Photoelectrical properties of graphene/doped GeSn vertical heterostructures

GeSn is a group IV alloy material with a narrow bandgap, making it favorable for applications in sensing and imaging. However, strong surface carrier recombination is a limiting factor. To overcome this, we investigate the broadband photoelectrical properties of graphene integrated with doped GeSn, from the visible to the near infrared. It is found that photo-generated carriers can be separated and transported with a higher efficiency by the introduction of the graphene layer. Considering two contrasting arrangements of graphene on p-type and n-type GeSn films, photocurrents were suppressed in graphene/p-type GeSn heterostructures but enhanced in graphene/n-type GeSn heterostructures when compared with control samples without graphene. Moreover, the enhancement (suppression) factor increases with excitation wavelength but decreases with laser power. An enhancement factor of 4 is achieved for an excitation wavelength of 1064 nm. Compared with previous studies, it is found that our graphene/n-type GeSn based photodetectors provide a much wider photodetection range, from 532 nm to 1832 nm, and maintain comparable responsivity. Our experimental findings highlight the importance of the induced bending profile on the charge separation and provides a way to design high performance broadband photodetectors.

The photoelectrical properties of graphene integrated with doped GeSn have been investigated and a high performance broadband photodetection can be achieved by integration of graphene with n-type GeSn.

Alteration of imaging properties of an optical pickup's objective lens by a nonrotationally symmetric coating

One can control the aberrations of an optical readout system by varying the width of a strip of antireflection coatings deposited upon plastic objective lenses. It is found that one can control the magnitude of the third-order astigmatism of the system by changing the coating width. This process has the advantage that it does not significantly cause other kinds of aberration such as coma and spherical aberrations to deteriorate. When these nonrotational symmetrically (NRS) coated lenses are used for off-axis operations such as tracking movements in optical drives, the change in the magnitude of the astigmatism (DAS) that is generated can be made smaller than those of symmetrically coated or noncoated lenses. As much as a 73% decrease in DAS was observed experimentally with a NRS-coated lens. Including the birefringence of the plastic material in the analysis yields a low and constant level of astigmatism generated by shifting of the objective lens.

Effects of an asymmetrically molded plastic objective lens on the push-pull tracking-error signal in an optical disk drive

The effects of a plastic objective lens's astigmatism on the push-pull tracking-error signal (TES) of an optical disk data storage system were investigated theoretically and experimentally. Astigmatism of plastic objective lenses arises commonly from the asymmetric deviation from their designed shape during the molding process. By carefully studying the aberration characteristics of the objective lens and including the astigmatism of the laser diode in the analysis, we can calculate the combined effects of astigmatism of these two components on the push-pull TES. It is found, from both the simulations and the experiments, that, by rotation of the objective lens about the optical axis, the peak-to-peak value of the push-pull TES varies with the lens's rotation angle, and a change as great as 340% in its value was observed in a given optical pickup.

Sludge-Grown Algae for Culturing Aquatic Organisms: Part II. Sludge-Grown Algae as Feeds for Aquatic Organisms

This project investigated the feasibility of using sewage sludge to culture microalgae (Chlorella-HKBU) and their subsequent usage as feeds for rearing different organisms. Part II of the project evaluated the results of applying the sludge-grown algae to feed Oreochromis mossambicus (fish), Macrobrachium hainenese (shrimp), and Moina macrocopa (cladocera). In general, the yields of the cultivated organisms were unsatisfactory when they were fed the sludge-grown algae directly. The body weights of O. mossambicus and M. macrocopa dropped 21% and 37%, respectively, although there was a slight increase (4.4%) in M. hainenese. However, when feeding the algal-fed cladocerans to fish and shrimp, the body weights of the fish and shrimp were increased 7% and 11% accordingly. Protein contents of the cultivated organisms were comparable to the control diet, although they contained a rather high amount of heavy metals. When comparing absolute heavy metal contents in the cultivated organisms, the following order was observed: alga > cladocera > shrimp, fish > sludge extracts. Bioelimination of heavy metals may account for the decreasing heavy metal concentrations in higher trophic organisms.

Sludge-Grown Algae for Culturing Aquatic Organisms: Part I. Algal Growth in Sludge Extracts

This project is aimed at studying the feasibility of using sewage sludge to prepare culture media for microalgae (Chlorella-HKBU) and the use of the sludge-grown algae as a feed for some aquatic organisms. Part I of the project included results on preparing sludge extracts and their use on algal culture. By comparing two culturing techniques, "aeration" and "shaking," it was noted that both lag and log phases were shortened in the aeration system. A subsequent experiment noted that algal growth subject to aeration rates of 1.0 and 1.5 liters/min had similar lag and log phases. In addition, both aeration rates had a significantly higher (P<0.05) final cell density than that of 0.5 liters/min. A detailed study on the variation of growth conditions on the algal growth was done. The results indicated that pH values of all the cultures declined below 5 at day 12. The removal rates of ammonia N ranged from 62% to 70%. The sludge-grown algae contained a rather substantial amount of heavy metals (&mgr;g/g): Zn 289-581, Cu 443-682, Ni 310-963, Mn 96-126, Cr 25-118, and Fe 438-653. This implied that the rather high levels of heavy metals may impose adverse effects on higher trophic organisms.

A Mechanistic Model of Wormhole Growth in Carbonate Matrix Acidizing and Acid Fracturing

Summary

A mathematical model that describes the growth and competition of wormholes during an acidizing treatment in a carbonate formation was developed. The model is initialized with the distribution of largest pores. Wormhole characteristics (size, length, and distribution) were found to be controlled by acid-injection, diffusion, and fluid-loss rates.

Introduction

Acidizing in a reservoir formation is a stimulation technique that has been widely used by the oil industry. Acidizing treatments fall into two categories: matrix acidizing and fracture acidizing. Fracture acidizing consists of injecting acid solution into a carbonate formation at a high enough pressure to break down the formation hydraulically. As the acid flows along the crack, it reacts with the rock and etches the face of the crack. These acid-etched patterns remain to provide flow channels when hydraulic pressure is released and the well is placed back on production. In matrix acidizing. acid is injected at a slow enough rate that the fracture pressure is not exceeded. Matrix treatments are generally aimed at the removal of near-wellbore damage; in an optimal treatment, the acid is expended primarily in the damaged region. A relatively strong solution of HCl (typically 15 wt%) is the most common acid system used in treating carbonates.

When acid is injected into a carbonate, the acid flows preferentially into the highest-permeability regions-the largest pores, vugs. or natural fractures. The rapid dissolution of the matrix material enlarges these initial flow paths so that the acid has soon formed large, highly conductive flow channels, called wormholes.

Wormholes are likely to occur in both matrix acidizing and fracture acidizing in carbonate formations. In fracture acidizing. wormholes are usually deleterious and must be minimized for live acid to penetrate deeply along the fracture opening. In a matrix-acidizing treatment, these wormholes may or may not be desired; if a deep damaged zone exists around the wellbore, wormholes enhance stimulation by increasing the acid penetration depth. If on the other hand, the damaged zone is shallow, wormhole formation leads to an inefficient treatment because most of the acid will be expended beyond the damaged region. To estimate the fluid leakoff rate in an acid fracture or the effect of matrix acidizing in carbonates, a prediction of the distribution, size, and length of wormholes that will be created is needed. Nierode and Williams developed a model to predict the length of a wormhole, but the number of wormholes and their sizes were not addressed. More recently, Hoefner and Fogiers used a network model to simulate the growth of wormholes in matrix acidizing. This model illustrates the relationship between reaction rate and diffusion rate in the formation of wormholes but does not account for fluid loss through the walls of the wormholes, which can be the controlling factor limiting their length.

To describe the physical process of this wormholing phenomenon, a mathematical model that accounts for the chemical kinetics and fluid hydrodynamics in a wormhole and the initial distribution of large pores was developed. Results from the model confirm that wormholes result from the heterogeneity of carbonate rock and reaction kinetics between HCl and carbonates. Wormhole characteristics are controlled by injection, diffusion, and fluid-loss rates.

The aetiology of gastric ulceration induced by electrical vagal stimulation in rats

Histamine and serotonin levels in gastric secretion and the effects of pharmacological antagonists were studied in rats in which stomach ulceration was induced by electrical vagal stimulation. Electrical vagal stimulation (2 and 5 V) produced a graded increase in haemorrhagic glandular mucosal ulcers. NaHCO3 perfusion completely neutralised the increased acid output but failed to prevent ulceration. Atropine inhibited gastric mast cell degranulation as well as histamine and serotonin release. Diphenhydramine, atropine and sub-diaphragmatic vagotomy antagonised the increase in intragastric pressure. Diphenhydramine, cimetidine, atropine or vagotomy but not methysergide reduced ulcer severity. It is concluded that gastric acid and serotonin do not play an important role in glandular ulceration induced by electrical vagal stimulation. The lesions probably result from increased intragastric pressure and release of gastric histamine which stimulates H1 and H2 receptors in the stomach. The similarities between the aetiologies of glandular ulcers due to electrical vagal stimulation and to stress are also discussed.