Ultrahigh Vacuum Atomic Force Microscope Research Articles

Initial stages of gold adsorption on silicon stepped surface at elevated temperatures

Experimental study performed by ultrahigh vacuum reflection electron microscopy and atomic force microscopy reveals step instability on Si(111) surface during gold deposition at elevated temperatures (higher than 900°C). Our results show that transformations of regular atomic steps into the system of step bunches and vice versa depend on the gold coverage and direction of the electrical current heating the sample. The mechanism and conditions of the surface morphology transformations are discussed.

Application of onion-like carbon to micro and nanotribology

The tribological properties of onion-like carbon (OLC) have been investigated on a nanometer scale using an ultra-high vacuum atomic force microscope. In addition, the tribological properties of OLC added to oil were investigated. A viewpoint of the practical application of OLC as a lubricating additive to oil, under both experimental conditions, OLC showed low friction. In the latter case, OLC transformed into a low friction material with long life, and appears to be a promising material as nanolubricant.

Adhesion modulation by quantum size effects in Pb∕Si(111) system

Ultrahigh vacuum atomic force microscopy is employed to measure the adhesive force of atomically flat (111) oriented Pb island on Si (111) surface at low temperature (∼60K). The experimental results indicate that for the same (111) surface the adhesive force changes and oscillates with the island thickness. This phenomenon is shown to originate from the electron structure modulation by strong quantum size effects along the island surface normal direction, which modifies surface energy and leads to the observed oscillatory adhesive forces.

Differentiation of molecules in a mixed self-assembled monolayer of H-and Cl-terminatedbicyclo[2.2.2]octane derivatives

H- and Cl-terminal groups of bicyclo[2.2.2]octane (BCO) derivatives in a mixedself-assembled monolayer (SAM) on Au(111) were imaged using a modified Si tip with aCaF2 nanocluster to differentiate the two terminals, which have different electronegativities.In order to achieve this we fabricated a new sample holder, on which aCaF2 single crystal and the mixed SAM on Au(111) could be mounted side byside. We transferred the holder with the two samples into a ultrahighvacuum (UHV) atomic force microscopy (AFM) chamber. Upon cleaving theCaF2 single crystal underUHV, a fresh and clean CaF2(111) surface parallel with the SAM surface appeared within 2 mm of the separation. Themodified Si tip was prepared by repeatedly making contact between a Si tip and theCaF2(111) surface. The resulting modified tip could image the atomic periodicity of aCa2+ and anF− sublatticeon the CaF2(111) surface depending on the sign of the tip-terminating ion, i.e. anF− anda Ca2+ ion, respectively, as reported previously (Foster et al 2002 Phys. Rev. B 66235417). Using the modified Si tip with the known tip-terminating ion, weobserved the Cl-terminal in the surrounding H-terminals in the mixed SAM bynoncontact (NC) AFM. Here, the Cl-terminal is negatively charged due to itselectronegativity and thus the BCO moiety with the Cl-terminal is terminated by aCδ+–Clδ− permanent dipole, while the H-terminal is almost neutral. The Cl-terminal appearedbrighter (more attractive) and darker (more repulsive) than the surrounding H-terminalsin NC-AFM images depending on the sign of the tip-terminating ion, i.e. aCa2+ andan F− on the modified tip, respectively, although the relationship between the image contrast andthe sign of the tip-terminating ion was not always perfect because of the instability of thetip-terminating ion on the nanocluster. The present method can be used to distinguishterminal groups with different electronegativities.

Growth mechanism of ZnSe single crystal by chemical vapour transport method

We attempted to grow ZnSe single crystals by the chemical vapour transport (CVT) method using the source material with different particle diameters. The purpose of this study is to examine the dependence the growth mechanism on the source particle diameter. We observed surface topographies of grown single crystals using the ultra-high vacuum atomic force microscopy (UHV-AFM) and investigated the growth mechanism. Dislocation densities were determined from etch pit density counts. It can be seen that the transport rate is decreasing with the increase in the source particle diameter. In the case of decreasing in the transport rate, transported atoms diffuse easily on the grown surface. Moreover, it turned out that the growth mechanism changed to the two-dimensional growth from the three-dimensional growth because the transport rate decreased. The average value of EPD of 3.0×10 3 cm −2 was obtained. We found that control of the source particle diameter is important for preparing high-quality ZnSe single crystals.

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.

Atomic resolution on MgO(001) by atomic force microscopy using a double quartz tuning fork sensor at low-temperature and ultrahigh vacuum

Atomic resolution with a double quartz tuning fork sensor for low-temperature ultrahigh vacuum atomic force and scanning tunneling microscopy is presented. The new features of the force sensor assembly are discussed. Atomically resolved images of MgO on Ag(001) have been obtained. Images acquired in the attractive and the repulsive regimes controlled to a constant frequency shift are shown, revealing contrast changes.

From tunneling to point contact: Correlation between forces and current

We used a combined ultrahigh vacuum scanning tunneling and atomic force microscope (STM/AFM) to study W tip-Au(111) sample interactions in the regimes from weak coupling to strong interaction and simultaneously measure current changes from picoamperes to microamperes. Close correlation between conductance and interaction forces in a STM configuration was observed. In particular, the electrical and mechanical points of contact are determined based on the observed barrier collapse and adhesive bond formation, respectively. These points of contact, as defined by force and current measurements, coincide within measurement error. Ab initio calculations of the current as a function of distance in the tunneling regime is in quantitative agreement with experimental results. The obtained results are discussed in the context of dissipation in noncontact AFM as well as electrical contact formation in molecular electronics.

Nanoscale morphological and electrical homogeneity of HfO2 and ZrO2 thin films studied by conducting atomic-force microscopy

The morphological and electrical evolution of HfO2 and ZrO2 thin films is investigated on the nanoscale using conducting atomic-force microscopy in ultrahigh vacuum. Films of different thicknesses have been grown by atomic layer deposition. With increasing film thickness the film structure changes from amorphous to polycrystalline. By conducting atomic-force microscopy using local current–voltage curve statistics and two-dimensional current imaging it is found that the formation of crystallites has different effects on the electrical properties of the two dielectrics. In the case of HfO2, the crystalline fraction causes weak spots in the oxide, whereas for the ZrO2 films the crystallites exhibit lower leakage currents compared to the amorphous matrix and leakage is mainly determined by thickness fluctuations.

A Si nanopillar grown on a Si tip by atomic force microscopy in ultrahigh vacuum for a high-quality scanning probe

We grow a Si nanopillar on a commercial Si tip on an atomic force microscopy (AFM) cantilever using AFM in ultrahigh vacuum for a high-quality scanning force probe, and observe noncontact-AFM (nc-AFM) images of Si(111)7×7 and Ge deposited Si(111) with the nanopillar. We observe it ex situ by transmission electron microscopy to confirm its growth and crystallinity. The nc-AFM image clearly showed the high performance of the nanopillar as a probe with respect to the spatial resolution, image stability, and reproducibility. This nanopillar growth technique can elongate the lifetime of the cantilever and be applied to other materials.

Few electrons injection in silicon nanocrystals probed by ultrahigh vacuum atomic force microscopy

Ultrahigh vacuum atomic force microscopy has been used to inject and detect charges in individual silicon nanocrystals. The sensitivity of our measurements is shown to be better than 2e. Injected charge saturates as a function of injection time for a given electric field. The potential of the charged nanocrystal as a function of the number of charges in the dot is in good agreement with a simple electrostatic model.

Double quartz tuning fork sensor for low temperature atomic force and scanning tunneling microscopy

A double quartz tuning fork sensor for low temperature ultrahigh vacuum atomic force and scanning tunneling microscopy is presented. The features of the new sensor are discussed and compared to a single asymmetric tuning fork assembly. In addition, a low temperature ac signal amplifier has been developed to pick up the oscillation amplitude of the tuning fork. Current consumption and amplification factor versus the supply voltage of the amplifier as well as the magnitude response of the sensor have been measured at room temperature, 77 and 4 K. Atomically resolved images of a Ag(111) surface and single Ag atoms on Ag(111) were recorded in the scanning tunneling microscopy mode. Initial atomic force measurements are shown that reveal step resolution on a NiAl(110) surface.

Epitaxial relation and island growth of perylene-3.4.9.10-tetracarboxylic dianhydride (PTCDA) thin film crystals on a hydrogen-terminated Si(1 1 1) substrate

Surface of an off-cut Si(1 1 1) substrate with an average step distance of 100 Å was terminated with monohydride. On this substrate, thin film crystals of organic semiconductor PTCDA were grown by molecular beam epitaxy (MBE). Surface of the thin film crystals on the hydrogen-terminated Si(1 1 1) surface (H-Si(1 1 1)) were observed using ultra-high vacuum scanning tunneling microscopy (UHV-STM) and atomic force microscopy (AFM). The PTCDA thin film crystals had an island shape of Volmer–Weber type. From the direction of the vicinal steps of the substrate [1 1 ̄ 0] , we determined the size and orientation of the PTCDA thin film crystals on the H-Si(1 1 1). Long-axis of the two-dimensional (2D) unit cell of the thin film crystals matches the vector (6, 2) of the H-Si(1 1 1) surface. We proposed two conceivable epitaxial relations: one is 6 2 2/3 11/3 and the other is point-on-line coincidence. 2D unit cells of the thin film crystals have widely stretched structure (2–10%), and the island growth of Volmer–Weber type is probably due to this large lattice misfit.

A low-temperature ultrahigh vacuum atomic force microscope for biological applications

We present an atomic force microscope (AFM) for operation at low temperatures under ultrahigh vacuum conditions. It uses the laser beam deflection method to measure the bending of the cantilever. The four quadrant photodiode allows the detection of vertical and lateral forces. The AFM has been developed for studying biological samples. Images of deoxyribonucleic acid plasmids have been obtained in contact mode.

Hamaker Constants in Integrated Circuit Metalization

A new method for determining Hamaker constants was examined for materials of interest in integrated circuit manufacture. An ultra-high vacuum atomic force microscope and an atomic force microscope operated in a nitrogen environment were used to measure the interaction forces between metals, dielectrics, and barriers used during the metalization portion of integrated circuit manufacturing. The materials studied included copper, silver, titanium nitride, silicon dioxide, poly(tetrafluoroethylene), and parylene-N. Spheres coated with a material of interest were mounted on AFM cantilevers and brought into contact with substrates of interest. The interaction force was measured as the cantilever approached the substrate but before the two surfaces came into contact, and also when the particle was pulled out of contact with the substrate. The Hamaker constant calculation from the contact measurement is based on an adhesion model that quantifies the contribution of geometrical, morphological and mechanical properties of materials to the measured adhesion force. Hamaker constants determined with this new approach were compared with values found by using the Derjaguin approximation for a sphere to describe the interaction force as the cantilever approaches the surface. Both approaches produced similar values for most of the systems studied, with variations of less than 10%.

Scanning-tunneling/atomic-force microscopy study of the growth of KBr films on InSb( [formula omitted

Thin epitaxial films of KBr have been grown on InSb(0 0 1) substrate. Scanning tunneling microscopy and non-contact atomic-force microscopy in ultra-high vacuum have been used to study the film surfaces for coverages ranging from 0.3 to 120 ML. It has been found that initially islands of monatomic thickness are formed. These islands are often cut along 〈1 1 0〉 crystallographic direction and the distribution of these islands on the substrate surface is anisotropic which reflects the anisotropic diffusion of KBr molecules during growth. At 1–1.5 ML coverage a wetting single-atomic KBr film is formed and the deposited material in excess of 1 ML forms rectangular islands with edges oriented along 〈1 0 0〉 and 〈0 1 0〉 crystallographic directions. The KBr/InSb interface is likely stabilized by a bond between the halide ion and A III atoms arranged in chains on c(8×2) InSb. For high (multilayer) KBr coverages the growth is basically a layer-by-layer type but, due to slow diffusion of KBr molecules down across steps, the ( n+1)th layer starts to grow before the completion of nth layer. In result, pyramidal structures of rectangular bases are formed on the surface. These rough films can be, with thermal annealing, converted to flat films exposing large (0 0 1) terraces.

Ultrahigh vacuum scanning probe microscopy studies of carbon onions

Carbon onions were prepared by DC arc charge method. The behavior and electronic properties of carbon onions on highly oriented pyrolytic graphite (HOPG) substrate were studied by ultrahigh vacuum atomic force microscopy and scanning tunneling microscopy (UHV AFM/STM). UHV AFM/STM images showed that these ellipsoidal carbon onions tended to aggregate into clusters on the surface of HOPG. The scanning tunneling spectroscopy indicated that the electrical properties of carbon onions were between graphite and single-shell fullerenes.

Formation and stability of II–VI self-assembled quantum dots revealed by in situ atomic force microscopy

A study on self-assembled CdSe quantum dots (QDs) on ZnSe is reported. The use of an in situ ultra-high vacuum atomic force microscope (UHV-AFM) provides the morphological evidence for the thermally activated reorganization of an initially two-dimensional CdSe film into an array of QDs. The QDs have heights between 1.5 and 3.5 nm and lateral diameters smaller than 10 nm. UHV-AFM images do not yield any evidence for Ostwald ripening. We assign the ultra-high stability to the existence of a kinetic barrier prohibiting the heterosystem to pass from the metastable QD state into the thermodynamic equilibrium.

Development of Low Temperature Ultrahigh Vacuum Atomic Force Microscope/Scanning Tunneling Microscope

A low temperature ultrahigh vacuum atomic force microscope (UHV-AFM) has been developed, which allows the sample to be cooled from room temperature to lower than 28 K during observation and can be attached to the conventional UHV-AFM. Atom resolved non-contact atomic force microscopy (NC-AFM) images of the Si(100) surface were obtained at 120 K and 50 K using the produced low temperature UHV-AFM/STM, and the Si(100) dimer structure was successfully observed for the first time by NC-AFM. It was found that the Si(100)-c(4×2) structure mainly covered the area at 120 K, but the p(2×2) structure area increased at 50 K.

Development of low temperature ultrahigh vacuum noncontact atomic force microscope with PZT cantilever

We constructed a low temperature (LT) ultrahigh vacuum (UHV) atomic force microscope (AFM) system using a frequency modulation (FM) technique. As the displacement sensor, we used a lead zirconate titanium (PZT) cantilever to detect the force interaction between the tip and sample. Although the PZT film can be used as the actuator to oscillate the cantilever, an external piezoactuator is used to oscillate the PZT cantilever for small oscillation amplitude stably. The PZT cantilever does not require any alignment mechanisms for displacement detection and does not have thermal source due to resistive heating. Therefore, it is one of the promising force sensors under LT environments. Attractive force interaction was detected by the PZT cantilever. We presented the preliminary result of imaging of atomic steps on Si(111) surface measured by noncontact-AFM at room temperature and LT environment.