Growth of bismuth tri‐iodide platelets by the physical vapor deposition method

The work reports the growth of single BI₃ crystals with platelets habit. Platelets were grown by physical vapor deposition (PVD) in a high vacuum atmosphere and with argon, polymer or iodine as additives. Crystals grew in the zone of maximum temperature gradient, perpendicular to the ampoule wall. Crystals grown with argon as additive show a very shining surface, have hexagonal (0 0 l) faces, sizes up to 20 x 10 mm² and thicknesses up to 100 μm. They were characterized by optical microscopy and scanning electron microscopy (SEM). Dendritic‐like structures were found to be their main surface defect. SEM indicates that they grow from the staking of hexagonal unities. Electrical properties of the crystals grown under different growth conditions were determined. Resistivities up to 2 x 10¹² Ωcm (the best reported value for monocrystals of this material) were obtained. X‐ray response was measured by irradiation of the platelets with a ²⁴¹Am source of 3.5 mR/h. A comparison of results according to the growth conditions was made. Properties of the crystals grown by this method are compared with the ones measured for others previously grown from the melt. Also, results for bismuth tri‐iodide platelets are compared with the ones obtained for mercuric and lead iodide platelets. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth of lead bromide polycrystalline films

Lead bromide polycrystalline films were grown by the physical vapor deposition method (PVD). Glass 1″x1″ in size, uncoated, and coated with Indium Tin Oxide (ITO), was used as substrate and rear contact. The starting material was evaporated at temperatures from 395°C to 530°C under high vacuum atmosphere (6 x 10^‐3 Pa) and during 8 days. The substrate temperature was prefixed from 190°C to 220°C. Film thickness yielded values from 40 to 90 μm. Optical microscopy and scanning electron microscopy (SEM) were performed on the films. Grain size resulted to be from 1.0 to 3.5 μm. SEM and X‐ray diffraction indicate that films grow with a preferred orientation with the (0 0 l) planes parallel to the substrate. The Texture Coefficient (TC) related to the plane (0 0 6) was 7.3. Resistivity values in the order of 10¹² Ωcm were obtained for the oriented samples, but a strong polarization indicates severe charge transport problems in the films. Film properties were correlated with the growth temperature and with previous results for films of other halides. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis of mercuric iodide and bismuth tri‐iodide nanoparticles for heavy metal iodide films nucleation

We synthesized mercuric iodide and bismuth tri‐iodide nanoparticles by suspension in octadecene, from Hg(NO₃)₂.H₂O and I₂, and from Bi(NO₃)₃.5H₂O and I₂, respectively. The best synthesis conditions were 2 h at 70‐80 °C, followed by 10 min at 110 °C for mercuric iodide nanoparticles, and 4 h at 80‐110 °C, followed by 10 min at 180‐210 °C for bismuth tri‐iodide ones. Nanoparticles were then washed and centrifuged with ether repeatedly. Compounds identity was confirmed by X‐ray diffraction (XRD) and energy dispersive spectrometry (EDS). We found shifts of the X‐ray diffraction maxima for nanoparticles of both compounds. We characterized the nanoparticles by transmission (TEM) and scanning (SEM) electron microscopy. We obtained disk‐like and squared mercuric iodide nanostructures, 80‐140 nm and 100‐125 nm in size respectively. We also obtained rounded and rod‐like bismuth tri‐iodide nanoparticles, 30‐500 nm in size. Acetonitrile and isopropanol suspensions of mercuric iodide nanoparticles, and acetonitrile suspension of bismuth tri‐iodide nanoparticles exhibited peak maxima shifts in their UV‐Vis spectra. We synthesized for the first time mercuric iodide and bismuth tri‐iodide nanoparticles by the suspension method, although we have not yet obtained uniform shape and size distributions. They offer interesting perspectives for crystalline film nucleation and for improving current applications of these materials, as well as for opening new ones. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Fine tuning of a triply resonant OPO for generating frequency degenerate CV entangled beams at low pump powers

A method of achieving triple resonance in a frequency-degenerate but polarization-non-degenerate optical parametric oscillator (OPO) with external mirrors is illustrated. The OPO is brought to the triple-resonance condition by tuning the non-linear crystal temperature and tilting the cavity axis. The interplay among temperature and tilting mismatch and gain is analyzed together with the allowed ranges of variation of these parameters. The method has been successfully applied to an OPO based on a periodically poled KTP crystal. Calibration of the system is reported together with parametric gain and squeezing measurements below threshold as a test of reliability and efficiency. PACS 42.50.-p; 42.65.-k     

Modeling the anatomical distribution of sunlights

One of the major technical challenges in calculating solar irradiance on the human form has been the complexity of the surface geometry (i.e. the surface-normal vis-a-vis the incident radiation). Over 80% of skin cancers occur on the face, head, neck and back of the hands. The quantification, as well as the mapping of the anatomical distribution of solar radiation on the human form, is essential if we are to study the etiology of skin cancers or cataracts or immune system suppression. Using advances in computer graphics, including high-resolution three-dimensional mathematical representations of the human form, the calculation of irradiance has been attained to subcentimeter precision. Lighting detail included partitioning of direct beam and diffuse skylight, shadowing effects and gradations of model surface illumination depending on model surface geometry and incident light angle. With the incorporation of ray-tracing and irradiance algorithms, the results are not only realistic renderings but also accurate representations of the distribution of light on the subject model. The calculation of light illumination at various receptor points across the anatomy provides information about differential radiant exposure as a function of subject posture, orientation relative to the sun and sun elevation. The integration of a geodesic sun-tracking model into the lighting module enabled simulation of specific sun exposure scenarios, with instantaneous irradiance, as well as the cumulative radiant exposure, calculated for a given latitude, date, time of day and duration. Illustration of instantaneous irradiance or cumulative radiant exposure is achieved using a false-color rendering--mapping light intensity to color--creating irradiance or exposure isopleths. This approach may find application in the determination of the reduction in exposure that one achieves by wearing a hat, shirt or sunglasses. More fundamentally, such an analysis tool could provide improved estimates of scenario-specific dose (i.e. absorbed radiant exposure) needed to develop dose-response functions for sunlight-induced disease.     

Harnack estimates for non-negative weak solutions of a class of singular parabolic equations

We prove forward, backward and elliptic Harnack type inequalities for non-negative local weak solutions of singular parabolic differential equations of type $$u_t={\rm div}{\bf A}(x, t, u, Du)$$ where A satisfies suitable structure conditions and a monotonicity assumption. The prototype is the parabolic p?Laplacian with 1 < p < 2. By using only the structure of the equation and the comparison principle, we generalize to a larger class of equations the estimates first proved by Bonforte et al. (Adv. Math. 224, 2151–2215, 2010) for the model equation.     

Pointwise gradient estimates in exterior domains

We consider a class of elliptic operators with unbounded coefficients in a smooth exterior domain Ω and we prove that the Cauchy-Neumann problem associated with admits, for any bounded and continuous initial datum, a unique bounded classical solution. We also provide pointwise gradient estimates for such a solution. Received: 5 July 2005; Revised: 20 December 2005     

Addition of an insulating element to the Modified Markov Method for CdTe single crystals growth

CdTe single crystal, were grown by the Modified Markov Method. We have introduced an insulating element between the furnace and the growth chamber, trying to minimize the radiation effects in the crystal. Numerical simulation of the heat transfer phenomena predict a reduction of the axial temperature gradient in the growth chamber, confirming by numerical experimental measurements, leading to a more uniform growth. An improvement in the surface morphology and in the crystal quality as well was achieved, evaluated by the combination of X‐ray rocking curves, etch pit density determination and low temperature photoluminescence. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Numerical analysis of heat transfer for the modified markov method

In this work, we present a theoretical analysis of the behaviour of the temperature profile for the Modified Markov Method. We have assumed different materials and different thicknesses for the insulating element. Comparison of preliminary results with experimental evidences, demonstrate us the reliability of our model. Discussion among our calculated results are provided and future outlooks for the experimental work are fixed. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)