Sample holder for scanning tunnelling microscopes

Abstract

The present invention relates to a sample holder for scanning tunnelling microscopes (STM), especially designed for PAN models, with a rail-based system that allows the sample holder to be quickly and easily removed without having to disassemble the microscope. The sample holder comprises a base (7) with a cut-out having a series of rails (7.1) and a panel (8) that can be removed via the rails (7.1). The panel (8) comprises a plate (11.2) having, on one face thereof, a protrusion (11.4), a through hole (8.1) on one side of the protrusion (11.4) and a tab (11.3) on the opposite side. The protrusion (11.4) comprises a frame (11.1) to hold the sample. The protrusion (11.4) may comprise a central hole (12.2) and a latch (12.1) inside the central hole (12.2). Alternatively, the protrusion (11.4) can also rotate on the plate (11.2).

Description

[0001] DESCRIPTION

[0002] SAMPLE HOLDER FOR TUNNEL SCANNING MICROSCOPES

[0003] Object of the invention

[0004] The present invention is based on a sample holder for surfaces and substrates for scanning tunneling microscopes (STM).

[0005] The invention falls within the field of scanning tunneling microscopy, primarily conducted and tested on microscopes at room temperature, although it is also suitable for low temperatures, down to 4.2 K (i.e., -269 °C). While based on designs printed with PLA (polylactic acid), it can also be manufactured in metals.

[0006] The invention not only facilitates sample changes within the same technique and methodology, but also allows switching from one technique to another simply by replacing a single part. Furthermore, it offers an efficient solution for scanning samples created using electrochemical methods, whose substrates are often difficult to place in STMs. Thanks to the plug-and-play design, we have created a simple and adaptable method for handling these types of samples.

[0007] The invention falls within the industrial sector related to the development of sample holders for microscopes and, specifically, for scanning tunneling microscopes.

[0008] Prior art

[0009] It is well known that operating a Scanning Tunneling Microscope (STM) is highly complex. This device consists of a delicate tip and a crucial surface for its operation. The tip is usually held in place by a spring system, while the sample-bearing surface is firmly anchored to the microscope base. However, changing the sample is a very complex process with many potential errors. It requires completely disassembling the device and removing the tip holder, which carries the risk of displacing the spring from its equilibrium position. Maintaining this equilibrium position is essential, as it is often difficult to find. If the spring is out of equilibrium, there is a risk of collision, which would damage the sample, or the tip could be repeatedly withdrawn, preventing the acquisition of an atomic-resolution topographic image.

[0010] Even under the most favorable circumstances, the assembly, disassembly, and verification of the operation of this intricate apparatus require in-depth knowledge of the microscope and consume considerable time.

[0011] The lack of versatility further complicates the operation of the STM. Designed specifically for a single sample, changing sample types requires replacing the entire device base, significantly limiting its usefulness and efficiency.

[0012] Furthermore, it is important to note that the device is typically constructed from titanium. This material is known for its strength but presents significant challenges in terms of machining and manufacturing. This results in additional difficulties in handling and maintaining the STM.

[0013] In summary, working with an STM is very complex, with many possibilities for error, therefore requiring extensive training and time.

[0014] The system of the invention is designed for PAN-type STM models (incorporating the development described in WO9319494), see Figure 1. Due to the architecture of these microscopes, changing samples has always been a tedious task involving extensive equipment handling, including disassembling the microscope to replace the sample (the surface to be scanned). It is based on a click-and-lock system that allows for sample holder removal and easy connection to the electronic system. Furthermore, it is not limited to 10 x 10 mm surfaces but can be adapted to any size thanks to its click-and-lock system, making it highly versatile. A system for mounting electrochemical samples has also been developed for this PAN-type STM model.

[0015] Finally, a manual rotation system is implemented, allowing scanning of other regions of the sample that would be inaccessible with a fixed system. Generally, the scanned surface is a few nanometers in size, but thanks to this system, a larger scannable area can be covered, reaching a usable scannable area of ​​several millimeters, but composed of multiple images. Figure 1 shows an illustration of a PAN-type STM from a front and rear perspective with its respective components. The identified elements are: (1) the STM body, (2) the tip holder or carriage (behind which is the locking spring), (3) the sample or surface to be scanned, (4) the sample anchor points, (5) the spring and sample holder base, and (6) the sample holder base to be replaced.

[0016] Figure 1 represents the initial operating state of the STM. To extract the sample, the carriage (2) must be removed and the body (1) detached from the base (6) where (2) and (3) are located. Then, the screws at the anchor points (4) must be loosened to remove the sample. To place a new sample, these steps must be reversed: first, place the new sample and secure it with the screws at the anchor points (4). Then, reattach the carriage (2) and sample (3) system to the body (1). Finally, position the carriage (2) on the body (1) and adjust the spring (5). Adjusting the spring can be tedious, as it is crucial that it is in its equilibrium position. If it is not in equilibrium, the tip could collide with the surface or constantly retract, making it impossible to obtain topographic images with atomic resolution.This step is crucial, and the STM operator may spend considerable time, even days, adjusting it correctly. Once positioned, all electrical connections must be reconnected to ensure the microscope is fully operational.

[0017] The applicant is unaware of any sample holder that is similar or as advantageous as the one described and claimed below.

[0018] Explanation of the invention

[0019] The invention is designed for use in scanning tunneling microscopes. In this respect, it solves the technical problem presented by the PAN microscope, which requires a technician with years of experience in handling the system, someone who can assemble and disassemble it in the shortest possible time to change the sample, while maintaining all components in proper working order. This highlights the need for a system that is easy to connect, disconnect, and properly position the sample / surface. Therefore, the invention incorporates this type of mechanism.

[0020] The invention replaces the substrate holder with a fixed system that includes heels, allowing the sample holder to be inserted and removed in seconds with a simple pull or push. This eliminates the need to disassemble the STM and change the spring configuration that anchors the tip holder or carriage. A major advantage of the invention is that all parts are 3D printed using PLA (polylactic acid), although other materials could be used. Systems manufactured with PLA using this invention exhibit high stability, capable of maintaining atomic precision. The choice of polylactic acid (PLA) for manufacturing the system parts is based on finite element simulations performed with FreeCAD and structural tests that confirmed its mechanical stability.

[0021] Various designs were explored and experimental tests were conducted until the current configurations were achieved, demonstrating the required precision. The use of PLA in 3D printing allows for rapid and efficient production, facilitating design iterations and reducing costs. Furthermore, it is biodegradable and environmentally friendly. In conclusion, PLA was chosen for its proven performance, structural stability, and atomic precision, as validated by the microscope calibration images. The quality of the images obtained during microscope calibration (Figures 8, 9, and 10 in this text) validates the reliability of the PLA parts. Moreover, the atomic distances shown in Figures 8, 9, and 10 coincide with those reported in the literature by L. Delgado, S. Chacón, C. Sabater, and G. Sáenz. Topographic measurements on atomically flat surfaces under ambient conditions using a scanning tunneling microscope: a didactic approach.Uniciencia 33 Issue 1 (2019) http: / / dx. doi. org / 10.15359 / ru .33-1.3.

[0022] In any case, the essential thing is the design; the corresponding parts can be manufactured with any metal, but PLA is preferred because it is easy to print, low cost and because it is a polymer derived from natural and renewable raw materials.

[0023] Although attempts have been made to standardize for typical samples of the most common metallic surfaces (for example, those marketed by the company Arrandee), as well as for HOPG samples (suppliers such as SPI, MikroMash) and sample standards prepared by electrochemical methods (such as those prepared by J. Clavilier, R. Faure, G. Guinet, R. Durand, Preparation of monocrystalline Pt microelectrodes and electrochemical study of the plane surfaces cut in the direction of the {111} and {110} planes, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, Volume 107, Issue 1, (1980) https: / / doi.org / 10.1016 / S0022-0728(79)80022-4; or in G. Melle, F. Scholten, JM Feliu, E. Herrero, BR Cuenya, RM Arán-Ais, Elucidating Interfacial parameters of platinum-palladium bulk alloy single crystals, Journal of Materials Chemistry A, on-line version, (2024), http: / / dx.doi.org / 10.1039 / D4TA01771 D), one of the advantages of manufacturing the parts with 3D printers is their low cost and ease of printing, which allows the sample holder to be adapted according to the user's needs or in case the suppliers change the formats.

[0024] The invention enables the acquisition of atomic-resolution images of samples, such as the so-called "herringbone" pattern of gold, which requires an exceptionally clean sample free of any type of electronic or mechanical interference. This invention eliminates unnecessary vibrations and reduces the handling required of the sample, thanks to the design of the press frame system, which incorporates a metal mask.

[0025] The sample holder allows for quick changes between different sample types without disassembling the microscope, thus reducing the time and complexity associated with sample changes. Regardless of the sample holder used, the spring's position is maintained once calibrated, as these are located in the unaffected part of the STM body. This eliminates the need to move or recalibrate the spring, ensuring a fast and effective way to obtain atomic-resolution images.

[0026] The sample holder provides high mechanical and electrical stability, allowing the capture of high-resolution images of metallic surfaces and samples prepared by electrochemical methods, even under ambient conditions.

[0027] It is important to mention that all the parts of the specimen holder shown in Figure 2 are made entirely of PLA. The specimen holder of our invention for scanning tunneling microscopes comprises a base with a cutout on one side containing a series of grooves. This base is fixed to the microscope body. It also has a removable panel that slides along these grooves. On one side of the base plate is a protrusion, a through-hole next to the protrusion, and a tab on the opposite side. The protrusion incorporates a retaining frame for the specimen.

[0028] This allows the plate to be removed from the side of the base to insert a new sample or replace the plate with another. Depending on the selected plate inserted, the type of sample and / or experiment can be changed. In the first plate variant, the protrusion includes a central hole and a latch inside the central hole. That is, the latch presses against any object inserted into the central hole to lock it in place.

[0029] In another type of flat bar, the protrusion is rotatable about an axis perpendicular to the flat bar. For example, the protrusion may be attached to the flat bar by means of a shaft that has a handle on the opposite side of the flat bar, that is, the side where the protrusion is not located.

[0030] It should be noted that, throughout the description and claims, the term "includes" and its variations are not intended to exclude other technical features or additional elements.

[0031] Brief description of the figures

[0032] In order to complete the description and to aid in a better understanding of the characteristics of the invention, a set of figures and drawings is presented which, for illustrative and non-limiting purposes, represent the following:

[0033] Figure 1: Front perspective view of an example of a PAN-type STM according to the prior art. Part (1) refers to the body of the STM, reference (2) to the bit holder or carriage, reference (4) refers to the screws or anchor points of the sample, and reference (5) corresponds to the spring.

[0034] Figure 2: Rear view of an example of a prior art PAN-type STM.

[0035] Figure 3: Detail of the sample holder from the previous PAN-type STM example. Element (3) is the sample to be scanned and (6) is the sample holder base.

[0036] Figure 4: shows an example of the base (7) where the panels (8) are attached.

[0037] Figure 5: shows a first embodiment of the plate (11.2) that can be assembled on the base (7). In this case, it is for measuring fixed samples. Figure 6: shows a second embodiment of the plate (11.2) that can be assembled on the base (7). In this case, it is for measuring samples generated by electrochemical methods.

[0038] Figure 7: shows a third embodiment of the plate (11.2) that can be assembled on the base (7). In this case, it is used for rotating samples.

[0039] Figure 8. Panels a), b), and c) described here were obtained using the first vahante: Panel a) is a topographic image of Au

[0111] with high stability. Panel b) is an enlargement of the indicated area, showing the Au in its

[0111] reconstruction. The high resolution allows visualization of the atomic steps, including the herringbone pattern, which is observed when the sample is very clean and mechanically and electrically stable. The arrow indicates the direction in which the height profile of the terraces and atomic steps was taken. Panel c) shows the height profile of the atomic steps from Figure b) in the direction indicated by the arrow.

[0040] Figure 9. Panels a), b), and c) described here were obtained using the second vahante. Panel a) is a topographic image of Pt. Panel b) is an enlargement of the highlighted area of ​​panel a), showing Pt in its crystallographic reconstruction

[0111] of this metal, and highlighting the atomic terraces and steps. The arrow indicates the direction in which the height profile of the atomic steps will be taken. Panel c) shows the height profile of the atomic steps from figure b) in the direction indicated by the arrow.

[0041] Figure 10. This figure is a topographic image of High Ordered Pyrolitic Graphite (HOPG) taken with the third vahante. In the left panel, a rectangular area is marked where only that area was scanned. In the right panel, the atomic resolution of the HOPG is shown.

[0042] Detailed explanation of some ways of carrying out the invention

[0043] Figure 2 schematically presents the invention. It consists of a base (7) comprising a cut on one side with a series of parallel grooves (7.1). A panel (8), which can be of different types, is placed on these grooves (7.1).

[0044] The panel (8) is part of a plate (11.2) that slides along the rails (7.1). A stepped protrusion (11.4) and a through-hole (8.1), shown as hexagonal, are defined on this plate (11.2). When the panel (8) is in position, the hole (8.1) is centered on the base (7), while the protrusion (11.4) is located to one side. This hole (8.1) has a threaded inner edge. In the depicted embodiment, it is hexagonal and contains a nut secured inside, which allows a screw to be inserted. This screw is electrically connected to the sample via a metallic conductive mask located on the frame (11.1). This screw, passing through the hole (8.1), allows the surface to be connected to the bias voltage.

[0045] A tab (11.3) on the other side of the protrusion (11.4) allows the user to remove the panel (8) and perform the sample or experiment change.

[0046] A frame (11.1) can be placed on the protrusion (11.4), supported on the step, to delimit the position of the sample, and to anchor it mechanically and electrically.

[0047] A second section of panel (8) also incorporates a central hole (12.2) in the protrusion (11.4) and a latch (12.1), represented as an “L”, positioned on the outside of the protrusion (11.4), on the opposite side of the hole (8.1) in panel (8). The latch (12.1) allows any electrochemical sample (or surface with a stud on its back) inserted into the central hole (12.2) to be secured. This solution is primarily intended for electrochemical samples. Electrochemical samples typically have a stud and a wider, truncated spherical head.

[0048] The bolt, which is usually already incorporated into the sample during manufacturing, is inserted into the central hole (12.2), where it is firmly secured by the L-shaped latch (12.1), ensuring that the sample does not move during surface scanning. Furthermore, the excess portion of the bolt allows for an electrical connection to the drill screw (8.1), thus guaranteeing the electrical contact necessary for measurements. This ensures standardization of the connection process, as electrochemical samples cannot be trapped by the frame (11.1).

[0049] This second vahante can be used as the first vahante as long as the samples are surface samples and not electrochemical samples, provided that the central hole (12.2) is left empty.

[0050] A third vane of the panel (8) allows the projection (11.4) to rotate about an axis perpendicular to its surface (Figure 7). For this purpose, the projection (11.4) is attached to the plate (11.2) by means of a shaft (13.1) with a handle on the underside of the plate (11.2). Movement of the handle rotates the projection (11.4), aligning different parts of the sample within the microscope. It is worth noting that in this case, the sample connection to the bias voltage requires considerably longer components compared to those used in the other two vanes. This is because the sample holder moves in this rotation system, and it is necessary to prevent the bias voltage connection cable from breaking due to twisting. In the first two vanes, the standard connection cable requires, for example, approximately 1 cm in length, while in the third vane, its length increases to between 2 and 3 cm to ensure its integrity and functionality.

[0051] To demonstrate the validity, reproducibility, and accuracy of the results obtained with our invention, we present Figures 8, 9, and 10. Multiple measurements were performed on both the same samples and different samples with identical properties, ensuring that the data obtained were consistent in all cases. These results agree with those reported in the scientific literature, reinforcing the reliability of the developed device.

[0052] The information provided is sufficient for a third party to replicate the experiments, ensuring the robustness and reliability of our invention, as well as the reproducibility of the tests performed.

[0053] Generally, a gold substrate (111) can be used to calibrate the z-axis distances of scanning tunneling microscopes because it exhibits atomic-sized steps (0.25 nm). To image these steps and measure their profile, good mechanical stability and low electronic noise are required. Furthermore, if the samples are sufficiently clean, herringbone patterns are observed. Thanks to the first vahante, we were able to obtain Figure (8).

[0054] Figure 8a) shows a topographic image of Au in the crystallographic direction (111), where the typical terraces, atomic steps, and herringbone patterns can be observed. Figure 8b) presents a magnification of the steps with an arrow indicating a height profile. As detailed in Figure 8c), the profile shows Au atomic steps with an approximate height of 0.25 nm. In addition to highlighting the good atomic resolution, we wish to emphasize the preservation of the herringbone patterns, which is due to the minimal sample handling and the resulting cleanliness.

[0055] Using an electrochemical sample, the second phase yielded Figure 9, corresponding to platinum (Pt). Figure 9a) shows a topographic image of Pt in the crystallographic direction (111), where typical features such as terraces and atomic steps can be distinguished. Figure 9b) shows a magnification of the steps, highlighting an arrow that indicates the height profile. In Figure 9c), the profile reveals that the atomic steps of Pt have an approximate height of 0.3 nm. We wish to emphasize not only the excellent atomic resolution obtained but also the preservation of the sample's cleanliness, which allowed for the observation of these structural features.

[0056] To ensure the reproducibility of the example presented in Figure 10, using the third vahante, a meticulous procedure was followed. A sample of Highly Oriented Pyrolytic Graphite (HOPG) was selected to allow observation of carbon atoms and thus calibrate the xy distances. The choice of HOPG was based on its ability to exhibit atomic resolution when the system is stable and clean, despite being more difficult to scan compared to gold.

[0057] Initially, the STM system was ensured to be in optimal mechanical stability. Instabilities introduced by the rotational system were avoided by leaving the system in a fixed position during scanning. This allowed for clear observation of atomic positions on a HOPG foil, where a large area of ​​approximately 10 x 10 nm was scanned.

[0058] Multiple scans were performed on different areas of the sample to verify the system's operation and stability. Once it was confirmed that the system was operating consistently and producing reproducible results, a specific region of the sample was selected for a more detailed scan, as shown in the labeled topographic sub-image. In this sub-image, the typical and expected positions of the carbon atoms in the HOPG can be clearly observed.

[0059] The system was then rotated and another similar area was scanned, thus validating the mechanical and electrical stability of the rotational invention. This system allows scanning in diverse environments and on HOPG samples, using an STM microscope with a titanium body and our invented component made of PLA. It is clear, therefore, that the invention maintains sufficient stability to obtain sharp images of the atomic steps and terraces of gold. This is especially noteworthy given that these images were obtained under common environmental conditions, where historically it has been difficult to obtain a clear herringbone pattern in gold. Furthermore, our invention has demonstrated the ease with which atoms can be observed in highly oriented pyrolytic graphite (HOPG). Additionally, images of atomic steps have also been obtained in samples prepared using electrochemical methods.

[0060] The ability to reproduce clear and detailed images of atomic structures in both HOPG and electrochemically prepared precious metals underscores the robustness and versatility of our technique. This not only opens new opportunities for advanced research in materials science and nanotechnology but also represents a significant advance in the accessibility and cost of scientific instrumentation. Using PLA, a common 3D printing material, to construct a STM is exceptional for several reasons: it not only significantly reduces manufacturing and maintenance costs but also simplifies the implementation and adaptation of advanced technologies for scientific applications. Furthermore, as a non-conductive polymer, PLA does not cause short circuits or interfere with bias voltage connections, making it an excellent choice for avoiding electrical problems.It is worth highlighting that the key feature of this invention is the design of the parts, which allows for easy interchangeability and minimal handling. Our invention guarantees the repeatability and reproducibility of experiments.

[0061] This approach not only expands the scope of atomic-scale research but also democratizes access to high-precision tools, opening new avenues for innovation and collaboration within the scientific community. The ability to achieve comparable results using more accessible materials like PLA highlights the innovative potential of adapting existing technologies to solve complex scientific challenges more efficiently and effectively.

[0062] This version expands further on the advantages of the proposed methodology and the innovation of using PLA in STM construction, highlighting its potential impact on scientific research and the academic community.

Claims

CLAIMS 1. Scanning tunneling microscope sample holder, characterized in that it comprises a base (7) comprising a cut on one side having a series of parallel helices (7.1), and a panel (8) removable by the helices (7.1) comprising a plate (11.2), on one face of which a protrusion (11.4) is defined, a drill (8.1) through one side of the protrusion (11.4) and a tab (11.3) on the opposite side, and where the protrusion (11.4) comprises a sample retention frame (11.1) containing a conductive mask for connecting the sample surface with the bias voltage. 2- Scanning tunnel microscope sample holders, according to claim 1, characterized in that the protrusion (11.4) comprises a central hole (12.2) and a latch (12.1) inside the central hole (12.2), for holding electrochemical samples. 3- Scanning tunnel microscope sample holders, according to claim 1, characterized in that the protrusion (11.4) is rotatable about an axis perpendicular to the plate (11.2). 4- Scanning tunnel microscope sample holders, according to claim 3, characterized in that the protrusion (11.4) is coupled to the plate (11.2) by means of a shaft (13.1) having a handle on the opposite side of the plate (11.2) to facilitate rotation. 5- Scanning tunneling microscope sample holders, according to claim 1, characterized in that the panel (8) is dimensionally printed with polylactic acid (PLA).

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