H. Tokumoto

Nanoscale investigation of fatigue effects in Pb(Zr,Ti)O3 films

Scanning force microscopy has been used to perform a comparative nanoscale study of domain structures and switching behavior of Pb(ZrxTi1−x)O3 (PZT) thin films integrated into heterostructures with different electrodes. The study revealed a significant difference between polarization state of as‐deposited PZT films on RuO2 and Pt electrodes. The PZT/RuO2 films exhibit polydomain crystallites and show almost symmetric switching behavior, while the PZT/Pt films are mainly in a single polarity state and exhibit highly asymmetric piezoelectric hysteresis loops. Formation of unswitchable polarization within the grains of submicron size as a result of fatigue process was directly observed.

Nanoscale imaging of domain dynamics and retention in ferroelectric thin films

We report results on the direct observation of the microscopic origins of backswitching in ferroelectric thin films. The piezoelectric response generated in the film by a biased atomic force microscope tip was used to obtain static and dynamic piezoelectric images of individual grains in a polycrystalline material. We demonstrate that polarization reversal occurs under no external field (i.e., loss of remanent polarization) via a dispersive continuous-time random walk process, identified by a stretched exponential decay of the remanent polarization.

Scanning force microscopy for the study of domain structure in ferroelectric thin films

A piezoresponse technique based on scanning force microscopy (SFM) has been used for studying domain structure in ferroelectric thin films. Studies were performed on Pb(Zrx,Ti1−x)O3(PZT) thin films produced by a sol–gel method. The piezoresponse images of the PZT films were taken before and after inducing polarization in the films by applying a direct current voltage between the bottom electrode and the SFM tip. Polarization induced patterns were written with 20 V pulses and subsequently imaged by the SFM piezoresponse technique. The effect of the film structure on the imaging resolution of domains is discussed.

Asymmetric nanoscale switching in ferroelectric thin films by scanning force microscopy

Scanning force microscopy (SFM) has been used to perform nanoscale studies of the switching behavior of Pb(Zr, Ti)O3 thin films via the direct observation of their domain structures. The study revealed a significant asymmetry of a switching pattern which is a function of the voltage polarity and original domain structure of individual grains. The phenomenon of asymmetric switching is attributed (1) to the presence of an internal built-in electric field at the bottom interface and (2) to the mechanical stress exerted by the SFM tip. The former effect results in incomplete 180° switching, while the latter effect leads to a 90° rotation of the polarization vector. The resulting shear stress deformation of the grain underneath the tip combined with the applied field effect propels polarization reversal in the adjacent grains.

Domain structure and polarization reversal in ferroelectrics studied by atomic force microscopy

The ferroelectric domain structure and its dynamics under applied electric field have been studied with nanoscale resolution by atomic force microscopy (AFM). Two mechanisms responsible for the contrast between opposite domains are proposed: large built‐in domains are delineated in friction mode due to the tip–sample electrostatic interaction, and small domains created by an external field are imaged in topography mode due to piezoelectric deformation of the crystal. The ability of effective control of ferroelectric domains by applying a voltage between the AFM tip and the bottom electrode is demonstrated. It is experimentally confirmed that the sidewise growth of domain proceeds through the nucleation process on the domain wall.

Direct observation of SiH3 on a 1%‐HF‐treated Si(111) surface by scanning tunneling microscopy

Scanning tunneling microscopy (STM) has been made on an as‐prepared Si(111) surface by the 1%‐HF treatment. The STM images for both the empty and filled states exhibit regular dots with the threefold symmetry on the flat parts of the surface: the distance between dots measures 2.2 A. The origin of these dots can be ascribed to the H atoms of the trihydride (SiH3) phase on the Si(111) surface. The electrons can tunnel from or to the tail states of the σ (filled) states or the σ* (empty) states around the H atoms for the SiH3 radicals, respectively.

Carbon nanotube tips for thermomechanical data storage

We report the demonstration of thermomechanical data storage in a poly(methylmethacrylate) film using a multiwalled carbon nanotube (MWCNT) tip. Indentation densities of >250 Gbits/in2 are achieved. The power efficiency of indent writing with MWCNT tips is found to be higher than that of conventional silicon tips owing to better heat transfer at the tip–polymer interface.

Molecular arrangement of copper phthalocyanine on hydrogen‐terminated Si(111): Influence of surface roughness

The molecular arrangement of copper phthalocyanine (CuPc) crystals on hydrogen‐terminated Si(111) surfaces was investigated by using both scanning probe microscopy and x‐ray diffraction in terms of influence of the surface roughness. On a rough surface with a root‐mean‐square roughness of 0.20 nm, the molecules were stacked so as to form an α crystal where the molecular column was parallel to the surface. On the other hand, a new crystal form with its column exactly perpendicular to the Si(111) plane was grown on a atomically flat surface. In this case, the molecules were stacked perpendicular to the substrate with the underlying molecules situated directly below. These molecular arrangements were independent of the growth temperature in the range of 60–180 °C. On the atomically flat surfaces, the strong interactive force between the surface and the planar CuPc molecule may result in the new growth behavior.

Nanometer-scale patterning of self-assembled monolayer films on native silicon oxide

A nanoscale-patterning method on silicon oxide using a self-assembled monolayer (SAM) was developed. The silicon surface with native oxide was additionally oxidized locally in dry nitrogen atmosphere by the field-induced oxidation (FIO) technique using an atomic force microscope with a conductive cantilever, and then immersed in octadecyltrichlorosilane (OTS) solution. The contact angle and topography image revealed that the OTS layer was formed only on the native oxide. In contrast, when FIO was performed under the humidity of 88%, OTS SAM was formed on both FIO and native oxide. These results indicate that SAM formation on silicon oxides can be locally suppressed by FIO in a dry environment. By using this technique, we could fabricate a line structure of OTS SAM as narrow as 22 nm.

Noncontact atomic force microscopy in liquid environment with quartz tuning fork and carbon nanotube probe

A force sensor for noncontact atomic force microscopy in liquid environment was developed by combining a multiwalled carbon nanotube (MWNT) probe with a quartz tuning fork. Solvation shells of octamethylcyclotetrasiloxane on a graphite surface were detected both in the frequency shift and dissipation. Due to the high aspect ratio of the CNT probe, the long-range background force was barely detectable in the solvation region.