Session X1: Focus Session: Advances in Scanned Probe Microscopy IV: New Instrumentation & Techniques

ezAFM: A low cost Atomic Force Microscope(AFM)

A low cost AFM, ezAFM is developed for educational purposes as well as research. Optical beam deflection method is used to measure the deflection of cantilever. ezAFM scanner is built using voice coil motors (VCM) with $\sim $50x50x6 $\mu $m scan area. The microscope uses alignment free cantilevers, which minimizes setup times. FPGA based AFM feedback Control electronics is developed. FPGA technology allows us to drive all peripherals in parallel. ezAFM Controller is connected to PC by USB 2.0 interface as well as Wi-Fi. We have achieved $<$5nm lateral and $\sim $0.01nm vertical resolution. ezAFM can image single atomic steps in HOPG and mica. An optical microscope with $<$3 $\mu $m resolution is also integrated into the system. ezAFM supports different AFM operation modes such as dynamic mode, contact mode, lateral force microscopy. Advanced modes like magnetic force microscopy and electric force microscopy will be implemented later on. The new ezAFM system provides, short learning times for student labs, quick setup and easy to transport for portable applications with the best price/performance ratio. The cost of the system starts from {\$}15,000, with system performance comparable with the traditional AFM systems.   

Compact Scanning Tunneling Microscope for Spin Polarization Measurements

We have built a low temperature scanning tunneling microscope for spin-polarized studies. An important aspect of our design philosophy is to keep everything small, starting with a one-inch STM body that fits in the UHV bore of a small superconducting solenoid that provides up to 8 Tesla parallel to the tip. This, in turn, makes the liquid helium and liquid nitrogen dewars smaller and leads to a compact UHV chamber. The largest flange in the system is 10 inches in outer diameter. The benefits of a smaller system include lower consumption of cryogens and a reduced footprint. The STM has been tested from 300K to 77K and has achieved atomic resolution. A test at 4.2K will be done soon. We have imaged cobalt clusters deposited in situ using a simple and compact design for an electron-beam evaporator. We have developed new electronics for z-approach and a novel magnetically-coupled manipulator with an actuated grabber for tip and sample exchange.   

Development of Molecular Beam Epitaxy/Pulsed Laser Deposition/Low Temperature Spin-Polarized Scanning Tunneling Microscopy System

Spin-polarized scanning tunneling microscopy and spectroscopy have shown tremendous abilities to obtain detailed spin information about surfaces down to the atomic scale. In order to take full advantage of this method for studying pristine, as-prepared sample surfaces, we couple an SP-STM system to a sophisticated ultra-high-vacuum growth facility. The hybrid molecular beam epitaxy/pulsed laser deposition/spin-polarized STM system is home-designed and constructed with many unique features. A wide variety of engineered spintronic materials can be grown in the 8 source growth chamber, or using the 9 source laser deposition system. Samples may be heated during growth to as high as 1300 K or cooled using LN2 to temperatures below 195 K, while being simultaneously probed using reflection high energy electron diffraction. The system is currently configured for nitride systems as well as transition metal or rare earth ultra-thin films. Prepared samples are transferred through a central distribution chamber to the LHe-cooled, spin-polarized STM operating inside an integral superconducting magnet (0-4.5 T). The system is outfitted with magnetic tip preparation. The magnetic field allows us to manipulate the magnetic structure of samples during SP-STM experiments.   

A Dual Tip STM for Imaging the Superconducting Phase Difference

We have built a dual tipped millikelvin STM with each tip capable of independently scanning a sample. We will use the STM to measure spatial variations of the gauge-invariant phase difference at the atomic scale. The two tips along with the superconducting sample constitute a SQUID. This configuration is designed to minimize fluctuations in the superconducting phase of one of the tips as it scans the sample, hence inhibiting supercurrent suppression. We are currently developing a technique to fabricate superconducting Niobium tips for use with this system.   

mK-Scanning Probe Microscope(mK-SPM) operating in a Cryogen-Free Dilution Refrigerator at 20mK

Dramatic increase in liquid helium price limits the usage of cryogenic equipment. Dry cryogen-free dilution refrigerators(DR) systems are promising platforms to run mK-Scanning Probe Microscopes(mK-SPM) systems with a number of operating modes: STM, AFM, MFM, EFM, SSRM, PFM, etc. We present the design of a mK-Scanning Probe Microscope (mK-SPM) operating in a cryogen-free DR. An Oxford Instrument cryogen-free DR(Triton DR200) with 200uW cooling power and 7mK base temperature is used for the experiments. A 1W Pulse Tube cryocooler is integrated into the DR. After wiring and attaching the microscope we achieved 20mK base temperature. Piezo driven Stick slip coarse approach mechanism is used to bring the sample in to close proximity of the sample. In these initial results we deliberately did not take any precautions to isolate the pumping lines, attached to the DR and the DR itself. The turbomolecular pump was attached directly to the top plate of the DR. We first tested our mK-SPM in Scanning Tunnelling Microscope (STM) mode as it is the most sensitive of the SPM techniques. An image, using a gold coated 6$\mu $m period calibration grating at 20mK, obtained under these rudimentary conditions.   

Atom-Specific Interaction Quantification and Identification by 3D-SPM

Entire scientific disciplines such as mechanics and chemistry are governed by the interactions between atoms and molecules. On surfaces, forces extending into the vacuum direct the behavior of phenomena such as thin film growth, nanotribology, and surface catalysis. To advance our knowledge of the fundamentals governing these topics, it is desirable to simultaneously map electron densities and quantify force interactions between the surface of interest and a probe with atomic resolution. Using the oxygen-reconstructed copper (100) surface as a model system, we demonstrate that much of this information can be derived from combining three-dimensional atomic force microscopy (3D-AFM) with simultaneous STM. The three-dimensional scanning probe microscopy (3D-SPM) variant resulting from this combination provides complementary information in the various interaction channels recorded. The surface oxide layer of copper (100) features defects and a distinct structure of the Cu and O sublattices that is ideally suited for such model investigations. The analysis of our experimental results allows for the identification of atomic species and defects on the sample surface through the comparison of simultaneously recorded force and current data.   

Laser Scanning Microscopy for Quantitative Measurement of the Local Microwave-Photonic Properties of Advanced Materials and Devices

We present laser scanning microscopy (LSM) as a non-contact and non-invasive instrumentation technique for quantitative measurement of the local microwave, optoelectronic, and optical properties of advanced materials including superconductors and graphene. Since an LSM setup may be configured in different modes of operation, we will focus on the photoresponse and reflectivity imaging modes. It will be discussed how an LSM, in the photoresponse measurement mode, may be used to image the distribution of rf/microwave currents at the surface of a superconducting device, such as a resonator. In addition, relevant techniques for distinguishing the kinetic and resistive parts of the superconductor's photoresponse as well as imaging its possible anisotropic microwave properties are addressed. With regard to the reflectivity mode, we will present how this method enables precise measurement of the topological features and optical properties of non-opaque samples. As an example, we will show how the thickness of few-layer graphene flakes may be measured by this method.   

Torsional tapping atomic force microscopy for molecular resolution imaging of soft matter

Despite considerable advances in image resolution on challenging, soft systems, a method for obtaining molecular resolution on `real' samples with significant surface roughness has remained elusive. Here we will show that a relatively new technique, torsional tapping AFM (TTAFM), is capable of imaging with resolution down to 3.7 Angrstrom on the surface of `bulk' polymer films [1]. In TTAFM T-shaped cantilevers are driven into torsional oscillation. As the tip is offset from the rotation axis this provides a tapping motion. Due to the high frequency and Q of the oscillation and relatively small increase in spring constant, improved cantilever dynamics and force sensitivity are obtained. As the tip offset from the torsional axis is relatively small (typically 25 microns), the optical lever sensitivity is considerably improved compared to flexural oscillation. Combined these give a reduction in noise floor by a factor of 12 just by changing the cantilever geometry. The ensuing low noise allows the use of ultra-sharp `whisker' tips with minimal blunting. As the cantilevers remain soft in the flexural axis, the force when imaging with error is also reduced, further protecting the tip. We will show that this combination allows routine imaging of the molecular structure of semicrystalline polymer films, including chain folds, loose loops and tie-chains in polyethylene, and the helical conformation of polypropylene within the crystal, using a standard, commercial AFM. \\[4pt] [1] N Mullin, JK Hobbs, PRL 107, 197801 (2011)   

Reference system for scanning probe tip fingerprinting

Knowledge of the chemical structure of the tip asperity in Non-Contact Atomic Force Microscopy (NC-AFM) is crucial as controlled manipulation of atoms and/or molecules on surfaces can only be performed if this information is available. However, a simple and robust protocol for ensuring a specific tip termination has not yet been developed. We propose a procedure for chemical tip finger printing and an example of a reference system, the oxygen-terminated Cu(110) surface, that enables one to ensure a specific tip termination with Si, Cu, or O atoms. To follow this up and unambiguously determine tip types, we performed a theoretical DFT study of the line scans with the tip models in question and found that the tip characterization made based on experimental results (Cu/O-terminated tip imaging Cu/O atoms) is in fact incorrect and the opposite is true (Cu/O-terminated tip imaging O/Cu atoms). This protocol allows the tip asperity's chemical structure to be verified and established both before as well as at any stage of the manipulation experiment when numerous tip changes may take place.   

Micromolding fabrication of ``T'' cross section SiC SPM probes

Micromolding techniques using pre-ceramic polymers have been demonstrated for the creating silicon carbide based AFM cantilevers. In an attempt to reach higher resonance frequencies without significantly increasing the spring constant of the cantilevers, a ``T'' cross section cantilever was fabricated. The resonant frequency and spring constant have been compared to the standard rectangular cross section cantilevers of both Sic and commercially available silicon cantilevers.   

Single molecule and single atom sensors for atomic resolution imaging of chemically complex surfaces

To resolve single atoms has always been a major goals of surface science. Mapping forces with a dynamic AFM, it is possible to reconstruct atomic resolution of various surfaces and of large organic molecules[1]. At the same time scanning tunneling hydrogen microscopy (STHM) reaches atomic scale resolution and reveals intermolecular interactions with much less technical effort[2]. Besides H$_{2}$ and D$_{2}$, also individual Xe atoms, single CO and CH$_{4}$ molecules adsorbed at the tip apex of an STM function as microscopic force sensors that change the tunneling current in response to the forces acting from the surface. An STM equipped with any of these sensors is able to image the Pauli repulsion and thus resolve the inner structure of organic adsorbates. The more rigidly bounded CO yields the strongest, least distorted contrast. Thus, the sensor functionality can be tailored by tuning the interaction between sensor particle and STM tip. Hence, STHM belongs to a wider family of atomic-sensor microscopy techniques.\\{[}1] L. Gross et al., Science 325, 1110 (2009)\\{[}2] R. Temirov et al., New J. Phys. 10, 053012 (2008); C. Weiss et al., Phys. Rev. Lett. 105, 086103 (2010), C. Weiss et al., J. Am. Chem. Soc. 132, 11864 (2010); G. Kichin et al., J. Am. Chem. Soc. 133, 16847 (2011)   

Detection of cantilever thermal motion and feedback cooling using a quantum point contact

Nanomechanical oscillators enable ultrasensitive detection of force, mass and displacement. In particular, recent measurements of oscillator displacement have achieved an imprecision below that at the standard quantum limit (SQL), using optical [1] or microwave techniques [2]. A quantum point contact (QPC) has been employed as a transducer of nanomechanical motion, thanks to the sensitive dependence of its conductance on electrostatic fields. Such an approach has been demonstrated in combination with an off-board micromechanical cantilever in a versatile design compatible with nanoscale oscillators and, in principle, with a variety of force-sensing applications, including magnetic resonance force microscopy [3]. The aim of the research we present here is to improve this technique and to approach the SQL by accessing a regime in which, due to the one-dimensional electron transport, quantum mechanical back-action effects emerge on the mechanical resonator. We demonstrate the use of different types of QPCs as sensitive detectors of the low-temperature thermal motion of an off-board cantilever and their ability to cool the cantilever oscillation mode through feedback.\\[4pt] [1] Phys. Rev. A 82, 061804 (2010)\newline [2] Nat. Nano. 4, 820 (2009)\newline [3] Nat. Phys. 4, 635 (2008)   

Design of a Self-Aligned, High Sensitivity Fiber Fabry-Perot Interferometer for Low Temperature Atomic Force Microscope/Magnetic Force Microscope

We describe the design of a high sensitivity fiber Fabry-Perot interferometer for low temperature atomic force microscope/magnetic force microscope. This is a self-aligned system utilizing an alignment chip and eliminating all tedious alignment procedures. Our interferometer cavity is composed of a cleaved fiber, which is coated using dielectric to increase the reflectivity of laser from fiber-air interface, and a cantilever. 50 percent of the incident laser beam is reflected at the end of the fiber. The transmitted light propagates from the fiber end and hits the cantilever. Multiple reflections occur between cantilever and the fiber then the beams go into the fiber again. These two beams interfere and generate a photocurrent at the PD which is used for deflection measurement. We designed a special stick-slip coarse approach mechanism using piezoelectric tube scanner of the microscope. We have measured 8fm per square root Hz noise level at 300K, while the shot noise limit was 2fm per square root Hz. Our previous Michelson interferometer design had 20 fm per square root Hz noise level and gave better than 10nm MFM resolution on hard disk. Our goal is to further enhance the noise levels and achieve 6 nm resolution for LT-MFM with this new interferometer.   

Next Generation SPM Control System with ppm precision

Novel scanning probe microscopy techniques, modes of operation, and advances in microscope hardware are pushing the boundary for signal resolution and flexibility in SPM measurements. Here we present a new state of the art SPM control system, which improves signal precision, resolution, bandwidth and noise performance by about one order of magnitude compared to current generation controllers. The controller incorporates the performance of expensive dedicated instruments in a compact modular multichannel package. In combination with the well proven and flexible Nanonis SPM control software, this next generation controller is the ideal platform for the most demanding microscopy, spectroscopy and transport measurements tasks, opening the door to a larger range of applications compared to current systems. Furthermore, the flexible and easily configurable user interface of the controller and the large number of measurement channels allows its operation as a high performance DC and AC source and measurement interface with ppm precision and multiple lock-in amplifiers, opening new perspectives for materials research.   

Conical spin-spiral ground state of a Mn double layer on W(110) driven by higher-order exchange interactions

The magnetic properties of transition-metal nanostructures are commonly explained based on the interplay of Heisenberg exchange, Dzyaloshinskii-Moriya (DM) interaction and magnetocrystalline anisotropy while higher order terms such as the biquadratic exchange and the four-spin interaction are typically neglected due to their small strength. Here, we demonstrate that higher-order terms can play a crucial role for the magnetic ground state and report as an example a transverse conical spin-spiral state in an ultra-thin film composed of two atomic layers of Mn on W(110). This spin structure is characterized by magnetic moments rotating on a cone that is perpendicular to the [001] propagation direction of the spin-spiral with a periodicity of 2.4 nm. The cones of nearest-neighbor Mn atoms point into opposite directions which results in nearly antiferromagnetic alignment. This intriguing spin structure has been resolved on the atomic-scale using spin-polarized scanning tunneling microscopy and confirmed to be the ground state by first-principles calculations based on DFT. Our calculations also reveal that the canting of the spins is induced by higher-order exchange interactions while the spiraling along the [001]-direction is due to frustrated Heisenberg exchange and DM interaction.