Atomic force microscopy of bamboo-shaped multiwalled carbon nanotube structures

Atomic force microscopy (AFM) was employed for the morphology measurements of bamboo-shaped multiwalled carbon nanotubes (BS-MWNTs) grown by thermal chemical vapor deposition on Fe catalyst deposited SiO₂/Ti substrates. Greater diameters and compartment distances of the bamboo structures were observed for the BS-MWNTs grown at 950 °C than for those grown at 850 °C.     

Factors affecting phase and height contrast of diblock copolymer PS-b-PEO thin films in dynamic force mode atomic force microscopy

Asymmetric PS-b-PEO block copolymer exhibits well-ordered cylindrical morphology with nanoscale domain sizes due to microphase separation. Since the PS and PEO blocks have large stiffness difference, this polymer system represents an ideal candidate for studies of the phase contrast behavior in atomic force microscopy (AFM). In this paper, PS-b-PEO films are investigated under different scanning conditions using two different atomic force microscopes. It is found that the phase contrast of the film can be well described in terms of energy dissipation, though the exact phase image may also depend on the scanning parameters (e.g., the repulsive versus attractive regimes) as well as the settings of the microscope. Height variation on sample surface does not have significant effect on phase contrast. However, in order to obtain true topography of the polymer film, care has to be taken to avoid damage to the sample by AFM. Under certain conditions, true topography can be obtained during the first scan in spite of the surface-damaging forces are used.     

Dynamic scanning force microscopy study of self-assembled DNA-protein nanostructures

Self-assembled oligomeric nanostructures consisting of bisbiotinylated DNA fragments connected by the protein streptavidin (STV) are studied by dynamic scanning force microscopy (SFM) operating in air. A comparison of the images taken in repulsive and attractive regimes is systematically made on DNA and STV structures. Stable and reproducible SFM images are obtained in the attractive regime by using a special feedback circuit, called Q-control. On the other hand, when SFM is operating in the repulsive regime, deformation of the structures that reduce the resolution and the image quality are clearly observable. The heights of both DNA and STV have been measured as a function of the tip/molecule interaction forces. This study offers the possibility to suggest a different mechanical behavior of DNA with respect to STV. Received: 24 July 2001 / Accepted: 3 December 2001 / Published online: 4 March 2002     

Nonlinear optical and electrostatic force microscopy for ferroelectric polarization imaging

Second-harmonic generation (SHG)-based nonlinear optical microscopy is used for spatially resolved imaging of the polarization switching in lead zirconium titanate ferroelectric thin films. The local SHG hysteresis loops reveal a strong dependence on film composition and structure. The SHG microscopy results are in good agreement with the efficiency of electrostatic force microscopy writing and allow us to predict the microscopic dielectric memory efficiency, both in contact and contact-less ways. Received: 15 January 2001 / Revised version: 27 March 2002 / Published online: 6 June 2002     

Phase-contrast scanning force microscopy and chemical heterogeneity of GR polysulfone ultrafiltration membranes

The surfaces of three commercial urea formaldehyde polysulfone membranes from Dow Denmark^TM (GR51, GR61 and GR81) are characterised both topographically and chemically. Their topography is studied by scanning force microscopy to obtain the corresponding pore-size distributions, which are in fair agreement with nominal molecular weight cut-offs. The composition of the surfaces of the membranes is analysed by X-ray photoelectron spectroscopy. The resulting percentage content of nitrogen, which could be attributed probably to an additive used in the manufacturing process, is shown to correlate with the portion of the total surface with different viscoelastic properties as investigated by using phase-contrast scanning force microscopy. Both parameters are increasing for membranes with decreasing molecular weight cut-off. Also, the additive seems to be more sparsely distributed for the membranes with bigger pores, according to fractal analysis. Finally, all the membranes are very similarly wettable. Received: 22 May 2001 / Accepted: 30 May 2001 / Published online: 25 July 2001     

Domain wall thickness variations of ferroelectric BaMgF4 single crystals in the tip fields of an atomic force microscope

The ferroelectric domain wall thickness of a fluoride BaMgF₄ single crystal was investigated by piezoresponse force microscopy. It was found that the domain wall thickness shows a strong spatial variation in the as‐grown crystal and the polarization reversal process. The original wall thickness is greater (about two to seven times) than that switched by the tip fields of the atomic force microscope. A significantly narrower domain wall was obtained in the higher tip‐field. The trapped defects at the domain wall play an important role in the spatial variation of the polarization width of 180° domain wall in the BaMgF₄ single crystal. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Manipulation and soldering of carbon nanotubes using atomic force microscope

Manipulation of carbon nanotubes (CNTs) by an atomic force microscope (AFM) and soldering of CNTs using Fe oxide nanoparticles are described. We succeeded to separate a CNT bundle into two CNTs or CNT bundles, to move the separated CNT to a desirable position, and to bind it to another bundle. For the accurate manipulation, load of the AFM cantilever and frequency of the scan were carefully selected. We soldered two CNTs using an Fe oxide nanoparticle prepared from a ferritin molecule. The adhesion forces between the soldered CNTs were examined by an AFM and it was found that the CNTs were bound, though the binding force was not strong.     

Morphology of compressed dipalmitoyl phosphatidylcholine monolayers investigated by atomic force microscopy

The effectiveness of a substitute of natural lung surfactants on replacement therapy strongly depends on the stability of the monolayer of those substitute molecules. An atomic force microscope is utilized to investigate the microstructure of the films of the major components of natural lung surfactants, dipalmitoyl phosphatidylcholine—DPPC, which are transferred to mica substrates by the Langmuir-Blodgett film technique. A concave deformation structure was first observed for DPPC in solid phase. The depth of the concave domain was about 6 nm and was remarkably uniform. For a collapsed DPPC monolayer, the surface film consists of a granular convex multilayer structure and a disc-like concave structure. Dynamic cyclic compression-expansion experiments indicate that the formation of the concave domain is a reversible process while the process for convex multilayer formation is irreversible. This gives direct evidence that convex grain is the collapsed structure of DPPC monolayer and the concave shallow disc corresponds to the elastic deformation of a DPPC solid film. Results of atomic force microscopy indicate that the nucleation and growth model instead of the fracture model can better describe the collapse behavior of a DPPC monolayer.     

Bias-induced spatially resolved growth and removal of Si-oxide by atomic force microscopy

Three mechanisms for spatially resolved growth and removal of oxide on silicon substrates have been investigated. Thermally grown oxide layers with thicknesses in the range 2–6 nm were the distinctive feature of the system. The layers were characterized and manipulated by methodologies based on atomic force microscopy (AFM) with conducting probes in a vacuum environment of 10^-2–10^-3 Pa. The probe is then effectively a travelling electrode that generates an electrostatic field between the tip and the substrate. Oxide growth was induced for a positive sample bias greater than 5 V, but below the level corresponding to dielectric breakdown. Application of a short pulse of amplitude marginally above that corresponding to dielectric breakdown, on the other hand, had the effect of producing pits of inner diameter of about 10 nm in the pre-existing oxide layer at the point of tip-to-oxide contact. Application of a low positive sample bias (less than that required for measurable oxide growth) in combination with high linear scan speed had the effect of removing a pre-existing oxide layer from the scanned field of view. The most plausible mechanisms are based on transverse ionic diffusion (for oxide growth), controlled dielectric breakdown (for formation of pits) and lateral transport of silicaceous species (for oxide removal). Received: 24 October 2001 / Accepted: 6 January 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +617-3875-7656, E-mail: s.myhra@sct.gu.edu.au     

Adsorption and manipulation of carbon onions on highly oriented pyrolytic graphite studied with atomic force microscopy

Carbon onions produced by DC arc discharge method were deposited on highly oriented pyrolytic graphite (HOPG) surface and their adsorption and manipulation was studied using an atomic force microscopy (AFM). Well-dispersed adsorption of carbon onions on HOPG surface was obtained and aggregations of onions were not observed. The van der Waals interaction between the onion and HOPG surface and that between two onions, were calculated and discussed using Hamaker's theory. The manipulation of adsorbed onions on HOPG surface was realized using the AFM in both the raster mode and the vector mode. The controllability and precision of two manipulation modes were compared and the vector mode manipulation was found superior, and is a useful technique for the construction of nano-scale devices based on carbon onions.     

Atomic force microscopy study of thermal stability of silver selenide thin films grown on silicon

Silver selenide thin films were grown on silicon substrates by the solid-state reaction of sequentially deposited Se and Ag films of suitable thickness. Transmission electron microscopy and particle-induced X-ray emission studies of the as-deposited films showed the formation of single phase polycrystalline silver selenide from the reaction of Ag and Se films. Atomic force microscopy images of the as-deposited and films annealed at different temperatures in argon showed the film morphology to evolve into an agglomerated state with annealing temperature. The results indicate that when annealed above 473 K, silver selenide films on silicon become unstable and agglomerate through holes generated at grain boundaries.     

Atomic force microscopy studies on phase transitions and surface morphology transformation of CMTC crystals

Atomic force microscopy (AFM) has been used to investigate the phase transitions and surface morphology transformation of cadmium mercury thiocyanate (CMTC) crystals, which are highly efficient nonlinear optical (NLO) materials for generating blue-violet light by laser frequency doubling. Amorphous aggregates at the crystalline steps become greatly contracted and much more crystalloid after the crystal was kept for one day. Elimination of dangling bonds, which lower the surface free energy at the crystal surface, and structural adjustment inside the crystal are assumed to cause this phase transition. Surface morphology transformations were also observed in CMTC crystals during and after multiple scanning by AFM tips. We have visualized the continuous translation process from two-dimensional nuclei to trigonal microcrystals with almost equal sizes during multiple scanning. In other cases, however, the surface morphology did not change at all during scanning, but became greatly altered hours after scanning. These experimental results suggest that reconstruction is a characteristic growth phenomenon on CMTC crystal surfaces. Reconstruction probably results from the formation of intervening metastable phases that have the potential to arrive at more stable stages; however, multiple scanning of AFM tips greatly affects this translation process. Received: 28 August 2001 / Accepted: 7 November 2001 / Published online: 29 May 2002     

A cryogenic scanning force microscope for the characterization of frozen biological samples

We present the design of a new scanning force microscope specially suited for the investigation of soft matter, particularly biological, in an ultrahigh vacuum. The key point is that the samples are immobilized by shock freezing in order to maintain their native structure before they are introduced into the vacuum system. The vacuum system itself consists of a transfer chamber, which allows an exchange of the cold sample with cryo-electron microscopes, a preparation chamber including a stage for in-situ freeze drying, freeze etching, or freeze fracturing, and the analysis chamber with the microscope. Sample cooling is maintained in all chambers. The microscope is mounted on a commercially available vibration isolation system; a flow cryostat cools the sample to the temperature of liquid nitrogen, while the tip is scanned. Besides measurements on test samples, which demonstrate the imaging capabilities of the instrument, first results on T4-bacteriophages (viruses) are shown. Received: 2 September 2002 / Accepted: 2 September 2002 / Published online: 5 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-40/42838-6188, E-mail: wiesendanger@physnet.uni-hamburg.de     

An electrostatic force microscope study of Si nanostructures on Si(1 0 0) as a function of post-annealing temperature and time

The evolution of Si nanostructures induced by Ar⁺ ion sputtering on Si(1 0 0) was studied with electrostatic force microscopy (EFM) as a function of post-annealing temperature (T = room temperature-800 °C) and time (t = 0-160 min). The post-annealing of the nanostructure was conducted in vacuum. It was found that with T increasing, the EFM contrast degraded steadily and became nearly undetectable at T = 800 °C; with t increasing at T = 800 °C, the EFM contrast fell down steadily as well. However, the surface morphology and roughness were much less affected after annealing. The results suggest that the as-formed Si nanostructures may not be epitaxially grown on Si(1 0 0) substrate as claimed before. A plane capacitance model supported this conclusion.     

Dependences of the surface and the optical properties on the O2/O2 + Ar flow-rate ratios for ZnO thin films grown on ZnO buffer layers

ZnO active layers on ZnO buffer layers were grown at various O₂/O₂ + Ar flow-rate ratios by using radio-frequency magnetron sputtering. Atomic force microscopy images showed that the surface roughnesses of the ZnO active layers grown on ZnO buffer layers decreased with decreasing O₂ atmosphere, indicative of an improvement in the ZnO surfaces. The type of the ZnO active layer was n-type, and the resistivity of the layer increased with increasing O₂ atmosphere. Photoluminescence spectra from the ZnO active layers grown on the ZnO buffer layers showed dominant peaks corresponding to local levels in the ZnO energy gap resulting from oxygen vacancies or interstitial zinc vacancies, and the peak positions changed significantly with the O₂/O₂ + Ar flow rate. These results can help improve understanding of the dependences of the surface and the optical properties on the O₂/O₂ + Ar ratio for ZnO thin films grown on ZnO buffer layers.     

Piezoelectric evaluation of ion beam etched Pb(Zr,Ti)O3 thin films by piezoresponse force microscopy

The evolution of piezoelectric properties of Pb(Zr,Ti)O₃ (PZT) thin films after ion beam etching have been investigated at the nanoscale level by piezoelectric force microscopy. A comparison of the piezoelectric properties on etched and unetched films is realized. Piezoelectric contrasts imaging evidences a modification of the domain architecture at the film surface. Local piezoelectric hysteresis loops measurements on grains indicate that the coercive voltage for switching is much higher for the etched films (2.3 V) compared to the unetched ones (1.0 V) while the average piezoelectric activity is slightly lower. The results are explained in terms of grain-damaging during etching and domain-wall pinning.     

Correlation of atomic force microscopy detecting local conductivity and micro‐Raman spectroscopy on polymer–fullerene composite films

Thin hetero‐junction composite films of polymer (electron donor) and fullerene (electron acceptor) are prepared on indium‐tin‐oxide coated glass by spin‐coating from solution in dichlorobenzene. Optimized atomic force microscopy (AFM) parameters allowed us to scan these soft composite films in contact mode and to measure their local conductivity with high lateral resolution by current‐sensing AFM. The morphology and local conductivity data are correlated with Kelvin force microscopy and micro‐Raman mapping and discussed with view to their photovoltaic properties. Regions with both compounds present are compared to areas where the components segregated, acting as shunts of the junction. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural investigation of n-hexadecanoic acid multilayers on mica surface: Atomic force microscopy study

The structure of n-hexadecanoic acid (HA) multilayers formed by spreading an ethanol solution containing this molecule onto a freshly cleaved mica surface has been studied by atomic force microscopy (AFM). AFM images of multilayers obtained with different coating time showed that HA molecules first formed some sporadic domains on mica surface. With the proceeding of the coating process, these domains gradually enlarged and coalesced, until formed a continuous film finally. It was observed that HA molecules were always adsorbed on mica surface with tilted even-numbered layers structure. The height of the repeated tilted bilayer film was measured to be approximately 3.8 ± 0.2 nm, which implied a ∼60° tilt molecular conformation of the HA bilayers on mica surface. Phase image confirmed that the HA multilayers terminated with the hydrophilic carboxylic acid groups. The formation mechanism of the HA multilayers was discussed in detail. Thus, resulted hydrophilic surfaces are of special interest for further study in biological or man-made member systems.     

Evaluations of carbon nanotube field emitters for electron microscopy

Brightness of carbon nanotube (CNT) emitters was already reported elsewhere. However, brightness of electron emitter is affected by a virtual source size of the emitter, which strongly depends on electron optical configuration around the emitter. In this work, I-V characteristics and brightness of a CNT emitter are measured under a practical field emission electron gun (e-gun) configuration to investigate availability of CNT for electron microscopy. As a result, it is obtained that an emission area of MWNT is smaller than its tip surface area, and the emission area corresponds to a five-membered-ring with 2nd nearest six-membered-rings on the MWNT cap surface. Reduced brightness of MWNT is measured as at least 2.6×10⁹ A/m² sr V. It is concluded that even a thick MWNT has enough brightness under a practical e-gun electrode configuration and suitable for electron microscopy.     

Fine-structured field emission images originating from coherently scattering of electrons within a multi-walled carbon nanotube

High resolution field emission image of a single multi-walled carbon nanotube was studied by field emission microscopy. The images contain patterns consisting of rather ordered bright fringes. We propose a model based on coherent electron scattering to explain the observed field emission image. The emitted electrons will undergo coherent scattering within the cap region of a multi-wall carbon nanotube, which may be viewed as elastic scattering by a polycrystalline structure with an infinite size. This study is helpful for understanding the physical mechanism of field emission of carbon nanotube.