| RESEARCH HIGHLIGHTS |


we needed a model that could predict ridership under the regional centers plan.” The team collected data from the city’s


smartcard system on people tapping in and out of individual bus and subway stations over a period of a week — more than 20 million journeys in total. The smartcard data was combined with


city-wide information on how land was being used — for business, industry, residence, water or greenery — and high-resolution maps that identified individual amenities within a set radius of each station. Monterola’s team


trialled three different machine-learning models — computer programs that train themselves through repeated simulations — to find one that first accurately reproduced, and then predicted, transport ridership across the city. “We found that a decision-tree model


performed best, with good accuracy, compu- tational efficiency and an easy-to-follow user display,” says Monterola. “Results indicated that an increase in amenities of up to 55 per cent across the city would increase ridership. Beyond this point, ridership begins to decline;


this is logical because if amenities are available locally, people walk instead.” The high-resolution amenity data proved a


much stronger predictor of ridership than general land-use details; a useful result for informing future urban planning and monitoring Singapore’s regional centers as they develop. The model could be applied to any city with access to similar high-resolution data, notes Monterola.


1. Hu, N., Legara, E. F., Lee, K. K., Hung, G. G. & Monterola, C. Impacts of land use and amenities on public transport use, urban planning and design. Land Use Policy 57, 356–367 (2016).


Nanomaterials


TURNING THE SCREW


A SIMPLE ETCHING TECHNIQUE OFFERS A MEANS FOR CREATING LEFT-HANDED AND RIGHT-HANDED NANOSTRUCTURES


A simple etch method can create nanoscale screws for useful for sensors.


What could be the world’s smallest screws have been fabricated by researchers from A*STAR. The thread on a screw is among the ‘chiral’


structures whose mirror image is different from the original. When reduced to the nanometer scale, these structures could have an important role in nanosensor technology. However, making a screw out of a straight wire is no small task, even in the macroscopic world. Making it on the nanoscale has previously used bottom-up methods that grow or assemble the structure in a gas or solution. But such approaches can be complicated, slow and expensive. Jun Wei from A*STAR’s Singapore Institute of Manufacturing Technology


www.astar-research.com


and co-workers from the A*STAR Institute of Materials Research and Engineering, Nanyang Technological University and Nanjing Tech University in China, devel- oped a simpler method that uses etching techniques to convert a straight nanowire into a screw. The team created 10-micrometer-long


silver nanowires, 80 nanometers in diameter and with five sides. The structures were attached to a silicon substrate and then placed into a solution of silver nitride in ethylene glycol at 80 degrees Celsius for 20 minutes. The sample was then rinsed clean, and the process was repeated five times.


When the resultant wires were imaged using


a scanning transmission electron microscope the team observed smooth ridges and grooves reminiscent of screw threads. Interestingly, such a structure was not evident when a single-step etch was used. Etching usually works along specific


crystallographic directions, leading to symmetric structures, so the team wanted to know how equivalent crystal facets could be etched in an anisotropic way. They propose that this unusual etching mode might begin with the creation of pits at the boundaries between the five crystallographic regions that make up the pentagonal nanowire. These pits


A*STAR RESEARCH 5


Adapted, with permission, from Ref. 1 © 2016 American Chemical Society


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