Cross-sectional Atomic Force Microscopy Research Articles

Nanohydrodynamic Local Compaction and Nanoplasmonic Form-Birefringence Inscription by Ultrashort Laser Pulses in Nanoporous Fused Silica.

The inscription regimes and formation mechanisms of form-birefringent microstructures inside nano-porous fused silica by tightly focused 1030- and 515-nm ultrashort laser pulses of variable energy levels and pulsewidths in the sub-filamentary regime were explored. Energy-dispersion X-ray micro-spectroscopy and 3D scanning confocal Raman micro-spectroscopy revealed the micro-tracks compacted by the multi-shot laser exposure with the nanopores hydrodynamically driven on a microscale to their periphery. Nearly homogeneous polarimetrically acquired subwavelength-scale form-birefringence (refractive index modulation ~10−3) was simultaneously produced as birefringent nanogratings inside the microtracks of wavelength-, energy- and pulsewidth-dependent lengths, enabling the scaling of their total retardance for perspective phase-modulation nanophotonic applications. The observed form-birefringence was related to the hierarchical multi-scale structure of the microtracks, envisioned by cross-sectional atomic-force microscopy and numerical modeling.

Realization of p-type conduction in compositionally graded quaternary AlInGaN

Quaternary AlInGaN alloy offers more degree of freedom in independently adjusting the band gap and lattice constant, and it shows the great potential in the fabrication of nitride optoelectronic and electronic devices. In this work, AlInGaN films were grown by metal-organic chemical vapor deposition and the p-type conduction was realized in compositionally graded quaternary AlInGaN. The element compositions in AlInGaN were confirmed by secondary ion mass spectrometry. The structural properties such as layer thickness, lattice polarity and surface morphology of the epilayers were characterized by cross-sectional scanning transmission electron microscopy and atomic force microscopy (AFM). The measured surface AFM images verify that the surface morphology of the quaternary AlInGaN film is smooth and it is free from V-shaped pits. Hall effect measurement results show that the grown graded AlInGaN is p-type with the hole concentration of ∼7.3 × 1016 cm−3 at room temperature. The realization of p-type conduction in AlInGaN can enrich the polarization engineering and the device structures of nitride semiconductors.

Direct Observation and Semiquantitative Analysis of Hierarchical Structures in Graft-Type Polymer Electrolyte Membranes Using the AFM Technique.

The structural features of a polymer electrolyte membrane (PEM), consisting of polystyrene sulfonic acid (PSSA) grafted onto poly(ethylene-co-tetrafluoroethylene) (ETFE), can be characterized semiquantitatively by atomic force microscopy (AFM). The cross-sectional AFM phase images are converted to the binarized image by fitting two Gaussian functions. The domains correspond to hydrophilic PSSA domains and hydrophobic ETFE crystalline and amorphous regions, respectively, at lower and higher phase shift values. The area fraction of PSSA domains was consistent with the volume fraction determined by the grafting degree (GD). The dependence of the radius and interdomain distance of the PSSA domains on the GDs of PEMs shows discontinuous features at the threshold GD (39%). The former slightly increased from 10 to 12 nm and significantly increased to 17 nm at a GD greater than 39%; the latter decreased from 140 to 54 nm with increases in GDs up to 39% but inversely increased to 78 nm at a GD of 46%. This discontinuous change in radius and interdomain distance should be caused by the fusion of adjacent PSSA domains to form a larger size and spacing and thus less connectivity between each large domain, thereby lowering the conductivity at GD greater than 39%. We were able to demonstrate the existence of an ion-conducting hydrophilic path with a radius of approximately 10 nm. Even though it has received little attention in the past, it is expected to enable the design of electrolyte membrane functions in the future.

Effect of substrate temperature on the near-infrared shielding properties of WO3-x thin films deposited by RF magnetron sputtering

Near-infrared (NIR) shielding materials emerge as potential candidates for energy conservation applications. In this work, tungsten oxide (WO3-x, 0 < x < 1) thin films were deposited on soda-lime glass substrate by RF magnetron sputtering at various substrate temperatures and NIR shielding properties of the films were studied. Structural, morphological and compositional properties of the films were also investigated. Optical constants, thickness and roughness of the films were extracted from spectroscopic ellipsometric (SE) measurements. The correlativity of SE with UV–Vis–NIR spectrophotometry, cross-sectional field emission scanning electron microscopy and atomic force microscopy was investigated. Of all the films, WO3-x thin films deposited at 500 °C showed the lowest NIR transmittance of about 23% with the highest visible transmittance of above 70% corresponding to a Tvis/Tsol ratio of 1.35 and NIR shielding value of 75%. WO3-x coated soda lime glass shows better NIR shielding performance compared to commercial soda lime glass in the thermal shielding test. The underlying reason for NIR absorption is the presence of oxygen vacancies which promote formation of W5+ ions and the resultant combined effects of polaron absorption and surface plasmon resonance. This work is significant in utilizing a facile technique for developing undoped WO3-x thin films for efficient NIR shielding applications.

Probing the effect of graphene surface chemistry on compatibility, crystallinity, and viscoelastic response of polylactic acid/polyvinylidene fluoride blends

Polylactic acid (PLA)/polyvinylidene fluoride (PVDF)/graphene nanocomposites were prepared by solution casting technique to study the effect of graphene surface chemistry on blend compatibility. Surface and cross-sectional morphology and atomic force microscopy analyses revealed that the addition of hydrophobic graphene resulted in the highest level of compatibility in PLA/PVDF blends. The untreated graphene exhibited a more compatibilizing effect than the hydrophilic graphene. According to X-ray diffraction (XRD) results, the crystallinity of polymers was enhanced in the presence of graphene for the compatibilized blends through the interfacial assisted nucleation mechanism. The formation of β-phase PVDF crystals was also increased in the presence of PLA and graphene, especially for the blend loaded with hydrophobic functionalization. Based on XRD and rheological results, the order of graphene dispersion quality was as follows: hydrophobic> untreated>hydrophilic. The improved dispersion quality of graphene nanoplatelets, along with the higher interfacial modulus for the compatibilized blends, were found responsible for the observed enhancement in the elasticity of the samples. The current study demonstrated the promising potential of graphene nanoplatelets with hydrophobic functionalization in compatibilizing the immiscible PLA/PVDF blends. Moreover, the enhanced β-phase crystals of PVDF and increased PLA crystallinity are highly useful in advanced applications that require piezoelectricity and biocompatibility.

Investigation of the Nanoscale Morphology in Industrially Relevant Clearcoats of Waterborne Polymer Colloids by Means of Variable-Angle Grazing Incidence Small-Angle X-ray Scattering

Soft polymer colloidal water suspensions are extremely important formulations for industrial applications such as water-based environmental-friendly coatings, paints, and adhesives. Homogeneity of the final coating at the micrometer and nanoscale is a crucial factor for optimal coating performance, such as barrier properties against solvent permeation. Here, we investigated the remnant nanostructure in slot-die-coated micrometer-sized thick clear coating films (clearcoats) of three different waterborne polymer colloids (pure soft, pure hard, and soft/hard multiphase), commonly utilized as primers in paint formulations [Mader et al. Prog. Org. Coat. 2011, 71, 123–135], using variable-angle grazing incidence small-angle X-ray scattering (GISAXS) complemented with cross-sectional atomic force microscopy (cs-AFM). After complete macroscopic drying, the coating films exhibit the presence of residual nanostructure with characteristic distance (d*) smaller than the original particle size and even smaller (≪d*) h...

Defect chemistry of Eu dopants in NaI scintillators studied by atomically resolved force microscopy

Activator impurities and their distribution in the host lattice play a key role in scintillation phenomena. Here a combination of cross-sectional noncontact atomic force microscopy (nc-AFM), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) was used to study the distribution of Eu2+ dopants in a NaI scintillator activated by 3% of EuI2. Two types of precipitate structures were found. First, needle-shaped EuI2 precipitates with a layered structure are likely responsible for scattering the scintillation light. In transparent crystals with good scintillation properties, precipitates with a cubic crystal structure and a size below 4 nm were found. A surprisingly low concentration of point defects was detected in all of the investigated samples. Upon annealing, Eu segregates towards the surface, which results in the formation of an ordered hexagonal overlayer with the EuI2 composition and a pronounced, unidirectional moire pattern.

Monolayer doping of silicon-germanium alloys: A balancing act between phosphorus incorporation and strain relaxation

This paper presents the application of monolayer doping (MLD) to silicon-germanium (SiGe). This study was carried out for phosphorus dopants on wafers of epitaxially grown thin films of strained SiGe on silicon with varying concentrations of Ge (18%, 30%, and 60%). The challenge presented here is achieving dopant incorporation while minimizing strain relaxation. The impact of high temperature annealing on the formation of defects due to strain relaxation of these layers was qualitatively monitored by cross-sectional transmission electron microscopy and atomic force microscopy prior to choosing an anneal temperature for the MLD drive-in. Though the bulk SiGe wafers provided are stated to have 18%, 30%, and 60% Ge in the epitaxial SiGe layers, it does not necessarily mean that the surface stoichiometry is the same, and this may impact the reaction conditions. X-ray photoelectron spectroscopy (XPS) and angle-resolved XPS were carried out to compare the bulk and surface stoichiometry of SiGe to allow tailoring of the reaction conditions for chemical functionalization. Finally, dopant profiling was carried out by secondary ion mass spectrometry to determine the impurity concentrations achieved by MLD. It is evident from the results that phosphorus incorporation decreases for increasing mole fraction of Ge, when the rapid thermal annealing temperature is a fixed amount below the melting temperature of each alloy.

Polarization Effects in Graded AlGaN Nanolayers Revealed by Current-Sensing and Kelvin Probe Microscopy.

We experimentally demonstrate that the conductivity of graded AlxGa1-xN increases as a function of the magnitude of the Al concentration gradient (%Al/nm) due to polarization doping effects, without the use of impurity dopants. Using three up/down-graded AlxGa1-xN nanolayers with Al gradients ranging from ∼0.16 to ∼0.28%Al/nm combined in one structure, the effects of polarization engineering for localized electric fields and current transport were investigated. Cross-sectional Kelvin probe force microscopy and conductive atomic force microscopy were used to directly probe the electrical properties of the films with spatial resolution along the thickness of the growth. The experimental profiles of the built-in electric fields and the spreading current found in the graded layers are shown to be consistent with simulations of the field distribution as well as of the electron and hole densities. Finally, it was directly observed that for gradients less than 0.28%Al/nm the native n-type donors still limit polarization-induced hole doping, making p-type conductivity still a challenge due to background impurities and defects.

The Role of Duty Cycle of Substrate Pulse Biasing in Filtered Cathodic Vacuum Arc Deposition of Amorphous Carbon Films

The effect of the duty cycle of substrate pulse biasing on the structure (hybridization), thickness, residual stress, and roughness of amorphous carbon (a-C) films deposited by filtered cathodic vacuum arc was examined in the light of high-resolution transmission electron microscopy, cross-sectional electron energy loss spectroscopy, Raman spectroscopy, residual stress, and atomic force microscopy measurements. The film structure and composition reveal a multilayer architecture consisting of an interface layer of C, Si, and (possibly) SiC, a buffer layer with varying sp3 fraction and relatively constant carbon concentration, a bulk layer with constant and high sp3 concentration, and a surface layer rich in sp2 hybridization. It is shown that a 65% duty cycle yields the smoothest and thinnest a-C films with relatively high sp3 content, whereas a 75% duty cycle produces relatively thicker and rougher a-C films with maximum sp3 carbon concentration and highest residual (compressive) stress. The results of this study have direct implications in high-density magnetic recording, where smooth, ultrathin a-C films with high sp3 content are of critical importance to the longevity and reliability of hard-disk drives.

Indium segregation during III–V quantum wire and quantum dot formation on patterned substrates

We report a model for metalorganic vapor-phase epitaxy on non-planar substrates, specifically V-grooves and pyramidal recesses, which we apply to the growth of InGaAs nanostructures. This model—based on a set of coupled reaction-diffusion equations, one for each facet in the system—accounts for the facet-dependence of all kinetic processes (e.g., precursor decomposition, adatom diffusion, and adatom lifetimes) and has been previously applied to account for the temperature-, concentration-, and temporal-dependence of AlGaAs nanostructures on GaAs (111)B surfaces with V-grooves and pyramidal recesses. In the present study, the growth of In0.12Ga0.88As quantum wires at the bottom of V-grooves is used to determine a set of optimized kinetic parameters. Based on these parameters, we have modeled the growth of In0.25Ga0.75As nanostructures formed in pyramidal site-controlled quantum-dot systems, successfully producing a qualitative explanation for the temperature-dependence of their optical properties, which have been reported in previous studies. Finally, we present scanning electron and cross-sectional atomic force microscopy images which show previously unreported facetting at the bottom of the pyramidal recesses that allow quantum dot formation.

Site specific isolated nanostructure array formation on a large area by broad ion beam without any mask and resist

We report the formation of isolated nanostructure arrays on a large area via broad ion beam implantation without the aid of any mask or resist. Desired ions have been implanted at specific locations of the prefabricated silicon ripple or triangular structures by exploiting the variation of local ion impact angles. We have shown that the implantation of Fe ions on an O+ ions induced pre fabricated triangular shaped patterned Si surface results in a self-organized periodic array of striped magnetic nanostructures having several micron length and about 50 nm width arranged with a spacial separation of ∼200 nm. The morphology, composition, crystalline structure, and magnetic property of these nanopatterns have been analyzed using high-resolution cross-sectional transmission electron microscopy and atomic force microscopy. A geometrical model has been proposed to explain the fundamental features of such ion-induced nanopattern structures.

Investigation of high hole mobility In0.41Ga0.59Sb/Al0.91Ga0.09Sb quantum well structures grown by molecular beam epitaxy

Modulation-doped In0.41Ga0.59Sb/Al0.91Ga0.09Sb quantum-well (QW) structures were grown by molecular beam epitaxy. Cross-sectional transmission electron microscopy and atomic force microscopy studies show high crystalline quality and smooth surface morphology. X-ray diffraction investigations confirm 1.94% compressive strain within In0.41Ga0.59Sb channel. High room temperature hole mobility with high sheet density of 1000 cm2/Vs, 0.877 × 1012/cm2, and 965 cm2/Vs, 1.112 × 1012/cm2 were obtained with different doping concentrations. Temperature dependent Hall measurements show different scattering mechanisms on hole mobility at different temperature range. The sheet hole density keeps almost constantly from 300 K to 77 K. This study shows great potential of In0.41Ga0.59Sb/Al0.91Ga0.09Sb QW for high-hole-mobility device applications.

Single Etch-Pit Shape on Off-Angled 4H-SiC(0001) Si-Face Formed by Chlorine Trifluoride

The etch pit shape of an off-angled 4H-SiC Si-face formed by chlorine trifluoride (ClF3) in nitrogen (N2) ambient has been studied. One type of etch pit with a crooked hexagonal shape was formed at an etching temperature below 500 °C. The angle of the etch pit measured from a cross-sectional atomic force microscopy image was about 10° from the [112̄0] view. The dislocation type of the etch pit was discussed in relation to the etch pit shape and an electron-beam-induced current image.

Photoluminescence and secondary ion mass spectrometry investigation of unintentional doping in epitaxial germanium thin films grown on III-V compound by metal-organic chemical vapor deposition

High quality epitaxial germanium (Ge) thin films grown on lattice matched and mismatched III-V compound may lead to development of new electronic and optoelectronic devices. Understanding the doping and electronic properties of these Ge thin films is the first step in this development. In this paper, we report on high-quality epitaxial Ge thin films grown on GaAs and AlAs by metal-organic chemical vapor deposition. Cross-sectional transmission electron microscopy and atomic force microscopy reveal the high structural quality of the Ge thin films. Using photoluminescence, secondary ion mass spectrometry, and spreading resistance analysis, we investigated the unintentional doping characteristics of the fabricated Ge-on-III-V thin films. We found that arsenic (n-type doping) concentration is determined by the background partial pressure of volatile As-species (e.g., As2 and As4), which incorporate into the Ge thin films via gas phase transport during the growth. Group III element (p-type doping) incorporation in the Ge thin films occurs during the growth through a surface exchange process. There exists a trade-off between Ge film structural quality and group III element “auto-doping.” III-V compound surfaces that are group III element-rich facilitate the initiation of Ge thin films with high crystalline quality and low surface roughness. However, the group-III-rich surfaces also result in high group III element (p-type doping) concentrations in the Ge thin films.

An atomic scale study on the effect of Sb during capping of MBE grown III–V semiconductor QDs

The use of Sb during the capping process of quantum dots (QDs) to tune the emission wavelength has been investigated by means of cross-sectional scanning tunneling microscopy (X-STM), photoluminescence (PL) measurements and atomic force microscopy. It is found that a capping layer of GaAsSb reduces InAs/GaAs QD decomposition during capping due to the smaller lattice mismatch with the QD and due to the surfactant effect of Sb. An Sb content of 14% in the GaAsSb layer has been found to be sufficient to eliminate the QD decomposition completely and improve the PL properties. Further increase in the Sb content causes a degradation of the PL properties. Reduction of the QD decomposition has also been achieved by soaking the QDs with Sb before the capping. Furthermore, it is shown that the segregation of In and of Sb in the Ga and the As sublattices respectively are two independent processes. Similar investigations to the use of Sb to tune InAs/InP (311B) QDs are reported. Also in this case, the use of Sb, either in the form of elemental Sb or as a GaAsSb alloy, has resulted in elimination of QD decomposition.

Deposition and characterization of low temperature silicon nitride films deposited by inductively coupled plasma CVD

Silicon nitride films have been deposited at a low temperature (70 °C) by inductively coupled plasma chemical vapor deposition (ICP-CVD) technique and their physical and chemical properties were studied. For a deposited SiN sample, β-phase was observed and refractive index of 2.1 at 13.18 nm/min deposition rate was obtained. The attained stress of 0.08 GPa is lower as compared to the reported value of 1.1 GPa for SiN thin films. To study the deposited film, characterization was performed using X-ray photoelectron spectra (XPS), X-ray diffraction (XRD), micro Raman spectroscopy, Fourier transfer infrared spectroscopy (FTIR), cross-section scanning electron microscopy (SEM) and atomic force microscopy (AFM).

Structure and properties of ZrN coatings deposited by high power impulse magnetron sputtering technology

Monolayer ZrN coatings were deposited exclusively by the novel high power impulse magnetron sputtering (HIPIMS) technology in an industrial scale physical vapour deposition (PVD) machine (HTC-1000-4 target system). Coatings were deposited on 1 μm polished M2 high speed steel, on 304L stainless steel (SS), and on Si (100) specimens. Prior to deposition, HIPIMS plasma sustained on a zirconium (Zr) target was utilized to pretreat the specimens. Coatings were deposited at 400 °C in a mixed N2 and Ar atmosphere using 2 magnetrons in HIPIMS mode and at three different substrate bias voltages (Ubias) keeping all other process parameters constant. The thicknesses of the coatings measured by the ball cratering technique were in the ranges of 1.84, 1.96, and 2.13 μm at bias voltages of −95, −75, and −65 V, respectively, where the difference in thickness can be attributed to the resputtering effect. X-ray diffraction experiments on SS specimens revealed a dominating 111 texture for all three coatings irrespective of the bias voltage. Cross-sectional transmission electron microscopy revealed extremely dense coating structures at all bias voltages, similar to the transition zone structure (zone T) reported by Thornton. The −95 bias voltage coatings appeared extremely smooth on the top and with no dome shaped structures often associated with low ion bombardment during deposition. HIPIMS pretreatment leads to high adhesion (LC) of the coatings to the substrate. A continuous ductile perforation of the coating was observed at progressive loads greater than 65 N; however, no spallation of the coating was observed up to loads of 100 N. High values of hardness (40.4 GPa), Young’s modulus (424 GPa), and compressive stress (10 GPa) were recorded for coatings deposited at −95 BV. The hardness and internal stress of the coating were found increasing with more negative bias voltages. All the coatings exhibited high dry sliding wear resistance (KC) in the range 6×10−15 m3 N−1 m−1. Cross-sectional transmission electron microscopy and atomic force microscopy analyses have been used to study the effect of ion bombardment obtained from HIPIMS on the structure of the coatings.

Hierarchical porous materials: Internal structure revealed by argon ion-beam cross-section polishing, HRSEM and AFM

High-resolution scanning electron microscopy coupled with argon ion-beam cross-section polishing and atomic force microscopy were used to reveal details about the interconnectivity of meso- and macroporous networks in silica hierarchical porous material. The material studied has ordered macro- and mesoporosity as well as a disordered microporosity that are created using organic templating agents. Latex spheres template the macro- porosity, mesophase self-assembly of block co-polymers template the mesoporosity and individual polymer chains template the microporosity. By polishing this composite material with an argon ion beam it is possible to reveal the internal structure with minimal structural damage. Direct imaging of the mesoporous network within such cross sections is possible using high-resolution scanning electron microscopy. These observations demonstrate a lack of interconnectivity between the mesoporous and macroporous networks.

Nanodots induced columnar growth of YBa2Cu3Ox films

We report columnar growth of YBa2Cu3Ox (YBCO) films by pulsed laser deposition using nanodots of Au, Ag and Pd deposited on SrTiO3 substrate prior to the growth of the film. The nanodots initiate formation of densely packed YBCO columns with c-axis along the length of the columns. The obtained columnar structure offers enhanced pinning of magnetic flux and provides high critical current density in a magnetic field directed along the c-axis of YBCO while keeping high critical current density for other directions of magnetic field. The evidence of columnar structure is given by the planar and cross-sectional atomic force microscopy, transmissional election microscopy and rotational transport measurements of critical current. By creating the columnar structure, an improvement of critical current up to about 200% is achieved comparable to the reference samples without nanodots in a wide range of magnetic field and thickness of the samples. The largest increase in critical current is observed in magnetic fields close to the matching field calculated from the density of nanoparticles or density of YBCO columns.