A model analysis of climate and CO2 controls on tree growth and carbon allocation in a semi-arid woodland

Many studies have failed to show an increase in the radial growth of trees in response to increasing atmospheric CO2 concentration [CO2] despite the expected enhancement of photosynthetic rates and water-use efficiency at high [CO2]. A global light use efficiency model of photosynthesis, coupled with a generic carbon allocation and tree-growth model based on mass balance and tree geometry principles, was used to simulate annual ring-width variations for the gymnosperm Callitris columellaris in the semi-arid Great Western Woodlands, Western Australia, over the past 100 years. Parameter values for the tree-growth model were derived from independent observations except for sapwood specific respiration rate, fine-root turnover time, fine-root specific respiration rate and the ratio of fine-root mass to foliage area (ζ), which were calibrated to the ring-width measurements by Bayesian optimization. This procedure imposed a strong constraint on ζ. Modelled and observed ring-widths showed quantitatively similar, positive responses to total annual photosynthetically active radiation and soil moisture, and similar negative responses to vapour pressure deficit. The model also produced enhanced radial growth in response to increasing [CO2] during recent decades, but the data do not show this. Recalibration in moving 30-year time windows produced temporal shifts in the estimated values of ζ, including an increase by ca 12% since the 1960s, and eliminated the [CO2]-induced increase in radial growth. The potential effect of CO2 on ring-width was thus shown to be small compared to effects of climate variability even in this semi-arid climate. It could be counteracted in the model by a modest allocation shift, as has been observed in field experiments with raised [CO2]

Visual modeling of dynamic gestures using 3D appearance and motion features

We present a novel 3-D gesture recognition scheme that combines the 3-D appearance of the hand and the motion dynamics of the gesture to classify manipulative and controlling gestures. Our method does not directly track the hand. Instead, we take an object-centered approach that efficiently computes 3-D appearance using a region-based coarse stereo matching algorithm. Motion cues are captured by differentiating the appearance feature with respect to time. An unsupervised learning scheme is carried out to capture the cluster structure of these features. Then, the image sequence of a gesture is converted to a series of symbols that indicate the cluster identities of each image pair. Two schemes, i.e., forward HMMs and neural networks, are used to model the dynamics of the gestures. We implemented a real-time system and performed gesture recognition experiments to analyze the performance with different combinations of the appearance and motion features. The system achieves recognition accuracy of over 96 % using both the appearance and motion cues.

Switching the current through molecular wires

The influence of Gaussian laser pulses on the transport through molecular wires is investigated within a tight-binding model for spinless electrons including correlation. Motivated by the phenomenon of coherent destruction of tunneling for monochromatic laser fields, situations are studied in which the maximum amplitude of the electric field fulfills the conditions for the destructive quantum effect. It is shown that, as for monochromatic laser pulses, the average current through the wire can be suppressed. For parameters of the model, which do not show a net current without any optical field, a Gaussian laser pulse can establish a temporary current. In addition, the effect of electron correlation on the current is investigated.Comment: 8 pages, 6 figure

Overcoming the performance limitations of industrial silicon solar cell by laser doping technology

Global photovoltaic (PV) market harvested rapid growth annually during the past two decades. As the primary part of this rapid growth, most of Silicon (Si) wafer-based solar cells employ screen printing(SP) technology for metallization on both polarities of P-type CZ Si wafers and the solar cell efficiency is around 18%. This efficiency is significantly lower than that of high efficiency solar cells developed in research laboratory. The fabrication of high efficiency solar cells in laboratory usually involves multiple expensive processes and high quality silicon material which are not suitable for production. Compared to the complicated and expensive laboratory process for high efficiency solar cell, SP technology is a simple and robust process. However, such technology also has some disadvantages such as poor blue response, high shading issue, low aspect ratio of metal fingers and insufficient rear surface passivation.Selective emitter (SE) technology is one possible way to further increase solar cell efficiency by harvesting more blue light. SE technology is of high interest to the PV industry in recent past. One of the cost effective method of achieving SE is by using laser doping. The laser doped selective emitter (LDSE) solar cell developed at the University of New South Wales (UNSW) combines laser doping and self-aligned light induced plating (LIP) technologies which make it one of the most feasible solutions for industrial selective emitter solar cell structure. The main advantage of LDSE technology is the simultaneously creation of dielectric layer patterning and localized heavy doping without an extra high temperature process or any other masking processes which are required by other technologies. In this thesis, LDSE technology was employed to improve the conversion efficiency of solar cells using a commercial available continuous wave (CW) green laser. The basic laser theory was reviewed and laser induced defects by CW laser were studied. The impact of different laser parameters and dielectric layer combinations on the morphology of laser scanned region was investigated. The process optimization of standard LDSE solar cells was presented. The focus of optimization was given to two key processes: laser doping and light induce plating in the LDSE solar cell process. Wide ranges of parameters were investigated in detail for each process, such that systematic improvements were demonstrated. The influences of different parameters on final solar cell devices were studied through the use of techniques such as light I-V curve, spectral response, scanning electron microscope, focus ion beam etc. As a result of optimizations, a final solar cell device with efficiency >19% was achieved on p-type 1 Ω cm Czochralski (CZ) silicon wafer.The other very important aspect of improving solar cell efficiency is the rear surface design. The aluminum (Al) back surface field (BSF) used in SP solar cells only has moderate passivation effect. Such Al BSF was identified as a limiting factor for standard LDSE solar cells. To overcome this problem, a stack dielectric layer of silicon dioxide (SiO2) and silicon oxynitride (SiOxNy) was developed. Such stack layer demonstrated excellent surface and bulk passivation ability on commercial grade p-type 1 Ω cm CZ Si wafer with carrier lifetime of 670µs and implied open circuit voltage (iVoc) of 735 mV. The significance of rear surface passivation has been realized by researchers worldwide as a key approach for high efficiency solar cell designs and several dielectric layers were successfully developed to passivate Si surface and get high carrier lifetime. However, challenge lies in making local contact opening on the dielectric layers or forming local BSF (LBSF) without massive jeopardize passivation ability using industry suitable approach rather than expensive photolithograph which is normally employed in laboratory. As a local heating process, laser doping could achieve dopants diffusion and dielectric layer patterning in step without causing massive degradation in the passivation ability of dielectric layer. In this thesis, CW laser system was used to create double sided laser doped (DSLD) solar cell by performing phosphorous and boron laser doping to front and rear surface of industry p-type CZ Si wafer passivated by SiNx/SiO2 stack layers on the front surface and SiO2/SiOxNy stack layers on the rear surface. By optimizing the thermal stability of the dielectric layers, emitter sheet resistance and the laser doping parameters, over 700mV iVoc was achieved on CZ Si samples after laser process, prior to metallization. As proof-of-concept, DSLD solar cells were made on CZ samples. Open circuit voltage (Voc) in the range of 660mV was achieved with high short circuit current density. However, at this stage, the solar cells efficiency is limited by low fill factor (FF). The origin of low FF was investigated and possible causes and solutions were discussed. At the end of this thesis, DSLD solar cells with Voc of 680mV was achieved on p-type 1 Ω cm CZ silicon wafers by optimizing rear surface metallization process