The development of future 3D-printed electronics relies on the access to highly conductive inexpensive materials that are printable at low temperatures (<100 ◦C). The implementation of available materials for these applications are, however, still limited by issues related to cost and printing quality. Here, we report on the simple hydrothermal growth of novel nanocomposites that are well suited for conductive printing applications. The nanocomposites comprise highly Al-doped ZnO nanorods grown on graphene nanoplatelets (GNPs). The ZnO nanorods play the two major roles of (i) preventing GNPs from agglomerating and (ii) promoting electrical conduction paths between the graphene platelets. The effect of two different ZnO-nanorod morphologies with varying Al-doping concentration on the nanocomposite conductivity and the graphene dispersity are investigated. Time-dependent absorption, photoluminescence and photoconductivity measurements show that growth in high pH solutions promotes a better graphene dispersity, higher doping levels and enhanced bonding between the graphene and the ZnO nanorods. Growth in low pH solutions yields samples characterized by a higher conductivity and a reduced number of surface defects. These samples also exhibit a large persistent photoconductivity attributed to an effective charge separation and transfer from the nanorods to the graphene platelets. Our findings can be used to tailor the conductivity of novel printable composites, or for fabrication of large volumes of inexpensive porous conjugated graphene-semiconductor composites.

As an intermediate band (IB) originating from discrete nitrogen (N) levels is formed in GaP:N with increasing N concentration, GaP1−xNx alloy is considered to be a promising candidate for IB‐type solar cells. We studied the IB luminescence of a GaP1−xNx with 0.56% N and detected carrier recombination (CR) levels by superposing a below‐gap excitation (BGE) light of 1.17 eV. We resolved a high‐energy component of 2.15 eV in the IB luminescence, Ihigh, from total luminescence intensity Iall. With increasing the BGE density at fixed temperature of 5 K, the amount of decrease in Ihigh was distinctly smaller than that of simple temperature rise without the BGE at the same Iall value. We conclude that the observed intensity change of the IB luminescence due to the BGE comes not from thermal activation, but from optical excitation among the IB, conduction band, and CR levels in GaP1−xNx. It is of primal importance to understand CR levels toward determining their origins and eliminating them for realization of efficient IB‐type solar cells.

Wave functions of N-isoelectronic levels in GaP overlap with increasing N concentration and form an intermediate band (IB) with a suitable energy allocation. Thus GaPN is a promising candidate for IB-type solar cells. Detailed characterization of defect originated recombination levels is the first step to realize the expected efficiency improvement. We have studied carrier recombination processes via the IB in GaPN with N concentration of 0.56% by two wavelength excited photoluminescence (TWEPL) measurement. Spectral behavior of high energy shoulder in the IB luminescence with increasing the density of below gap excitation (BGE) light (hv BGE =1.17eV) discriminated the effect of optical excitation from that of thermal activation. The BGE effect revealed the presence of different carrier recombination levels which are connected with conduction band and with the IB, respectively.

The growth of thick InGaN layers on free-standing GaN (0001¯) substrates was studied using tri-halide vapor phase epitaxy. It was found that high-indium-content InGaN can be grown under higher InCl3 input partial pressure at higher growth temperature, which allows the fabrication of a high crystalline quality InGaN layer with a smooth surface morphology. Using the growth conditions of high InCl3 input partial pressure and high growth temperature, crack- and droplet-free InGaN layers with a thickness of over 10µm and with an indium fraction of 0.05 were successfully grown. Although the surface showed many hillocks, the number of hillocks was reduced upon growth of thicker InGaN layers. Photoluminescence measurements confirm that thick InGaN layers could be successfully grown without degradation of the crystalline quality.

Direct measurements of carrier diffusion in GaN nanorods with a designed InGaN/GaN layer-in-a-wire structure by scanning near-field optical microscopy (SNOM) were performed at liquid-helium temperatures of 10 K. Without an applied voltage, intrinsic diffusion lengths of photo-excited carriers were measured as the diameters of the nanorods differ from 50 to 800 nm. The critical diameter of nanorods for carrier diffusion is concluded as 170 nm with a statistical approach. Photoluminescence spectra were acquired for different positions of the SNOM tip on the nanorod, corresponding to the origins of the well-defined luminescence peaks, each being related to recombination-centers. The phenomenon originated from surface oxide by direct comparison of two nanorods with similar diameters in a single map has been observed and investigated.

High symmetry epitaxial quantum dots (QDs) with three or more symmetry planes provide a very promising route for the generation of entangled photons for quantum information applications. The great challenge to fabricate nanoscopic high symmetry QDs is further complicated by the lack of structural characterization techniques able to resolve small symmetry breaking. In this work, we present an approach for identifying and analyzing the signatures of symmetry breaking in the optical spectra of QDs. Exciton complexes in InGaAs/AlGaAs QDs grown along the [111]B crystalline axis in inverted tetrahedral pyramids are studied by polarization resolved photoluminescence spectroscopy combined with lattice temperature dependence, excitation power dependence and temporal photon correlation measurements. By combining such a systematic experimental approach with a simple theoretical approach based on a point-group symmetry analysis of the polarized emission patterns of each exciton complex, we demonstrate that it is possible to achieve a strict and coherent identification of all the observable spectral patterns of numerous exciton complexes and a quantitative determination of the fine structure splittings of their quantum states. This analysis is found to be particularly powerful for selecting QDs with the highest degree of symmetry (C3v and D 3 h ?> ) for potential applications of these QDs as polarization entangled photon sources. We exhibit the optical spectra when evolving towards asymmetrical QDs, and show the higher sensitivity of certain exciton complexes to symmetry breaking.

We report on excitonic single photon emission and biexcitonic photon bunching from an InGaN quantum dot formed on the apex of a hexagonal GaN micropyramid.An approach to suppress uncorrelated emission from the pyramid base is proposed, a metal film is demonstrated to effectively screen background emission and thereby significantly enhance the signal-to-background ratio of the quantum dot emission.As a result, the second order coherence function at zero time delay g (2) (0) is significantly reduced (to g (2) (0) = 0.24, raw value) for the excitonic autocorrelation at a temperature of 12 K under continuous wave excitation, and a dominating single photon emission is demonstrated to survive up to 50 K.The deterioration of the g (2) (0)-value at elevated temperatures is well understood as the combined effect of reduced signal-to-background ratio and limited time resolution of the setup.This result underlines the great potential of site controlled pyramidal dots as sources of fast polarized single photons.

The selected InGaAsN/GaAs T-shaped quantum wire (T-QWR) fabricated by metal organic vapor phase epitaxy has been investigated by microphotoluminescence (m-PL) and excitation-power-dependent mu-PL. The optical characteristics of one-dimensional structure were taken at low-temperature (4 K) and room temperature (RT) to clarify the intersection of two familiar quantum wells (QWs) in the [001] and [110] directions, named QW1 and QW2, respectively. For the excitation-power-dependent measurement, the intensity of the excitation source was used in the range of 0.001I(0) to I-0. The result shows that all of emissions related to QW1 and QWR peaks have a nonsymmetric line shape as evidenced by the tailing on the lower-energy side. All peaks shift to higher-energy side (blueshift) with the increase of the excitation power intensity. The blueshift and the low-energy tailing of PL peaks are attributed to the alloying effect. However, the emission peak related to QWR region shows a larger blueshift rate than that of QW1 on increasing of the excitation power intensity. This is an anomalous characteristic for the low-dimensional structure, affected by the large fluctuation state in the QWR region. This fluctuation state is combined of both edges of QWs (QW1 and QW2). (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We review the research done in recent years in our group on the effects of nanostructure symmetry, and outline its relevance both for nanostructure physics and for computations of their electronic and optical properties. The exemples of C3v and C2v quantum dots are used. A number of surprises and non-trivial aspects are outlined, and a few symmetry-based tools for computing and analysis are shortly presented.

The recombination dynamics of neutral donor bound excitons (DoX: I4, I6/6a) and near band edge defect-related emission in solution grown ZnO nanorods are investigated using steady state and time-resolved photoluminescence (PL) measurements. The effects of annealing are also studied. Low temperature steady state PL shows a systematic removal of the I4 line after annealing at 450 °C and the subsequent domination of I6a in these PL spectra. Additionally, the time decay of the I4, I6/6a, free exciton (FX), and basal plane stacking fault-related (BSF) PL transitions are studied as a function of annealing temperature. For the various annealing temperatures studied, the PL decay is described by a bi-exponential profile with a fast component (contribution from the surface) and slow component (related to bulk recombination). The fast component dominates in the case of as-grown and low temperature annealed samples (anneal temperatures up to 300 °C), suggesting the presence of surface adsorbed impurities. For samples annealed above 400 °C, the effects of the surface are reduced. The sample annealed at 850 °C produced an overall enhancement of the crystal quality. The underlying mechanisms for the observed PL characteristics are discussed based on near surface band bending caused by surface impurities.

We report on the observation of linearly polarized single photon antibunching in the excitonic emission from a site-controlled InGaN quantum dot. The measured second order coherence function exhibits a significant dip at zero time difference, corresponding to gm2(0)=0.90 under continuous laser excitation. This relatively high value of gm2(0) is well understood by a model as the combination of short exciton life time (320 ps), limited experimental timing resolution and the presence of an uncorrelated broadband background emission from the sample. Our result provides the first rigorous evidence of InGaN quantum dot formation on hexagonal GaN pyramids, and it highlights a great potential in these dots as fast polarized single photon emitters if the background emission can be eliminated.

Indium segregation in a narrow InGaN single quantum well creates quantum dot (QD) like exciton localization centers. Cross-section transmission electron microscopy reveals varying shapes and lateral sizes in the range ∼1-5 nm of the QD-like features, while scanning near field optical microscopy demonstrates a highly inhomogeneous spatial distribution of optically active individual localization centers. Microphotoluminescence spectroscopy confirms the spectrally inhomogeneous distribution of localization centers, in which the exciton and the biexciton related emissions from single centers of varying geometry could be identified by means of excitation power dependencies. Interestingly, the biexciton binding energy (E(b)xx) was found to vary from center to center, between 3 to -22 meV, in correlation with the exciton emission energy. Negative binding energies are only justified by a three-dimensional quantum confinement, which confirms QD-like properties of the localization centers. The observed energy correlation is proposed to be understood as variations of the lateral extension of the confinement potential, which would yield smaller values of E(b)xx for reduced lateral extension and higher exciton emission energy. The proposed relation between lateral extension and E(b)xx is further supported by the exciton and the biexciton recombination lifetimes of a single QD, which suggest a lateral extension of merely ∼3 nm for a QD with strongly negative E(b)xx = -15.5 meV.

Patients receiving home care are becoming increasingly dependent upon competent caregivers’ 24-h availability due to their substantial care needs, often with advanced care and home care technology included. In Sweden, care is often carried out by municipality-employed paraprofessionals such as health care assistants (HC assistants) with limited or no health care training, performing advanced care without formal training or support. The aim of this study was to investigate the work experience of the HC assistants and to explore how they manage when delivering 24-h home care to patients with substantial care needs. Grounded theory methodology involving multiple data sources comprising interviews with HC assistants (n 19) and field observations in patients’ homes was used to collect data and constant comparative analysis was used for analysis. The initial analysis revealed a number of barriers, competence gap; trapped in the home setting; poor supervision and unconnected to the patient care system, describing the risks associated with the situations of HC assistants working in home care, thus affecting their working conditions as well as the patient care. The core process identified was the HC assistants’ strivings to combine safe home care with good working conditions by using compensatory processes. The four identified compensatory processes were: day-by-day learning; balancing relations with the patient; self-managing; and navigating the patient care system. By actively employing the compensatory processes, the HC assistants could be said to adopt an inclusive approach, by compensating for their own barriers as well as those of their colleagues’ and taking overall responsibility for their workplace. In conclusion, the importance of supporting HC assistants in relation to their needs for training, supervision,and support from health care professionals must be addressed when organising 24-h home care to patients with substantial care needs in the future.

Semiconductor quantum dots (QDs) have been demonstrated viable for efficient light emission applications, in particular for the emission of single photons on demand. However, the preparation of QDs emitting photons with predefined and deterministic polarization vectors has proven arduous. Access to linearly polarized photons is essential for various applications. In this report, a novel concept to directly generate linearly-polarized photons is presented. This concept is based on InGaN QDs grown on top of elongated GaN hexagonal pyramids, by which the predefined elongation determines the polarization vectors of the emitted photons from the QDs. This growth scheme should allow fabrication of ultracompact arrays of photon emitters, with a controlled polarization direction for each individual emitter.

Quasi-one-dimensional AlGaAs quantum wires (QWRs) with parabolic heterostructure profiles along their axis were fabricated using metallorganic vapor phase epitaxy (MOVPE) on patterned (111)B GaAs substrates. Tailoring of the confined electronic states via modification in the parabolic potential profile is demonstrated using model calculations and photoluminescence spectroscopy. These novel nanostructures are useful for studying the optical properties of systems with dimensionality between zero and one.

The influence of the source gas supply sequence prior to growth and the NH3 input partial pressure (P-NH3 degrees) on the nucleation of InN islands during the initial stages of hydride vapor phase epitaxy on a nitrided (0001) sapphire substrate was investigated. The crystalline quality of the InN layer after subsequent lateral growth was also examined. When NH3 was flowed prior to growth, single-crystal hexagonal InN islands formed. When InN was grown with a higher P-NH3 degrees, the number of InN islands decreased remarkably while their diameter increased. The crystalline quality of InN grown on the hexagonal islands with a high P-NH3 degrees significantly improved with increasing growth time. A strong PL spectrum was observed only from InN layers grown with a high P-NH3 degrees. It was thus revealed that an NH3 preflow and a high P-NH3 degrees are effective for producing InN with high crystalline quality and good optical and electrical properties. (C) 2013 The Japan Society of Applied Physics

In this experimental study of single InGaAs/GaAs quantum dots (QDs) the photoluminescence intensity of the second order LO-phonon replica of the excitonic interband recombination was measured along ...