Low-temperature Atomic Force Microscope Research Articles

Design, fabrication, and characterization of kinetic-inductive force sensors for scanning probe applications.

We describe a transducer for low-temperature atomic force microscopy based on electromechanical coupling due to a strain-dependent kinetic inductance of a superconducting nanowire. The force sensor is a bending triangular plate (cantilever) whose deflection is measured via a shift in the resonant frequency of a high-Q superconducting microwave resonator at 4.5 GHz. We present design simulations including mechanical finite-element modeling of surface strain and electromagnetic simulations of meandering nanowires with large kinetic inductance. We discuss a lumped-element model of the force sensor and describe the role of an additional shunt inductance for tuning the coupling to the transmission line used to measure the microwave resonance. A detailed description of our fabrication is presented, including information about the process parameters used for each layer. We also discuss the fabrication of sharp tips on the cantilever using focused electron beam-induced deposition of platinum. Finally, we present measurements that characterize the spread of mechanical resonant frequency, the temperature dependence of the microwave resonance, and the sensor's operation as an electromechanical transducer of force.

A cantilever-based, ultrahigh-vacuum, low-temperature scanning probe instrument for multidimensional scanning force microscopy.

Cantilever-based atomic force microscopy (AFM) performed under ambient conditions has become an important tool to characterize new material systems as well as devices. Current instruments permit robust scanning over large areas, atomic-scale lateral resolution, and the characterization of various sample properties using multifrequency and multimodal AFM operation modes. Research of new quantum materials and devices, however, often requires low temperatures and ultrahigh vacuum (UHV) conditions and, more specifically, AFM instrumentation providing atomic resolution. For this, AFM instrumentation based on a tuning fork force sensor became increasingly popular. In comparison to microfabricated cantilevers, the more macroscopic tuning forks, however, lack sensitivity, which limits the measurement bandwidth. Moreover, multimodal and multifrequency techniques, such as those available in cantilever-based AFM carried out under ambient conditions, are challenging to implement. In this article, we describe a cantilever-based low-temperature UHV AFM setup that allows one to transfer the versatile AFM techniques developed for ambient conditions to UHV and low-temperature conditions. We demonstrate that such a cantilever-based AFM offers experimental flexibility by permitting multimodal or multifrequency operations with superior force derivative sensitivities and bandwidths. Our instrument has a sub-picometer gap stability and can simultaneously map not only vertical and lateral forces with atomic-scale resolution, but also perform rapid overview scans with the tip kept at larger tip–sample distances for robust imaging.

Single hydrogen atom manipulation for reversible deprotonation of water on a rutile TiO2 (110) surface

The discovery of hydrogen atoms on the TiO2 surface is crucial for many practical applications, including photocatalytic water splitting. Electronically activating interfacial hydrogen atoms on the TiO2 surface is a common way to control their reactivity. Modulating the potential landscape is another way, but dedicated studies for such an activation are limited. Here we show the single hydrogen atom manipulation, and on-surface facilitated water deprotonation process on a rutile TiO2 (110) surface using low temperature atomic force microscopy and Kelvin probe force spectroscopy. The configuration of the hydrogen atom is manipulated on this surface step by step using the local field. Furthermore, we quantify the force needed to relocate the hydrogen atom on this surface using force spectroscopy and density functional theory. Reliable control of hydrogen atoms provides a new mechanistic insight of the water molecules on a metal oxide surface.

Characterization of two- and one-dimensional water networks on Ni(111) via atomic force microscopy

Determination of the adsorption structure of water molecules on metal surfaces is an imperative challenge to understanding the mechanisms of the wetting process and water-related heterogeneous catalysis. We identify water monolayers formed on Ni(111) via low-temperature atomic force microscopy, which enables the visualization of individual water molecules in monolayers with higher spatial resolution than scanning tunneling microscopy. On the terraces of Ni(111) at 150 K, water forms monolayers comprising fused pentagonal, hexagonal, and heptagonal rings. Water adsorbates on step sites assemble in a different manner, forming a hydrogen-bonding network with fused pentagonal and octagonal rings aligned in the step direction. Because similar water networks with pentagonal rings have been proposed in monolayers or their defect sites on other metal surfaces, our structural characterization of H2O/Ni(111) provides an insight into water adsorption structures on metals.Received 30 June 2019DOI:https://doi.org/10.1103/PhysRevMaterials.3.093001©2019 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasAdsorptionPhysical SystemsSurfacesTechniquesNoncontact atomic force microscopyScanning tunneling microscopyCondensed Matter, Materials & Applied Physics

Bond-Level Imaging of the 3D Conformation of Adsorbed Organic Molecules Using Atomic Force Microscopy with Simultaneous Tunneling Feedback.

The chemical structure and orientation of molecules on surfaces can be visualized using low temperature atomic force microscopy with CO-functionalized tips. Conventionally, this is done in constant-height mode by measuring the frequency shift of the oscillating force sensor. However, this method is unsuitable for analyzing 3D objects. We are using the tunneling current to track the topography while simultaneously obtaining submolecular resolution from the frequency shift signal. Thereby, the conformation of 3D molecules and the adsorption sites on the atomic lattice can be reliably determined.

Symmetry breakdown of 4,4\u2033-diamino-p-terphenyl on a Cu(111) surface by lattice mismatch

Site-selective functionalization of only one of two identical chemical groups within one molecule is highly challenging, which hinders the production of complex organic macromolecules. Here we demonstrate that adsorption of 4,4″-diamino-p-terphenyl on a metal surface leads to a dissymmetric binding affinity. With low temperature atomic force microscopy, using CO-tip functionalization, we reveal the asymmetric adsorption geometries of 4,4″-diamino-p-terphenyl on Cu(111), while on Au(111) the symmetry is retained. This symmetry breaking on Cu(111) is caused by a lattice mismatch and interactions with the subsurface atomic layer. The dissymmetry results in a change of the binding affinity of one of the amine groups, leading to a non-stationary behavior under the influence of the scanning tip. Finally, we exploit this dissymmetric binding affinity for on-surface self-assembly with 4,4″-diamino-p-terphenyl for side-preferential attachment of 2-triphenylenecarbaldehyde. Our findings provide a new route towards surface-induced dissymmetric activation of a symmetric compound.

Assigning the absolute configuration of single aliphatic molecules by visual inspection

Deciphering absolute configuration of a single molecule by direct visual inspection is the next step in compound identification, with far-reaching implications for medicinal chemistry, pharmacology, and natural product synthesis. We demonstrate the feasibility of this approach utilizing low temperature atomic force microscopy (AFM) with a CO-functionalized tip to determine the absolute configuration and orientation of a single, adsorbed [123]tetramantane molecule, the smallest chiral diamondoid. We differentiate between single enantiomers on Cu(111) by direct visual inspection, and furthermore identify molecular dimers and molecular clusters. The experimental results are confirmed by a computational study that allowed quantification of the corresponding intermolecular interactions. The unique toolset of absolute configuration determination combined with AFM tip manipulation opens a route for studying molecular nucleation, including chirality-driven assembly or reaction mechanisms.

Development of low temperature atomic force microscopy with an optical beam deflection system capable of simultaneously detecting the lateral and vertical forces.

The atomic force microscopy (AFM) is a very important tool for imaging and investigating the complex force interactions on sample surfaces with high spatial resolution. In the AFM, two types of detection systems of the tip-sample interaction forces have been used: an optical detection system and an electrical detection system. In optical detection systems, such as optical beam deflection system or optical fiber interferometer system, both the lateral and the vertical tip-sample forces can be measured simultaneously. In electrical detection systems, such as qPlus or Kolibri sensors, either the lateral or vertical forces can be measured. Simultaneous measurement of the lateral and vertical interaction forces effectively allows investigation of force interactions because the force is a vector with magnitude and direction. In this study, we developed a low-temperature, frequency-modulation AFM using an optical beam deflection system to simultaneously measure the vertical and lateral forces. In this system, the heat sources, such as a laser diode and a current-to-voltage converter, for measuring the photocurrent of the four-segmented photodiode are located outside the observation chamber to avoid a temperature increase of the AFM unit. The focused optical beam is three-dimensionally adjustable on the back side of the cantilever. We demonstrate low-noise displacement measurement of the cantilever and successful atomic resolution imaging using the vertical and lateral forces at low temperatures.

Field ion microscopy characterized tips in noncontact atomic force microscopy: Quantification of long-range force interactions

Direct comparison of tip-sample forces obtained by dynamic force spectroscopy experiments with theoretical simulations is extremely difficult, since the precise tip shape and chemical identity of the apex atoms of the force sensing tip remain unknown in most experiments. Here, we present force curves measured with a tungsten tip on a Ag(111) surface obtained in a low-temperature atomic force microscope using tips that were analyzed by field ion microscopy down to atomic levels. The resulting van der Waals and electrostatic forces were found to be in quantitative agreement with analytical models, if the tip shape parameters from the field ion microscopy analysis were used. Furthermore, our analysis shows an additional long-range force interaction at tip-sample distances above 1.3 nm. We suggest that this unexpected force is related to patch charges arising from the inhomogeneous work function distribution on the surface of highly faceted sharp tips.

In2o3 Thinfilm Analysis by Low Temperature Atomic Force Microscope

Scanning Probe Microscopy (SPM) define a broad group of instruments used to image and measure properties of material, chemical and biological surfaces. Despite the success of SPM, it has some limitation. Because it uses the tunneling current between a biased tip and a sample as the feedback signal, it requires that the sample must be electrically conductive. Also it is difficult to achieve high-resolution images with AFM. The solution to this limitation is to maintain cryogenic temperature for the sample. The cryogenic AFM appears to be the most promising approach, because it will work on all macromolecules. Lowering the sample temperature will increase the mechanical stability of the molecules and will decrease its thermal induced motion. Lower temperature will reduce the thermal induced motion of the cantilever and also the overall noise of the instrument. Lowering the sample temperature up to 137K will decrease RMS area and surface roughness. In2O3 thin film has been used as a sample. Low temperature has been providing by liquid nitrogen container. To avoid condensation on the tip of AFM as well as on the sample Vacuum chamber has been provided. The entire set up must be isolated from surrounding vibration that can damage the tip of AFM. RMS roughness area of In2O3 thin film decreases from 13.682nm to 1.984nm and also line roughness from 2.889nm from 1.443nm. The films exhibit a finer microstructure

Energy Dissipation in Dynamic Force Spectroscopy of PTCDA on Ag-Si(111) .RAD.3*.RAD.3

The energy dissipation of different phases of PTCDA (3,4,9,10-perylene-tetracarboxylic-dianhydride) on Ag-Si(111) √3×√3 was investigated by means of atomic force spectroscopy using a custom-built low-temperature atomic force microscope (LT-AFM). The energy dissipation is due to non-conservative forces between the tip and sample. We present preliminary energy dissipation data measured when the tip was placed above four different PTCDA superstructures. While one of them showed energy dissipation, the others did not. The dissipation process seems to be critically influenced by the junction geometry; a chemical bound between the tip apex and a specific site of the molecular layer might be one channel for the dissipation. [DOI: 10.1380/ejssnt.2011.21]

Demonstration of low-temperature atomic force microscope with atomic resolution using piezoresistive cantilevers

Compared to current optical-lever methods adopted in atomic force microscopes, nonoptical methods such as piezoresistive methods and quartz fork methods can be more advantageous due to their smaller installation size and the lack of electromagnetic effects from laser light during electrical conductivity experiments. As a technological demonstration of nonoptical methods, a low-temperature atomic force microscope using piezoresistive cantilevers was developed and operated at liquid-He4 temperatures (5K). The cantilever and sample can be transferred from atmosphere to the microscope head operating at low temperatures. Both contact mode and noncontact mode were used for testing the system while carrying out atomic resolution studies on clean Si(111) and clean Si(100) surfaces.

Versatile low-temperature atomic force microscope with in situ piezomotor controls, charge-coupled device vision, and tip-gated transport measurement capability

A versatile cryogenic (5 K) ultrahigh-vacuum (UHV) atomic force microscope (AFM) with tip-gated transport measurement capability has been developed. Using high-resolution (<1.5μm) plan-view charge-coupled device (CCD) optics, and three planar piezomotors we achieved visually guided in situ alignments of a sample position with respect to the AFM tip, and the laser beam position with respect to the cantilever and the quadrant photodiode. We made optical fiber feedthroughs and a laser lens assembly to bring external laser light and CCD illuminating light onto the cantilever and the sample. A sample holder with an embedded temperature sensor and eight transport electrodes is detachably mounted on a piezotube scanner. The generic cantilever mount can be easily replaced with a tuning-fork mount or a piezoresistive cantilever mount for experiments where stray laser light should be avoided. To our knowledge, this is the first Dewar-immersion type cryogenic AFM with laser beam deflection sensing capability and high-resolution plan-view CCD optics.

In situ observation of freeze-fractured and deep-etched red blood cells with a high-vacuum low-temperature atomic force microscope

A high-vacuum low-temperature atomic force microscope (AFM) for the direct observation of freeze-fracture samples has been developed. This AFM has a freeze-fracture mechanism inside the vacuum chamber. With this AFM it is possible to observe the fractured surface directly without both fabricating a replica and exposure to the ambient atmosphere. Both sandwich and knife fracture methods have been achieved to obtain freeze-fracture surfaces and after deep etching. A fine structure of the fractured red blood cell membrane has been observed using both methods. These are relatively quick and easy methods for the observation of freeze-fracture surfaces without introducing replica artifacts.

In Situ Observation of Freeze Fractured Red Blood Cells with a High-Vacuum Low-Temperature Atomic Force Microscope

Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.

Revealing the hidden atom in graphite by low-temperature atomic force microscopy

Carbon, the backbone material of life on Earth, comes in three modifications: diamond, graphite, and fullerenes. Diamond develops tetrahedral sp3 bonds, forming a cubic crystal structure, whereas graphite and fullerenes are characterized by planar sp2 bonds. Polycrystalline graphite is the basis for many products of everyday life: pencils, lubricants, batteries, arc lamps, and brushes for electric motors. In crystalline form, highly oriented pyrolytic graphite is used as a diffracting element in monochromators for x-ray and neutron scattering and as a calibration standard for scanning tunneling microscopy (STM). The graphite surface is easily prepared as a clean atomically flat surface by cleavage. This feature is attractive and is used in many laboratories as the surface of choice for "seeing atoms." Despite the proverbial ease of imaging graphite by STM with atomic resolution, every second atom in the hexagonal surface unit cell remains hidden, and STM images show only a single atom in the unit cell. Here we present measurements with a low-temperature atomic force microscope with pico-Newton force sensitivity that reveal the hidden surface atom.

Study of Probes and Substrates for Low Temperature Atomic Force Microscopy and Biological Applications

The atomic force microscopy in ultrahigh vacuum and at low temperature demonstrated its excellent capability to reach atomic resolution. Nevertheless in the case of biological samples high resolution has been achieved only in very few cases. We demonstrated here the importance of the appropriate choice of probes and substrates in order to image DNA at low temperature with high resolution. We investigated properties of three types of cantilevers and they were studied by scanning electron microscopy as a function of temperature. A large bending of cantilevers, which were coated from both sides, was observed at low temperatures. Therefore uncoated cantilevers pare strongly recommended for low temperature applications. Different methods for immobilization of DNA on the substrate are examined at low temperatures. First images of linear DNA on graphite at 82 K under ultrahigh vacuum conditions are presented.

Quantitative measurement of short-range chemical bonding forces.

We report direct force measurements of the formation of a chemical bond. The experiments were performed using a low-temperature atomic force microscope, a silicon tip, and a silicon (111) 7x7 surface. The measured site-dependent attractive short-range force, which attains a maximum value of 2.1 nanonewtons, is in good agreement with first-principles calculations of an incipient covalent bond in an analogous model system. The resolution was sufficient to distinguish differences in the interaction potential between inequivalent adatoms, demonstrating the ability of atomic force microscopy to provide quantitative, atomic-scale information on surface chemical reactivity.

Development of low-temperature and high vacuum atomic force microscope with freeze–fracture function

A high vacuum low temperature atomic force microscope (AFM) for the direct observation of biological freeze–fracture samples has been developed. This AFM has an integrated vacuum system and a freeze–fracture mechanism inside the vacuum chamber. It is possible to observe the fractured sample surface without exposing the freshly fractured surface to the ambient atmosphere. The design is described in detail. A sample temperature up to −175 °C is achieved, with the temperature fluctuation less than 0.1 °C. Temperature control is possible from −175 to 0 °C for deep etching. An ultimate vacuum pressure of 8.7×10−6 Pa has been achieved. Freeze-fractured human red blood cell has been examined, and fine structure within the cell has been observed. It is demonstrated that this is convenient and useful for the direct observation of a biological freeze–fractured surface.

Scanned potential microscopy of edge states in a quantum Hall liquid

Using a low-temperature atomic force microscope as a local voltmeter, we measure the Hall voltage profile in a quantum Hall conductor in the presence of a gate-induced non-equilibrium edge state population at ν=3. We observe sharp voltage drops at the sample edges which are suppressed by re-equilibrating the edge states.