Ultrahigh Vacuum Atomic Force Microscope Research Articles

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.

Observation of robust superlubricity of MoS2 on Au(111) in ultrahigh vacuum

The structural and superlubric properties of single layer MoS2 on Au(111) forming moiré superlattice structures have been investigated by means of ultrahigh vacuum atomic force microscope with bimodal and contact modes. We synthesize epitaxial monolayer MoS2 flakes on the Au(111) surface in ultrahigh vacuum. Using friction force microscopy, atomic friction measurements indicate a superlubric regime between the tip apex and the moiré corrugated MoS2 surface in which the friction force remains at an ultralow value and is independent from normal load. Superlubricity conditions are observed for different loads and velocities which indicates the absence of out-of-plane deformations. We find that the MoS2 layer including the moiré superlattice modulation originating from the natural misfit between MoS2 and the Au(111) substrate is relatively rigid. We also demonstrate a low friction coefficient of the MoS2 surface crossing a single Au(111) step. Our results open up a new avenue for minimizing friction in nanoscale electronic devices and other dry rigid contacts used in aerospace lubrication.

2D KBr/Graphene Heterostructures-Influence on Work Function and Friction.

The intercalation of graphene is an effective approach to modify the electronic properties of two-dimensional heterostructures for attractive phenomena and applications. In this work, we characterize the growth and surface properties of ionic KBr layers altered by graphene using ultra-high vacuum atomic force microscopy at room temperature. We observed a strong rippling of the KBr islands on Ir(111), which is induced by a specific layer reconstruction but disappears when graphene is introduced in between. The latter causes a consistent change in both the work function and the frictional forces measured by Kelvin probe force microscopy and frictional force microscopy, respectively. Systematic density functional theory calculations of the different systems show that the change in work function is induced by the formation of a surface dipole moment while the friction force is dominated by adhesion forces.

Bottom-tracking: the possibilities and physical meaning of keeping the bottom of the frequency shift in atomic force microscopy

In ultrahigh vacuum atomic force microscopy, the frequency of an oscillator holding the scanning tip takes a typical Lennard–Jones like curve with a local minima just before contact. We demonstrate here the application of a control scheme to keep this local minima, or the bottom of the frequency shift curve (FC) as the working point for imaging in UHV, and discuss its physical meaning and possible applications. Tip-sample distance modulation and Lock-in detection were used to obtain the gradient of the FC, where null control of the gradient signal resulted in implementing the designated control. Atomic resolution was confirmed for Si(111) and solder. The histogram of minimum frequency shift on the apex of atomic features on solder showed two to four peaks, implying the ability of the method to map characteristic differences of the depth of the FC per site. The method is an alternative to the existing constant-frequency-shift mode and constant-height mode, with the possibility to access chemical information on-the-fly.

Morphological Transitions in Organic Ultrathin Film Growth Imaged by In Situ Step-by-Step Atomic Force Microscopy

In situ atomic force microscopy (AFM) allowed us to investigate the evolution at the early stages of the growth of organic thin films. An ultrahigh-vacuum atomic force microscope, integrated with a...

Size investigation of silicon nanoclusters deposited on HOPG using noncontact atomic force microscopy

The sizes of silicon nanoparticles produced using two different novel methods are investigated in this report. The method of production used to generate silicon oxide nanoparticles was achieved via gas aggregation codeposition with water on a cold target, and also via a liquid jet method. The nanoparticles were drop-cast on a highly oriented pyrolytic graphite (HOPG) substrate and assessed using ultrahigh vacuum atomic force microscopy (AFM). Noncontact constant force mode was used in the AFM investigations. The silicon nanoparticles produced using the gas aggregation and water codeposition method were found to be smaller than 2 nm. A degree of deviation in the measured sizes of the silicon nanoparticles in different layers was detected. The size deviation was attributed to surface-nanoparticle, surface-tip, and nanoparticle-tip interactions. Silicon nanoparticles produced in alcohol using the liquid jet method were also found to be smaller than 2 nm. The solvent used for the silicon nanoparticles was varied in our investigations. When water was used as a solvent, a size deviation for silicon nanoparticles in different layers was also observed.

Influence of O2 exposure on the interaction between CH4 and amorphous AlYB14

The influence of surface oxidation on the interaction between CH4 and amorphous AlYB14 (a-AlYB14) has been studied theoretically by using density functional theory and experimentally by ultra-high vacuum atomic force microscopy (UHV-AFM). CH4 mimics the –CH3 termination and aliphatic subunits of a polymer chain. Low-energy ion scattering measurements of magnetron sputtered thin films suggest that the bonding at the surfaces of pristine a-AlYB14 and O2 exposed a-AlYB14 (O2//a-AlYB14) is metal-boron and metal-oxygen dominated, respectively. Based on the ab initio calculations the adsorption energies of CH4 on a-AlYB14 and O2//a-AlYB14 decreases from −0.07 to −0.30eV, respectively. This trend is consistent with experimental data obtained by colloidal probe UHV-AFM studies with a polyethylene sphere, where larger adhesion forces for the O2 exposed surface as compared to the pristine a-AlYB14 surface were measured. No charge transfer takes place between CH4 and the pristine as well as the O2 exposed a-AlYB14. Oxygen chemisorption induces changes in surface bonding. States at the Fermi level are depleted upon oxidation, hence the surface bonding becomes more semiconducting causing a charge redistribution within the adsorbed CH4 molecule. Hence, these data serve as proof of concept for exploring the effect of O2 exposure on the interaction between aliphatic polymers and a-AlYB14 using a correlative experimental and theoretical research approach.

Effect of low-temperature-grown GaSb layer on formation of high-density and small GaSb islands on Si(100) substrate

The effect of a low-temperature-grown gallium antimonide (LT-GaSb) layer on the formation of high-density GaSb islands on a Si(100) substrate was studied using ultrahigh-vacuum scanning tunneling microscopy (UHV-STM) and atomic force microscopy. By using an LT-GaSb layer in the initial growth stage (new growth method), high-density and small GaSb islands were formed at 500 °C regardless of the type of the reconstructed surface. The density and size of the GaSb islands with the LT-GaSb layer were estimated to be ∼1.0 × 1011 cm−2 and ∼20 nm, respectively. The STM results showed that the LT-GaSb layer was composed of an Sb-rich amorphous GaSb layer. With a rapid increase in temperature under the Sb flux, the LT-GaSb layer immediately changed into GaSb nuclei with a high density (2.1 × 1011 cm−2) and small size (11 nm). It is suggested that these high-density and small GaSb islands grow from the GaSb nuclei, which act as crystal seeds.

A Reexamination of Phonon Transport Through a Nanoscale Point Contact in Vacuum

Using a silicon nitride cantilever with an integral silicon tip and a microfabricated platinum–carbon resistance thermometer located close to the tip, a method is developed to concurrently measure both the heat transfer through and adhesion energy of a nanoscale point contact formed between the sharp silicon tip and a silicon substrate in an ultrahigh vacuum atomic force microscope at near room temperature. Several models are used to evaluate the contact area critical for interpreting the interfacial resistance. Near field-thermal radiation conductance was found to be negligible compared to the measured interface thermal conductance determined based on the possible contact area range. If the largest possible contact area is assumed, the obtained thermal interface contact resistance can be explained by a nanoconstriction model that allows the transmission of phonons from the whole Brillouin zone of bulk Si with an average finite transmissivity larger than 0.125. In addition, an examination of the quantum thermal conductance expression suggests the inaccuracy of such a model for explaining measurement results obtained at above room temperature.

Size dependent polarization reversal at nanoscale metal-ferroelectric interfaces

Ferroelectric oxides possess a unique set of physical properties that have not been maximally exploited due to a lack of understanding of the origins of the switching behavior. Currently, controversy exists over whether switching in ferroelectric oxides is due to polarization or defect mediated switching. Here, we use ultra-high vacuum scanning tunneling microscopy and atomic force microscopy to investigate the process of polarization reversal and to assess the effect of the electronic structure and conductance on the switching behavior of single crystal BaTiO3 (001). Measurements of the local density of states revealed the surface switching behavior was initially due to polarization and that a transition occurred to defected mediated switching after repeated switching cycles. Atomic force microscopy investigations of the size effects at nanoscale Au-BaTiO3 (001) interfaces revealed asymmetric polarization reversal at large interfaces with a convergence to symmetric behavior as the size scale decreased, in agreement with the tunneling-based scanning tunneling microscopy technique.

Piezoelectric force microscopy of crystalline oxide-semiconductor heterostructures

Thin films of epitaxial SrTiO3 grown on silicon exhibit compressive in-plane strain that may stabilize ferroelectricity in this normally non-ferroelectric material. We investigate this possibility by using an ultra-high vacuum atomic force microscope to measure the local force response of coherently strained SrTiO3 films on silicon to an applied ac electric field. The observed cantilever response is different in regions that were previously written with positive and negative voltages, but the frequency dependence of this response indicates that the dominant forces are related to electrostatic charging rather than ferroelectricity.

Thermomechanical stability of ultrananocrystalline diamond

We have measured mechanical stiffness and dissipation in ultrananocrystalline diamond (UNCD) from 63 K to 450 K using microcantilever resonators in a custom ultrahigh vacuum (UHV) atomic force microscope. UNCD exhibits a temperature coefficient of modulus that is found to be extremely low: −26 ppm/K, which is close to the previously measured value of −24 ppm/K for single crystal diamond. The magnitude and the temperature dependence of dissipation are consistent with the behavior of disordered systems. The results indicate that defects, most likely at the grain boundaries, create the dominant contribution to mechanical dissipation. These measurements of modulus and dissipation versus temperature in this temperature range in UNCD establish the nanostructure’s effect on the thermomechanical stability and suggest routes for tailoring these properties.

Atomic-scale nanoindentation: detection and identification of single glide events in threedimensions by force microscopy

Indentation experiments on the nanometre scale have been performed by means of atomicforce microscopy in ultra-high vacuum on KBr(100) surfaces. The surfaces yield in the formof discrete surface displacements with a typical length scale of 1 Å. These surfacedisplacements are detected in both normal and lateral directions. Measurement of thelateral tip displacement requires a load-dependent calibration due to the load dependenceof the effective lateral compliance. Correlation of the lateral and normal displacementsfor each glide event allow identification of the activated slip system. The resultsare discussed in terms of the resolved shear stress in indentation experimentsand of typical results in atomistic simulations of nanometre-scale indentation.

Atomic layer deposition reactor for fabrication of metal oxides

AbstractWe developed an ultra high vacuum (UHV) compatible atomic layer deposition (ALD) reactor for studying the growth of metal oxides. The compatibility of the ALD reactor with the UHV system allows the use of surface sensitive techniques for characterizing the substrate/thin film complex after each ALD cycle. For investigation of the growth of metal oxides we used synchrotron radiation X‐ray photoelectron spectroscopy, UHV atomic force microscopy and a combined system consisting of electron energy loss spectroscopy and ultraviolet photoelectron spectroscopy. The samples were analyzed after each ALD cycle, composed of the subsequent exposure of the surface to metal oxides precursors and H2O (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Atomically resolved surface structures of vapor deposited amorphous silicon–carbon alloys: An atomic force microscopy and spectroscopic study

Silicon carbide alloys are widely used in high-tech applications due to their interesting combination of chemical, mechanical and electronic properties. Growing thin films of this material in a simple and controlled way is a hot topic in modern material's science. In particular, the possibility to tailor the film properties just by tuning the deposition temperature would be an important progress. In the present work amorphous silicon–carbon alloys thin films have been deposited by electron beam sublimation of a poly-crystalline silicon carbide target in vacuum environment. The deposition temperature was varied from Room Temperature to about 1300 K. The resulting films were analyzed by means of Ultra High Vacuum–Atomic Force Microscopy (UHV–AFM) down to even atomic resolution. The observed features agree with literature data, e.g. interatomic bond lengths, as achieved by others methods, and the structural arrangements of silicon and carbon atoms as concluded from IR and Raman spectroscopy measurements carried out on the same samples. The results not only allow a correlation between film properties and deposition temperature but also support the notion of the UHV –AFM images of the amorphous surfaces being atomically resolved.

Temperature dependence of single-asperity friction for a diamond on diamondlike carbon interface

A variable temperature, ultrahigh vacuum atomic force microscope (AFM) was used to characterize interfacial friction for a single-asperity diamond contact on a diamondlike carbon (DLC) substrate over a nominal substrate temperature range of 90 to 275 K. Calibrated friction force measurements were obtained by analyzing lateral force hysteresis loops as a function of normal force. For sufficiently large normal forces, the lateral force was proportional to the normal force, and a friction coefficient μ could be identified. μ varied approximately linearly with substrate temperature, with μ=0.28 at T=90 K and μ=0.38 at 275 K. These results are compared to other recent variable temperature AFM friction measurements and to theoretical calculations based on the Tomlinson model. This comparison is obscured by large, experimentally uncontrolled temperature differences between the tip and the substrate which inevitably exist in conventional, variable temperature AFMs. A thermal model which can be used to quantitatively estimate these temperature differences is presented.

In situ characterization of initial growth of HfO2

The initial growth of HfO2 on Si (111) is monitored in situ by ultrahigh vacuum (UHV) scanning probe microscopy. UHV scanning tunneling microscopy and UHV atomic force microscopy reveal the topography of HfO2 films in the initial stage. The chemical composition is further confirmed by x-ray photoelectron spectroscopy. Scanning tunneling spectroscopy is utilized to inspect the evolution of the bandgap. When the film thickness is less than 0.6 nm, the bandgap of HfO2 is not completely formed. A continuous usable HfO2 film with thickness of about 1.2 nm is presented in this work.

Growth and Characterization of Ultrathin Fe Films on Molecule-Adsorbed MgO Surfaces

Effects of the molecule-adsorption on a MgO surface on the growth of ultra-thin Fe films were investigated. Surface observation by reflection high-energy electron diffraction and ultra-high vacuum atomic force microscopy showed that the adsorption of molecules on the MgO surface affected the morphology of Fe films grown on the MgO surface. It was revealed that the surface flatness of Fe films was improved by appropriate molecule-adsorption on the MgO surface.

Interfacial potential barrier theory of friction and wear

An interfacial potential barrier theory to calculate friction and wear is proposed by considering the micro interaction of frictional surfaces. The theory suggests that the performance of friction and wear depends on the magnitude and distribution of the interfacial potential barrier on contact surfaces. The calculation methods of the interfacial potential barrier and standard interfacial potential barrier are then studied and the formulas to calculate the friction force, friction coefficient, and quantity of adhesion wear are derived based on the theory. With its independence and stability, the standard interfacial potential barrier can be used as an index to describe the frictional performance of materials. The calculation results of the friction force with some existing experimental data are consistent with the experimental results performed with an ultra high vacuum atomic-force microscope, which proves that the theory and method are feasible.

Charge retention characteristics of silicon nanocrystal layers by ultrahigh vacuum atomic force microscopy

The nanoscale charge retention characteristics of both electrons and holes in SiO2 layers containing silicon nanocrystals were investigated with ultrahigh vacuum conductive-tip noncontact atomic force microscopy. The results revealed much longer hole retention time (e.g., >1 day) than that of electrons (e.g., ∼1 h). A three-dimensional electrostatic model was developed for charge quantification and analysis of charge dissipation. Based on the superior retention characteristics of holes, a p-channel nanocrystal memory working with holes is suggested to be an interesting choice in improving data retention or in further device scaling.