SCANNING ELECTROCHEMICAL MICROSCOPY WITH OSCILLATING PROBE TIP

Abstract

A new scanning electrochemical microscopy tip positioning method that allows topography and surface activity to be resolved independently is presented. A SECM tip is oscillated relative to the surface of interest. Changes in the oscillation amplitude, caused by the intermittent contact of the SECM tip with the surface of interest, are used to detect the surface of interest, and as a feedback signal for various types of imaging

Claims

1. A method comprising: oscillating, using a piezoelectric positioner, a scanning microscopy ultramicroelectrode probe tip in height relative to a surface of interest; detecting damping of an amplitude of the oscillation of the probe tip by detecting a decrease in the amplitude of the oscillation of the probe tip as compared to an amplitude of oscillation of the probe tip in a bulk solution, the decrease in the amplitude of the oscillation of the probe tip indicating intermittent contact with the surface of interest; using the detected decrease in the amplitude to detect the surface of interest; and using the probe tip to measure or modify activity of the surface of interest simultaneously with detecting damping.

2. A method as claimed in claim 1, comprising detecting the decrease in the amplitude of the oscillation of the probe tip during an approach curve measurement.

3. A method as claimed in claim 2, comprising terminating the approach curve measurement when intermittent contact is detected.

4. A method as claimed in claim 2, comprising terminating the approach curve measurement on detecting a decrease in sensor tip oscillation amplitude as compared to oscillation amplitude in a reference medium.

5. A method as claimed in claim 2, comprising terminating the approach curve measurement on detecting a decrease of between 0.5% and 15% in sensor tip oscillation amplitude as compared to oscillation amplitude in a reference medium.

6. A method as claimed in claim 2, comprising terminating the approach curve measurement on detecting a decrease of about 5-10% in tip sensor oscillation amplitude as compared to oscillation amplitude in a reference medium.

7. A method as claimed in claim 1, further comprising constructing an image using a series of line scans, each line scan including a forward intermittent contact scan and a reverse constant distance scan.

8. A method according to claim 1, further comprising using a measured oscillation amplitude to control the probe tip movement relative to the surface of interest.

9. A method as claimed in claim 1, wherein oscillating the probe tip comprises oscillating the probe tip with a magnitude of between 1% and 2% of the radius of an active electrode of the probe tip.

10. A method according to claim 1, wherein the oscillation to the probe tip is selected from the group of: sinusoidal oscillation, sawtooth oscillation, and square oscillation.

11.-12. (canceled)

13. A method according to claim 1, in which a frequency of oscillation is selected from the group: between 5 and 100,000 Hz, between 5 and 5,000 Hz, and between 30 and 110 Hz.

14.-15. (canceled)

16. A method according to claim 1, in which an amplitude of oscillation is selected from the group: between 0.1 nm and 1 μm, between 5 nm and 500 nm, and between 15 nm and 250 nm.

17.-22. (canceled)

23. A method according to claim 1, further comprising using a measured electrochemical response of the probe to provide information about the surface of interest.

24. A method according to claim 23, in which the electrochemical response of the probe tip is the current generated at the probe tip when held at a potential to interact with a species of interest.

25. A method according to claim 23, in which the electrochemical response of the probe tip is the potential generated at the probe tip when interacting with a species of interest.

26. A method according to claim 23, comprising using the electrochemical response of the probe tip to deliver chemical species to the surface of interest.

27. A method as claimed in claim 1, wherein oscillating of the probe tip is normal or generally normal to the surface of interest.

28. (canceled)

29. A non-transitory computer readable medium having stored thereon machine readable code that when executed by a processor of a scanning microscopy apparatus cause the apparatus to perform a method comprising: oscillating an ultramicroelectrode probe tip in height relative to a surface of interest using a piezoelectric positioner; detecting damping of an amplitude of the oscillation of the probe tip by detecting a decrease in the amplitude of the oscillation of the probe tip as compared to an amplitude of oscillation of the probe tip in a bulk solution, the decrease in the amplitude of the oscillation of the probe tip indicating intermittent contact with the surface of interest; using the detected damping to detect the surface of interest; and using the probe tip to measure or modify activity of the surface of interest simultaneously with detecting damping.

30. Apparatus configured: to oscillate a scanning microscopy ultramicroelectrode probe tip in height relative to a surface of interest using a piezoelectric positioner; to detect damping of an amplitude of the oscillation of the probe tip by detecting a decrease in the amplitude of the oscillation of the probe tip as compared to an amplitude of oscillation of the probe tip in a bulk solution, the decrease in the amplitude of the oscillation of the probe tip indicating intermittent contact with the surface of interest; to use the detected damping to detect the surface of interest; and to use the probe tip to measure or modify activity of the surface of interest simultaneously with detecting damping.

31. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

[0044] FIG. 1 shows a UME and a substrate surface as used with embodiments of the present invention;

[0045] FIGS. 2 and 3 show movement of SECM tips according to embodiments of the invention; and

[0046] FIG. 4 shows an implementation of intermittent contact SECM embodying aspects of the invention.

DETAILED DESCRIPTION

[0047] Embodiments of the invention involve the application of an oscillatory perturbation of typically 0.1 nm to 1 μm at typically 5 to 100 000 Hz to a SECM tip. The SECM tip may be a UME, for instance in the form of a metal wire A, with a radius of 0.002 to 12.5 μm, sealed in a glass capillary B relative, typically normal, to a surface of interest C. With this oscillation the amplitude of the oscillation becomes damped as the tip encounters the surface. The oscillation is typically sinusoidal, though other oscillation types can be used.

[0048] FIG. 1 shows a cross section of an UME and the oscillation of the UME. FIG. 2 shows the movement of the SECM tip when in bulk solution (i.e. not in contact with the surface). FIG. 3 shows the movement of the SECM tip when in intermittent contact with the substrate surface. Here, it can be seen that the uppermost half of the waveform, i.e. the parts above position 0, are substantially the same as for FIG. 2, however the lowermost half of the waveform is damped by contact with the surface C. The damping is detected and provides absolute information on the tip-surface separation. This can be used as a measure of tip-surface separation for approach curve measurements (where the tip is translated towards or away from the substrate, usually in a perpendicular direction) and as a set point to maintain a fixed distance between the tip and substrate surface during imaging (where the tip and/or the substrate are moved laterally with respect to each other). The SECM tip electrochemical signal (e.g. current and/or potential) is measured throughout and provides information about the surface activity.

[0049] A detailed description of an implementation of the invention is now given. Embodiments of the invention are described with amperometric approach curve measurements and amperometric and potentiometric imaging. The amperometric configuration for an electrically unbiased sample is shown schematically in FIG. 4. Note that if the sample is a conductor of semiconducting material, it can also be connected up as an electrode (potential and/or current control).

[0050] Apparatus and Instrumentation

[0051] Coarse control of the SECM tip B (a Pt disk UME), which is typically mounted perpendicular to the substrate surface 52 but can be mounted at different angles, is realized by a three-dimensional manual x,y,z stage controlled by manipulator screws 56. Note that other means of achieving this positioning are also possible. Fine control is realized by three (x, y, z) piezoelectric positioners 42 fitted with strain gauge sensors. The piezoelectric positioners 42, operated in closed loop, are controlled by an amplifier/servo 47. The piezoelectric positioner amplifier/servo is controlled by a personal computer 45. An ac signal provided by an AC generator 49 is added to the z piezoelectric positioner control by a signal adder 48. The ac signal creates a sinusoidal oscillation of:

δ*sin(2π*ƒ*t)

[0052] in the height of the SECM tip B about the average tip height, but other oscillation profiles can be used.

[0053] The computer 45 includes processing means, comprising one or more processors, memory means, comprising one or more memories, and a computer program stored in the memory means. The processing means, under control of the computer program, performs various actions that are described below, including measuring, detecting and controlling operations.

[0054] Embodiments of the invention are typically implemented in a Faraday cage 41 on a vibration isolation table 55 in a two electrode arrangement with the metal wire A or UME tip B as the working or active electrode and a quasi-reference electrode 51. However, a three electrode (working, reference and counter electrodes) or a four electrode bipotentiostatic setup can be used, among other well known electrochemical setups.

[0055] The SECM tip current and the location of the piezoelectric positioners are recorded. The SECM is operated in a diffusion-limited configuration; with the SECM tip held at a potential to electrolyse a target chemical.

[0056] Intermittent contact SECM (IC-SECM) Approach Curves

[0057] The SECM tip is moved close to the substrate surface using the manipulator screws 56. Approach curve measurements are carried out by translating the SECM tip towards the substrate using the z one of the x,y,z piezoelectric positioners 42. Simultaneously the SECM tip is typically oscillated at a frequency of 70 Hz with a magnitude of 1-2% (10 nm-150 nm) of the active electrode radius. The oscillation magnitude may, however, take any value between 0.001% and 50% of the active electrode radius. The IC-SECM approach curve is terminated when intermittent contact is detected. Intermittent contact here is defined as a sustained decrease in the z piezoelectric positioner strain gauge sensor (z-SGS) tip oscillation amplitude as compared to the z-SGS oscillation amplitude in the bulk solution (for example a 1 to 15% sustained decrease).

[0058] As the UME tip B approaches a surface C the mean current decreases if the surface C is an insulating substrate. When approached to a surface C the mean current increases if the surface C is a conducting substrate. The magnitude of the z-SGS oscillation remains constant for most of the approach curve, only changing when intermittent contact is made between the UME tip B and the substrate surface C.

[0059] Although the oscillation frequency here is 70 Hz, it may take any value between 5 and 100 000 Hz. The oscillation frequency may be between 5 and 5000 Hz. The oscillation frequency may be between 30 and 110 Hz.

[0060] The oscillation amplitude may between 0.1 nm and 1 μm. The oscillation amplitude may be between 5 nm and 500 nm. The oscillation amplitude may be between 15 nm and 250 nm.

[0061] The oscillation amplitude of the SECM tip is monitored, and the measured oscillation amplitude is used to control the SECM tip movement relative to the surface of interest.

[0062] Instead of a sinusoidal oscillation, it may take some other form. For instance, a square oscillation may be applied to the SECM tip. The oscillation frequency of the square wave may be between 5 and 100 000 Hz, or may take some other value. The oscillation amplitude may be between 0.1 nm and 1 μm.

[0063] A sawtooth oscillation may be applied to the SECM tip. The oscillation frequency of the sawtooth signal may be between 5 and 100 000 Hz. Here, the oscillation amplitude may be between 0.1 nm and 1 μm.

[0064] The electrochemical response of the SECM tip is measured to provide information about the surface of interest. The electrochemical response of the SECM tip may be the current generated at, or flowing through, the SECM tip when held at a potential to interact with a species of interest. Alternatively it may be the potential generated at the SECM tip when interacting with a species of interest. It may alternatively be the potential when a current is applied to the tip, via galvanostatic control. It may alternatively be a conductance current.

[0065] The electrochemical response of the SECM tip may be used to deliver chemical species to the surface of interest.

[0066] The surface of interest in these embodiments is an interface between two substances, a surface of a solid or liquid, or a boundary between two phases (i.e. solid and liquid, liquid and gas, or solid and gas) of a substance, although it could be another surface such as a surface of a living cell or tissue.

[0067] The SECM tip is oscillated normal, or substantially normal, to the surface of interest.

[0068] Intermittent Contact SECM Imaging

[0069] The SECM tip B is engaged to the surface C using an Intermittent Contact (IC)-SECM approach curve which halts when intermittent contact is detected. An image is constructed typically using a series of line scans, although other scan methods are possible. Each line scan consists of a forward intermittent contact scan and a reverse constant distance scan. The forward scan is done while maintaining intermittent contact with the substrate surface C. The reverse scan is done at a constant distance away from the substrate surface C, which is identified by the z measurements of the tip position B in the forward scan. This separation is typically in the range 0.1-2 μm for a 2 μm active radius tip B. During the intermittent contact scan the SECM tip height is updated by a proportional controller, implemented on the computer 45. Other forms of controller, for instance a PID (proportional-integral-derivative) controller, can be used instead.

[0070] The proportional controller takes the form:

[0071] z.sub.new=z.sub.old+P*(z.sup.SGSAmplitude−0.9*z.sup.SGSBulkAmplitude),

[0072] where z.sub.new and z.sub.old are the new2 and old SECM tip height respectively, z.sub.SGSAmplitude is the z-SGS oscillation amplitude and z.sup.SGSBulkAmplitude is the z-SGS oscillation amplitude in the bulk solution.

[0073] A ten percent decrease in the z-SGS oscillation amplitude is used as a set point for scanning, although other values can be used. The SECM tip current is measured during the line scans. The images of chemical activity (from the various tip current measurements) and substrate height (from the location of the z piezoelectric positioner) are thus constructed simultaneously.

[0074] On a substrate with conducting and insulating regions, IC-SECM imaging, when the UME is operated in an amperometric feedback mode, produces an image with an increase in mean current over the conducting regions (positive diffusional feedback) and a decrease in mean current over the insulating regions (negative diffusional feedback). The mean current can be recorded during both the intermittent contact lines scans and the constant distance lines scans. The same pattern of increases and decreases in mean current is observed in both the intermittent contact and constant distance images. However the intermittent contact mean current shows a greater variability than the constant distance mean current. The substrate surface is identified by the computer 45 by the position of the z piezoelectric positioner during the intermittent contact lines scans. In addition, the oscillating component of the current can be isolated. The magnitude and phase of the oscillating component of the current is used by the computer 45 to construct images of the substrate surface activity.

[0075] IC-SECM imaging when operating an UME as a potentiometric tip (e.g. a pH-sensitive or C1-selective electrode or similar) produces an image of the concentration of the species of interest. In this case a two-electrode potentiometric electrode set up is used (with indicator and reference electrodes) and the potential of the indicator electrode is measured. This can be converted to a local concentration of the species of interest at the location of the tip. As for amperometric imaging described above, a key advantage of this method is that the topography of the sample and the tip-substrate separation is determined from the damping of the tip oscillation. Potentiometric electrodes can also be deployed into the IC-SECM mode for approach curve measurements.