Ultrahigh Vacuum Atomic Force Microscope Research Articles

Electrostatic force-feedback force sensor incorporated in an ultrahigh vacuum force microscope

A force sensor based on a fiber-optic interferometric displacement transducer incorporated in an ultrahigh vacuum atomic force microscope is described. The operation of the sensor is based on balancing the tip-sample interfacial force using an electrostatic actuator. The electrodes of the actuator are formed by the grounded W cantilever and the metallized end facet of the optical fiber used by the interferometer. Chemical reduction of Ag by a wet chemical method is used for metal coating of the fiber end. A special masking procedure is used to obtain a window hole in the metal coating at the position of the fiber core to allow for optical beam output. Using a window instead of a semitransparent metal film allows us to save the low-finesse characteristics of the interferometer which facilitates the calibration of cantilever displacement. The performance of the sensor is discussed and exemplified by experimental results from force-separation measurements on the W–Au system in ultrahigh vacuum.

In situ monitoring of the growth of oxide thin films at high oxygen pressure

Pulsed laser deposition is a well established deposition technique for oxide thin films and heterostructures. The knowledge and control of the growth processes of oxide materials are important for the fabrication of high-quality epitaxial and heteroepitaxial thin-film structures. Because of the high oxygen pressure during the growth process standard reflection high-energy electron diffraction (RHEED) analysis is not possible. To overcome this problem we have used a two-stage differential pumping system. In this way, the electron path in the high-pressure oxygen atmosphere within the deposition chamber has been reduced considerably. The specular beam intensity of the RHEED system was sufficiently high to allow the observation of intensity oscillations during the growth of oxide materials such as SrTiO3 and La2/3Ca1/3MnO3. In addition to the RHEED investigation the surface morphology of the deposited films has been characterized in situ by ultrahigh-vacuum atomic force microscopy.

Load-Independent Friction: MoO3 Nanocrystal Lubricants

The behavior of the model lubricant system molybdenum disulfide (MoS2) has been characterized as a function of thermal oxidation on the atomic to nanometer scales with scanning tunneling microscopy (STM) and atomic force microscopy (AFM) in ultrahigh vacuum (UHV). STM studies of single-crystal MoS2 surfaces showed that the initial stages of thermal oxidation produce atomic-scale pits and then molybdenum oxide (MoO3) nanocrystals 1−5 nm in diameter. The densities of pits and MoO3 nanocrystals and the size of MoO3 nanocrystals increased with increasing oxidation time. UHV AFM friction measurements made on these same samples exhibited a direct relationship between defect density and friction for short oxidation times. After longer oxidation times when well-defined MoO3 nanocrystals were present, load-independent friction was observed. A model, which involves adhesion of MoO3 nanocrystals to the AFM probe tip end, has been developed to explain this first observation of load-independent friction. The implications of these results are discussed.

Comparative Surface Studies at Atomic Resolution with Ultrahigh Vacuum Variable-Temperature Atomic Force and Scanning Tunneling Microscopes.

: With the ultrahigh vacuum variable-temperature scanning tunneling microscope (UHV-VT-STM), atomic-level observation has been achieved. An ultrahigh vacuum atomic force microscope (UHV-AFM) has also been developed, with success in obtaining atom images where observation in noncontact (NC) mode with a frequency modulation (FM) detection method was attempted. Using the FM detection method in the constant oscillation amplitude of the cantilever excitation mode, we have obtained atomic-resolution images of Si(111) 7 x 7 structures and Si(100) 2 x 1 structures and other structures together with STM images in an ultrahigh vacuum environment. Also shown here are contact potential difference (CPD) images using the NC-AFM method.

Tip cleaning and sharpening processes for noncontact atomic force microscope in ultrahigh vacuum

Tip cleaning and sharpening processes for noncontact atomic force microscope (AFM) operated in ultrahigh vacuum (UHV) were carried out and evaluated by a scanning Auger microscope (SAM) with a field emission electron gun and a noncontact AFM in UHV combined with a scanning tunneling microscope and a field emission microscope. The cantilever used in this study was piezoresistive, which can be heated by passing a current through the resistive legs of the cantilever. As a pretreatment, the tip was irradiated with ultraviolet light in oxygen to remove carbon contaminants. It was heated at about 750°C to form a clean oxide layer in oxygen of 5×10 −5 Torr in an SAM chamber. The desorption of the layer can make a remained tip apex sharper by heating under electron beam irradiation. A thermally oxidized layer was also eliminated by HF etching to sharpen the tip apex. The procedures are useful to obtain a well-defined Si tip suitable for a noncontact AFM.

Annealing of CaF 2 adlayers grown on Si(111): investigations of the morphology by atomic force microscopy

The successive annealing of CaF 2 adlayers deposited at low temperature (400°C) on vicinal Si(111) has been studied in situ by ultra high vacuum atomic force microscopy (UHV-AFM). Monoatomic islands on top of closed CaF 2 layers observed for the as-deposited adlayers vanish during the first annealing steps (<600°C). In addition notches of monoatomic height are formed at the previous straight adlayer steps of the CaF 2 terraces. They have triangular shape in agreement with the non-polar {111} facets of the B-orientated CaF 2 adlayer. For higher annealing temperatures the CaF 2 is partly dissociated so that Ca clusters are formed on top of the adlayer steps. This extraordinary nucleation site is attributed to the partial lateral relaxation of the CaF 2 adlayer.

Fundamental measurements of the friction of clean and oxygen-covered VC(100) with ultrahigh vacuum atomic force microscopy: evidence for electronic contributions to interfacial friction

Atomic force microscopy (AFM) has been employed in an ultrahigh vacuum (UHV) environment to measure the frictional properties of single-crystal vanadium carbide (VC) as a function of reaction with oxygen. The VC(100) surface, prepared by sputtering and annealing in vacuum, was found to be free of oxygen and to exhibit good crystallographic order with atomically flat terraces ∼100–200 Å in width. The coefficient of friction between a silicon nitride probe tip and the clean VC surface was measured as 0.52±0.04. Chemical modification of this surface was accomplished by exposure to molecular oxygen which produced a saturation coverage of chemisorbed atomic oxygen. The coefficient of friction between the same silicon nitride tip and the “oxidized” VC surface in the wearless regime was measured as 0.32±0.05, representing a ∼40% reduction in friction. This reduction in friction is associated with a reduction in the density of metal d electrons nearest the Fermi level which occurs upon oxygen adsorption. These changes provide evidence for the contribution of an electronic mechanism of energy dissipation at the sliding interface of a solid–solid point contact.

A Surface Science Study of Model Catalysts. 1. Quantitative Surface Analysis of Wet-Chemically Prepared Cu/SiO2 Model Catalysts

Cu/SiO2 model catalysts containing nanometer-sized Cu particles on a flat silica model support were wet-chemically prepared and characterized in detail by a variety of surface science techniques. The particle size and shape, particle number density, metal surface coverage, total metal loading, and oxidation state of the particles were determined by ultrahigh vacuum atomic force microscopy, electron microscopy, low-energy ion scattering, Rutherford backscattering spectrometry, and X-ray photoelectron spectroscopy. Deposition of a Cu precursor on a flat Si wafer with a SiO2 top layer by spin-coating was followed by calcination in air at 450 °C. This preparation method produces both homogeneously distributed hemispherical CuO particles with an average height of 8 nm and highly dispersed oxidic Cu species. Subsequent reduction in hydrogen at 250 °C results in metallic and more rounded Cu particles with an average height of 8 nm.

Atomic Force Microscopy Study of an Ideally Hard Contact: The Diamond(111)/Tungsten Carbide Interface

A comprehensive nanotribological study of a hydrogen-terminated diamond(111)/tungsten carbide interface has been performed using ultrahigh vacuum atomic force microscopy. Both contact conductance, which is proportional to contact area, and friction have been measured as a function of applied load. We demonstrate for the first time that the load dependence of the contact area in UHV for this extremely hard single asperity contact is described by the Derjaguin-M\"uller-Toporov continuum mechanics model. Furthermore, the frictional force is found to be directly proportional to the contact area.

Using a Moderate Vacuum, Hot/Cryo-Stage Equipped AFM for In-Situ Observation of α-Phase Growth In 60SN40PB Hypoeutectic Solder

Abstract Atomic force microscopy (AFM) was introduced in 1984, and proved to be more versatile than scanning tunneling microscopy (STM) due to the AFM's capabilities to scan non-conductive samples under atmospheric conditions and achieve atomic resolution. Ultra high vacuum (UHV) AFM has been used in surface science applications when control of oxidation and corrosion of a sample's surface are required. Expensive equipment and time consuming sample exchanges are two drawbacks of the UHV AFM system that limit its use. Until recently, no hot/cryo-stage, moderate vacuum, controlled gas environment AFM was commonly available. We have demonstrated that phase transformations are easily observable in metal alloys and polymers with the use of a moderate vacuum AFM that has in-situ heating/cooling capabilities and quick (within minutes) sample exchange times. This talk will describe the results of experiments involving a wide range of samples designed to make use of the full capabilities of a hot/cryo-stage, controlled gas environment AFM.

Nucleation and surface morphology evolution of ferroelectric SrBi 2Ta 2O 9 films studied by atomic force microscopy

Nucleation stages and surface morphology evolution of epitaxial SrBi 2Ta 2O 9 thin films on SrTiO 3 (001) substrates have been studied by ultra-high vacuum atomic force microscopy. The SrBi 2Ta 2O 9 nucleates and grows half-unit-cell by half-unit-cell as well-defined 2D islands at low coverage. After 2D growth stage, which proceeds up to several half unit-cell layers, a transition from 2D to 3D island growth is observed. After coalescence of small 3D islands into larger islands, clear columnar structures, consisting of spirals, emerge. The surfaces of films with their thickness above 1500 Å exhibit round hills consisting of curved terraces with sizes of steps ranging from 6 Å to 12.5 Å.

Simultaneous force and conduction measurements in atomic force microscopy

We used an ultrahigh-vacuum atomic force microscope (AFM) to measure lateral forces and conductivity simultaneously as a function of the applied normal force for nanometer-sized elastic contacts. Metal-coated or bare Si AFM tips are used on cleaved ${\mathrm{NbSe}}_{2}$ or graphite surfaces. Results are used to compare various means of obtaining the tip-sample contact area ${(A}_{0}).$ We find that simple continuum models can give a reasonable description of the mechanical behavior of the contact. Specifically, the Maugis-Dugdale model [D. Maugis, J. Colloid Interface Sci. 150, 243 (1992)] provides a good basis for describing the elastic contact between an AFM tip and a smooth sample. The theoretical variation in contact radius with load is in good agreement with the experimental variation in friction force, conductivity, and lateral tip-sample contact stiffness. To find the contact area ${A}_{0},$ the best approaches appear to be either to fit the applied force data to an appropriate continuum model (provided the contact is elastic) or to measure the lateral tip-sample contact stiffness. In principle we show that conduction AFM methods can be used to find ${A}_{0}$ for Ohmic contacts. However, the uncertainty in the conduction properties of available AFM tips means that at present the absolute value of ${A}_{0}$ cannot be found with confidence. In this regard the use of metal-coated tips can often be misleading for conduction and mechanical measurements because metal wears rapidly off all or some of the tip apex.

Atomic-scale friction on diamond(111) studied by ultra-high vacuum atomic force microscopy

The frictional properties of a diamond(111) single-crystal surface have been studied using an ultra-high vacuum atomic force microscope, while the presence or absence of hydrogen on the surface was monitored by low-energy electron diffraction (LEED). We have observed, for the first time, atomic-scale stick-slip features of a silicon tip on the hydrogen-terminated (1 × 1) diamond(111) surface. The distance between the stick-slip rows, measured perpendicularly to the rows, is typically 2–3 Å, consistent with the lattice parameters of diamond. Removal of the hydrogen from the surface, indicated by a change in LEED pattern from (1 × 1) to (2 × 1), gives rise to enormous stick-slip features at larger scale. The average friction coefficient on the hydrogen-free surface is found to be more than two orders of magnitude larger than on the hydrogen-terminated surface for loads up to 30 nN.

Development of ultrahigh vacuum-atomic force microscopy with frequency modulation detection and its application to electrostatic force measurement

We succeeded in high resolution force measurements by using a noncontact ultrahigh vacuum-atomic force microscope (UHV-AFM) with frequency modulation (FM) detection. We clearly observed adatoms and corner holes on the Si(111)7×7 reconstructed surface. Then we applied the noncontact UHV-AFM with FM detection to the high resolution measurement of the electrostatic force. We prevented deterioration of the spatial resolution of the topography by isolating the electrostatic interaction from van der Waals interaction. By simultaneous measurements of the topography and electrostatic force on a silicon oxide, a spatial resolution ∼15 Å of the electrostatic force was achieved.

Preparation and characterization of MgO(100) surfaces

The preparation of crystalline magnesium oxide surfaces by means of mechanical polishing, acid etching, oxidative annealing and vacuum annealing has been studied on an atomic scale using low energy electron diffraction (LEED) and ultrahigh vacuum (UHV) atomic force microscopy (AFM). Mechanical polishing results in a microscopically smooth and an atomically disordered surface. Although acid etching of the magnesium oxide surface results in LEED patterns indicating a crystalline nature, AFM characterization of these surfaces reveals that surface layers are removed in an anisotropic fashion resulting in a surface roughness on the order of 100 Å (which is not probed by the LEED studies). Annealing this same surface in vacuum only modestly reduces the surface roughness while high temperature anneals of an acid etched surface in a 1 atm oxygen environment produces a well ordered surface characterized by 1000 Å terraces separated by single and double layer step heights. Further annealing these terraced surfaces to 1000°C under vacuum produced well-ordered and crystallographic surfaces on an atomic scale. Topographic images of the MgO(100) surface obtained under UHV conditions reveal rows of oxygen atoms oriented in a 〈110〉 direction with a spacing of 3.0 Å and indicate that this surface preparation results in an unreconstructed, bulk termination of the rock salt structure.

Re-entrant behavior of 2D to 3D morphology change and 3D island lateral size equalization via mass exchange in Stranski—Krastanow growth: InAs on GaAs(001)

Abstract Using in-situ, ultrahigh vacuum scanning tunneling and atomic force microscopy and ex-situ photoluminescence (PL) and PL excitation we show that the growth of highly strained InAs on GaAs(001) proceeds via a re-entrant behavior of the 2D to 3D morphology change. Quasi-3D clusters of heights 0.6–1.2 nm first appear at an InAs delivery (θ) of ∼ 1.25 ML, only to disappear for θ ∼ 1.3 ML and reappear again at θ ∼ 1.45 ML, prior to 3D island initiation at ∼ 1.57 ML. With increasing θ, an observed tendency of the 3D islands towards lateral size equalization is shown to be accompanied by mass exchange between the 2D and 3D surface features. Molecular dynamics simulations reveal a decreasing binding energy for atoms at island edges as a function of increasing island size and island—island interaction, thereby suggesting a role for atom detachment from larger islands in the size equalization process.

True atomic resolution imaging with noncontact atomic force microscopy

With an atomic force microscope (AFM) operating in the noncontact mode in an ultrahigh vacuum (UHV), the InP(110)1 × 1 surface and the Si(111)7 × 7 reconstructed surface were observed. The force gradient acting on the tip was detected by frequency modulation method. Rectangle lattice on the InP(110)1 × 1 surface, the adatoms and the corner holes on the Si(111)7 × 7 surface have been clearly and reproducibly resolved, including the atomic-scale point defects. The motion of the defects was observed on the InP(110) surface at room temperature, but not on the Si(111)7 × 7 surface. These results clearly show that the noncontact UHV AFM has true atomic-scale lateral resolution and is quite effective for atomic surface structure analysis in real space.

Ultrahigh vacuum atomic force microscope study of 10–30 nm scale GaAs ridge structure formation by molecular beam epitaxy

The evolution of GaAs ridge structure formation by molecular beam epitaxy on a patterned substrate has been investigated using an ultrahigh vacuum atomic force microscope. It is found that the morphology of ridges can be quite irregular with random formation of various facets in the intermediate phase of growth, but self-smoothing processes of the lateral facets take place later on, leading to very sharp and smooth ridge structures in the end. The ridge top is quite sharp and straight with the height fluctuation of within 1–2 nm over the length of 1.4 μm. The role of the Ga atom flows from the side (111)B surfaces to the top (001) surface and their local modulations are considered to account for these observations.

In situ, atomic force microscope studies of the evolution of InAs three-dimensional islands on GaAs(001)

The size evolution of molecular beam epitaxy-grown strained InAs three-dimensional (3D) islands on GaAs(001) is examined using in situ ultrahigh vacuum atomic force microscopy. Remarkably, just after the initiation of well-formed 3D islands at ∼1.57 ML InAs deposition, the lateral size dispersion and average value are found to first increase drastically with the smallest amount (∼0.05 ML) of additional InAs deposition and then decrease and saturate, indicating the onset of a natural tendency for size equalization, including through loss of material from the initially formed largest islands. These observations are found to be consistent with the previously suggested island-separation dependent influence of the evolving island-induced substrate strain fields on the adatom migration and incorporation/detachment kinetics that control the evolution of the islands.

Strained coherent InAs quantum box islands on GaAs(100): Size equalization, vertical self-organization, and optical properties

Evolution of the size distribution of InAs islands is examined via in situ ultrahigh vacuum atomic force microscope for samples grown via molecular beam epitaxy with InAs deposition at and just above the critical value for two- to three-dimensional growth mode transition. An initial independent island formation and growth followed by a global tendency towards island lateral size equalization is observed. The strain fields induced by the InAs islands in the subsequently grown GaAs cap layer are exploited to fabricate vertically self-organized multiple sets of islands. Optical properties of such vertically self-organized quantum box islands are examined via photoluminescence spectroscopy. Microtwins of very low linear density (&amp;lt;1/μm), originating from the edge of the topmost set of islands are found for the samples with multiple sets of islands and are shown to be the type of defect that appears first to relax the accumulated strain around the multiply stacked islands.