Nano Focus: Scanning thermoelectric microscopy locates extra electrons outside quantum dots
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emiconducting quantum dots (QDs) can be used to enhance the performance of a variety of devices encompassing optoelectronic, thermoelectric, and alternative energy technologies. Often, a small amount of another element must be added to the semiconducting QDs to provide extra electrons and improve conductivity. At the nanoscale, these extra electrons are difficult to locate. Rachel Goldman and her colleagues at the University of Michigan have now shown the feasibility of such measurements. As reported in the May 11 issue of Applied Physics Letters (DOI: 10. 1063/1.4919919; 192101), the researchers used scanning thermoelectric microscopy to locate electrons in semiconducting QDs on surfaces. Beginning with high-purity solid Ga, As2, and In as the raw materials, they used molecular beam epitaxy and employed Stranski– Krastanov growth to fabricate InAs QDs on a GaAs substrate. Such structures can be doped with another element, such as Si, to provide extra electrons. However, it is difficult to predict how many of the dopants will incorporate into a QD rather than into the surrounding layers. Each QD is believed to contain fewer than 10
In pursuit of novel superconductors: Four years and ~1000 materials
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ver 100 previously unknown superconducting materials ranging from new iron-based superconductors (IBSCs or pnictide semiconductors) to titaniumand cobalt-based ones are listed in a review article by Hideo Hosono and colleagues, published in the June issue of Science and Technology of Advanced Materials (DOI: 10.1088/1468-6996/16/3/033503). The article summarizes the results of a four-year long research project funded as part of FIRST (Funding Program for
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dopant atoms, making them particularly challenging to locate. The researchers then used a scanning thermoelectric microscope (SThEM) with a specially prepared tungsten tip. The measurements were performed in ultrahigh vacuum, and the sample was heated a few Kelvins above room temperature for several hours to achieve a uniform temperature in the QD, surrounding layers, and substrate. Upon contact, the SThEM tip locally cools the QD, causing the extra electrons in the hot sample to travel toward the cold tip, generating a thermoelectric voltage. Since this voltage depends on the number of electrons, the research team was able to locate those extra electrons using measurements at several points across the QD. Specifically, the dependence of the thermoelectric voltage on the thermopower allowed the researchers to locate the extra electrons: they found fewer electrons within the interior of the QD than in the surrounding substrate, which could mean that the silicon “dopants” prefer to stay outside the QD. “We were really interested in measuring the thermopower, since QDs are considered promising for thermoelectrics,” doctoral student and NSF Fellow Jenna Walrath wrote of her original goals for the research. Goldman and Walrath were surprised by the unusual behavior of the QD thermopower, and their efforts to explai
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