Atomic Force Microscopy Profiles Research Articles

Correlative Analysis of the Dimensional Properties of Bipyramidal Titania Nanoparticles by Complementing Electron Microscopy with Other Methods.

In this paper, the accurate determination of the size and size distribution of bipyramidal anatase nanoparticles (NPs) after deposition as single particles on a silicon substrate by correlative Scanning Electron Microscopy (SEM) with Atomic Force Microscopy (AFM) analysis is described as a new measurement procedure for metrological purposes. The knowledge of the exact orientation of the NPs is a crucial step in extracting the real 3D dimensions of the particles. Two approaches are proposed to determine the geometrical orientation of individual nano-bipyramides: (i) AFM profiling along the long bipyramid axis and (ii) stage tilting followed by SEM imaging. Furthermore, a recently developed method, Transmission Kikuchi Diffraction (TKD), which needs preparation of the crystalline NPs on electron-transparent substrates such as TEM grids, has been tested with respect to its capability of identifying the geometrical orientation of the individual NPs. With the NPs prepared homogeneously on a TEM grid, the transmission mode in a SEM, i.e., STEM-in-SEM (or T-SEM), can be also applied to extract accurate projection dimensions of the nanoparticles from the same sample area as that analysed by SEM, TKD and possibly AFM. Finally, Small Angle X-ray Scattering (SAXS) can be used as an ensemble technique able to measure the NPs in liquid suspension and, with ab-initio knowledge of the NP shape from the descriptive imaging techniques, to provide traceable NP size distribution and particle concentration.

Epitaxial lift-off CdTe/MgCdTe double heterostructures for thin-film and flexible solar cells applications

This paper reports an improved epitaxial lift-off (ELO) technology for monocrystalline CdTe/MgCdTe double-heterostructure (DH) thin films using water-soluble and nearly lattice-matched MgTe as a sacrificial layer. Employing hard-baked photoresist as the superstrates with appropriate surface tension, the lift-off thin films show smooth and flat surfaces, confirmed by atomic-force microscopy profiles. Photoluminescence (PL) measurements reveal further enhancement of the light extraction from the ELO thin films with a coated Ag back reflective mirror. The increased PL intensity also confirms that the CdTe/MgCdTe DHs maintain high optical quality after ELO. External luminescence quantum efficiency (ηext) is quantitatively measured and used to calculate the implied open-circuit voltage (iVOC). A 0.5-μm-thick lift-off CdTe/MgCdTe DH with a back mirror demonstrates an ηext value of 5.35% and an iVOC value of 1.152 V. The devices based on this structure are also expected to have an improved fill factor and a short-circuit current density (JSC) of 24.7 mA/cm2 according to the simulation results, promising to achieve CdTe solar cells with a record power conversion efficiency.

Nanoscale dielectric grating polarizers tuned to 4.43 eV for ultraviolet polarimetry.

Transmissive dielectric wire grid polarizers tuned to 4.43 eV (Mg II line, 280 nm), an important diagnostic line for solar physics, are presented in this communication. The polarizers are based on TiO2 gratings and designed with a period of ∼140 nm (7143 lines/mm), 40 nm line width (duty cycle of 0.286), and 100 nm line height. Several gratings are fabricated through electron beam lithography combined with reactive ion etching, whereby two parameters in the nanofabrication process are explored: e-beam dosage on the photoresist and TiO2 etching time. Polarization of samples is optically characterized using a spectroscopic ellipsometer in transmission mode, achieving the best result with an extinction ratio of ∼109 and a transmittance of 16.4% at the target energy of 4.43 eV. The shape of the gratings is characterized through atomic force microscopy (AFM) and scanning electron microscopy (SEM); the measured AFM profiles are distorted by the tip geometry, hence a simple deconvolution procedure is implemented to retrieve the real profile. By analysing the AFM and SEM profiles, we find that the real shapes of the different gratings are close to the design, but with a larger duty cycle than the intended value. With the real grating geometry, an improved model of the best sample was built with a finite-difference time-domain (FDTD) method that matches the result obtained through optical characterization.

Length Scale Matters: Real-Time Elastography versus Nanomechanical Profiling by Atomic Force Microscopy for the Diagnosis of Breast Lesions.

Real-time elastography (RTE) is a noninvasive imaging modality where tumor-associated changes in tissue architecture are recognized as increased stiffness of the lesion compared to surrounding normal tissue. In contrast to this macroscopic appraisal, quantifying stiffness properties at the subcellular level by atomic force microscopy (AFM) reveals aggressive cancer cells to be soft. We compared RTE and AFM profiling of the same breast lesion to explore the diagnostic potential of tissue elasticity at different length scales. Patients were recruited from women who were scheduled for a biopsy in the outpatient breast clinic of the University Hospital Basel, Switzerland. RTE was performed as part of a standard breast work-up. Individual elastograms were characterized based on the Tsukuba elasticity score. Additionally, lesion elasticity was semiquantitatively assessed by the strain ratio. Core biopsies were obtained for histologic diagnosis and nanomechanical profiling by AFM under near-physiological conditions. Bulk stiffness evaluation by RTE does not always allow for a clear distinction between benign and malignant lesions and may result in the false assessment of breast lesions. AFM on the other hand enables quantitative stiffness measurements at higher spatial, i.e., subcellular, and force resolution. Consequently, lesions that were false positive or false negative by RTE were correctly identified by their nanomechanical AFM profiles as confirmed by histological diagnosis. Nanomechanical measurements can be used as unique markers of benign and cancerous breast lesions by providing relevant information at the molecular level. This is of particular significance considering the heterogeneity of tumors and may improve diagnostic accuracy compared to RTE.

Triaxial compressive strain in bilayer graphene enabled by nitride stressor layer

A technique that can be used to viably create triaxially strained bilayer graphene on any desirable location by simple patterning was developed. Unlike the conventional graphene strain engineering methods, the photolithographically defined spoke patterns and compressive strained Si3N4 layer deposited by plasma-enhanced chemical vapor deposition (PECVD) system enable the creation of locally confined triaxial strained bilayer graphene at the desire location by forming a unique tristar shaped wrinkle. The tristar shaped wrinkle was investigated with high resolution micro-Raman spectroscopy and atomic force microscopy (AFM) analyses, and confirmed that 0.38% of maximum triaxial compressive strain was created. Mechanical simulation was used to verify the strain distribution and confirm the strain value which was calculated from the Raman spectroscopy and AFM profile. The technique presented here not only provides a practical route to the creation of strained graphene at desired locations but also offers the potential of the creation of multiaxial strain in the graphene for various types of graphene-based electronic and optoelectronic devices.

Microstructure evolution of Al-doped zinc oxide and Sn-doped indium oxide deposited by radio-frequency magnetron sputtering: A comparison

The microstructure and morphology evolution of Al-doped zinc oxide (AZO) and Sn-doped indium oxide (ITO) thin films on borosilicate glass substrates deposited by radio-frequency magnetron sputtering at room temperature (RT) and 300 °C were investigated by X-ray diffraction and atomic force microscopy (AFM). One-dimensional power spectral density (1DPSD) functions derived from the AFM profiles, which can be used to distinguish different growth mechanisms, were used to compare the microstructure scaling behavior of the thin films. The rms roughness Rq evolves with film thickness as a power law, Rq ∼ dfβ, and different growth exponents β were found for AZO and ITO films. For AZO films, β of 1.47 and 0.56 are obtained for RT and 300 °C depositions, respectively, which are caused by the high compressive stress in the film at RT and relaxation of the stress at 300 °C. While for ITO films, β1 = 0.14 and β2 = 0.64 for RT, and β1 = 0.89 and β2 = 0.3 for 300 °C deposition are obtained, respectively, which is related to the strong competition between the surface diffusion and shadowing effect and/or grain growth. Electrical properties of both materials as a function of film thickness were also compared. By the modified Fuchs-Sondheimer model fitting of the electrical transport in both materials, different nucleation states are pointed out for both types of films.

Impact of microcrystalline-silicon surface-morphology on film quality and solar cell performance

Atomic force microscopy (AFM) and Raman and infrared spectroscopy were used to measure and compare hydrogenated microcrystalline silicon (µc-Si:H) thin films. The height-height correlation function was calculated from the AFM profiles, and roughness parameters of surface roughness w, longitudinal correlation length ξ and roughness component α were estimated for µc-Si:H thin films with various film thicknesses. Results for film thickness dependence showed w and α to be more sensitive to changes in film quality than Xc. A rapid increase in w and/or α indicated the formation of defective regions with low Xc in µc-Si:H thin film, due to the shadowing effect. The Raman crystallinity Xc dependences of roughness parameters and the stretching modes (SM) of Si–H bonds were investigated for µc-Si:H thin films with almost the same film thickness. The properties of µc-Si:H thin film (high SM fraction, Xc, α) were compared with cell parameters of a hydrogenated amorphous silicon (a-Si:H)/µc-Si:H tandem cell. Consequently, α was a better index of a-Si:H/µc-Si:H tandem solar cell performance than Xc or high SM fractions. We consider w and α to be promising new indexes of film and solar cell quality in addition to the conventional Xc and SM.

Mechanical Tomography of Human Corneocytes with a Nanoneedle

Atomic force microscopy (AFM) can image biological samples and characterize their mechanical properties. However, the low aspect ratio of standard AFM probes typically limits these measurements to surface properties. Here, the intracellular mechanical behavior of human corneocytes is determined using "nanoneedle" AFM probes. The method evaluates the forces experienced by a nanoneedle as it is pushed into and then retracted from the cell. Indentation loops yield the stiffness profile and information on the elastic and nonelastic mechanical properties at a specific depth below the surface of the corneocytes. A clear difference between the softer ∼50-nm-thick external layer and the more rigid internal structure of corneocytes is apparent, which is consistent with the current understanding of the structure of these cells. There are also significant variations in the mechanical properties of corneocytes from different volunteers. The small diameter of the nanoneedle allows this "mechanical tomography" to be performed with high spatial resolution, potentially offering an opportunity to detect biomechanical changes in corneocytes because of, e.g., environmental factors, aging, or dermatological pathologies.

Growth and morphology of thin Cu(In,Ga)S2 films during reactive magnetron co-sputtering

The roughness and the morphology of chalcopyrite Cu(In,Ga)S2 (CIGS) films on different substrates prepared by dc reactive magnetron co-sputtering with thicknesses ranging from 10 to 1000nm were investigated by X-ray diffraction, scanning electron microscopy and atomic force microscopy (AFM). One-dimensional power spectral density (1DPSD) functions, derived from the AFM profiles, were used to compare the microstructure scaling behavior of the thin films. For CIGS films on Mo coated glass substrates, the variation of the grain size (dg) obtained from the AFM measurements, with film thickness (df) in the columnar grained films was dg(AFM)~df0.33. The growth exponent β, characterizing the roughness evolution of the films, changes with the film thickness. The root-mean-square roughness (Rrms) increases only slightly as Rrms~dfβ (β=0.13±0.09) in the initial growth stage, while Rrms increases with β=0.56±0.03, when the film thickness is larger than 50nm. 1DPSD analysis showed three spatial-frequency regimes of the roughness evolution during room temperature deposition, where the intermediate region disappears when the substrate temperature is increased. The nucleation and growth mechanisms are discussed in terms of surface diffusion, grain growth and shadowing effects. The substrate morphology does not significantly affect CIGS growth, which was proved for TiN and float glass. From the viewpoint of surface roughness and film growth, TiN is comparable to molybdenum and is suited as a potential back contact for chalcopyrite solar cell applications.

Influence of chitosan concentration on cell viability and proliferation in vitro by changing film topography

Chitosan is a natural polysaccharide which can form gels and scaffolds that support its use as a biomaterial in various tissue engineering applications. A useful feature of chitosan polymer is that you can manipulate its properties easily. Thus, in this work we studied the effect of varying chitosan concentration in the topography and the biological properties of the chitosan films, as well as the effects in the structure of 3D gels in order to be used as nerve bridges. Analysis of film topographies were addressed by swelling test and atomic force microscopy (AFM). In vitro biological properties were assessed through MTT viability assays on cultures of blood-brain barrier forming endothelial (bEnd5), glioma (C6) and postmitotic neuron (NGF-differentiated PC12) cell lines. The structure of tridimensional gels was studied by environmental scanning electron microscopy. Topography of 1% chitosan films showed a AFM profile with higher nano-roughness profile than that observed in 2% films, which was smoother. Moreover, swelling rate was not affected. Topography changes affected cell viability as shown by the MTT assays. Our results showed that 2% chitosan films promoted higher proliferation and viability of C6 and PC12 respectively than 1% films. Conversely, neither 1% nor 2% films promoted viability of bEnd5 cells. In order to establish the feasibility of both type of chitosan solutions as nerve bridges, we constructed 3D gels by alkaline precipitation. Resulting gels showed that only 2% gels were rigid enough to be effectively used as nerve bridges. These results establish that changes in chitosan concentration affects the polymer surface topography, which has a direct effect in the growing cell behavior. Additionally, higher concentration of chitosan gels are required to be used in neural tissue engineering.

Nanoscale Chemical Imaging of Zinc Oxide Nanowire Corrosion

The ability to monitor corrosion on a nanometer scale is a powerful tool for a fundamental understanding of surface chemical processes. Nanoscale chemical images of individual bare and alumina-coated zinc oxide nanowires (NWs) were recorded using tip-enhanced second harmonic generation (SHG) spectroscopy before and after exposure to carbon dioxide and water vapor. Images were collected for the same bare nanowire after each successive two-day exposure period. Corrosion of the bare ZnO NW to zinc carbonate was evident from far-field and near-field SHG images and simultaneously recorded atomic force microscopy (AFM) data. The expected zinc carbonate corrosion product is SHG inactive. The AFM profile of the NW showed vertical and lateral expansion in different regions of the nanowire. The lower resolution far-field SHG signal decreased gradually and uniformly. The near-field SHG signal provided a profile of the evolving NW with a spatial resolution approaching 100 nm. In contrast, exposed alumina-coated ZnO NWs showed reduced, but still observable, degradation. The 3 nm thick alumina protective layer may have been insufficient to fully protect the NW, or the coating may have been incomplete. Thicker coatings preclude the tip-enhanced method. Further nanometer-scale imaging should lead to the discovery of protective layers to prevent or delay ZnO degradation.

A comparative study of heat treatment temperature influence on the thickness of zirconia sol–gel thin films by three different techniques: SWE, SEM and AFM

A clear ethanol based precursor sol obtained using zirconium acetate hydroxide was utilized for the deposition of nanometer ZrO2 thin films on 316L stainless steel substrates by a sol–gel dip coating process. The influence of heat treatment temperature on the structural evolution of the films was examined using X-ray diffractometry (XRD) and Fourier transform-infrared spectroscopy (FT-IR). The results indicated that the adopted production route led to the formation of an amorphous structure at 200–300°C and surface tetragonal (t-ZrO2) and monoclinic (m-ZrO2) zirconia phases at 700°C and 900°C temperatures, respectively. Microstructural studies by a scanning electron microscopy (SEM) showed that the morphology and size of the particles of ZrO2 films depend upon the heat treatment temperature. The effect of firing temperature on the thickness of ZrO2 nanometer thin films was studied by three different techniques: single-wavelength ellipsometry (SWE), SEM and atomic force microscopy (AFM). The film thickness measured by the SWE and SEM techniques, as a function of heat treatment temperature, was in close agreement, elucidating a major decrease when heated up to 700°C, and a slight increase between 700 and 900°C. As determined by the AFM studies, the thickness changes were similar to those of SWE and SEM results during the former step, up to 700°C, but it appeared to be constant in the later temperature interval. The AFM profile images showed an increase in surface roughness of the films with rising of the heat treatment temperature.

Ultrathin Films of Variable Polarity and Crystallinity Obtained from 1,2-Polybutadiene Nanoparticle Dispersions

Characterization of ultrathin films of different polymer nanoparticles obtained at room temperature via spin-coating of aqueous dispersions and their morphology are described. Very small nanoparticles of semicrystalline 1,2-polybutadiene (PB), noncrystalline poly(1-butene) (PH), and poly(1-butenal) (PHF) were prepared via catalytic emulsion polymerization and subsequent hydrogenation or hydroformylation. The prefabricated nanoparticles were used as building blocks. The thin films obtained are continuous and transparent (n=1.5; κ=0). The properties of these films, formed from different constituents, are analyzed. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) images show that the PB-films are very smooth (rms roughness=10 nm) and polycrystalline. Recrystallization of these PB films reveals that edge-on lamellae are the constituent units. Films with very low roughness values (rms roughness <2 nm) are obtained with PH nanoparticles, due to the soft character of the nanoparticles. The AFM profile of the PHF films reveals that the surface remains structured after drying due to the high degree of the internal cross-linking that occurs in the nanoparticles. Quantification of the films' polarity (I(3)/I(1)=0.89, 1.3, and 2.1 for PHF, PB, and PH, respectively) agrees well with the previous values obtained for the polymer dispersions. Surfactant molecules are desorbed during the film formation; however, these aggregates can be removed by rinsing with water with no undesirable effects observed on the films.

Fractal in fracture of bulk metallic glass

We investigate the nanoscale periodic corrugation (NPC) structures on the dynamic fracture surface of a typical tough bulk metallic glass, submitted to high-velocity plate impact and scanned by atomic force microscopy (AFM). The detrended fluctuation analysis (DFA) of the recorded AFM profiles reveals that the valley landscapes of the NPC are nearly memoryless, characterized by Hurst parameter of 0.52 and exhibiting a self-similar fractal character with the dimension of about 1.48. Our findings confirm the existence of the “quasi-cleavage” fracture underpinned by tension transformation zones (TTZs) in metallic glasses.

Three-dimensional liquid surfaces through nanoparticle self-assembly

Nanoparticles blended into thin polymer films are driven to assemble at interfaces by both entropic and enthalpic forces. When nanoparticles assemble at a substrate–polymer film interface, these forces are so large that they enable the film to follow surface protrusions and form a three-dimensional surface, instead of dewetting from the substrate. In other words, disassembling the nanoparticle layer requires more energy than that gained by the film dewetting from the rough surface. Here we studied blends of linear polystyrene and CdSe nanoparticles spin coated onto silicon substrates containing sparsely distributed SiO2 particles (ca. 120 nm diameter). The films were then thermally annealed for periods of up to 24 h, well above the glass transition temperature of the polymer. The profiles of different film thicknesses (40–180 nm) were characterized using atomic force microscopy (AFM) before and after being annealed and were found to become much smoother, yet remain three-dimensional after annealing with a profile that gradually decayed away from the SiO2 particles. Calculations based on a continuum theory using a balance of assembly, dispersion and surface tension forces were performed and found to be in agreement with the AFM profile data demonstrating the strength of the assembly forces.

AFM stochastic analysis of surface twisted nanograin chains of iron oxide: a kinetic study

We have studied the stochastic parameters of surface iron oxide nanograin chains, 97 nm in diameter and 2.4 µm in length, prepared at different annealing temperatures, using atomic force microscopy (AFM) spectral analysis. In this regard, the roughness of the thin films including self-assembled twisted nanograin chains has been analysed and characterized using the height–height correlation function, the roughness exponent as well as the power spectrum density of the AFM profiles and their gradient, for the different annealing temperatures. The tip convolution effect on the stochastic parameters under study has also been investigated. The kinetics of the formation of nanograins on the film surface has been obtained using the AFM spectral analysis of the profiles and their gradient. The activation energy needed for the formation of surface nanograin chains was found to be 0.55 eV. It has been shown that the tip–surface interaction affects mainly the diffusion parameters obtained by using the surface roughness analysis of the profiles, while use of the surface roughness analysis of the gradient of the profiles results in a nearly independent tip convolution effect on the diffusion parameters. Hence, this work also provides a method for calculating the required activation energy for the formation of self-assembled nanostructures affecting the roughness of a surface.

Origin of intragranular crystallographic misorientations in hot-dip Al–Zn–Si coatings

The origin of intragranular variations of the crystallographic orientation in hot-dip Al–Zn–Si coatings is discussed based on new experimental results and modelling. The solidification microstructure in as-received 55Al–43.4Zn–1.6Si (in wt.%) coatings deposited on steel plates in an industrial production line was analyzed by electron backscattered diffraction, glow-discharge optical emission spectroscopy and atomic force microscopy (AFM). The results were compared with those obtained in coatings re-solidified under different cooling and mechanical loading conditions. Continuous variations of the crystallographic orientation as large as 35° were observed within individual grains of Al–Zn–Si, consistent with previous studies. However, the mechanisms previously proposed for the origin of intragranular crystallographic misorientations had to be revisited. The new experimental data acquired during this study indicate that the solidification shrinkage accumulating in the area of the grain envelope is the driving force for the formation of intragranular misorientations. The solidification shrinkage leads to the development of tensile stresses in the oxide film covering the coating while it solidifies. Estimations based on AFM profiles and phase field simulations of the dendritic structure indicate that the stresses applied on the dendrite network are sufficient to deform plastically the dendrite arms during solidification.

Study of poly(o-Ethoxyaniline) interactions with herbicides and evaluation of conductive polymer potential used in electrochemical sensors

Interactions of four aromatic nitrogen-heterocyclic herbicides (atrazine, imazaquin, metribuzin and paraquat) with the conductive polymer poly(o-ethoxyaniline)-POEA, were studied with atomic force microscopy (AFM), UV-visible spectrophotometry (UV-Vis) and electrochemical impedance spectroscopy. AFM profiles of self-assembled (SA) films of POEA revealed that the polymer surface became rougher (on the nanoscale) when exposed to atrazine, imazaquin and metribuzin, but slightly smoother in contact with paraquat. This suggested that specific chemical interactions, possibly electroscopic, have occurred between nitrogen atoms in the polymer chain and the dissimilar groups in the various herbicide molecules, during adsorption of the latter onto the film. The UV-Vis analyses indicated a stronger interaction of POEA in solution with imazaquin, that has a special importance for the intended application. Sensors produced by coating microelectrodes with SA films of POEA were used to perform impedance spectroscopy in aqueous solutions of each herbicide. With the resulting data, it was possible to distinguish and set detection limits for each herbicide in water, corroborating AFM and UV-Vis results.

Rheology of Thin Asphalt Films in Contact with Aggregate

ABSTRACT A specially designed shear viscometer fixture was built to investigate the physical properties of thin asphalt films between aggregate plates. Viscosities were measured using the specially designed fixture in an Instron instrument. The properties of thin asphalt films are not predictable from thick (1.0 mm) films on steel plates, which are used in the current Superpave® specification. Rheological properties of thin films (20 microns thick) of neat SHRP asphalt AAD-1, PAV-aged AAD-1 (20 hours at 100°C), and decanedicarboxylic acid-modified AAD-1 were determined at 25°C using the specially designed fixture. Atomic force microscopy (AFM) was applied to investigate the surface roughness profiles of different substrates including glass and aggregate plates. It was found that the roughness of the glass plate was only 20nm disparity in height, approximately one one-thousandth of the film thickness being studied. The AFM profiles of the polished aggregate plates used in the study showed that they had a surface roughness of approximately two microns, i.e., approximately ten percent of the film thickness. The results showed that thin film physical properties are dependent upon asphalt composition, aggregate type, oxidation, and asphalt modification. Separately, preliminary results from a water soaking experiment show that the specially designed fixture may be a useful tool for the characterization of moisture damage

Evaluation of surface preparation methods for glass

AbstractIn this study, methods of surface preparation by polishing, fiber‐drawing, melt‐casting and chemical etching were evaluated with x‐ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) for a variety of alkali and alkaline earth aluminosilicate glasses. Freshly polished glass surfaces were exposed to a variety of chemical etchants (NaOH, NH4OH and HF) and the resulting surface composition and morphology are reported. The polishing and etching parameters were optimized to obtain a smooth surface (root‐mean‐square (RMS) roughness of ≤3 nm) and a surface composition as close as possible to the bulk composition. In addition, glass melt and fiber surfaces were examined to investigate the effect of thermal processes on surface composition and morphology. It is shown that polishing can alter the surface composition of these multicomponent glasses, and that post‐etching of the polished surface to expose the bulk composition is plagued by preferential leaching, contamination and/or roughening. The most significant conclusion is that there is a strong dependence of these surface compositional modifications upon the specific glass; even within specific glass systems, the polish/etch procedure must be modified to yield a surface with the bulk composition. On the other hand, the smoothness of fibers and melt‐cast surfaces is ideally suited to AFM and depth profiling; their surface compositions can be close to the bulk values, but careful attention to the annealing conditions is required to achieve this. Copyright © 2001 John Wiley &amp; Sons, Ltd.