Scanning tunneling microscope and method

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

[Technical Field]

[0001] The present invention relates to a scanning tunneling microscope and method for detecting a tunneling current induced by optical excitation of a sample. [Background technology]

[0002] Conventionally, a scanning tunneling microscope (STM) has been known in which a sharp metal probe is brought close to a sample and the probe is moved horizontally over the sample while detecting the tunneling current flowing between the probe and the sample (see, for example, Patent Document 1). Here, the tunneling current is measured while maintaining a constant distance between the probe and the sample, or the distance between the probe and the sample is controlled to maintain a constant tunneling current. STMs can observe a sample three-dimensionally with atomic-molecular resolution by using a piezoelectric element to drive the probe horizontally and vertically with atomic-molecular resolution. Patent Document 1: International Publication No. 2014 / 054741 Summary of the Invention [Problem to be solved by the invention]

[0003] The present invention provides a scanning tunneling microscope and method that enables observation with high resolution, particularly with a resolution smaller than the atomic or molecular size, such as sub-nano order (also called atomic resolution), by detecting a tunneling current (also called photocurrent) induced by optically exciting a sample. [Means for solving the problem]

[0004] In a first aspect of the present invention, there is provided a scanning tunneling microscope that detects a tunneling current induced by optically exciting a sample, the scanning tunneling microscope comprising: a positioning device that positions a probe in a direction away from the sample; an irradiation unit that irradiates the sample with excitation light; and a detection unit that applies a bias voltage between a substrate that supports the sample and the probe, and detects the tunneling current that flows between the substrate and the probe.

[0005] In a second aspect of the present invention, there is provided a method for detecting a tunneling current induced by optical excitation of a sample, the method comprising the steps of positioning a probe in a direction away from the sample, applying a bias voltage between the probe and a substrate supporting the sample, irradiating the sample with excitation light, and detecting the tunneling current flowing between the substrate and the probe.

[0006] The above summary of the invention does not list all of the features of the present invention, and subcombinations of these features may also be inventions. [Brief explanation of the drawings]

[0007] [Figure 1A] 1 shows a schematic configuration of a scanning tunneling microscope according to the present embodiment. [Figure 1B] 1 shows the configuration of the detection unit. [Figure 1C] The control system configuration is shown below. [Figure 2] 10 shows the results of measuring photocurrent as a function of the distance between the sample and the probe using the scanning tunneling microscope according to this embodiment. [Figure 3] 10 shows a flow of a photocurrent detection operation by the scanning tunneling microscope according to the present embodiment. [Figure 4A] 1 shows the results of measuring photocurrent from a sample or substrate under irradiation with excitation light. [Figure 4B] 1 shows the measurement results of photocurrent from a sample versus wavelength of excitation light. [Figure 5] Photocurrent / dark current images of the sample detected with the excitation light on and off are shown. [Figure 6A] 1 shows the bias voltage dependence of dark current when the excitation light is off. [Figure 6B] A dark current image of the sample with the excitation light off is shown. [Figure 6C] A photocurrent image of the sample with excitation light on is shown. [Figure 7A]1 shows the bias voltage dependence of the photocurrent from the sample with excitation light on. [Figure 7B] The photocurrent image of the sample detected at a bias voltage of −0.25 V is shown. [Figure 8] The contributions of three photocurrent generation channels to the bias voltage dependence of the photocurrent are shown. [Figure 9A] A model of the photocurrent generation process (P1 channel) is shown. [Figure 9B] A model of the photocurrent generation process (N1 channel) is shown. [Figure 9C] A model of the photocurrent generation process (N2 channel) is shown. [Figure 10A] 1 shows the tip shape of a first comparative probe. [Figure 10B] 1 shows the measurement results of the emission spectrum of plasmons generated using the first comparative probe. [Figure 10C] 1 shows a photocurrent image of the sample measured using the first comparative probe. [Figure 11A] The tip shape of a second comparative probe is shown. [Figure 11B] 10 shows the measurement results of the emission spectrum of plasmons generated using a second comparative probe. [Figure 11C] A photocurrent image of the sample measured using a second comparative probe is shown. [Figure 12A] 1 shows the tip shape of a probe according to an embodiment. [Figure 12B] 10 shows the measurement results of the emission spectrum of plasmons generated using the probe according to the example. [Figure 12C] 10 shows a photocurrent image of a sample measured using a probe according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0008] The present invention will be described below through embodiments of the invention, but the following embodiments do not limit the scope of the invention according to the claims. Furthermore, not all of the combinations of features described in the embodiments are necessarily essential to the solution of the invention.

[0009] 1A, 1B, and 1C show the schematic configuration of a scanning tunneling microscope (STM) 100 according to this embodiment, the configuration of a detection unit 20, and the configuration of a control system, respectively. The STM 100 is a scanning tunneling microscope that detects a tunneling current (also called a photocurrent) induced by optical excitation of a sample, and includes a vacuum chamber 90, an irradiation unit 10, a detection unit 20, a photodetection unit 30, a positioning device 40, and a control unit 50. A substrate 9 supporting a sample S is housed within the vacuum chamber 90. The sample S can be made of a conductive material such as a conductive substrate or a polymer. In this embodiment, metal-free phthalocyanine (FBPc) is used as the sample S.

[0010] The substrate 9 is a plate-like member that supports the sample S, and is made of a noble metal including gold or silver. In this embodiment, as an example, a silver single crystal (111) plane is used. This makes it easier to generate localized plasmons P between the substrate 9 and the probe 21. Furthermore, in this embodiment, the surface of the substrate 9 that supports the sample S is covered with an insulating layer 9a.

[0011] The insulating layer 9a is formed on the surface of the substrate 9 using, for example, sodium chloride (NaCl) to a thickness of 4 monolayers (atomic layers). By insulating the sample S from the substrate 9 by the insulating layer 9a, it is possible to suppress energy dissipation of the photoexcited sample S. The insulating layer 9a may be formed of an insulating material suitable for supporting the sample S, and may be, for example, a part of the sample S, and may not necessarily cover the entire surface of the substrate 9 but only the part that supports the sample S.

[0012] The vacuum chamber 90 is a container that holds the substrate 9 supporting the sample S under low-temperature vacuum, and is connected to a vacuum pump 91 and includes a cryostat 92 therein. The inside of the vacuum chamber 90 is heated by the vacuum pump 91 to, for example, 10 -10 The vacuum level is maintained at Torr and the temperature is maintained at, for example, liquid helium temperature (approximately 4 K) by the cryostat 92. Placing the sample S at a low temperature and under vacuum enables photocurrent detection with little noise. Depending on the type of sample S and the target resolution, the sample S may be placed under low temperature and atmospheric pressure, under vacuum at room temperature, or under atmospheric pressure at room temperature.

[0013] 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 that matches the excitation energy of the sample S. In this embodiment, for example, a tunable laser such as a dye laser or semiconductor laser that generates laser light having a wavelength within a wavelength range of 0.32 to 1.2 μm is used. The irradiation optical system 12 includes a lens element that focuses light, a filter element that cuts noise light, etc., and shapes the excitation light using these optical elements, and sends the excitation light toward the sample S on the substrate 9 through a window (not shown) provided in the vacuum chamber 90. The irradiation optical system 12 may also include an openable and closable shutter that blocks the excitation light.

[0014] 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, and detects the tunneling current that flows between the substrate 9 and the probe 21, and includes the probe 21, a voltage source 22, and an ammeter 23. Here, the probe 21 is clamped to the ground, and the voltage source 22 and the ammeter 23 are connected in series between the probe 21 and the substrate 9. Note that the substrate 9 may also be clamped to the ground.

[0015] The probe 21 is a probe for detecting tunneling current, and is made of a metal containing gold or silver. By forming the probe 21 from a metal, localized plasmon P is more likely to be generated between the probe 21 and the substrate 9. The probe 21 includes, for example, a cylindrical base end and a tip end that extends from the base end in a tapered cone shape. The shape and method of forming the probe 21, which can efficiently detect tunneling current, will be described later.

[0016] The probe 21 can be selected, adjusted, or formed so that the wavelength (energy) range of the emission spectrum of localized plasmons P generated when a bias voltage Vb is applied between the probe 21 and the substrate 9 includes the excitation energy of the target sample S. Here, it is desirable that the emission spectrum distribution of the localized plasmons P exhibits a peak approximately at the excitation energy of the sample S. It is even more desirable that the peak energy of the emission spectrum distribution of the localized plasmons P coincides with the excitation energy of the target sample S. The tip shape of the probe 21 may be adjusted by piercing and retracting the tip of the probe 21 into the surface of the substrate 9, or by repeating this process. Alternatively, an appropriate probe 21 may be selected from multiple probes 21. Alternatively, the tip of the probe 21 may be processed using a focused ion beam. The emission spectrum can be detected by a light detection unit 30, which will be described later.

[0017] 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 a control unit 50, which will be described later.

[0018] The ammeter 23 measures the tunnel current flowing between the substrate 9 and the probe 21. The measurement result is sent to the control unit 50, which will be described later.

[0019] The light detection unit 30 is a unit that detects light emitted from the sample S, and includes a detection optical system 31 and a spectroscope 32. The detection optical system 31 includes a lens element that focuses light, a filter element that cuts excitation light and noise light, and receives light (emitted light) emitted from the sample S through a window (not shown) provided in the vacuum chamber 90, shapes the emitted light using these optical elements, and sends it to the spectroscope 32. The spectroscope 32 performs spectroscopic measurement of the emitted light from the sample S using, for example, a CCD element. The measurement results are sent to the control unit 50, which will be described later.

[0020] Positioning device 40 is a device that drives probe 21 in three-dimensional directions, namely, the direction away from sample S or substrate 9 supporting it (vertical direction) and the horizontal direction, and positions it at a target position, and includes driving element 41. In this embodiment, probe 21 is driven relative to sample S. Driving element 41 has a Z driving element for driving probe 21 in the vertical direction (Z-axis direction) relative to sample S, and an X driving element and a Y driving element for driving probe 21 in two mutually orthogonal horizontal directions (X-axis direction and Y-axis direction). For example, piezoelectric elements can be used as the Z driving element, X driving element, and Y driving element, respectively.

[0021] The control unit 50 is a unit that controls the operation of the STM 100. The control unit 50 performs its function, namely, the function of detecting photocurrent by linking the irradiation unit 10, detection unit 20, photodetection unit 30, and positioning device 40, by executing a dedicated program on a computing device such as a personal computer. The photocurrent detection operation of the STM 100 performed by the control unit 50 will be described later. The control unit 50 also performs control functions such as controlling the bias voltage Vb during photocurrent detection and positioning the probe 21. The control unit 50 has input devices 51, such as a keyboard, mouse, or touch panel, that receive command inputs for the user to operate the STM 100, and an output device 52, such as a display, that displays measurement results.

[0022] In controlling the bias voltage Vb, the control unit 50 controls the direction and magnitude of the bias voltage Vb, thereby controlling the photocurrent generation process, as will be described later.

[0023] In controlling the positioning of probe 21, control unit 50 controls drive element 41 to drive probe 21 to each target position in the X-axis, Y-axis, and Z-axis directions, and positions probe 21 at each target position.

[0024] Figure 2 shows the photocurrent measurement results for various distances between the substrate 9 and the probe 21 using the STM 100 during the photocurrent detection operation described below. The probe 21 was positioned on a node of the sample S, FBPc. The current value was nearly zero, i.e., almost no photocurrent was detected, within the distance range of 0.53 to 1.11 nm. However, the current value exponentially increased within the distance range of 0.35 to 0.53 nm. This suggests that the tunneling probability increased as the distance decreased, and localized plasmons P containing the excitation energy of the sample S within the wavelength (energy) range of the emission spectrum were generated and enhanced, thereby enhancing the generation of photocurrent. Therefore, by positioning the probe 21 at an appropriate distance from the sample S, for example, within the range of 0.35 to 0.53 nm, the photocurrent flowing between the probe 21 and the substrate 9 can be increased. Furthermore, by driving the probe 21 in a direction parallel to the surface of the substrate 9 and sequentially positioning it above any local portion of the sample S to measure the photocurrent, the sample S can be scanned two-dimensionally with high precision.

[0025] FIG. 3 shows a flow of the photocurrent detection operation by the STM 100 according to this embodiment.

[0026] In step S101, an appropriate probe 21 is selected from a plurality of probes prepared in advance.

[0027] In step S102, 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 maintain the substrate 9 in the vacuum chamber 90 under low-temperature vacuum.

[0028] In step S104, the probe 21 is adjusted. First, the control unit 50 controls the positioning device 40 to drive the probe 21, and adjusts the shape of the tip of the probe 21 by inserting and withdrawing the tip of the probe 21 into the surface of the substrate 9. Next, the control unit 50 controls the positioning device 40 to position the probe 21 at an appropriate distance (for example, 0.35 to 0.53 nm) from the sample S, controls the detection unit 20 to apply a bias voltage Vb between the probe 21 and the substrate 9 to generate localized plasmons P, and controls the photodetection unit 30 to measure the emission spectrum of the localized plasmons P. The results are displayed on the output device 52. The user confirms that the wavelength (energy) range of the measured emission spectrum of the localized plasmon P includes the excitation energy of the target sample, preferably that the emission spectrum distribution of the localized plasmon P exhibits a peak approximately at the excitation energy of the target sample S (for example, the excitation energy of the sample S is included within a half-value range of the peak value of the emission spectrum, preferably within a 60% value range, more preferably within a 70% value range, and even more preferably within an 80% value range), and more preferably that 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 to the input device 51 to select that probe 21, and the process moves to the next step.

[0029] Instead of or in addition to adjusting the tip shape of probe 21 by piercing and withdrawing the tip of probe 21 from the surface of substrate 9, the tip of probe 21 may be processed using a focused ion beam, as described below.

[0030] 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 again measures the emission spectrum of the localized plasmon P. This makes it possible to select a probe 21 suitable for photocurrent detection.

[0031] If a suitable probe 21 has already been selected, step S104 may be omitted.

[0032] In step S106, the control unit 50 controls the detection unit 20 to apply a bias voltage Vb between the substrate 9 and the probe 21. Here, the control unit 50 controls the direction (positive or negative) and magnitude (voltage value) of the bias voltage Vb to select a direction and magnitude suitable for photocurrent detection within the range of −2.25 to +0.9 V for FBPc, which is the sample S, for example.

[0033] In step S108, the control unit 50 controls the positioning device 40 to drive the probe 21 in the direction away from the sample S (Z-axis direction) and / or in the horizontal direction (X-axis direction and Y-axis direction), and position it over any local area of ​​the sample S on the substrate 9.

[0034] In step S110, the control unit 50 controls the irradiation unit 10 to irradiate excitation light onto the sample S supported on the substrate 9. Here, the excitation light has a wavelength that matches the excitation energy of the sample S from its ground state to the excited state of the detection target.

[0035] In step S112, the control unit 50 controls the detection unit 20 to detect a tunneling current (which may include a photocurrent component and a dark current component) flowing between the substrate 9 and the probe 21. The value of the detected tunneling current is transmitted to the control unit 50 together with the position of the probe 21 relative to the substrate 9 in the horizontal direction.

[0036] In step S114, it is determined whether or not scanning is to be ended. If not, the process proceeds to step S116. If yes, the process proceeds to step S118.

[0037] In step S116, control unit 50 controls positioning device 40 to drive probe 21 in the horizontal direction (X-axis direction and Y-axis direction) relative to sample S, and position it above the next local portion of sample S on substrate 9. After positioning, the process proceeds to step S110.

[0038] By repeating steps S110 to S116, the sample S is scanned two-dimensionally.

[0039] 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, and displays it on the output device 52. Once the control unit 50 has saved the processed data in a storage device (not shown), the flow ends.

[0040] We will now explain the results of measuring the photocurrent using the photocurrent detection operation of the STM 100. As the sample S, FBPc (see Figure 1B for its molecular structure) is used.

[0041] Figure 4A shows the results of measuring photocurrent under irradiation with excitation light. Here, the positioning device 40 was used to position the probe 21 on the sample S and the substrate 9 (insulating layer 9a), respectively. At each probe position, the irradiation unit 10 was used to open and close a shutter (not shown) to intermittently irradiate the sample S (FBPc) with excitation light, for example, at a frequency of 2 Hz. The detection unit 20 then detected the tunneling current (i.e., photocurrent) flowing between the probe 21 and the substrate 9. The separation distance between the probe 21 and the sample S was 0.48 nm, and the bias voltage Vb was −2.0 V. The excitation light energy was selected to be 1816 meV, the excitation energy from the ground state (S0) of FBPc to the excited state (S1). When the probe 21 was positioned on the sample S, a photocurrent was detected in response to irradiation (ON) of excitation light. However, when the probe 21 was irradiated onto the substrate 9 (insulating layer 9a), only dark current (noise) was detected. This result indicates that a tunnel current (i.e., photocurrent) induced by excitation of the sample S due to irradiation with excitation light (i.e., photoexcitation) was detected.

[0042] Figure 4B shows the measurement results of the photocurrent as a function of the wavelength of the excitation light. Here, the energy (wavelength) of the excitation light was adjusted in the range of 1800 to 1820 meV (689 to 681 nm), and the probe 21 was positioned above the sample S as described above to measure the photocurrent. The measured photocurrent value exhibited a peak at an energy that matched the excitation energy of FBPc (1816 meV). The half-width of this peak was a very narrow 1.45 meV. This result suggests that the photocurrent increases when the energy of the excitation light matches the excitation energy of the sample S, thereby enabling efficient detection of the photocurrent.

[0043] Figure 5 shows an image of the tunneling current (photocurrent / dark current) of the sample S detected by irradiating (ON) / blocking (OFF) the excitation light. Here, the positioning device 40 was controlled to drive the probe 21 by a unit pitch in the horizontal direction (X-axis direction and Y-axis direction) relative to the sample S on the substrate 9, and the photocurrent was detected while successively positioning it above the next local part of the sample S on the substrate 9. The distance between the probe 21 and the sample S was 0.5 nm, and the bias voltage Vb was -2.0 V. The unit pitch was 0.06 nm. Therefore, the size of one pixel was 0.06 nm. 2 When the excitation light is blocked (no irradiation), only noise associated with the dark current can be confirmed, but when excitation light is irradiated, the current value distribution resulting from the two-dimensional structure of FBPc can be confirmed, particularly the center with low current value (i.e., density of states), the high current lobes located at each vertex of the octagon, and the low current nodes located between them. This result means that photocurrent measurements have enabled us to obtain two-dimensional imaging of the spatial distribution of sample S, FBPc, with atomic resolution.

[0044] Figure 6A shows the bias voltage dependence of the tunneling current (dark current) with the excitation light off. Here, the tunneling current was measured by positioning the probe 21 above the sample S as described above while varying the bias voltage Vb between 2.0 and -3.0 V. At bias voltages Vb between -2.0 and 0.7 V, no dark current was detected; only noise was detected. In contrast, at bias voltages below -2.1 V, the dark current increased in the negative direction (i.e., negative dark current was detected). At bias voltages above 0.7 V, the dark current increased in the positive direction (i.e., positive dark current was detected). Note that dI / dVb peaks at bias voltages of 0.9 V and -2.25 V.

[0045] Figure 6B shows tunneling current images (i.e., dark current images) of sample S detected at (1) a bias voltage Vb = 0.75 V and (2) a bias voltage Vb = -2.1 V with the excitation light off. The measurement method was 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 sample S. Therefore, the above results indicate that a dark current channel via the LUMO of sample S opens at bias voltages Vb = 0.7 V or higher. Note that the LUMO has a finite density of states at the nodes indicated by the four arrows in the figure. 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 indicate that a dark current channel via the HOMO of sample S opens at bias voltages Vb = -2.25 V or lower. Note that the HOMO has a nearly zero density of states at the nodes indicated by the four arrows in the figure. These results suggest that by selecting the bias voltage Vb, it is possible to select a dark current channel via a specific molecular orbital and thus select the state of sample S to be observed.

[0046] Figure 6C shows photocurrent images of sample S detected with excitation light on at (3) a bias voltage Vb = -2.0 V and (4) -0.0 V. The photocurrent image obtained in (3) reflects the density of states of the LUMO of sample S due to the negative current distribution. The photocurrent image obtained in (4) reflects the density of states of the HOMO of 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, suggesting that it is possible to select the photocurrent generation channel and the state of sample S to be observed.

[0047] Figure 7A shows the bias voltage dependence of photocurrent with excitation light on. Here, the photocurrent was measured by positioning the probe 21 above the sample S as described above while varying the bias voltage Vb between -0.5 and 0.0 V. 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. In (A), the photocurrent I increases as the bias voltage Vb increases, crosses the zero line at approximately -0.33 V, and then gradually increases as the bias voltage Vb increases further. In (B), the photocurrent I also increases as the bias voltage Vb increases, crosses the zero line at approximately -0.16 V, and saturates to approximately zero as the bias voltage Vb increases further. These results show that the zero cross voltage of the photocurrent I varies depending on the position of the probe 21, that is, the direction of the photocurrent changes depending on the position of the probe 21 when the bias voltage Vb is between -0.33 and -0.16V.

[0048] Figure 7B shows a photocurrent image of sample S detected at a bias voltage of Vb = -0.25 V. The average photocurrent across the entire two-dimensional image of sample S (FBPc) is zero, but there are positive currents at eight lobes resulting from the HOMO density of states distribution of FBPc, and negative currents at four nodes and the center resulting from the LUMO density of states distribution. This shows that even within a single molecule, the direction of the detected photocurrent changes (i.e., locally) depending on the positioning of the tip 21. This suggests that multiple photocurrent generation channels exist within a molecule, each with a different spatial distribution. Therefore, depending on the position of the tip 21, one of the generation channels becomes dominant, suggesting that it is possible to image the molecular orbitals associated with that generation channel.

[0049] Figure 8 shows the contributions of the three photocurrent generation channels to the bias voltage dependence of the photocurrent. The three photocurrent generation channels are known to be the P1 channel via the HOMO of sample S, the N1 channel via the LUMO of sample S, and the N2 channel. The P1 channel generates a nearly constant positive current with little dependence on the bias voltage Vb. The positive current reflects that it is a HOMO-mediated channel. The N1 channel generates a nearly constant negative current with little dependence on the bias voltage Vb. The N2 channel has a threshold voltage specific to sample S (e.g., approximately -0.2 V for FBPc), and increases the photocurrent in the negative direction when the bias voltage is below this threshold. The negative current reflects that it is a LUMO-mediated channel. The bias voltage dependence of the photocurrent is determined by the sum of the contributions of these three generation channels. Therefore, the photocurrent is nearly constant for bias voltages above the threshold and increases in the negative direction for bias voltages below the threshold.

[0050] Figure 9A shows a model of the P1 channel. Assume that the Fermi levels of the substrate 9 and the probe 21 are the same. First, upon absorption of excitation light, one electron occupying the HOMO of sample S transitions to the LUMO, exciting sample S from the ground state (S0) to the excited state (S1). Next, an electron transfers from the probe 21 to the HOMO of sample S, causing sample S to assume an anionic (negative ion) state. Due to the Coulomb force between multiple electrons, including the transferred electron, the electron orbital of sample S rises, raising the LUMO above the Fermi level of the substrate 9. Finally, the electron occupying the LUMO transfers to the substrate 9. Therefore, in the P1 channel, a nearly constant positive current is generated, almost independent of the bias voltage Vb, in the range of -0.5 to 0 V.

[0051] Figure 9B shows a model of the N1 channel. Assume that the Fermi levels of the substrate 9 and the probe 21 are the same. First, upon absorption of excitation light, one electron occupying the HOMO of sample S transitions to the LUMO, exciting sample S from the ground state (S0) to the excited state (S1). Next, an electron transfers from the substrate 9 to the HOMO of sample S, causing sample S to assume an anionic (negative ion) state. Due to the Coulomb force between multiple electrons, including the transferred electron, the electron orbital of sample S rises, raising the LUMO above the Fermi level of the probe 21. Finally, the electron occupying the LUMO transfers to the probe 21. Therefore, in the N1 channel, a nearly constant negative current is generated, almost independent of the bias voltage Vb, in the range of -0.5 to 0 V.

[0052] Figure 9C shows a model of the N2 channel. Assume that applying a negative bias voltage Vb lowers the Fermi level of the tip 21 relative to the Fermi level of the substrate 9 and further lowers it below the LUMO of the sample S. First, by absorbing excitation light, one electron occupying the HOMO of the sample S transitions to the LUMO, exciting the sample S from the ground state (S0) to the excited state (S1). Next, the electron occupying the LUMO transfers to the tip 21, which places the sample in a cationic (positive ion) state. The Coulomb force between the remaining electrons, excluding the transferred electron, causes the electron orbital of the sample S to descend, lowering the LUMO below the Fermi level of the tip 21. Finally, an electron transfers 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. When the bias voltage lowers the Fermi level of the tip 21, a photocurrent that increases in the negative direction is generated.

[0053] When the sum of the photocurrents from each of the three photocurrent generation channels is calculated by giving a given LUMO and HOMO state ratio (bonding ratio between the tip 21 and the molecular orbital), the I-Vb characteristics when the LUMO state ratio is relatively large are shown in the measurement results obtained by positioning the tip 21 on the lobe (Figure 7A (A)), and when the HOMO state ratio is relatively large are shown in the measurement results obtained by positioning the tip 21 on the node (Figure 7A (B)). The spatial distribution of the photocurrent and its bias voltage dependence revealed the mechanism by which the photocurrent is generated. Furthermore, by visualizing the spatial distribution of the molecular orbitals of sample S, it was found that the excited state to be observed can be selected by the wavelength of the excitation light, and that the molecular orbital to be visualized can be selected by controlling the bias voltage Vb.

[0054] Therefore, the control unit 50 can control the bias voltage Vb between two voltages (for example, −2.0 to 0.5 V in FIG. 6A) at which the dark current begins to increase in the positive and negative directions with increasing or decreasing the bias voltage without irradiating excitation light. As a result, by selecting the bias voltage Vb so that the N2 channel in particular opens, irradiating excitation light, and detecting the positive photocurrent via the HOMO and the negative photocurrent via the LUMO of the sample S, it is possible to perform two-dimensional imaging of the spatial distribution of the molecular orbitals of the sample, such as the HOMO or LUMO, or both, as shown in FIGS. 6C and 7B.

[0055] We will now consider the shape of the probe 21 that can generate an appropriate localized plasmon between the probe 21 and the substrate 9 and efficiently detect the photocurrent. Such a probe 21 must be sharp on an atomic or molecular scale, smooth on the order of at least 10 nm, and symmetrical with respect to the central axis.

[0056] Figures 10A, 10B, and 10C show, respectively, the tip shape of the first comparative probe photographed with a scanning electron microscope, the measurement results of the localized plasmon emission spectrum generated when a bias voltage Vb of +2.5 V was applied between the first comparative probe and the substrate 9, and a photocurrent image of a sample measured using the first comparative probe. FBPc was used as the sample. The bias voltage Vb was set to -2.0 V. The first comparative probe was made of gold. However, its tip shape was blunt, uneven, and asymmetrical about the central axis, on an atomic or molecular scale. The plasmon emission spectrum included the excitation energy of the sample (1816 meV) within its distribution range. The photocurrent image of the sample was inhomogeneous in intensity and unclear. The absence of a symmetrical structure of the sample suggests that the central axis symmetry of the probe is important for photocurrent imaging.

[0057] Figures 11A, 11B, and 11C show, respectively, the tip shape of the second comparative probe photographed with a scanning electron microscope, the measurement results of the localized plasmon emission spectrum generated when a bias voltage Vb of +2.5 V was applied between the second comparative probe and the substrate 9, and a photocurrent image of the sample measured with the second comparative probe. FBPc was used as the sample. The bias voltage Vb was set to -0.25 V. The second comparative probe was fabricated using gold by electrochemical etching, and the tip shape was further adjusted using the method in step S104. The tip shape is sharp and symmetrical about the central axis, with some irregularities, at atomic molecular size. The plasmon emission spectrum includes the excitation energy of the sample (1816 meV) within its distribution range, but the center of the spectrum is away from the excitation energy. The photocurrent image of the sample shows a symmetric intensity distribution, indicating the symmetric structure of the sample, but is somewhat blurred.

[0058] Figures 12A, 12B, and 12C show, respectively, the tip shape of the example probe photographed with a scanning electron microscope, the measurement results of the localized plasmon emission spectrum generated when a bias voltage Vb of +2.5 V was applied between the probe and the substrate 9, and a photocurrent image of a sample measured with the comparative probe. FBPc was used as the sample. The bias voltage Vb was set to -0.25 V. The example probe was fabricated using gold by electrochemical etching and then processed by irradiating the tip with a focused ion beam from the front in a doughnut-shaped pattern. The tip is formed in the shape of a truncated cone (a cross-sectional diameter of approximately 60 nm at a distance of approximately 100 nm from the tip) supporting a hemisphere (a diameter of approximately 30 nm in this example) on its upper surface. In other words, it is smooth on the order of at least 10 nm and symmetrical about the central axis. Furthermore, the tip shape was adjusted to be sharp on the atomic molecular scale using the method of step S104. The plasmon emission spectrum includes the excitation energy of the sample (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 included within approximately 80% of the peak value of the emission spectrum. The photocurrent image of the sample has a symmetrical intensity distribution, clearly showing the symmetrical structure of the sample. Therefore, it can be seen that the probe 21 with the tip shape described above is suitable for obtaining clear photocurrent imaging.

[0059] The STM 100 according to this embodiment is a scanning tunneling microscope that detects a tunneling current (photocurrent) induced by optical excitation of a sample, and includes a positioning device 40 that positions a probe 21 in a direction away from the sample S, an irradiation unit 10 that irradiates excitation light onto the sample S supported on a substrate 9, and a detection unit 20 that applies a bias voltage Vb between the substrate 9 and the probe 21 to detect the tunneling current flowing between the substrate 9 and the probe 21. In this configuration, the irradiation unit 10 irradiates the sample S supported on the substrate 9 with excitation light, and the detection unit 20 uses the probe 21 positioned on the substrate 9 to detect the tunneling current (i.e., photocurrent) flowing between the probe 21 and a local portion of the sample S facing it. By applying a bias voltage Vb between the substrate 9 and the probe 21 or irradiating light between them, localized plasmons P are generated, which increases the photocurrent due to interaction between the localized plasmons P and the sample S. This enables atomic-resolution observation of the sample S using the photocurrent, particularly two-dimensional image detection.

[0060] The method according to this embodiment is a method for detecting a tunneling current (photocurrent) induced by optical excitation of a sample, and includes the steps of positioning a probe in a direction away from the sample S, applying a bias voltage Vb between the substrate 9 and the probe 21, irradiating the sample S supported on the substrate 9 with excitation light, and detecting the tunneling current flowing between the substrate 9 and the probe 21. According to this method, by positioning the probe 21 in a direction away from the sample S, applying a bias voltage Vb between the substrate 9 and the probe 21 or irradiating the sample S with excitation light to generate localized plasmons P, and irradiating the sample S supported on the substrate 9 with excitation light and detecting the tunneling current (photocurrent) flowing between the substrate 9 and the probe 21, the photocurrent can be increased by the interaction between the localized plasmons P and the sample S, thereby enabling atomic resolution observation of the sample S using the photocurrent, particularly two-dimensional image detection.

[0061] Although the present invention has been described above using embodiments, the technical scope of the present invention is not limited to the scope described in the above embodiments. It will be apparent to those skilled in the art that various modifications and improvements can be made to the above embodiments. It is clear from the claims that such modifications and improvements can also be included within the technical scope of the present invention.

[0062] It should be noted that the execution order of each process, such as operations, procedures, steps, and stages, in the devices, systems, programs, and methods shown in the claims, specifications, and drawings is not specifically stated as "before," "prior to," etc., and that the processes can be performed in any order unless the output of a previous process is used in a subsequent process. Even if the operational flow in the claims, specifications, and drawings is described using "first," "next," etc. for convenience, this does not mean that the processes must be performed in this order. [Explanation of symbols]

[0063] 9...substrate, 9a...insulating layer, 10...irradiation unit, 11...light source, 12...irradiation optical system, 20...detection unit, 21...probe, 22...voltage source, 23...current meter, 30...light detection unit, 31...detection optical system, 32...spectroscope, 40...positioning device, 41...driving element, 50...control unit, 51...input device, 52...output device, 90...vacuum chamber, 91...vacuum pump, 92...cryostat, P...localized plasmon, S...sample, Vb...bias voltage.

Claims

1. 1. A scanning tunneling microscope for detecting a tunneling current induced by optical excitation of a sample, comprising: a positioning device for positioning the probe in a direction away from the sample; an irradiation unit that irradiates the sample with excitation light for the photoexcitation; a detection unit that applies a bias voltage between the probe and a substrate that supports the sample, and detects a tunneling current that flows between the substrate and the probe in a state in which the sample is irradiated with the excitation light and localized plasmons are generated between the substrate and the probe; a control unit that processes the detection result of the tunneling current detected by the detection unit with respect to the position of the probe on the substrate in a horizontal direction; A scanning tunneling microscope comprising:

2. 2. The scanning tunneling microscope according to claim 1, wherein the control unit further controls the direction and magnitude of the bias voltage.

3. 3. The scanning tunneling microscope according to claim 2, wherein the control unit controls the bias voltage between two voltages at which the tunneling current begins 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.

4. 4. The scanning tunneling microscope according to claim 1, wherein the probe is formed from a metal including gold or silver.

5. 5. The scanning tunneling microscope according to claim 1, wherein the probe has a tip formed in the shape of a truncated cone that supports a hemisphere on its upper surface.

6. 6. The scanning tunneling microscope according to claim 1, wherein the probe is selected, adjusted, or formed so that an energy range of an emission spectrum of localized plasmons generated when the bias voltage is applied between the probe and the substrate includes an excitation energy of the sample.

7. 7. A scanning tunneling microscope according to claim 1, wherein the substrate is formed from a noble metal including gold or silver, and the surface supporting the sample is covered with an insulating material.

8. 8. The scanning tunneling microscope according to claim 1, wherein the illumination unit comprises a light source that generates the excitation light having a wavelength that matches the excitation energy of the sample.

9. The scanning tunneling microscope according to claim 1 , wherein the positioning device further positions the probe in two dimensions relative to the sample on the substrate.

10. 9. A scanning tunneling microscope according to claim 1, further comprising a chamber for holding the substrate at low temperature and / or under vacuum.

11. 1. A method for detecting a tunneling current induced by optical excitation of a sample, comprising: positioning the probe in a spaced apart direction relative to the sample; applying a bias voltage between a substrate supporting the sample and the probe; irradiating the sample with excitation light for the photoexcitation; detecting a tunneling current flowing between the substrate and the probe in a state in which the excitation light is irradiated onto the sample and localized plasmons are generated between the substrate and the probe; processing the detection result of the tunneling current detected in the detecting step with respect to the position of the probe on the substrate in a horizontal direction; A method for providing

12. The method of claim 11 further comprising controlling the direction and magnitude of the bias voltage.

13. The method of claim 11 or 12, further comprising selecting or adjusting the probe so that an energy range of an emission spectrum of localized plasmons generated when the bias voltage is applied between the probe and the substrate includes an excitation energy of the sample.

14. 14. The method of claim 11, further comprising positioning the probe in two dimensions relative to a sample on the substrate.

15. 15. The method of any one of claims 11 to 14, further comprising holding the substrate at low temperature and / or under vacuum in a chamber.

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