A novel scanning tunnelling microscope head with a double‐piezo‐tube nested scanner for operation in a vibrational magnet

Abstract

Coarse stepping motor and imaging scanner are the most important components of the scanning tunnelling microscope (STM). In traditional STMs, a single small‐sized piezoelectric tube is fixed directly to the motor and used as the imaging scanner, the small‐sized scanner can only provide a small imaging area. As a result, those STMs need equipped with X–Y piezo stage to move sample. While the noise generated from coarse stepping motor and X–Y piezo stage, as well as the complex structure will greatly reduce the STM's imaging stability. In this paper, we present an independent scanner structure featuring a double‐piezo‐tube nested design. In this design, both two piezoelectric tubes can be responsible for adjusting the tip‐sample distance; the external scan tube can be used to adjust the position of the sample. The small tip‐sample mechanical loop and compact structure of the scanner ensure the high imaging stability of the new STM. Finally, we constructed a non‐magnetic STM head based on above novel scanner. High‐quality atomic images of the highly ordered pyrolytic graphite (HOPG) and low drift rates in both X–Y plane and Z directions were obtained at room temperature, demonstrating the high imaging stability of our new STM even at 300 K. Additionally, atomic images and the dI/dV spectra of NbSe₂ obtained at 1.7 K, as well as the atomic images of HOPG obtained at changing magnetic fields from 0 T to 12 T in a vibrational magnet, proving new STM's high stability under low temperature and high magnetic fields.

1. INTRODUCTION

The scanning tunnelling microscope (STM) possesses real‐space atomic resolution when probing sample surfaces, and simultaneously enabling high‐resolution dI/dV spectra measurements. The first STM ¹ was invented by G. Binnig et al., followed by the successful development of numerous STM systems operating under cryogenic, ultra‐high vacuum (UHV), extreme low temperature, and high magnetic fields. ² , ³ , ⁴ , ⁵ Among these, the low‐temperature UHV STM developed by Pan et al. is a representative one, ⁵ with its STM head constructed using a scanner mounted on a Pan‐type piezoelectric motor. Due to the simplicity of manufacturing Pan‐type motor and its excellent compatibility with low‐temperature and vacuum environments, it was rapidly adopted by many researchers to build their own STM systems, ⁶ , ⁷ , ⁸ and were even utilised in some commercial STMs. These STMs achieved significant successes in the fields of materials science, particularly in studying the superconducting materials. ⁹ , ¹⁰ With the discovery of novel physical phenomena like the charge density waves, ¹¹ , ¹² Majorana zero‐energy modes ¹³ , ¹⁴ , ¹⁵ and quantum Hall effect, ¹⁶ , ¹⁷ STM systems operating under low temperature and high magnetic field conditions became increasingly important. The early high magnetic field STMs used traditional liquid‐helium immersed superconducting magnet as the experimental platform, which produced minimal vibrations during operation, making them highly suitable for the STM measurements. ¹⁸ However, the high cost and scarcity of helium limited their long‐term use. To address this issue, cryogen‐free superconducting magnet (also named dry magnet) was developed and quickly replaced traditional superconducting magnet. Unlike the quiet environment provided by traditional magnet, dry magnet requires continuous operation of cooler and compressor, generating significant vibration interference. Additionally, the sample space of dry magnet is typically around 50 mm, some even as small as 30 mm. Therefore, achieving atomic‐resolution imaging within the confined sample space and huge vibration environment of dry magnet necessitates meticulous design of structural compactness, material selection, and vibration damping of core STM head.

Although STMs based on Pan‐type motor have been widely used in traditional superconducting magnet, their complicated STM structure, multi‐channel control signals, and relatively large size make them less suitable for use in the narrow space and huge vibration featured dry magnet. The first STM developed in dry magnet ¹⁹ was reported by Meng et al. in 2015, achieving atomic imaging at a base temperature of 1.6 K and a maximum magnetic field of 8 T. Subsequently, STMs with different functions, such as in‐plane magnetic field STM ²⁰ and rotating STM, ²¹ were developed. In recent years, multiple STMs ²² , ²³ operating in water‐cooled magnet have been successfully developed, setting a world record for STM atomic imaging at 35 T. ²⁴ Above reported STMs are construct by using Spider‐drive motor, where the motor and scanner are not contact directly, which can effectively prevent the noise from the motor entering into the tip‐sample junction. Although this design has been proven to be capable of atomic imaging in narrow space and vibrational conditions, it still has many problems that need to be solved urgently. Firstly, the imaging area that a single small‐sized piezoelectric tube can achieve is often relatively small. Due to the small sample space in dry magnet, the X–Y piezo stage cannot be installed in their STM head, thus unable to move the sample position. Secondly, the scanner part in their STM incorporates the metal, zirconia or sapphire frame, and the piezoelectric tube. The inconsistent thermal expansion coefficients between these two different materials can lead to low stability of the STM, especially in the high magnetic field conditions. Finally, the imaging piezoelectric tube in above STMs are fixed to a moveable shaft, not an independent and highly stable scanning structure, the imaging stability also needs to be improved. All of the above restrictions will limit the STM's high‐stable atomic imaging in low‐temperature and high magnetic field environments.

To construct a high‐stable STM head that capable of atomic imaging under the challenging conditions of narrow space, huge vibrations and high magnetic fields, multiple problems need to be solved. Firstly, the core scanner part should be designed with small size and high compactness. The mechanical loop between the tip and sample should be as small as possible, smaller size helps improve the imaging stability. Secondly, the materials used in STM head should be without non‐magnetic. Otherwise, the STM head will be attracted by the magnetic field and come into contact with the inner wall of the magnet, transmitting the vibration interference into the tip‐sample junction, thereby damaging the atomic imaging. Finally, the probe that connecting dry magnet and STM head needs to have good shock absorption; otherwise, the vibrations will also be transmitted through the rigid probe to the STM head.

In this work, we proposed the new design of a double‐piezo‐tube nested scanner, as well as the non‐magnetic, highly stable STM head. The inner smaller‐sized piezo tube in scanner is responsible for the atomic imaging, while the outer larger‐sized piezo tube serves as the scanner's framework and can also adjust the position of sample. This design improves the regulating range of tip‐sample distance and imaging area without increasing the mechanical loop between the tip and sample. Additionally, the two piezo tubes in the scanner have same thermal drift coefficients, resulting in higher imaging stability. All materials used in whole STM head are non‐magnetic zirconia and sapphire, which also have extremely low thermal drift coefficients, further enhancing the STM's imaging stability under low temperature and high magnetic field conditions. The STM head is suspended at the bottom of a two‐stage vibration damping rod, preventing the external vibration interference entering into the sensitive tip‐sample junction. The new built STM is capable of operating in the narrow space and huge vibration conditions of a 12 T dry magnet. Using this STM, we first obtained the high‐quality atomic images of HOPG at 300 K, demonstrating its atomic imaging capability at room temperature. Then, we performed the continuous scanning of a large area with boundary defects on HOPG surface at 300 K, low drift rates as low as 17.9 pm/min in the X–Y plane and 16.4 pm/min in the Z direction were determined. Atomic images and dI/dV spectra of NbSe₂ at 1.7 K demonstrated the STM's applicability at low temperature. Finally, we presented the atomic images at changing magnetic fields from 0 T to 12 T, proving the device's high immunity to high magnetic field conditions. These results demonstrate that our new STM is capable of performing high‐quality atomic‐resolution characterisation in the narrow space, huge vibration, low‐temperature, and high magnetic field environments.

2. INSTRUMENT DESIGN AND CONTROL

2.1. STM head

The new STM head is designed based on an improved Spider‐drive piezo motor. Figure 1a–c illustrates the detailed structure of the STM head with main view, cross‐sectional view, and exploded view. STM head consists of three main components: a piezo motor, a double‐piezo‐tube nested scanner, and a zirconia frame. The motor utilises a piezo tube (marked with PZT#3, EBL3# type from EBL PRODUCTS INC.) with an outer diameter of 8.2 mm and a length of 32 mm as the driving source. One end of PZT#3 is fixed to the motor base; the square shaft is elastically pressed against the inner wall of PZT#3 via a spring piece (beryllium‐copper). The friction force between shaft and PZT#3 is set at approximately 1.5 N by adjusting the curvature of the spring. The inner wall of PZT#3 is comparatively rough, whereas the shaft surface is very smooth. Consequently, when the shaft is driven upward or downward by PZT#3, the spring piece remains in an almost stationary state. The scanner part is constructed using two piezo tubes: PZT#2 (8.2 mm‐outer diameter, and 18 mm‐length) and PZT#1 (4.5 mm outer diameter, and 9 mm length). One ends of PZT#1 and PZT#2 are fixed coaxially to the scanner base. A square shaft is elastically pressed against the inner wall of PZT#1 via a spring; the friction force is approximately 0.7 N. The tip is mounted on the top of the scanner shaft. PZT#2 serves as the frame of the independent scanner; its free end is partially removed along the axial direction, and a circular sample holder is fixed to the top end of PZT#2. A cylindrical frame connects the motor base and the scanner base. To enable the forward and backward motion of the scanner shaft, a connecting rod is used between the motor shaft and scanner shaft. All components, including the motor base, frame, scanner base, shafts, connecting rod, and sample holder, are made of zirconia to ensure the imaging stability in high magnetic field conditions.

FIGURE 1.

Three‐dimensional schematic diagram of the new STM head with (a) main view, (b) section view, and (c) exploded view.

2.2. Control signals

The STM controller used in our experiments was developed based on a PCIe‐7851R data acquisition card from National Instruments (NI). The controller consists of five modules: a power supply module, a low‐voltage signal processing module, a high‐voltage signal amplification module, the data acquisition card module, and a control computer. The controller supports a maximum output voltage of 200 V, with a base noise smaller than 10 mV. The signal amplifier, developed in‐house, is fully soldered onto a sapphire substrate to minimise leakage current. The waveforms used to control the motor approaching and retracting are shown in Figure 2. When driving the tip move close to the sample for approaching, a pair of triangular signals are applied on PZT#1 and PZT#2. The above waveforms cause PZT#1 to extend, moving the tip upward, while PZT#2 retracts, moving the sample downward. If no tunnelling current is detected, PZT#1 and PZT#2 return to their initial lengths, and a sharp pulse signal is applied to PZT#3, which drives the shaft and tip upward by one step. When retracting the tip from the sample surface, two reverse voltage signals are firstly used to drive the tip move downward and drive the sample move upward, increasing the tip‐sample distance to the maximum. Then we applied continuous sharp pulse signals on PZT#3 to drive the shaft and tip move downward step by step. The enlarged tip‐sample gap ensures a safer tip retraction process. Unlike traditional single‐tube featured scanner, both PZT#1 and PZT#2 in our design can be used to adjust the tip‐sample distance, significantly enhancing STM's performance in extreme conditions of changing temperatures and magnetic fields.

FIGURE 2.

Driving signals for motor stepping and tip retraction.

2.3. Assembly process

Figure 3 shows detailed assembly process of the STM head. Figure 3a is the cross‐sectional view of 3D diagram of the STM head and shows the glue points between each component. Figure 3b and c shows the photos during the assembly process. To polish the square shaft, we pressed it firmly against sandpapers of decreasing roughness (W70, W50, W28, W20, W10, and W7) and repeated the polishing process on each of the four surfaces multiple times. Finally, the four corners of the shaft were polished with W7 sandpaper. After polishing, two shafts are inserted into the piezo tubes for smoothness testing. Next, the polished spring pieces are inserted between the shafts and the inner walls of the piezo tubes. The sliding friction forces to move sliders in PZT#1 and PZT#3 are adjusted to 0.7 N and 1.5 N, respectively. (2) One end of PZT#3 is fixed to the motor base using an insulating glue (H74); the other end used to drive the shaft. Similarly, one ends of PZT#1 and PZT#2 are fixed to the scanner base. (3) The assembled motor part is inserted into the frame using H74 glue. Similarly, the assembled scanner is embedded into the top surface of the frame, and fixed at the step position using H74 glue. (4) The tip holder and sample holder are fixed to the top of the scanner shaft and PZT#2 using H74 glue. The connecting rod is inserted into the bottom hole of scanner shaft and fixed with H74 glue. Finally, a platinum wire with diameter of 40 µm is used to connect the electrodes of three piezo tubes with external interface, the wires are fixed inside the slots in the frame surface using H74 glue.

FIGURE 3.

Assembly process of the STM head. (a) Cross‐sectional view of the STM head, in which the detailed glue points are marked. (b) Photos of STM components taken during the assembly process. (c) Photos of STM head after assembly is complete.

2.4. Experiment conditions

The low temperature and high magnetic field environment used in experiments is provided by a dry magnet purchased from Oxford Instruments with a bore diameter of 50 mm, as shown in Figure 4a. The dry magnet operates via a cold head and compressor, capable of cooling the sample chamber down to 1.6 K and generating a maximum magnetic field of 12 T. To reduce the impact of the huge vibrations generated when running the dry magnet, we designed a probe with two‐stage vibration damping, as shown in Figure 4b. The STM head is hanging at the bottom of the probe by using a spring. Figure 4c is the photo of how the STM head is connected. After inserting the STM probe into the sample chamber of dry magnet, the pre‐amplifier box is connected to the dry magnet via a KF50 flange. A small amount of static helium exchange gas is filled into the sample chamber to cool down the STM and sample. The chamber pressure is controlled to approximately 10⁻¹ torr at 300 K. The tip used for atomic imaging were manually fabricated by cutting a 0.25 mm diameter platinum‐iridium wire (purchased from Goodfellow) with a cutting angle of approximately 15°. Before cutting, the platinum‐iridium wire was subjected to a 10‐s annealing process.

FIGURE 4.

(a) Photo of the dry magnet STM system. (b) Schematic diagram of the STM probe. (c) A local photo shows the connection between the STM head and probe.

3. RESULTS

Using the newly constructed STM, we first conducted atomic‐resolution testing at room temperature. Figure 5a displays the large‐area STM image (raw data) of HOPG surface under constant height mode. Figure 5b is the small area atomic image obtained by using the zoom‐in scan, the below curve is the tunnelling current measured along the black dashed line indicated in upper image. Compared with the low‐temperature environment, atomic imaging at room temperature is more challenging. Many commercial STM devices have difficulty in obtaining high‐quality atomic image at room temperature. Our raw‐data results obtained at 300 K demonstrate the excellent atomic‐resolution imaging capability of our new STM device.

FIGURE 5.

(a) Large‐area atomic image obtained on HOPG surface under constant height mode. (b) Atomic image (upper) was obtained on HOPG by scanning a small‐area, below tunnelling current was obtained along the dashed line in upper image. Measure conditions: average tunnelling current is 10 nA, bias voltage is +100 mV applied on sample.

To quantitatively characterise the new STM's stability, we performed the large‐area imaging on HOPG surface to search defect features that could serve as drift reference. Figure 6a–c shows morphological images of the HOPG surface obtained under constant current mode, in which two intersecting boundary defects are clearly shown. We continuously imaged this region for 200 min at 300 K and tracked the displacement of the boundary intersection point in the X–Y plane, a low drift rate of 17.9 pm/min is acquired. To determine STM's drift rate in the Z direction, we recorded the displacement of tip‐sample distance with feedback loop open, the tunnelling current was set at 2.5 nA. Figure 6d shows the variation of the displacement between tip and sample over 100 min; the calculation results reveal a low drift rate of 16.4 pm/min in the Z direction. These low drift rates in both the X–Y plane and Z direction confirm the structural stability of our STM.

FIGURE 6.

(a–c) Continuous STM images obtained on HOPG surface under constant current mode, the tunnelling current is set at 2.5 nA. (d) Curves of the drift rate in Z direction, the tunnelling current is set at 2.5 nA, the feedback loop to control the tip‐sample junction is open.

To demonstrate the scanner's (PZT#2) ability to move the sample position, we applied two voltage signals with opposite polarity on the external electrodes of PZT#2. The opposite polarity voltages are used for driving PZT#2 to create bending deformation. Figure 7a and b shows STM topographic images obtained before and after adjusting the position of the HOPG sample. We marked the intersection position of the two boundaries in Figure 7a with a dotted circle, and the solid circle in Figure 7b marked the new position of the intersection position after the sample moved. Our results show that a 1 V voltage can cause the sample to move approximately 12 nm.

FIGURE 7.

Morphological images of HOPG surface obtained before (a) and after (b) moving the sample position using PZT#2, the tunnelling current is set at 2.5 nA, bias voltage is +100 mV.

Subsequently, we characterised the atomic imaging and spectroscopic capabilities of the new STM at low temperature. Figure 8a shows an atomic‐resolution image acquired on a freshly cleaved NbSe₂ (purchased from SixCarbon Technology Shenzhen) surface at 1.7 K. Due to the high activity of NbSe₂ surface, the sample cleaving process was quickly performed in air. Then we immediately insert the STM probe and STM head into the sample chamber of the 12 T dry magnet within 10 min. Figure 8b shows the dI/dV spectra acquired at 1.7 K, in which the superconducting gap of NbSe₂ and its coherence peaks are well resolved. The amplitude and frequency of modulation signal were set at 3 mV and 371 Hz, respectively. The atomic‐resolution image and high‐resolution dI/dV spectra obtained at 1.7 K demonstrate the high spatial and energy resolution of our new STM. Figure 9 displays the noise spectra of the tunnelling current at 1.7 K. Although a few interference peaks exist at low frequencies, their amplitudes are not significant and do not substantially affect atomic imaging.

FIGURE 8.

(a) Atomic image obtained on NbSe₂ surface under constant height mode, the average tunnelling current is 7.5 nA. (b) dI/dV spectra obtained with tip position at the marked position in (a), the amplitude and frequency of modulating signal are set at 3 mV and 371 Hz.

FIGURE 9.

Noise spectra acquired with the tunnelling current set at 2.5 nA.

Finally, we performed atomic imaging with the new STM under high magnetic field conditions. Raw‐data STM images of HOPG (GRADE ZYH, purchased SPI Supplies) presented in Figure 10 are obtained at varying magnetic field strength of 0 T, 2 T, 4 T, 6 T, 9 T, and 12 T. The quality of atomic images shows minimal degradation under magnetic fields, highlighting the new STM's high immunity to magnetic fields.

FIGURE 10.

Raw‐data atomic images of HOPG obtained at 1.7 K low temperature and changing magnetic fields, the average current is set at 10 nA.

Our new STM has been proven to be capable of performing stable atomic‐resolution imaging in harsh environments. However, the atomic imaging in dry magnet needs static exchange gas to cool down the STM and sample, unable to measure samples with extremely active surface. In future, we expect to design a standalone insert chamber that can be cooled within the magnet. The STM head will be thermally linked to this new insert via copper braids. This design will enable the low‐temperature and high‐vacuum measurements inside the dry magnet.

4. CONCLUSION

We present the design and the construction of a novel scanner featured STM head, which capable of realising the high‐quality atomic imaging and spectroscopic measurement within a vibrational dry magnet. Unlike conventional imaging scanners, our design employs two piezo tubes to control the tip‐sample distance and the position of sample. The outer tube serves as the frame of scanner; it does not increase the length of the tip‐sample mechanical loop. Furthermore, the main components within the scanner are two piezoelectric tubes with the same thermal expansion coefficient. Therefore, the imaging stability of this new scanner is extremely high. Using the newly constructed non‐magnetic STM, we obtained high‐quality atomic images of HOPG at 300 K and measured low drift rates in both the X–Y plane and Z direction, demonstrating its high‐stable imaging capability at room temperature. Subsequently, high‐resolution atomic imaging and dI/dV spectra measurement of NbSe₂ performed at 1.7 K within the dry magnet confirmed the STM's cryogenic compatibility and applicability. Finally, raw‐data STM images obtained under magnetic fields ranging from 0 T to 12 T proves the new STM's excellent immunity to the high magnetic field environments. These results comprehensively demonstrate the superior capabilities of our new STM. This new device is highly suitable for studying the novel physical phenomena under low temperature and high magnetic field conditions.

CONFLICT OF INTEREST STATEMENT

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.