Scanning Tunneling Microscopy

Scanning Tunneling Microscopy

  • Alexander Loskutov added an answer:
    Is there a better approach than the Simmons analysis for MIM diodes having small electrode gaps?

    In 1963 Simmons [1] gave a general expression (Eq. 20) with a separate expression for low voltages (Eq. 24) for the tunneling current in a MIM diode as a function of the applied bias.  These equations have frequently been applied in the analysis of experimental data for MIM diodes as in scanning tunneling microscopy.   Both of these equations fail when the electrode spacing "d" is small, which may be understood because they were derived using the WKB approximation which assumes a quasi-opaque barrier.   As the spacing becomes small Eq. (20) has a change in sign followed by a singularity of order 1/d^2, whereas Eq. (24) has no sign change but a singularity of order 1/d.  Others have used the Landauer formula or other approaches to obtain closed-form expressions for the tunneling current with ultrathin barriers.  I am interested in applications for modeling a STM, and have introduced the spreading resistance of the sample in Eq. (24).   For a tungsten tip and sample I find that a typical spreading resistance of about 300 Ohms, corresponding to the spot size for the tunneling current, avoids the singularity in Eq. (24) and gives I-V curves that are consistent with the observation that the current does not have a dramatic increase in tip-crash.  I question whether it is necessary to simulate the effects of ultrathin barriers because this effect would be dominated by the spreading resistance. My questions are as follows:

    1. Is it appropriate to allow for a spreading resistance, and if so what would be an appropriate spot size?  That is, beyond what radius could we consider only classical transport of electrons within the metal sample of an STM? 

    2. Is it reasonable to consider that adding the spreading resistance to the model  could provide a "patch", as I have suggested, to connect the contact mode (where the tip and sample make electrical contact) to tip-sample distances where the WKB approximation is reasonable?

    I would appreciate any answers, comments, or suggestions.

    Mark

    [1] J.G. Simmons,  J. Appl. Phys. 34 (1963) 1793-1803.

    Alexander Loskutov · Moscow State Technological University Stankin

    Very interesting question. Indeed eq. 20 in Simmons article  for ultrathin barriers is not suitable. Two main reasons. The first - the inadequacy of the WKB approximation of the tunneling probability for thin barriers. The reason is that accurate wave function inside the barrier, i.e. in the classically inaccessible region, contains the sum of two exponents - descending and ascending. Usually one always leaves the first descending, but the second ascending for broad barriers neglects. The coefficients in front of them are determined by the usual boundary conditions at the edges of the barrier: the continuity of the wave function and its first derivative. It is important not to confuse the two exponents with the eq. (20), because (20) shows the sum of the currents in the two opposite directions - on the probe with the sample and vice versa, wherein each in WKB approximation. In the exact general expression there should be four such terms. In this case the signs will be all right. Of course, the sign of the current, ie its direction,  is determined by the sign of U. In principle, with one-dimensional approximation for a rectangular barrier, this problem is considered in any good textbook on quantum mechanics (eg Landau). The only problem is that the exact wave function in the probe and, especially in the sample, which should be stapled at the boundaries of the barrier to the sum of two exponents, we do not know. In the probe, according to Tersoff and Haman, it can take as s-wave, ie, decreasing depth probe exponent. As for the sample we must know Bloch functions for surface superstructure, which is practically impossible. Furthermore, in the electric field, these wave functions are deformed, which also must be considered carefully. Therefore, the only way - to use the approximate solutions. Here we come to the second reason - inadequate Simmons  eq.- neglecting the pre-exponential factor, whose influence on the current and the shape of the CVC even more than tunneling exponents. It is easy to see from the eq. for low voltages, in which exponents simply do not contain U, so purely resistive dependence is realized . To learn how you can take into account the pre-exponential factor for the different types of symmetry of surface electronic states – see for example: Journal of Computational and Theoretical Nanoscience . 01/2015; 12 (1-8). DOI: 10.1166 / jctn.2015.4078. If you enter a "spreading resistance" related to the current gathering area - it is possible to take into account the multidimensional problems, but, of course,  the situation with the correct limiting transition to small d does not improve. Coanswer with Arkadii Mandel.

  • Shahnaz Khosravian asked a question:
    Is there any experimental result by STM technique for deposition of oxygen on Cu(111) at 100-190 K?

    Ph.D fellow

  • Ahmad Razmdideh added an answer:
    How can we calculate current of cnt fet with stone walls defect?

    A Stone–Wales defect is the rearrangement of the six-membered rings of graphene into pentagons and heptagons.[ This rearrangement is a result of π/2 (90°) rotation of a C–C bond. The density of Stone–Wales defects is usually small due to the high activation barrier of several eV for the bond rotation. However, the defects has been imaged using scanning tunneling microscopy as well as resonant–vibrational spectroscopy techniques[clarifyA number of theoretical studies have shown that the absorption energies and charge transfer energies for single-walled carbon nanotubes are larger than the corresponding values on pristine carbon nanotubesThe diminished resonance and higher strain energy of this defect increased the probability of nucleophilic attack which provides a possible explanation for the enhanced reactivity. Research has also shown the incorporation of defects along a carbon-nanotube network can program a carbon-nanotube circuit to enhance the conductance along a specific path. These defects lead to a charge de localization which redirects and incoming electron down a given trajectory/as shown as a attachment file

    now how we can calculate current of cnt fet with stone walls defect?

    Ahmad Razmdideh · Urmia University

    tanks my dear zahedi but intrest for me analiticalay calculate with out soft ware.. best regards

  • William Cullen added an answer:
    Is scanning tunneling microscopy in air like "snow plowing" through oxides?

    My simulations suggest that if analog PI or integral (I) feedback control is used a recoverable "tip bounce" is likely to occur. I would expect this problem to be more prominent in air than in vacuum where the sample and tip are both cleaned.  Our by SEM images show marked disfigurement of the tip (like a corkscrew) immediately after measurements in air with a commercial STM.    

    William Cullen · University of Maryland, College Park

    I think the answer is in many cases "yes".

    I was told many years ago that the conventional wisdom was to avoid tungsten tips for ambient STM work - use Pt-Ir instead. I think your observation of "corkscrewing" the tip is not surprising. The clean gold surface should be robust for ambient tunneling, I think, but probably not the tungsten tip. I would be curious to know if your experiment is possible using Pt-Ir tips without damaging them.

    Simulations involving temporal response of the STM feedback loop (digital, analog, I vs. PI, whatever) may be missing the point here. As pointed out in the replies by Gan and Giessibl, the issue is force at the tunnel junction. Unfortunately, STM feedback has no sensitivity to forces but the forces are there nonetheless. I think it is a question of what forces are involved in bringing the juntion into the required conductance (set by the current setpoint and voltage bias). STM will always drive the tip in until the current setpoint is maintained.
    If the tungsten tip is sufficiently oxidized, then the tip and sample may have to be in mechanical contact before current can flow at, say 100 pA level. If the tip is initially sharp - a high-aspect-ratio needle - then it will almost certainly bend over and curl up under this force. I've seen it many times, even in UHV. But my point is that if a moderately strong repulsive force is needed to establish current flow, then STM feedback won't change the behavior no matter how good it is - AFM feedback should be used instead.

  • Michael Leitner added an answer:
    Is there any commercial AFM that uses piezoelectric cantilever for tapping mode deflection measurements?

    I was wondering if any commercial Atomic Force Microscope (AFM) has been made by any company that only uses piezoelectric cantilevers instead of laser beam sensor for deflection measurement of the cantilever tip.

    Michael Leitner · Center for Advanced Bioanalysis

    There is an Austrian company (http://www.sclsensortech.com/5.0.html) producing and selling self sensing cantilevers based on piezo resistive sensing and thermal actuation. They are especially designed to be used for purposes where laser read out is not possible or where free space above the cantilever is required. For a prototpye this cantilevers have been implemented on a Keysight (Agilent) 5500 AFM at JKU and used for conatct and tapping mode imaging in dry state as well as in liquid. An additional application for this cantilevers comes from GETec (http://www.getec-afm.com/index.php?id=10) using them for an AFM-SEM combination by using a special commercial available scanner.

  • Mohd Qasim added an answer:
    What is the perfect substrate to study gold nanoplates by STM?

    I want to study gold nanoplates by STM and I need to individually disperse on conductive substrate.

    What is the perfect substrate to study gold nanoplates by STM?

    HOPG? Silicon?

    Mohd Qasim · University of Hyderabad

    drop casting or spin coating of Au dispersion on substrate can be used.

    U can Make dilute dispersion of nano materials in some solvent and they put a drop on substrate. Uniformity of film matter. or do a spin coating. 

  • Seyyedali Hosseini added an answer:
    How can I individually dispersed a gold nanoplate on HOPG?

    I want to study Gold nanoplates by STM and I need to individually disperse on conductive substrate similar to HOPG.
    How can I dispersed gold on HOPG?
    Thanks.

    Seyyedali Hosseini · Sharif University of Technology

    Do you have suggestions on coating the nanoplates on the HOPG?

    thanks.

  • Saeed Hasani added an answer:
    How can I calculate the volume of hexagonal structured nano particles?

    From XRD data the nano particle structure is hexagonal. How can I calculate a,c and volume if my two theta value is slightly deviated from standard value?

    Saeed Hasani · Isfahan University of Technology

    The parameter lattice extracted from XRD is associated with the error. Because, the location of peaks is affected by tension or by nano-sized particle.

    TEM and its diffraction patterns is the best method for determine of parameter lattice e.g. a, and c in hcp materials.

    With Bests

  • Ramesh Mohan Thamankar added an answer:
    For a LHe cooled STM sample, what is the temperature right under the tunnelling junction for RT tips?

    Some STM manufacturers propose variable temperature STMs in which the sample is cooled but the tip is not. It is easy to monitor the overall sample temperature, but given that the tip is very close to the sample and at RT, the tip will certainly heat the tunnelling junction area. Would this temperature increase be significant (more than say 5 Kelvin)? 

    Ramesh Mohan Thamankar · Agency for Science, Technology and Research

    if it a perfectly low temperature, that means both the tip-sample are cooled, then the junction temperature should be same as the sample. But if you are using variable temperature STM, then only sample is cooled. Tip is always at room temperature. Thus the junction temperature is not equal to sample temperature. One needs to be careful while mentioning the temperature. It is neither  helium temperature, nor 300K. There was a study done and published in PRL about this. check this for example Phys. Rev. Lett. 105, 166101.

  • Lars Diekhöner added an answer:
    What is the difference between Surface band gap and bulk band gap?

    I want to correlate band gap values got from two technique 

    1) Tauc plot which will give overall band gap

    2) Scanning tunneling spectra (STS) or Scanning tunneling microscopy

    Lars Diekhöner · Aalborg University

    Let me add a comment on STS: In a band gap measurement using STS you can't directly distinguish between bulk- and surface band gaps. 

  • Hubert Klein added an answer:
    What is the current density for atomic resolution in scanning tunneling microscopy?
    If a current of 10 nA passes through the tunneling junction, which may be of atomic size, a simple calculation suggests a local current density exceeding 10^11 A/sq.mtr- which would ablate the sample if the current flow were one-dimensional. I have not seen any analysis considering the effects of current spreading, short-range ballistic transport, nanoscale heat transfer, etc.
    Hubert Klein · CINaM - Centre Interdisciplinaire de Nanoscience de Marseille

    Following the first answer, there is no dissipation in a tunnel junction, as transmission comes from evanescent waves, there is thus no diffusion and nor heating in the junction. Heating, due to electron phonon interactions, can occur in the surrounding of the junction, but not in the junction itself.

    Noteworthy inelastic processes may occur in the junction, leading for instance to radiative emission by coupling to surface plasmons modes, but quantum yeld of such processes are very low (tipycally 10^-7).

    Even in the case of propagating waves, as in metallic point contacts, where the transport is ballistic, electron-phonon interaction length (typically a few tenth of nm) are larger than the size of the junction. Thus, the electrons cannot release heat in the contact, but rather in the macroscopic electrodes far from the contact. I attach a review of that field.

  • Rui Zhang added an answer:
    How can I make a gauge plate for STM?

    I need a calibration (gauge) plate in order to check a scanning tunneling microscope. For that means I would need two atoms isolated from other atoms in an area of 50 nm (or more) radius on a glass plate? These two atoms must be at a distance of 10-30 nm. I will then look to resolve them.

    My questions are:

    1. Do they sell such plates?

    2, If not how to build one?

    I have an idea to dissolve a small amount of say Cu and then dilute to reach 1 atom in 1 ml. Then pour it on a glass plate and dry. Would this give me the gauge needed?

    Rui Zhang · KTH Royal Institute of Technology

    As the former ( above) said, just using the Si(111) of Au(111) surfaces for STM calibration should be more proper and standard.

  • Christian Wagner added an answer:
    During Current-Voltage measurement, the sample bias and the set current, keeps the STM tip-sample separation fixed. How can I measure the separation?
    When we carry out spectroscopy measurement, at first we stop STM. At that time the sample bias and tunneling set current keeps the tip-sample separation constant. As soon as we do the I-V measurement the feedback loop is switched off and the voltage sweeps across the range and corresponding tunneling current is recorded. If you change the sample bias or set current tip-sample separation changes accordingly. So, how can I measure this tip-sample separation?
    Christian Wagner · Forschungszentrum Jülich
    If you mean the absolute tip-sample separation, this is a highly non-trivial issue and notoriously difficult to measure. One can estimate a lower bound for this distance by going into point contact. Since the tip usually elongates under the influence of short ranged forces this method can only provide said lower estimate. It is in general not possible to obtain the distance from the I, V setpoint alone. If you just want to know the relative tip displacement when changing the setpoint, just look at the change in z-piezo voltage and calculate it from the piezo constant. Your STM software might also have a direct readout for this relative displacement.
  • Jimmie Miller added an answer:
    How would a (commercial) STM behave in high vacuum (~10^(-5)Pa)?
    Scanning Tunneling Microscopy (STM) is usually done in UHV (and sometimes in Air). Doing STM in UHV has many advantages but also imposes greater constraints. In some applications, possibly working in HV only could be more advantageous. So, I wonder exactly what one loses by performing STM in HV (apart from no having perfect surface cleanness). Would the current stability (and tip behaviour) be much worse? What drawbacks would high vacuum imply? Please indicate relevant literature on the subject.
    Jimmie Miller · University of North Carolina at Charlotte
    Don't forget that if you use tungsten tips they oxidize faster in HV environments which will change your ability to image. Also attached is a file showing surface degradation of Si before and after six days of UHV (10-11 Torr) from my ancient dissertation which can be found at the attached link
  • sunil kumar Samji added an answer:
    Is it possible to study the local density of states from phase image of tapping mode? If so how can I do it or interpret it?
    In scanning tunneling microscopy (STM) a contrast in image corresponds to variation in local density of states (LDOS). I learnt that phase mapping (image) in tapping mode is very sensitive technique, is it possible to speak about variation in LDOS from phase contrast , if so, how can I understand it, or interpret it?
    sunil kumar Samji · IIT Madras
    Hi, Swaminathan, Vincent and Nicolas Thanks a lot for the answers.
    I am trying to re frame my question, it requires a bit more understanding will get back after that, Thank you
    @ Nicolas: Thank you very much fro the reference will go through it.
  • Rupali G. Mane added an answer:
    Is it possible to measure the work function of a metal surface with adsorbed gas molecules using STM (scanning tunneling microscopy)?
    We are looking for methods to measure the work function of metal surfaces with polyatomic gas molecules.
    Rupali G. Mane · Bhabha Atomic Research Centre
    Thanks to Jaun De Miguel
  • Ali Salehizadeh asked a question:
    Are there any available lab positions for collaboration in SEM and TEM?
    I am a Physics Engineering PhD Student at the University of Aveiro, Portugal, and looking for an expert lab for electron microscopy, preferably in Europe or the US, to do SEM or TEM and publish a paper. I should mention that my sample is iron oxide nano particles embedded in silica shells.
  • Rainer Christoph added an answer:
    What is the best vibration isolation solution for AFM?
    Are the isolation tables using nitrogen or compressed air supply the best? What do you think about the active isolation platforms available from Herzan and Accurion that runs simply on electricity?
    Rainer Christoph · Nanotecnia
    Whilst working with Heini Rohrer and Christoph Gerber, I learnt that when a hanging suspension elongates 25cm under the combined weight of AFM & support platform, the system oscillates with a main resonance frequency of aprox. 1 s-1. This can be calculated and measured.
  • Daniil Bratashov added an answer:
    Why do I get these streaks in the background in all my AFM images on Mica substrate?
    Please see this attached image. I am using tapping mode in air. I have tried different probes, but no help.
    Daniil Bratashov · Saratov State University
    It is harmonic generation with period(s) defined by feedback loop delay and usually it is perpendicular to fast scanning axis (if you clear the highest harmonic from image above, you can see slight vertical lines on Fourier spectrum). Beside this generation, the ill-tuned scanning system can catch another sources of noise (50/60 Hz harmonics from electrical noise, around a 1-3 kHz in acoustic noise from ventilation and electrical from inverter and so on). I have seen up to 5 different noise frequencies in very bad tuned scan in my practice. As noise is sampled with regular interval, it frequency can move to the low freqs. region as difference between sampling and noise freqs. (an so it will appear along slow scanning axis).
  • Gopalakrishnan Ramalingam added an answer:
    Is metal contact between the epitaxial graphene sample and the metal sample substrate imperative in order to take STM images?
    I use a UHV growth chamber for High-T graphitization and the SiC substrate is placed on a metal plate (sample holder). I supposed to transfer the sample into the STM chamber connecting to the growth chamber directly after the graphitization. but i couldn't get stable scan condition, therefore no images. I wonder if it is necessary to make metal contact between the sample and the metal plate(the same sample holder used for graphitization)
    Gopalakrishnan Ramalingam · University of Virginia
    You can use Ag paste in UHV. You just need to pump your sample longer in the loadlock before transferring into the chamber so that the volatiles from the paste are outgassed. We generally outgas our samples overnight to avoid destroying the UHV; try not to use more than necessary. We use Ag paste all the time for good contact between graphite and our Mo-sample plate. Good luck!
  • Meysam Nobakht added an answer:
    Can anyone help me to find a comprehensive review papers about applications of synchrotron?
    I want to know what the advantages are of synchrotron radiation for material analysis in comparison with conventional methods?
    I searched through the internet but I couldn't find anything helpful.
    Meysam Nobakht · University of Isfahan
    thank you dear Timothy Hyde. you help me so much.
  • Sara Ataran added an answer:
    How can I make the thin film top of the surface with STM preparation chamber?
    I used evaporation in preparation chamber, but it was thicker than thin film.
    Sara Ataran · Lund University
    Thank you all for your answers, these are so useful for me.
  • Andrzej Sikora added an answer:
    What are the state of art resolution, operation speed, largest scanable size for main scanning techniques (STM, AFM, TEM)?
    What about the lithography and deposition techniques regarding their resolution, smoothness of surfaces, growth speed, and the largest possible area?
    Andrzej Sikora · Electrotechnical Institute, Wroclaw
    Dear Aref,
    I think that the state of the art in case of AFM is a bit better than Vladimir described. In terms of the resolution, few nanometers may be relatively achieved in the plane, and subnanometer in the vertical scale (please note, that the quantitative height information is delivered only by AFM - the roughness information). But the resolution is also strongly determined by the sample, scanning mode and the settings. The maximum scanning range typically reaches 100 x 100 micrometers in plane and up to 8 micrometers in height.
    There is a significant progress in scanning speed lately. You can purchase the systems allowing to acquire a single image within a minute (at least two producers from USA). It causes however the scanning area.
    I agree with Sridhar, that TEM provides a number of additional analysis. AFM on the other hand also provides imaging of the mechanical, magnetic or electrical properties. Additionally, AFM allows to perform the imaging in the wide range of the environmental conditions (vacuum, gas, liquids, low and high temperatures, magnetic fields). In some experiments its major advantage is the simple preparation (almost none), which would alter the properties of the sample. All those techniques have some potential and in the best solution are used together to provide a complementary source of information as the base on various physical phenomena. So you need to know what your needs are (and the budget, as the prices of the equipment may differ significantly as well).
  • Matúš Dubecký added an answer:
    Does anyone know how Au to Au will be build a schottky contact?
    I have read a artical from Surface Science 600 (2006) 442–447
    which describes that STM tip of Au was used to probe a Au island ,between the tip and island will form a Schottky contact when appropriate gold thickness (such as 0.2nm).
  • Lijo Francis added an answer:
    After a hydrogen molecule, do you think we can see a bigger molecule soon or it is the limit?
    Few days ago, it was announced that researchers in China reported the first visualisation of a hydrogen bond using atomic force microscopy (AFM).In May, Felix Fischer and colleagues at the University of California Berkeley in the US used AFM to image molecules before and after a chemical transformation. The remarkable images showed the formation of covalent bonds in a cyclisation reaction.

    In the latest study to visualise molecules, Xiaohui Qiu and colleagues at the National Centre for Nanoscience and Technology, China, went one step further. They used the same non-contact AFM as Fischer, but instead of looking for covalent bonds they tweaked it to look for weaker interactions.
    Do you think we had reached the limit of magnification or we can see bigger molecule soon?
    http://www.rsc.org/chemistryworld/2013/09/first-pictures-hydrogen-bonds-unveiled-afm
  • A. Bensouici asked a question:
    What are the parameters to be considered in account to obtain accurate results in I-V measurements using home-made STM?
    STM measurements of semiconducting surfaces.
  • Régis Bisson asked a question:
    Can you suggest a power inverter to protect instruments against overvoltage/power-cut without inducing spurious signal in discriminator/electrometer?
    I am looking for power inverter (220V, 50Hz) suitable for protection of sensitive instruments like STM / EELS / AES / QMS (power consumption ~250W) against overvoltage from power-provider and power cut up to 4-5 hours. It should specifically not induce any spurious signal in measurements from discriminators / electrometers when running on battery during a power cut.
    I would very much appreciate any advice and feedback on any devices you may have tried.

About Scanning Tunneling Microscopy

A scanning tunneling microscope (STM) is an instrument for imaging surfaces at the atomic level.

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