DNA-decorated graphene chemical sensors

Graphene is a true two dimensional material with exceptional electronic properties and enormous potential for practical applications. Graphene's promise as a chemical sensor material has been noted but there has been relatively little work on practical chemical sensing using graphene, and in particular how chemical functionalization may be used to sensitize graphene to chemical vapors. Here we show one route towards improving the ability of graphene to work as a chemical sensor by using single stranded DNA as a sensitizing agent. The resulting broad response devices show fast response times, complete and rapid recovery to baseline at room temperature, and discrimination between several similar vapor analytes.Comment: 7 pages, To appear in Applied Physics Letter

High On/Off Ratio Graphene Nanoconstriction Field Effect Transistor

We report a method to pattern monolayer graphene nanoconstriction field effect transistors (NCFETs) with critical dimensions below 10 nm. NCFET fabrication is enabled by the use of feedback controlled electromigration (FCE) to form a constriction in a gold etch mask that is first patterned using conventional lithographic techniques. The use of FCE allows the etch mask to be patterned on size scales below the limit of conventional nanolithography. We observe the opening of a confinement-induced energy gap as the NCFET width is reduced, as evidenced by a sharp increase in the NCFET on/off ratio. The on/off ratios we obtain with this procedure can be larger than 1000 at room temperature for the narrowest devices; this is the first report of such large room temperature on/off ratios for patterned graphene FETs.Comment: 18 pages, 6 figures, to appear in Smal

Analysis of Picker Blocking in Narrow-aisle Batch Picking

This study analyzes impacts by batch picking on picker blocking in narrow-aisle order picking, and determines appropriate batch formations for a better order picking throughput. We present multiple-pick analytical models to offer insights about picker blocking in batch picking. Several simulation studies over a variety of batching situations scrutinize order batching situations which give throughput benefits in narrow-aisle configurations by satisfying the analytical results. Our results highlight three findings for narrow-aisle batch picking processes: 1) variation of pick probability in batch picking is inevitable and is a primary driver of picker blocking; 2) a near-optimal distance-based batching algorithm can experience less picker blocking than expected, because it reduces both the number of aisles visited and the variation in the number of picks per aisle; and 3) the sorting strategy itself (i.e., pick-then-sort or sort-while-pick) causes varying amounts of congestion

Order Batching With Time Constraints in a Parallel-aisle Warehouse: a Multiple-policy Approach

A commitment of delivery time is critical in some online businesses (De Koster, 2003). An important challenge to meeting customers’ needs is timely order picking which is also relevant to worker safety, item freshness, overall operational synchronization, and reduced overtime. We analyze an order batch picking situation where a trip is constrained by vehicle capacity and must be completed within a specified time. We develop a model which partitions orders to batches to minimize the total travel time such that each trip meets the orders’ time constraints and capacity limit, and also determines a suitable operational policy for each batch. Each policy is characterized by routing method, travel speed, capacity, and pick time. The proposed batching model can simultaneously group orders and can select a best policy among possible policy choices for each batch. To solve the proposed batching procedure, an exact algorithm is implemented based on a branch-and-price method. Our multiple-policy approach experiences 2.1~7.0% reductions in retrieval time compared to a best single-policy approach. The experimental results emphasize that when time constraints are enforced in order batching, a multiple-policy is preferable to a single-policy approach, because allows additional flexibility

What Do Students Experience as Peer Leaders of Learning Teams?

In a course for engineering freshmen, peer leaders facilitated optional study sessions, which implemented peer-led team learning workshops. Some leaders were paid teaching assistants, but most were undergraduate volunteers. To understand the experiences of the peer leaders, we asked them to keep weekly reflective journals. By performing a basic qualitative analysis of fourteen journals from two semesters, we developed a description of the experience of leading peer-led team learning workshops over the course of the semester. At the beginning of the semester, the leaders were apprehensive about teaching and concerned with correctly answering students’ questions. As the semester progressed, the leaders were often frustrated with the difficulty of teaching, and the leaders tried new ways of encouraging student participation. At the end of the semester, the leaders reported that they increased self-confidence, developed an appreciation for intellectual diversity, and gained an increased interest in teaching

Evidence for 20th Century Climate Warming and Wetland Drying in the North American Prairie Pothole Region

The Prairie Pothole Region (PPR) of North America is a globally important resource that provides abundant and valuable ecosystem goods and services in the form of biodiversity, groundwater recharge, water purification, flood attenuation, and water and forage for agriculture. Numerous studies have found these wetlands, which number in the millions, to be highly sensitive to climate variability. Here, we compare wetland conditions between two 30-year periods (1946–1975; 1976–2005) using a hindcast simulation approach to determine if recent climate warming in the region has already resulted in changes in wetland condition. Simulations using the WETLANDSCAPE model show that 20th century climate change may have been sufficient to have a significant impact on wetland cover cycling. Modeled wetlands in the PPR’s western Canadian prairies show the most dramatic effects: a recent trend toward shorter hydroperiods and less dynamic vegetation cycles, which already may have reduced the productivity of hundreds of wetland-dependent species