Dynamic Mode Atomic Force Microscopy Research Articles

Japan AFM roadmap 2006

This article reviews recent progress in Japan in the techniques and applications of dynamicmode atomic force microscopy (AFM), specifically focusing on three topics: (1)mechanical discrimination of intermixed two-atom species and their manipulation byfrequency modulation (FM) AFM under ultrahigh vacuum; (2) sub-molecular andatomic-resolution imaging in liquids by FM-AFM; and (3) high-speed imaging ofbiological macromolecules in solution by amplitude modulation (AM) AFM. Afterreviewing the state of the art of these high-performance AFMs and various techniquesthat have enabled them, we present future prospects for these AFMs and fortechnological innovations that will be led by them. These prospects are given fromthe viewpoint of Japanese groups that have been involved in these three topics.

Atomic Force Microscopy in Liquid and Its Q Control for Biological Observation

The atomic force microscope (AFM) has advantages in creating topographic images of the sample surface at high resolution even in a liquid environment, and has been expected to be used as a powerful tool for observing biological samples under the condition closer to the physiological state. However, in contrast with the advances in AFM imaging under ambient conditions, it is still difficult to obtain clear images of biological samples by AFM in liquid, because the samples are usually very soft and uneven, and easily deformed during AFM operation. In this article, based upon our experience, we showed some techniques suitable for imaging biological soft samples by AFM in a liquid environment. Special attention was paid to the dynamic mode AFM for biological studies in liquid. We also introduced a Q-control technique of dynamic mode AFM in liquid, which is helpful for increasing the image quality of soft biological sample.

Investigations of Organic Materials by Kelvin Probe Force Microscopy using Frequency Modulation Method

Kelvin probe force microscopy (KFM), which has been a common method for studying electrical properties of nanometer-scale structures, is a dynamic-mode atomic force microscopy (AFM) technique combined with a traditional Kelvin probe method used for measuring macroscopic contact potential differences. It allows us to map electron affinity or work function on a nanometer scale. In this article KFM using a frequency modulation detection method (FM-KFM) is first explained briefly. The frequency modulation method plays an essential role in high-sensitive interaction force detection. We also describe applications of FM-KFM to the nanometer-scale surface potential investigations of organic materials including phase-separated self-assembled monolayer films (PS-SAM films) and single polymer crystal surfaces. Furthermore, recent results on the local surface potential study of carbon nanotube field effect transistors using FM-KFM is presented.

In-Situ AFM Studies of the Phase-Transition Behavior of Single Thermoresponsive Hydrogel Particles

The volume phase transition (VPT) behavior of individual thermally responsive poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAm-co-AAc) hydrogel microparticles was studied by in-situ dynamic mode atomic force microscopy (AFM) and force spectroscopy during heating and cooling cycles. Hydrogel samples were prepared by electrostatic immobilization of microparticles to amine-modified gold surfaces. The AFM studies of particle deswelling were performed by varying the force applied on the particles during imaging as a function of the geometry and material of the AFM probe. Aluminum-coated silicon cantilevers were found to influence substantially the behavior of the particles during the VPT, leading to a significant shape change. Low force impact magnetic excitation of the AFM probe (MAC mode) during dynamic mode measurements resulted in an undisturbed deswelling behavior enabling observation of the expected volume changes of the particles without significant tip-sample interaction. Hence, MAC-mode AFM was determined to be the most suitable technique for in-situ AFM studies on volume and shape changes at single hydrogel particles during VPT. Elasticity measurements performed at single particles at temperatures below and above the VPT revealed a 15-fold increase in the Young's modulus after passing the VPT, indicating the transition from a soft, swollen network to a stiffer, deswollen state.

Self-Assembled Two-Dimensional Ordered Arrays of Tripod-Type Molecules on Graphite

Tripod-type molecules with long alkyl chains, 1,1,1-tris(4-alkoxyphenyl)ethanes with octadecyloxy or docosyloxy chains, self-assemble into two-dimensional crystallites on drop-casting onto the surface of highly oriented pyrolytic graphite. In the two-dimensional crystalline domain, the molecules are organized in a mortise-and-tenon motif, as revealed by scanning tunneling microscopy. The time evolution of the crystallite formation has been followed by the dynamic force mode atomic force microscopy. The tripods may be used as a basis for the extension of a two-dimensional order into three-dimensional molecular architectures.

Direct tip-sample interaction force control for the dynamic mode atomic force microscopy

A control method, in which the tip-sample interaction force of each tapping cycle is directly regulated, is proposed for dynamic mode atomic force microscopy. It does not rely on the steady-state relationship between the cantilever’s oscillation amplitude and tip-to-sample distance, and therefore the cantilever’s transient dynamics and the time delay of rms-dc converter are irrelevant. Experimental results clearly demonstrate that the proposed method regulates the tip-sample interaction force for each tapping cycle and time delay effect is eliminated. Computer simulations also show that the proposed method reconstructs a step change in topography within two tapping cycles, independent of the cantilever’s transient dynamics.

The interference effect in an optical beam deflection detection system of a dynamic mode AFM

Optical beam deflection detection is one of the main techniques used to detect the vibrating amplitude of dynamic mode atomic force microscope cantilevers. Due to the limitations of optical beam deflection detection systems and cantilevers, light leakage of the incident laser beam around the cantilevers can occur. An interference effect between the reflected beam from the cantilever and some scattered light from the specimen surface occurs, and an interference error in the probe tip–specimen approaching curve arises from this effect. In this paper, the interference effect in a dynamic atomic force microscope is analysed and observed in different conditions, and the micro-profile measurement error caused by the optical interference is deduced and calculated mathematically. The influence of the interference effect on a grating pattern measurement is then simulated.

Atomic Force Microscopy in Dynamic Mode with Displacement Current Detection in Double Cantilever Devices

A cantilever array for dynamic mode atomic force microscopy (AFM) is presented, the vertical displacement of which is analyzed by the detection of displacement currents in the electrodes. Each cantilever in the array consists of an actuation part that allows an independent vertical movement, and a sensor part. The lateral distance between the tips of the different cantilevers is fixed to 10 µm. When operated as an actuator, a voltage is applied between the silicon membrane and the underlaying electrode. Due to the resulting coulomb forces, the vertical position of the tip is controllable. The reaction time in this mode is shorter than the response time of a piezostack. The sensor part, on the other hand, allows the device to work in dynamic mode without a laser deflection system. The vertical resolution achieved is below 1 nm. The dependence of force distance curves on the excitation amplitude is shown.

Atomic Force Microscopy Utilizing SubAngstrom Cantilever Amplitudes

A major problem of dynamic mode atomic force microscopy has been the necessity to employ relatively large amplitude of drive of the cantilever in the few Angstrom to the nm range to allow for sustaining self-excitation, and to obtain sufficient signal to noise ratio for frequency or amplitude shift measurement. The use of laser Doppler interferometery and super heterodyne signal processing has enabled clear atomic resolution imaging using subAngstrom cantilever amplitudes in the MHz regime. Due to the small amplitude, and the fact that the tip apex was always within the field of force gradient, long life of the tip was obtained, and frequency shift could more readily be interpreted as that coming from the short range forces. [DOI: 10.1380/ejssnt.2006.110]

Au cluster growth on ZnO thin films by pulsed laser deposition

Abstract Nanostructures formed by Au nanoparticles on ZnO thin film surface are of interest for applications which include medical implants, gas-sensors, and catalytic systems. A frequency tripled Nd:YAG laser ( λ = 355 nm, τ FWHM ∼ 10 ns) was used for the successive irradiation of the Zn and Au targets. The ZnO films were synthesized in 20 Pa oxygen pressure while the subsequent Au coverage was grown in vacuum. The obtained structures surface morphology, crystalline quality, and chemical composition depth profile were investigated by acoustic (dynamic) mode atomic force microscopy, X-ray diffraction, and wavelength dispersive X-ray spectroscopy. The surface is characterized by a granular morphology, with average grain diameters of a few tens of nanometers. The surface roughness decreases with the increase of the number of laser pulses applied for the irradiation of the Au target. The Au coverage reveals a predominant (1 1 1) texture, whereas the underlying ZnO films are c -axis oriented. A linear dependence was established between the thickness of the Au coverage and the number of laser pulses applied for the irradiation of the Au target.

AFM characterization of solid-supported lipid multilayers prepared by spin-coating

Lipids are the principal components of biologically relevant structures as cellular membranes. They have been the subject of many studies due to their biological relevance and their potential applications. Different techniques, such as Langmuir–Blodgett and vesicle-fusion deposition, are available to deposit ordered lipid films on etched surfaces. Recently, a new technique of lipid film deposition has been proposed in which stacks of a small and well-controlled number of bilayers are prepared on a suitable substrate using a spin-coater. We studied the morphological properties of multi-layers made of cationic and neutral lipids (DOTAP and DOPC) and mixtures of them using dynamic mode atomic force microscopy (AFM). After adapting and optimizing, the spin-coating technique to deposit lipids on a chemically etched Silicon (1,0,0) substrate, a morphological nanometer-scale characterization of the aforementioned samples has been provided. The AFM study showed that an initial layer of ordered vesicles is formed and, afterward, depending on details of the spin-coating preparation protocol and to the dimension of the silicon substrate, vesicle fusion and structural rearrangements of the lipid layers may occur. The present data disclose the possibility to control the lipid's structures by acting on spin-coating parameters with promising perspectives for novel applications of lipid films.

Programmed Hyperhelical Supramolecular Assembly of Nickel Phthalocyanine Bearing Enantiopure 1-(p-Tolyl)ethylaminocarbonyl Groups

The present paper reports uniqueness of a simple, programmed design of disk-shaped homochiral nickel phthalocyanine (Pc) molecules bearing four enantiomerically pure 1-(p-tolyl)ethylaminocarbonyl groups at their peripheral positions, (Pc-(R) and Pc-(S)), and their controlled self-organization into mesoscopic supramolecular helical fibers with a preferential handedness in solution and onto solid surfaces. A combination of four fundamental intermolecular interactions, including quadruple hydrogen bonding, pi-pi stacking, homochiral interactions of the enantiopure bulky aralkyl entities, and noncoordinating nature of nickel ion of the Pc molecules afforded a high thermal stability of the Pc self-assembly in chloroform (CHCl(3)), tetrahydrofuran, and o-dichlorobenzene and onto hydrophilic mica and hydrophobic HOPG surfaces. A higher-ordered helical self-assembly of Pc disks was observed in these solutions (approximately 200 Pc molecules), while the self-assembly was completely dissociated into monomeric species in N,N-dimethylformamide due to a loss of hydrogen-bonding interactions between Pc molecules. Supramolecular chirality in the hierarchical self-assembly of Pc molecules originated from the presence of (R)- or (S)-chiral centers in the peripheral tails, which rotate noncovalently linked molecular building blocks to effectively form the helical architectures. The helical Pc nanofibers dissolved in CHCl(3), estimated to be ca. 70 nm from peak molecular weight obtained by SEC analysis, acts as a building block for higher-order helical fibers (ca. 1 microm) at single molecular level on the solid surfaces, as demonstrated by the dynamic force mode atomic force microscopy. Regardless of hydrophilic and hydrophobic substrates, the interaction between these Pc molecules and the solid surfaces could not affect the morphology of helical assemblies, indicating a unique robustness of these assemblies.

Thermal Stripping of Supramolecular Structures: C60 Nanorods

The investigated ionic C60 derivative self-assembles into nanorods. When the functional side groups are removed by heating the nanorods to 623 K, they retain their shape. Utilization of lithographic markers allows the study of identical nanostructures before and after heat treatment by dynamic mode atomic force microscopy. Various independent techniques, including Raman spectroscopy and mass spectroscopy demonstrate that the shape-preserving mechanism is a thermal-stripping process, stabilizing the original supramolecular morphology. The latter implies two coherent sub-processes: detachment of the side groups and oligopolymerization running in parallel, eventually yielding rod-shaped C60 polymers. Synthesizing fullerenic polymers in this way can lead to several applications.

Surface Heterogeneity of Polystyrene Latex Particles Determined by Dynamic Force Microscopy

Atomic force microscopy (AFM) was employed to characterize the surface chemistry distribution on individual polystyrene latex particles. The particles were obtained by surfactant-free emulsion polymerization and contained hydrophilic quaternary ammonium chloride, sodium sulfonate, or hydroxyethyl groups. The phase shift in dynamic force mode AFM is sensitive to charge/chemical interactions between an oscillating atomic force microscope tip and a sample surface. In this work, the phase imaging technique distinguished phase domains of 50-100 nm on the surfaces of dried latex particles in ambient air. The domains are attributed to the separation of ion-rich and ion-poor components of the polymer on the particle surface.

Investigations of Nanoparticles by Scanning Near-Field Optical Microscopy Combined with Kelvin Probe Force Microscopy Using a Piezoelectric Cantilever

A microfabricated cantilever with a lead zirconate titanate (PZT) piezoelectric thin film used as an integrated deflection sensor was applied to dynamic-mode atomic force microscopy (AFM). This probe can be used for scanning near-field optical microscopy (SNOM) and Kelvin-probe force microscopy (KFM). We adopted a frequency modulation (FM) detection method not only to stabilize imaging conditions in dynamic-mode AFM but also to enhance sensitivity for the detection of electrostatic forces in KFM measurement. We investigated local optical and electrical properties of gold and polystyrene nanoparticles dispersed on an indium tin oxide (ITO) substrate. We successfully identified the species of each particle by different contrasts obtained in SNOM and KFM images.

Measurements of metal–polymer adhesion properties with dynamic force spectroscopy

We investigate nanoscale interface properties using dynamic mode atomic force microscopy (AFM) operated in the frequency modulation mode in ultrahigh vacuum. The AFM tip was was functionalised by a thin layer of aluminium and the polymer was treated by plasma-etching. In the spectroscopy mode we could measure the adhesion properties between the metal and the polymer surfaces. We found that plasma-etching of the polymer resulted in strongly enhanced force interactions, indicating a chemical activation of the polymer surface. Values for the adhesion force and work of adhesion were measured on the nanometer scale and are compared to conventional macroscopic adhesion failure tests.

Nanoscale Investigation of Optical and Electrical Properties by Dynamic-Mode Atomic Force Microscopy Using a Piezoelectric Cantilever

We demonstrated a novel application of a microfabricated force-sensing cantilever with a lead zirconate titanate (PZT) thin film as an integrated deflection sensor for a dynamic-mode atomic force microscope (AFM) combined with a scanning near-field optical microscope (SNOM). This experimental setup was also applied to a Kelvin-probe force microscope (KFM) for the investigation of local electrical properties. A frequency modulation (FM) detection method was used for enhancing the detection sensitivity of electrostatic forces between a probe tip and a sample. Local optical properties of a PbTiO3 single crystal were investigated using the conductive PZT cantilever as an AFM/SNOM probe. Furthermore, studies on local optical and electrical properties of ferroelectric copolymer films were also presented.

Simulation of section curve by phase constant dynamic mode atomic force microscopy in non-contact situation

We investigate the behavior of an oscillating tip operating in tapping mode, e.g. with fixed drive frequency and drive excitation while scanning horizontally an ideal flat surface holding a small corrugation. The numerical simulations are performed to compare the relative vertical and lateral sensitivity of the oscillator when the feedback loop controlling the tip–sample distance keeps constant the oscillator phase instead of the amplitude. In these simulations, a sphere lies on a flat surface and a Van der Waals force is assumed between the tip, the sphere and the substrate. Once the tip has approached from the substrate far from the sphere, the tip is scanned towards the sphere. It is found that the phase constant mode makes smaller indentation and better response height probably without larger damage by using wider setpoint value (SPV) than the cases of amplitude constant mode.

Self-sensing and self-actuating probe based on quartz tuning fork combined with microfabricated cantilever for dynamic mode atomic force microscopy

A novel probe based on a commercial quartz tuning fork and a microfabricated cantilever is presented. The U-shaped cantilever with a monolithic tip is combined with the tuning fork in a symmetrical arrangement, such that each of the two legs of the cantilever is fixed to one of the prongs of the tuning fork. The tuning fork is used as an oscillatory force sensor. Its frequency and amplitude governs that of the vibrating tip, while the cantilever determines the spring constant of the whole probe. Several probes with different cantilevers in terms of material and spring constant are studied. A typical silicon nitride cantilever has a spring constant of 0.1 N/m and those made of silicon have 1.2–319 N/m. Monoatomic terraces of highly oriented pyrolytic graphite (HOPG) was successfully imaged in the intermittent contact mode with the silicon probes. Both types of probes are suited for batch fabrication and assembling and, therefore enlarges the applications of the tuning fork based AFM.

Nanoscopic imaging of human chromosomes via a scanning near-field optical/atomic-force microscopy (SNOAM)

Scanning near-field optical/atomic-force microscopy (SNOAM) provided us with simultaneous topographic and fluorescence images of human chromosomes. The SNOAM uses a bent optical fiber simultaneously as a dynamic mode atomic force microscopy cantilever. Optical resolution was approximately 50—100 nm in fluorescence mode. Conventional karyotyping information was linked with SNOAM topographic analyses such as location of centromere and length of individual chromosomes. The height profile clearly indicated higher teromere regions. The SNOAM fluorescence images were different shapes from topographic images probably due to results from the combination of fluorescence dye and chromosome DNA.