Showing papers in "Physica Status Solidi B-basic Solid State Physics in 2010"
TL;DR: In this paper, a didactical review of optical properties of ZnO has been presented, focusing mainly on optical properties but presenting shortly also a few aspects of other fields like transport or magnetic properties.
Abstract: The research on ZnO has a long history but experiences an extremely vivid revival during the last 10 years. We critically discuss in this didactical review old and new results concentrating on optical properties but presenting shortly also a few aspects of other fields like transport or magnetic properties. We start generally with the properties of bulk samples, proceed then to epitaxial layers and nanorods, which have in many respects properties identical to bulk samples and end in several cases with data on quantum wells or nano crystallites. Since it is a didactical review, we present explicitly misconceptions found frequently in submitted or published papers, with the aim to help young scientists entering this field to improve the quality of their submitted manuscripts. We finish with an appendix on quasi two- and one-dimensional exciton cavity polaritons.
348 citations
TL;DR: Using friction force microscopy, Wang et al. as discussed by the authors have investigated the frictional behavior of graphene deposited on various substrates as well as over micro-fabricated wells.
Abstract: Using friction force microscopy, we have investigated the frictional behavior of graphene deposited on various substrates as well as over micro-fabricated wells. Both graphene on SiO 2 / Si substrates and graphene freely suspended over the wells showed a trend of increasing friction with decreasing number of atomic layers of graphene. However, this trend with thickness was absent for graphene deposited on mica, where the graphene is strongly bonded to the substrate. Measurements together with a mechanics model suggest that mechanical confinement to the substrate plays an important role in the frictional behavior of these atomically thin graphite sheets. Loosely bound or suspended graphene sheets can pucker in the out-of-plane direction due to tip-graphene adhesion. This increases contact area, and also allows further defonnation of the graphene when sliding, leading to higher friction. Since thinner samples have lower bending stiffness, the puckering effect and frictional resistance are greater. However, if the graphene is strongly bound to the substrate, the puckering effect will be suppressed and no thickness dependence should be observed. The results can provide potentially useful guidelines in the rational design and use of graphene for nano-mechanical applications, including nano-lubricants and components in micro- and nanodevices.
215 citations
TL;DR: In this paper, the authors exfoliate graphite in both aqueous and non-aqueous environments through mild sonication followed by centrifugation and enrich the dispersions with monolayers.
Abstract: We exfoliate graphite in both aqueous and non-aqueous environments through mild sonication followed by centrifugation. The dispersions are enriched with monolayers. We mix them with polymers, followed by slow evaporation to produce optical quality composites. Nonlinear optical measurements show similar to 5% saturable absorption. The composites are then integrated into fiber laser cavities to generate 630 fs pulses at 1.56 mu m. This shows the viability of solution phase processing for graphene based photonic devices. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
201 citations
TL;DR: In this paper, the authors analyse the evolution of the disorder induced D-band (∼1350 cm−1) and the first-order allowed G-band in the Raman spectra of ion bombarded graphene.
Abstract: Here we analyse the evolution of the disorder induced D-band (∼1350 cm−1) and of the first-order allowed G-band (∼1584 cm−1) in the Raman spectra of ion bombarded graphene. By increasing the bombardment time, we increase the disorder and, consequently, decrease the average distance (LD) between defects. We describe how the intensity, full width at half maximum (FWHM) and integrated area vary for the D and G bands as a function of LD. Finally, we compare the evolution of the intensity ratio ID/IG and of the integrated area ratio AD/AG between the D and G bands as a method for quantifying disorder in graphene. For practical use and inter-laboratorial comparison, the authors advise using the intensity ratio for a more suitable measure for analysing defect density.
181 citations
TL;DR: In this article, the authors provide an overview of the latest results on the study of luminescence, absorption and scintillation properties of materials selected for rare event searches so far, and discuss the advantages and limitations of different materials with emphasis on their application as cryogenic phonon-scintillation detectors (CPSD).
Abstract: An increasing number of applications of scintillators at low temperatures, particularly in cryogenic experiments searching for rare events, has motivated the investigation of scintillation properties of materials over a wide temperature range. This paper provides an overview of the latest results on the study of luminescence, absorption and scintillation properties of materials selected for rare event searches so far. These include CaWO 4 , ZnWO 4 , CdWO 4 , MgWO 4 , CaMo0 4 , CdMoO 4 , Bi 4 Ge 3 O 12 , CaF 2 , MgF 2 , ZnSe and Al 2 O 3 -Ti. We discuss the progress achieved in research and development of these scintillators, both in material preparation and in the understanding of scintillation mechanisms, as well as the underlying physics. To understand the origin of the performance limitation of self-activated scintillators we employed a semi-empirical model of conversion of high energy radiation into light and made appropriate provision for effects of temperature and energy transfer. We conclude that the low-temperature value of the light yield of some modern scintillators, namely CaWO 4 , CdW0 4 and Bi 4 Ge 3 O 12 , is close to the theoretical limit. Finally, we discuss the advantages and limitations of different materials with emphasis on their application as cryogenic phonon-scintillation detectors (CPSD) in rare event search experiments.
153 citations
TL;DR: In this article, the structure and microstructure of magnetically grown elemental (Fe, Ni, Co) and alloy (Co-Pt) magnetic nanowires are compared.
Abstract: Magnetic nanowire arrays allow studying magnetism at the nanoscale and have broad application areas. Here we review our recent experiments on tailoring the structure and microstructure of electrochemically grown elemental (Fe, Ni, Co) and alloy (Co-Pt) magnetic nanowires. The comparison of these different materials allows identifying the role of shape, magnetocrystal-line and magnetoelastic anisotropies as well as magnetostatic interactions.
139 citations
TL;DR: In this article, a self-designed 3D-auxetic structure is constructed using selective electron-beam melting (SEBM), which is a rapid prototyping/manufacturing technique allowing for the direct translation of CAD models to real world objects.
Abstract: This paper is concerned with the build up and characterization of well-defined auxetic structures (negative Poisson ratio) from Ti-6A1-4V through selective electron-beam melting (SEBM). SEBM is a rapid prototyping/manufacturing technique allowing for the direct translation of CAD models to real world objects. Using SEBM we are able to produce structures of arbitrary geometry in a well-defined manner. Here, we introduce a self-designed 3D-auxetic structure and determine its mechanical properties. We also address the dependence of Young's modulus on relative density.
135 citations
TL;DR: In this article, the CdSe:Cr crystal was pumped at the room temperature (RT) by a continuous wave (CW) 1.908-μm thulium fiber laser, and the output laser power at 2.6 μm was increased up to 1.7 W.
Abstract: Seeded free growth method with physical transport was used for preparation of large-size II—VI single crystals uniformly doped by transition metals directly during the growth. The grown crystals possess small intrinsic losses. Based on these crystals new results on development of mid-IR lasers were achieved. With the CdSe:Cr crystal pumped at the room temperature (RT) by a continuous wave (CW) 1.908-μm thulium fiber laser, output laser power at 2.6 μm was increased up to 1.7 W. CW lasing from the ZnSe:Fe crystal was achieved using the 2.97-μm CW Cr 2+ :CdSe laser as a pump source. In this case the output power as high as 0.2 W was obtained at 4.1 μm and T=80 K. Broad spectral tuning of the Cr 2+ :CdSe (2.26-3.61 μm) and Fe 2+ :ZnSe (3.95-5.05 μm) lasers was achieved at RT in the pulsed operation. The RT laser action from CdS:Cr and CdSe:Fe was demonstrated for the first time. The output power of the CW Cr +2 :CdS laser was 0.81 W. The pulsed Fe 2+ :CdSe laser was tuned in the 4.6-5.9 μm spectral range.
101 citations
TL;DR: In this article, the conductivity of n-type GaN was investigated to demonstrate the feasibility of novel optical and microelectromechanical system devices for photonic applications, where optical index contrast can be achieved by selective etching or nanoporous formation of GaN.
Abstract: Electrochemical etching having large selectivity based on the conductivity of n-type GaN was investigated to demonstrate the feasibility of novel optical and microelectromechanical system devices. The electrochemical etching exhibited two regimes with different etching characteristics, i.e., nanoporous and electropolishing, depending on the doping concentration and applied voltage. For photonic applications, GaN microdisks and distributed Bragg reflectors were fabricated where optical index contrast can be achieved by selective etching or nanoporous formation of GaN. Stimulated emission of GaN microdisk was observed under pulsed optical pumping. In addition, a GaN cantilever was formed and its resonance frequency was measured at ∼ 120 kHz.
94 citations
TL;DR: In this paper, the current state of the field of structure prediction on the ab initio level is reviewed, and it is shown that it is necessary to employ ab- initio energy functions during the global optimization phase of the structure prediction.
Abstract: Predicting which crystalline modifications can exist in a chemical system requires the global exploration of its energy landscape. Due to the large computational effort involved, in the past this search for sufficiently stable minima has been performed employing a variety of empirical potentials and cost functions followed by a local optimization on the ab initio level. However, this might introduce some bias favoring certain types of chemical bonding and entails the risk of overlooking important modifications that are not modeled accurately using empirical potentials. In order to overcome this critical limitation, it is necessary to employ ab initio energy functions during the global optimization phase of the structure prediction. In this paper, we review the current state of the field of structure prediction on the ab initio level.
86 citations
TL;DR: In this article, it was shown that low temperature experiments within the superconducting phase have not shown convincing signatures of such an optimal doping QCP at hole density x = x m, near optimal doping.
Abstract: Transport measurements in the hole-doped cuprates show a "strange metal" normal state with an electrical resistance which varies linearly with temperature. This strange metal phase is often identified with the quantum critical region of a zero temperature quantum critical point (QCP) at hole density x = x m , near optimal doping. A long-standing problem with this picture is that low temperature experiments within the superconducting phase have not shown convincing signatures of such an optimal doping QCP (except in some cuprates with small superconducting critical temperatures). I review theoretical work which proposes a simple resolution of this enigma. The crossovers in the normal state are argued to be controlled by a QCP at x m linked to the onset of spin density wave (SDW) order in a "large" Fermi surface metal, leading to small Fermi pockets for x < x m . A key effect is that the onset of superconductivity at low temperatures disrupts the simplest canonical quantum critical crossover phase diagram. In particular, the competition between superconductivity and SDW order shifts the actual QCP to a lower doping x s < x m in the underdoped regime, so that SDW order is only present for x < x s . I review the phase transitions and crossovers associated with the QCPs at x m and x s : the resulting phase diagram as a function of x, temperature, and applied magnetic field consistently explains a number of recent experiments.
TL;DR: In this article, the plasmonic electromagnetic nearfield enhancement is exploited to increase the sensitivity of infrared (IR) vibration spectroscopy by several orders of magnitude, which cannot be achieved by randomly grown metal island films that on average may show already signal enhancement of the order of 1000.
Abstract: The sensitivity of infrared (IR) vibration spectroscopy can be enhanced by several orders of magnitude if plasmonic electromagnetic nearfield enhancement is exploited. For maximum enhancement the plasmonic resonance needs to be strong, which cannot be achieved by randomly grown metal island films that on average may show already signal enhancement of the order of 1000. Metal nanowires may give sufficiently strong antenna-like plasmonic resonances in the IR that can be adjusted to the molecular vibration frequencies of interest via the wire length. With individual gold nanowires we obtained vibration-signal enhancement up to 500,000 for molecular monolayers adsorbed on gold nanowires. The already obtained enhancement is not a limit. Since antennae coupled via nanogaps should enable strong electromagnetic nearfield enhancement in these gaps, respective arrays would be ideal for surface enhanced IR spectroscopy. First experiments prove the tendency of increasing vibration-signal enhancement with decreasing gap size.
TL;DR: In this paper, the authors give an overview on growth principles and mechanistic aspects of self-organized TiO 2 nanotubular layers and related transition metal oxide nanostructures.
Abstract: Self-organized oxide nanostructures grown by controlled anodic oxidation of a metal substrate attracted wide scientific interest due to a broad number of potential applications. The present work gives an overview on growth principles and mechanistic aspects of self-organized TiO 2 nanotubular layers and related transition metal oxide nanostructures. In particular, key electrochemical factors that control tube geometry and routes to fabricate advanced TiO 2 nanotube geometries and morphologies are discussed.
TL;DR: In this article, the whispering gallery effect was used to determine the refractive index of the wires as a function of the photon energy and temperature, and two methods for calculating the complex resonant modes were presented: a simple plane wave model and the numerical solution of the Helmholtz equation for the given resonator geometry.
Abstract: Optical whispering gallery mode (WGM) resonances have been observed in zinc oxide micro- and nanowire cavities. Using model calculations, the experimentally observed mode spectrum was reproduced. The effect has been observed for wire radii between 100 nm and 10 µm corresponding to angular mode numbers from 1 to about 250. The whispering gallery effect was used to determine the refractive index of the wires as a function of the photon energy and temperature. Under high excitation conditions, WGM lasing was observed. Two methods for calculating the complex resonant modes are presented: a simple plane wave model and the numerical solution of the Helmholtz equation for the given resonator geometry.
A typical photoluminescence (PL) spectrum showing WGM resonances and (inset) a scanning electron microscopy (SEM) image of a zinc oxide (ZnO) microwire.
TL;DR: In this paper, two types of antiferromagnetic quantum critical points (QCPs) have been explicitly observed in the heavy fermion metals, and they have been discussed in a number of strongly correlated electron systems.
Abstract: Quantum criticality describes the collective fluctuations of matter undergoing a second-order phase transition at zero temperature. It is being discussed in a number of strongly correlated electron systems. A prototype case occurs in the heavy fermion metals, in which antiferromagnetic quantum critical points (QCPs) have been explicitly observed. Here, I address two types of antiferromagnetic QCPs. In addition to the standard description based on the fluctuations of the antiferromagnetic order, a local QCP is also considered. It contains inherently quantum modes that are associated with a critical breakdown of the Kondo effect. Across such a QCP, there is a sudden collapse of a large Fermi surface to a small one. I also consider the proximate antiferromagnetic and paramagnetic phases, and these considerations lead to a global phase diagram. Finally, I discuss the pertinent experiments and outline some directions for future studies.
TL;DR: In this article, the authors calculate the variations of this contribution as a function of the beam incidence angle and of the droplet contact angle for a given incident directional flux, as pertains to molecular beam epitaxy, which can be directly incorporated in growth models.
Abstract: During nanowire growth by the vapor-liquid-solid method, part of the atoms incorporated in the nanowire originate from the direct impingement of a vapor beam on the droplet sitting at the top of the nanowire. For a given incident directional flux, as pertains to molecular beam epitaxy, we calculate the variations of this contribution as a function of the beam incidence angle and of the droplet contact angle. This provides a quantitative evaluation of a major contribution to the supply of atoms to the growing nanowire, which can be directly incorporated in growth models.
TL;DR: In this article, the authors present details on a recently installed hot-wall reactor for graphene growth on silicon carbide (SiC) in argon under atmospheric pressure, where both preparation steps, i.e., the preconditioning of the SiC substrate by hydrogen etching and the graphene growth are performed in this setup in a fully automated manner, thus ensuring the preparation of high quality graphene on an everyday basis.
Abstract: In this contribution, we present details on a recently installed hot-wall reactor for graphene growth on silicon carbide (SiC) in argon under atmospheric pressure. Both preparation steps, i.e., the preconditioning of the SiC substrate by hydrogen etching and the graphene growth are performed in this setup in a fully automated manner thus ensuring the preparation of high-quality graphene on an everyday basis. Samples were characterized by atomic force microscopy and X-ray induced photoelectron spectroscopy. We present results on the optimization of the hydrogen etching procedure. The thickness distribution of graphene samples grown in the automated process is Gaussian with a mean value of 1.1 monolayers and a standard deviation of 0.17 monolayers. This indicates a highly controlled process.
TL;DR: In this article, the complex dynamics of a quantum dot (QD) laser subjected to optical feedback from a short external cavity is studied. But the model consists of a Lang-Kobayashi-like model for the electric field combined with a microscopically based rate equation system.
Abstract: We present a systematic study of the complex dynamics of a quantum dot (QD) laser subjected to optical feedback from a short external cavity. Our model consists of a Lang―Kobayashi like model for the electric field combined with a microscopically based rate equation system. We separately treat electron and hole dynamics in the QDs and the surrounding wetting layer (WL). By tuning the phase―amplitude coupling and the optical confinement factor we are able to discuss various scenarios of the dynamics on the route towards conventional quantum well (QW) lasers. Due to the optical feedback, multistability occurs in our model in form of external cavity modes (ECMs) or delay-induced intensity pulsations. In dependence of the feedback strength we analyze complex bifurcation scenarios for the intensity of the emitted laser light as well as time series, power spectra, and phase portraits of all dynamic variables in order to elucidate the internal dynamics of the laser.
TL;DR: In this paper, the linear and third-order nonlinear optical absorption coefficients of intersublevel transitions have been calculated for a spherical CdS/SiO 2 quantum dot (QD) with infinite and different finite confining potential by using the density matrix formalism.
Abstract: In this study, the linear and third-order nonlinear optical absorption coefficients of intersublevel transitions have been calculated for a spherical CdS/SiO 2 quantum dot (QD) with infinite and different finite confining potential by using the density matrix formalism. The electron eigenenergies and the corresponding wavefunctions have been determined by the variational method under the effective mass approximation. We have investigated the effects of the incident optical intensity, QD radius, and confinement potential on the intersublevel optical absorption coefficients. It is found that the size of the dot and depth of the confinement have a great effect on the linear, the nonlinear, and the total absorption coefficients. Also, the total optical absorption saturation intensity is controlled by the strength of the confinement or the incident optical intensity.
TL;DR: In this paper, first-principles simulations of single grain boundary reflectivity of electrons in noble metals, such as Cu and Ag, were presented using non-equilibrium Green's function and first principles methods.
Abstract: We present first-principles simulations of single grain boundary reflectivity of electrons in noble metals, Cu and Ag. We examine twin and non-twin grain boundaries using non-equilibrium Green's function and first principles methods. We also investigate the determinants of reflectivity in grain boundaries by modeling atomic vacancies, disorder, and orientation and find that both the change in grain orientation and disorder in the boundary itself contribute significantly to reflectivity. We find that grain boundary reflectivity may vary widely depending on the grain boundary structure, consistent with published experimental results. Finally, we examine the reflectivity from multiple grain boundaries and find that grain boundary reflectivity may depend on neighboring grain boundaries. This study raises some potential limitations in the independent grain boundary assumptions of the Mayadas-Shatzkes (MS) model.
TL;DR: In this paper, the Stokes shift has been found to be ∼ 50 meV at room temperature, which is much smaller than the polarization-related electric field in this system and results from the Fermi level pining at the AlGaN surface and the Fermani level position at the InGaN/GaN interface.
Abstract: Contactless electroreflectance (CER) spectroscopy was applied to study (i) the type of surface band bending, (ii) the built-in electric field, and (iii) the Stokes shift in selected III-nitrides samples. The surface band bending was studied for truly bulk GaN crystals with various types of conductivity obtained by the ammonothermal method and GaN epilayers doped by Si and Mg. It has been shown that the shape of CER resonance, which is related to the band-to-band absorption, is sensitive to the type of surface band bending. This feature of semiconductor surface can be determined in contactless manner from CER measurements. The built-in electric field was studied for AlGaN/GaN heterojunction structures. From the period of AlGaN-related Franz-Keldysh oscillation the value of electric field has been determined to be 0.27 MV/cm. This value is much smaller that the polarization-related electric field in this system and results from the Fermi level pining at AlGaN surface and the Fermi level position at AlGaN/GaN interface. The Stokes shift was studied for InGaN/GaN multi quantum wells. This shift has been found to be ∼ 50 meV at room temperature.
TL;DR: In this article, a new concept of the energy transfer process from Ce 3+ to Tb 3+ in LaPO 4 via host lattice states was suggested and elucidated by proposing several possible models.
Abstract: The energy transfer mechanisms between Ce 3+ and Tb 3+ in LaPO 4 :Ce,Tb nanocrystals have been studied by means of time-resolved luminescence spectroscopy in a wide temperature range (10-300K). Special attention was paid to detailed comparative analysis of both rise and decay emission components of both Ce 3+ and Tb 3+ . Surprisingly, a relatively slow rise (several microseconds) of Tb 3+ emission under 266-nm laser excitation was detected, which corresponds to the 4f-5d transition of Ce 3+ in LaPO 4 . It was shown that this rise of Tb 3+ emission could not have arisen due to relaxation of Ce 3+ ions, whose excited state has a lifetime of about 20ns. It was demonstrated that the generally accepted concept of a resonant energy transfer from Ce 3+ to Tb 3+ in LaPO 4 could not explain the time-resolved luminescence characteristics as well as the observed temperature dependence. Hence, a new concept of the energy transfer process from Ce 3+ to Tb 3+ in LaPO 4 via host lattice states was suggested and elucidated by proposing several possible models.
TL;DR: In this article, multi-walled carbon nanotubes (MWNT) of different average diameters with ppm level purity were produced by a low temperature C2H4 pyrolysis on Fe-Co type catalysts combined with graphitization at 2200-2800°C in argon flow.
Abstract: Here multi-walled carbon nanotubes (MWNT) of different average diameters with ppm level purity were produced by a low temperature C2H4 pyrolysis on Fe-Co type catalysts combined with forthcoming graphitization at 2200–2800 °C in argon flow. Annealed nanotubes were characterized with X-ray fluorescent analysis, BET surface measurements, HR TEM, X-ray diffraction, DTA analysis, and measurements of temperature and magnetic field dependences of conductivity. The graphitization of MWNT results in removal of residual catalyst metal impurities, reduction of the wall defects, and closure of nanotube tips. It was found that extent of these effects depends on MWNT diameters. It was proposed that the graphitization is caused by a significant thermal displacement and diffusion of carbon atoms at temperatures higher the Debye temperature.
TL;DR: In this paper, the authors present the key ingredients of the model, i.e., the algorithm, and some methods implemented to determine the cohesive energy as well as the activation energy.
Abstract: The evolution of alloy microstructures under non-equilibrium conditions such as irradiation is an important academic as well-industrial issue. Atomistic kinetic Monte Carlo is one of the most versatile method which can be used to simulate the evolution of a complex microstructure at the atomic scale, dealing with elementary atomic mechanisms. It was developed more than 40 years ago to investigate diffusion events via the motion of a single vacancy, and the introduction of hetero-interstitials or self-interstitials in the models is yet under development. This paper presents the key ingredients of the model, i.e. the algorithm, and some methods implemented to determine the cohesive energy as well as the activation energy. For purpose of simplicity and speed of calculations, most of the models developed so far apply to idealized lattices and model alloys, for example binary alloys. The extension of the models to more complex alloys is recent and an example of the simulation of multi-component Fe-CuNiMnSi alloys representative of pressure vessel steels is thus presented in more details. In particular, the adjustment procedures of the cohesive model are demonstrated as well as the validation of the model for vacancies and self-interstitials on thermal ageing and isochronal annealing experiments.
TL;DR: In this article, the effects of tin interstitial (Sni), oxygen vacancy (VO), and Sni+VO defect pair on the electronic structure and magnetic properties of undoped SnO2 are investigated by means of density functional calculations.
Abstract: The effects of tin interstitial (Sni), oxygen vacancy (VO), and Sni + VO defect pair on the electronic structure and magnetic properties of undoped SnO2 are investigated by means of density functional calculations. Only single positively charged O vacancies V can induce local magnetic moments in bulk SnO2. The magnetic coupling between two V calculated by generalized gradient approximation (GGA) is ferromagnetic. Self-consistent bandgap correction, which is achieved by adding a Coulomb U on O-2s orbital, results in the full occupation of the spin-up gap state of V. Consequently, the magnetic coupling calculated by GGA + US becomes antiferromagnetic, which shows that a self-consistent bandgap correction is essential for the correct description of the magnetism in widegap SnO2. The results indicate that O vacancy in bulk SnO2 cannot induce ferromagnetism, which suggests that the atoms or defects located at the surface or substrate interface may play a key role in turning the ferromagnetism observed in undoped SnO2.
TL;DR: In this article, a two-step pulsed laser deposition and direct carbothermal growth was used to grow nano-and micro-structures with controlled orientation, size, and lateral density.
Abstract: ZnO-based nano- and micro-structures with controlled orientation, size, and lateral density were grown by specially designed two-step pulsed laser deposition and direct carbothermal growth, respectively. Different substrate orientations and nucleation layers allow well-defined tuning of growth direction and lateral arrangement of the wires. As a result, axial MgZnO-ZnO nano quantum dots and homogeneous radial nano coreshell quantum well structures are demonstrated. Donor and acceptor doping of nano- and microwires influence considerably both electrical characteristics as well as morphology including branching. Field effect transistors with n- and p-type wire channel and nanowire p-n junctions seem to prove a reproducible p-type conductivity of phosphorous-doped ZnO wires.
TL;DR: In this article, the imperative analog functional block (IMF) was used to create a reference voltage for various biasing and sensing circuits, and the voltage reference generator achieved less than 0.5 mV/°C drift.
Abstract: GaN smart power chip technology has been realized on the GaN-on-Si platform, featuring monolithically integrated high-voltage power devices, and low-voltage peripheral devices for mixed-signal functional blocks. In particular, this paper presents the imperative analog functional block - the voltage reference generator for smart power applications with wide-temperature-range stability. These circuits are shown to be capable of proper functions from room temperature (RT) up to 250 °C, featuring a negative reference voltage of -2.21 V at RT, and -2.13 V at 250 °C. The optimized voltage reference generator achieved less than 0.5 mV/°C drift. It can be used to create a reference voltage for various biasing and sensing circuits.
TL;DR: In this article, a microscopic theory describing the charge carrier and light emission dynamics in quantum dot (QD) light emitters is presented, which is based on QD Bloch equations including microscopically calculated Coulomb and electron-phonon scattering rates between bound QD, continuous wetting layer (WL) and bulk states.
Abstract: We present a microscopic theory describing the charge carrier and light emission dynamics in quantum dot (QD) light emitters. The theory covers non-classical light emission (fluorescence and Raman emission) in the low carrier injection limit as well as laser emission and pulse amplification in the high carrier injection limit. The theoretical approach is based on QD Bloch equations including microscopically calculated Coulomb and electron―phonon scattering rates between bound QD, continuous wetting layer (WL) and bulk states. In the low carrier density limit, multi-phonon relaxation is the dominant process, while at high charge carrier densities, Coulomb scattering dominates the dynamics. Using an equation of motion approach, we address (i) time-resolved fluorescence and Raman emission, (ii) electrical injection and charge carrier transfer from bulk into WL and QD states, (iii) single photon emission and (iv) gain dynamics of QD amplifiers and lasing dynamics in QD vertical-cavity surface-emitting lasers (VCSELs) at high injection currents.
TL;DR: In this article, a Raman spectroscopy study of multi-wall carbon nanotubes (MWNTs) of different diameters (with a small controllable number of walls) produced using FeCo catalyst with a variable size of the same active component has been performed.
Abstract: Raman spectroscopy study of multi-wall carbon nanotubes (MWNTs) of different diameters (with a small controllable number of walls) produced using Fe–Co catalyst with a variable size of the same active component has been performed. We have characterized as produced MWNTs with different diameters (series 1) and two types of tubes with fixed mean diameters (∼10 and 20 nm) heated in a flow of pure argon at various temperatures (2200, 2600 and 2800 °C – series 2). The Raman spectra of MWNTs have been registered in three spectral regions, corresponding to D (disorder-induced), G (graphite) and 2D (two-phonon scattering) bands. A ratio of intensities I2D/ID for tubes of series 1 has demonstrated almost a linear dependence on the nanotube diameter. After heating (series 2), D (disorder-induced) Raman band has shown a substantial decrease in intensity. The variation of the Raman spectra parameters is discussed in terms of defectiveness of nanotubes.
TL;DR: In this article, the authors investigated flexoelectric charge separation in centrosymmetric dielectric solids with truncated pyramids and found that the effective piezoelectrics coefficient is strongly enhanced by size reduction.
Abstract: Flexoelectric charge separation and the associated size dependent piezoelectricity are investigated in centrosymmetric dielectric solids. Direct piezoelectricity can exist as external mechanical stress is applied to non-piezoelectric dielectrics with shapes such as truncated pyramids, due to elastic strain gradient induced flexoelectric polarization. Effective piezoelectric coefficient is analyzed in truncated pyramids, which is strongly enhanced by size reduction and depends on flexoelectricity, elastic compliance, and aspect ratio of the non-piezoelectric dielectric solids.