Method for manufacturing single-molecule device using nanogap electrode, and single-molecule device

Abstract

This method for manufacturing a single-molecule device comprises forming a nanogap electrode having a gap length such that the nanogap electrode is bridgeable by a nanomaterial single molecule having two terminal ends, immersing the nanogap electrode in a solution in which the nanomaterial single molecules is dispersed, and irradiating the nanogap electrode with light in the solution to adsorb the nanomaterial single molecule to the nanogap electrode. The irradiation light during light irradiation is linearly polarized pulsed light, and the direction of the polarization axis of the linearly polarized light is parallel to the gap length direction of the nanogap electrode.

Description

Method for fabricating a single-molecule device using a nano-gap electrode and a single-molecule device

[0001] The present disclosure relates to a method for fabricating a single-molecule device in which a single molecule is disposed on a nano-gap electrode and a single-molecule device.

[0002] It is known that quantum effects appear when a substance is reduced to a nano-scale size. For example, a π-conjugated system molecule with a π-conjugated group length of 20 nm or less has a discrete state density of electrons and has a band gap as a semiconductor or an energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).

[0003] In order to fabricate a single-molecule device using such a molecule having a nano-scale size, it is necessary to controllably arrange a single molecule at a predetermined position. For example, as a method of arranging a predetermined single molecule between a pair of electrodes, a break junction method using a scanning tunneling microscope (STM) and a mechanically controllable break junction method using a leaf spring-shaped substrate have been disclosed (see Non-Patent Document 1).

[0004] Xiaohui Li, Zhibing Tan, Xiaojuan Huang, Jie Bai, Junyang Liu, Wenjing Hong, “Experimental investigation of quantum interference in charge transport through molecular architectures”, Journal of Materials Chemistry C 7 (41), 2019, 12790-12808.

[0005] Since the break junction method and the mechanically controllable break junction method disclosed in Non-Patent Document 1 arrange molecules by a movable mechanism, it is difficult to integrate elements at a high density like a silicon semiconductor integrated circuit, and it is a problem that it cannot be said to be a highly productive technology. In order to fabricate a single-molecule device using a single molecule, it is necessary to establish a new manipulation technology capable of handling materials at the molecular level.

[0006] A method for fabricating a single-molecule device according to one embodiment of the present disclosure includes forming a nanogap electrode having a gap length to which a single molecule having two ends can be crosslinked, immersing the nanogap electrode in a solution in which single molecules are dispersed, and irradiating the nanogap electrode with light in the solution to adsorb the single molecules onto the nanogap electrode.

[0007] A method for fabricating a quantum single-molecule device according to one embodiment of this disclosure includes irradiating a nanogap electrode with pulsed light. The pulsed light may be linearly polarized pulsed light. The polarization axis of the linearly polarized light may be parallel to the gap length direction of the nanogap electrode. The pulsed light may be infrared laser light.

[0008] A method for fabricating a quantum single-molecule device according to one embodiment of the present disclosure includes adsorbing at least one terminal group of a single molecule onto a nanogap electrode, and crosslinking the single molecule between the gaps of the nanogap electrode.

[0009] In a method for fabricating a quantum single-molecule device according to one embodiment of the present disclosure, the single molecule is a π-conjugated molecule, and the π-conjugated molecule may contain a linker group and an anchor group at one end and the other end of the π-conjugated skeleton. The linker group may contain a different number of phenylene groups and methylene groups, and the anchor group may contain a thiol group.

[0010] In a method for fabricating a quantum single-molecule device according to one embodiment of the present disclosure, the concentration of the solution may be from 1 × 10⁻⁵ M to 1 × 10⁻⁹ M.

[0011] A monomolecule device according to one embodiment of the present disclosure comprises a nanogap electrode in which one electrode and the other electrode are arranged to have a nanoscale gap, a monomolecule placed in the gap of the nanogap electrode, and a gate electrode that applies an electric field to the gap of the nanogap electrode. The monomolecule includes a π-conjugated skeleton and terminal groups at one end and the other end of the π-conjugated skeleton, wherein the structures of the terminal groups at one end and the other end are different.

[0012] In a monomolecule device according to one embodiment of the present disclosure, the terminal group includes a linker group that bonds to a π-conjugated skeleton and an anchor group that bonds to the linker group, and the molecular weight of at least the linker group may differ between one terminal and the other terminal.

[0013] In a monomolecule device according to one embodiment of the present disclosure, one of the terminals may contain biphenyl as a linker group.

[0014] In a monomolecule device according to one embodiment of the present disclosure, one terminal linker group may be terphenyl and the other terminal linker group may be biphenyl.

[0015] According to a method for fabricating a single-molecule device as embodied in this disclosure, plasmon resonance is generated on the metal surface of a nanogap electrode by light irradiation, and the resulting photoelectric field is applied to the single molecule, thereby crosslinking the single molecule between the nanogap electrodes and improving the yield of the single-molecule device.

[0016] This is a plan view showing the structure of a single-molecule device according to one embodiment of this disclosure. This is a cross-sectional view showing the structure of a single-molecule device according to one embodiment of this disclosure. This shows a structure in which a single molecule is crosslinked to a nanogap electrode in a single-molecule device according to one embodiment of this disclosure. This is a diagram illustrating the principle of photoelectric field orientation crosslinking in a method for fabricating a single-molecule device according to one embodiment of this disclosure. This is a graph showing the relationship between the laser beam diameter and pulse energy in the photoelectric field orientation treatment of a single molecule in a method for fabricating a single-molecule device according to one embodiment of this disclosure, with the intensity of the photoelectric field as a parameter. This is a diagram illustrating the polarization direction of light irradiated onto the nanogap electrode in a method for fabricating a single-molecule device according to one embodiment of this disclosure. This is a diagram showing the results obtained by determining the strength of the photoelectric field generated between nanogaps and the distribution of the electric field using the finite difference time-domain method (FDTD method) in a method for fabricating a single-molecule device according to one embodiment of this disclosure. This is a diagram illustrating the principle of a method for fabricating a single-molecule device according to one embodiment of this disclosure, illustrating the relationship between the direction of the electric field and the central axis of the molecule. This shows the results of calculating the principle of a method for fabricating a single-molecule device according to one embodiment of this disclosure using first-principles calculations based on density functional theory, showing the relationship between the direction of the photoelectric field and the total energy change of the molecule. This figure illustrates the molecular structure used when calculating the principle of a single-molecule device fabrication method according to one embodiment of this disclosure using first-principles calculations based on density functional theory. This figure shows the results of calculating the principle of a single-molecule device fabrication method according to one embodiment of this disclosure using first-principles calculations based on density functional theory, and shows the relationship between bond rotation at the methylene group portion and the total energy change. This figure shows a structure in which protecting groups are attached to the two ends of a single molecule that can be used in a single-molecule device according to one embodiment of this disclosure. This figure illustrates the phenomenon of protective groups detaching from the single molecule shown in Figure 11A upon light irradiation. This figure illustrates a single-molecule device fabrication method according to one embodiment of this disclosure. This figure illustrates a single-molecule device fabrication method according to one embodiment of this disclosure. This figure illustrates a single-molecule device fabrication method according to one embodiment of this disclosure. This figure illustrates a light irradiation system that can be used when fabricating a single-molecule device according to one embodiment of this disclosure.This is a graph showing the current-voltage characteristics of the nanogap electrode during photoelectric field orientation treatment, representing the characteristics of a single-molecule device according to one embodiment of this disclosure. This is a graph showing the current-voltage characteristics of the nanogap electrode during photoelectric field orientation treatment, representing the characteristics of a single-molecule device according to one embodiment of this disclosure. This is a graph showing the dependence of the single-molecule device according to one embodiment of this disclosure on fabrication conditions. This is a graph showing the dependence of the single-molecule device according to one embodiment of this disclosure on fabrication conditions. This is a graph showing the dependence of the single-molecule device according to one embodiment of this disclosure on fabrication conditions. This is a graph showing the dependence of the single-molecule device according to one embodiment of this disclosure on fabrication conditions. This is a graph showing the dependence of the single-molecule device according to one embodiment of this disclosure on fabrication conditions. This shows the results of in-situ observation of the process when a single molecule is introduced into the nanogap electrode when fabricating a single-molecule device according to one embodiment of this disclosure. This shows the spatial extent of the HOMO orbital of the π-conjugated derivative used in a single-molecule device (single-molecule transistor) according to one embodiment of this disclosure. This shows the transmittance of the π-conjugated derivative used in a single-molecule device (single-molecule transistor) according to one embodiment of this disclosure. This shows the transmittance of the π-conjugated derivative used in a single-molecule device (single-molecule transistor) according to one embodiment of this disclosure. This shows the drain current (Id)-drain voltage (Vd) characteristics of a single-molecule device (single-molecule transistor) according to one embodiment of this disclosure. This shows the drain current (Id)-drain voltage (Vd) characteristics of a single-molecule device (single-molecule transistor) according to one embodiment of this disclosure. This shows the drain current (Id)-gate voltage (Vg) characteristics of a single-molecule device (single-molecule transistor) according to one embodiment of this disclosure. This shows the drain current (Id)-drain voltage (Vg) characteristics of a single-molecule device (single-molecule transistor) according to one embodiment of this disclosure, and the differential conductance (dId / dVd)-drain voltage (Vd) curve calculated from these characteristics. This shows the temperature dependence of the drain current (Id)-gate voltage (Vg) characteristics of a single-molecule device (single-molecule transistor) according to one embodiment of this disclosure.This shows the temperature dependence of the drain current (Id) on and off currents, and the temperature dependence of the on / off ratio of a single-molecule device (single-molecule transistor) according to one embodiment of this disclosure. This shows the temperature dependence of the subthreshold swing (SS) of a single-molecule device (single-molecule transistor) according to one embodiment of this disclosure.

[0017] The embodiments of this disclosure will be described below with reference to the drawings and other figures. However, this disclosure can be implemented in many different ways and should not be interpreted as being limited to the embodiments described below. In order to make the explanation clearer, the drawings may schematically represent the width, thickness, shape, etc. of each part compared to the actual embodiments, but these are merely examples and should not limit the interpretation of this disclosure. In addition, in this specification and each figure, elements similar to those described above with respect to previously shown figures will be denoted with the same reference numerals (or numerals followed by A, B, etc.), and detailed explanations may be omitted as appropriate. Furthermore, the letters "First," "Second," etc., attached to each element are convenient indicators used to distinguish each element and have no further meaning unless specifically explained.

[0018] In this specification, when a member or region is said to be "above (or below)" another member or region, unless otherwise specified, this includes not only cases where it is directly above (or directly below) the other member or region, but also cases where it is above (or below) the other member or region, that is, cases where another component is included between them above (or below) the other member or region.

[0019] In one embodiment of this disclosure, "single-molecule device" refers to a device that can operate in conjunction with quantum mechanical phenomena such as resonant tunneling, and which controls the electronic state of a molecule to realize functions such as a transistor, diode, light-emitting device, or sensor. A single-molecule device according to one embodiment of this disclosure includes a nanogap electrode and a single molecule disposed in the gap of the nanogap electrode.

[0020] In one embodiment of this disclosure, "nanogap electrode" refers to an electrode having a nanoscale gap (nanogap length) between a pair of electrodes, unless otherwise specified. The nanogap length refers to a length of 10 nm or less, preferably 6 nm or less, for example, 0.5 nm to 5 nm.

[0021] 1. Structure of the Monomolecule Device Figures 1A and 1B show the structure of a monomolecule device 100 according to one embodiment of the present disclosure. Figure 1A shows a plan view of the monomolecule device 100, and Figure 1B shows a cross-sectional view corresponding to the area A1-A2 shown in the plan view.

[0022] The single-molecule device 100 includes a nanogap electrode 102 and a single molecule 104 disposed in the gap of the nanogap electrode 102. The nanogap electrode 102 has a structure in which the tip of a first electrode 102A and the tip of a second electrode 102B are facing each other and spaced apart. The distance between the tips of the first electrode 102A and the tips of the second electrode 102B (gap length LG) is 10 nm or less, preferably 6 nm or less, for example, 0.5 nm to 5 nm. The gap length LG of the nanogap electrode 102 is not simply a matter of being small; it is preferable that the gap length LG is such that a single molecule 104 is disposed in the gap of the nanogap electrode 102, and ideally a cross-linked structure is formed in which chemical bonds are formed between the first electrode 102A and the single molecule 104, and further between the same single molecule 104 as the second electrode 102B.

[0023] The nanogap electrode 102 can be fabricated by combining electron beam lithography patterning and electroless plating, as described later, and the gap length LG can be adjusted to match the single molecule 104.

[0024] Figures 1A and 1B show an example in which a nanogap electrode 102 is arranged on a substrate 150, with the first electrode 102A being composed of a first electrode layer 1021A and first metal particles 1022A, and the second electrode 102B being composed of a second electrode layer 1021B and second metal particles 1022B. The first electrode layer 1021A and the second electrode layer 1021B have a stripe-like pattern extending along the X-axis direction shown in the figure when viewed from above. The width W of the first electrode layer 1021A and the second electrode layer 1021B is 20 nm or less, preferably 15 nm or less, and more preferably 10 nm or less. The first metal particles 1022A and the second metal particles 1022B are provided at the opposing tip portions of the first electrode layer 1021A and the second electrode layer 1021B, respectively.

[0025] The first metal particle 1022A forms a single clump (or island-like region) on the surface of the first electrode layer 1021A. The first metal particle 1022A has a hemispherical appearance, like a water droplet dropped onto a hydrophobic surface. Note that "hemispherical" refers to a spherical surface with continuous curved surfaces, and is not limited to a perfectly spherical surface. In a plan view, the length of the first metal particle 1022A from one end to the other is 20 nm or less, preferably 15 nm or less, and more preferably 10 nm or less. The first electrode 102A has a structure in which the first metal particle 1022A protrudes outward from the leading edge of the first electrode layer 1021A. When this structure is viewed in cross-section, as shown in Figure 1B, the first metal particle 1022A does not contact the underlying surface of the first electrode layer 1021A, but is separated upward (in the Z-axis direction).

[0026] The first metal particle 1022A was described above, but the morphology of the second metal particle 1022B is similar.

[0027] The first metal particle 1022A and the second metal particle 1022B are arranged at a distance from each other, and the length of this distance corresponds to the gap length LG. In other words, the shortest distance from one end of the first metal particle 1022A to one end of the second metal particle 1022B corresponds to the gap length LG.

[0028] Although not shown in Figures 1A and 1B, more metal particles may be formed on the surfaces of the first electrode layer 1021A and the second electrode layer 1021B. The first metal particles 1022A and the second metal particles 1022B, schematically shown, are components that define the gap length LG within the nanogap electrode 102.

[0029] The first electrode layer 1021A and the second electrode layer 1021B are formed of a first metallic material, and the first metallic particles 1022A and the second metallic particles 1022B are formed of a second metallic material. The first metallic material and the second metallic material are different metallic materials. The combination of the first metallic material and the second metallic material can be selected as appropriate, but it is preferable that the combination of the first metallic material and the second metallic material forms a metallic bond and also forms an alloy. An example of the first metallic material is platinum (Pt), and an example of the second metallic material is gold (Au). Palladium (Pd) may also be used as the first metallic material.

[0030] As shown in Figure 1A, the single-molecule device 100 may be provided with a third electrode 102C that applies an electric field between the nanogap. In other words, the first electrode 102A and the second electrode 102B that form the nanogap electrode 102 may be arranged in the X-axis direction, and a third electrode 102C extending in the Y-axis direction may be provided to apply an electric field between the nanogap. The third electrode 102C is an electrode that does not form a bond with the single molecule 104 and can be used as a gate electrode in the single-molecule device 100.

[0031] The single molecule 104 has a uniquely defined structure and possesses a specific molecular orbital. The single molecule 104 is, for example, a π-conjugated molecule, and it is preferable that the π-conjugated molecule has a rigid and flat structure with π-conjugated groups that are resistant to twisting. As such a π-conjugated molecule, for example, carbon-bridged π-conjugated molecules and silicon-bridged π-conjugated molecules can be used. For example, as carbon-bridged π-conjugated molecules, carbon-bridged phenylene vinylene and carbon-bridged oligophenylene vinylene can be used, and as silicon-bridged π-conjugated molecules, quinoid-type fused ring oligosilols can be used.

[0032] The single molecule 104 has a rod-like, elongated shape and has ends on both sides in the direction of its long axis. When the single molecule 104 is placed between the electrodes of the nanogap electrode 102, at least one of the two ends of the single molecule 104 is chemically adsorbed to the first electrode 102A or the second electrode 102B. Preferably, one of the two ends of the single molecule 104 is chemically adsorbed to the first electrode 102A and the other is chemically adsorbed to the second electrode 102B, forming a cross-linked structure.

[0033] The single molecule 104 has the aforementioned molecule as its backbone S1, and further includes anchor groups Z1 and Z2, and linker groups Y1 and Y2 that connect the anchor groups Z1 and Z2 to the backbone S1. Figure 2 shows a structure in which such a single molecule 104 is crosslinked to a nanogap electrode 102. In detail, the structure shown in Figure 2 has linker group Y1 and anchor group Z1 connected in series between the backbone S1 and the first electrode 102A, and linker group Y2 and anchor group Z2 connected in series between the backbone S1 and the second electrode 102B.

[0034] The anchor groups Z1 and Z2 contain atoms that form chemical bonds with the nanogap electrode 102. For example, when the first metal particle 1022A and the second metal particle 1022B are formed of gold (Au), it is preferable that the anchor groups Z1 and Z2 contain thiol groups or acetylthio groups.

[0035] Linker groups Y1 and Y2 are groups that connect the anchor groups Z1 and Z2 to the backbone S1, and are formed, for example, by a linear chain. In molecular design, the width of the two ends of the monomolecule 104 (the length from one end to the other) can be adjusted by changing the lengths of the linker groups Y1 and Y2. For example, methylene groups (-(CH2)n-) and perfluoroalkyl groups (-(CF2)n-) can be used as linker groups Y1 and Y2. Alternatively, for example, p-phenylene groups (-(C6H4)n-) and combinations of p-phenylene and methylene groups can be used as linker groups Y1 and Y2. In this case, part of the methylene group may be an oxygen atom. Linker groups Y1 and Y2 may be the same or different.

[0036] In a single molecule 104, when the framework S1 is formed of π-conjugated molecules, the π-conjugated molecules have molecular orbitals (Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO)). In contrast, the anchor groups Z1 and Z2 and linker groups Y1 and Y2 substantially form a tunnel barrier. Therefore, as shown in the energy band diagram inserted in Figure 2, a quantum well is formed when the framework S1 is sandwiched between the tunnel barrier formed by the anchor groups Z1 and Z2 and the linker groups Y1 and Y2. The molecular orbitals of the π-conjugated molecules (HOMO-1, HOMO, LUMO, LUMO+1, etc.) become quantized within the quantum well.

[0037] The tunnel barrier formed by the anchor group Z1 and the linker group Y1 has a resistive component called tunnel resistance. Similarly, the tunnel barrier formed by the anchor group Z2 and the linker group Y2 also has tunnel resistance. Here, if the two tunnel resistances are greater than the quantization resistance R (= h / e² = 25.8 kΩ, where h is Planck's constant and e is the unit charge), the single-molecule device 100 can operate as a single-electron transistor. A single-electron transistor exhibits the Coulomb blockade phenomenon, where the flow of current is inhibited in the Coulomb blockade region. To prevent the Coulomb blockade phenomenon from occurring, it is desirable that at least one of the tunnel resistances is smaller than the quantization resistance.

[0038] On the other hand, if the tunnel resistance is smaller than the quantization resistance, the overlap of the wave functions with the single molecules forming the framework S1 increases, causing a pinning effect, and the voltage is no longer distributed across the tunnel barrier. If the pinning effect occurs on only one side of the tunnel barrier, the voltage applied between the electrodes of the nanogap electrode 102 is applied to the tunnel barrier side where the pinning effect does not occur, thus reducing the operating voltage. On the other hand, if the pinning effect occurs on both sides of the tunnel barrier, it is undesirable because it hinders current modulation by the gate voltage.

[0039] 2. Introduction of Single Molecules to Nanogap Electrodes In order to realize a single-molecule device 100 using a nanogap electrode 102, it is necessary to control the arrangement of single molecules 104 between the nanogap of the nanogap electrode 102. In this embodiment, the orientation control of single molecules by light irradiation will be described.

[0040] 2-1. Orientation Control by Light Irradiation The introduction of single molecules 104 into the nanogap electrode 102 is performed by immersing the nanogap electrode 102 in a solution in which single molecules 104 are dispersed, and then irradiating it with light. As described above, single molecules 104 have ends to which anchor groups Z1 and Z2 are attached. Of these two anchor groups Z1 and Z2, at least one anchor group is adsorbed onto the first metal particle 1022A or the second metal particle 1022B that forms the nanogap electrode 102. For example, if the anchor groups Z1 and Z2 contain thiol groups, and the first metal particle 1022A and the second metal particle 1022B are gold (Au), then when the anchor groups Z1 and Z2 are adsorbed onto the first metal particle 1022A and the second metal particle 1022, an Au-S bond is formed.

[0041] There are two forms in which a single molecule 104 is adsorbed onto the nanogap electrode 102: one where one of the two ends of the single molecule 104 is adsorbed onto one electrode of the nanogap electrode 102 (hereinafter also referred to as "one-sided adsorption"), and another where both ends are adsorbed in a manner that bridges the nanogaps of the nanogap electrode 102 (hereinafter also referred to as "two-sided adsorption"). The ideal structure for a single molecule device 100 is one in which both ends of the single molecule 104 are bridged between the nanogaps of the nanogap electrode 102, as shown in Figure 2. Having such a bridged structure stabilizes the structure even at room temperature, thereby reducing structural and energy level instability due to thermal fluctuations and increasing the current density of the resonant tunneling current. On the other hand, even if the single molecule 104 is adsorbed onto one side of the nanogap electrode 102, it will function as a single molecule device 100 if the other end that is not adsorbed is close enough to the nanogap electrode 102 that a tunneling current can flow.

[0042] In the solution into which the nanogap electrode 102 is immersed, the single molecules 104 are oriented and dispersed in random directions. When the single molecules 104 adsorb to the nanogap electrode 102, it is desirable that the two ends bond so as to bridge the nanogap, but as shown in Figure 3, one end may adsorb to one electrode and become stable. If one end of the single molecule 104 is adsorbed to only one side of the nanogap electrode 102, and the other end is located away from the nanogap electrode 102, the resonance tunneling phenomenon becomes less likely to occur. Therefore, it is considered preferable not only to immerse the nanogap electrode 102 in the solution in which the single molecules 104 are dispersed, but also to apply energy from the outside so that the single molecules 104 are oriented while immersed. For example, by irradiating the nanogap electrode 102 with light while it is immersed in the solution in which the single molecules 104 are dispersed, it is possible to control the orientation state of the single molecules 104.

[0043] The nanogap electrode 102 is made of metal, and when light is shone on the metal, the free electrons on the surface are affected and undergo plasma oscillations. Since electrons are negatively charged particles, when plasma oscillations occur on the metal surface, an electric field is induced around them. When this induced electric field resonates with the incident light, light of a specific wavelength is strongly absorbed or scattered, and surface plasmon resonance (SPR) occurs. As mentioned above, the first metal particle 1022A and the second metal particle 1022B of the nanogap electrode 102 are nanoparticles of 20 nm or less, so the collective oscillation of free electrons induces polarization in the nanoparticles, and the generated plasmons become localized on their surface. That is, localized surface plasmon resonance (LSPR) occurs in the first metal particle 1022A and the second metal particle 1022B. Due to the localization of surface plasmon resonance, strong electric fields are generated on the surfaces of the first metal particle 1022A and the second metal particle 1022B, and strong electric fields are also induced in their vicinity. The electric field becomes particularly strong between the nanogap electrodes. In the vicinity region where these localized plasmons exist, the interaction between the photoelectric field and the molecular polarization corresponding to the photoelectric field frequency is significantly amplified.

[0044] The π-conjugated group of the π-conjugated molecule used as the single molecule 104 has a long axis and a short axis. The molecular polarizability in the long axis direction is larger than that in the short axis direction, and it has anisotropy of molecular polarizability. Molecular polarization is given by the product of the optical electric field and the molecular polarizability. Since stabilization energy is given by the product of molecular polarization and the optical electric field, the state where the long axis direction is parallel to the optical electric field is more energy-stabilized than the state where the long axis direction is perpendicular to the optical electric field, that is, parallel to the short axis direction. As a result, between the nano-gap electrodes, as shown in FIG. 3, an electric field is formed that attempts to orient parallel to the nano-gap between the nano-gap electrodes 102 with respect to the single molecule 104 adsorbed on one side of the nano-gap electrode 102.

[0045] FIG. 3 shows a state where the anchor group Z2 of the single molecule 104 is adsorbed on the second metal particle 1022B of the nano-gap electrode 102. When the anchor group Z2 and the second metal particle 1022B form an Au—S bond, this bonding energy is large, so the bond is not broken by the action of the electric field. In a state where the anchor group Z2 of the single molecule 104 is bonded to the second metal particle 1022B at a predetermined angle and the anchor group Z1 is not bonded to the first metal particle 1022A, when a strong electric field is generated between the nano-gaps of the nano-gap electrode 102 by light irradiation, a force acts on the single molecule 104 in the direction of the arrow F shown in FIG. In other words, torque is applied to the single molecule 104 so as to rotate in the direction of the arrow F shown in FIG. 3 with the bonding portion between the anchor group Z2 and the second metal particle 1022B as a fulcrum. As a result, the anchor group Z1 can be bonded to the first metal particle 1022A, and the single molecule 104 can be arranged so as to bridge the nano-gap electrode 102. Thus, the orientation state of the single molecule 104 can be controlled by light irradiation and can be bridged between the nano-gaps of the nano-gap electrode 102. In the present embodiment, such a phenomenon is referred to as "optoelectric field orientation", and such a process is referred to as "optoelectric field orientation process".

[0046] When the first metal particle 1022A and the second metal particle 1022B are gold (Au), localized surface plasmon resonance occurs in the visible to near-infrared region. Therefore, it is preferable to irradiate with light of wavelength 500 nm to 1200 nm. It is known that when localized surface plasmon resonance occurs between the metal particles and their gaps, a strong photoelectric field is generated. This photoelectric field is localized near the gap of the nanogap electrode 102 of the particles and occurs in a spatially limited region. Therefore, the single molecules 104 that can be subjected to photoelectric field orientation treatment are limited to single molecules 104 that exist in a spatially limited region, making it possible to perform orientation treatment on a single single molecule 104. In the calculation results shown below, the localized photoelectric field is amplified 93 times for incident light with a wavelength of 1 μm, and its region is limited to a diameter of 4 nm and a width of 2 nm.

[0047] The required light irradiation intensity to orient a single monomolecule 104 placed on the nanogap electrode 102 with the photoelectric field is estimated. Photoelectric field orientation utilizes the fact that π-conjugated molecules have anisotropy in their polarizability. For example, the polarizability α of one of the π-conjugated molecules used here is 2.71 × 10⁻³⁸ C²m² / J in the long axis direction and 1.04 × 10⁻¹⁸ C²m² / J in the short axis direction, so the anisotropy Δα is Δα = 1.67 × 10⁻³⁸ C²m² / J. Here, if the polarization P due to the photoelectric field is P = αE [Cm] (1), then the stabilization energy W resulting from the anisotropy ΔP of polarization due to the anisotropy of the molecule can be expressed by equation (2) as W = (1 / 2)ΔPE = (1 / 2)ΔαE² [J] (2). In order to orient a single molecule 104 using the photoelectric field, the stabilization energy W must be sufficiently higher than the energy at room temperature (T = 300 K), and the following relationship must be satisfied: W > kBT = 4.14 × 10⁻²¹ J = 26 [meV] (3) Here, kB is the Boltzmann constant and 26 meV is the energy at room temperature.

[0048] Next, it is known that the radiation intensity I of light is expressed by Equation (4) using the effective value E of the electric field. I = E0H0 / 2 = nE2 / (2Z0) [W / m2] (4) Here, E0 is the amplitude of the electric field, H0 is the amplitude of the magnetic field, n is the refractive index of the surrounding medium, and Z0 is the impedance of vacuum, which is 377 Ω. Consider the case of using pulsed laser light of a Q-switched YAG laser with a wavelength of 1064 nm as the light source. Let the optical energy per pulse be P0 [J], the pulse width be Δt = 9 nsec, and the beam diameter be D. The peak value Ip of the energy of the pulsed light is given by Equation (5). Ip = P0 / (Δt×π(D / 2)2) (5) From the above, when the localized optical electric field is enhanced by the enhancement factor A, the condition for the stabilization energy by the optical electric field to exceed the thermal energy at room temperature is as shown in Equation (6). P0 > (Δtn×πkBT D2 / (2Z0ΔαA2) (6)

[0049] The results of plotting the values on the right side of Equation (6) as a function of the beam diameter D for the cases where the enhancement factor A of the optical electric field is 5, 15, and 25 are shown in FIG. 4. The beam diameter D can be changed from about 0.001 mm to 20 mm by condensing light using a lens. The refractive index of the medium around the single molecule 104 is assumed to be the refractive index of water, 1.33. From this calculation result, it can be seen that a high enhancement factor of the optical electric field is extremely effective in reducing the optical energy required to satisfy Equation (6). That is, it is effective in avoiding damage to the nano-gap electrode by the incident light.

[0050] When the electric field amplification A = 25, even with a beam diameter of 1 mm, P0 is approximately 20 mJ, indicating that the conditions of equation (6) are satisfied at low energy. These conditions can be achieved not only with nanosecond pulse width Nd:YAG lasers but also with mode-locked femtosecond lasers such as TiSa lasers. However, when using a TiSa laser with a repetition frequency of 100 MHz and a pulse width of 100 fsec, an average power of about 5 W of femtosecond laser light is required to achieve the same peak value as the nanosecond laser mentioned above, which is 100 times greater than the average power of 0.05 W when the repetition frequency of the nanosecond laser was 10 Hz. Furthermore, by reducing the beam diameter through focusing, similar conditions can be created with a continuous-wave (CW) laser. However, in this case, it is necessary to reduce the average power using an optical shutter or optical chopper to avoid optical damage.

[0051] When the single molecule 104 has polarizability anisotropy, it is considered preferable to irradiate the nanogap electrode 102 with linearly polarized light. As shown in Figure 5, the direction of the polarization axis of linearly polarized light can be broadly classified into two types: light Po1 polarized in a direction parallel to the direction of the gap length LG of the nanogap electrode 102, and light Po2 polarized in a direction perpendicular to it. Polarized light Po1 has a polarization axis parallel to the direction of the gap length LG (X-axis direction) and its propagation direction is in the Z-axis direction. Polarized light Po2 has a polarization axis perpendicular to the direction of the gap length LG (Y-axis direction) and its propagation direction is in the Z-axis direction.

[0052] When a π-conjugated molecule used as a single molecule 104 has a polarizability α1 in the long axis direction and a polarizability α2 in the short axis direction, and the relationship α1 > α2 holds, the π-conjugated molecule will be oriented such that the direction of higher polarizability is parallel to the direction of the external electric field. Therefore, it is considered preferable to irradiate the nanogap electrode 102 with light (polarized Po1) whose polarization direction is parallel to the direction of the gap length LG.

[0053] The strength and distribution of the electric field generated between the gaps can be determined using the finite difference time-domain method (FDTD). As shown in Figure 6, the calculation model assumed that the gap between the first metal particle 1022A and the second metal particle 1022B of the nanogap electrode 102 was 2 nm. As explained with reference to Figures 1A and 1B, the first metal particle 1022A is provided in the first electrode layer 1021A and the second metal particle 1022B is provided in the second electrode layer 1021B. The first electrode layer 1021A and the second electrode layer 1021B were assumed to have a width of 12 nm, a length of 12 nm, and a thickness of 13 nm. Furthermore, the influence of the substrate on which the nanogap electrode 102 is provided is small and therefore not considered here.

[0054] As shown in Figure 6, the incident light was assumed to be linearly polarized light with a wavelength of 1 μm irradiated onto the nanogap electrode 102. In the calculation, the incident light (linearly polarized light) was a plane wave whose polarization was parallel to the gap length direction (X-axis direction) and whose propagation direction was the Z-axis direction. The Yee cell size used in the calculation was 0.25 nm × 0.25 nm × 0.25 nm, with periodic boundary conditions in the X-axis and Y-axis directions and a perfect absorption layer (PML) boundary condition in the Z-axis direction.

[0055] Figure 6 is a plot of the electric field intensity in the X-axis direction, showing a snapshot of the electric field intensity at its strongest. The electric field intensity is indicated by different colors, and a color scale is shown on the right (the color scale is shown in blue when the electric field intensity is weak, and changes continuously to green and red as it increases). Figure 6 shows that the photoelectric field is localized to about 93 times the strength near the gap of the nanogap electrode 102, and that this region is limited to a diameter of 4 nm and a width of 2 nm. Such high intensity enhancement and spatial localization of the electric field are considered to be extremely effective in photoelectric field orientation of single molecules 104.

[0056] 2-2. Verification of the Photoelectric Field Orientation Bridging Method by First-Principles Calculations The torque acting on molecules under a photoelectric field was calculated using first-principles calculations based on density functional theory. Figure 7 shows the state when the direction of the molecular axis and the electric field E are parallel (θ = 0 rad) and perpendicular (θ = π / 2 rad). The light source is a YAG laser, and the photoelectric field E is assumed to be generated under the conditions of pulse energy PL = 200 mJ, pulse width = 9 nsec, and irradiation diameter D = 1 mm. Figure 8 shows the relationship between the direction of the photoelectric field and the total energy change of the molecule. From the relationship between the total energy and the direction of the photoelectric field in Figure 8, it can be seen that when the molecule is oriented parallel to the photoelectric field (θ = 0 rad), it is thermodynamically 260 meV more stable than when it is oriented perpendicularly (θ = π / 2 rad) (E⊥ - E / / = 260 meV), so the molecule oriented in a direction parallel to the photoelectric field.

[0057] Furthermore, under the YAG laser irradiation conditions described above, when the sulfur (S) anchor group is adsorbed on one side of the nanogap electrode, a torque of 0.29 nN·Å acts from that one-sided adsorption point. On the other hand, Figure 9 shows a state in which the sulfur (S) anchor group is bonded to the methylene group constituting the linker group. When the torque required for bond rotation at the methylene group portion was calculated, the torque was found to be a maximum of 0.16 nN·Å based on the relationship between the total energy and the bond angle φ shown in Figure 10. From this result, it was found that in the state shown in Figure 3, the torque acting from the one-sided adsorption point of the molecule is greater than the torque required for bond rotation at the methylene group portion. Therefore, the molecule adsorbed on one electrode of the nanogap electrode 102 (the second electrode 102B in the example shown in Figure 3) undergoes deformation and crosslinking at the bond between the anchor group Z2 and the linker group Y2.

[0058] 3. The single molecule 104 is a particle or material that can be considered as a particle, with a size of 100 nm or less, preferably 20 nm or less. A π-conjugated molecule is an example of the single molecule 104. Below are examples of single molecules 104 that can be applied to the single molecule device 100 of this embodiment.

[0059] 3-1. A π-conjugated molecule is an example of a single molecule 104. As mentioned above, it is preferable that the π-conjugated molecule has a rigid and flat structure in which the π-conjugated group is rigid and has a shape that is resistant to twisting. Such a π-conjugated molecule may have two ends (anchor groups) and be symmetrical, or it may be a π-conjugated molecule in which the configuration of the linker group differs at the two ends and is asymmetrical.

[0060] As a monomolecule 104 consisting of an asymmetric π-conjugated molecule, for example, as shown in structural formulas [1] to [7], one can be used which has a rigid silicon-bridged π-conjugated molecule with different numbers of phenylene groups and methylene groups as linker groups at both ends and a thiol group as an anchor group.

[0061]

[0062] Structural formula [1] shows an example of an asymmetric π-conjugated derivative, in which one linker group of the Si2×2π-conjugated skeleton is terphenyl, the other linker group is phenyl, and the terminal is terminated with a thiol. Hereafter, this structure will be referred to as TphCH2-phCH2.

[0063]

[0064] Structural formula [2] shows another example of an asymmetric π-conjugated derivative, in which biphenyl is used as one of the linker groups of the Si 2 × 2π-conjugated skeleton, and the terminal end is terminated with a thiol. Hereafter, this structure will be referred to as BphCH2-(CH2)2.

[0065]

[0066] Structural formula [3] shows an example of an asymmetric π-conjugated derivative, which has a structure with Bph(CH2)2-(CH2)2 on a Si4π-conjugated skeleton.

[0067]

[0068] Structural formula [4] shows an example of an asymmetric π-conjugated derivative, in which one linker group of the Si 2 × 2π-conjugated skeleton is terphenyl, the other linker group is phenyl, and the terminal is terminated with a thiol. Hereafter, this structure will be referred to as Tph-ph.

[0069]

[0070] Structural formula [5] shows an example of an asymmetric π-conjugated derivative, in which biphenyl is used as one of the linker groups of the Si 2 × 2π-conjugated skeleton, and the terminal end is terminated with a thiol. Hereafter, this structure will be referred to as Bph.

[0071]

[0072] Structural formula [6] shows an example of an asymmetric π-conjugated derivative, in which biphenyl is used as one of the linker groups of the Si 2 × 2π-conjugated skeleton, and the terminal end is terminated with a thiol. Hereafter, this structure will be referred to as Bph(CH2)2-(CH2)2.

[0073]

[0074] Structural formula [H] shows an example of an asymmetric π-conjugated derivative, in which one linker group of the Si 2 × 2π-conjugated skeleton is biphenyl, the other linker group is phenyl, and the terminal is terminated with a thiol. Hereafter, this structure will be referred to as Bph(CH2)2-ph(CH2)2.

[0075] If the two ends of a π-conjugated molecule are symmetrical, the molecular orbitals will also be symmetrical when the π-conjugated molecule is positioned to bridge the nanogap electrode 102. In this case, when a voltage is applied to the nanogap electrode 102, the voltage is evenly divided across the portion forming the tunnel barrier. This results in a halving of the effect on the quantized energy levels. For example, when a transistor is constructed by using one end of the nanogap electrode 102 as the source and the other as the drain, and adding a gate electrode, the effect of the drain voltage will be halved. Therefore, it is preferable to use a π-conjugated molecule that has asymmetry, such as those shown in structural formulas [1] to [3], where the linker groups at both ends of the π-conjugated skeleton have different configurations (molecular structure and molecular weight).

[0076] As an example of monomolecule 104, a symmetrical π-conjugated molecule can be used, for example, as shown in structural formulas [8] to

[11] , which has the same linker group at both ends of a rigid silicon-bridged π-conjugated molecule and a thiol group as an anchor group.

[0077]

[0078] Structural formula [8] shows an example of a symmetrical π-conjugated derivative, in which both linker groups of the Si2×2π-conjugated skeleton are alkyl ethers with five carbon chains, and the terminal is terminated with a thiol. Hereafter, this structure will be referred to as O(CH2)5-O(CH2)5.

[0079]

[0080] Structural formula [9] shows an example of a symmetrical π-conjugated derivative, in which both linker groups of the Si2×2π-conjugated skeleton are biphenyl, and the terminal is terminated with a thiol. Hereafter, this structure will be referred to as Bph-bph.

[0081]

[0082] Structural formula

[10] shows an example of a symmetrical π-conjugated derivative, in which both linker groups of the Si2×2π-conjugated skeleton are biphenyl, and the terminal is terminated with a thiol. Hereafter, this structure will be referred to as Bph(CH2)2-bph(CH2)2.

[0083]

[0084] Structural formula

[11] shows an example of a symmetrical π-conjugated derivative, in which both linker groups of the Si2×2π-conjugated skeleton are ethynylphenyl, and the terminal is terminated with a thiol. Hereafter, this structure will be referred to as Eph(CH2)2-eph(CH2)2.

[0085] When chemically adsorbing a single molecule 104 onto the nanogap electrode 102, methylene groups are inserted between the linker groups Y1, Y2 and the anchor groups Z1, Z2 so that the terminal atoms of the anchor groups Z1, Z2 can move to a position where they can form chemical bonds with atoms on the electrode surface. The number of methylene groups to be inserted is preferably determined considering the on-current of the transistor, the rigidity of the bridging molecule, and the mobility of the anchor groups.

[0086] 3-2. Protecting Groups By immersing the nanogap electrode 102 in a solution in which π-conjugated molecules are dispersed and irradiating it with light, the π-conjugated molecules can be adsorbed onto the nanogap electrode 102. Since the π-conjugated molecules are simply dispersed in the solvent, it is preferable to limit random adsorption to the nanogap electrode 102 in order to improve the reproducibility and productivity of the single-molecule device 100.

[0087] Figure 11A shows a monomolecule 104 with protecting groups P1 and P2 attached to its ends. The protecting groups P1 and P2 are bonded to anchor groups Z1 and Z2, so that in this state the anchor groups Z1 and Z2 do not directly adsorb to the nanogap electrode 102. On the other hand, the protecting groups P1 and P2 interfere with the crosslinking of the monomolecule 104 to the nanogap electrode 102, so it is preferable that they be detached from the anchor groups Z1 and Z2 relatively easily by some action.

[0088] For example, as shown in Figure 11B, it is preferable that the protecting groups P1 and P2 break their bond with the anchor groups Z1 and Z2 upon light irradiation. By using a single molecule 104 with such protecting groups P1 and P2 attached, the nanogap electrode 102 will not bond simply by being immersed in the solution, but will only be able to bond upon light irradiation.

[0089] As protecting groups P1 and P2 that are cleaved from anchor groups Z1 and Z2 by light irradiation, for example, structures like those shown in structural formulas

[12] and

[13] can be used. Preferably, the protecting groups P1 and P2 shown in structural formulas

[12] and

[13] have substituents R1 and R2 that allow the wavelength of light absorbed to be adjusted, and the anchor groups Z1 and Z2 can only be bonded to the electrode after the removal of the protecting groups P1 and P2.

[0090]

[0091]

[0092] 4. Method for Fabricating a Single-Molecule Device Next, a method for fabricating a single-molecule device 100 using photoelectric field orientation treatment is shown. This fabrication method includes the steps of fabricating a nanogap electrode 102 and chemically adsorbing a single molecule 104 onto the nanogap electrode 102.

[0093] 4-1. Fabrication of Nanogap Electrode Figure 12A shows the step of forming a first electrode layer 1021A and a second electrode layer 1021B on a substrate 150. The substrate 150 preferably has an insulating surface, and in order to form a fine pattern, it is desirable to have excellent flatness and small warpage. For example, a silicon wafer on which an insulating film such as a silicon oxide film has been formed can be used as the substrate 150. The insulating layer 152 formed on the surface of the silicon wafer by thermal oxidation is dense and has excellent uniformity of film thickness, making it suitable. Furthermore, if the insulating layer 152 is made of a high dielectric constant material such as a hafnium oxide film, it is suitable because it makes it easier to apply a gate electric field between the nanogap electrodes. In addition, as the substrate 150, a quartz substrate, an alkali-free glass substrate, a ceramic substrate formed of an insulating oxide material such as alumina or zirconia can be used.

[0094] The first electrode layer 1021A and the second electrode layer 1021B are formed of a metallic material. The structure of the first electrode layer 1021A and the second electrode layer 1021B is not limited and may have a single-layer structure or a laminated structure. For example, the first electrode layer 1021A and the second electrode layer 1021B may have a structure in which a first metal layer 1023 and a second metal layer 1024 are laminated. The first metal layer 1023 may be a layer formed of titanium (Ti), for example, and the second metal layer 1024 may be a layer formed of platinum (Pt), for example. The thickness of the first metal layer 1023 is 2 nm to 10 nm, for example 3 nm, and the thickness of the second metal layer 1024 is 5 nm to 20 nm, for example 10 nm. Note that the first metal layer 1023 is not an essential component and may be provided as appropriate to improve the adhesion of the second metal layer 1024. The second metal layer 1024 is a matrix layer for growing the first metal particles 1022A and the second metal particles 1022B. The first metal layer 1023 and the second metal layer 1024 can be deposited by electron beam deposition, sputtering, or other methods.

[0095] The first electrode layer 1021A and the second electrode layer 1021B are fabricated by first forming a coating of the metal material described above over the entire surface of the substrate 150, and then patterning it into a predetermined electrode shape using photolithography or electron beam lithography technology. The first electrode layer 1021A and the second electrode layer 1021B are formed in stripe-like patterns with a width of 20 nm or less, preferably 15 nm or less, in the main components that form the nanogap electrode (excluding the pads for applying the probe and the contact areas for connecting to other wiring). The distance L1 between the tip of the first electrode layer 1021A and the tip of the second electrode layer 1021B is 20 nm or less, preferably 15 nm or less, for example, 7.5 nm.

[0096] Figure 12B shows the steps for forming the first metal particle 1022A and the second metal particle 1022B. In the following description, the case where the first metal particle 1022A and the second metal particle 1022B are formed from gold (Au) is shown. The gold (Au) particles are produced by electroless plating. As the plating solution, a solution of gold foil dissolved in iodine tincture (a solution of I2 and KI2- dissolved in ethanol solvent) and L(+)-ascorbic acid (C6H8O6) as a reducing agent are used. By performing electroless plating using such a plating solution, the first metal particle 1022A and the second metal particle 1022B can be grown on the first electrode layer 1021A and the second electrode layer 1021B formed from platinum (Pt).

[0097] In electroless plating, gold (Au) particles grow at arbitrary positions on the surfaces of the first electrode layer 1021A and the second electrode layer 1021B. However, because the first electrode layer 1021A and the second electrode layer 1021B have a width of 20 nm or less, nucleation is preferred, especially at the leading edge of the electrode layer pattern. As a result, first metal particles 1022A grow at the edge of the first electrode layer 1021A, and second metal particles 1022B grow at the edge of the second electrode layer 1021B. During the growth process of electroless gold plating, monovalent positive ions of ascorbic acid and gold are present on the surfaces of the first electrode layer 1021A and the second electrode layer 1021B, and since ascorbic acid acts as a reducing agent, electrons are present. On the surfaces of the first electrode layer 1021A and the second electrode layer 1021B, gold ions are reduced to gold (Au) by a surface autocatalytic reaction, and electroless plating proceeds.

[0098] Then, as shown in Figure 12B, the first metal particles 1022A and the second metal particles 1022B grow at the respective ends of the first electrode layer 1021A and the second electrode layer 1021B. As the first metal particles 1022A and the second metal particles 1022B grow, the distance between the two opposing metal particles narrows. As a result, a Helmholtz layer (a layer of solvent, solute molecules, and solute ions adsorbed on the electrode surface) is formed between the first metal particles 1022A and the second metal particles 1022B, preventing gold ions from entering the gap. Consequently, when the distance between the first metal particles 1022A and the second metal particles 1022B narrows, electroless plating stops. In other words, by utilizing a diffusion-controlled reaction system, a self-stopping function is activated to control the gap.

[0099] Gold (Au) deposited by reduction on the surface of the platinum (Pt) layers formed as the first electrode layer 1021A and the second electrode layer 1021B forms metallic bonds with the platinum (Pt). As a result, the lateral diffusion of gold (Au) on the platinum (Pt) surface is suppressed, and it grows to form a hemispherical surface. The width from one end to the other of the first metal particle 1022A and the second metal particle 1022B having a hemispherical surface is 20 nm or less. In addition, the radius of curvature of the first metal particle 1022A and the second metal particle 1022B is 12 nm or less. The width from one end to the other and the radius of curvature of the first metal particle 1022A and the second metal particle 1022B can be controlled by the electroless plating process time. In this way, by performing electroless gold plating on the surface of the platinum (Pt) layer, a nanogap electrode 102 can be fabricated in which the first metal particles 1022A and the second metal particles 1022B are arranged in close proximity with a gap of 10 nm or less, as shown in Figure 12B.

[0100] Further details regarding the production of metal particles by electroless plating are disclosed in detail in the specification of International Publication No. 2019 / 007941.

[0101] In this embodiment, the process of forming first metal particles 1022A and second metal particles 1022B on the first electrode layer 1021A and second electrode layer 1021B has been described, but the structure of the nanogap electrode 102 is not limited to the structure shown in Figure 12B. Depending on the electroless gold plating conditions, gold (Au) can be grown in layers on the surface of the first electrode layer 1021A and second metal layer 1024B, and the nanogap electrode 102 can also be fabricated with such a structure.

[0102] 4-2. Single-molecule crosslinking process by photoelectric field orientation Figure 13 shows the step of adsorbing single molecules 104 onto the nanogap electrode 102 by photoelectric field orientation treatment. The photoelectric field orientation treatment is performed with the nanogap electrode 102 immersed in a solution containing single molecules 104. The solution uses π-conjugated molecules as shown in the structural formulas [1] to

[11] above as single molecules 104, and an organic solvent such as toluene is used as the solvent. If the concentration of this solution is too high, multiple single molecules will be adsorbed onto the nanogap electrode at once, which is undesirable, and if it is too low, the frequency of single molecules adsorbing onto the nanogap electrode will decrease, which is also undesirable. Therefore, it is preferable to set the concentration of the solution within a predetermined range, for example, it can be in the range of 1 × 10⁻⁵ M to 1 × 10⁻⁹ M.

[0103] An infrared laser is used as the light source for light irradiation. Specifically, an Nd:YAG laser with a wavelength of 1064 nm is used. The range of light irradiation may be adjusted by the optical system. Preferably, the direction of the polarization axis of the irradiated light is adjusted using a polarizer so that it is parallel to the axis of the nanogap electrode (gap length direction). Preferably, the light irradiation intensity is adjusted in advance with a power meter to 5 kW / m² to 50 kW / m², for example, 13 kW / m². Under these conditions, light irradiation is performed continuously for several tens of minutes to several hours, for example, 1 hour, with the nanogap electrode 102 immersed in the solution.

[0104] Under an electric field enhanced by light irradiation, the π-conjugated molecule used as a single molecule 104 aligns in the polarization direction of the laser light. Then, the single π-conjugated molecule is attracted to the nanogap of the nanogap electrode 102, forming a bridge structure with the first metal particle 1022A and the second metal particle 1022B.

[0105] When performing photoelectric field orientation processing, the crosslinking state of the single molecule 104 can be determined by applying an AC or DC voltage to the nanogap electrode 102 and monitoring the current. As shown in Figure 14, when the single molecule 104 is not bonded to the nanogap electrode 102 (first state), almost no current flows between the first electrode 102A and the second electrode 102B. However, when the single molecule 104 is adsorbed to one of the electrodes (second state), a current of 10 to 200 pA can be measured when an AC or DC voltage of 0.1 V is applied. Furthermore, when the single molecule 104 is crosslinked to the nanogap electrode 102 (third state), a current more than ten times greater can be measured. In this way, the crosslinking state of the single molecule 104 can be determined by applying a voltage to the nanogap electrode 102 simultaneously with light irradiation and observing the change in the current value.

[0106] 5. Characteristics of Monomolecule Devices This section describes the characteristics of monomolecule devices using π-conjugated derivatives that have asymmetry in which the linker groups at both ends of the π-conjugated skeleton have different configurations. As shown in Figure 1A, the monomolecule device has a structure in which a monomolecule 104 is placed in the gap of a nanogap electrode 102, and a third electrode 102C is provided to apply an electric field to the monomolecule 104. Since a monomolecule device having such a configuration exhibits transistor characteristics, it will be referred to as a monomolecule transistor in the following description.

[0107] Figure 20A shows the results of theoretical calculations of the Kohn-Sham molecular orbitals in π-conjugated derivatives, illustrating the HOMO orbitals in the neutral state (left) and cationized state (right) of the π-conjugated derivatives shown in structural formulas [1] (TphCH2-phCH2) and [3] (Bph(CH2)2-(CH2)2). As is clear from Figure 20A, in both states, the HOMO orbitals are localized on the Si2×2π-conjugated skeleton in the neutral state, whereas in the cationic state, the HOMO orbitals extend to the adjacent linker groups (phenyl group, biphenyl group, terphenyl group). Since the HOMO orbitals extend throughout the entire linker group, the effect of the number of phenyl groups on the decrease in on-current is limited.

[0108] Figure 20B shows the results of theoretical calculations of the transmittance, which indicates the tunneling probability of the π-conjugated derivative shown in structural formula [1] (TphCH2-phCH2), and Figure 20C shows the results of theoretical calculations of the transmittance of the π-conjugated derivative shown in structural formula [3] (Bph(CH2)2-(CH2)2). As shown in Figures 20B and 20C, due to the spatial expansion of the HOMO orbital, the transmittance in the cation state is higher than that in the neutral state, and the transmittance peak is broadened. This result suggests an increase in conductance.

[0109] As explained with reference to Figure 2, the operating mechanism of a single-molecule transistor involves a π-conjugated derivative providing an electron transport channel. When the applied bias causes the HOMO or LUMO level to coincide with the Fermi level of the metal forming the nanogap electrode, the tunneling probability increases, and a resonant tunneling current flows. In the π-conjugated derivatives shown in structural formulas [1] and [3], the HOMO level is generally provided, and it is thought that a resonant tunneling current flows when the applied bias causes the HOMO level to coincide with the Fermi level of the metal. The transmittance, which represents the electron tunneling probability, can be approximated as a Lorentz type, and the peak width shown in Figures 20B and 20C is determined by the sum of the electron bonds with the left and right electrodes constituting the nanogap electrode. Since the spatial molecular orbitals of the π-conjugated derivative are localized in the central skeleton, it is thought that the electronic bonding with the electrode is mainly determined by the structure of the linker group.

[0110] Therefore, as shown in Figure 20A, when cationization occurs by applied bias, the frontier orbital expands at the linker group, which is thought to increase the transmittance and significantly increase the resonance tunneling probability, as shown in Figures 20B and 20C.

[0111] Figure 21 shows the results of measuring the electrical characteristics of a single-molecule transistor. The measurements were performed in a vacuum (~10⁻⁵ Pa) at 9 K. Figure 21A shows the drain current (Id)-drain voltage (Vd) characteristics of a single-molecule transistor using the π-conjugated derivative shown in structural formula [1] (TphCH₂-phCH₂), and Figure 21B shows the drain current (Id)-drain voltage (Vd) characteristics of a single-molecule transistor using the π-conjugated derivative shown in structural formula [3] (Bph(CH₂)₂-(CH₂)₂). As is clear from the characteristics shown in Figures 21A and 21B, a steep increase in drain current (Id) is observed when the drain voltage (Vd) is positive.

[0112] When the drain voltage (Vd) is negative, the characteristics of the single-molecule transistor using the π-conjugated derivative of structural formula [1] (TphCH2-phCH2) shown in Figure 21A show that no rapid increase in drain current (Id) is observed up to -0.5V, and the drain current (Id) changes gradually. In contrast, in the single-molecule transistor using the π-conjugated derivative of structural formula [3] (Bph(CH2)2-(CH2)2) shown in Figure 21B, there is almost no drain current (Id) in the drain voltage (Vd) range of 0V ± 0.17V, and a significant increase in drain current (Id) is observed from -0.5V to -0.7V.

[0113] The HOMO level of the π-conjugated derivative shown in structural formula [1] (TphCH2-phCH2) is thought to be closer to the Fermi level (EF) than the HOMO level of the π-conjugated derivative shown in structural formula [3] (Bph(CH2)2-(CH2)2), and therefore the drain voltage (Vd) required to eliminate the Coulomb blockade in Figure 21A is thought to be smaller.

[0114] Figure 21C shows the drain current (Id)-gate voltage (Vg) characteristics of a single-molecule transistor using the π-conjugated derivative shown in structural formula [1] (TphCH2-phCH2), and Figure 21D shows the drain current (Id)-gate voltage (Vg) characteristics of a single-molecule transistor using the π-conjugated derivative shown in structural formula [3] (Bph(CH2)2-(CH2)2). The gate voltage is the voltage applied to the third electrode 102C shown in Figure 1A.

[0115] As shown in Figures 21C and 21D, the single-molecule transistor exhibits similar characteristics to a p-channel transistor, transitioning from an ON state where a high current level drain current (Id) flows during a sweep of the gate voltage (Vg) from negative to positive, to an OFF state where a low current level drain current (Id) flows, after a current decrease of more than two orders of magnitude.

[0116] The on / off ratios were 235 for the single-molecule transistor using the π-conjugated derivative shown in structural formula [1] (TphCH2-phCH2) and 305 for the single-molecule transistor using the π-conjugated derivative of structural formula [3] (Bph(CH2)2-(CH2)2). Furthermore, the subthreshold swing (SS), which indicates the switching characteristics of the single-molecule transistor, is more beneficial when it is small. A value of 380 mV / dec was obtained for the single-molecule transistor using the π-conjugated derivative shown in structural formula [1] (TphCH2-phCH2), and a value of 85 mV / dec was obtained for the single-molecule transistor using the π-conjugated derivative of structural formula [3] (Bph(CH2)2-(CH2)2). In the device using the π-conjugated derivative of structural formula [3], a 50 nm thermally oxidized SiO2 was used as the gate oxide film. The subthreshold swing (SS) value is proportional to the gate oxide film thickness and inversely proportional to the relative permittivity. Therefore, it can be reduced by lowering the gate oxide film thickness or by using a gate oxide with a high dielectric constant, such as hafnium oxide (HfO2). For example, if the gate oxide film SiO2 (film thickness: 50 nm, relative permittivity: 3.9) of this device (SS value 85 mV / dec) is changed to HfO2 (film thickness: 5 nm, relative permittivity: 20), the subthreshold swing (SS) value can be reduced to 1.7 mV / dec. Such a high on / off ratio and small subthreshold swing (SS) value can be said to reflect the high field-effect performance of the single-molecule transistor in this embodiment. Furthermore, hysteresis is small whether the applied bias voltage is swept in the forward or reverse direction, and it exhibits highly reproducible and stable characteristics.

[0117] In Si2×2π conjugated molecules, the HOMO level is thought to be closer to the Fermi level than the LUMO level. Therefore, in single-molecule transistors using Si2×2π conjugated derivatives, the HOMO level is thought to act as a carrier transport channel at low bias.

[0118] Furthermore, in the characteristics shown in Figure 21D, an increase in drain current (Id) is observed again when the gate voltage (Vg) increases. This suggests that the resonant tunneling effect due to the LUMO level occurs when the positive gate voltage (Vg) is sufficiently large.

[0119] From the characteristics shown in Figure 21, it is considered that the Fermier levels (EF) of the source and drain electrodes formed by the nanogap electrode are located between the HOMO and LUMO levels of the Si2×2π conjugated derivative. In the characteristics shown in Figures 21A and 21B, when the drain voltage (Vd) is low, almost no drain current (Id) flows due to the effect of Coulomb blockade. However, when the drain voltage (Vd) is increased, the Coulomb blockade condition is broken, and drain current (Id) begins to flow. It is thought that when the Fermi level (EF) coincides with the HOMO level, the drain current (Id) increases sharply. Furthermore, when a positive gate voltage (Vg) is applied, the HOMO level is pushed down relative to the Fermier level (EF). Therefore, a larger drain voltage (Vd) is required to eliminate Coulomb blockade, and it is thought that the voltage at which the drain current (Id) increases in a stepwise manner shifts to the positive side. In contrast, when Coulomb blockade is overcome, a tendency is observed for the stepwise change in drain current (Id) to disappear at negative gate voltages (Vg). This asymmetric Id-Vd characteristic is thought to be due to the asymmetric structure of the linker groups of the Si2×2π conjugated derivatives on both sides.

[0120] Figure 22A shows the drain current (Id)-drain voltage (Vd) characteristics of a single-molecule transistor using the π-conjugated derivative shown in structural formula [1] (TphCH2-phCH2), and the differential conductance (dId / dVd)-drain voltage (Vd) curve calculated from these characteristics. The drain current (Id) of a single-molecule transistor is basically temperature-independent because the current due to resonant tunneling is dominant. However, in the region where the drain current is affected by Coulomb blockade around 0V, there is a temperature dependence, and the increase in current induced by resonant tunneling changes from a steep increase to a gradual increase as the temperature rises. This change in drain current (Id) is clearly shown in the differential conductance characteristics, and the differential conductance peak around 0.125V drain voltage (Vd) changes to a broader profile at 50K compared to the characteristics at 9K.

[0121] As shown in Figure 22B, a significant temperature dependence is also observed in the drain current (Id)-gate voltage (Vg) characteristics, with the slope in the subthreshold region decreasing as the temperature increases.

[0122] Figure 22C shows the temperature dependence of the drain current (Id) on and off currents, and the temperature dependence of the on / off ratio. The on current hardly changes, but the off current increases slightly with increasing temperature, resulting in a decrease in the on / off ratio from 220 at 9K to 140 at 50K. Figure 22D shows the temperature dependence of the subthreshold swing (SS), showing that the subthreshold swing (SS) increases proportionally with increasing temperature. This linear correlation is consistent with the thermionic limit of a classical field-effect transistor.

[0123] The small subthreshold swing (SS) obtained in single-molecule transistors using the π-conjugated derivative shown in structural formula [1] (TphCH2-phCH2) and the π-conjugated derivative shown in structural formula [3] (Bph(CH2)2-(CH2)2) is thought to be due to the cationization of the π-conjugated derivative, which extends the HOMO orbital to the linker group and increases the conductance, as explained with reference to Figure 20A.

[0124] In this embodiment, we show an example using the π-conjugated derivatives shown in structural formulas [1] and [3] as the single molecule 104. However, it is possible to similarly realize a single molecule transistor with a small subthreshold swing (SS) by using the π-conjugated derivatives shown in structural formulas [2], [4] to

[11] .

[0125] The following describes, as an example of the present disclosure, the conditions for adsorbing a single molecule onto a nanogap electrode and the data obtained during the fabrication process.

[0126] Figure 15 shows an example of a light irradiation system that generates an electric field on a nanogap electrode 102. An Nd:YAG laser with a wavelength of 1064 nm is used as the light source 160. The light emitted from the light source 160 is polarized and focused by the optical system 164 and irradiated onto a sample (nanogap electrode) held in a container 166. The configuration of the optical system 164 is arbitrary, and may include a mirror 1641 to change the direction of the optical axis, optical elements that polarize the light (polarizer 1642, phase difference plate 1643), and a lens 1644 to focus the light. Also, since the light emitted from the light source 160 is infrared light, the optical system 164 is configured so that light from a laser light source 162 (for example, a HeNe laser) that emits visible light is irradiated coaxially to confirm the irradiation position. The container 166 contains a solution containing a single molecule, and the sample 168 on which the nanogap electrode is formed is immersed in the solution. To observe the adsorption state of single molecules in situ, a measurement system 170 is added for applying a bias voltage to the nanogap electrode and measuring the current. The measurement system 170 consists of an oscillator 171, a current amplifier 172, a lock-in amplifier 173, a semiconductor parameter analyzer 174, and the like.

[0127] As a sample, a nanogap electrode having the same structure as the nanogap electrode 102 having the structure shown in Figure 1B was fabricated. The sample used in this example consisted of multiple nanogap electrodes arranged on a single substrate, and the layout was such that laser light was simultaneously irradiated onto multiple nanogap electrodes. The portions corresponding to the first electrode layer 1021A and the second electrode layer 1021B were formed by patterning a platinum (Pt) film formed on the substrate using electron beam lithography. The portions corresponding to the first metal particle 1022A and the second metal particle 1022B were formed by heteroepitaxial growth of gold particles by electroless gold plating on a platinum nanogap electrode layer made of platinum (Pt), thereby further narrowing the gap between the platinum nanogap electrode layers. The gap length LG of the fabricated nanogap electrode was adjusted to match the length of the single molecule to be crosslinked by adjusting the electroless gold plating time, and in this example, it was set to 3.5 nm.

[0128] The solution containing the monomolecule used was a π-conjugated derivative having an anchor group and a linker group at both ends of the π-conjugated molecule shown in structural formulas [1] to

[11] , and toluene was used as the solvent. The solvent is preferably smaller than the dielectric constant of the π-conjugated molecule (5.5), and in addition to toluene, the solvents shown in Table 1 can be used. The solution concentration was used between 1 × 10⁻⁵ and 1 × 10⁻⁹ M.

[0129]

[0130] The photoelectric field orientation treatment was performed at room temperature. Multiple nanogap electrodes were irradiated with infrared laser light polarized at a wavelength of 1064 nm at an irradiation intensity of 13 kW / m². Since energy is concentrated in the nanogap region of the nanogap electrodes during irradiation, the temperature of the nanogap region is expected to rise above room temperature. However, the solvent did not boil, and the shape of the nanogap electrodes remained unchanged after irradiation, as confirmed by scanning electron microscopy.

[0131] In the photoelectric field orientation treatment, as explained with reference to Figure 6, the photoelectric field is amplified by approximately 100 times, and the Si2×2π conjugated molecules used as single molecules align in a direction parallel to the polarization direction of the laser light. The Si2×2π conjugated molecules are attracted to the nanogap electrode, and when the terminal groups at both ends are chemically adsorbed to bridge the nanogap, an Au-Si2×2-S-Au bond is formed. Whether or not a single molecule was chemically adsorbed onto the nanogap electrode was evaluated by measuring the current-voltage characteristics of the nanogap electrode using the measurement system 170 shown in Figure 15.

[0132] Figures 16A and 16B show the current-voltage characteristics of a nanogap electrode during photoelectric field orientation treatment. Figure 16A shows the current-voltage characteristics before and after a single molecule (Si2×2π conjugated molecule) is chemically adsorbed and crosslinked between the nanogap of the nanogap electrode. Before chemical adsorption and crosslinking of the single molecule, no current flows through the nanogap electrode, whereas after crosslinking, a current flows in accordance with the applied voltage. Figure 16B shows the characteristics when a single molecule is adsorbed on only one side of the nanogap electrode (unilateral adsorption). In the characteristics of the device with unilateral adsorption, a current of 1 nA is observed for an applied voltage of 2 V, while in the characteristics of the device with adsorption on both sides, a current of 100 nA is observed for an applied voltage of 1 V. Thus, the characteristics of a device equipped with a nanogap electrode differ depending on the adsorption state of the single molecule chemically adsorbed between the nanogap, and it can be seen that more current flows when a single molecule is chemically adsorbed so as to crosslink between the electrodes on both sides of the nanogap electrode.

[0133] Figures 17A, 17B, and 17C show the results of investigating the effects of the presence or absence of irradiation light and the polarization direction on the photoelectric field orientation process. The graph in Figure 17A shows the yield of single-molecule elements (the percentage of elements in which a single molecule is considered to have chemically adsorbed onto the nanogap electrode) when the polarization axis of the irradiated laser light is parallel to the gap length direction of the nanogap electrode, when the polarization axis of the irradiated laser light is perpendicular to the gap length direction of the nanogap electrode, and when there is no laser irradiation. Si2×2_BphCH2SH-CH2CH2SH was used as the single molecule.

[0134] Referring to the graph shown in Figure 17A, the yield of single-molecule devices was 35% when the polarization axis of the irradiated laser light was parallel to the gap length direction of the nanogap electrode, 16% when it was perpendicular, and 23% when there was no light irradiation. These results indicate that when the polarization axis of the irradiated laser light is parallel to the gap length direction of the nanogap electrode 102, the long axis of the π-conjugated molecule introduced as a single molecule 104 in the solution is oriented in the direction of the nanogap, making it easier to form a single-molecule junction. On the other hand, when the polarization axis of the irradiated laser light is perpendicular, the long axis of the π-conjugated molecule is perpendicular to the nanogap direction, resulting in a lower yield of single-molecule junctions than when there was no laser irradiation. From the results shown in the graph in Figure 17A, it was found that in the photoelectric field orientation treatment, by using polarized light as the irradiation light and aligning the polarization axis parallel to the gap length direction of the nanogap electrode 102, chemiadsorption of the π-conjugated molecule used as a single molecule 104 can be promoted, thereby increasing the yield of the device.

[0135] The graphs in Figures 17B and 17C show the yield when the concentration of the solution containing the single molecule is varied in the range of 1 × 10⁻⁵ to 1 × 10⁻⁹ M. In this case, the polarization direction of the irradiated laser light was parallel to the gap length direction of the nanogap electrode 102, and light irradiation was performed for 1 hour. The graph in Figure 17B shows the yield of elements adsorbed on one side, and the graph in Figure 17C shows the yield of elements cross-linked by single molecules. From these results, it can be seen that the yield is highest at a solution concentration of 1 × 10⁻⁷ M.

[0136] The graphs shown in Figures 18A and 18B show the yield of devices fabricated by photoelectric field orientation treatment. The graph in Figure 18A has the gap length of the nanogap electrode on the horizontal axis and shows the yield when there is no laser light irradiation and when laser light polarized in a direction parallel to the gap length of the nanogap electrode is irradiated. It can be seen that when there is no laser light irradiation, the yield peak is at a gap length of 3.4 nm, and when polarized laser light is irradiated, the yield peak is at a gap length of 3.2 nm.

[0137] The graph in Figure 18B shows the yield of devices (nanogap electrodes) for which device characteristics were not observed after photoelectric field orientation treatment. In this evaluation, when there is no laser light irradiation, the histogram peak is at 3.4 nm, but there is a large variation with respect to the gap length. In contrast, when polarized laser light is irradiated, the histogram peak is distributed at a gap length of 6.3 nm, and the number of nanogap electrodes with a gap length of 3 nm to 4 nm that can crosslink single molecules for which device characteristics could not be observed is small. From the characteristics shown in Figures 18A and 18B, it can be seen that in photoelectric field orientation treatment, irradiation with polarized laser light preferentially forms single-molecule devices in nanogap electrodes with a gap length of 3 nm to 4 nm for which single molecules can be crosslinked.

[0138] Figure 19 shows the results of in-situ observation of the process when a single molecule is introduced into a nanogap electrode. The measurement used the same configuration as the measurement system 170 shown in Figure 15. An oscillator 171 (function synthesizer) was connected to one electrode of the nanogap electrode to apply an AC voltage, and a current amplifier 172 (sensitivity: 10⁻⁵ A / V) and a lock-in amplifier 173 (sensitivity: 1 V, time constant: 300 ms) were connected to the other electrode. The electrical signal was finally measured with a semiconductor parameter analyzer 174, and the waveform was observed with an oscilloscope 175. The repetition frequency of the irradiated laser light was set to 10 Hz and the pulse width to 10 nsec, so a sinusoidal AC voltage (1000 Hz, Vp-p = 2 V) was set immediately after the laser pulse trigger, and the oscillator was able to observe both the laser pulse and the AC voltage. Continuously applying an AC voltage to the nanogap electrode introduces disturbances due to the applied voltage. Therefore, in this study, we used a burst mode method, applying the AC voltage only once per second for 50 msec.

[0139] As shown in characteristic A of Figure 19, the current value does not change unless a single molecule is introduced into the nanogap electrode. On the other hand, as shown in characteristic B, when a single molecule is introduced into the nanogap electrode, the amplitude R increases, which means that the tunnel current increases. This method makes it possible to capture the state in which molecules are introduced into the nanogap before chemical bonds are formed.

[0140] 100: Single-molecule device, 102: Nanogap electrode, 102A: First electrode, 1021A: First electrode layer, 1022A: First metal particle, 102B: Second electrode, 1021B: Second electrode layer, 1022B: Second metal particle, 102C: Third electrode, 1023: First metal layer, 1024: Second metal layer, 104: Single molecule, 150: Substrate, 152: Insulating layer, 160: Light source, 162: Ray 164: Light source, 1641: Optical system, 1642: Mirror, 1643: Polarizer, 1644: Phase difference plate, 1644: Lens, 166: Container, 168: Sample, 170: Measurement system, 172: Current amplifier, 173: Lock-in amplifier, 174: Semiconductor parameter analyzer, 175: Oscilloscope, S1: Skeleton, Y1, Y2: Linker group, Z1, Z2: Anchor group, LG: Gap length

Claims

1. A method for producing a single-molecule device, characterized by forming a nanogap electrode having a gap length to which a single molecule having two ends can be crosslinked, immersing the nanogap electrode in a solution in which the single molecule is dispersed, and irradiating the nanogap electrode with light in the solution to adsorb the single molecule onto the nanogap electrode.

2. A method for fabricating a single-molecule device according to claim 1, wherein pulsed light is irradiated onto the nanogap electrode.

3. The method for fabricating a single-molecule device according to claim 2, wherein the pulsed light is linearly polarized pulsed light.

4. The method for fabricating a single-molecule device according to claim 3, wherein the polarization axis of the linearly polarized light is parallel to the gap length direction of the nanogap electrode.

5. The method for fabricating a single-molecule device according to claim 2, wherein the pulsed light is infrared laser light.

6. A method for fabricating a monomolecule device according to claim 4, wherein at least one terminal group of the monomolecule is adsorbed onto the nanogap electrode.

7. A method for fabricating a monomolecule device according to claim 4, wherein the monomolecule is crosslinked between the gaps of the nanogap electrode.

8. The method for producing a monomolecule device according to claim 1, wherein the monomolecule is a π-conjugated molecule, and the π-conjugated molecule includes a linker group and an anchor group at one end and the other end of a π-conjugated skeleton.

9. The method for producing a monomolecule device according to claim 8, wherein the linker group comprises a different number of phenylene groups and methylene groups, and the anchor group comprises a thiol group.

10. The method for producing a single-molecule device according to claim 1, wherein the concentration of the solution is 1 × 10⁻⁵ M to 1 × 10⁻⁹ M.

11. A monomolecule device comprising: a nanogap electrode in which one electrode and the other electrode are arranged to have a nanoscale gap; a monomolecule disposed in the gap of the nanogap electrode; and a gate electrode that applies an electric field to the gap of the nanogap electrode, wherein the monomolecule comprises a π-conjugated skeleton and end groups at one end and the other end of the π-conjugated skeleton, and the structures of the end groups at the one end and the other end are different.

12. The monomolecule device according to claim 11, wherein the terminal group comprises a linker group that bonds to the π-conjugated skeleton and an anchor group that bonds to the linker group, and the molecular weight of at least the linker group differs between one terminal and the other terminal.

13. The monomolecule device according to claim 12, wherein one of the two terminals contains biphenyl as the linker group.

14. The monomolecule device according to claim 13, wherein one of the linker groups at the terminal is terphenyl and the other linker group at the terminal is biphenyl.

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