Hirofumi Yamada

Analog frequency modulation detector for dynamic force microscopy

A new analog frequency modulation (FM) detector (demodulator) for dynamic force microscopy (DFM) is presented. The detector is designed for DFM by utilizing the FM detection method where the resonance frequency shift of the force sensor is kept constant to regulate the distance between a tip and a sample surface. The FM detector employs a phase-locked loop (PLL) circuit using a voltage-controlled crystal oscillator (VCXO) so that the thermal drift of the output signal is negligibly reduced. The PLL is used together with a frequency conversion (heterodyne) circuit allowing the FM detector to be used for a wide variety of force sensors with the resonance frequency ranging from 10 kHz to 10 MHz. The minimum detectable frequency shift was as small as 0.1 Hz at the detection bandwidth of 1 kHz. The detector can track a resonance frequency shift as large as 1 kHz. We also present some experimental results including the observations of the Si(111)-7×7 reconstructed surface and fullerene molecules deposited on th...

True-molecular resolution imaging by frequency modulation atomic force microscopy in various environments

In this study, we discuss the relationship between Q factor of the cantilever in various environments and frequency noise in frequency modulation atomic force microscopy (FM-AFM). We first present true-molecular resolution FM-AFM images of alkanethiol self-assembled monolayers taken in a moderate vacuum environment (vacuum pressure: 6 Pa) and in air (cantilever Q factor: 390) using FM-AFM with a low noise cantilever deflection sensor. The results reveal that the minimum Q factor to obtain true-molecular resolution in FM-AFM can be less than a few hundred.

Nanometer-Scale Characterization of Ferroelectric Polymer Thin Films by Variable-Temperature Atomic Force Microscopy

The morphological changes of ferroelectric polymer thin films during the annealing process were directly imaged by variable-temperature atomic force microscopy (AFM). The growing process of the crystallites underlying the surface amorphous layer was observed as well as the morphological changes using intermittent-contact AFM. We found that fine structures appeared in the rod like grains at a temperature corresponding to the Curie point of the bulk copolymer in the cooling process. In addition, local piezoelectric response was mapped in order to investigate the relationship between the structural change and the ferroelectric phase transition. The local polarization prepared at room temperature disappeared after the film was heated above 120°C, which agreed with both the Curie temperature and the temperature of the observed structural change.

Strong substrate effect in local poling of ultrathin ferroelectric polymer films

By locally poling and measuring ultrathin films of the ferroelectric copolymer vinylidenefluoride/trifluoroethylene (VDF/TrFE), we show that the substrates on which these films are adsorbed play a strong role in orienting and poling the films. We carried out both poling and measurements using the Au-coated probe tip of the cantilever of an atomic force microscope. For ultrathin films of about 20 nm of the copolymer spin-coated on graphite, only a fraction of the thickness of the copolymer film can be repoled, so that the net local piezoelectric effect is enhanced or reduced, depending upon whether poling is parallel or antiparallel to that induced by the substrate, respectively. For sufficiently thick films, this substrate effect is negligible. The apparent lateral resolution with which information can be stored by poling in ultrathin films depends strongly on the film thickness and poling direction with respect to the substrate effect.

Dopant profiling on semiconducting sample by scanning capacitance force microscopy

Scanning capacitance force microscopy (SCFM) capable of mapping differential capacitance (∂C/∂V) on semiconducting sample based on atomic force microscopy (AFM) without an external capacitance sensor is introduced. While an electric field alternating at an angular frequency ω is applied between the tip and the sample, an induced electrostatic force (ESF) oscillating at its third harmonic frequency (3ω) is detected by a lock-in amplifier. Owing to the fact that the magnitude of the induced ESF is proportional to the square of the magnitude of the applied electric field and the fact that the capacitance of the semiconducting sample is also modulated at ω, the amplitude and the phase of the induced ESF oscillating at 3ω contain information on ∂C/∂V. We present ∂C/∂V images on a Si test sample obtained by SCFM using both contact-mode AFM and dynamic-mode AFM, showing clear contrasts depending on species and density of dopants.

Orientation control of poly(vinylidenefluoride-trifluoroethylene) crystals and molecules using atomic force microscopy

We have developed an aligning technique for polymer crystals and molecular chains utilizing contact-mode atomic force microscopy (AFM). We have aligned lamellar crystals and molecular chains of poly(vinylidenefluoride-trifluoroethylene) thin films. By scanning the film surface using an AFM cantilever tip at the temperature range of 70–100u200a°C, we aligned the crystals to the scan direction. Moreover, we successfully aligned the molecular chains to the scan direction by scanning at a higher temperature (135u200a°C). The aligned chains subsequently formed large lamellar crystals, which were still ferroelectric.

Investigation of surface potential of ferroelectric organic molecules by scanning probe microscopy

The surface potential of ferroelectric copolymer films of vinylidene fluoride and trifluoroethylene was investigated by scanning probe microscopy. The results reveal that the as-deposited copolymer films on a graphite substrate show a negative surface potential. The surface potential is ascribed to the substrate-induced effect, which causes frozen dipoles in the copolymer films. We made locally poled areas in the films using a conductive cantilever tip and measured the piezoelectric response of the films with a modified scanning force microscope. The observed asymmetric behavior of the piezoelectric response of the thin films can be consistently explained by the frozen ferroelectric layer in the films. Both surface potential and piezoresponse images of locally polarized areas were also obtained. The results indicate that the charge transferred from the tip overcompensated for the oriented dipoles and that the surface potential of the poled regions was attributed to the excess charge.

Investigations of C60 molecules deposited on Si(111) by noncontact atomic force microscopy

We demonstrated the high resolution imaging of the organic molecules using noncontact atomic force microscopy in ultrahigh vacuum. The sample was C60 molecules deposited on the Si(111)-7×7 reconstructed surface. When the thickness of the C60 film was submonolayer, we could image some isolated C60 molecules and the reconstructed Si surface simultaneously. However, the imaging was highly unstable not only because of the large structure but also due to the large difference between the interaction forces on the molecules and on the Si surface. On the other hand, when the thickness of the C60 molecules was almost monolayer, individual molecules could be stably imaged.

Structures and Ferroelectric Natures of Epitaxially Grown Vinylidene Fluoride Oligomer Thin Films

Structural and electrical properties of newly synthesized vinylidene fluoride (VDF) oligomer thin film have been investigated. The FTIR spectrum showed that the epitaxially grown film on KBr(001) substrate consists of form I (β phase) crystals and their c axes (molecular axes) and b axes (polar axes) are arranged parallel to the KBr substrate. To make electrical measurements possible, this film was transferred onto a gold bottom electrode without causing any changes in the crystalline structures. By using a modified atomic force microscope, we succeeded in the formation of local polarized domains as well as the clear observation of piezoresponse hysteresis curves in this sample. The coercive field and piezoelectric coefficient (d33) for the 37-nm-thick film were about 200 MV/m and -3 pm/V, respectively. It was suggested that the b axis in the as-grown film rotated from the parallel to the perpendicular direction to the film surface during the poling process. This study reveals the ferroelectric characteristics in the VDF oligomer thin films for the first time.

Lead zirconate titanate cantilever for noncontact atomic force microscopy

Noncontact atomic force microscopy with frequency modulation detection is a promising technique for surface observation with true atomic resolution. The piezoelectric material itself can be an actuator and sensor of the oscillating probe simultaneously, without the need for additional electro-mechanical transducers or other measurement systems. A vertical resolution of 0.01 nm rms has been achieved using a microfabricated cantilever with lead zirconate titanate thin film in noncontact mode frequency modulation detection. The cantilever also has a sharpened pyramidal stylus with a radius of about 10 nm for noncontact atomic force microscopy.