Number Of Stacked Layers Research Articles

Polarization controlled emisson from closely stacked InAs/GaAs quantum dots

AbstractWe have controlled the electronic states of closely‐stacked InAs/GaAs quantum dots with a 4.0 nm spacer layer and investigated the optical gain characteristics. With an increase in the stacking‐layer number (SLN), the [001] transverse‐magnetic (TM) polarization component increases as well as the linear polarization anisotropy in the (001) plane becomes remarkable. These SLN‐dependent polarization characteristics result from the valence‐band mixing induced by the vertically‐coupled electronic states in stacked QDs. We have systematically studied polarized electroluminescence properties of a semiconductor‐optical amplifier devise containing 30‐stacked InAs/GaAs QDs. The net modal gain was analyzed by using the Hakki‐Paoli method. The injection current dependence of the gain spectra shows a state filling effect and a change in the contribution of the TM polarization component. The polarization insensitive gain feature within ±1 dB has been achieved in the low injection current condition. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

X-ray diffraction: a powerful tool to probe and understand the structure of nanocrystalline calcium silicate hydrates

X-ray diffraction (XRD) patterns were calculated and compared to literature data with the aim of investigating the crystal structure of nanocrystalline calcium silicate hydrates (C-S-H), the main binding phase in hydrated Portland cement pastes. Published XRD patterns from C-S-H of Ca/Si ratios ranging from ~ 0.6 to ~ 1.7 are fully compatible with nanocrystalline and turbostratic tobermorite. Even at a ratio close or slightly higher than that of jennite (Ca/Si = 1.5) this latter mineral, which is required in some models to describe the structure of C-S-H, is not detected in the experimental XRD patterns. The 001 basal reflection from C-S-H, positioned at ~ 13.5 Å when the C-S-H structural Ca/Si ratio is low (< 0.9), shifts towards smaller d values and sharpens with increasing Ca/Si ratio, to reach ~ 11.2 Å when the Ca/Si ratio is higher than 1.5. Calculations indicate that the sharpening of the 001 reflection may be related to a crystallite size along c* (i.e. a mean number of stacked layers) increasing with the C-S-H Ca/Si ratio. Such an increase would contribute to the observed shift of the 001 reflection, but fails to quantitatively explain it. It is proposed that the observed shift could result from interstratification of at least two tobermorite-like layers, one having a high and the other a low Ca/Si ratio with a basal spacing of 11.3 and 14 Å, respectively.

Chemical beam epitaxy growth and optimization of InAs/GaAs quantum dot multilayers

This paper reports on an in-situ growth process used to optimize InAs/GaAs quantum dot (QD) multilayer structures grown on (001) GaAs substrate by chemical beam epitaxy (CBE). Defects related to the incoherently relaxed InAs clusters are found to alter the QD nucleation mechanism on the subsequent layers, leading to reduced QD density and photoluminescence intensity. The formation of poor crystalline quality clusters is avoided by growing the GaAs spacer layers in a two-step process. The technique consists in covering the InAs QD layer with a 10nm-thick GaAs layer grown at 465°C, and then removing the excess indium contained in the uncapped portion of the clusters by increasing the temperature to 565°C for 10min before the deposition of the remaining GaAs spacer layer. Morphological investigation shows that the QD density and size distribution obtained in the first layer are preserved up to the tenth layer. The QD integrated photoluminescence intensity is found to increase linearly with the number of stacked layers. These results are very promising for chemical beam growth of high performance intermediate-band solar cells.

Exploring the behavior of glass fiber reinforcements under vibration-assisted compaction

In this study, a new vibration-assisted approach is proposed for compacting dry fibrous reinforcements in liquid composite molding (LCM). The first steps of LCM processes involve fiber bed lay-up within a rigid mold, followed by the closure of the counter-mold and subsequent fiber compaction. Traditionally, static compaction has been applied to level the thickness of the reinforcement and achieve high fiber volume contents. However, fibrous materials under transverse compression exhibit viscoelastic characteristics that can be exploited through dynamic loading in order to achieve a higher level of compaction. In this work, a vibration-assisted compaction technique has been developed, in which static and dynamic stresses are combined to provide specific compaction conditions. A series of tests allowed exploring the impact of different loading conditions (i.e. static force, dynamic force, and vibratory frequency) on the compaction response of fibrous preforms. The effects of vibration-assisted compaction on various fabric architectures and different numbers of stacked layers were explored as well. The analysis of experimental outputs allows identifying the governing parameters of this new approach. The fiber volume contents obtained reveal that the use of a complex compressive load, composed of static and dynamic forces, can be very effective for the compaction of dry fibrous materials. Although vibration-assisted compaction remains a technology in its preliminary stage, this paper contributes to a better understanding of its potential and provides tools for eventual applications.

Polymeric Frameworks as Organic Semiconductors with Controlled Electronic Properties

The rational assembly of monomers, in principle, enables the design of a specific periodicity of polymeric frameworks, leading to a tailored set of electronic structure properties in these solid-state materials. The further development of these emerging systems requires a combination of both experimental and theoretical studies. Here, we investigated the electronic structures of two-dimensional polymeric frameworks based on triazine and benzene rings by means of electrochemical techniques. The experimental density of states was obtained from quasi-open-circuit voltage measurements through a galvanostatic intermittent titration technique, which we show to be in excellent agreement with first-principles calculations performed for two- and three-dimensional structures of these polymeric frameworks. These findings suggest that the electronic properties depend not only on the number of stacked layers but also on the ratio of the different aromatic rings.

One-dimensional miniband formation in closely stacked InAs/GaAs quantum dots

We have studied the electronic states of closely stacked InAs/GaAs quantum dots (QDs) with a 4.0-nm spacer layer using linearly polarized photoluminescence (PL) and time-resolved PL measurements. An increase in the stacking-layer number (SLN) leads to an increase in the linear polarization anisotropy in the (001) plane; the [$\ensuremath{-}$110]-polarization component becomes dominant. These SLN-dependent polarization characteristics result from the valence-band mixing induced by the vertically coupled electronic states. The PL spectrum of the stacked QDs shows clear blueshifts with an increase in the excitation power because of the band filling. In addition, the radiative recombination lifetime has been found to obey the ${T}^{1/2}$ dependence, which directly confirms the one-dimensional translational motion of excitons in the closely stacked QDs.

Magnetoelectric effect in functionalized few-layer graphene

We show that the spin moment induced by $s{p}^{3}$-type defects created by different covalent functionalizations on a few-layer graphene structure can be controlled by an external electric field. Based on ab initio density functional calculations, including van der Waals interactions, we find that this effect has a dependence on the number of stacked layers and concentration of point defects, but the interplay of both with the electric field drives the system to a half-metallic state. The calculated magnetoelectric coefficient $\ensuremath{\alpha}$ has a value comparable to those found for ferromagnetic thin films (e.g., Fe, Co, Ni) and magnetoelectric surfaces (e.g., CrO${}_{2}$). The value of $\ensuremath{\alpha}$ also agrees with the universal value predicted for ferromagnetic half-metals and also points to a novel route to induce half-metallicity in graphene using surface decoration.

Transient absorption microscopy studies of energy relaxation in graphene oxide thin film

Spatial mapping of energy relaxation in graphene oxide (GO) thin films has been imaged using transient absorption microscopy (TAM). Correlated AFM images allow us to accurately determine the thickness of the GO films. In contrast to previous studies, correlated TAM–AFM allows determination of the effect of interactions of GO with the substrate and between stacked GO layers on the relaxation dynamics. Our results show that energy relaxation in GO flakes has little dependence on the substrate, number of stacked layers, and excitation intensity. This is in direct contrast to pristine graphene, where these factors have great consequences in energy relaxation. This suggests intrinsic factors rather than extrinsic ones dominate the excited state dynamics of GO films.

Efficient one pot synthesis of mesoporous NiMo–Al2O3 catalysts for dibenzothiophene hydrodesulfurization

With the aim of disposing hazardous sulphur-containing chemicals, an experimental investigation on the hydrodesulfurization (HDS) of the model compound dibenzothiophene (DBT) is presented. Mesoporous NiMo–Al2O3 catalysts for HDS were prepared by efficient and simple one-pot synthesis method using aluminum chloride as start material, urea as precipitant and starch as template. The catalysts were characterized by means of N2 physisorption, XRD, SEM, TPR, XPS and HRTEM. The characterization results show that, compared with the NiMo/Al2O3 catalysts prepared by impregnation method, the as-prepared NiMo–Al2O3 catalysts exhibit smaller nano-sized spherical particles of alumina, higher dispersions of Ni and Mo species, shorter length and lower stacking layer number of MoS2 slabs. Thus, NiMo–Al2O3 catalysts possess higher catalytic performance in the HDS of DBT than those of impregnated catalysts. Therefore, one pot synthesis is a promising method for the development of hydrotreating catalysts.

Selective UV Reflecting Mirrors Based on Nanoparticle Multilayers

AbstractA new type of nanostructured selective ultraviolet (UV) reflecting mirror is presented. Periodic porous multilayers with photonic crystal properties are built by spin‐coating‐assisted layer‐by‐layer deposition of colloidal suspensions of nanoparticles of ZrO2 and SiO2 (electronic band gap at λ &lt; 220 nm). These optical filters are designed to block well‐defined wavelength ranges of the UVA, UVB, and UVC regions of the electromagnetic spectrum while preserving transparency in the visible. The shielding against those spectral regions arises exclusively from optical interference phenomena and depends only on the number of stacked layers and the refractive index contrast between them. In addition, it is shown that the accessible pore network of the as‐deposited multilayer allows preparing thin, flexible, self‐standing, transferable, and adaptable selective UV filters by polymer infiltration, without significantly losing reflectance intensity, i.e., preserving the dielectric contrast. These films offer a degree of protection comparable to that of traditional ones, without any foreseeable unwanted secondary effects, such as photodegradation, increase of local temperature or, as is the case for organic absorbers, generation of free radicals, all of which are caused by light absorption.

F223 高耐圧マイクロチャネル積層型熱交換器のヒートポンプシステムへの適応可能性評価(OS-6:次世代ヒートポンプ・冷凍システム(3))

It has been required to develop small and high efficient heat exchangers in order to manufacture high efficiency in small energy equipments such as heat pump system using CO_2 refrigerant. Thus, stacked high pressure resistance micro-channel heat exchangers have been developed. The purpose of this study is to reveal heat transfer properties of the stacked high pressure resistance micro-channel heat exchangers when water and steam are respectively supplied test fluids entrance of low and high temperature fluid with. In conclusion, it is found that the longer length of a straight section is, the smaller size of each channel and the more number of stacked layers are, the higher heat transfer rate is.

Probing structures of soot formed in premixed flames of methane, ethylene and benzene

HR-TEM analysis was used in conjunction with elemental analysis and UV–Visible and FT-IR spectroscopy to infer the structural features of soot formed along the axis of premixed flames of methane, ethylene and benzene flames burning at different temperatures (1600–1800K) and rather similar sooting conditions. The order degree and the main structural parameters of soot particles (layer length, number of stacked layers, interspatial distance, etc.) were investigated by HR-TEM analysis. The aromatic and aliphatic character of soot was instead inquired mainly by FT-IR and UV–Visible spectroscopic analysis.HR-TEM image analysis showed small differences in the soot structure indicating a slight increase of order extent for effect of the higher flame temperature, longer residence time and passing from methane to benzene. The dehydrogenation and graphitization processes occurring during soot formation were successfully followed by measuring the H/C ratio of soot and considering peculiar spectroscopic parameters as the intensity ratio of the isolated/adjacent aromatic hydrogens, the specific absorption coefficient and the optical band gap.The very low and quasi-invariant hydrogen content and optical band gap of benzene soot were found in contrast with the strong variations of the absorption mass coefficient. On the opposite, methane soot showed very high H/C atomic ratios and optical band gaps, both largely decreasing during soot formation, in contradiction with the small variation of the absorption coefficient. Generally, by relating the H/C ratio to the specific absorption, it was found that the specific absorption slightly increases for H/C variations from 0.3 down to 0.1, whereas the strong increase of the absorption coefficient occurred below the H/C value of 0.1.Some insights on the soot structure and chemical environment effect on soot inception, dehydrogenation and graphitization processes were obtained on the basis of gathered structural data.

Enhanced transmission with a graphene-dielectric microstructure at low-terahertz frequencies

Here, we report on the transmissivity of electromagnetic waves through a stack of monolayer graphene sheets separated by dielectric slabs at low-terahertz frequencies. It is observed that the multilayer structure possesses band-gap properties and supports a series of bandpass and band-stop regions, similar to the cases of stacked metallic meshes separated by dielectric slabs at microwave/THz frequencies and a metal-dielectric stack at optical frequencies. The transmission resonances in the bandpass region are identified as coupled Fabry-P\'erot resonances associated with the individual cavities of dielectric slabs loaded with graphene sheets. It is also noticed that these resonances lie within a certain characteristic frequency band, independent of the number of layers in the graphene-dielectric stack. The study is carried out using a simple analytical transfer-matrix approach or, equivalently, a circuit-theory model, resulting in the exact solution for the multiple dielectric/graphene sheet surface-conductivity model. Also, an independent verification of the observed phenomena is obtained with commercial numerical simulations.

Adsorption and stacking behaviour of zwitterionic porphyrin on the clay surface

AbstractZwitterionic porphyrin (tetrakis {4-(2-carboxyethyl)pyridinio}porphyrin (TPyCP)) with four pyridinium and carboxyl groups in the molecule was synthesized. The adsorption behaviour of TPyCP on the montmorillonite clay surface was examined in an aqueous colloidal solution. While the adsorption maximum λmax of TPyCP was 424 nm in water, it shifted to longer wavelengths on complex formation with clay. The λmax on the exfoliated clay surface was 450 nm, and that in the stacked clay sheets was 471 nm, respectively. Under alkaline conditions ([NaOH] = 2 × 10–4 M), the stacking behaviour of the clay was completely suppressed. It emerged that anionic parts in the porphyrin molecule can suppress the stacking of clay sheets. Judging from the quantitative analysis, the maximum adsorption amount of TPyCP was 100% vs. cation exchange capacity (CEC) of the clay. As the adsorption density of TPyCP increased, λmax shifted slightly to longer wavelengths due to the interactions between adjacent porphyrins. When the loading level of TPyCP was 200% vs. CEC, the stacking of TPyCP was indicated. The average stacking layer number was calculated to be 1.48. On the other hand, it is known that tetra cationic porphyrins without anionic parts do not form such stacking structures. Thus, it seems that zwitterionic porphyrin has the potential to form a three dimensional structure through electrostatic interactions between porphyrins on the clay surface.

Van der Waals Epitaxy of InAs Nanowires Vertically Aligned on Single-Layer Graphene

Semiconductor nanowire arrays integrated vertically on graphene films offer significant advantages for many sophisticated device applications. We report on van der Waals (VDW) epitaxy of InAs nanowires vertically aligned on graphene substrates using metal-organic chemical vapor deposition. The strong correlation between the growth direction of InAs nanowires and surface roughness of graphene substrates was investigated using various graphene films with different numbers of stacked layers. Notably, vertically well-aligned InAs nanowire arrays were obtained easily on single-layer graphene substrates with sufficiently strong VDW attraction. This study presents a considerable advance toward the VDW heteroepitaxy of inorganic nanostructures on chemical vapor-deposited large-area graphenes. More importantly, this work demonstrates the thinnest epitaxial substrate material that yields vertical nanowire arrays by the VDW epitaxy method.

Ultra-high stacks of InGaAs/GaAs quantum dots for high efficiency solar cells

We report ultra-high stacked InGaAs/GaAs quantum dot (QD) solar cells fabricated by the intermittent deposition of In0.4Ga0.6As under an As2 source using molecular beam epitaxy. We obtain a 400-stack In0.4Ga0.6As QD structure without using a strain balancing technique, in which the total number of QDs reaches 2 × 1013 cm−2. Photoluminescence and cross-sectional scanning transmission electron microscope measurements indicate that the In0.4Ga0.6As QD structure exhibits no degradation in crystal quality, no dislocations and no crystal defects even after the stacking of 400 QD layers. The external quantum efficiency and the short-circuit current density of multi-stacked In0.4Ga0.6As QD solar cells increase as the number of stacked layers is increased to 150. Such ultra-high stacks and good cell performance have not been reported for QD solar cells using other material systems. The performance of the ultra-high stacked QD solar cells indicates that InGaAs QDs are suitable for use in high efficiency solar cells requiring thick QD layers for sufficient light absorption.

Interactions of nanoparticles with purple membrane films

Lamellar structures self-assembled from purple membranes (PM) of Halobacterium salinarum are promising building units for bio-electronic devices, due to proton pumping ability of the PM. The functionality and durability of such devices are hinged on the structural integrity of PM lamellae. Using X-ray diffraction, we examined the structure of PM multilayers on silicon when challenged with two types of nanoparticles (NPs): carboxymethyl-dextran coated magnetite (2.4 nm core size) and citrate-stabilised gold (5 nm core size). We tried to infiltrate the PM multilayers with the NPs using two alternative routes: facile penetration (FP) and co-assembly (CS) by solution mixing. We found that under all conditions the NPs did not disrupt the overall lamellar structure of the PM films or enter the inter-lamellar space, although the presence of NPs affected the self-assembly process of the PM films. This caused an increase in the disorder in the film structure, as assessed by the decreasing number of layers in the multilayer stack as the NP concentration increased. Despite this, UV-Vis spectroscopic measurements showed that the conformation of the retinal residue within the protein was intact so the proton pumping functionality of PM multilayers would be retained in all samples with added NPs. Our results show that the effects of NPs on the PM structure and functionality are subtle and complex, and we will discuss the structural integrity of lipid-protein composite PM films against NP infiltration in terms of their high bending modulus as compared with that of fluid lipid bilayers.

A peptide of 17 aminoacids from the N-terminal region of maize plastidial transglutaminase is essential for chloroplast targeting

Transglutaminases (TGases, EC 2.3.2.13) catalyse posttranslational modification of proteins by establishing ε-(γ-glutamyl) links and covalent conjugation of polyamines. In plants, the functionality of these enzymes is scarcely known. The maize transglutaminase gene (tgz), the only cloned plant TGase, produces major alterations in thylakoid membrane architecture when the transglutaminase (chlTGZ) protein was over-expressed in tobacco chloroplasts, significantly increasing the number of grana stacked layers. Here we demonstrate that nuclear transformation of rice plants starting from a tgz gene truncated in 17 N-terminal aas (tgzt) non altered chloroplast thylakoid structures. F3 transformed plants were analysed for TGase activity, chlTGZ presence and tgzt transcription levels. Transformed plants exhibited double the in vitro TGase activity of the non-transformed plants. Immunoblot and quantitative RT-PCR analysis results of tgzt-rice plants grown under different illumination periods revealed that chlTGZ maintains its differential expression depending on the light regime. Nevertheless, the maize protein was localised by confocal microscopy in the cell wall of transformed rice cells. TEM analyses of the transformed cells showed normal, non-altered chloroplast thylakoid structures with the maize protein preferentially located in the cell walls. The results confirmed that the tgz eliminated sequence is essential for chloroplast targeting, being its absence sufficient to the lack of protein expression in its original plastidal compartment. Interestingly, the immunolocalization of a putative endogenous rice TGase protein is also showed. These data give further information on plant TGase functionality and its relationship to photosynthetic membranes.

Multilayer Graphene-Based Carbon Interconnect

ABSTRACT3D stacked (or uncorrelated) multilayer graphene (s-MLG) is investigated as a potential material platform for carbon-based on-chip interconnects. S-MLG samples are prepared by repeatedly transferring and stacking the large-area CVD-grown graphene monolayers, followed by wire patterning and oxygen plasma etching of graphene. We observed superior wire conduction of s-MLG over that of monolayer graphene or ABAB-stacked multilayer graphene. Further reduction of s-MLG resistivity is anticipated with increasing number of stacked layers. Electrical stress-induced doping technique is used to engineer the Dirac point, as well as to reduce graphene-to-metal contact resistance, improving the overall performance metrics of the s-MLG system. Breakdown experiments show that the current-carrying capacity of s-MLG is significantly enhanced as compared with that of monolayer graphene.

Experimental and atomistic theoretical study of degree of polarization from multilayer InAs/GaAs quantum dot stacks

Recent experimental measurements, without any theoretical guidance, showed that isotropic polarization response can be achieved by increasing the number of quantum-dot (QD) layers in a QD stack. Here we analyze the polarization response of multilayer QD stacks containing up to nine QD layers by linearly polarized photoluminescence (PL) measurements and by carrying out a systematic set of multimillion atom simulations. The atomistic modeling and simulations allow us to include correct symmetry properties in the calculations of the optical spectra, a factor critical to explain the experimental evidence. The values of the degree of polarization (DOP) calculated from our model follows the trends of the experimental data. We also present detailed physical insight by examining strain profiles, band edges diagrams, and wave function plots. Multidirectional PL measurements and calculations of the DOP reveal a unique property of InAs QD stacks that the TE response is anisotropic in the plane of the stacks. Therefore, a single value of the DOP is not sufficient to fully characterize the polarization response. We explain this anisotropy of the TE modes by orientation of hole-wave functions along the [$\overline{1}10$] direction. Our results provide a new insight that isotropic polarization response measured in the experimental PL spectra is due to two factors: (i) TM${}_{001}$-mode contributions increase due to enhanced intermixing of HH and LH bands, and (ii) TE${}_{110}$-mode contributions reduce significantly due to hole-wave function alignment along the [$\overline{1}10$] direction. We also present optical spectra for various geometry configurations of QD stacks to provide a guide to experimentalists for the design of multilayer QD stacks for optical devices. Our results predict that the QD stacks with identical layers will exhibit lower values of the DOP than the stacks with nonidentical layers.