Ultrathin Highly Luminescent Two‐Monolayer Colloidal CdSe Nanoplatelets

Surface effects in atomically flat colloidal CdSe nanoplatelets (NLPs) are significantly and increasingly important with their thickness being reduced to subnanometer level, generating strong surface related deep trap photoluminescence emission alongside the bandedge emission. Herein, colloidal synthesis of highly luminescent two‐monolayer (2ML) CdSe NPLs and a systematic investigation of carrier dynamics in these NPLs exhibiting broad photoluminescence emission covering the visible region with quantum yields reaching 90% in solution and 85% in a polymer matrix is shown. The astonishingly efficient Stokes‐shifted broadband photoluminescence (PL) emission with a lifetime of ≈100 ns and the extremely short PL lifetime of around 0.16 ns at the bandedge signify the participation of radiative midgap surface centers in the recombination process associated with the underpassivated Se sites. Also, a proof‐of‐concept hybrid LED employing 2ML CdSe NPLs is developed as color converters, which exhibits luminous efficacy reaching 300 lm W_opt^?1. The intrinsic absorption of the 2ML CdSe NPLs (≈2.15 × 10⁶ cm^?1) reported in this study is significantly larger than that of CdSe quantum dots (≈2.8 × 10⁵ cm^?1) at their first exciton signifying the presence of giant oscillator strength and hence making them favorable candidates for next‐generation light‐emitting and light‐harvesting applications.     

Cascode Monolithic DC-DC Converter for Reliable Operation at High Input Voltages

A CMOS OTA-C low-pass notch filter for EEG application is described. The pass-band covers four bands of brain wave and provides more than 65 dB attenuation for the 50 Hz power line interference. The OTA works in the weak inversion region and a low transconductance of 3 nA/V is achieved. The low transconductance enables using small capacitors in the OTA-C filter so that the filter is suitable for the multi-channel EEG integrated circuits. The measured results show the good performance of the filter for filtering the noise in acquired EEG signals. Xinbo Qian received the B.Sc. degree from Beijing Institute of Technology, P.R. China, in 1991 and M.Sc. degree from Institute of Physics, Chinese Academy of Sciences, in 1996. From 1996 to 1999, she was a research engineer in the Institute of Acoustics, Chinese Academy of Sciences, worked on the sonar signal receiving and processing systems. Since 1999, she has been pursuing the Ph.D. degree in Electrical and Computer Engineering department, National University of Singapore, with research direction on on-chip readout circuits for microbolometer focal plane arrays. Now she is employed by Department of Mechanical Engineering and Division of Bioengineering, National University of Singapore as a research fellow. Her research interest is low-noise integrated circuits design and bio-medical sensor electronics, including electroencephalography IC, magnetocardiography IC, low-noise amplifier, filter and data converters etc. Yong Ping Xu graduated from Nanjing University, P.R. China in 1977. He received his Ph.D. from University of New South Wales (UNSW) Australia, in 1994. From 1978 to 1987, he was with Qingdao Semiconductor Research Institute, P.R. China, initially as an IC design engineer, and later the deputy R&D manager and the Director. From 1989 to 1992, he was working on silicon diode based infrared detectors towards his Ph.D. at School of Electrical Engineering, UNSW Australia. From 1993 to 1995, he worked on an industry collaboration project with GEC Marconi, Sydney, Australia, at the same university, involved in design of sigma-delta ADCs. He was a lecturer at University of South Australia, Adelaide, Australia from 1996 to 1998. He has been with the Department of Electrical and Computer Engineering, National University of Singapore since June 1998 and is now an Associate Professor. His general research interests are in the areas of mixed-signal and RF integrated circuits, and integrated MEMS and sensing systems. His current focuses are high-speed wideband ADC, UWB front-end circuits and low-power low-voltage integrated circuits for biomedical applications. He is a Senior Member of IEEE. Xiaoping Li received his Ph.D. degree from Department of Mechanical and Manufacturing Engineering, University of New South Wales, Australia in 1991, and joined the National University of Singapore in 1992, where he is currently an Associate Professor with the Department of Mechanical Engineering and Division of Bioengineering. He was a visiting professor of Tokyo Institute of Technology, Japan in 2000, and visiting professor of Georgia Institute of Technology, USA in 2001. He is a member of American Society of Mechanical Engineers (ASME), a senior member of Society of Manufacturing Engineering (SME) and a senior member of North American Manufacturing Research Institute/SME, and is currently the Chairman of SME Singapore Chapter. His current research interests include neurosensors and nanomachining. He is a guest editor of International Journal of Computer Applications in Technology, USA. He is a regular reviewer of the ASME Journal of Manufacturing Engineering, USA, Transactions of NAMRI/SME, USA, Journal of materials processing technology, UK, International Journal of Machine Tools and Manufacture, UK, and IMechE Journal of Engineering Manufacture, UK.     

Coreless Fiber‐Based Whispering‐Gallery‐Mode Assisted Lasing from Colloidal Quantum Well Solids

Whispering gallery mode (WGM) resonators are shown to hold great promise to achieve high‐performance lasing using colloidal semiconductor nanocrystals (NCs) in solution phase. However, the low packing density of such colloidal gain media in the solution phase results in increased lasing thresholds and poor lasing stability in these WGM lasers. To address these issues, here optical gain in colloidal quantum wells (CQWs) is proposed and shown in the form of high‐density close‐packed solid films constructed around a coreless fiber incorporating the resulting whispering gallery modes to induce gain and waveguiding modes of the fiber to funnel and collect light. In this work, a practical method is presented to produce the first CQW‐WGM laser using an optical fiber as the WGM cavity platform operating at low thresholds of ≈188 µJ cm^?2 and ≈1.39 mJ cm^?2 under one‐ and two‐photon absorption pumped, respectively, accompanied with a record low waveguide loss coefficient of ≈7 cm^?1 and a high net modal gain coefficient of ≈485 cm^?1. The spectral characteristics of the proposed CQW‐WGM resonator are supported with a numerical model of full electromagnetic solution. This unique CQW‐WGM cavity architecture offers new opportunities to achieve simple high‐performance optical resonators for colloidal lasers.     

Polyester composites reinforced with noncrimp stitched carbon fabrics: Mechanical characterization of composites and investigation on the interaction between polyester and carbon fiber

The primary purpose of the study is to investigate the anisotropic behavior of different noncrimp stitched fabric (NCF) reinforced polyester composites. Carbon fiber composite laminates were manufactured by vacuum infusion of polyester resin into two commonly used advanced noncrimp stitched carbon fabric types, unidirectional and biaxial carbon fabric. The effects of geometric variables on composite structural integrity and strength were illustrated. Hence, tensile and three‐point bending flexural tests were conducted up to failure on specimens strengthened with different layouts of fibrous plies in NCF. In this article an important practical problem in fibrous composites, interlaminar shear strength as measured in short beam shear tests, is discussed. The fabric composites were tested in three directions: at 0°, 45°, and 90°. Extensive photomicrographs of multilayered composites resulting from a variety of uniaxial loading conditions were presented. It was observed that broken fibers recede within the matrix in composites with weak interfacial bond. Another aim of the present work was to investigate the interaction between carbon fiber and polyester matrix. The experiments, in conjunction with scanning electron photomicrographs of fractured surfaces of composites, were interpreted in an attempt to explain the instability of polyester‐resin–carbon‐fiber interfaces. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 4554–4564, 2006     

Energy-optimal trajectory planning for car-like robots

When a battery-powered robot needs to operate for a long period of time, optimizing its energy consumption becomes critical. Driving motors are a major source of power consumption for mobile robots. In this paper, we study the problem of finding optimal paths and velocity profiles for car-like robots so as to minimize the energy consumed during motion. We start with an established model for energy consumption of DC motors. We first study the problem of finding the energy optimal velocity profiles, given a path for the robot. We present closed form solutions for the unconstrained case and for the case where there is a bound on maximum velocity. We then study a general problem of finding an energy optimal path along with a velocity profile, given a starting and goal position and orientation for the robot. Along the path, the instantaneous velocity of the robot may be bounded as a function of its turning radius, which in turn affects the energy consumption. Unlike minimum length paths, minimum energy paths may contain circular segments of varying radii. We show how to efficiently construct a graph which generalizes Dubins’ paths by including segments with arbitrary radii. Our algorithm uses the closed-form solution for the optimal velocity profiles as a subroutine to find the minimum energy trajectories, up to a fine discretization. We investigate the structure of energy-optimal paths and highlight instances where these paths deviate from the minimum length Dubins’ curves. In addition, we present a calibration method to find energy model parameters. Finally, we present results from experiments conducted on a custom-built robot for following optimal velocity profiles.     

Matrix Recipes for Hard Thresholding Methods

In this paper, we present and analyze a new set of low-rank recovery algorithms for linear inverse problems within the class of hard thresholding methods. We provide strategies on how to set up these algorithms via basic ingredients for different configurations to achieve complexity vs. accuracy tradeoffs. Moreover, we study acceleration schemes via memory-based techniques and randomized, ?-approximate matrix projections to decrease the computational costs in the recovery process. For most of the configurations, we present theoretical analysis that guarantees convergence under mild problem conditions. Simulation results demonstrate notable performance improvements as compared to state-of-the-art algorithms both in terms of reconstruction accuracy and computational complexity.     

Energy‐efficient Path Planning for Solar‐powered Mobile Robots*

We explore the problem of energy‐efficient, time‐constrained path planning of a solar‐powered robot embedded in a terrestrial environment. Because of the effects of changing weather conditions, as well as sensing concerns in complex environments, a new method for solar power prediction is desirable. We present a method that uses Gaussian Process regression to build a solar map in a data‐driven fashion. Using this map and an empirical model for energy consumption, we perform dynamic programming to find energy‐minimal paths. We validate our map construction and path‐planning algorithms with outdoor experiments, and we perform simulations on our solar maps to further determine the limits of our approach. Our results show that we can effectively construct a solar map using only a simple current measurement circuit and basic GPS localization, and this solar map can be used for energy‐efficient navigation. This establishes informed solar harvesting as a viable option for extending system lifetime even in complex environments with low‐cost commercial solar panels.     

Artificial neural networks–based backcalculation of the structural properties of a typical flexible pavement

Pavement evaluation is one of the foremost phases in all pavement engineering activities. In the backcalculation process, the researcher or the engineer varies the structural properties of the layers until the theoretical (calculated) deflections and the obtained (measured) deflections from FWD tests are closely matched to each other within a tolerable limit. However, this process is substantially time-consuming and poses some difficulties due to inherent inaccuracies in the results. In this study, synthetically derived deflections from a typical flexible pavement are used to estimate asphaltic concrete layer’s elastic modulus, Poisson’s ratio and thickness. Furthermore, artificial neural network (ANN) is utilized to determine the structural parameters, and it can be clearly seen that satisfactory results are obtained. ANN estimation of the three pavement layer characteristic parameters, that is, layer elastic modulus, Poisson’s ratio and layer thickness, was carried out for the first time in the study.     

A Novel Nonlinear Acoustic Health Monitoring Approach for Detecting Loose Bolts

To date, sensors have been the inevitable component of structural health monitoring (SHM) systems. Typically, sensory signals are digitized, processed by computers, and then the information is presented to the operator with plots or warnings depending on the sophistication of the system. This study proposes a novel nonlinear acoustic health monitoring (NAHM) approach for detection of loose bolts, which can work with and without any sensors. The structure is excited with bitonal excitations, which their difference is in the audible range. When the bolts are well tightened, the structure remains silent. But, the structure creates audible sound or verbal warnings in the presence of one or more loose bolts. There is no need for sensor(s), A/D converters or computers between the operator and the structure. However, it is also possible to attach a piezoelectric sensor or to use a microphone/sound level meter for further analysis of the structure’s response. The feasibility of the concept was demonstrated by detecting the loose bolt in a bolted plate system. For demonstrating the industrial potential of the proposed NAHM system, the concept was implemented for two simple washers held with nuts and bolts. Additionally, the intensities of the audible alarms were studied at different torque levels. The proposed NAHM may be used as a low-cost sensor-free SHM or as a backup for conventional nonlinear SHM systems.     

A geographic information system‐based model for economical and eco‐friendly high‐speed railway route determination using analytic hierarchy process and least‐cost‐path analysis

In this study, geographic information system (GIS)‐based network analysis and analytic hierarchy process (AHP) methods were applied to the Erzincan–Trabzon segment of the Turkish high‐speed railway (HSR) project. A new hybrid route, which considers both economic and environmental criteria, was created and compared with three different routes from various preconstruction studies. The route‐generating analysis included the eight parameters (criteria) of slope, geology, soil quality, rivers, protected areas, roads, land cover, and lakes. The optimum route for connecting the port of Trabzon to Turkey's interior via HSR was generated. The weights for determining the route were tested for sensitivity. The study demonstrated the efficiency of GIS and AHP integration in generating optimum routes for HSR projects according to given databases. All these GIS and AHP analyses were applied automatically with our novel GIS extension for determining optimum HSR routes. The final route has preserved cultural and historical structures, environmental integrity. The new hybrid route also decreased construction costs by approximately 12%, and the least‐cost‐path analysis confirmed the environmental efficiency of the route.