Patent ID: 12253538

DETAILED DESCRIPTION

The method according to an aspect of the present invention can be realized on any conventional microscope having at least one scanning tunneling probe, which can include a scanning tunneling microscope (STM), an atomic force microscope (AFM) combined with an STM (such apparatuses are usually called AFM/STM) etc. To be able to perform the method according to an aspect of the present invention, the apparatus must have at least one scanning tunneling probe. In a preferred embodiment, the apparatus is equipped to change the tip-to-sample distance sufficiently fast.

A typical STM is shown inFIG.2a. The sample1is placed on a sample stage2attached to a stage actuator3of XYZ or XY type, wherein XYZ are conventional cartesian coordinates. To illustrate the embodiments of this invention, Z axis is chosen as vertical and X and Y axes are horizontal. However, any other axis orientation is also possible. The origin of the coordinate system can also be arbitrarily chosen. In this description, a global coordinate system which does not move with the tip and/or sample will be considered. The tip4of the STM probe is placed into a tip holder5, which is attached to a tip actuator6of XYZ or XY type. The sample1is grounded by a first wire7. The potential is applied to the tip4by a second wire8. The tunneling current TC is enhanced by a current preamplifier9, measured in a signal processing module10and analyzed by a computer11. The commands, required to move the sample1and/or the tip4, are send by a controller12to a voltage amplifier13and to the corresponding actuators3and6.

Definitions

Acronym STM is used for both Scanning Tunneling Microscopy and Scanning Tunneling Microscope.

A relative movement of the tip4and of the sample1can be carried out by either moving the stage2with the sample1alone, by moving the tip4alone, or by moving both, i. e. by moving the stage2with the sample1along with moving the tip4. It is commonplace to change the distance between the tip4and the sample1by moving the sample1or the tip4. In this application the tip-to-sample distance D changes will be described by the tip4movement. As this is the most current scenario, the wording “tip approach” will be used hereinafter to describe more briefly “reduction of the tip-to-sample distance D” or “decrease in the tip-to-sample distance D”. Similarly, “tip retraction” should be understood also as “extension of the tip-to-sample distance D” or “increase in the tip-to-sample distance D”. As for the movements in the horizontal plane, the most current embodiment is that the tip4stays fixed in the horizontal plane and it is only the stage2that moves in the horizontal plane.

For sake of simplicity, one global Cartesian coordinate system is selected to describe the LDOS coordinates of the tip xtip, ytip, ztip, of the sample coordinates xsample, ysample, zsampleand the LDOST coordinates of the sample xtopo, ytopo, ztopo. Any global Cartesian system, as the one ofFIG.2a, can be selected. This Cartesian coordinate system has one vertical axis Z and two horizontal axes X, Y. However, with an appropriate calibration, all these sets of coordinates can have their own coordinate systems. In the case of the coordinate system according toFIG.2a, the tip-to-sample distance D is changed in the vertical direction, i. e. in the direction of the Z-axis. The origin of the system can be placed anywhere aside from the moving parts of the microscope.

Hereinafter, the tip coordinates xtip, ytip, ztip, the sample coordinates xsample, ysample, zsampleand the LDOST coordinates of the sample xtopo, ytopo, ztopowill be described in the same global Cartesian coordinate system similar to that ofFIG.2a.

The tip coordinates xtip, ytip, ztipare the coordinates of the tip4in term of LDOS. Coordinate xtipand ytipare lateral positions of the tip4. Coordinate ztipis a vertical position of the tip4(seeFIG.2a) corresponding to tip surface LDOS. Coordinates xtip, ytipand ztipare defined by the voltages applied to the tip actuator8.

The sample coordinates xsample, ysample, zsampleare the coordinates of an arbitrary spot of the sample in the same Cartesian coordinate system. Any spot of the sample, including inside the sample, can be chosen as the one for which the sample coordinates xsample, ysample, zsamplewill be expressed. These sample coordinates are used to describe the movement of the sample as a whole, when it moves together with the stage2driven by the stage actuator3. As it is usually not possible to measure the sample coordinates xsample, ysample, zsampledirectly, the voltages on the actuator3with corresponding calibration are used to define the sample coordinates.

The Local Density of Electronic States Topography, or more briefly Local Density of States Topography, with acronym LDOST, is also one of the topics of an aspect of this invention. It provides information about positions of atoms during electron tunneling events.

The goal of STM measurements is to probe the LDOS of the sample surface. In order to avoid confusion, we distinguish LDOS, which is probed during conventional constant current and constant height STM measurements, and LDOST, which is evaluated by presented method.

The sample1height coordinate corresponding to sample surface LDOS, i.e. LDOST of the sample1, will be noted ztopo. The definition of this coordinate is shown in theFIGS.2band2cfor one surface point of the sample. The ztopocoordinate can be attributed to all surface points of the sample. The lateral coordinates of the surface points of the sample will be noted as xtopo, ytopo.

It is convenient to have the zero-level of Z axis at the lowest surface point for the sample1placed on the fixed stage2and describe the relative movement by the tip4movement only, as it is done in present application.

Tip-to sample distance D is defined in the above-described global coordinate system ofFIG.2aas D=ztip−zsample, i. e. as a difference between the tip coordinate ztipalong the vertical axis Z and the sample coordinate zsamplealong the vertical axis Z. Tip-to sample distance D is defined by voltages, applied to actuators3and6.

The tip-to-sample surface distance DS in terms of LDOS is defined as potential barrier width for electrons occupying the Fermi level, as shown in theFIG.2c. DS=ztip−ztopo. The tip-to-sample surface distance DS is defined by fitting of TC(D) curve in each studied point of the sample by equation 1.

The sample height SH is a Local Density of Electronic States Topography of the sample surface. It is defined as SH=D—DS=ztopo−zsample.

The potential barrier φ is the averaged barrier heightφ≡φ, i.e. the averaged barrier height between the tip4and the surface point of the sample1, as shown in theFIG.2c.

Relative tip-to-sample planar coordinates xrel≡xtip−xsample=xtip−xtopoand yrel=ytip−ysample=ytip−ytopoare the differences between corresponding tip4and sample1coordinates along horizontal axes X and Y. The relative position of the tip4and the sample1in a horizontal plane is important to determine above which surface point of the sample1is placed the tip4, particularly during the scanning.

The tunneling current TC is the current that can be detected between the tip4and the sample1for appropriate tip-to-sample distances D and applied voltage V.

The time dependencies of all variables showing such dependency will be noted by the sign of the variable followed by (t), t meaning the time. Eg. TC(t) is the time dependency of the tunneling current, xrel(t) is the time dependency of the first relative tip-to-sample planar coordinate, yrel(t) is the time dependency of the second relative tip-to-sample planar coordinate, D(t) is the time dependency of the tip-to-sample distance, etc.

If the variation of the tip-to-sample distance is set as a quasi-periodical oscillation of periodical oscillation, an amplitude A and frequency f are introduced. As it is usually the tip4that moves in vertical direction above the sample1to change the tip-to-sample distance, the amplitude A and the frequency f will most often correspond to the tip4oscillations. The changes in sample properties can cause the changes in average tip-to-sample distance D.

The setpoint tunneling current STC, is the highest allowable tunneling current, it is defined by the operator or by the microscope software.

The setpoint tip-to-sample distance SD(t) is the distance between the tip4and the sample1when TC(t)=STC. This distance inherently varies depending on the sample1properties and morphology due to the feedback keeping maximum TC(t)=STC.

The constant current CC is a current value used to reconstruct the constant current map.

The constant height CH is a value of the tip-to-sample distance D used to reconstruct the constant height map.

The constant current CC and the constant height CH values are selected by the operator or by the microscope software according to information that should be retrieved from the measurement.

InFIG.1a, the state-of-art constant current method is explained, i. e. it is explained how the constant current maps are constructed. InFIG.1b, the state-of-art constant height method is explained, i. e. it is explained how the constant height maps are constructed.

The time dependency of sample1LDOST coordinate ztopois plotted as a topography curve14, the time dependency of the tip4coordinate ztipis plotted as tip curve15, and the tunneling current TC time dependency is plotted as tunneling current curve16.

FIG.1aandFIG.1billustrate how the STM controller12in the state-of-art methods deals with the increase in sample topography. This increase is registered as an increase in the sample surface point z-coordinate of ztopowhen moving from one point of the sample to another, e.g. by scanning the sample surface. The time dependency ztopo(t) of ztopowhen moving in the horizontal direction between two regions of the sample with different topography, is plotted as topography curve14. In this example, ztopochanges from 0 nm to 100 pm. During the preliminary time interval17from 0 to 500 μs, ztopo=0 nm. Then, during the transition time interval18with 50 μs duration, ztopovaries from 0 nm to 100 μm. Then, during the feedback time interval19with 50 μs duration and during the final time interval20, ztopo=100 pm. For both images zsample=0 nm during all four time intervals17,18,19,20, i. e. the sample does not move in the vertical direction.

In the constant current method (FIG.1a) the decrease of the tip-to-sample surface distance DS due to the increase in the sample LDOS topography causes the increase of tunneling current during the transition time interval18, see the tunneling current dependency curve16. The constant tunneling current CC is set. There is a feedback that forces the current to return to the constant current CC value by initiating an upward tip movement during the feedback time interval19which lasts until the tunneling current TC returns to its initial value CC. Vice versa, the feedback would move the tip downwards for a depression in the sample LDOS topography. In this example, the feedback time interval19lasts from 550 μs to 600 μs, see the tip curve15. The tip-to-sample distance D is thus increased and at 600 μs, the tunneling current TC returns to its original constant current CC value, see the tunneling current curve16. During the preliminary time interval17and the final time interval20, the tunneling current TC has thus the same value, the constant current value CC. When scanning the sample across multiple surface points, a constant current map38can be created as a map of D=ztip−zsamplecoordinates for which the recorded tunneling current TC was equal to the constant current CC.

In the constant height method (FIG.1b) the position of the tip ztipis constant, ztip=CH+zsample, as shown by the tip curve15, and the tip-to-sample distance D is constant as well and equal to CH, thus the reduction of tip-to-sample surface distance DS causes the increase of tunneling current TC during the transition time interval18, see the tunneling current dependency curve16. When scanning the sample across multiple surface points, a constant height map39can be created as a map of tunneling currents TC obtained when scanning the sample with the tip coordinate ztipset constant and equal to CH+zsample.

It is obvious that according to the state-of-art methods, to obtain the constant current map, it is necessary to scan the sample when keeping the tunneling current TC constant, whereas to obtain the constant height map, it is necessary to rescan it again, this time while keeping the tip-to-sample distance D=ztip−zsampleconstant.

According to an aspect of the present invention, all maps, i. e. the constant current map38, the constant height map39, the LDOST map40and the potential barrier map41can be obtained from a single scan of the sample.

To this aim, the new method comprises carrying out the following steps for at least two surface points of the sample1:placing the tip4successively above said surface points of the sample1, which is typically done by moving the tip4and/or by moving the sample1in the horizontal direction;above each of said surface points of the sample, performing a distance varying step33comprising varying the tip-to-sample distance D, andconcurrently with the distance varying step33, performing a time dependencies recording step35comprising: recording time dependency TC(t) of the tunneling current TC, recording time dependencies xrel(t), yrel(t) of the relative tip-to-sample planar coordinates xrel, yrel, and recording time dependency D(t) of the tip-to-sample distance D.

The method works for at least two surface points of the sample, while the most advantageous embodiment is the one when a scanning movement is performed, i. e. when the tip4is placed successively above a plurality of surface points of the sample1by scanning movement34of the sample1and/or of the tip4in a horizontal plane and wherein while performing the scanning movement34, the distance varying step33and the time dependencies recording step35are carried out.

The relative tip-to-sample planar coordinates xrel=xtopo−xsample, yrel=ytopo−ysample, can be used to calculate the planar coordinates xtopo, ytopoof each sample surface point above which the tip4was placed, in a coordinate system related to the sample.

A map denotes a 3D image with at least two couples of planar coordinates xrel, yrelof at least two map points. Each of the map points has a space coordinate corresponding to some value interesting from the point of view of sample properties. It can be e. g. the tunneling current, tip-to-sample distance, sample height or potential barrier.

Above each surface point of the sample, for which map points of at least two maps should be created from a single measurement, it is necessary to vary a tip-to-sample distance D at least to some extent. It is not necessary to have the same span of the tip-to-sample distance D for all surface points of the sample. A tip approach can continue above more than one surface point of the sample1, and similarly, a tip retraction can continue above more than one surface point of the sample1. However, it is advantageous to perform the distance varying step33with approximately the same span, i. e. with approximately the same difference between the minimum and the maximum distance, above each of the surface points of the sample for which one of the maps should be created, the maps including the constant current, constant height, LDOST and potential barrier map.

To avoid the contact between the tip4and the sample1that can result in destroying the tip4and/or the sample1surface, in a preferred embodiment, the operator or the microscope software determine a setpoint tunneling current STCas the highest allowable tunneling current.

In a preferred embodiment, a preliminary approach32between the tip4and the sample1is carried out before performing the distance varying step33for the first time. The preliminary approach is stopped when the tunneling current TC reaches the value of the setpoint tunneling current STC. Then the tip-to-sample distance D is increased for A.

To avoid collision between the tip4and the sample1, a feedback regime is set up so that when the tunneling current TC becomes greater than the setpoint tunneling current STC, the tip-to-sample distance D is extended.

This feedback regime is illustrated inFIG.4. A preferred embodiment with zsample=0 and ztipvarying periodically is shown. However, similar feedback can be used for any embodiment in which the tip-to-sample distance D is varied in any manner.FIG.4shows how the feedback regime deals with the increase in sample topography ztopo, as shown in the topography curve14, from 0 nm during the preliminary time interval17from 0 μs at 500 μs to 100 pm at 550 μs. The time axis is common for all ztopo, ztipand TC dependencies. In this preferred embodiment, the position of the tip4is oscillating above the sample surface, see the tip curve15. The tunneling current (TC) is thus oscillating too, see the tunneling current curve16. While oscillating, the tip4moves from a region of the sample1where the surface points have ztopo=0 nm, to a region with surface points having greater ztopo=100 μm. The decrease of tip-to-sample surface distance DS causes the increase of the tunneling current TC with a maximum at 550 μs during the transition time interval18. During feedback time interval19feedback forces the upward movement of the tip4, see the tip curve15, at 600 μs and returning of tip-to-sample surface distance DS and tunneling current16to same oscillations during final time interval20, i.e. maximum tunneling current TC in each oscillation cycle is equal to threshold value STC.

In yet another aspect of the invention, that can be advantageously combined with the above-mentioned feedback regime, varying the tip-to-sample distance D comprises setting the amplitude A of the tip-to-sample distance D variation and then repeatedly decreasing and increasing the tip-to-sample distance D between the setpoint tip-to-sample distance SD(t) and the setpoint tip-to-sample distance SD(t) plus twice the amplitude A.

It is possible to vary tip-to-sample distance D mechanically by actuators3or6or by additional high frequency actuator, which will manage the tip-to-sample distance oscillations. Such additional actuator can be helpful as in this case fast tip-to-sample distance oscillations managed by high frequency actuator will not interfere with slow changes in the tip-to-sample distance caused by feedback and managed by actuators3and6.

In one preferred embodiment, the amplitude A falls in the range of 100 pm to 10 nm.

A frequency f can be set and the tip-to-sample distance D can be varied with said frequency f. The changes in tip-to-sample distance are then periodical or quasi-periodical.

In one preferred embodiment, the frequency f falls in the range of 1 kHz to 1000 KHz.

Thanks to varying the tip-to-sample distance D while recording the time dependencies TC(t), xrel(t), yrel(t), D(t), two or more maps (selected from a group comprising constant current map, constant height map, LDOST map and potential barrier map) can be reconstructed from just one scan of the sample. More details about how each of the maps is reconstructed are disclosed hereinafter.

The constant current map is created from the recorded time dependencies TC(t), xrel(t), yrel(t), D(t) for at least two surface points of the sample1above which the tip4was placed when the tip-to-sample distance D was varied. In a preferred embodiment, the constant current map is created for a plurality of surface points of the sample1above which the tip4was placed during the scanning movement34of the sample1and/or of the tip4in a horizontal plane.

First, an operator or the microscope software determines a constant current CC value within the range of the recorded tunneling currents TC.

Then, the recorded time dependency TC(t) is examined and among the values of time t, constant current times tCCare found as the times for which the recorded value of the tunneling current TC(tCC) was equal to the constant current CC. In this way, a plurality of values of constant current times tCCis retrieved.

Consequently, the constant current map is created as a plurality of constant current map points with planar coordinates of each constant current map point equal to relative tip-to-sample planar coordinates xrel(tCC), yrel(tCC), recorded when time t was equal to one of the constant current times tCC, and with space coordinate of each constant current map point equal to tip-to-sample distance D(tCC) recorded when time t was equal to one of the constant current times tCC. The horizontal coordinates xrel(tCC), yrel(tCC) and the space coordinate D(tCC) of the same constant current map point have the same constant current time tCC.

FIG.6illustrates an example of how the constant current map38can be created from the time dependencies recorded in the time dependency recording step35according to an aspect of the invention. For sake of simplicity, inFIG.6, we consider zsample=0. The measured tip curve15, showing the dependency ztip(t), is thus equivalent to the curve of the time dependency D(t) of the tip-to-sample distance D. The measured tunneling current curve16showing the time dependency TC(t) of the tunneling current, is also shown. The constant current CC is set as CC=25 pA. The ztipand thus also tip-to-sample distance D values that were recorded at each time when the tunneling current TC was equal to CC, are marked with first squares21on the tip curve15. These values are plotted as space coordinates in the constant current map. To each value of the tip-to-sample distance D, or to each value of ztipin this example with zsample=0, corresponding to one of the first squares21, planar coordinates of the same constant current map point are assigned as the relative tip-to-sample planar coordinates xrel(t), yrel(t) recorded at the same time as the tip-to-sample distance D corresponding to one of the first squares21.

The constant height map is created from the recorded time dependencies TC(t), xrel(t), yrel(t), D(t) for at least two surface points of the sample1above which the tip4was placed when the tip-to-sample distance D was varied. In a preferred embodiment, the constant height map is created for a plurality of surface points of the sample1above which the tip4was placed during the scanning movement34of the sample1and/or of the tip4in a horizontal plane.

First, an operator or the microscope software determines a height tip-to-sample distance CH within the range of the recorded tip-to-sample distances D.

Then, the recorded time dependency D(t) is examined and among the values of time t, constant height times tCHare found as the times for which the recorded value of tip-to-sample distance D(tCH) was equal to the constant height tip-to-sample distance CH. In this way, a plurality of values of constant height times tCHis retrieved.

Consequently, the constant height map is created as a plurality of constant height map points wherein planar coordinates of each constant height map point are equal to relative tip-to-sample planar coordinates xrel(tCH), yrel(tCH) recorded when time t was equal to one of the constant height times tCH, and wherein space coordinate of each constant height map point correspond to tunneling currents TC(ICH) recorded when time t was equal to one of the values of the constant height time tCH. The planar coordinates and the space coordinate of the same constant height map point have the same constant height time tCH.

FIG.7illustrates an example of how the constant height map can be created from the time dependencies recorded during the time dependencies recording step35according to an aspect of the invention. For sake of simplicity, inFIG.7, we consider zsample=0. The measured tip curve15is thus a curve of the time dependency D(t) of the tip-to-sample distance D. The constant height is set as CH=1.15 nm. The current values reached each time when the tip-to-sample distance D was equal to the constant height CH, are marked with second squares22on the tunneling current curve16. These values are plotted as space coordinates in the constant height map. To each value of current corresponding to one of the second squares22, planar coordinates of the same constant height map point are assigned as the relative tip-to-sample planar coordinates xrel(t), yrel(t) recorded at the same time as the tunneling current TC corresponding to one of the second squares22.

Based on the recorded time dependencies TC(t), xrel(t), yrel(t), D(t) for at least two surface points of the sample1above which the tip4was placed when the tip-to-sample distance D was varied, a Local Density of States Topography map40and a potential barrier map41can be created.

The values of sample height SH and of potential barrier φ can be retrieved from the recorded time dependencies TC(t), xrel(t), yrel(t), D(t) by carrying out the following sequence of steps for at least two surface points of the sample1above which the tip4was placed when the tip-to-sample distance D was varied:determining a sample surface point specific time interval <t1, t2> as a time interval during which the tip4remained above this surface point of the sample1andusing the time dependencies TC(t), D(t) recorded during the surface point specific time interval <t1, t2> to reconstruct the dependency TC(D) of the tunneling current TC on the tip-to sample distance D for this surface point of the sample1, andfinding a sample height SH and a potential barrier φ by fitting the dependency TC(D), obtained for the sample surface point specific time interval <t1, t2> by equation:

TC⁡(D)=γ⁢σ⁢φ⁢VD-SH⁢exp⁡(-B⁡(D-SH)⁢φ),1

where potential barrier φ is equal to the averaged barrier height

φ_,γ=e⁢2⁢m4⁢β⁢π2⁢ℏ2,B=2⁢β⁢2⁢mℏ2,

β is a dimensionless factor, V is a voltage between the tip4and the sample1, σ is a tunneling area, m is free electron mass, e is elementary charge, ℏ is Planck's constant.

Equation 1 can be found in e. g. https://www.ntmdt-si.com/resources/spm-theory/theoretical-background-of-spm/1-scanning-tunnel-microscopy-(stm)/13-observed-physical-quantities-in-stm/132-current-distance-characteristic.

Dimensionless factor β˜1. Its more precise value can be found in e. g. https://www.ntmdt-si.com/resources/spm-theory/theoretical-background-of-spm/1-scanning-tunnel-microscopy-(stm)/12-tunnel-current-in-mim-system/121-appendix.

More detailed definitions of averaged barrier heightφ≡φ and potential barrier width for electrons occupying the Fermi level δz≡DS can be found in e. g. https://www.ntmdt-si.com/resources/spm-theory/theoretical-background-of-spm/1-scanning-tunnel-microscopy-(stm)/12-tunnel-current-in-mim-system/121-john-g-simmons-formula.

The first time t1limiting the sample surface point specific time interval <t1, t2> is defined as a time when the tip4arrives above the surface point of the sample1for which by fitting the dependency TC(D) by equation 1 is carried out. The second time t2limiting the sample surface point specific time interval <t1, t2> is defined a time when the tip4leaves the place above this surface point of the sample1. Thus, in each sample surface point specific time interval <t1, t2> coordinates xrel(t) and yrel(t) are constant, D(t) and TC(t) vary in time, but fitted SH and φ values are constant.

If the above-described fitting procedure is done for a plurality of surface points of the sample1, a Local Density of States Topography map40can be created as a plurality of Local Density of States Topography map points. Each of the Local Density of States Topography map points has planar coordinates xrel(t) and yrel(t) and a space coordinate which corresponds to one of the sample heights SH. For each of Local Density of States Topography map points, time t in xrel(t), yrel(t) is a time arbitrary chosen from one of the sample surface point specific time intervals <t1, t2>, and the sample height SH attributed as a space coordinate to the same Local Density of States Topography map point is retrieved from the dependency TC(D) reconstructed from the time dependencies TC(t), D(t) recorded during the same sample surface point specific time interval <t1, t2>. In other words, each LDOST map point assigns to one of the surface points of the sample its characteristic value of sample height SH.

By analogy, if the above-described fitting procedure is done for a plurality of surface points of the sample, also a potential barrier map41can be created as a plurality of potential barrier map points. Each of the potential barrier map points has planar coordinates xrel(t) and yrel(t) and a space coordinate which corresponds to one of the potential barriers φ. For each of the potential barrier map points, time t in xrel(t), yrel(t) is a time arbitrary chosen from one of the sample surface point specific time intervals <t1, t2>, and the potential barrier φ attributed as a space coordinate to the same potential barrier map point is retrieved from the dependency TC(D) reconstructed from the time dependencies TC(t), D(t) recorded during the same sample surface point specific time interval <t1, t2>. In other words, each potential barrier map point assigns to one of the surface points of the sample its characteristic value of potential barrier.

FIG.5illustrates the dependence between the tunneling current TC and distance between the tip and the sample surface DS. Data were calculated using formula TC=81.5485/DS×exp(−DS), where TC is in pA and DS is in nm. According to an aspect of the present invention, four maps can be reconstructed from a single sample scan. In an example shown inFIG.5, the constant current map is reconstructed for constant current CC=25 pA, DS=1.1 nm. Constant height map is reconstructed for the constant height CH=1.15 nm, i.e. for distances DS=1.05-1.15 nm. Based on the dependency TC(D) reconstructed from the time dependencies TC(t) and D(t), the sample height SH=D—DS and the potential barrier φ are determined in each point of the sample by fitting the measured dependency TC(D) by equation 1. Result is the same as the bottom image inFIG.4, i.e. ztopodata and φ=1/B2.

FIG.8illustrates how by fitting the measured dependence TC(D) by equation 1 the sample height SH=D−DS and potential barrier φ (it is constant in this case) are defined for the preliminary time interval17when SH=0 nm, and for the final time interval20, when SH=100 μm.

FIG.3illustrates the most complex embodiment of the mechanically oscillating STM method according to an aspect of the present invention, where all the four maps are created, i. e. the constant current map38, the constant height map39, LDOST map40and the potential barrier map41. Block31describes installation of the tip4, setting up the frequency f and amplitude A of the tip-to-sample distance D oscillations. Then the preliminary approach32is presented. Block33describes the most important step of an aspect of this invention, the distance varying step. After starting the tip-to-sample distance oscillations the scanning movement of the sample1and/or tip4in X and Y direction34is performed. Time dependencies recording step35, feedback step36and fine tuning step37are taking place at the same time during measurements. Based on analysis of TC(t), D(t), xrel(t) and yrel(t) dependencies all the four maps are created, i. e. the constant current map38, the constant height map39, LDOST map40and the potential barrier map41. By “and/or” oval block we mean that operator can choose 1, 2, 3 or 4 maps to be reconstructed.

Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, apparatuses, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, apparatuses, means, methods, or steps.

The method of examining a sample1in a scanning tunneling microscope according to an aspect of the present invention can be realized on all commercially available STMs without the need to modify hardware. In principle, only software modification is sufficient for implementation of this method.

However, additional high frequency actuator, which will manage the high frequency tip-to-sample distance D oscillations, can be helpful as in this case fast tip-to-sample distance D oscillations managed by high frequency actuator will not interfere with slow changes in the tip-to-sample distance caused by feedback and managed by actuators3and6.

Additional processor, which will analyze the TC(D) dependencies in real time, can be useful too. Using such processor, it will not be necessary to record large datasets with TC(t), D(t), xrel(t) and yrel(t) dependencies. In this case only much smaller datasets, with the constant current map38, the constant height map39, LDOST map40and the potential barrier map41coordinates can be recorded.