Scanning Tunneling Microscope Head Research Articles

Quick-connect scanning tunneling microscope head with nested piezoelectric coarse walkers.

To meet changing research demands, new scanning tunneling microscope (STM) features must constantly evolve. We describe the design, development, and performance of a modular plug-in STM, which is compact and stable. The STM head is equipped with a quick-connect socket that is matched to a universal connector plug, enabling it to be transferred between systems. This head can be introduced into a vacuum system via a load-lock and transferred to various sites equipped with the connector plug, permitting multi-site STM operation. Its design allows for reliable operation in a variety of experimental conditions, including a broad temperature range, ultra-high vacuum, high magnetic fields, and closed-cycle pulse-tube cooling. The STM's compact size is achieved by a novel nested piezoelectric coarse walker design, which allows for large orthogonal travel in the X, Y, and Z directions, ideal for studying both bulk and thin film samples ranging in size from mm to μm. Its stability and noise tolerance are demonstrated by achieving atomic resolution under ambient conditions on a laboratory desktop with no vibrational or acoustic isolation. The operation of the nested coarse walkers is demonstrated by successful navigation to a μm-sized 2D sample.

Atomic imaging with a 12 T magnetic field perpendicular or parallel to the sample surface by an ultra-stable scanning tunneling microscope

We present the first nonmetallic scanning tunneling microscope (STM) featuring an ultra-stable tip-sample mechanical loop and capable of atomic-resolution imaging within a 12 T magnetic field that could be either perpendicular or parallel to the sample surface. This is also the first STM with an ultra-stable tip-sample mechanical loop but without a standalone scanner. The STM head is constructed only with two parts: an improved spider-drive motor and a zirconia tip holder. The motor performs both the coarse approach and atomic imaging. A supporting spring is set at the fixed end of the motor tube to decrease the tip-sample mechanical loop. The zirconia tip holder performs as the frame of the whole STM head. With the novel design, the STM head in three dimensions can be as small as 7.9 mm × 7.9 mm × 26.5 mm. The device's excellent performance is demonstrated by atomic-resolution images of graphite and NbSe2 obtained at 300 K and 2 K, as well as the high-resolution dI/dV spectrums of NbSe2 at variable temperatures. Low drift rates in the X-Y plane and Z direction further prove the imaging stability of our new STM. High-quality imaging of the Charge Density Wave (CDW) structure on a TaS2 surface shows the STM's good application capability. Continuous atomic images obtained in magnetic fields rangs from 0 T to 12 T with the direction of the magnetic field perpendicular or parallel to the sample surface show the STM's good immunity to high magnetic fields. Our results illustrate the new STM's broad application ability in extreme conditions of low temperature and high magnetic field.

A Novel Atomically Resolved Scanning Tunneling Microscope Capable of Working in Cryogen-Free Superconducting Magnet.

We present a novel homebuilt scanning tunneling microscope (STM) with atomic resolution integrated into a cryogen-free superconducting magnet system with a variable temperature insert. The STM head is designed as a nested structure of double piezoelectric tubes (PTs), which are connected coaxially through a sapphire frame whose top has a sample stage. A single shaft made of tantalum, with the STM tip on top, is held firmly by a spring strip inside the internal PT. The external PT drives the shaft to the tip-sample junction based on the SpiderDrive principle, and the internal PT completes the subsequent scanning and imaging work. The STM head is simple, compact, and easy to assemble. The excellent performance of the device was demonstrated by obtaining atomic-resolution images of graphite and low drift rates of 30.2 pm/min and 41.4 pm/min in the X-Y plane and Z direction, respectively, at 300K. In addition, we cooled the sample to 1.6 K and took atomic-resolution images of graphite and NbSe2. Finally, we performed a magnetic field sweep test from 0 T to 9 T at 70 K, obtaining distinct graphite images with atomic resolution under varying magnetic fields. These experiments show our newly developed STM's high stability, vibration resistance, and immunity to high magnetic fields.

Atomically resolved low-temperature scanning tunneling microscope operating in a 22 T water-cooled magnet

We present the design and construction of a nonmetallic tip-sample mechanical loop featured Scanning Tunneling Microscope (STM) that operates in a 22 T water-cooled magnet at a low temperature of l.8 K. The STM head mainly consists of a spider-drive motor, stand-alone scanner, moveable sapphire sample holder, and sapphire frame. All parts exist in the tip-sample mechanical loop are made of sapphire to reduce the interference from high magnetic fields. Except for the necessary movement of the tip and scanner, all STM parts are stationary. More importantly, the tip-sample mechanical loop is separate from the motor after detecting the tunneling current, which helps prevent the high voltage signal interference from entering the tip-sample junction, leading to a high stable imaging. A Janis liquid helium cryostat is used to obtain a variable temperature range from 1.8 K to 300 K, and the STM head is cooled down via helium exchange gas. The STM head hangs at the bottom of a probe with a two-stage spring suspension to prevent the huge vibration generated by the water-cooled magnet from entering the tip-sample junction. The performance is demonstrated by atomically resolved STM images of graphite surface at 0 T and 22.8 T under room temperature. Furthermore, the obtained atomic-resolution images of NbSe2 at 1.8 K and 22 T, as well as high-resolution dI/dV spectrums at temperatures from 1.8 K to 8.5 K and magnetic fields from 0 T to 22 T are displayed. This is the first STM capable of atomic-resolution imaging and dI/dV measurement at 1.8 K in a 22 T water-cooled magnet. The high immunity to the magnetic field makes the nonmetallic tip-sample mechanical loop widely useable for atomic-resolution STM imaging in ultra-high magnetic field conditions.

Millikelvin scanning tunneling microscope at 20/22T with a graphite enabled stick-slip approach and an energy resolution below 8 μeV: Application to conductance quantization at 20T in single atom pointcontacts of Al and Au and to the charge density wave of 2H-NbSe2.

We describe a scanning tunneling microscope (STM) that operates at magnetic fields up to 22T and temperatures down to 80 mK. We discuss the designof the STM head, with an improved coarse approach, the vibration isolation system, and efforts to improve the energy resolution using compact filters for multiple lines. We measure the superconducting gap and Josephson effect in aluminum and show that we can resolve features in the density of states as small as 8 μeV. We measure the quantization of conductance in atomic size contacts and make atomic resolution and density of states images in the layered material 2H-NbSe2. The latter experiments are performed by continuously operating the STM at magnetic fields of 20T in periods of several days without interruption.

A millikelvin scanning tunneling microscope in ultra-high vacuum with adiabatic demagnetization refrigeration.

We present the design and performance of an ultra-high vacuum scanning tunneling microscope (STM) that uses adiabatic demagnetization of electron magnetic moments for controlling its operating temperature ranging between 30 mK and 1K with an accuracy of up to 7 μK rms. At the same time, high magnetic fields of up to 8T can be applied perpendicular to the sample surface. The time available for STM experiments at 50 mK is longer than 20 h, at 100 mK about 40 h. The single-shot adiabatic demagnetization refrigerator can be regenerated automatically within 7 h while keeping the STM temperature below 5K. The whole setup is located in a vibrationally isolated, electromagnetically shielded laboratory with no mechanical pumping lines penetrating its isolation walls. The 1K pot of the adiabatic demagnetization refrigeration cryostat can be operated silently for more than 20 days in a single-shot mode using a custom-built high-capacity cryopump. A high degree of vibrational decoupling together with the use of a specially designed minimalistic STM head provides outstanding mechanical stability, demonstrated by the tunneling current noise, STM imaging, and scanning tunneling spectroscopy measurements, all performed on an atomically clean Al(100) surface.

A hybrid magnet based scanning tunneling microscope.

In this paper, a scanning tunneling microscope (STM) is presented that operates in a 27.5 T magnetic field within a hybrid magnet. The coarse approach of the STM is realized by using an inertial piezoelectric motor, and the scanning is realized by using a miniature scanner, which stands alone on a sapphire base. A combined vibration isolation system consisting of a brick-rubber-brick stack and two springs is used to isolate the vibration generated from the magnet. An enclosed copper shield is used to prevent sound from entering the tip-sample junction. The sound and vibration isolation measures highly improve the stability of the STM imaging. All the materials selected to construct the STM head are nonmagnetic. The drift rates of the STM in the X-Y plane and Z direction are as low as 26.2 pm/min and 34.6 pm/min, respectively, under ambient conditions. The high performance of the homebuilt STM was demonstrated by graphite hexagonal lattice images obtained in magnet fields ranging from 0 T to 27.5 T even without the protection of a vacuum and low temperatures. As far as known, this is the first STM that operates in a hybrid magnet. It is also the first STM that can obtain graphite hexagonal lattice images in magnetic fields up to 27.5 T. Our results greatly contribute to the further STM studies under ambient conditions and ultrahigh magnetic fields.

A variable-temperature scanning tunneling microscope operated in a continuous flow cryostat.

We present a continuous flow cryostat scanning tunneling microscope (STM) which can be operated at temperatures between 4.9 K and 300 K. A variable temperature insert with 20 mm aperture is installed in the cryostat. A base temperature of 4.9 K at the STM head was achieved with helium exchange gas cooling. By using a novel design of zirconia structure, the size of the STM head was minimized to 9.2 mm in outer diameter. The high compactness and rigidity of the STM head make it less sensitive to vibrations, contributing to the high stability of the tip-sample junction. The drifting rates of the STM at 4.9 K in the X-Y plane and Z direction are 1.96 and 3.05 pm/min, respectively. The performance of the STM was demonstrated by atomically resolved imaging of graphite and NbSe2 without using external vibrational isolation. Furthermore, the dI/dV spectra of NbSe2 were resolved near its critical temperature, illustrating the formation process of the superconducting gap as a function of temperature. This STM is ultrahigh vacuum and strong magnetic field compatible, making it promising for direct application in the extreme conditions involving water-cooled magnets and hybrid magnets.

The design and the performance of an ultrahigh vacuum 3He fridge-based scanning tunneling microscope with a double deck sample stage for in-situ tip treatment

Scanning tunneling microscope (STM) is a powerful tool for studying the structural and electronic properties of materials at the atomic scale. The combination of low temperature and high magnetic field for STM and related spectroscopy techniques allows us to investigate the novel physical properties of materials at these extreme conditions with high energy resolution. Here, we present the construction and the performance of an ultrahigh vacuum 3He fridge-based STM system with a 7 Tesla superconducting magnet. It features a double deck sample stage on the STM head so we can clean the tip by field emission or prepare a spin-polarized tip in situ without removing the sample from the STM. It is also capable of in situ sample and tip exchange and preparation. The energy resolution of scanning tunneling spectroscopy at T = 310 mK is determined to be 400 mK by measuring the superconducting gap with a niobium tip on a gold surface. We demonstrate the performance of this STM system by imaging the bicollinear magnetic order of Fe1+xTe at T = 5 K

An ultra-high vacuum scanning tunneling microscope operating at sub-Kelvin temperatures and high magnetic fields for spin-resolved measurements.

We present the construction and performance of an ultra-low-temperature scanning tunneling microscope (STM), working in ultra-high vacuum (UHV) conditions and in high magnetic fields up to 9 T. The cryogenic environment of the STM is generated by a single-shot 3He magnet cryostat in combination with a 4He dewar system. At a base temperature (300 mK), the cryostat has an operation time of approximately 80 h. The special design of the microscope allows the transfer of the STM head from the cryostat to a UHV chamber system, where samples and STM tips can be easily exchanged. The UHV chambers are equipped with specific surface science treatment tools for the functionalization of samples and tips, including high-temperature treatments and thin film deposition. This, in particular, enables spin-resolved tunneling measurements. We present test measurements using well-known samples and tips based on superconductors and metallic materials such as LiFeAs, Nb, Fe, and W. The measurements demonstrate the outstanding performance of the STM with high spatial and energy resolution as well as the spin-resolved capability.

Compact low temperature scanning tunneling microscope with in-situ sample preparation capability.

We report on the design of a compact low temperature scanning tunneling microscope (STM) having in-situ sample preparation capability. The in-situ sample preparation chamber was designed to be compact allowing quick transfer of samples to the STM stage, which is ideal for preparing temperature sensitive samples such as ultra-thin metal films on semiconductor substrates. Conventional spring suspensions on the STM head often cause mechanical issues. To address this problem, we developed a simple vibration damper consisting of welded metal bellows and rubber pads. In addition, we developed a novel technique to ensure an ultra-high-vacuum (UHV) seal between the copper and stainless steel, which provides excellent reliability for cryostats operating in UHV. The performance of the STM was tested from 2 K to 77 K by using epitaxial thin Pb films on Si. Very high mechanical stability was achieved with clear atomic resolution even when using cryostats operating at 77 K. At 2 K, a clean superconducting gap was observed, and the spectrum was easily fit using the BCS density of states with negligible broadening.

A 30 mK, 13.5 T scanning tunneling microscope with two independent tips.

We describe the design, construction, and performance of an ultra-low temperature, high-field scanning tunneling microscope (STM) with two independent tips. The STM is mounted on a dilution refrigerator and operates at a base temperature of 30 mK with magnetic fields of up to 13.5 T. We focus on the design of the two-tip STM head, as well as the sample transfer mechanism, which allows in situ transfer from an ultra high vacuum preparation chamber while the STM is at 1.5 K. Other design details such as the vibration isolation and rf-filtered wiring are also described. Their effectiveness is demonstrated via spectral current noise characteristics and the root mean square roughness of atomic resolution images. The high-field capability is shown by the magnetic field dependence of the superconducting gap of CuxBi2Se3. Finally, we present images and spectroscopy taken with superconducting Nb tips with the refrigerator at 35 mK that indicate that the effective temperature of our tips/sample is approximately 184 mK, corresponding to an energy resolution of 16 μeV.

A 350 mK, 9 T scanning tunneling microscope for the study of superconducting thin films on insulating substrates and single crystals

We report the construction and performance of a low temperature, high field scanning tunneling microscope (STM) operating down to 350 mK and in magnetic fields up to 9 T, with thin film deposition and in situ single crystal cleaving capabilities. The main focus lies on the simple design of STM head and a sample holder design that allows us to get spectroscopic data on superconducting thin films grown in situ on insulating substrates. Other design details on sample transport, sample preparation chamber, and vibration isolation schemes are also described. We demonstrate the capability of our instrument through the atomic resolution imaging and spectroscopy on NbSe2 single crystal and spectroscopic maps obtained on homogeneously disordered NbN thin film.

18/20 T high magnetic field scanning tunneling microscope with fully low voltage operability, high current resolution, and large scale searching ability

We present a home-built 18/20 T high magnetic field scanning tunneling microscope (STM) featuring fully low voltage (lower than ±15 V) operability in low temperatures, large scale searching ability, and 20 fA high current resolution (measured by using a 100 GOhm dummy resistor to replace the tip-sample junction) with a bandwidth of 3.03 kHz. To accomplish low voltage operation which is important in achieving high precision, low noise, and low interference with the strong magnetic field, the coarse approach is implemented with an inertial slider driven by the lateral bending of a piezoelectric scanner tube (PST) whose inner electrode is axially split into two for enhanced bending per volt. The PST can also drive the same sliding piece to inertial slide in the other bending direction (along the sample surface) of the PST, which realizes the large area searching ability. The STM head is housed in a three segment tubular chamber, which is detachable near the STM head for the convenience of sample and tip changes. Atomic resolution images of a graphite sample taken under 17.6 T and 18.0001 T are presented to show its performance.

A versatile variable-temperature scanning tunneling microscope for molecular growth

We describe and discuss the design of a variable-temperature scanning tunneling microscope (STM) system for the study of molecules at temperatures between 18 and 300 K in ultrahigh vacuum. The STM head is a refinement of a very rigid design developed and successfully operated in Hamburg. In the current version, the head is connected to a liquid helium flow cryostat, thereby reaching a base temperature of 18 K. To minimize the heat load on the STM head, a helium back flow cooled radiation shield is installed. The dimensions and the choice of materials are based on simulations of the heat dissipation. The STM is galvanically isolated from the vacuum chamber to minimize electronic noise and mechanically decoupled by means of springs and an eddy current damping stage. Additionally, the design of the STM head allows the deposition of several molecular materials onto the same cold sample surface. The operation of the STM in imaging mode is demonstrated for TPP/Cu(111) and FePCNaClCu(111). Spectroscopic capabilities of the system are shown for electronic states on NaClCu(111) and TPP/Cu(111).

An ultrahigh vacuum fast-scanning and variable temperature scanning tunneling microscope for large scale imaging

We describe the design and performance of a fast-scanning, variable temperature scanning tunneling microscope (STM) operating from 80 to 700 K in ultrahigh vacuum (UHV), which routinely achieves large scale atomically resolved imaging of compact metallic surfaces. An efficient in-vacuum vibration isolation and cryogenic system allows for no external vibration isolation of the UHV chamber. The design of the sample holder and STM head permits imaging of the same nanometer-size area of the sample before and after sample preparation outside the STM base. Refractory metal samples are frequently annealed up to 2000 K and their cooldown time from room temperature to 80 K is 15 min. The vertical resolution of the instrument was found to be about 2 pm at room temperature. The coarse motor design allows both translation and rotation of the scanner tube. The total scanning area is about 8 x 8 microm(2). The sample temperature can be adjusted by a few tens of degrees while scanning over the same sample area.

4 K , ultrahigh vacuum scanning tunneling microscope having two orthogonal tips with tunnel junctions as close as a few nanometers

An instrument that incorporates two scanning tunneling microscope (STM) tips which can have their tunnel junctions as close together as a few nanometers was designed and built. The sample is fixed and can be imaged simultaneously and independently with both STM tips. The tips and sample can be positioned and angled to image the same surface or perpendicular surfaces. The entire STM head is cooled with liquid helium to about 4K while in an ultrahigh vacuum environment. Macroscopic positioning of the tips is accomplished using piezoelectric “stick-slip” coarse motion stages, whereas atomic positioning is accomplished with piezoelectric tube scanners. This instrument addresses the critical need to locally characterize individual nanostructures and heterostructures.

Variable Structure Control of a Piezoelectric Actuator for a Scanning Tunneling Microscope

Scanning probe microscopes are now widely used in the field of material science and engineering for surface imaging at atomic scale. Their principle is based on the surface probing by a sharp tip approached at a nanometric distance of the surface. The probe is fixed to piezoelectric actuators allowing its displacement above the surface. An electronic command of a scanning tunneling microscope (STM) has been designed and tested. The regulation feedback loop of the tunnel current includes an integral controller, as is the case in commercial equipment. An extra control by variable-structure system has been implemented on this electronic command. Its principle is based on the commutation of the feedback sign. The effect on the system performance of the variable structure control is presented and discussed. An STM head has been modeled and all the model parameters have been determined. The model has been validated by comparison of the experimental and simulated responses of the system under excitation.

Development of ultralow temperature scanning tunneling microscope cooled by a dilution refrigerator

An ultralow temperature scanning tunneling microscope (STM) was developed by installing an STM head in a dilution refrigerator. Using the apparatus, STM images and the differential tunneling conductance spectra were taken at the sample temperature of 90 mK on a cleaved surface of 2H-NbSe2. The atomically-resolved and the CDW-modulated STM images, and the superconduting gap as well as the CDW gap in the tunneling conduction (dI/dV) spectra were observed. [DOI: 10.1380/ejssnt.2004.151]

Direct measurement instrument for lattice spacing on regular crystalline surfaces using a scanning tunneling microscope and laser interferometry

An instrument for direct measurement of the lattice spacing on regular crystalline surfaces, which incorporates a scanning tunneling microscope (STM) and a phase modulation homodyne interferometer (PMHI), was developed. Our aim was to verify the applicability of the length measurement method in which the lattice spacing on the crystalline surface obtained with the STM is used as a fine scale and the optical interference fringe, i.e., wavelength λ, of the PMHI is used as a coarse scale. The instrument consists of a STM head with a YZ axes tip scanner, a precise X-axis sample stage with flexure springs, and the PMHI with a four-path differential configuration. Combining the movements of the YZ-axes tip scanner and the X-axis sample stage, the instrument can perform long atomic STM imaging of the crystalline surface along the X axis, which is also the fast scanning axis for eliminating thermal drift. The relative displacement of the X-axis sample stage between optical interference dark fringes (=null points) of the PMHI, which is λ/16 times the integer value in the design, can be measured with a resolution of 10 pm or less using the phase modulation technique. The lattice spacing on a highly oriented pyrolytic graphite (HOPG) crystalline surface was measured by comparing the number of atoms in the atomic STM image of 100 nm length with the optical fringes of the PMHI. The mean and expanded uncertainty (k=2) of the lattice spacing between α sites of the HOPG surface were 0.246 nm and 7 pm, respectively. The mean value was very close to that reported by Park and Quate [Sang-II Park and C. F. Quate, Appl. Phys. Lett. 48, 112 (1986)]. The experimental results also show the feasibility of realizing length measurement using the lattice spacing on the crystalline surface and the PMHI.