Symmetry of k·p Hamiltonian in pyramidal InAs/GaAs quantum dots: Application to the calculation of electronic structure

A method for the calculation of the electronic structure of pyramidal self-assembled InAs/GaAs quantum dots is presented. The method is based on exploiting the C-4 symmetry of the 8-band k·p Hamiltonian with the strain taken into account via the continuum mechanical model. The operators representing symmetry group elements were represented in the plane wave basis and the group projectors were used to find the symmetry adapted basis in which the corresponding Hamiltonian matrix is block diagonal with four blocks of approximately equal size. The quantum number of total quasiangular momentum is introduced and the states are classified according to its value. Selection rules for interaction with electromagnetic field in the dipole approximation are derived. The method was applied to calculate electron and hole quasibound states in a periodic array of vertically stacked pyramidal self-assembled InAs/GaAs quantum dots for different values of the distance between the dots and external axial magnetic field. As the distance between the dots in an array is varied, an interesting effect of simultaneous change of ground hole state symmetry, type, and the sign of miniband effective mass is predicted. This effect is explained in terms of the change of biaxial strain. It is also found that the magnetic field splitting of Kramer's double degenerate states is most prominent for the first and second excited state in the conduction band and that the magnetic field can both separate otherwise overlapping minibands and concatenate otherwise nonoverlapping minibands

Transport Properties of Electron-doped Sr1-xLaxCuOδ Superconducting Thin Films

An important, still open question in solid state physics is the mechanism of high-temperature superconductivity observed in cuprate compounds, discovered 28 years ago. They are characterized by a layered structure - superconducting CuO2 planes separated by doping layers. Superconductivity emerges from an insulating antiferromagnetic state by doping the CuO2 planes with holes or electrons. Structurally, the simplest cuprate family is only composed of CuO2 planes separated by Sr planes and it might be considered as a model cuprate. However, it appeared that such compound is difficult to prepare: no single crystals were produced to date. Ceramic samples are prepared under high pressure. Epitaxial films are also difficult to synthesize. We have studied Sr1-xLaxCuOδ (SLCO) compound which is electron-doped by partial substitution Sr2+/La3+. As most of research is focused on hole-doped cuprates, our goal was to make stable superconducting SLCO thin films to perform the first measurements of magnetic and electrical transport properties of this "model" cuprate family. C-axis oriented films were deposited by rf magnetron sputtering technique on heated substrates suitable for epitaxial growth. We determined basic superconducting properties of these films, namely the critical temperature and upper critical fields, and the temperature and field dependence of the critical currents. Normal state transport properties were also investigated. In particular a strong temperature dependence of the Hall number was observed, with a change of sign., indicating that two types of carriers are present. Negative in-plane magnetoresistance was also detected, indicative of some remaining antiferromagnetism. Any theory that concerns the mechanism of superconductivity of cuprates must be able to explain these properties

Interaction of UV irradiation with thin films of organic molecules

There is an ongoing interest in organic materials due to their application in various organic electronic devices. However stability of organic materials limits their potential use. They are prone to degradation both during the working life and storage. One of the main causes is extrinsic degradation, under the influence of oxygen and moisture. This problem can be solved by encapsulation of devices. However no encapsulation is perfect. In the first part of this work a study of degradation of thin films of N,N′-bis(3-methylphenyl)-N,N′-bis(phenyl)benzidine (TPD) and 4,4′-bis(2,2-diphenylvinyl)-1,1′-biphenyl (DPVBi) under UV irradiation in air is given. Films of both materials are stable in vacuum, but readily degrade in the presence of oxygen. Thus, the necessary condition for degradation is the simultaneous presence of UV light and oxygen. Chemical analysis of irradiated films by NMR, mass and infrared spectroscopy revealed presence of oxidized species (impurities). These impurities are responsible for increased morphological stability of irradiated films and quenching of photoluminescence. Only small amount of impurities, 0.4 % (0.2 %) for TPD (DPVBi), causes 50 % decrease of photoluminescence. This implies a non-trivial mechanism of quenching. For both molecules it was found that distance between impurities is smaller or equal to exciton diffusion length, which is the necessary condition for quenching. Following mechanism of quenching is proposed: exciton diffuses by hopping form one DPVBi (TPD) to another through FRET in a random walk manner. If, during its lifetime, it comes to proximity of an impurity, a Dexter-type energy transfer occurs and PL is quenched. Findings of DPVBi study are important because they show that even a small amount of oxygen that penetrates a DPVBi layer would impair luminescence efficiency of a device. Moreover, the absorption of own radiation (for DPVBi and TPD both) would additionally contribute to the rate of degradation of a device. It is reasonable to expect that transport properties would also be affected when materials are used as a hole-transporting layer in OLEDs.Serbian Ceramic Society Conference ADVANCED CERAMICS AND APPLICATION IV New Frontiers in Multifunctional Material Science and Processing Serbian Academy of Sciences and Arts, Knez Mihailova 35 Serbia, Belgrade, 21-23. September 2015

Magnetic Field Dependence Of Anisotropy Of In-plane Angular Magnetoresistance Of Electron-doped Sr1-xLaxCuO2 Thin Films

We studied the normal state magnetoresistance of underdoped superconducting epitaxial Sr1-xLaxCuO2 thin films by applying a high magnetic field up to 22 T parallel to the CuO2 planes and by varying the orientation of a field of given intensity in order to probe the underlying spin system. This infinite layer compound which has the simplest structure of all the cuprates presents a monotonic negative in-plane magnetoresistance with an anisotropic angular dependence which depends on the doping level [1] and on the field intensity [2]. Angular dependence of the in-plane magnetoresistance at highest magnetic fields is the same for films with different doping levels [2]. We compare our observations with the corresponding ones for the other electron-doped family Ln2-xCexCuO4 (Ln=Nd, Pr, La) and we attribute them to a manifestation of antiferromagnetism which appears to be only due to spins in the CuO2 planes

Tunnel Junction Sensing of TATP Explosive at the Single-Molecule Level

Triacetone triperoxide (TATP) is a homemade, potent explosive and, unfortunately, is used in many terrorist attacks. It is hard to detect, and present techniques for its sensing do not offer portability. Fortunately, TATP is volatile, and gas-sensing-based devices for TATP detection would provide a higher level of safety. Here, we explore the possibility of single molecule TATP detection in the air by tunneling current measurement in the N-terminated carbon-based nanogaps, at the DFT+NEGF level of theory. We found TATP averaged current amplitude of tens nano amperes, with a discrimination ratio with respect to prevalent indoor volatile organic compounds (VOC) of a few orders of magnitude. That high tunneling current is due to specific TATP HOMO contributions to electronic transport. The transport facilitates the strong, in-gap electrical field generated by N-C polar bonds from electrode ends and TATP electrode hybridization, spurred by oxygen atoms from a probed molecule

Single-Molecule Probing By Rectification in a Nanogap

Here in this talk, we propose the simultaneous measurement of rectification and amplitude of tunneling current during electrical probing of a molecule in a nanogap for efficient single-molecule detection. Also, we suggest the application of nitrogen-terminated graphene or CNT nanogaps due to their inherent outstanding features. With DFT and Non-Equilibrium Green's Function formalism, we show that tunneling current through various molecules, including ssDNA, TATP, or small organics placed in those nanogaps, exhibits unique rectification behavior under square pulses of alternating bias. The rectification arises by on-off switching of electronic transport through the molecular HOMO or LUMO levels, sustained by partial charging of the probed molecule, generated by asymmetric hybridization of that level with Bloch states from one of the electrodes. An effect that mimics local gating, i. e. an interaction between the molecule and the nitrogen-induced dipole moment located at the N-C interface of the electrode ends, strongly influences the rectification. The simultaneous measurement of rectification and amplitude of tunneling current could be applied to gas-phase single-molecule detection, as shown in the example case of the TATP. The TATP (triacetone triperoxide) is a volatile, potent, and hard-to-detect explosive made from commonly available chemicals, a terrorist weapon of choice in the last two decades. The rectification could also be applied in the liquid phase, offering the possibility of high-throughput and precise DNA sequencing. We found that the environment (neighboring nucleotides, water molecules, and counterions) does not mask ssDNA rectification while ssDNA traverses the nanogap

Micro-rods of oxidized pentacene obtained by thermal annealing in air of pentacene thin films

The influence of thermal annealing (in air and nitrogen at ambient pressure) on optical properties of pentacene films, well-known material widely used in organic electronic devices, was studied. Pentacene films, whose thickness varies an order of magnitude (30 – 300 nm) depending on the position on the substrate, were polycrystalline at all thicknesses. Raman and UV-vis absorption spectra depend on the position on film implies changes of the film structure with the thickness. These spectra are not largely affected by annealing if it is not performed in air at temperatures higher than 100°C. Prolonged annealing in air, at temperatures higher than 100°C, leads to formation of nano- and micro-scale rod-shaped structures on film surface. Based on scanning electron microscopy measurements, it is supposed that these structures are crystalline. Their UV-vis absorbance indicates that they are composed of more than one species of oxidized pentacene molecules, including 6,13-pentacenequinone. Further study is necessary to precisely determine composition and structure of micro-rods, as well as the mechanism of their formation.Serbian Ceramic Society Conference ADVANCED CERAMICS AND APPLICATION V New Frontiers in Multifunctional Material Science and Processing Serbian Academy of Sciences and Arts, Knez Mihailova 35 Serbia, Belgrade, 21st-23rd September 201

Analysis of 4,4′-bis(2,2′diphenyl vinyl)-1,1′- biphenyl using the atmospheric-pressure solids analysis probe for ionization

An Atmospheric pressure Solids Analysis Probe (ASAP) mass spectrometer are used for investigation the ionization mechanism and fragmentation pathways of 4,4′-bis(2,2′diphenyl vinyl)-1,1′-biphenyl (DPVBi). DPVBi is material used in OLEDs (organic light-emitting diode). Results obtained indicate that by controlling ion source conditions it is possible to optimize forming of desired precursor ion, primarily radical cation and in less content protonated ion of DPVBi. The results presented illustrate the usefulness of ASAP MS in the characterization of DPVBi compounds