Characteristic Feature Size Research Articles

Lignite Particle Size Distribution and its Effect on Briquetting

Huolinhe(HLH) Lignite was used for this experiment. Among the several factors that influence lignite briquetting, particle size distribution was investigated in detail. Rosin- Rammler- Bennet equation (RRB) was used to describe particle size distribution. Uniformity index n and characteristic feature size De were also calculated. Compaction curves of the lignite shows that the whole compaction process can be divided into the particle re-arrangement stage, the plastic deformation stage and the whole deformation stage. More reasonable uniformity of particle size distribution and characteristic feature size would have higher compact density. The granularity of lignite had a distinct effect on the briquette strength. In order to increase the mechanical stability of briquettes small particle size lignite should be used.

The fabrication of tunable nanoporous oxide surfaces by block copolymer lithography andatomic layer deposition

Patterned nanoscale materials with controllable characteristic feature sizes andperiodicity are of considerable interest in a wide range of fields, with various possibleapplications ranging from biomedical to nanoelectronic devices. Block-copolymer(BC)-based lithography is a powerful tool for the fabrication of uniform, densely spacednanometer-scale features over large areas. Following this bottom-up approach,nanoporous polymeric films can be deposited on any type of substrate. Thenanoporous periodic template can be transferred to the underlying substrate bydry anisotropic etching. Nevertheless the physical sizes of the polymeric maskrepresent an important limitation in the implementation of suitable lithographicprotocols based on BC technology, since the diameter and the center-to-centerdistance of the pores cannot be varied independently in this class of materials.This problem could be overcome by combining block copolymer technology withatomic layer deposition (ALD): by means of BC-based lithography a nanoporousSiO2 template, with well-reproducible characteristic dimensions, can be fabricated and subsequentlyused as a backbone for the growth of perfectly conformal thin oxide films by ALD. In thiswork polystyrene-b-poly(methylmethacrylate) (PS-b-PMMA) BC and reactive ion etchingare used to fabricate hexagonally packed 23 nm wide nanopores in a 50 nm thickSiO2 matrix. By ALDdeposition of Al2O3 thin filmsonto the nanoporous SiO2 templates, nanostructured Al2O3 surfaces are obtained. By properly adjusting the thickness of theAl2O3 film the dimension of the pores in the oxide films is progressively reduced, withnanometer precision, from the original size down to complete filling of the pores,thus providing a simple and fast strategy for the fabrication of nanoporousAl2O3 surfaces with well-controllable feature size.

The Characteristics and Applications of Metamaterials

The past 10 years have seen remarkable developments in microwave, optical and acoustic methods that make use of the unique wave transmission properties of a class of ordered composites known as metamaterials. These materials differ from regular materials in that wave propagation is dramatically affected by the size and arrangement of their small-scale structure, and not just by the choice of constituent materials. Their characteristic feature sizes can be much less than, or on the same order as a wavelength. They have a variety of potential applications ranging from super-lenses, to enabling subwavelength imaging resolution, to the creation of a Harry Potter-like ‘invisibility cloak’. In this review we provide an overview of some of these achievements, much of which has been proposed and demonstrated with optical and electromagnetic waves. However, more recently these ideas have been extended to acoustic waves, offering the opportunity of controlling ultrasound wave propagation in a unique manner.

Effect of the Structural Scale of Plasma-Sprayed Alumina Coatings on Their Friction Coefficients

The objective of this study is to compare the tribological properties of alumina coatings with two different structural scales, a micrometer-sized one manufactured by atmospheric plasma spraying and a sub-micrometer-sized one manufactured by suspension plasma spraying. Coating architectures were analyzed and their friction coefficients in dry sliding mode measured. Sub-micrometer-sized structured coatings present a lower friction coefficient than micrometer ones, thanks to their higher cohesion and smaller characteristic structural feature sizes.

Transitions from oscillatory to smooth fracture propagation in brittle metallic glasses

We present a simple model to explain the transition from oscillatory to smooth crack propagation in brittle metallic glasses. We demonstrate that the smooth fracture propagation that is characteristic for higher temperature or higher crack opening velocities (for type 1 crack propagation) becomes unstable and oscillatory behavior is being observed. The characteristic feature size of the crack propagation may be at the nanometer scale and grows as the opening velocity decreases.

Enhanced size-dependent piezoelectricity and elasticity in nanostructures due to the flexoelectric effect

Crystalline piezoelectric dielectrics electrically polarize upon application of uniform mechanical strain. Inhomogeneous strain, however, locally breaks inversion symmetry and can potentially polarize even nonpiezoelectric (centrosymmetric) dielectrics. Flexoelectricity\char22{}the coupling of strain gradient to polarization\char22{}is expected to show a strong size dependency due to the scaling of strain gradients with structural feature size. In this study, using a combination of atomistic and theoretical approaches, we investigate the ``effective'' size-dependent piezoelectric and elastic behavior of inhomogeneously strained nonpiezoelectric and piezoelectric nanostructures. In particular, to obtain analytical results and tease out physical insights, we analyze a paradigmatic nanoscale cantilever beam. We find that in materials that are intrinsically piezoelectric, the flexoelectricity and piezoelectricity effects do not add linearly and exhibit a nonlinear interaction. The latter leads to a strong size-dependent enhancement of the apparent piezoelectric coefficient resulting in, for example, a ``giant'' 500% enhancement over bulk properties in $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$ for a beam thickness of $5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. Correspondingly, for nonpiezoelectric materials also, the enhancement is nontrivial (e.g., 80% for $5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ size in paraelectric $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$ phase). Flexoelectricity also modifies the apparent elastic modulus of nanostructures, exhibiting an asymptotic scaling of $1∕{h}^{2}$, where $h$ is the characteristic feature size. Our major predictions are verified by quantum mechanically derived force-field-based molecular dynamics for two phases (cubic and tetragonal) of $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$.

Indentation variability of natural nanocomposite materials

Small-scale depth-sensing indentation (nanoindentation) is a popular technique for measuring the mechanical properties of a wide range of materials. Contact mechanics solutions used in data analysis are based on the indentation of a homogeneous half-space, but the experiments are frequently conducted on mineralized biological tissues—biocomposite materials with nanometer-scale features—such as bone and dentin. The current study examines the experimental indentation response of bone across orders of magnitude in contact dimension length-scale, from nanometers to micrometers. Scaling arguments are used to establish the need for nanoscale simulations of mineralized tissue indentation. A finite element model of an inhomogeneous contact problem is developed and used to interpret experimental indentation data on bone and dentin. Both experimental data and modeling results demonstrate a convergence in apparent elastic modulus at increasing contact length-scales. Models results are used to estimate a feature size associated with inhomogeneity of the indentation response; for experiments conducted here the characteristic feature size is found to be substantially larger for bone than for dentin, and in both cases larger than for individual nanometer-scale mineral platelets.

Profile Control in Silicon Nanostructures Using Fluorine-Enhanced Oxide Passivation

Nanostructures with controllable geometries are increasingly needed for mechanosensor applications due to their superior capabilities in measuring minute mechanical forces at small scales. Previous nanofabrication technologies are deficient in creating such structures due to high cost, complicated process and poor geometric control. In this paper, we introduce an innovative use of fluorine enhanced oxidation to realize a segment by segment sidewall control strategy for creating silicon nanostructures. This method is based on fluorine reactive ion etching, where the rich amount of fluorine species accumulated on the surface are utilized to enhance surface oxidation during an additional oxygen exposure. The experimental results showed good repeatability. The characteristic feature size of the resulting nanostructures is about 200 nm. The minimum feature size goes down to 50 nm. This method requires minimal use of equipment, and demonstrates good control of sidewall profiles. It thus has technical and economic significance in development of functional nanomechanosensors, which are potent for quantitative mechanical sensing and actuating at small scales.

Surface microstructure of GeSbSe chalcogenide thin films grown at oblique angle

Surface analysis of GeSbSe chalcogenide thin films grown at normal incidence and at four different oblique evaporation angles (45°, 75°, 80°, and 85°) was performed by a combination of scanning electron microscopy and atomic force microscopy. Additionally, quantitative roughness and microstructural analyses of the GeSbSe chalcogenide thin films were performed. Increasing roughness and surface area for increasingly oblique evaporation angle were observed, following an exponential relationship in both cases. Two-dimensional power spectral density analysis further supported the exponential behavior of the surface characteristic feature size. The notable increase of roughness and surface area with increasing evaporation oblique angle can have a significant effect on the further development of optical sensors.

Solidification processing of alloys in the pseudo-binary PbTe–Sb 2Te 3 system

The effects of composition and cooling rate on the microstructures of alloys in the pseudo-binary PbTe–Sb 2Te 3 system were investigated as a first step towards the design of nanostructured materials with enhanced thermoelectric properties. Liquid alloys of three different compositions were cooled in three distinct ways: water quenching, air cooling and furnace cooling. The resultant structures and phases were examined by electron microscopy, electron microprobe chemical analysis and electron backscatter diffraction. The compound Pb 2Sb 6Te 11 precipitated as a metastable phase (in conjunction with PbTe and/or Sb 2Te 3) under all conditions. Furthermore, whereas PbTe exhibited dendritic morphology, Sb 2Te 3 and Pb 2Sb 6Te 11 crystallized as lamellar platelets with preferred (0 0 1) orientation. The range of cooling rates was from ∼1 to 26 K/s, while the characteristic microstructural feature size ranged from 10 to 35 μm for dendrites, and from 15 to 50 μm for lamella. The prospects for achieving nanoscale structure are discussed.

Training effects in Gd5Ge4: role of microstructure

Detailed optical metallographic studies at room temperature on polycrystallineGd5Ge4 are presented for two cases: (a) a sample in the as-cast condition and (b) the samesample subjected to a known number of temperature and magnetic field cycles. Aherringbone- (criss-cross-) like pattern whose characteristic feature size extends to≈20 µm is observed in the as-cast sample. The herringbone pattern at room temperature isinterpreted as arising due to a long-lived (kinetically arrested) metastable phase. Thisherringbone pattern can be trained to form a pattern having a different morphology bycycling the sample through the low-temperature field induced magneto-structuraltransition. Training effects are also observed in magneto-transport and magnetizationmeasurements as the sample is subjected to temperature and field cycles. Based onheuristic arguments, a model is proposed which self-consistently explains anomalies intransport and magnetization properties in terms of changes occurring in the microstructureof the sample. These results highlight the need for analysing the interesting properties ofGd5Ge4 and its family of alloys with microstructure as an important component.

Reversible Shape Transition of Pb Islands on Cu(111)

Using low-energy electron microscopy, we have observed a reversible transition in the shape of Pb adatom and vacancy islands on Cu(111). With increasing temperature, circular islands become elongated in one direction. In previous work we have shown that surface stress domain patterns are observed in this system with a characteristic feature size which decreases with increasing temperature. We show that the island shape transition occurs when the ratio of the island size to this characteristic feature size reaches a particular value. The observed critical ratio matches the value expected from stress domains.

Reconstruction of pattern images from scanning electron microscope images

As the characteristic feature sizes of integrated circuits continue to shrink, both research and manufacturing are increasingly reliant on the scanning electron microscope (SEM) images. In this article, we propose a robust algorithm that automatically extracts the full edge contours from the SEM images and then converts the SEM images to pattern images even when the SEM images are corrupted by strong noise and lossy compression. This algorithm is expected to have many applications. An example of application—defect self-inspection—based on SEM conversion using this algorithm, is also shown.

Effective slip on textured superhydrophobic surfaces

We study fluid flow in the vicinity of textured and superhydrophobically coated surfaces with characteristic texture sizes on the order of 10μm. Both for droplets moving down an inclined surface and for an external flow near the surface (hydrofoil), there is evidence of appreciable drag reduction in the presence of surface texture combined with superhydrophobic coating. On textured inclined surfaces, the drops roll faster than on a coated untextured surface at the same angle. The highest drop velocities are achieved on surfaces with irregular textures with characteristic feature size ∼8μm. Application of the same texture and coating to the surface of a hydrofoil in a water tunnel results in drag reduction on the order of 10% or higher. This behavior is explained by the reduction of the contact area between the surface and the fluid, which can be interpreted in terms of changing the macroscopic boundary condition to allow nonzero slip velocity.

Surface morphology evolution during electrodeposition of amorphous CoP films

We have used atomic force microscopy to measure the roughness of electrodeposited amorphous CoP as a function of length scale and film thickness. In contrast to the power law scaling usually observed for polycrystalline electrodeposited films in the absence of additives, the roughness decreases with increasing film thickness. For films grown on Au on glass substrates, the characteristic lateral feature size is ∼250 nm, independent of the CoP thickness. This is consistent with columnar growth. Films of thickness 4 μm have a saturation roughness of only ∼1 nm, which is less than that of the substrates.

Electrodes and electrolytes in micro-SOFCs: a discussion of geometrical constraints

Miniaturized solid oxide fuel cells (SOFCs), whose designs are compatible with conventional semiconductor fabrication technology, are considered from a theoretical point of view given their distinctive electrode and electrolyte geometries. Micro-SOFCs may be operated in either a single-chamber mode for which both electrodes are located on one side of a thin film electrolyte or in the more conventional two-chamber mode, in which electrodes are located on opposite sides of the electrolyte. The electrode geometries and film thicknesses necessary to achieve sufficiently low polarization resistances are evaluated for the single-chamber micro-SOFC. Interdigitated (comb-like) electrodes with characteristic feature sizes of a few micrometers are found to be essential to insure low polarization resistance. Requirements with respect to electrochemical properties of the electrodes are addressed as well. Numerical calculations are used to examine the correlation between decreasing electrolyte film thickness and ohmic polarization in two-chamber fuel cells. Current constriction at electrode particles or three phase boundaries render very low film thicknesses (below say 300 nm) not very sensible.

Post-chemical mechanical polishing cleaning of silicon wafers with laser-induced plasma

During chemical-mechanical polishing (CMP), wafers are subjected to various particle sources such as slurry, polishing pads and polishing machines. Consequently, wafer post-CMP cleaning is crucial in micro- and nano-manufacturing to improve yield. In conventional cleaning process, the wafer is subjected to forces whose magnitudes are large enough to potentially cause substrate damage. This damage concern is becoming more severe as the characteristic feature size shrinks to sub-100-nm region. The development of dry, rapid, non-contact and non-destructive particle removal methods has been emerging as a critical requirement for post-CMP cleaning. In recent years a novel technique for particle removal using the pressure field generated by pulsed laser-induced plasma (LIP) has been introduced for cleaning surfaces and trenches. In the current study, it is demonstrated that the LIP removal technique is capable of removing ceria CMP particles with size of 100 nm and above without any substrate damage. The results reported in this study prove that LIP can also be applied over extended areas for post-CMP cleaning. It is possible to remove smaller particles at lower gap distances but the minimum particle size that can be removed is limited by the risk of substrate damage. Results reported in this study indicate that the LIP particle removal technique has significant potential for post-CMP cleaning in the near future.

Raman spectroscopy of electrochemically self-assembled CdS quantum dots

We report a Raman spectroscopy investigation of electrochemically self-assembled quasiperiodic arrays of CdS quantum dots with characteristic feature size of 10 nm. The dots were synthesized using electrochemical deposition of CdS into a porous anodized alumina film. Polarization-dependent Raman scattering study over an extended frequency range reveals the quantization of electronic states in the conduction band and intersubband transitions. Raman peaks observed at 2919 and 3050 cm−1 are attributed to transitions between the lowest two subbands. The results suggest that quantum dot arrays, produced by inexpensive robust electrochemical means, may be suitable for infrared detector applications.

Growth of Patterned Surfaces

During epitaxial crystal growth a pattern that has initially been imprinted on a surface approximately reproduces itself after the deposition of an integer number of monolayers. Computer simulations of the one-dimensional case show that the quality of reproduction decays exponentially with a characteristic time which is linear in the activation energy of surface diffusion. We argue that this lifetime of a pattern is optimized if the characteristic feature size of the pattern is larger than $(D/F{)}^{1/(d+2)}$, where $D$ is the surface diffusion constant, $F$ the deposition rate, and $d$ the surface dimension.

Nanoscale Characterization of Materials

One of the dominant trends in current research in materials science and related fields is the fabrication, characterization, and application of materials and device structures whose characteristic feature sizes are at or near the nanometer scale. Achieving an understanding of—and ultimately control over—the properties and behavior of a wide range of materials at the nanometer scale has therefore become a major theme in materials research. As our ability to synthesize materials and fabricate structures in this size regime improves, effective characterization of materials at the nanometer scale will continue to increase in importance.Central to this activity are the development and application of effective experimental techniques for performing characterization of structural, electronic, magnetic, optical, and other properties of materials with nanometer-scale spatial resolution. Two classes of experimental methods have proven to be particularly effective: scanning-probe techniques and electron microscopy. In this issue of MRS Bulletin, we have included eight articles that illustrate the elucidation of various aspects of nanometer-scale material properties using advanced scanningprobe or electron-microscopy techniques. Because the range of both experimental techniques and applications is extremely broad—and rapidly increasing—our intent is to provide several examples rather than a comprehensive treatment of this extremely active and rapidly growing field of research.