Properties Of Low-dimensional Structures Research Articles

Enhanced Optical Absorption and Slowed Light of Reduced-Dimensional CsPbBr3 Nanowire Crystal by Exciton-Polariton.

Metallic halide perovskites are promising for low-cost, low-consumption, flexible optoelectronic devices. However, research is lacking on light propagation and dielectric behaviors as fundamental properties for optoelectronic perovskite applications, particularly the mechanism supporting a strong light-matter interaction and the different properties of low-dimensional structures from their bulk counterparts. We use spatially resolved photoluminescence (SRPL) spectroscopy to explore light propagation and measure the refractive index of CsPbBr3 nanowires (NWs). Owing to strong exciton-photon interactions, light is guided as an exciton-polariton inside the NWs at room temperature. Remarkable spatial dispersion is confirmed, in which both the real and imaginary parts of the refractive index increase dramatically approaching exciton resonance, thus slowing light and enhancing absorption, respectively. Reducing the NWs dimension increases exciton-photon coupling and the exciton fraction, increasing the light absorption coefficient and group index 5- and 3-fold, respectively, relative to those of bulk films and slowing the light group velocity by ∼74%. Furthermore, dispersive absorption induces an energy redshift to the propagating PL at 4.1-5.5 meV μm-1 until the bottleneck region. These findings clarify light-matter interaction in confined perovskite structures to improve their optoelectronic device performance.

Thermoelectric Applications of Low-Dimensional Carbon Nanostructures with Acoustically Mismatched Boundaries

Thermoelectric applications of low-dimensional carbon nanostructures offer new ways to manipulate the electron and phonon properties of a given material. High-resolution Raman spectroscopy of ultra-thin silicon-on-insulator structures reveals multiple peaks in the spectral range from 50 cm−1 to 160 cm−1. The peak positions are consistent with the theoretical predictions and indicate the confined nature of phonon-transport in thin films and superlattices with a finite acoustic mismatch between layers. This opens up a novel turning capability for optimization of the thermoelectric properties of low-dimensional structures.

Space charge layer effects in silicon studied by in situ surface transport

Electronic properties of low dimensional structures on surfaces can be comprehensively explored by surface transport experiments. However, the surface sensitivity of this technique to atomic structures comes along with the control of bulk related electron paths and internal interfaces. Here we analyzed the role of Schottky-barriers and space charge layers for Si-surfaces. By means of a metal submonolayer coverage deposited on vicinal Si(1 1 1), we reliably accessed subsurface transport channels via angle- and temperature-dependent in situ transport measurements. In particular, high temperature treatments performed under ultra high vacuum conditions led to the formation of surface-near bulk defects, e.g. SiC-interstitials. Obviously, these defects act as p-type dopants and easily overcompensate lightly n-doped Si substrates.

Thermal vibration of a double-layered graphene sheet with initial stress at low temperature

Thermal fluctuations play an important role in the resonance properties of low dimensional structures as they serve as acoustic and vibration sensors of micro and nano machine. The quantum effects will be obvious during the thermal vibration of nanostructures because of the high natural frequency. Graphene has novel electronic properties and superior mechanical strength. The thermal vibration problems of graphene, which can be used as nano electronic components, have attracted a lot of research interest. The influence of quantum effects for the thermal vibration of single-layered graphene has been investigated. The dynamical behaviors of the double-layered graphene are more complicated due to the van der Waals force between two layers. However, few studies about the thermal vibration of double-layered rectangle graphene considering quantum effects have been found. To this end, an equivalent plate model with quantum effects of a double-layered graphene is established. The van der Waals interaction between two adjacent layers can be simplified as Winkler type spring because the thermal amplitude of graphene is small when the temperature is very low. The nonlocal effects and initial stress of the graphene are also taken into consideration. The natural frequency and the root of mean squared (RMS) amplitude of the double-layered graphene with different initial stress in different temperatures are calculated. The in-phase vibration mode and anti-phase vibration mode can be obtained from thermal vibration of the double-layered graphene. The natural frequency of the in-phase vibration of the double-layered graphene is the same as the natural frequency of the single-layered graphene with the same size. The natural frequency of the anti-phase vibration is larger than the in-phase vibration frequency. The RMS amplitude obtained by the plate model considering quantum effects is smaller than that obtained by the plate model together with the law of energy equipartition. The different of these two models becomes more obvious at the lower temperature. The RMS amplitude of the anti-phase vibration is smaller than the in-phase vibration RMS amplitude. The peak values of the RMS amplitude of the anti-phase vibration in different models are very close because the natural frequency of anti-phase vibration increases slowly with the increasing of model. The natural frequency of the in-phase vibration will increase with increasing of the initial stress, while the RMS amplitude of the in-phase vibration will decrease. However, the changes of the natural frequency and the RMS amplitude of the anti-phase vibration are very small when the initial stress changes. The initial stress has more significant influence for the in-phase vibration than the anti-phase vibration. The effect of zero-point energy for the thermal vibration of the double-layered graphene is also investigated. If the zero-point energy is taken into consideration to the mean modal energy, the RMS amplitude obtained by the plate model considering quantum effects is larger than that obtained by the plate model together with the law of energy equipartition in a wide range. The difference is obvious when the temperature is very low. Besides, the RMS amplitude of the graphene is not zero at the temperature of 0 K.

Effect of medium pH on the optical properties of CdTe nanocrystals at colloidal synthesis and postsynthetic treatment

Physicochemical properties of low-dimensional structures based on CdTe obtained by colloidal synthesis method were studied. Effect of thioglycolic, hydroxyethylidenediphosphonic, and ethylenediaminetetraacetic acids as well as L-cysteine and thiourea on the processes of formation of CdTe nanocrystals in aqueous solutions was examined. Medium pH was shown to have a considerable effect on the process of their colloidal synthesis. The features of preparation of concentrated colloidal solutions of CdTe nanocrystals stabilized by thioglycolic acid and further sedimentational precipitation of monodisperse fractions from polydisperse solutions were studied. Optical absorption and photoluminescence spectra of the studied materials immediately after synthesis completion and after postsynthetic treatment were obtained and analyzed.

Perovskite Oxide Nanowires: Synthesis, Property and Structural Characterization

Perovskite oxide materials display a wide spectrum of functional properties, including switchable polarization, piezoelectricity, pyroelectricity, and non-linear dielectric behavior. These properties are indispensable for application in electronic devices such as non-volatile memories, sensors, microactuators, infrared detectors, microwave phase filters, and so on. Recent advances in science and technology of perovskite oxide materials have resulted in the feature sizes of perovskite oxides-based electronic devices entering into nanoscale dimensions. At nanoscale perovskite oxide materials exhibit a pronounced size effect manifesting itself in a significant deviation of the properties of low-dimensional structures from the bulk and film counterparts. In the last decade low-dimensional perovskite nanosized oxides have been received much attention because of their superior physical and chemical properties. Among them, perovskite oxide nanowires are especially attractive for nanoscience studies and nanotechnology applications. Compared to other low-dimensional perovskite oxide systems, perovskite oxide nanowires are not only used as the building blocks of future nanodevices, but also they offer fundamental scientific opportunities for investigating the intrinsic size effects of physical properties. In the recent years, much progress has been made both in synthesis and physical property testing of perovskite oxide nanowires, which have a profound impact on the nanoelectronics. In this work, an overview of the state of art in perovskite oxide nanowires is presented, which covers their synthesis, property, and structural characterization. In the first part, the recent literatures for fabricating perovskite oxide nanowires with promising features, are critically reviewed. The second part deals with the recent advances on the physical property testing of perovskite oxide nanowires. The third part summarizes the recent progress on microstructural characterizations of perovskite oxide nanowires, to improve their crystalline quality, morphology and uniformity. Finally, this review concludes with some perspectives and outlook on the future developments of perovskite oxide nanowires.

Optical properties of low-dimensional structures formed by irradiation of laser

Some kinds of low-dimensional nanostructures can be formed by the irradiation of laser on a pure silicon sample and SiGe alloy sample. We have studied the photoluminescence (PL) of the hole-net structure of silicon and the porous structure of SiGe where the PL intensity at 706 nm and 725 nm wavelength increases obviously. The effect of intensity-enhancing in the PL peaks cannot be explained within the quantum confinement alone. We propose a mechanism on the increasing PL emission in the above structures, in which the trap state of the interface between SiO2 and nanocrystal plays an important role.

Phonon Spectra and Thermodynamic Properties Of Crystalline Nanowires

AbstractPhonon spectra and allowed phonon states, as well as thermodynamic characteristics of nanowires of simple cubic crystalline structure, are analyzed using the method of two-time dependent Green's functions, adjusted to bounded crystalline structures. Poles of Green's functions, defining phonon spectra, can be found by solving of the secular equation. For different boundary parameters, this problem is presented graphically. The presence of boundaries as well as the change of boundary parameters leads to appearance of new properties of low dimensional structures (thin film and nanowire). The most important feature is that beside allowed energy zones (which are continuous as in the bulk structure), zones of forbidden states appear. Different values of boundary parameters lead to appearance of lower and upper energy gaps, or dispersion branches spreading out of bulk energy zone. The correlation with spectra of phonons in corresponding unbounded structures is maintained in the work. Determination of phonon spectra and allowed phonon energies has great importance for kinetic and thermodynamic properties of semiconductive nanostructures and devices. The temperature behavior of nanowire thermal capacitance is compared to that of bulk structures. It is shown that at extremely low temperature nanowire thermal capacitance is considerably lower than the thermal capacitance of bulk sample. It was discussed what are the consequences of this fact to the thermal, conducting and superconducting properties of materials.

Preface: phys. stat. sol. (c) 1/S2

The papers in this special issue of physica status solidi (c) are selected manuscripts including diverse research lines presently in development in the ambit of the NanoSemiMat network in Brazil. The 3rd Workshop on Semiconductor Nanodevices and Nanostructured Materials (NanoSemiMat-3) took place in Salvador, Bahia, Brazil, at the Catussaba Resort Hotel, during the period of 24–27 March 2004. The NanoSemiMat network is part of the Brazilian Initiative on Nanoscience and Nanotechnology (N&N), with strategic cooperative research support in this area. The initiative started in 2001, through the formation of four research networks nationwide in different scientific fields associated to NanoScience and Nanotechnology (N&N). The 3rd Workshop on Nanodevices and Nanostructured Materials (NanoSemiMat-3) is an evolution of the two previous meetings which were held in Recife, PE, Brazil and Natal, RN, Brazil in 2002 and 2003, respectively. The meeting comprised 16 invited plenary talks, each 30 minutes long, given by eminent researchers from Brazil, Canada, France, Germany and the United States of America. These invited talks extend through different topics of N&N associated to Nanodevices and Nanostructured Materials: Photodetectors, Lasers and LEDs, Porous Materials, New Materials, and New Technologies, among others. There were short talks presented by representatives of the other N&N networks in Brazil dealing with Molecular Technology and Interfaces, Nanostructured Materials and Nanobiotechnology. Also a poster session, with about 60 presentations, highlighted the main research activities presently being developed by the network members at the different sites which constitute the NanoSemiMat network. The presentations reflected theoretical and experimental research lines which lead to the development of basic and applied research in nanostructured semiconductor materials such as III–V and II–VI, Si and SiC based nanodevices, wide gap materials, ceramics, polymers, porous materials, optical and transport properties of low dimensional structures, magnetic nanostructures and structures under the influence of high fields, spintronics and sensor applications. The participants of the workshop came from 20 research institutions within Brazil and from 7 research laboratories and universities in Europe and North America. In total about 120 researchers, members of the network, invited researchers, representatives of supporting and funding agencies in Brazil, undergraduate and graduate students, technical staff and supporting personal as well as researchers from complementary fields were present. The realization of the NanoSemiMat-3 was possible due to the financial support of the Brazilian Ministry of Science and Technology (MCT) and the Brazilian National Research Council (CNPq) and the logistic support of Federal University of Bahia. All activities during the NanoSemiMat-3 were open to the general public with interest in nanoscience and nanotechnology. In this third workshop of the series, we highlight the expansion of its format, with plenary and invited talks, poster sessions, as well as the presence of seven invited speakers from abroad. We expect that the continuation of the NanoSemiMat series will be a forum for discussions of state-of-the-art research developed in Brazil on N&N and the multidisciplinary field of semiconductor nanodevices and nanostructured materials as well as its superposition to other branches of science. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermoelectric Properties of PbSr(Se,Te)-Based Low Dimensional Structures

ABSTRACTThermoelectric properties of low dimensional structures based on PbTe/PbSrTe-multiple quantum-well (MQW)-structures with regard to the structural dimensions, doping profiles and levels are presented. Interband transition energies and barrier band-gap are determined from IR-transmission spectra and compared with Kronig-Penney calculations. The influence of the data evaluation method to obtain the 2D power factor will be discussed. The thermoelectrical data of our layers show a more modest enhancement in the power factor σS2 compared with former publications and are in good agreement with calculated data from Broido et al. [5]. The maximum allowed doping level for modulation doped MQW structures is determined. Thermal conductivity measurements show that a ZT enhancement can be achieved by reducing the thermal conductivity due to interface scattering. Additionally promising lead chalcogenide based superlattices for an increased 3D figure of merit are presented.

The Influence of Process-Induced Surface Defects on Luminescence and Transport Properties of Low-Dimensional Structures

The Influence of Process-Induced Surface Defects on Luminescence and Transport Properties of Low-Dimensional Structures