JP2023042371A - Scanning tunneling microscope and method - Google Patents

Abstract

To provide a scanning tunneling microscope being observable at atomic resolution.SOLUTION: A scanning tunneling microscope for detecting a tunnel current (i.e., the photocurrent) induced by photoexciting a sample, includes a positioning device for positioning a probe 21 in a direction separating from the sample S, an irradiation part for applying excitation light to the sample supported on a substrate, and a detection part applying a bias voltage Vb between the substrate and the probe to detect the tunnel current flowing between the substrate and the probe. Thereby, the scanning tunneling microscope can cause the irradiation part to apply the excitation light to the sample supported on the substrate, cause the detection part to use the probe positioned on the substrate and detect the tunnel current flowing between the probe and a local area of the sample facing thereto, and at that time, apply the bias voltage between the substrate and the probe or generate localized plasmon P by light irradiation, so as to be able to increase the photocurrent by interaction between the localized plasmon and the sample.SELECTED DRAWING: Figure 1B

Description

The present invention relates to scanning tunneling microscopes and methods for detecting tunneling currents induced by photoexcitation of a sample.

Conventionally, a scanning tunneling microscope (STM) is known, in which a metal probe with a sharp tip is brought close to a sample, the probe is horizontally moved over the sample, and a tunnel current flowing between the probe and the sample is detected. (See, for example, Patent Document 1). Here, the tunnel current is measured while keeping the distance between the probe and the sample constant, or the distance between the probe and the sample is controlled and measured so as to keep the tunnel current constant. The STM can observe a sample three-dimensionally with atomic-molecular-size resolution by driving a probe horizontally and vertically with an atomic-molecular-size precision using a piezoelectric element.
Patent Document 1 International Publication No. 2014/054741

In the present invention, by detecting the tunneling current (also called photocurrent) induced by photoexcitation of the sample, high resolution, especially at a resolution smaller than the size of an atomic molecule such as sub-nano order (also called atomic resolution) can be achieved. A scanning tunneling microscope that enables observation and its method are provided.

In a first aspect of the present invention, there is provided a scanning tunneling microscope for detecting tunneling current induced by photoexciting a sample, comprising: a positioning device for positioning a probe in a direction away from the sample; A scanning type comprising an irradiation unit that irradiates excitation light, and a detection unit that applies a bias voltage between a substrate supporting a sample and the probe and detects a tunnel current flowing between the substrate and the probe. A tunneling microscope is provided.

In a second aspect of the invention, a method of detecting a tunneling current induced by photoexcitation of a sample comprises the steps of positioning a probe in a direction away from the sample, and a substrate supporting the sample. A method is provided comprising: applying a bias voltage between the probe, illuminating the sample with excitation light, and detecting a tunneling current flowing between the substrate and the probe.

It should be noted that the above summary of the invention does not list all the features of the invention. Subcombinations of these feature groups can also be inventions.

1 shows a schematic configuration of a scanning tunneling microscope according to an embodiment; 4 shows the configuration of a detection unit; The configuration of the control system is shown. FIG. 10 shows measurement results of photocurrent measured with respect to the separation distance between the sample and the probe using the scanning tunneling microscope according to the present embodiment. FIG. 4 shows a flow of photocurrent detection operation by the scanning tunneling microscope according to the present embodiment. 4 shows the measurement results of photocurrent from a sample or substrate under irradiation of excitation light. Fig. 3 shows the measurement results of the photocurrent from the sample with respect to the wavelength of the excitation light; Photocurrent/dark current images of samples detected with excitation light on/off are shown. Dependence of dark current on bias voltage when excitation light is off. A dark current image of the sample with the excitation light off is shown. A photocurrent image of the sample with the excitation light on is shown. The bias voltage dependence of the photocurrent from the sample with the excitation light on is shown. A photocurrent image of the sample detected at bias voltage = -0.25V is shown. Contribution of three photocurrent generation channels to the bias voltage dependence of the photocurrent is shown. A model of the photocurrent generation process (P1 channel) is shown. A model of the photocurrent generation process (N1 channel) is shown. A model of the photocurrent generation process (N2 channel) is shown. 4 shows the shape of the tip of the first comparative probe; FIG. 10 shows measurement results of emission spectra of plasmons generated using the first comparative probe; FIG. Fig. 2 shows a photocurrent image of a sample measured using the first comparative probe; FIG. 10 shows the shape of the tip of a second comparative probe; FIG. FIG. 10 shows measurement results of emission spectra of plasmons generated using the second comparative probe; FIG. Fig. 2 shows a photocurrent image of a sample measured using a second comparative probe; 4 shows the shape of the tip of the probe according to the example. FIG. 10 shows measurement results of emission spectra of plasmons generated using the probe according to the example. FIG. FIG. 4 shows a photocurrent image of a sample measured using a probe according to an example. FIG.

Hereinafter, the present invention will be described through embodiments of the invention, but the following embodiments do not limit the invention according to the claims. Also, not all combinations of features described in the embodiments are essential for the solution of the invention.

1A, 1B, and 1C respectively show a schematic configuration of a scanning tunneling microscope (STM) 100, a configuration of a detection unit 20, and a configuration of a control system according to this embodiment. The STM 100 is a scanning tunneling microscope that detects a tunneling current (also called photocurrent) induced by photoexciting a sample. , and a control unit 50 . A substrate 9 supporting the sample S is accommodated in the vacuum chamber 90 . As the sample S, a conductive substrate, a conductive substance such as a polymer can be selected. In this embodiment, as sample S, metal-free phthalocyanine (FBPc) is used.

The substrate 9 is a plate-like member that supports the sample S, and is made of noble metal including gold or silver. In this embodiment, the (111) plane of a silver single crystal is used as an example. As a result, localized plasmons P are easily generated between the substrate 9 and the probe 21 . Furthermore, in this embodiment, the surface of the substrate 9 supporting the sample S is covered with the insulating layer 9a.

The insulating layer 9a is formed on the surface of the substrate 9 with a thickness of four monolayers (atomic layers) using sodium chloride (NaCl), for example. By insulating the sample S from the substrate 9 by the insulating layer 9a, the energy dissipation of the photoexcited sample S can be suppressed. The insulating layer 9a may be formed of an insulating material suitable for supporting the sample S, and may be a part of the sample S, for example. It is also possible to cover only a part that supports the .

The vacuum chamber 90 is a container that holds the substrate 9 supporting the sample S under low-temperature vacuum, is connected to a vacuum pump 91 and includes a cryostat 92 therein. The interior of the vacuum chamber 90 is maintained at, for example, a liquid helium temperature (approximately 4 K) by a vacuum pump 91 and a cryostat 92 at a vacuum level of, for example, ˜10 ⁻¹⁰ Torr. By placing the sample S at a low temperature and in a vacuum, it is possible to detect the photocurrent with little noise. Depending on the type of the sample S and the target resolution, the sample S may be placed under low temperature atmospheric pressure, room temperature vacuum, or room temperature atmospheric pressure.

The irradiation unit 10 is a unit that irradiates the sample S with excitation light, and includes a light source 11 and an irradiation optical system 12 . The light source 11 is a device that generates excitation light having a wavelength matching the excitation energy of the sample S. In this embodiment, for example, a dye laser that generates laser light having a wavelength within the wavelength range of 0.32 to 1.2 μm. , a wavelength tunable laser such as a semiconductor laser is adopted. The irradiation optical system 12 includes a lens element that converges light, a filter element that cuts noise light, and the like. to send excitation light toward the sample S on the substrate 9 . In addition, the irradiation optical system 12 may include an openable and closable shutter that blocks the excitation light.

The detection unit 20 is a unit that applies a bias voltage Vb between the substrate 9 and the probe 21 that can be positioned thereon to detect a tunnel current flowing between the substrate 9 and the probe 21. It has a needle 21 , a voltage source 22 and an ammeter 23 . Here, the probe 21 is clamped to ground, and a voltage source 22 and an ammeter 23 are connected in series between the probe 21 and the substrate 9 . Note that the substrate 9 may be clamped to the ground.

The probe 21 is a probe for detecting tunnel current, and is made of metal containing gold or silver. By forming the probe 21 from metal, the localized plasmons P are easily generated between the probe 21 and the substrate 9 . The probe 21 includes, for example, a cylindrical proximal end and a distal end extending conically and taperingly from the cylindrical proximal end. The shape and forming method of the probe 21 that can efficiently detect the tunnel current will be described later.

The probe 21 is selected and adjusted so that the wavelength (energy) range of the emission spectrum of the localized plasmon P generated when the bias voltage Vb is applied between the probe 21 and the substrate 9 includes the target excitation energy of the sample S. , or can be formed. Here, it is desirable that the emission spectrum distribution of the localized plasmon P approximately peaks at the excitation energy of the sample S. Furthermore, it is more desirable that the peak energy of the emission spectrum distribution of the localized plasmon P matches the target excitation energy of the sample S. The shape of the tip may be adjusted by piercing and pulling the tip of the probe 21 from the surface of the substrate 9 and repeating these steps. Alternatively, an appropriate probe 21 may be selected from a plurality of probes 21 . Alternatively, the tip of the probe 21 may be processed using a focused ion beam. The emission spectrum can be detected by the photodetector 30, which will be described later.

A voltage source 22 applies a bias voltage Vb between the substrate 9 and the probe 21 . The magnitude (voltage value) and direction of the voltage are controlled by the controller 50, which will be described later.

Ammeter 23 measures the tunnel current flowing between substrate 9 and probe 21 . The measurement result is transmitted to the control unit 50, which will be described later.

The photodetector 30 is a unit that detects light emitted by the sample S, and includes a detection optical system 31 and a spectroscope 32 . The detection optical system 31 includes a lens element for converging light, a filter element for cutting excitation light and noise light, and the like. ), the emitted light is shaped by these optical elements and sent to the spectroscope 32 . The spectroscope 32 spectrally measures the emitted light from the sample S using, for example, a CCD element. The measurement result is transmitted to the control unit 50, which will be described later.

The positioning device 40 is a device that drives the probe 21 in three-dimensional directions, i.e., the separation direction (vertical direction) and horizontal direction, with respect to the sample S or the substrate 9 that supports it, and positions it at a target position. 41 included. It should be noted that the probe 21 is driven with respect to the sample S in this embodiment. The driving element 41 includes a Z driving element for driving the probe 21 in the vertical direction (Z-axis direction) with respect to the sample S, and two mutually orthogonal horizontal directions (X-axis direction and Y-axis direction). It has an X drive element and a Y drive element for driving . For example, piezoelectric (piezo) elements can be employed as the Z driving element, the X driving element, and the Y driving element.

A control unit 50 is a unit that controls the operation of the STM 100 . The control unit 50 causes an arithmetic device such as a personal computer to execute a dedicated program to perform its function, that is, the function of detecting the photocurrent by interlocking the irradiation unit 10, the detection unit 20, the light detection unit 30, and the positioning device 40. Express. The photocurrent detection operation of the STM 100 by the controller 50 will be described later. In addition, control functions such as control of the bias voltage Vb in the photocurrent detection operation and positioning control of the probe 21 are realized. The control unit 50 has an input device 51 such as a keyboard, mouse, touch panel, etc. for receiving command inputs for the user to operate the STM 100, and an output device 52, such as a display, for displaying the measurement results.

In controlling the bias voltage Vb, the controller 50 controls the direction and magnitude of the bias voltage Vb. Thereby, the photocurrent generation process can be controlled, as will be described later.

In the positioning control of the probe 21, the control unit 50 controls the driving element 41 to drive the probe 21 to target positions in the X-axis direction, the Y-axis direction, and the Z-axis direction, and to move the probe 21 to each target position. position to the target position.

FIG. 2 shows the measurement result of the photocurrent measured with respect to the separation distance between the substrate 9 and the probe 21 by the photocurrent detection operation, which will be described later, using the STM 100 . The probe 21 was positioned on the node of the sample S, FBPc. In the separation distance range of 0.53 to 1.11 nm, the current value was almost zero, that is, almost no photocurrent was detected. have understood. This is because the tunneling probability increases as the separation distance decreases, and the localized plasmon P containing the excitation energy of the sample S within the wavelength (energy) range of the emission spectrum is generated and enhanced, thereby generating a photocurrent. This suggests that the Therefore, the photocurrent flowing between the probe 21 and the substrate 9 can be increased by positioning the probe 21 with respect to the sample S at an appropriate separation distance, for example, within the range of 0.35 to 0.53 nm. can. Further, the probe 21 is driven in a direction parallel to the surface of the substrate 9 and sequentially positioned above an arbitrary local portion of the sample S to measure the photocurrent, thereby scanning the sample S two-dimensionally with high accuracy. be able to.

FIG. 3 shows a flow of photocurrent detection operation by the STM 100 according to this embodiment.

In step S101, an appropriate probe 21 is selected from a plurality of probes prepared in advance.

In step S<b>102 , the user places the substrate 9 in the vacuum chamber 90 and supports the sample S on the substrate 9 . Next, the control unit 50 controls the vacuum pump 91 and the cryostat 92 to hold the substrate 9 in the vacuum chamber 90 under low-temperature vacuum.

In step S104, the probe 21 is adjusted. First, the control unit 50 controls the positioning device 40 to drive the probe 21 and pierce the surface of the substrate 9 with the tip of the probe 21 and pull it out, thereby adjusting the shape of the tip. Next, the control unit 50 controls the positioning device 40 to position the probe 21 at an appropriate separation distance (for example, 0.35 to 0.53 nm) with respect to the sample S, and controls the detection unit 20 to control the substrate. 9 to generate a localized plasmon P, control the photodetector 30, and measure the emission spectrum of the localized plasmon P. FIG. The results are displayed on the output device 52 . The user can determine whether the measured wavelength (energy) range of the emission spectrum of the localized plasmons P includes the target excitation energy of the sample, preferably the emission spectrum distribution of the localized plasmons P corresponds to the target excitation energy of the sample S. (e.g., within the half-value range of the peak value of the emission spectrum, preferably within the range of 60%, more preferably within the range of 70%, more preferably within the range of 80% of the sample S) (including excitation energy), more preferably the peak energy of the emission spectrum distribution of the localized plasmon P matches the excitation energy of the sample S. After confirmation, the user inputs a selection command for selecting the probe 21 to the input device 51, and the process proceeds to the next step.

Instead of or in addition to adjusting the shape of the tip by piercing and withdrawing the tip of the probe 21 from the surface of the substrate 9, a focused ion beam may be used as described later. The tip of the can be processed.

When the user inputs a retry command to the input device 51, the control unit 50 redoes the selection or adjustment of the probe 21 and measures the emission spectrum of the localized plasmon P again. Thereby, the probe 21 suitable for photocurrent detection can be selected.

Note that step S104 may be omitted when a suitable probe 21 has already been selected.

At step S<b>106 , the control unit 50 controls the detection unit 20 to apply the bias voltage Vb between the substrate 9 and the probe 21 . Here, the control unit 50 controls the direction (positive/negative) and magnitude (voltage value) of the bias voltage Vb so that the light is applied to FBPc, which is the sample S, within the range of −2.25 to +0.9 V, for example. Choose a suitable orientation and magnitude for current sensing.

In step S108, the control unit 50 controls the positioning device 40 to drive the probe 21 in the separation direction (Z-axis direction) and/or in the horizontal direction (X-axis direction and Y-axis direction) with respect to the sample S, It is positioned on any localized portion of the sample S on the substrate 9 .

In step S110, the control unit 50 controls the irradiation unit 10 to irradiate the sample S supported by the substrate 9 with excitation light. Here, the excitation light has a wavelength that corresponds to the excitation energy from the ground state of the sample S to the excited state to be detected.

In step S112, the control unit 50 controls the detection unit 20 to detect the tunnel current (which may include a photocurrent component and a dark current component) flowing between the substrate 9 and the probe 21. FIG. The detected tunneling current value is sent to the control unit 50 together with the horizontal position of the probe 21 with respect to the substrate 9 .

In step S114, it is determined whether or not to end scanning. If not, the process proceeds to step S116. When finished, the process proceeds to step S118.

In step S116, the control unit 50 controls the positioning device 40 to drive the probe 21 in the horizontal direction (X-axis direction and Y-axis direction) with respect to the sample S, so that the next local portion of the sample S on the substrate 9 is moved. position above. After positioning, the process moves to step S110.

By repeating steps S110 to S116, the sample S is two-dimensionally scanned.

In step S118, the control unit 50 performs data processing. The control unit 50 processes the photocurrent measurement results to generate, for example, a two-dimensional image of the sample S, which is displayed on the output device 52 . When the data processed by the control unit 50 is saved in a storage device (not shown), the flow ends.

A photocurrent measurement result obtained by the above photocurrent detection operation of the STM 100 will be described. As sample S, FBPc (see FIG. 1B for its molecular structure) is used.

FIG. 4A shows the measurement results of the photocurrent under excitation light irradiation. Here, the positioning device 40 is controlled to position the probe 21 on the sample S and the substrate 9 (insulating layer 9a), respectively, and the irradiation unit 10 is controlled to open the shutter (not shown) at each probe position. By opening and closing, the sample S (FBPc) is intermittently irradiated with the excitation light at a cycle of 2 Hz, for example, and a tunnel current (i.e., photocurrent) flows between the probe 21 and the substrate 9 by the detection unit 20. detected. The distance between the probe 21 and the sample S was 0.48 nm, and the bias voltage Vb was -2.0V. The energy of the excitation light was selected to be the excitation energy (1816 meV) from the ground state (S ₀ ) to the excited state (S ₁ ) of FBPc. When the probe 21 was positioned on the sample S, a photocurrent was detected in response to irradiation (on) of the excitation light. Only dark current (noise) was detected. From this result, it can be seen that a tunneling current (ie, photocurrent) induced by the excitation of the sample S (ie, photoexcitation) accompanying the irradiation of the excitation light was detected.

FIG. 4B shows the measurement results of the photocurrent with respect to the wavelength of the excitation light. Here, while adjusting the energy (wavelength) of the excitation light in the range of 1800 to 1820 meV (689 to 681 nm), the probe 21 was positioned on the sample S as described above to measure the photocurrent. The measured photocurrent value exhibited a peak at an energy matching the excitation energy of FBPc (1816 meV). The half width of the peak is as narrow as 1.45 meV. This result suggests that matching the energy of the excitation light to the excitation energy of the sample S increases the photocurrent, thereby enabling efficient detection of the photocurrent.

FIG. 5 shows tunneling current (photocurrent/dark current) images of the sample S detected by irradiation (on)/blocking (off) of excitation light. Here, the positioning device 40 is controlled to drive the probe 21 by a unit pitch in the horizontal direction (X-axis direction and Y-axis direction) with respect to the sample S on the substrate 9, and The photocurrent was detected while being sequentially positioned on the local area. The distance between the probe 21 and the sample S was 0.5 nm, and the bias voltage Vb was -2.0V. A unit pitch was set to 0.06 nm. Therefore, the size of one pixel is 0.06 ^nm2 . When the excitation light is blocked (no irradiation), only noise associated with dark current can be confirmed, but when the excitation light is irradiated, the current value distribution derived from the two-dimensional structure of FBPc, especially the current value (i.e., It can be confirmed that there are lobes with high current values located at the center of the low density of states, the vertices of the octagon, and nodes with low current values located between them. This result means that the spatial distribution of FBPc, which is the sample S, could be two-dimensionally imaged with atomic resolution by photocurrent measurement.

FIG. 6A shows the bias voltage dependence of the tunnel current (dark current) when the excitation light is turned off. Here, the tunnel current was measured by positioning the probe 21 on the sample S as described above while changing the bias voltage Vb in the range of 2.0 to -3.0V. When the bias voltage Vb was -2.0 to 0.7 V, no dark current was detected, only noise was detected. In contrast, the dark current increased in the negative direction (that is, a negative dark current was detected) at a bias voltage of -2.1 V or less. Also, the dark current increased in the positive direction at a bias voltage of 0.7 V or more (that is, a positive dark current was detected). Note that dI/dVb peaks at bias voltages of 0.9V and -2.25V.

FIG. 6B shows tunneling current images (ie, dark current images) of Sample S detected at (1) bias voltage Vb=0.75 V and (2) −2.1 V with excitation light off. The measurement method is as described above. The dark current image obtained in (1) corresponds to the density of states distribution of the lowest unoccupied molecular orbital (LUMO) of the sample S. Therefore, the above results imply that the dark current channel through the LUMO of the sample S was opened above the bias voltage Vb=0.7V. LUMO has a finite density of states at node positions indicated by four arrows in the figure. Also, the dark current image obtained in (2) corresponds to the density of states distribution of the highest occupied molecular orbital (HOMO). Therefore, the above results imply that the dark current channel through the HOMO of the sample S is opened below the bias voltage Vb=-2.25V. The HOMO has almost zero density of states at the node positions indicated by the four arrows in the figure. These results suggest that selection of the bias voltage Vb enables selection of a dark current channel through a specific molecular orbital and selection of the state of the sample S to be observed.

FIG. 6C shows photocurrent images of sample S detected at (3) bias voltage Vb=−2.0 V and (4) −0.0 V with the excitation light turned on. The photocurrent image obtained in (3) reflects the density of states distribution of the LUMO of the sample S due to the negative current distribution. The photocurrent image obtained in (4) reflects the HOMO density of states distribution of the sample S due to the positive current distribution. Therefore, by adjusting the bias voltage Vb within the range of −2.0 to 0.0 V, the shape of the two-dimensional image changes along with the direction of the photocurrent. It is suggested that the state of the sample S of .

FIG. 7A shows the bias voltage dependence of the photocurrent when the excitation light is turned on. Here, the photocurrent was measured by positioning the probe 21 on the sample S as described above while changing the bias voltage Vb in the range of -0.5 to 0.0V. The distance between the probe 21 and the sample S was 0.49 nm, and the probe 21 was positioned on the (A) lobe and (B) node of FBPc to measure the photocurrent. At (A), the photocurrent I increases with increasing bias voltage Vb, crosses the zero line at about −0.33 V, and then increases slowly with increasing bias voltage Vb. Similarly, in (B), the photocurrent I increases as the bias voltage Vb increases, crosses the zero line at about -0.16 V, and saturates to approximately zero as the bias voltage Vb further increases. From these results, it can be seen that the zero-cross voltage of the photocurrent I varies depending on the position of the probe 21. is known to change.

FIG. 7B shows a photocurrent image of sample S detected at a bias voltage Vb=-0.25V. The average photocurrent in the entire two-dimensional image of FBPc, which is sample S, is zero, but the positive current in the eight lobes derived from the HOMO density of states distribution of FBPc, the four nodes derived from the LUMO density of states distribution, and The current is negative at the center. Thus, it can be seen that even within a single molecule, the direction of the detected photocurrent changes depending on the positioning position of the probe 21 (that is, locally). This suggests that there are multiple photocurrent generation channels in the molecule and their spatial distributions are different. Therefore, depending on the position of the probe 21, it is suggested that one or the other production channel will dominate and that the molecular orbitals involving that production channel can be imaged.

FIG. 8 shows the contribution of the three photocurrent generation channels to the bias voltage dependence of the photocurrent. It is known that there are three photocurrent generation channels, the P1 channel via the HOMO of the sample S, the N1 channel and the N2 channel via the LUMO of the sample S. The P1 channel produces approximately constant positive current with little dependence on the bias voltage Vb. A positive current reflects channeling through the HOMO. The N1 channel produces a roughly constant negative current with little dependence on the bias voltage Vb. The N2 channel has a threshold voltage characteristic of sample S (eg, about −0.2 V vs. FBPc), below which the bias voltage increases the photocurrent in the negative direction. Negative currents reflect LUMO-mediated channels. The bias voltage dependence of the photocurrent is determined by the total contribution of these three generation channels. Therefore, the photocurrent is approximately constant for bias voltages above the threshold and increases in the negative direction for bias voltages below the threshold.

FIG. 9A shows a model of the P1 channel. It is assumed that the substrate 9 and the probe 21 have the same Fermi level. First, one electron occupying the HOMO of the sample S transitions to the LUMO by absorbing the excitation light, and the sample S is excited from the ground state (S ₀ ) to the excited state (S ₁ ). Next, electrons are transferred from the probe 21 to the HOMO of the sample S, and the sample S forms an anion (negative ion) state. The Coulomb force between a plurality of electrons, including the transferred electrons, raises the electron trajectories of the sample S and raises the LUMO above the Fermi level of the substrate 9 . Finally, the electrons occupying the LUMO migrate to the substrate 9 . Therefore, in the P1 channel, a substantially constant positive current is generated in the range of -0.5 to 0 V, irrespective of the bias voltage Vb.

A model of the N1 channel is shown in FIG. 9B. It is assumed that the substrate 9 and the probe 21 have the same Fermi level. First, one electron occupying the HOMO of the sample S transitions to the LUMO by absorbing the excitation light, and the sample S is excited from the ground state (S ₀ ) to the excited state (S ₁ ). Electrons are then transferred from the substrate 9 to the HOMO of the sample S, and the sample S forms an anion (negative ion) state. The electron trajectory of the sample S rises due to the Coulomb force between a plurality of electrons including the transferred electrons, and the LUMO becomes higher than the Fermi level of the probe 21 . Finally, the electrons occupying the LUMO migrate to the probe 21 . Therefore, in the N1 channel, a substantially constant negative current is generated in the range of -0.5 to 0 V, irrespective of the bias voltage Vb.

A model of the N2 channel is shown in FIG. 9C. Suppose that the Fermi level of the probe 21 is lowered with respect to the Fermi level of the substrate 9 by applying the negative bias voltage Vb, and is lower than the LUMO of the sample S. First, one electron occupying the HOMO of the sample S transitions to the LUMO by absorbing the excitation light, and the sample S is excited from the ground state (S ₀ ) to the excited state (S ₁ ). Electrons occupying the LUMO then migrate to the probe 21 . The sample thereby becomes a cationic (positive ion) state. The electron trajectory of the sample S descends due to the Coulomb force between the remaining electrons excluding the transferred electrons, and the LUMO becomes lower than the Fermi level of the probe 21 . Finally, electrons are transferred from the substrate 9 to the HOMO or LUMO of the sample S. Therefore, the N2 channel has a threshold voltage corresponding to the level of the occupied energy of the LUMO of the sample S, and the photocurrent increases in the negative direction when the Fermi level of the probe 21 is lowered by the bias voltage. occurs.

Given the state ratio of LUMO and HOMO (bonding ratio between the probe 21 and the molecular orbital) and calculating the sum of the photocurrents from each of the three photocurrent generation channels, when the LUMO state ratio is relatively large The I-Vb characteristic of explains the measurement result of the I-Vb characteristic obtained by positioning the probe 21 on the lobe ((A) in FIG. 7A), and the I-Vb characteristic when the HOMO state ratio is relatively large. -Vb Characteristics Measurement results of IVb characteristics obtained by positioning the probe 21 on the node ((B) in FIG. 7A) will be described. The spatial distribution of the photocurrent and the dependence of the bias voltage on the photocurrent clarified the mechanism by which the photocurrent was generated, and the spatial distribution of the molecular orbitals of the sample S was visualized. It has been found that the molecular orbitals to be visualized can be selected by selecting and controlling the bias voltage Vb.

Therefore, the control unit 50 sets the bias voltage Vb between two voltages (for example, FIG. -2.0 to 0.5 V at 6 A). Thereby, the bias voltage Vb is selected so that the N2 channel is particularly opened, the excitation light is irradiated, and the positive photocurrent through the HOMO and the negative photocurrent through the LUMO of the sample S are detected. The spatial distribution of HOMO or LUMO or both molecular orbitals of a sample can be two-dimensionally imaged as shown in FIGS. 6C and 7B.

A shape of the probe 21 that can generate an appropriate localized plasmon between itself and the substrate 9 and efficiently detect the photocurrent will be examined. Such a probe 21 should be sharp on the atomic molecular size, smooth on the order of at least 10 nm, and symmetrical with respect to the central axis.

10A, 10B, and 10C respectively show the shape of the tip of the first comparative probe photographed with a scanning electron microscope, and the bias voltage Vb= between the first comparative probe and the substrate 9 using the first comparative probe. FIG. 10 shows a photocurrent image of a sample measured using the first comparative probe as a measurement result of the emission spectrum of localized plasmons generated when +2.5 V is applied. FIG. FBPc was used as a sample. The bias voltage Vb was -2.0V. A first comparative probe was formed using gold. However, the tip shape is blunt in atomic molecular size, has irregularities, and is not symmetrical with respect to the central axis. The plasmon emission spectrum contains the sample excitation energy (1816 meV) within its distribution range. The photocurrent image of the sample is uneven in intensity and blurry. The fact that the symmetrical structure of the sample does not appear suggests that the axial symmetry of the tip is important for photocurrent imaging.

11A, 11B, and 11C respectively show the shape of the tip of the second comparative probe photographed with a scanning electron microscope, and the bias voltage Vb= between the substrate 9 and the second comparative probe using the second comparative probe. FIG. 10 shows the measurement result of the emission spectrum of localized plasmons generated when +2.5 V is applied, and the photocurrent image of the sample measured using the second comparative probe. FBPc was used as a sample. The bias voltage Vb was -0.25V. A second comparative probe was made by electrochemical etching using gold, and the shape of the tip was adjusted by the method of step S104. Its tip shape is sharp in atomic molecular size and symmetrical with respect to the central axis, but has some unevenness. The plasmon emission spectrum contains the sample excitation energy (1816 meV) within its distribution range, but the center of the spectrum is away from the excitation energy. The photocurrent image of the sample is somewhat blurred, although the intensity distribution is symmetrical and reveals the symmetrical structure of the sample.

12A, 12B, and 12C respectively show the shape of the tip of the probe according to the example photographed by a scanning electron microscope, and the bias voltage Vb=+2.5 V between the probe and the substrate 9 using the probe. 2 shows the measurement result of the emission spectrum of the localized plasmon generated when , and the photocurrent image of the sample measured using the comparison probe. FBPc was used as a sample. The bias voltage Vb was -0.25V. The probe according to the example was produced by an electrochemical etching method using gold, and further processed by irradiating the tip with a focused ion beam in a doughnut-shaped pattern from the front direction. The tip is shaped like a truncated cone (about 60 nm in cross-sectional diameter at about 100 nm distance from the tip) supporting a hemisphere (about 30 nm in diameter in this example) on the top surface, i.e. smooth at least on the order of 10 nm. and is symmetrical about the central axis. Furthermore, the tip shape was adjusted to be sharp in atomic molecular size by the method of step S104. The plasmon emission spectrum contains the sample excitation energy (1816 meV) within its distribution range and the center of the spectrum is close to the excitation energy. More specifically, the excitation energy of the sample is contained within approximately 80% of the peak value of the emission spectrum. The photocurrent image of the sample has a symmetrical intensity distribution and clearly represents the symmetrical structure of the sample. Therefore, it can be seen that the probe 21 having the tip shape described above is suitable for obtaining clear photocurrent imaging.

The STM 100 according to this embodiment is a scanning tunneling microscope that detects a tunneling current (photocurrent) induced by photoexciting a sample, and is a positioning device that positions the probe 21 in the separation direction with respect to the sample S. 40, an irradiation unit 10 that irradiates the sample S supported on the substrate 9 with excitation light, and a bias voltage Vb is applied between the substrate 9 and the probe 21, thereby and a detection unit 20 for detecting a tunnel current flowing through. According to this, the irradiation unit 10 irradiates the sample S supported on the substrate 9 with the excitation light, and the detection unit 20 uses the probe 21 positioned on the substrate 9 to detect the probe 21 and the probe 21 . A tunnel current (that is, a photocurrent) flowing between the facing sample S and a local portion is detected, and at that time, a bias voltage Vb is applied between the substrate 9 and the probe 21, or light is irradiated to localize the current. By generating the plasmon P, the photocurrent can be increased by the interaction between the localized plasmon P and the sample S, thereby enabling atomic resolution observation of the sample S by the photocurrent, particularly two-dimensional image detection. It becomes possible.

The method according to the present embodiment is a method for detecting a tunneling current (photocurrent) induced by photoexciting a sample, comprising the steps of positioning a probe in a direction away from the sample S; applying a bias voltage Vb to the probe 21; irradiating the sample S supported by the substrate 9 with excitation light; and detecting a tunnel current flowing between the substrate 9 and the probe 21. And prepare. According to this method, the probe 21 is positioned in the separation direction with respect to the sample S, a bias voltage Vb is applied between the substrate 9 and the probe 21, or light is irradiated to generate the localized plasmons P, and the substrate By irradiating the sample S supported by 9 with excitation light and detecting the tunnel current (photocurrent) flowing between the substrate 9 and the probe 21, the interaction between the localized plasmons P and the sample S The photocurrent can be increased, which enables atomic resolution observation of the sample S by means of the photocurrent, in particular two-dimensional image detection.

Although the present invention has been described above using the embodiments, the technical scope of the present invention is not limited to the scope described in the above embodiments. It is obvious to those skilled in the art that various modifications and improvements can be made to the above embodiments. It is clear from the description of the scope of claims that forms with such modifications or improvements can also be included in the technical scope of the present invention.

The execution order of each process such as actions, procedures, steps, and stages in the devices, systems, programs, and methods shown in the claims, the specification, and the drawings is particularly "before", "before etc., and it should be noted that they can be implemented in any order unless the output of the previous process is used in the subsequent process. Regarding the operation flow in the claims, the specification, and the drawings, even if the description is made using "first," "next," etc. for the sake of convenience, it means that it is essential to carry out in this order. not a thing

DESCRIPTION OF SYMBOLS 9... Substrate 9a... Insulating layer 10... Irradiation part 11... Light source 12... Irradiation optical system 20... Detection part 21... Probe 22... Voltage source 23... Ammeter 30... Light detection part, 31... Detection optical system, 32... Spectroscope, 40... Positioning device, 41... Driving element, 50... Control section, 51... Input device, 52... Output device, 90... Vacuum chamber, 91... Vacuum pump, 92... Cryostat, P... localized plasmon, S... sample, Vb... bias voltage.

Claims (15)

A scanning tunneling microscope for detecting tunneling currents induced by photoexcitation of a sample, comprising:
a positioning device that positions the probe in a separation direction with respect to the sample;
an irradiation unit that irradiates the sample with excitation light;
a detection unit that applies a bias voltage between the substrate supporting the sample and the probe and detects a tunnel current flowing between the substrate and the probe;
scanning tunneling microscope. 2. The scanning tunneling microscope according to claim 1, further comprising a controller for controlling direction and magnitude of said bias voltage. The controller controls the bias voltage between two voltages at which the tunnel current starts to increase in a positive current direction and a negative current direction in response to an increase or decrease in the bias voltage in a state in which the excitation light is not irradiated. 3. The scanning tunneling microscope of claim 2, wherein 4. A scanning tunneling microscope according to any one of claims 1 to 3, wherein said probe is made of metal containing gold or silver. 5. The scanning tunneling microscope according to any one of claims 1 to 4, wherein said probe has a truncated conical tip supporting a hemisphere on its upper surface. The probe is selected, adjusted, or formed so that the energy range of the emission spectrum of the localized plasmon when the bias voltage is applied between the probe and the substrate includes the excitation energy of the sample. 6. The scanning tunneling microscope according to any one of 1 to 5. 7. The scanning tunneling microscope according to any one of claims 1 to 6, wherein said substrate is made of a noble metal including gold or silver, and the surface supporting said sample is coated with an insulating material. 8. The scanning tunneling microscope according to any one of claims 1 to 7, wherein said irradiation section has a light source that generates said excitation light having a wavelength matching excitation energy of said sample. 9. The scanning tunneling microscope according to any one of claims 1 to 8, wherein said positioning device further positions said probe in two-dimensional directions with respect to said sample on said substrate. 9. The scanning tunneling microscope according to any one of claims 1 to 8, further comprising a chamber for holding said substrate under low temperature and/or under vacuum. A method of detecting a tunneling current induced by photoexcitation of a sample, comprising:
positioning the probe in a spaced apart direction with respect to the sample;
applying a bias voltage between a substrate supporting the sample and the probe;
irradiating the sample with excitation light;
detecting a tunneling current flowing between the substrate and the probe;
How to prepare. 12. The method of claim 11, further comprising controlling the direction and magnitude of said bias voltage. 11. The step of selecting or adjusting the probe such that the energy range of the emission spectrum of the localized plasmon when the bias voltage is applied between the probe and the substrate includes the excitation energy of the sample. Or the method according to 12. 14. The method of any one of claims 11-13, further comprising positioning the probe in two dimensions with respect to a sample on the substrate. 15. The method of any one of claims 11-14, further comprising holding the substrate in a chamber under low temperature and/or vacuum.

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