Number Of Stacked Layers Research Articles

Convective assembly of linear gold nanoparticle arrays at the micron scale for surface enhanced Raman scattering

A convective assembly technique at the micron scale analogous to the writing action of a “pipette pen” has been developed for the linear assembly of gold nanoparticle strips with micron scale width and millimeter scale length for surface enhanced Raman scattering (SERS). The arrays with interparticle gaps smaller than 3 nm are hexagonally stacked in the vicinity of the pipette tip. Variable numbers of stacked layers and clean surfaces of the assembled nanoparticles are obtained by optimizing the velocity of the pipette tip. The SERS properties of the assembled nanoparticle arrays rely on their stacking number and surface cleanliness. Open image in new window

Electronic Structures and Charge Transport of Stacked Annelated β-Trithiophenes

Density functional theory (DFT) calculations were performed using a MPWB1K functional to interpret the charge transport (CT) properties of stacked annelated β-trithiophene molecules. The goal was to help understand how the chemical composition, face-to-face stacking, intra- and intermolecular correlations, and the applied electric field affect the CT properties of this organic semiconductor compared with those of the edge-to-face "herringbone" motif. The variations in the frontier molecular orbitals, energy gap, nonadiabatic electron attachment energies (EAE), and vertical detachment energies (VDE) were investigated under an external electric field as a function of the number of stacked layers (n). Two possible CT pathways, monomer-to-monomer (MM) and dimer-to-dimer (DD) transports, were postulated to determine the charge carrier conductivities of these molecules. The results highlight that the intermolecular electronic couplings and electrostatic interactions can significantly affect the stacking geometry, even in more extended structures; displaced stacks with smaller interlayer spacing resulted in more compact stacking and, thus, higher CT efficiency; face-to-face stacking geometries can help to reduce the energy gap and VDE. Despite the fact that common thiophene-based oligomers adopting edge-to-face herringbone motifs exhibited p-type (hole-transporting) characters, the face-to-face stacked models based on annelated β-trithiophenes exhibited remarkably increased n-type (electron-transporting) performances. The electric field in the z-direction produced a small influence on the DD charge transport, whereas both electron and hole mobilities decreased dramatically in the MM case. More importantly, in MM charge transport, the electric field increases the hole mobility so that it is higher than that of the electron.

Optical bistability in a two-section InAs quantum-dot laser

Room temperature, continuous-wave bistable operation is achieved in two-section 1.24 μm InAs quantum-dot (QD) lasers with integrated intracavity QD saturable absorbers (SA). It is found that the hysteresis width is narrowed with increasing reverse bias voltage, and broadened with increasing length of saturable absorber. This can be explained by the competition between QD absorption and electroabsorption in the SA section. In addition, a larger hysteresis width is realized than other reports so far, which can be attributed to a greater number of stacked layers of active region in our case. The experimental results can be explained by a modified threshold current model.

Fabrication of ultra‐high‐density InAs quantum dots using the strain‐compensation technique

AbstractHighly‐stacked InAs quantum dots (QDs) were successfully grown on InP(311)B substrates using a novel strain‐compensation technique. The number of stacked layer was increased to 300, the density of the QDs reaches 2 × 1013/cm2; this value cannot be obtained using conventional QD fabrication techniques. In a highly stacked sample, the QDs show good size uniformity with a lateral and vertical ordered structure. In addition, this sample exhibits strong 1.55 µm photoluminescence (PL) emission at room temperature.

Vertical stacking of InAs quantum dots for polarization-insensitive semiconductor optical amplifiers

In this paper, we report a control of the polarization property in quantum dot semiconductor optical amplifiers (QD-SOAs) using vertically-stacked, electronically-coupled InAs/GaAs QDs grown by molecular beam epitaxy. By optimizing the number of stacked layers and intermediate GaAs thickness, the 9-stacked QDs demonstrated the polarization-insensitive operation within 1.2 dB in the 1.3-mm optical communication band. Our results demonstrate that the electronically-coupled QDs are useful to realize the polarization-insensitive QD-SOAs.

Strain relaxation effect on electronic properties of compressively strained InGaAs/InP vertically stacked multiple quantum wires

Electronic properties of compressively strained InGaAs/InP vertically stacked multiple quantum wires were investigated using an six-band strain-dependent k⋅p Hamiltonian. The strain tensor ϵyy (ϵxx) is found to relax from its initial strain. The amount of relaxation is dependent on the number of wire layers in the vertical stack and increases with the number of wire layers. The interband transition energy also decreases with the number of wire layers. This is mainly attributed to the decrease in the conduction band energy because subband energies in the valence band are nearly independent of the strain. The matrix element is shown to slightly decrease with increasing number of the wire layer in the vertical stack.

Magnetic patterns in dipolar films with close packed structures

It is described and explained the dynamics of the transition from in-plane to perpendicular magnetization for dipolar films with close packed structures when the c/ a ratio, with c the interlayer separation and a the two-dimensional lattice constant, is varied. Several arrangements of distinct number of stacked layers are considered. Magnetization switching follows a mechanism based on the role of the interlayer coupling, which is understood as the contribution of the interaction between dipoles of different layers to the studied effects. Calculations for sufficiently low temperatures show that each magnetic configuration transition leaves a fingerprint as an entropy maximum when the latter is plotted as a function of c/ a. Thus the entropy allows to determine the c/ a range of stability of the different magnetic patterns.

Temperature-dependent energy gap variation in InAs/GaAs quantum dots

In this letter, we report a photoluminescence (PL) study of the temperature-dependent energy gap variation in InAs/GaAs quantum dots (QD). Energy gaps E(T) of different InAs/GaAs QD samples with various numbers of QD stacking layers were measured from the ground state PL emissions at various sample temperatures. For each of the QD samples, linear dependences between [E(T)−E0](β+T) and T (where E0=0.42 eV and β=−550 K) is obtained in low and high temperature regions. The transition temperatures between the two temperature regions are found to be related to the numbers of QD stacking layers. A linear relation between the number of the QDs and the phonon densities at the corresponding transition temperatures is obtained.

Stacking of polycyclic aromatic hydrocarbons as prototype for graphene multilayers, studied using density functional theory augmented with a dispersion term

The interlayer pi-pi interaction between finite-size models of graphene sheets was investigated by using a density functional theory method, augmented with an empirical R(-6) term for the description of long-range dispersive interaction; these were calibrated by studying the pi-pi interaction between various benzene dimer configurations and comparing the results with previous calculations. For stacked bilayers (dimers) and multilayers of polyaromatic hydrocarbons, which serve as molecular models of graphene sheets, we found that binding energies and energy gaps are strongly dependent on their sizes, while the stacking order and the number of stacked layers have a minor influence. The remarkably broad variation of the energy gap, ranging from 1.0 to 2.5 eV, due mainly to variation of the model size, suggests the potential of broadband luminescence in the visible range for carbon-based nanomaterials that have pi-pi interacting.

Multi-layer stackable polymer memory devices

Multi-layer stackable polymer memory architecture is an interesting new direction for polymer memory. The memory density can be increased by increasing the number of stacked layers without reducing the minimum feature size. To achieve multi-level stacking, the polymer used must be able to be cross-linked so that it will not be dissolved upon deposition of additional layers. This requirement also makes the polymer robust enough to withstand conventional lithographic processes. In this paper, the various approaches to achieve cross-linkable polymer memory are discussed. Device fabrication and performance are also reported.

Towards stacked zone plates

Fresnel zone plates are the key optical elements for soft and hard x-ray microscopy. For short exposure times and minimum radiation load of the specimen the diffraction efficiency of the zone plate objectives has to be maximized. As the efficiency strongly depends on the height of the diffracting zone structures the achievable aspect ratio of the nanostructures determines these limits. To reach aspect ratios ≥ 20:1 for high efficient optics we propose to superimpose zone plates on top of each other. With this multiplication approach the final aspect ratio is only limited by the number of stacked zone plate layers. For the stack process several nanostructuring process steps have to be developed and/or improved. Our results show for the first time two layers of zone plates stacked on top of each other.

Remodeling of tobacco thylakoids by over-expression of maize plastidial transglutaminase

Transglutaminases (TGases, EC 2.3.2.13) are intra- and extra-cellular enzymes that catalyze post-translational modification of proteins by establishing ɛ-(γ-glutamyl) links and covalent conjugation of polyamines. In chloroplast it is well established that TGases specifically polyaminylate the light-harvesting antenna of Photosystem (PS) II (LHCII, CP29, CP26, CP24) and therefore a role in photosynthesis has been hypothesised (Della Mea et al. [23] and refs therein). However, the role of TGases in chloroplast is not yet fully understood. Here we report the effect of the over-expression of maize ( Zea mays) chloroplast TGase in tobacco ( Nicotiana tabacum var. Petit Havana) chloroplasts. The transglutaminase activity in over-expressers was increased 4 times in comparison to the wild-type tobacco plants, which in turn increased the thylakoid associated polyamines about 90%. Functional comparison between Wt tobacco and tgz over-expressers is shown in terms of fast fluorescence induction kinetics, non-photochemical quenching of the singlet excited state of chlorophyll a and antenna heterogeneity of PSII. Both in vivo probing and electron microscopy studies verified thylakoid remodeling. PSII antenna heterogeneity in vivo changes in the over-expressers to a great extent, with an increase of the centers located in grana-appressed regions (PSIIα) at the expense of centers located mainly in stroma thylakoids (PSIIβ). A major increase in the granum size (i.e. increase of the number of stacked layers) with a concomitant decrease of stroma thylakoids is reported for the TGase over-expressers.

Tunable Effect of Double-Connective Dendritic Left-Handed Metamaterials Based on Electrorheological Fluids

We present double-connective dendritic unit pairs which exhibit the left-handed property for electromagnetic wave normal incidence. Based on the tunable characteristics of electrorheological fluids (ERF) as the electric field, we experimentally study the influence of the distance of electrodes, the number of stacked layers, and the intensity of the external electric field upon the ERF to the left-handed transmission peak of the double-connective dendritic structure. The results show that the transmission could be enhanced with the increase of distance or the number of layers. Furthermore, by changing the intensity of the electric fields, the left-handed transmission peak can be modulated actively, and the maximum shift is up to 160 MHz.

Bit Cost Scalable (BiCS) Flash Technology for Future Ultra High Density Storage Devices

We've proposed Bit Cost Scalable (BiCS) technology which realizes a 3D multi-stacked memory array with a few critical lithography steps regardless of number of stacked layers to keep a drastically continuous reduction of bit cost. The cell array concept, fabrication process, and key features are presented.

Statistical characteristics of out‐of‐plane permeability for plain‐woven structure

AbstractSince out‐of‐plane permeability of fiber preforms is a function of the number and arrangement of stacked layers, either many layers of preforms or numerous experiments are required to obtain an exact out‐of‐plane permeability experimentally. The reason is that there exist nesting and phase shifting when the preforms are laid up. From a statistical viewpoint, the effect of the number of preform layers on the out‐of‐plane permeability was analyzed by adopting an analytical model proposed in this study. Numerical simulation for a unit‐cell constructed based on geometry of the preform was carried out to validate the analytical model as well as experimental measurements of the permeability. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers

Optical polarization in columnar InAs/GaAs quantum dots: 8-bandk⋅pcalculations

We have theoretically studied the optical polarization in columnar InAs/GaAs quantum dots (QDs), in which the self-assembled QDs are vertically stacked with no interdot spacing. The model structure of the columnar QDs consists of truncated-cone-shaped InAs QDs with the stacking-layer numbers (SLNs) of 1, 3, 5, 7, and 9. We used the valence-force-field model to calculate the strain distribution. We find that the biaxial strain in the middle layers of the columnar QDs decreases with increasing SLN and becomes negative for $\text{SLN}=9$. This is due to the condition that the vertical lattice constant of InAs in these layers has to match that of the side GaAs. By using the strain-dependent 8-band $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ theory for the electronic states, we calculated the transverse-electric (TE)- and transverse-magnetic (TM)-mode intensities for the electron-hole transitions. The piezoelectric effect is included in the calculations. For $\text{SLN}=1$ and 3, only the TE-mode transition occurs. With increasing SLN beyond 3, the TM-mode intensity increases while the TE-mode one decreases. Consequently, when SLN changes from 7 to 9, the dominant polarization character changes from the TE mode to the TM mode. This dominant polarization change is attributed to the increase of the light-hole character in the wave function of the ground hole state, which is the consequence of the negative biaxial strain in the middle layers for $\text{SLN}=9$. The change in the optical polarization calculated in this study is in good agreement with the photoluminescence experiment reported by Kita et al. [Jpn. J. Appl. Phys., Part 2 41, L1143 (2002)].

Ultrathick and High-Aspect-Ratio Nickel Microgyroscope Using EFAB Multilayer Additive Electroforming

This paper presents a new approach for the development of a microgyroscope that has a 240-mum-thick multilayer electroformed-nickel structural mass and a lateral aspect ratio greater than 100. The gyroscope is fabricated using commercial multilayer additive electroforming process EFAB of Microfabrica, Inc., which allows defining the thickness of different structural regions, such as suspensions, proof mass, and capacitive electrodes, unlike many classical surface-micromachining technologies that require a uniform thickness for the structural features. The capacitive gaps of the gyroscope are designed to be laying parallel to the substrate plane and can be set as small as 4 mum, and therefore, the lateral aspect ratio, which is defined as the ratio of the overlap length of capacitor plates to the gap spacing, can easily exceed 100. The high capacitive aspect ratio, multilayer capacitors, and thick proof mass altogether yield a highly sensitive gyroscope for in-plane angular-rate measurements. Characterization of the fabricated gyroscope using a very simple capacitive interface circuit constructed from standard IC components yields a measured mechanical sensitivity of 65 muV/(deg/s) and a noise equivalent rate of 0.086deg/s at atmospheric pressure in a bandwidth of 1 Hz. The response bandwidth is limited to 100 Hz when the resonance frequencies of the drive and sense modes are matched by electrostatic tuning. The measured stability of the drive-mode resonance frequency is better than 0.1% within a 40-h period, demonstrating reliability of electroformed nickel of EFAB process. In addition, the mechanical quality factor of the sense mode reaches 2000 at 20-mtorr vacuum, verifying the quality of structural nickel. The resonance-frequency variations of the drive and sense modes with respect to increasing temperature are characterized to be smaller than 1 and -2 Hz/degC, respectively, in the -40-degC to +85-degC measurement range. The gyroscope performance can be further increased by reducing the minimum capacitive gaps, by increasing the number of stacked layers, by using a nickel-alloy structural layer with better mechanical properties, and by using a dedicated CMOS-application-specified integrated circuit which are under consideration.

Nonradiative Recombination in Multiple Layer In(Ga)As Quantum-Dot Lasers

The segmented contact technique has been used to study the effects on nonradiative recombination of stacking multiple quantum-dot layers. As the number of stacked layers is increased a shift in the balance of dots in the bimodal dot distribution is observed due to a reduction in the number in the smaller dot size subset. This is attributed to an increase in the density of defect islands, as the number of layers is increased, that preferentially take material from the smaller dots, and lead to an increased level of nonradiative recombination per layer at low injection level. A second nonradiative process is apparent at higher injection level, which is related to the population of the small dot size subset. Spontaneous radiative efficiency was improved in a five-layer sample where the large to small dot size energy separation and relative density of the large dot size subset were increased.

Ab-initio calculation on Co substitution into NiSi

Vertical interconnect technology called 3D stacking has been a major focus of the next generation of IC industries. 3D stacked devices in the vertical dimension give several important advantages over conventional two-dimensional scaling. The most eminent advantage is its performance improvement. Vertical device stacking enhances a performance such as inter-die bandwidth improvements, RC delay mitigation and geometrical routing and placement advantages. At present memory stacking options are of great interest to many industries and research institutes. However, these options are more focused on a form factor reduction rather than the high performance improvements. In order to improve a stacked device performance significantly vertical interconnect technology with wafer level stacking needs to be much more progressed with reduction in inter-wafer pitch and increases in the number of stacked layers. Even though 3D wafer level stacking technology offers many opportunities both in the short term and long term, the full performance benefits of 3D wafer level stacking require technological developments beyond simply the wafer stacking technology itself.

Modeling and simulation for the enhancement of electron storage in a stacked multilayer nanocrystallite silicon floating gate memory

In this article, we investigate the storage enhancement mechanism of stacked multilayer nanocrystallite silicon (nc-Si) structures in a master-equation-based equivalent circuit model. As a theoretical extension from our previous experimental works, we reveal the detail injection sequence of electrons into the multilayer nc-Si structure via a direct tunneling process, and how the retention property is enhanced by the stacked structures. Seeking a further improvement in the multilayer nc-Si-based nonvolatile memory structure, we compare two major approaches for that purpose, i.e. (1) by further increasing the number of stacked layers or (2) by adopting an asymmetric double-layer structure. It is shown that the latter is more promising for achieving better nonvolatile storage property and shows a more effective threshold shifting, while retaining the virtues of direct tunneling process like fast write/erase and low operation voltage. We suggest that these results provide important guides for practical design of memory devices based on multilayer nc-Si floating gate structures.