synonymTunnel scanning microscope(Tunnel scanning microscope) generally refers to scanning tunnel microscope
Scanning tunneling microscope (STM for short) is a scanning probe microscopy tool. STM allows scientists to observe and locate single atoms, which has more advantages than its counterpartsatomic force microscopeHigher resolution.
In addition, the scanning tunneling microscope（4K）The probe tip can be used to manipulate atoms precisely, so it canNanotechnologyBoth importantMeasuring toolsIt is also a processing tool.
STM makes it possible for human beings to observe the arrangement of single atoms on the surface of a substance and the physicochemical properties related to the electronic behavior of the surface in real time for the first timeSurface science、material science , life science and other fields have great significance and broad application prospects, and are recognized by the international scientific community as one of the world's top ten scientific and technological achievements in the 1980s[2]。
Tunnel tip
The structure of tunnel needle tip is scanning tunnelMicroscopyOne of the main problems to be solved.The size, shape and chemical identity of the tip not only affect the resolution and shape of the image of the scanning tunneling microscope, but also affect the measured electronic state.
Scanning tunneling microscope structure
NeedlepointMacrostructureThe needle tip should be highly curvedresonance frequency, which can reducePhase lagTo improve the acquisition speed.If the tip of the tip has only one stable atom instead of multiple tips, then the tunnel current will be very stable, and atomic resolution images can be obtained.NeedlepointChemical purityHigh, there will be no series barrier involved.For example, if there is an oxide layer on the surface of the needle tip, its resistance may be higher than the resistance value of the tunnel gap, which will lead to collision between the needle tip and the sample before the tunnel current is generated[2]。
The materials for preparing the tip mainly include metalTungsten wire, Platinum-Iridium alloySilk, etc.Commonly used in preparation of tungsten tipelectrochemical corrosion Law.The platinum iridium alloy needle tip is usually mechanically formed, which is usually cut directly with scissors.No matter which kind of needle tip, its surface is often covered with an oxide layer, or some impurities are adsorbed, which often causes instability of tunnel current, high noise and unpredictability of scanning tunneling microscope image.Therefore, before each experiment, the needle tip should be treated withChemical methodClean and remove the oxide layer and impurities on the surface to ensure good conductivity of the needle tip[2]。
3D scanning controller
Due to the need to control the needle tip on the sample surface in the instrumenthigh-precisionIt is difficult to achieve this requirement with common mechanical control.
Scanned nanoscale images[3]
Piezoelectric ceramicsUtilizedPiezoelectric phenomenon。The so-called piezoelectric phenomenon refers to the phenomenon that a certain type of crystal will produce an electric field when it is deformed by mechanical force, or the crystal will produce physical deformation when an electric field is added to the crystal.Single crystals of many compounds, such as quartz, have piezoelectric properties, but polycrystals are widely usedceramic material , for exampletitaniumZirconic acidlead[Pb(Ti,Zr)O3](short forPZT）Andbarium titanateEtc.Piezoelectric ceramic materials can convert 1mV-1000V voltage signal into displacement of one tenth of a nanometer to several microns in a simple way[2]。
Made of piezoelectric ceramic materials3D scanningThe controllers mainly include the following
①tripodThree independent long prismatic piezoelectric ceramic materials are combined in orthogonal directions. The needle tip is placed on the top of the tripod, and the three legs are extended and contracted independently, so that the needle tip moves along the x-y-z three directions.
② Single tube type, the external electrode of the ceramic tube is divided into four parts with equal area, and the inner wall is a whole electrode. When voltage is applied to one of the electrodes, this part of the tube will stretch or shrink (depending on the positive and negative voltagePiezoelectric ceramicsOfPolarization directionDecision), causing the ceramic tube to bend in a direction perpendicular to the tube axis.By applying voltage on two adjacent electrodes in a certain order, the x-y direction can be mutually connectedVertical movement。The movement in the z direction is to apply voltage to the electrode on the inner wall of the tube to make the tubeOverall shrinkageImplemented.The other two electrodes on the outer wall of the pipe can simultaneously apply the voltage of opposite symbols to make one side of the pipe expand, and the opposite side shrink, increasingScanning range, you can also addDC Offset To adjust the scanning area.
③ The cross is matched with a single tube type, and the movement in the z direction is completed by a piezoelectric ceramic tube in the center of the "cross" shape. The x and y scan voltages are respectively applied to a pair of x, - x, y, - y in the same size and opposite symbols.The x-y scanning unit with this structure is a complementary structure, which can compensate for the influence of thermal drift to a certain extent.
In addition to piezoelectric ceramics, some 3D scanning controllers usescrew、reed, motor, etc.
Shock absorption system
Since the distance between the working tip of the instrument and the sample is generally less than 1nm, and the tunnel current is exponentially related to the tunnel gap, any small vibration will affect the stability of the instrument.The two types of disturbances that must be isolated are vibration and shock, of which vibration isolation is the most important.The isolation of vibration is mainly based on the consideration of the frequency of external vibration and thenatural frequencystart with[2]。
Electronic control system
The scanning tunneling microscope is a nanoscaleServo systemTherefore, electronic control system is also an important part.Scanning tunneling microscope shall be controlled by computerDrive of stepping motorMake the probe approach the sample, enter the tunnel area, and then continuously collect the tunnel current. In the constant current modeSet valueBy comparison, the feedback system controls the advance and retreat of the probe to maintain the stability of the tunnel current.All these functions are realized by electronic control system.Figure 1 shows the electronic control system of scanning tunneling microscopeblock diagram[2]。
Online scanning control system
Dna taken by scanning tunneling microscope
In the software control system of scanning tunneling microscope, the role of computer software is mainly divided into "online scanning control" and "offline scanning control"Data analysis”Two parts.
Online scanning control
① Parameter setting function
In the scanning tunneling microscope experiment, the computer software mainly realizes someBasic parametersSetting, adjusting, acquiring, displaying and recording the scanned data image, etc.The computer software will becomputer interfaceImplementation andElectronic equipment roomCoordination ofWorking together。The setting functions of some parameters in online scanning control are as follows:
⑴ The value of "current setting" means the value to be maintained in constant current modeConstant current, also represents constantCurrent sweepThe constant distance between the needle tip and the sample surface during the process.The greater the value setting, the smaller the constant distance.During measurement, the "current setting" is generally within the range of "0.5-1.0nA".
(2) "Tip bias voltage" refers to the voltage applied between the tip and the sample to generate tunnel currentTrue value。The larger this value is set, the easier it is to generate tunnel current between the needle tip and the sample. The larger the constant distance maintained in the constant current mode, the higher the constant heightScan ModeThe greater the tunnel current generated in the.The value of "needle tip bias" is generally set in the range of "50-100mV".
⑶ "Z voltage" refers to the real voltage applied to the piezoelectric ceramic material of the 3D scanning controller.The initial value of Z voltage determines the initial state of piezoelectric ceramics, which will change with the scanning.The initial value of "Z voltage" when the probe is far away from the sample is generally set at "- 150.0mV - - 200.0mV"[3]。
⑷ "Acquisition target" includes two options: "height" and "tunnel current". Select whether the information collected during scanning is the information about the change of sample surface height or the change of tunnel current.
(7) "Angle direction" refers to the deflection direction of horizontal movement of the probe. Changing the angle value will cause the scanned image to rotate.
"Blood cells" photographed under "scanning tunneling microscope"[4]
(8) "Size" is to set the size of probe scanning area, which is adjustedMaximumIt is determined by measuring range.The smaller the size is, the higher the scanning accuracy is. Changing the size can enlarge and reduce the scanning image.
(9) "Center offset" refers to the offset distance between the starting position of scanning and the sample and the tip when they are just put in place. Changing the value of center offset can make the tip shift in a small scale.Maximum center offsetOffsetIs the maximum size determined by the current range.
(10) "Working mode" determines whether the scanning mode is constant current mode or constant height mode.
⑾ “Inclined planeCalibration "refers to the software calibration made by the probe when scanning along the inclined sample surface.
(12) "Reciprocating scanning" determines whether to perform reciprocating scanning.
(13) "Range" refers to the detection accuracy and maximumScan sizeSize of.
When using software to control the motor to make the needle tip approach the sample, first ensure thatElectric motorThe red button of the controller is in the pop-up state, otherwise the probe part is only controlled by the electronic control system, and the computer software does not work on the control of the motor.motorcontrol software Control the electric motor with a smallstepRotate so that the needle tip slowly approaches the sample until it enters the tunnel area.
The operation mode of motor control is: select "Advance" for "Motor control", click "Continuous" button to carry out continuous approximation. When the detected tunnel current reaches a certain value, the computer will give a warning prompt, and automatically stop approximation. At this time, click "Single step" button, until the value of "Z voltage" is close to zero, stop approximation, and complete the motor control operation[2]。
Offline data analysis software
Offline data analysis refers to various analysis and processing work for the saved image data after leaving the scanning process.Common image analysis and processing functions include: smoothing, filteringFourier transformation Image inversion, data statistics, 3D generation, etc.
(1) Smoothing. The main function of smoothing is to smooth the high and low changes in the image and eliminate the sudden changes of data points.
⑵wave filteringThe basic function of filtering is to cut too low and level too low in a series of data.Therefore, formeasurement process A lot of burrs caused by tip jitter or other disturbances can be eliminated by filtering.
⑶ Fourier transform,fast fourier transform It is very effective for studying the periodicity of atomic images.
⑷ Image inversion: inverting the image in black and white will bring unexpected visual effects.
⑸ Data statistics: statistical analysis of image data.
(6) Three dimensional generation, based on the two-dimensional image of the scanned surface appearance, to generate intuitive and beautiful3D image。
Most of the software also provides many other functions, and uses various data processing methods to finally get a satisfactory image[2]。
working principle
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When the atomic scale tip scans the sample at a height of less than one nanometerElectronic cloudOverlap, apply a voltage (2mV~2V), and generate between the tip and the sampleTunnel effectAnd some electrons escape, forming a tunnelelectric current。The current intensity and the distance between the tip and the sample areFunctional relationWhen the probe scans along the surface of the substance at a given height, the distance between the probe and the surface of the substance is constantly changing due to the unevenness of the atoms on the surface of the sample, resulting in the constant change of the current.This change of current can be imaged to show the concave and convex shape at the atomic level.
Operating mode
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Constant current mode
Images taken with scanning tunneling microscope
A set of electronic feedback circuit is used to control the tunnel current I to keep it constant.Then through the computerSystem controlThe needle scanning on the sample surface is to make the needle move in two dimensions along the x and y directions.Since the tunnel current I needs to be controlled unchanged, the local height between the needle tip and the sample surface will also remain unchanged, so the needle tip will be the same as the rise and fall of the sample surfaceUndulatory motionThe information of height is reflected from this.That is to say, STM gets the three-dimensional information of the sample surface.This method can obtain comprehensive image information, high quality microscopic images, and is widely used.
Constant altitude mode
Operating principle of STM
Keep the needle tipAbsolute heightunchanged;Then the local distance between the tip and the sample surface will change, and the tunnel current I will also change;The change of tunnel current is recorded by computer and converted into image signal for display, and then the STM microscopic image is obtained.This working mode is only applicable to the case where the sample surface is relatively flat and the composition is single (such as composed of the same atom).From the working principle of STM, we can see that STM works by scanning the sample surface with a needle tip and obtaining microscopic images through tunnel current, without the need for light sources and lenses.This is the reason why it is named "scanning tunneling microscope".
Specific application
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scanning
When STM works, the probe will fully approach the sample to produce a height space limitedelectron beamTherefore, STM has a very highspatial resolution And scientific observation can be carried out.
Flaw detection and repair
STM can process the surface in real timesurface topography Imaging is used to find defects and damages on various structures of the surface, anddepositionAnd etching to eliminate defects and achieve the purpose of repair, and then STM can be used for imaging to check the repair results.
When STM is in the field emission mode, the needle tip is still quite close to the sample. At this time, a sufficiently high electric field can be generated with a low applied voltage (as low as about 10V), under which electrons will pass through the needle tipbarrierLaunch into space.These electrons have a certain beam current and energy. Because their distance in space is very small and there is no time to diverge from the sample, the beam diameter is very small, generallyNanometreMagnitude, so it may causeChemical bondFracturechemical reaction。
Moving the "IBM" pattern emitted by xenon atoms with STM
Move and write samples
When STM works under constant current state, the distance between the tip and the sample is suddenly shortened orBias voltageWhen a pulse is added, pits and mounds of nanometer scale will appear in the micro area of the sample surface under the tipIsostructureChange on.The tip of the needle is generally not damaged after the writing operation. It can still be used to image the surface atoms to check the quality of the writing results in real time.
There are two main ways to move the needle tip to write
① OnFeedback circuitDuring normal operation, adjust the reference current orBias voltageTo adjust thecontact resistanceTo control the movement of the needle tip.When the reference current is increased or decreasedbiasIn order to ensure constant current operation, the feedback will move the control needle tip to the sample, thus reducing the contact resistance.
② When STM is in tunnel stateoutput signal, turn off feedback, and then change the controlZ directionAthleticpiezoelectricityThe voltage applied on the ceramic can change the distance between the tip and the sample. This method can control the change of tunnel junction width more linearly than the former, which is relatively ideal.
The result of engraving is closely related to the cleanness of the needle tip.A small pit will be generated after the polluted tip contacts the surface;Unused clean tip contact surfaces produce a bump.The reason why the cleaning tip produces hills on the surface is that it hasBondingAt this time, if you want to restore the distance between the tip and the sample to that before contact with the surface, the tip must return more, which means that the bonding of the tip has caused a bulge on the surface from another angle.The pollution of the needle tip will prevent it from bonding to the surface, so the used needle tip will be engraved with a small pit after contacting the surface, and there will be originalIn pitThe atoms of[2]。
Au, Ag, etc. at room temperaturegoldThe micro structures engraved on the surface are always unstable at room temperature. Due to the diffusion of metal atoms, these structures will be blurred or even disappear in a few hours at most.
stayOther materialsFor example, it is possible to use STM to carve stable structures on Si (110), Si (100) and other surfaces.When writing, when the tip moves 2 nm into the sample, the depth of the small pit (calculated from the edge) is 0.7 nm.At room temperature andUltrahigh vacuumThese figures are highly stable and will not change after a long time.
STM canMetallic glassThe size of the mound increases with the increase of bias voltage.The cause of hillocks is generally considered to be highcurrent densityCausedsubstrateThe local melting of these molten materials at the negative bias of the needle tipelectrostatic fieldUnder the action, a protruding Taylor cone will be formed. After the current is removed, the cone will cool down immediately and form a hill on the surface... Not all surfaces can form hills like this.The melting point of the substrate determines the heat required for local melting;aboutpoint sourceThe actual heat gain of electron beam and substrate is not only related to the current density, but also depends on theMean free pathAnd the thermal conductivity of the substrate used;For disordered metallized glassRhtwenty-fiveZr75, because the average free path of the electron in it iscrystalAnd polycrystalline metal is 100 times smaller, and the melting point is not very high, 1340K. Therefore, when the electron beam is incident, it gets more heat and is relatively easy to be melted, so it is easy to form hills on it.
Observation of Ultrafine Metal Particles by STM Constant Current Operation Mode[5]
① It has atomic level high resolution. The resolution of STM in the direction parallel to the sample surface can reach 0.1 angstrom, which means that a single atom can be distinguished.
② Available in real timeReal spaceOf the sample surface in3D image, can be used for periodic or non periodicsurface structure This real-time observable performance can be used toSurface diffusionThe study of isodynamic process.
③ The local surface structure of a single atomic layer can be observed instead of the average properties of the bulk phase or the whole surface, so it can be directly observedSurface defect。Surface reconstruction, the shape and position of the surface adsorbent, and theSurface reconstructionEtc.
④ It can work in vacuum, atmosphere, room temperature and other different environments. The sample can even be immersed in water and other solutions without special sample preparation technology and the detection process will not damage the sample. These characteristics are particularly suitable for researchBiological sampleAnd evaluation of sample surface under different experimental conditions, such asHeterogeneous catalysisMechanism, super all-in-one geo creation, monitoring of electrode surface changes during electrochemical reaction, etc.
Although STM hasEM, FIM and other instruments, but the limitations caused by the working mode of the instrument itself are also obvious.This is mainly reflected in the following two aspects
① Constant current of STMOperating modeSometimes, it can not accurately detect some grooves between particles on the sample surface, and the resolution related to this is poor.In principle, this limitation will be improved under the constant height working mode.However, this defect can only be avoided by using a very sharp probe whose tip radius should be far less than the distance between particles.This is particularly important when observing the diffusion of ultrafine metal particles[5]。
② The sample observed by STM must have a certain degree ofConductivityFor semiconductors, the observation effect is worse than that of conductors;aboutinsulatorIt is impossible to observe directly.If the sample surface is covered with a conductive layer, the resolution of the image to the real surface is limited due to the particle size and uniformity of the conductive layer.Developed by Binny et al. in 1986AFMIt can make up for the shortcomings of STM[5]。
In addition, the commonly used (including commercial) STM instruments are generally not equipped with a FIM, so the needle shapeUncertaintyIt often brings many uncertain factors to the authentication and interpretation of instrument resolution and image[5]。
Product evaluation
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In 1981, with the invention of scanning tunneling microscope, physicists made a breakthroughZurichGerd Bining andHeinrich Rohrer (Heinrich Rohrer) wonNobel Prize。
Suddenly, physicists got an amazing "image" of individual atoms arranged in the chemistry book, which was onceAtomic theoryA situation that critics of.Brilliant photographs of atoms arranged in crystals and metals are now possible.The chemical formula often used by scientists contains a series of complex atoms wrapped in a molecule, which can be seen with the naked eye.In addition, scanning tunneling microscopy makes it possible to manipulate individual atoms.In fact, the three letters "IBM" were spelled out with atoms, creating a sensation in the scientific community.Scientists are manipulating individualAtomic timeNo longer at a loss, but can actually see them and play with them.
Working principle conclusion
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In the process of scanning tunneling microscope (STM) observation of sample surface, the structure of scanning probe plays an important role.NeedlelikeRadius of curvatureIs the impactLateral resolutionKey factors of;The size, shape and chemical identity of the tip not only affect the resolution of STM image, but also relate to the measurement of electronic structure.Therefore, accurately observe thegeometryAnd electronic characteristics have important reference value for the evaluation of experimental quality.Researchers of scanning tunneling microscope (STM) have used some other technical means to observe the microscopic morphology of scanning tunneling microscope (STM) tip, such asSEM、TEM, FIM, etc.SEM can only provide micron orSubmicronThe morphology information of the atomic levelMicrostructureObservation is far from enough.Although the sample image at atomic level can be obtained by using high-resolution TEM, it is difficult to observe the tip of scanning tunneling microscope (STM), and its atomic level resolution is barely achievable.Only FIM can observe the top morphology of the metal tip of the scanning tunneling microscope (STM) at atomic resolution, so it becomes an effective observation tool of the scanning tunneling microscope (STM) tip.Sakurai Lifu et al. of Tohoku University in Japan made use of this advantage of FIM to make a FIM-STM coupling device (called FI-STM by researchers), which can observe the geometry of scanning tunneling microscope (STM) tip at the atomic level through FIM, which enables people to carry out experiments when the state of scanning tunneling microscope (STM) tip is known,So as to improve theEfficiency[2]。
