Origins of Photoluminescence Decay Kinetics in CdTe Colloidal Quantum Dots

Recent experimental studies have identified at least two nonradiative components in the fluorescence decay of solutions of CdTe colloidal quantum dots (CQDs). The lifetimes reported by different groups, however, differed by orders of magnitude, raising the question of whether different types of traps were at play in the different samples and experimental conditions and even whether different types of charge carriers were involved in the different trapping processes. Considering that the use of these nanomaterials in biology, optoelectronics, photonics, and photovoltaics is becoming widespread, such a gap in our understanding of carrier dynamics in these systems needs addressing. This is what we do here. Using the state-of-the-art atomistic semiempirical pseudopotential method, we calculate trapping times and nonradiative population decay curves for different CQD sizes considering up to 268 surface traps. We show that the seemingly discrepant experimental results are consistent with the trapping of the hole at unsaturated Te bonds on the dot surface in the presence of different dielectric environments. In particular, the observed increase in the trapping times following air exposure is attributed to the formation of an oxide shell on the dot surface, which increases the dielectric constant of the dot environment. Two types of traps are identified, depending on whether the unsaturated bond is single (type I) or part of a pair of dangling bonds on the same Te atom (type II). The energy landscape relative to transitions to these traps is found to be markedly different in the two cases. As a consequence, the trapping times associated with the different types of traps exhibit a strikingly contrasting sensitivity to variations in the dot environment. Based on these characteristics, we predict the presence of a sub-nanosecond component in all photoluminescence decay curves of CdTe CQDs in the size range considered here if both trap types are present. The absence of such a component is attributed to the suppression of type I traps

Augmenting the Eye of the Beholder: Exploring the Strategic Potential of Augmented Reality to Enhance Online Service Experiences

Driven by the proliferation of augmented reality (AR) technologies, many firms are pursuing a strategy of service augmentation to enhance customers’ online service experiences. Drawing on situated cognition theory, the authors show that AR - based service augmentation enhances customer value perceptions by simultaneously providing simulated physical control and environmental embedding. The resulting authentic situated experience, manifested in a feeling of spatial presence, funct ions as a mediator and also predicts customer decision comfort. Furthermore, the effect of spatial presence on utilitarian value perceptions is greater for customers who are disposed toward verbal rather than visual information processing, and the positive effect on decision comfort is attenuated by customers’ privacy concerns

A new online state-of-charge estimation and monitoring system for sealed lead-acid batteries in telecommunication power supplies

This paper presents a new, online, battery state-of-charge estimation and monitoring system for both the discharge and charge periods of sealed lead-acid batteries, commonly used in telecommunication power systems as backup power supplies. The proposed monitoring system is based on discharge time versus discharge rate data given in manufacturers' data sheets, and coulometric measurements, without the need for any battery voltage measurement under open-circuit conditions. The corresponding battery management system can perform both the online monitoring and battery charging tasks by the use of the same low-cost 8-bit microcontroller. Battery state-of-charge monitoring tests have been carried out under both constant and dynamic load conditions and for alternate charge-discharge cycles of lead-acid batteries of different capacities, and for different operating temperatures. The test results have shown that a very good accuracy (better than 3%-4%) in the, estimation of available capacity is obtained for various charge-discharge strategies corresponding to practical operating conditions. Changes in battery characteristics which occur due to aging are also taken into account by estimating the actual capacity of the battery and adopting the corresponding recharacterization of the aged cells automatically

Digital control of universal telecommunication power supplies using dual 8-bit microcontrollers

Design and implementation of a digital controller for universal telecommunication power supply modules are presented. The paper emphasises on converter control strategy, and its implementation by means of parallel operated, dual 8-bit microcontrollers. One of the microcontrollers is employed for user interface purposes, such as long term records, display and alarm facilities, which are inherently slow processes. The fast processing speed required by output voltage setting, current limit, and load current sharing however, is fulfilled by the second microcontroller, which adjusts the output voltages of modules. The proposed dual-processor based digital controller provides an extended operating output voltage range, user programmable current limit setting, serial-communi cation based active load current sharing between modules, and automatic derating without extra hardware requirement. Overload and output short-circuit protection features are also software-controlled. The controller is tested successfully on six parallel operating, 1.8 M, 0-70V telecommunication power supplies

DOUBLE-OUTPUT INDUCTION GENERATOR OPERATING AT SUBSYNCHRONOUS AND SUPERSYNCHRONOUS SPEEDS - STEADY-STATE PERFORMANCE OPTIMIZATION AND WIND-ENERGY RECOVERY

The paper covers the steady-state analysis of a wound-rotor induction generator operated at varying shaft speeds in the subsynchronous and supersynchronous regions, by control of both the magnitude and direction of slip power. A modified equivalent circuit is used in the analyses in which core losses and harmonics are ignored. The resulting nonlinear algebraic equations are solved numerically. An optimum control strategy, which maximises the total electrical power output of the double output induction generator is determined, and theoretical results are verified experimentally with particular emphasis on the system including naturally commutated convertors. The limitations of naturally commutated convertor circuits and their effects on the output power characteristic of the system are also discussed. This system shows considerable advantage in the field of wind-energy conversion. Its performance is optimised on the basis of annual energy production by calculating optimum values of gear ratio and generator size for the given turbine characteristic and site wind regime. Also, a comparison is made among optimised versions of alternative induction generator schemes used in wind-energy conversion systems, on the bases of annual energy production and transfer characteristic for the same site and turbine

Performance evaluation of a wind driven DOIG using a hybrid model

This paper presents the performance analysis of a wind driven double output induction generator (DOIG) operating at varying shaft speeds. A periodic transient state analysis of DOIG equipped with two controlled converters is carried out using a hybrid induction machine model. It is shown that practical aspects of converters such as overlap and harmonics reduce the power output of the DOIG system and limit the operating shaft speed range, especially in the subsynchronous region near the synchronous speed. Validity of the mathematical model used in the analyses and the corresponding computer simulation results are verified experimentally

Active clamped ZVS forward converter with soft-switched synchronous rectifier for high efficiency, low output voltage applications

The analysis, design, and implementation of an active clamped ZVS forward converter equipped with a soft-switched synchronous rectifier (ACFC-SR), proposed for high-efficiency low output voltage DC-DC converter applications, is presented. The converter efficiency is maximised due to soft switching of the main, active clamp, synchronous rectifier, and freewheeling MOSFET switches. The operating principles of the ACFC-SR are analysed in detail, and the converter performance is compared with that of alternative forward converter schemes employed in low output voltage DC-DC converters, in view of their power conversion efficiency. Experimental results are presented for a 50 W ACFC-SR converter with a DC input voltage of 48 V, an output voltage of 5 V, and operating at a switching frequency of 120 kHz. In general, good agreement is observed between the theoretical and experimental results

COMMUTATION ANGLE ANALYSIS OF A DOUBLE OUTPUT INDUCTION GENERATOR OPERATING IN SUB-SYNCHRONOUS AND SUPER-SYNCHRONOUS MODES

A rigourous performance analysis of the double autpt indoction generator system with two controlled convaters hs been carried out including the effects of both the rotor side amvukr’s commutation an& and harmonics as well as the supply side converter’s harmonics. This is achieved by means of the hybrid induction machine model which allows for the changing states of conduction of the rotor side converter’s thyristors. The variations in overlapping angle of the rotor side convertex is obtained for a wide range of shaft speeds and for various firing delay angles

Active clamped ZVS forward converter with soft-switched synchronous rectifier for maximum efficiency operation

An active-clamped, zero-voltage switched forward converter equipped with a soft-switched synchronous rectifier is designed and implemented for some low output voltage applications where maximized efficiency is of utmost importance. The converter efficiency is maximized due to soft-switching of the main, active clamp and the synchronous rectifier MOSFET switches. Experimental results are presented for a converter with a de input voltage of 48V, an output voltage of 5V and a de electronic load up to 10A. The measured efficiency was 92% for an output power of 50 W