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28 SMT Magazine • January 2015 all required connections are completed. While there are nuance elements missing from this description, the practical results are fundamen- tally a working electronic assembly built in a matter of a couple of hours rather than days, weeks or months. In summary, 3D printing is coming on strong as a method for making rapid prototypes for many purposes and markets. Electronic products are unlikely to be left out of consider- ation when the power of time conservation gets the attention it so richly deserves. SMT 3D PrInTInG In ELECTrOnICS—a PErSPECTIvE continues Verdant Electronics Founder and president Joseph (Joe) Fjelstad is a four-decade vet- eran of the electronics industry and an international authority and innovator in the field of electronic interconnection and packaging technologies. Fjelstad has more than 250 u.S. and international patents issued or pending and is the author of Flexible Circuit Technology. a team of researchers led by north carolina State university has found that stacking ma- terials that are only one atom thick can create semiconductor junctions that transfer charge efficiently, regardless of whether the crystalline structure of the materials is mismatched—lower- ing the manufacturing cost for a wide variety of semiconductor devices such as solar cells, lasers, and lEDs. "This work demonstrates that by stacking multiple 2D materials in random ways, we can create semiconductor junctions that are as func- tional as those with perfect alignment," says Dr. linyou cao, senior author of a paper on the work and an assistant professor of materials sci- ence and engineering at NC State. "This could make the manufacture of semi- conductor devices an order of magnitude less expensive." For most semi- conductor electronic or photonic devices to work, they need to have a junction, which is where two semiconductor ma- terials are bound to- gether. For example, in photonic devices like solar cells, lasers and lEDs, the junction is where photons are converted into electrons, or vice versa. all semiconductor junctions rely on efficient charge transfer between materials, to ensure that current flows smoothly and that a minimum of energy is lost during the transfer. To do that in conventional semiconductor junctions, the crystalline structures of both materials need to match. However, that limits the materials that can be used, because you need to make sure the crystalline structures are compatible. and that limited number of material matches restricts the complexity and range of possible functions for semiconductor junctions. "But we found that the crystalline structure doesn't matter if you use atomically thin, 2D ma- terials," cao says. "We used molybdenum sulfide and tungsten sulfide for this experiment, but this is a fundamental discovery that we think applies to any 2D semiconductor material." 2D Materials Stacked for Cheaper Semiconductor Devices Feature

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