A 2-D PVDF force sensing system for micro-manipulation and micro-assembly

Despite the enormous research efforts in creating new applications with MEMS, the research efforts at the backend such as packaging and assembly are relatively limited. One reason for this is the level of difficulty involved. One fundamental challenge lies in the fact that at micro-scale, micro mechanical structures are fragile and easy to break - they typically will break at the micro-Newton (μN or 10-6N) force range, which is a range that cannot be felt by human operators. In this paper, we will present our ongoing development of a polyvinylidence fluoride (PVDF) multi-direction micro-force sensing system that can be potentially used for force-reflective manipulation of micro-mechanical devices or micro-organisms over remote distances. Thus far, we have successfully demonstrated 1D and 2D sensing systems that are able to sense force information when a micro-manipulation probe-tip is used to lift a micro mass supported by 2μm×30μm×200μm polysilicon beams. Hence, we have shown that force detection in the 50μN range is possible with PVDF sensors integrated with commercial micro-manipulation probe-tips. We believe this project will eventually make a great impact to the globalization of MEMS foundries because it will allow global users to micro-assemble and micro-manipulate surface micromachined devices from their laboratories, and therefore, reduce the time from design to production significantly.Link_to_subscribed_fulltex

Improving the detectability of CNT based infrared sensors using multi-gate field effect transistor

Carbon nanotube (CNT) is a novel one dimensional (1D) material that has unique electrical and optoelectronic properties. Photo-sensors using CNT can sense infrared signals by using Schottky barriers between metal and nanotube, which are able to separate photo-generated electron-hole pairs in order to generate photocurrent or photovoltage for detection and quantification. It has been demonstrated that both asymmetric metal structure and electrical field can improve the performance of the sensors by manipulating the energy alignment between metal and CNT. However, it is not clear how to optimize the design of the CNT photo-sensors. An asymmetric multi-gate field effect transistor based infrared detector was fabricated, integrating with asymmetric metal structure (Au-CNT-Al) and multiple gates, which allow for controlling the doping level at source, drain and channel independently. It was found that dark current was suppressed and photocurrent was enhanced by applying negative gate voltages, thus improving sensor's performance. The CNT detector exhibited similar photo-response when modulating the doping level of CNT segments at source, drain and bulk. We ascribe this to the charge distribution that has a long tail extending over the whole tube. ©2010 IEEE.Link_to_subscribed_fulltex

Development of infrared detectors using single carbon-nanotube-based field-effect transistors

Carbon nanotube is a promising material to fabricate high-performance nanoscale-optoelectronic devices owing to its unique 1-D structure. In particular, different types of carbonnanotube- infrared detectors have been developed. However, most previous reported carbon-nanotube-IR detectors showed poor device characteristics due to limited understanding of their working principles. In this paper, three types of IR detectors were fabricated using carbon-nanotube field effect transistors (CNTFETs) to investigate their performance: 1) symmetric Au-CNT-Au CNTFET IR detector; 2) symmetric Ag-CNT-Ag CNTFET IR detector; and 3) asymmetric Ag-CNT-Au CNTFET IR detector. The theoretical analyses and experimental results have shown that the IR detector using an individual single-wall carbon nanotube (SWCNT), with asymmetric Ag-CNT-Au CNTFET structure, can suppress dark current and increase photocurrent by electrostatic doping. As a result, an open-circuit voltage of 0.45 V under IR illumination was generated, which is the highest value reported to date for an individual SWCNT-based photodetector. The results reported in this paper have demonstrated that the CNTFET can be used to develop high-performance IR sensors.Link_to_subscribed_fulltex

Transmission of multimedia information on micro environment via Internet

Recent developments in the Internet have significantly increased the human capability to obtain information from and reach remote locations. In parallel, the Micro Mechanical and Electronic Systems (MEMS or Microsystems) technology makes it possible for the human to sense and act in a microenvironment. Combined, the Internet and MEMS, will produce a new technology for humans to sense and act in a remote micro-environment. This new technology has potential impact on several fields one of which is biomedical engineering. In this paper, we will present our ongoing development of polyvinylidence fluoride (PVDF) micro-tips as rate-of-force sensors for tele-micromanipulation force-feedback control of micro-mechanical devices over the Internet. Internet based teleoperation between Hong Kong and Michigan State was demonstrated using an event-based control scheme, which ensured stability and synchronization. Control was enhanced by multimedia feedback, which included, in addition to the traditional video, force feedback. The force fed back augmented the limited information supplied by visual feedback. However, the use of force feedback for microenvironments has difficulties, such as proper scaling, that should be carefully studied. We believe this project will eventually make a great impact to the globalisation of MEMS foundries because it will allow global users to micro-assemble and manipulate surface micromachined devices from their laboratories, and hence, reduce the time from design to production significantly.Link_to_subscribed_fulltex

Development of carbon nanotube based spectrum infrared sensors

Carbon nanotube (CNT) has been found to be one of the promising semi-conducting materials for nanoelectronic devices and sensors due to its unique electrical properties. Our group has developed a spectrum sensor using a single CNT and demonstrated its performance. In this paper, a steady and high-yield CNT bandgap engineering will be discussed and it has been used to manufacture an appropriate CNT for spectrum infrared detection. The fabrication and experimental result of the CNT based spectrum sensor will be presented. The results indicated the CNT based spectrum sensors are capable to sense near-infrared (IR) signal and middle-wave IR signal in room temperature environment. © 2008 IEEE.Link_to_subscribed_fulltex

Analysis and design of carbon nanotube based field effect transistors for nano infrared sensors

It has been demonstrated that carbon nanotube field effect transistor (CNTFET) is a promising device to improve the performance of carbon nanotube (CNT) infrared (IR) sensors by modulating the doping level of the CNT channel. However, how tooptimize the performance of the CNTFET based IR sensor is not well understood. In particularly, there was limited study on thedesign of transistor's gate structure, which determines its energyband profile. In order to improve our understating of its detection mechanism and guide for an optimized design, a multi-gateCNTFET was fabricated, which can control the doping level of the source, drain and nanotube channel independently. Photovoltaic characteristics were observed in bias dependent measurement by varying the intensity of incident IR source. In addition, the polarization dependent measurement was conducted, the detector showed clearly polarization anisotropy, and highest photocurrent was obtained when the polarization is parallel to the nanotube. Its temporal photocurrent responses using different gates were investigated. The results showed that photocurrent had similar photo-responses by applying voltages to the gates at different positions, implying the sensor design can be simplified into a single gate. © 2010 IEEE.Link_to_subscribed_fulltex

Carbon nanotube-based color IR detectors

Carbon nanotube (CNT) has been found to be one of the promising materials for efficient detection and used in different nanoelectronic devices due to its unique electrical properties. Recently, the applications of nanostructural material to infrared (IR) sensors are considered. Our group developed a color detector using a single CNT and demonstrated the good sensitivity of CNT to the infrared light in different ranges. In this report, the CNT bandgap engineering was discussed. The design, fabrication and experimental result of the CNT based color detector were described. The results indicated the heterogeneous electrode structure increased the signal-to-dark current ratio. Moreover, the CNT based color detectors were capable to sense near-infrared signal and middle-infrared signal in room temperature environment.Link_to_subscribed_fulltex

Perceived peer victimization and ethnic school context: the impact of interethnic climate

In western countries, there is concern about ethnic school segregation, as recent studies find indications that it is unfavorable for educational achievement (eg. Sweden: Szulkin & Jonsson, 2006; the Netherlands: Karsten et al., 2006). However, other studies have pointed to the flip side of this picture, arguing that school desegregation might have negative consequences for non-cognitive outcomes, such as peer victimization (Hanish & Guerra, 2000). When educational policies do not consider these potential adverse consequences, they run the risk to fail. Therefore, educational research should identify and explain these negative effects of desegregation, in order to counteract them and make desegregation policies work. In this contribution, we focus on perceived peer victimization as a possible effect of ethnic school composition. Studies investigating this, have found widely divergent results, and have still not come to an understanding of this relationship (Graham, 2006; Hanish & Guerra, 2000; Juvonen, et al., 2006; Verkuyten & Thijs, 2002; Vervoort, Scholte, & Overbeek, 2008). One of the factors responsible for this, can be the differing operationalization of ethnic composition – some use the proportion of students from a certain ethnic group (Vervoort et al., 2008), others a calculated index of heterogeneity (Graham, 2006; Juvonen et al., 2006). To counter this, we use both constructs simultaneously. For our analyses, we draw upon three theoretical frameworks. Firstly, we use the ‘Imbalance of Power’-thesis, that states that victimization is most likely to occur when ethnic groups are not equally large (Graham, 2006; Juvonen et al., 2006). This explanation starts from the most defining characteristic of victimization: the imbalance of power that exists between bully and victim. These actors are said to draw their power from the numerical strength of their ethnic group: if the own group is larger, they have more power, and vice versa. However, this new line of reasoning counters the well-established approach of group threat theory (Blalock, 1967), that states that interethnic conflict is the highest in schools where the ethnic groups are roughly as large: in those contexts, it is not clear which group is in charge, and, as bullying is a strategy to acquire social dominance (Demanet, 2008), it is used to resolve the matter. As a third theory, we use constrict theory (Putnam, 2007). According to this theory, the amount of ethnic diversity in a given context triggers social anomie or social isolation. As such, this theory can be used to hypothesize that victimization rates are higher in schools with a larger ethnic heterogeneity. The above stated theories contend that the relation between ethnic composition and peer victimization is not a mechanical one, but is moderated by intermediate processes. Especially certain factors of the school interethnic climate are said to mediate the relationship between ethnic composition of the school and victimization. In our contribution, we test these hypotheses as well. More specifically, we test whether the amount of interethnic conflict at school, the number of interethnic friendships in a school, and the amount of multicultural education at school indeed act as mediating mechanisms. We used data gathered in 2008–2009 from 2485 pupils in a sample of 68 primary schools in Flanders as part of Segregation in Primary Education in Flanders (SIPEF) project. Given the data at different levels - variables of ethnic school composition as determinants at school level and perceived peer victimization as outcome at pupil level, the use of multilevel modeling is appropriate. The models are estimated separately for migrants and natives. The determinants are entered stepwise in the model, to determine if selection and mediating effects occur. In the first step, we estimate effects of the composition variables – ethnic minority concentration and heterogeneity -, followed by control variables in the second. In the third model we add the amount of interethnic conflict and friendships at school; in the fourth the amount of multicultural education. In the fifth, these interethnic climate variables are entered simultaneously. The results indicate that ethnic composition indeed has its influences on the rates of peer victimization, if only for migrants: for natives, it exerts no influence whatsoever. Migrants have a significantly lower chance of being victimized in schools where they comprise 80% to 100% of the student population, compared to schools where they are largely outnumbered by native students. This supports the expectations from the ‘Imbalance of Power’-thesis. The ethnic heterogeneity appears to have no relation to victimization, contradicting our expectations from constrict theory. There are indications that interethnic climate is in part responsible for the observed effects. Especially the amount of interethnic conflict at school appears as an important moderator: there seems to be less interethnic conflict in schools with a majority of migrant students, which is responsible for the decreased rates of victimization in those contexts. Counter intuitively, the more multicultural education there is in a school, the more students report to be bullied, probably because the presence of multicultural education in those schools raise the students’ awareness to racist victimization. This finding is in line with the study by Verkuyten and Thys (2002), who give the same plausible explanation