JP2017044664A - Liquid atomic force microscope - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an in-liquid atomic force microscope having improved force detection performance.SOLUTION: An in-liquid atomic force microscope 10 for observing a sample placed in liquid comprises: a cantilever 11 having a probe part 12 at a tip; a displacement measuring part 13 which measures displacement of the cantilever 11 which is caused by interaction force between the sample 20 and a tip of the probe part 12; a feedback control part 14 which performs control of maintaining a distance between the sample 20 and the tip of the probe part 12 at a constant level based on a displacement measured by the displacement measuring part 13; and a thin film 23 which has an opening 23a, and covers a liquid surface of liquid 21, in which the probe part 12 is fixed to the cantilever 11 so as to penetrate the opening 23a and such that only the tip is immersed in the liquid 21 of the probe part 12.SELECTED DRAWING: Figure 7

Description

  The present invention relates to an in-liquid atomic force microscope for observing a sample placed in a liquid.

  An atomic force microscope (hereinafter also referred to as an AFM (Atomic Force Microscope)) is an analysis technique that can measure surface shapes and surface properties with nanoscale spatial resolution. AFM can be used for analysis of structures and phenomena at the solid-liquid interface, especially because it can be analyzed regardless of the conductivity of the sample, and it can be operated in a liquid environment. It has been actively implemented (see, for example, Patent Document 1).

JP-A-8-129017

  However, in the submerged AFM, there is a case where a problem of lowering the force detection performance by the cantilever occurs due to the dipping of the cantilever in the liquid, which hinders the development of the submerged AFM technique.

  Therefore, the present invention has been made in view of such a problem, and an object thereof is to provide an in-liquid atomic force microscope having improved force detection performance as compared with the prior art.

  In order to achieve the above object, an atomic force microscope in liquid according to an aspect of the present invention is an atomic force microscope in liquid for observing a sample placed in a liquid, and a probe at the tip. A cantilever having a portion, a displacement measuring unit for measuring displacement of the cantilever due to an interaction force between the sample and the tip of the probe unit, and the sample and the probe based on the displacement measured by the displacement measuring unit. A feedback control unit that performs control to maintain a constant distance from the tip of the needle unit, and a thin film that has an opening and covers the liquid surface of the liquid, and the probe unit penetrates the opening. The tip of the probe portion is fixed to the cantilever so that only the tip is immersed in the liquid.

  Here, the probe portion is composed of a glass probe bonded to the tip of the cantilever and a probe formed at the tip of the glass probe, and the probe portion is, of the probe portion, The tip of the cantilever may be fixed so that only the tip including the probe is immersed in the liquid.

  The thin film may be a membrane made of silicon nitride.

  Furthermore, a spacer that is sandwiched between the thin film and the sample and is in contact with the liquid may be provided.

  According to this, the atomic force microscope in liquid can be arranged by a thin film so that the tip of the probe part is immersed in the liquid and the part other than the tip of the probe part is located in the atmosphere. it can. Therefore, the submerged atomic force microscope can improve the force detection performance. Thus, an in-liquid atomic force microscope with improved force detection performance than before is provided.

  According to the present invention, an in-liquid atomic force microscope with improved force detection performance than before is provided.

It is a schematic diagram which shows the operation principle of AFM. It is explanatory drawing of the interaction force which acts between a probe and a sample. It is a schematic diagram which shows the displacement detection method by AFM. It is a schematic diagram which shows the problem of the submerged AFM in a prior art. It is a schematic diagram which shows the displacement detection method by the atomic force microscope in a liquid in a prior art. It is a schematic diagram which shows the displacement detection method by the atomic force microscope in liquid in embodiment. It is a block diagram which shows the structure of the atomic force microscope in a liquid in embodiment. It is a schematic diagram which shows the special probe of the atomic force microscope in a liquid in embodiment. It is an image figure which shows the atomic image in the liquid of the mica acquired by the atomic force microscope in liquid in embodiment.

  Before describing the embodiments of the present invention, the problems in the prior art described in the background art will be described in detail, and then the submerged atomic force microscope according to the present invention will be described in detail.

[Principle of scanning probe microscope]
In a scanning probe microscope (SPM (Scanning Probe Microscope)), a sharply sharpened probe is brought close to a sample to detect an interaction (such as a tunnel current or an interaction force) acting between the probe and the sample, The distance between the probe and the sample (z position of the probe) is feedback-controlled so as to keep this interaction constant. Further, when the probe (or sample) is scanned in the horizontal direction (xy direction) while maintaining this feedback control, the probe (or sample) moves up and down according to the unevenness of the sample surface. If the locus of the vertical movement of the probe is recorded with respect to the horizontal position, an uneven image on the sample surface can be obtained.

[Atomic Force Microscope (AFM)]
FIG. 1 is a schematic diagram showing the operating principle of the AFM. FIG. 2 is an explanatory diagram of the interaction force acting between the probe and the sample.

  An atomic force microscope (AFM) is a type of SPM that detects the interaction force acting between a probe and a sample and keeps the distance between the probe and the sample constant. The position is controlled (FIG. 1). In AFM, a cantilever (cantilever beam) provided with a sharply pointed tip is used as a force detector. Normally, when a probe is brought close to a sample, an attractive interaction force caused by a van der Waals force and an electrostatic force acts in the atmosphere or in a vacuum as shown in FIG. When the probe is further brought closer to the sample, the strong repulsive force due to the chemical interaction force exceeds these forces. In the AFM, the z position of the probe is feedback-controlled so that the change in attractive force (or repulsive force) applied to the probe when the probe is brought close to the sample surface is kept constant. By scanning the probe in the horizontal direction in this state, an uneven image on the sample surface is obtained as described above.

[Problems of AFM force detection method (prior art)]
FIG. 3 is a schematic diagram showing a displacement detection method by AFM.

  As shown in FIG. 3, the AFM detects the reflected light of the laser irradiated on the back surface of the cantilever with a photodiode, and thereby the interaction force received by the cantilever from the tip of the probe from the displacement (deflection amount) of the cantilever. Measure. Conventionally, atomic resolution measurement using AFM has been achieved only in a vacuum. However, in recent years, by greatly improving force detectors and force detection techniques, it has become possible to perform atomic resolution AFM observation in a liquid environment where high resolution observation is considered difficult. This makes it possible to directly visualize the solid-liquid interface phenomenon with AFM, and is considered to contribute to the development of a wide range of research fields. However, submerged AFM has the following problems, which hinders the development of submerged AFM technology.

(1) Problem of stability of AFM measurement FIG. 4 is a schematic diagram showing a problem of submerged AFM in the prior art.

  In the liquid AFM, since the cantilever is immersed in the liquid, impurities (such as precipitates or undissolved particles) contained in the observation solution may adhere to the back surface of the cantilever. Further, in AFM observation such as catalytic reaction and battery electrode reaction, bubbles generated from the sample surface may adhere to the cantilever as shown in FIG. As a result, the reflected light of the laser is irregularly reflected, and stable AFM measurement often becomes impossible. Such a problem becomes a factor of narrowing the application range of the observation sample of the AFM in liquid and the solution.

(2) Influence of laser light irradiation Some reactions in a biological sample or catalyst in liquid are generated or accelerated by laser light irradiation. Since the current AFM apparatus setup uses a force detection method using laser light, there is a problem that the observation sample changes during the AFM measurement in liquid due to the laser light. This problem is also a factor that narrows the application range of AFM in liquid.

(3) Problem of long-distance force in potential measurement In recent years, a potential measurement technique on the surface of a sample in liquid has been developed as a measurement technique applying the AFM technique in liquid. Currently, in order to obtain the local potential distribution (short-range force) at the solid-liquid interface, it is necessary to extract only the electrostatic force acting between the probe tip and the sample from the experimentally obtained electrostatic force. . For that purpose, it is necessary to subtract the long-range force (mainly the force acting on the whole cantilever and the sample surface) from the experimental data, but the separation of the short-range force and the long-range force is very It is difficult to.

(4) Problem of lowering Q value In the liquid, water exists around the cantilever, and the viscosity of the cantilever greatly increases due to its viscosity. Along with that, the Q value of the resonance of the cantilever is greatly reduced as compared to the atmosphere. An approximate value of the minimum force detection limit F _min of the cantilever is given by the following (Equation 1). Here, k is the spring constant of the cantilever, k _B is the Boltzmann constant, T is the temperature, B is the measurement band, and f ₀ is the resonance frequency of the cantilever. From (Equation 1), when the Q value decreases, F _min increases and the force detection performance deteriorates. In the liquid, the Q value is reduced from about 100 to 1000 to about 1 to 10 as compared to the atmosphere, so that the force detection performance is greatly deteriorated.

(Embodiment)
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that each of the embodiments described below shows a specific example of the present invention. Numerical values, shapes, materials, components, arrangement positions and connection forms of components, steps, order of steps, and the like shown in the following embodiments are merely examples, and are not intended to limit the present invention. In addition, among the constituent elements in the following embodiments, constituent elements that are not described in the independent claims indicating the highest concept of the present invention are described as optional constituent elements.

[Improved AFM Force Detection Method (Technical Content of the Present Invention)]
In order to solve the above problems, the present invention proposes a scanning probe microscope having a thin film shield mechanism. This scanning probe microscope will be described in comparison with the prior art.

  FIG. 5 is a schematic diagram showing a displacement detection method using a submerged atomic force microscope in the prior art. FIG. 6 is a schematic diagram showing a displacement detection method using an in-liquid atomic force microscope in the present embodiment.

  In the prior art, as shown in FIG. 5, the entire cantilever is immersed in the observation solution. On the other hand, in this embodiment, a thin film is installed between the cantilever and the observation sample, and only the tip of the probe is immersed in the observation solution from the hole formed in the thin film. As a result, it is possible to suppress contaminants and bubbles generated in the liquid from adhering to the cantilever, thereby enabling stable in-liquid AFM observation. Moreover, the shielding effect by a shielding thin film can be expected for light irradiation and potential measurement. Regarding the influence of the observation sample surface by light irradiation, irradiation of the force detection laser to the observation sample can be prevented by the shield thin film. In the potential measurement, the influence of the long-distance force acting between the cantilever and the observation sample can be blocked by using a metallic shield thin film.

  Furthermore, since the force detector is operated in the atmosphere in this method, there is an effect of suppressing the reduction of the Q value, which has been a problem in the conventional submerged AFM measurement. Therefore, in principle, it is possible to improve the force detection sensitivity as compared with conventional measurement in liquid.

[Structure of atomic force microscope in liquid]
FIG. 7 is a block diagram showing a configuration of the submerged atomic force microscope 10 in the present embodiment.

  The in-liquid atomic force microscope 10 is an in-liquid observation atomic force microscope for observing a sample 20 placed in a liquid 21, and includes a cantilever 11 having a probe unit 12, a displacement measuring unit 13, A feedback control unit 14, a PC (personal computer) 15, an XY drive unit 16, a stage 17 and a sample holder 18 are provided. As shown in the lower left of the figure, the vertical upper direction is defined as the Z axis, and the two orthogonal axes orthogonal to the Z axis are defined as the X axis and the Y axis.

  The cantilever 11 is a cantilever having a tip 12 bonded to the tip, and functions as a force detector that detects an interaction force between the sample 20 and the tip of the tip 12.

  The displacement measuring unit 13 is a circuit that measures the displacement of the cantilever 11 due to the interaction force between the sample 20 and the tip of the probe unit 12, and detects the displacement of the cantilever 11 in the Z-axis direction based on the principle shown in FIG. For this purpose, it has an LD (Laser Diode) 13a, a PD (Photodiode) 13b, and a preamplifier 13c. That is, the laser emitted from the LD 13a is reflected by the back surface of the cantilever 11, becomes reflected light and enters the PD 13b, and becomes an electric signal indicating the displacement of the cantilever 11 in the Z-axis direction by the PD 13b, and the electric signal is the preamplifier 13c. And output to the feedback control unit 14.

  The feedback control unit 14 is a circuit that performs control to maintain a constant distance between the sample 20 and the tip of the probe unit 12 based on the displacement measured by the displacement measurement unit 13, and the electric power sent from the preamplifier 13c. A signal for driving the stage 17 in the Z-axis direction (Z-axis drive signal) for maintaining the displacement of the cantilever 11 indicated by the signal in the Z-axis direction constant is generated and output to the stage 17 and the PC 15.

  The PC 15 outputs an XY drive signal for scanning the sample 20 in the X-axis and Y-axis directions to the XY drive unit 16, and receives a Z-axis drive signal sent from the feedback control unit 14, and drives these XY drives This is a device that generates and displays a two-dimensional image showing the unevenness of the surface of the sample 20 based on the signal and the Z-axis drive signal.

  The XY drive unit 16 drives the stage 17 in the X-axis direction and the Y-axis direction by outputting the X-axis drive signal and the Y-axis drive signal to the stage 17 in accordance with the XY drive signal sent from the PC 15, thereby The sample 20 is scanned in the X-axis direction and the Y-axis direction.

  The stage 17 moves the sample holder 18 placed thereon according to the Z-axis drive signal sent from the feedback control unit 14 and the X-axis drive signal and Y-axis drive signal sent from the XY drive unit 16. By moving in the X-axis direction, the Y-axis direction, and the Z-axis direction, a piezo element that moves the sample 20 in the X-axis direction, the Y-axis direction, and the Z-axis direction relative to the tip of the probe unit 12 This is a scanner.

  The sample holder 18 is a device that holds the sample 20 placed in the liquid 21, and includes a container 24, a sample 20, a liquid 21, a spacer 22, and a thin film 23. The container 24 is filled with the liquid 21, and the sample 20 is placed therein, and a thin film 23 having an opening 23a is provided on the liquid surface of the liquid 21. The opening 23 a allows the probe unit 12 to penetrate the opening 23 a, and only the tip of the probe unit 12 can be immersed in the liquid 21. A spacer 22 in contact with the liquid 21 is provided at a position sandwiched between the thin film 23 and the sample 20. The spacer 22 is made of, for example, gold deposited on the surface of the thin film 23 in contact with the liquid surface of the liquid 21.

  The submerged atomic force microscope 10 is only a static mode AFM of a type that detects the interaction force between the probe and the sample from the displacement of the cantilever 11 caused by the interaction force between the probe and the sample. The vibration amplitude, frequency, or frequency generated by the interaction force between the probe and the sample when the cantilever 11 is scanned in the horizontal direction with respect to the sample while mechanically vibrating the cantilever 11 at a frequency near the resonance frequency is not limited thereto. It may be a dynamic mode AFM that detects an interaction force between the probe and the sample from a change in phase. Further, the feedback control unit 14 may displace the probe unit 12 in the Z-axis direction instead of driving the stage 17 in the Z-axis direction.

[Novelty of the present invention]
A technique for immersing only the tip of the probe in the liquid and operating the force detector in the atmosphere has already been realized by a technique called a tuning fork or a colibri sensor. However, a method for separating a force detector from a liquid by installing a thin film between the force detector and an observation sample has not been reported so far.

  The tuning fork does not precisely control the length of the probe immersed in the liquid, and the Q value varies greatly for each AFM experiment. On the other hand, with this method, it is possible to precisely control the thickness of the water film by controlling the height of the spacer between the metal thin film and the observation sample, facilitating the reproducibility of the experimental environment. Can be obtained.

  The colibri sensor has a Q value of 6000 or more even in a liquid environment, and since the length of the probe immersed in the liquid is always constant, the Q value does not vary. However, as with the Q value, the spring constant is very high, and there is a risk that the sample will be destroyed when the probe is brought into contact with the sample. On the other hand, the technique of the present invention can maintain the spring constant at the same level as that of a conventional AFM cantilever, and can be applied to AFM observation of a relatively soft sample such as a biosample. is there.

  Further, in the colibri sensor, the increase in the Q value is offset by the increase in the spring constant from (Equation 1), and the force detection performance is not so improved as compared with the conventional AFM measurement. On the other hand, the technique of the present invention not only can expect an increase in the Q value by operating the cantilever in the atmosphere, but also can maintain the spring constant at the same level as the conventional one. Can be improved.

[Detailed structure of probe section]
FIG. 8 is a schematic diagram showing a special probe of an in-liquid atomic force microscope in the present embodiment. Specifically, FIG. 8 is a diagram showing a detailed structure of the probe portion 12 bonded to the tip of the cantilever 11 in FIG.

  As shown in FIG. 8, a glass probe 12a having a length of about 100 to 200 μm is adhered to the tip of the cantilever 11, and a probe by electron beam deposition (EBD (Electron Beam Induced Deposition)) is further attached to the tip of the glass probe 12a. 12b is formed. That is, the probe unit 12 includes a glass probe 12a and a probe 12b. Then, the probe unit 12 is attached to the cantilever 11 so that only a tip portion (for example, a portion of about 50 μm from the tip of the probe unit 12) including the probe 12b of the probe unit 12 is immersed in the liquid 21. It is fixed.

[Observation example (AFM observation of mica / phosphate buffer solution interface)]
In this observation example, AFM observation of the mica / phosphate buffer solution interface was performed using a thin film shield mechanism. The cleaved surface of mica can be easily cleaved with a cellophane tape or the like, and a flat and clean surface at the atomic level can be obtained immediately before AFM observation. Further, since the mica surface is hydrophilic and does not dissolve or grow in pure water, it is a system often used as a model for in-liquid AFM measurement. Based on this measurement result, the stability of the AFM measurement in liquid when using the thin film shield mechanism and the increase in the Q value were verified.

The thin film shield mechanism used a silicon nitride (Si ₃ N ₄ ) membrane used as an electron microscope window. The opening 23a was about 50 μm square.

In general, a membrane window for an electron microscope is manufactured by depositing a Si ₃ N ₄ thin film to a thickness of 20 to 500 nm on a Si wafer and etching only the central portion of the Si wafer. Since this membrane has relatively high heat resistance and mechanical strength to withstand imaging with an electron microscope, it is considered durable enough to withstand water pressure even when used in the shield mechanism of this time. It is done. Spacers were formed on the bottom of the membrane by gold mask vapor deposition to form a space between the membrane and the observation sample. By keeping extra observation solution in contact with the side of the membrane, even if the observation solution evaporates from the hole in the membrane, the observation solution is always replenished through the space between the side and the solution environment is maintained during AFM observation. It is possible to continue.

  In this case, since the thickness of the spacer 22 is about 50 μm, there is a possibility that the cantilever may come into contact with the membrane when approaching the sample surface with the length of the probe of the general cantilever (10 μm). Therefore, a special probe having a cantilever tip extended with a glass probe (length: 150 μm) was used (FIG. 8).

  FIG. 9 is an image diagram showing an in-liquid atomic image of mica obtained by the in-liquid atomic force microscope 10 according to the present embodiment.

  FIG. 9 shows that the periodic structure reflecting the crystal structure of mica is reproduced in the in-liquid AFM image. In this observation example, in-liquid AFM measurement was continuously performed for about 3 hours, but the observation solution was not dried, and imaging could be performed stably for a long time. Further, the Q value increased to about 15 to 30, and succeeded in improving it by more than twice as compared with conventional observation in liquid. The Q value can be further increased by shortening the length of the glass probe used this time, or by reducing the height of the spacer formed on the bottom of the membrane to reduce the thickness of the water film. is expected.

  From the above experimental results, it was proved that AFM observation on an atomic scale is possible with the atomic force microscope 10 in liquid according to the present embodiment. As a result, in-solution AFM observation can be performed in a solution environment containing a large amount of impurities or in severe conditions such as in a bubble generation environment. Further, it is possible to shield the influence of laser light or electric field with a shield thin film.

  Therefore, it is possible to observe in-liquid AFM of an observation sample that was impossible with the conventional method, and the in-liquid AFM measurement technique can be applied to a wide range of fields. Furthermore, by operating the force detector in the atmosphere, it is possible to improve the Q value over the conventional method and improve the force detection performance. From these facts, this measurement technique is a technology that advances the solid-liquid interface research in the future, and has a very large inventive step.

[Summary]
In an atomic force microscope (AFM) in liquid, a configuration in which the entire cantilever is immersed in liquid is common. However, this not only greatly reduces the Q value of the cantilever, but also has some problems in application. For example, when gas is generated by a catalytic reaction or an electrode reaction, it cannot be stably measured because it adheres to the cantilever. When a bias voltage is applied between the probe and the sample, a large electrostatic force is applied between the cantilever and the sample.

The in-liquid atomic force microscope in the present embodiment includes a thin film shield mechanism in order to solve these problems. In this configuration, a hole of about 50 μm is formed on the Si ₃ N ₄ membrane developed for an electron microscope using a focused ion beam (FIB), and only the tip of the probe is immersed in the liquid through the hole, and AFM observation is performed. I do.

  Initially, the gap between the membrane and the sample was planned to be adjusted by the thickness (about 500 nm) of the gold thin film formed on the frame portion supporting the membrane. However, even though the thickness of the membrane is about 500 nm, which is sufficiently shorter than the probe length (about 15 μm), the membrane is pushed up by the sealed solution, which causes a problem of contact with the cantilever. .

  In order to solve this problem, we bonded a glass probe having a length of about 100 to 200 μm to the tip of the cantilever, and further formed an electron beam deposition (EBD) probe on the tip (FIG. 8). By using this, we succeeded in observing the atomic resolution of the mica surface stably with only the probe immersed. When the probe is immersed in the liquid, the Q value decreases to about 20 to 50, so that a large improvement in force sensitivity cannot be obtained. However, effects such as stabilization of measurement in a gas generation environment and suppression of electrostatic force acting between the cantilever and the sample can be sufficiently expected.

  As described above, the atomic force microscope in liquid according to the present invention has been described based on the embodiment, but the present invention is not limited to this embodiment. Without departing from the gist of the present invention, various modifications conceived by those skilled in the art have been made in the present embodiment, and other forms constructed by combining some components in the embodiment are also within the scope of the present invention. Contained within.

  INDUSTRIAL APPLICABILITY The present invention can be used for an in-liquid atomic force microscope with improved force detection performance than before.

10 In-liquid atomic force microscope 11 Cantilever 12 Probe unit 12a Glass probe 12b Probe 13 Displacement measurement unit 13a LD
13b PD
13c Preamplifier 14 Feedback control unit 15 PC
16 XY drive unit 17 stage 18 sample holder 20 sample 21 liquid 22 spacer 23 thin film 23a opening 24 container

Claims (4)

A submerged atomic force microscope for observing a sample placed in a liquid,
A cantilever having a probe at the tip;
A displacement measuring unit that measures the displacement of the cantilever due to the interaction force between the sample and the tip of the probe unit;
A feedback control unit that performs control to maintain a constant distance between the sample and the tip of the probe unit based on the displacement measured by the displacement measurement unit;
A thin film having an opening and covering the liquid surface of the liquid,
The probe unit is fixed to the cantilever so as to penetrate the opening and only the tip of the probe unit is immersed in the liquid. The probe portion is composed of a glass probe bonded to the tip of the cantilever and a probe formed at the tip of the glass probe,
The submerged atomic force microscope according to claim 1, wherein the probe unit is fixed to the cantilever so that only a tip portion including the probe is immersed in the liquid. The submerged atomic force microscope according to claim 1, wherein the thin film is a membrane made of silicon nitride. The submerged atomic force microscope according to claim 1, further comprising a spacer sandwiched between the thin film and the sample and in contact with the liquid.