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.<ref name="Pan">{{Cite journal| last= Pan| first = S. H.| year = 2000| journal = Nature| volume = 403| pages = 746–750|doi=10.1038/35001534| pmid= 10693798| first2= EW| first3= KM| first4= H| first5= S| first6= JC| title= Imaging the effects of individual zinc impurity atoms on superconductivity in Bi2Sr2CaCu2O8+delta| issue= 6771| last2= Hudson| last3= Lang| last4= Eisaki| last5= Uchida| last6= Davis|arxiv = cond-mat/9909365 |bibcode = 2000Natur.403..746P }}</ref>
| |
| | |
| Framerates of at least 1 Hz enable so called Video-STM (up to 50 Hz is possible).<ref>{{Cite journal|author=G. Schitter, M. J. Rost|year=2008|title=Scanning probe microscopy at video-rate|journal=Materials Today|volume=11|issue=special issue|pages=40–48|publisher=Elsevier|location=UK|issn=1369-7021|doi=10.1016/S1369-7021(09)70006-9|url=http://www.materialstoday.com/view/2194/scanning-probe-microscopy-at-videorate/|format=PDF}}</ref><ref>{{Cite journal|author=R. V. Lapshin, O. V. Obyedkov|year=1993|title=Fast-acting piezoactuator and digital feedback loop for scanning tunneling microscopes|journal=Review of Scientific Instruments|volume=64|issue=10|pages=2883–2887|doi=10.1063/1.1144377|url=http://www.lapshin.fast-page.org/publications.htm#fast1993|format=PDF|bibcode = 1993RScI...64.2883L }}</ref> This can be used to scan surface [[diffusion]].<ref>{{Cite journal|author=B. S. Swartzentruber|year=1996|title=Direct measurement of surface diffusion using atom-tracking scanning tunneling microscopy|journal=Physical Review Letters|volume=76|issue=3|pages=459–462|doi=10.1103/PhysRevLett.76.459|pmid=10061462|bibcode=1996PhRvL..76..459S}}</ref>
| |
| | |
| ==Instrumentation==
| |
| [[File:ScanningTunnelingMicroscope schematic.png|thumb|400px|right|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.<ref name="Oura"/>
| |
| | |
| The [[Image resolution|resolution]] of an image is limited by the [[radius of curvature (applications)|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.<ref name="Bai"/> Therefore it has been essential to develop processes for consistently obtaining sharp, usable tips. Recently, [[carbon nanotube]]s have been used in this instance.<ref>{{Cite journal|doi=10.1016/j.sna.2005.02.036|title=STM carbon nanotube tips fabrication for critical dimension measurements|year=2005|journal=Sensors and Actuators A: Physical|volume=123-124|pages=655}}</ref>
| |
| | |
| The tip is often made of [[tungsten]] or platinum-iridium, though [[gold]] is also used.<ref name="Bai"/> Tungsten tips are usually made by electrochemical etching, and platinum-iridium tips by mechanical shearing.<ref name="Bai"/>
| |
| | |
| Due to the extreme sensitivity of tunnel current to height, proper vibration isolation 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.<ref name="Chen"/> 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.<ref name="Oura"/> The computer may also be used for enhancing the image with the help of [[image processing]]<ref>{{Cite journal|author=R. V. Lapshin|year=1995|title=Analytical model for the approximation of hysteresis loop and its application to the scanning tunneling microscope|journal=Review of Scientific Instruments|volume=66|issue=9|pages=4718–4730|doi=10.1063/1.1145314|url=http://www.lapshin.fast-page.org/publications.htm#analytical1995|format=PDF|bibcode = 1995RScI...66.4718L }} ([http://www.lapshin.fast-page.org/publications.htm#analytical1995 Russian translation] is available).</ref><ref>{{Cite journal|author=R. V. Lapshin|year=2007|title=Automatic drift elimination in probe microscope images based on techniques of counter-scanning and topography feature recognition|journal=Measurement Science and Technology|volume=18|issue=3|pages=907–927|doi=10.1088/0957-0233/18/3/046|url=http://www.lapshin.fast-page.org/publications.htm#automatic2007|format=PDF|bibcode = 2007MeScT..18..907L }}</ref> as well as performing quantitative measurements.<ref name="feature2004">{{Cite journal|author=R. V. Lapshin|year=2004|title=Feature-oriented scanning methodology for probe microscopy and nanotechnology|journal=Nanotechnology|volume=15|issue=9|pages=1135–1151|doi=10.1088/0957-4484/15/9/006|url=http://www.lapshin.fast-page.org/publications.htm#feature2004|format=PDF|bibcode = 2004Nanot..15.1135L }}</ref><ref name="fospm2011">{{cite book|author=R. V. Lapshin|year=2011|contribution=Feature-oriented scanning probe microscopy|title=Encyclopedia of Nanoscience and Nanotechnology|editor=H. S. Nalwa|volume=14|pages=105–115|publisher=American Scientific Publishers|location=USA|isbn=1-58883-163-9|url=http://www.lapshin.fast-page.org/publications.htm#fospm2011|format=PDF}}</ref>
| |
| | |
| ===Probe tips===
| |
| STM tips are usually made from W (tungsten) metal or Pt/Ir alloy where at the very end of the tip (called apex), there is one atom of the material.<ref>Abelev, E; Sezin, N.; Ein-Eli, Y.; Rev. of Sci. Inst. 2005, 76, 106105.</ref>
| |
| | |
| ==Other STM related studies==
| |
| [[File:Cens nanomanipulation3d Trixler.jpg|thumb|Nanomanipulation via STM of a [[Self-assembly|self-assembled]] [[organic semiconductor]] [[monolayer]] (here: PTCDA molecules) on [[graphite]], in which the logo of the [[Center for NanoScience]] (CeNS), [[Ludwig Maximilian University of Munich|LMU]] has been written.]]
| |
| Many other microscopy techniques have been developed based upon STM. These include [[photon scanning microscopy]] (PSTM), which uses an optical tip to tunnel photons;<ref name="Bai"/> scanning tunneling potentiometry (STP), which measures electric potential across a surface;<ref name="Bai"/> [[spin polarized scanning tunneling microscopy]] (SPSTM), which uses a [[ferromagnetic]] tip to tunnel spin-polarized electrons into a magnetic sample,<ref name="Wiesendanger">{{Cite journal|author=R. Wiesendanger, I. V. Shvets, D. Bürgler, G. Tarrach, H.-J. Güntherodt, and J.M.D. Coey|title=Recent advances in spin-polarized scanning tunneling microscopy|journal= Ultramicroscopy|volume=42-44|year=1992|page=338|doi=10.1016/0304-3991(92)90289-V}}</ref> 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.<ref name="Bai"/> 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 (Au, Ag, W, etc.) with any desired (pre-programmed) pattern, which can be used as contacts to nanodevices or as nanodevices themselves.
| |
| | |
| 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.
| |
| | |
| ==Principle of operation==
| |
| [[File:Scanning Tunnelling Microscope made by W.A. Technology of Cambridge in 1986 (9669013645).jpg|right|thumb|The first STM produced commercially, 1986.]]
| |
| {{Technical|date=September 2010}}
| |
| 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 [[Quantum realm|very small mass]], such as the [[electron]], have [[Wave-particle duality|wavelike]] characteristics which permit such an event, referred to as [[Quantum tunnelling|tunneling]].
| |
| | |
| Electrons behave as beams of energy, and in the presence of a potential ''U''(''z''), assuming 1-dimensional case, the energy levels ''ψ<sub>n</sub>''(''z'') of the electrons are given by solutions to [[Schrodinger Equation|Schrödinger’s equation]],
| |
| | |
| ::<math>- \frac{\hbar^2}{2m} \frac{\partial^2\psi_n (z)}{\partial z^2} + U(z) \psi_n (z) = E\psi_n (z) </math>
| |
| | |
| where ''ħ'' is the reduced Planck’s constant, ''z'' is the position, and ''m'' is the mass of an electron.<ref name="Chen"/> 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,
| |
| | |
| ::<math>\psi_n (z) = \psi_n (0)e^{\pm ikz}</math>
| |
| | |
| where
| |
| | |
| ::<math> k=\frac{\sqrt{2m(E-U(z))}}{\hbar}</math>
| |
| if ''E'' > ''U''(''z''), which is true for a wave function inside the tip or inside the sample.<ref name="Chen"/> Inside a barrier, ''E'' < ''U''(''z'') so the wave functions which satisfy this are decaying waves,
| |
| | |
| ::<math>\psi_n (z) = \psi_n (0)e^{\pm \kappa z}</math>
| |
| | |
| where
| |
| | |
| ::<math> \kappa = \frac{\sqrt{2m(U-E)}}{\hbar}</math>
| |
| quantifies the decay of the wave inside the barrier, with the barrier in the +''z'' direction for <math> -\kappa </math>.<ref name="Chen"/>
| |
| | |
| {{Condensed matter experiments}}
| |
| 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.<ref name="Chen"/> 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,
| |
| | |
| ::<math>P \propto |\psi_n (0)|^2 e^{-2 \kappa W} </math>.<ref name="Chen"/>
| |
| | |
| 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.<ref name="Chen"/> These excited electrons can tunnel across the barrier. In other words, tunneling occurs mainly with electrons of energies near the Fermi level.
| |
| | |
| [[File:STM at the London Centre for Nanotechnology.jpg|thumb|A large scanning tunneling microscope, in the labs of the London Centre for Nanotechnology]]
| |
| | |
| 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 ''E''<sub>f</sub> 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.<ref name="Chen"/> 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.<ref name="Chen"/>
| |
| | |
| Mathematically, this tunneling current is given by
| |
| | |
| ::<math> I \propto \sum_{E_f-eV}^{E_f} |\psi_n (0)|^2 e^{-2 \kappa W} </math>.
| |
| | |
| One can sum the probability over energies between ''E''<sub>f</sub> − ''eV'' and ''E''<sub>f</sub> 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.<ref name="Chen"/> The LDOS near some energy ''E'' in an interval ''ε'' is given by
| |
| | |
| ::<math> \rho_s (z,E) = \frac{1}{\epsilon} \sum_{E- \epsilon}^{E}|\psi_n (z)|^2 </math>,
| |
| | |
| 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.<ref name="Chen"/> It is desirable to use LDOS to express the current because this value does not change as the volume changes, while probability density does.<ref name="Chen"/> Thus the tunneling current is given by
| |
| | |
| ::<math> I \propto V \rho_s (0, E_f) e^{-2 \kappa W} </math>
| |
| | |
| where ρ<sub>s</sub>(0,''E''<sub>f</sub>) is the LDOS near the Fermi level of the sample at the sample surface.<ref name="Chen"/> This current can also be expressed in terms of the LDOS near the Fermi level of the sample at the tip surface,
| |
| | |
| ::<math> I \propto V \rho_s (W, E_f) </math>
| |
| | |
| 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 Ǻ, the current increases by an order of magnitude, and vice versa.<ref name="Bonnell">{{Cite book|author=D. A. Bonnell and B. D. Huey|chapter=Basic principles of scanning probe microscopy|title=Scanning probe microscopy and spectroscopy: Theory, techniques, and applications|edition=2|editor=D. A. Bonnell|publisher=Wiley-VCH|location=New York|year=2001|isbn=0-471-24824-X}}</ref>
| |
| | |
| 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.<ref name="Bardeen">{{Cite journal|doi=10.1103/PhysRevLett.6.57|author=J. Bardeen|title=Tunneling from a many particle point of view|journal=Phys. Rev. Lett.|volume=6|issue=2|pages=57–59|year=1961|bibcode=1961PhRvL...6...57B}}</ref> 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.<ref name="Chen"/> 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.<ref name="Chen"/>
| |
| | |
| Now, Fermi’s Golden Rule gives the rate for electron transfer across the barrier, and is written
| |
| | |
| ::<math> w = \frac{2 \pi}{\hbar} |M|^2 \delta (E_{\psi} - E_{\chi} ) </math>,
| |
| | |
| where δ(E<sub>ψ</sub>–E<sub>χ</sub>) restricts tunneling to occur only between electron levels with the same energy.<ref name="Chen"/> The tunnel matrix element, given by
| |
| | |
| ::<math> M= \frac{\hbar ^2}{2 m} \int_{z=z_0} ( \chi*\frac {\partial \psi}{\partial z}-\psi \frac{\partial \chi*}{\partial z}) dS </math>,
| |
| | |
| is a description of the lower energy associated with the interaction of wave functions at the overlap, also called the resonance energy.<ref name="Chen"/>
| |
| | |
| Summing over all the states gives the tunneling current as
| |
| | |
| ::<math> I = \frac{4 \pi e}{\hbar}\int_{-\infty}^{+\infty} [f(E_f -eV + \epsilon) - f(E_f + \epsilon)] \rho_s (E_f - eV + \epsilon) \rho_T (E_f + \epsilon)|M|^2 d \epsilon </math>,
| |
| | |
| where ''f'' is the [[Fermi function]], ρ<sub>s</sub> and ρ<sub>T</sub> are the density of states in the sample and tip, respectively.<ref name="Chen"/> The Fermi distribution function describes the filling of electron levels at a given temperature T.
| |
| | |
| ==Early invention==
| |
| An earlier, similar invention, the ''Topografiner'' of R. Young, J. Ward, and F. Scire from the [[NIST]],<ref>{{Cite journal|author=R. Young, J. Ward, F. Scire|title=The Topografiner: An Instrument for Measuring Surface Microtopography|doi=10.1063/1.1685846 |url=http://www.nanoworld.org/museum/young2.pdf|journal=Rev. Sci. Instrum.|volume=43|page=999|year=1972}}</ref> 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.<ref>{{Cite news|url=http://nvl.nist.gov/pub/nistpubs/sp958-lide/214-218.pdf|title=The Topografiner: An Instrument for Measuring Surface Microtopography|publisher=NIST}}</ref>
| |
| | |
| ==See also==
| |
| {{colbegin}}
| |
| *[[Microscopy]]
| |
| *[[Scanning electron microscope]]
| |
| *[[Scanning probe microscopy]]
| |
| *[[Scanning tunneling spectroscopy]]
| |
| *[[Electrochemical scanning tunneling microscope]]
| |
| *[[Atomic force microscope]]
| |
| *[[Electron microscope]]
| |
| *[[Spin polarized scanning tunneling microscopy]]
| |
| {{colend}}
| |
| | |
| ==References==
| |
| {{Reflist|2}}
| |
| | |
| ==Further reading==
| |
| *Tersoff, J.: Hamann, D. R.: Theory of the scanning tunneling microscope, [http://dx.doi.org/10.1103/PhysRevB.31.805 Physical Review B 31, 1985, p. 805 - 813].
| |
| *Bardeen, J.: Tunnelling from a many-particle point of view, [http://dx.doi.org/10.1103/PhysRevLett.6.57 Physical Review Letters 6 (2), 1961, p. 57-59].
| |
| *Chen, C. J.: Origin of Atomic Resolution on Metal Surfaces in Scanning Tunneling Microscopy, [http://dx.doi.org/10.1103/PhysRevLett.65.448 Physical Review Letters 65 (4), 1990, p. 448-451]
| |
| *G. Binnig, H. Rohrer, Ch. Gerber, and E. Weibel, [http://dx.doi.org/10.1103/PhysRevLett.50.120 Phys. Rev. Lett. 50, 120 - 123 (1983)]
| |
| *G. Binnig, H. Rohrer, Ch. Gerber, and E. Weibel, [http://dx.doi.org/10.1103/PhysRevLett.49.57 Phys. Rev. Lett. 49, 57 - 61 (1982)]
| |
| *G. Binnig, H. Rohrer, Ch. Gerber, and E. Weibel, [http://dx.doi.org/10.1063/1.92999 Appl. Phys. Lett., Vol. 40, Issue 2, pp. 178-180 (1982)]
| |
| *{{cite journal|author=R. V. Lapshin|year=2004|title=Feature-oriented scanning methodology for probe microscopy and nanotechnology|journal=Nanotechnology|volume=15|issue=9|pages=1135–1151|publisher=IOP|location=UK|issn=0957-4484|doi=10.1088/0957-4484/15/9/006|url=http://www.lapshin.fast-page.org/publications.htm#feature2004|format=PDF|bibcode=2004Nanot..15.1135L}}
| |
| *{{cite book|author=R. V. Lapshin|year=2011|contribution=Feature-oriented scanning probe microscopy|title=Encyclopedia of Nanoscience and Nanotechnology|editor=H. S. Nalwa|volume=14|pages=105–115|publisher=American Scientific Publishers|location=USA|isbn=1-58883-163-9|url=http://www.lapshin.fast-page.org/publications.htm#fospm2011|format=PDF}}
| |
| *D. Fujita and K. Sagisaka, Topical review: Active nanocharacterization of nanofunctional materials by scanning tunneling microscopy [http://dx.doi.org/10.1088/1468-6996/9/1/013003 Sci. Technol. Adv. Mater. 9, 013003(9pp) (2008)] (free download).
| |
| *{{Cite book|url=http://books.google.com/?id=EXae0pjS2vwC&printsec=frontcover|title=Scanning probe microscopy and spectroscopy: methods and applications|author=Roland Wiesendanger|publisher=Cambridge University Press|year=1994|isbn=0-521-42847-5}}
| |
| *''Theory of STM and Related Scanning Probe Methods.'' Springer Series in Surface Sciences, Band 3. Springer, Berlin 1998
| |
| *''Stabilization of Large Adsorbates by Rotational Entropy: A Time-Resolved Variable-Temperature STM Study'', ChemPhysChem, doi: 10.1002/cphc.201200531
| |
| | |
| ==External links==
| |
| {{Wikibooks|The Opensource Handbook of Nanoscience and Nanotechnology}}
| |
| {{Commons category|Scanning tunneling microscope}}
| |
| *[http://www.fz-juelich.de/pgi/pgi-3/microscope A microscope is filming a microscope] (Mpeg, AVI movies)
| |
| *[http://www.nano.geo.uni-muenchen.de/SW/images/zoom.html Zooming into the NanoWorld] (Animation with measured STM images)
| |
| *[http://nobelprize.org/educational_games/physics/microscopes/scanning/index.html NobelPrize.org website about STM], including an interactive STM simulator.
| |
| *[http://www.mobot.org/jwcross/spm/ SPM - Scanning Probe Microscopy Website]
| |
| *[http://www.almaden.ibm.com/vis/stm/gallery.html STM Image Gallery at IBM Almaden Research Center]
| |
| *[http://www.iap.tuwien.ac.at/www/surface/STM_Gallery/ STM Gallery at Vienna University of technology]
| |
| *[http://www.lapshin.fast-page.org/gallery.htm SPM gallery: surface scans, collages, artworks, desktop wallpapers]
| |
| *[http://web.archive.org/web/20091028073926/http://www.geocities.com/spm_stm/Project.html Build a simple STM with a cost of materials less than $100.00 excluding oscilloscope]
| |
| *[http://www.nanotimes-corp.com Nanotimes Simulation engine of scanning tunneling microscope]
| |
| *[http://www.uni-ulm.de/~hhoster/personal/self_assembly.htm Structure and Dynamics of Organic Nanostructures discovered by STM]
| |
| *[http://www.uni-ulm.de/~hhoster/personal/metal_organic.htm Metal organic coordination networks of oligopyridines and Cu on graphite investigated by STM]
| |
| *[http://www.uni-ulm.de/~hhoster/personal/surface_alloys.html Surface Alloys discovered by STM]
| |
| *[http://molecularmodelingbasics.blogspot.com/2009/09/tunneling-and-stm.html Animated illustration of tunneling and STM]
| |
| *[http://nanohub.org/resources/2620 60 second movie clip with an introduction to Scanning Tunneling Microscopy(STM)]
| |
| * [http://www.toutestquantique.fr/#atome, applications and research linked to atomic orbitals and other quantum phenomena] (Université Paris Sud)
| |
| | |
| {{SPM2}}
| |
| {{IBM}}
| |
| | |
| {{DEFAULTSORT:Scanning Tunneling Microscope}}
| |
| [[Category:Scanning probe microscopy]]
| |
| [[Category:Swiss inventions]]
| |
| [[Category:German inventions]]
| |
| [[Category:Microscopes]]
| |
| [[Category:1981 introductions]]
| |
| | |
| {{Link GA|de}}
| |
| {{Link FA|pl}}
| |