Nanocrystals selfpurification mechanisms explained by energetics

first_imgIn an example of doped nanocrystals, this charge density plot shows magnesium impurities in cadmium-selenium nanocrystals at two different gap levels: (a) resonant and (b) hybrid. Image credit: Gustavo Dalpian and James Chelikowsky. On the macroscale, doping led to the transistor. On the nanoscale, scientists believe that doping could lead to an assortment of technologies, including solar cells, electroluminescent devices and electronic devices. Doping, which means adding impurities containing electrons, can enable electric conductance in a controlled way. But the miniscule size of semiconductor nanocrystals – also described as one-dimensional “quantum dots” – means that scientists must explore new methods for doping. Because nanocrystals have very little interior volume and are virtually all surface, scientists in the past believed that inner impurities can easily migrate the short distance to the surface and be ejected. “People used to believe that nanocrystals had fewer defects due to their limited size,” explains Gustavo Dalpian, coauthor with James Chelikowsky of a recent publication in Physical Review Letters on the energetics of doping. “People believed that defects could be annealed away from the nanocrystal in an easy way due to its limited size. After a few jumps, the impurity will be out of the nanocrystal.”In 2005, scientists (Erwin et al.) proposed that the difficulties in doping nanocrystals could be explained by the crystals’ surface topology and how easily impurities could bind to the surface. For these reasons, these scientists determined that the smaller the size of a nanocrystal, the less binding energy, and the more difficult doping becomes.Dalpian and Chelikowsky, from the University of Texas, have shown that understanding doping in semiconductor nanocrystals requires an understanding of both kinetic and thermodynamic/energetic properties. By explaining nanocrystals’ tendency toward self-purification in terms of the energy needed to form impurities in nanocrystals, the scientists hope to find new ways to increase the dopability of these materials.“Annealing was basically a kinetic argument,” Dalpian said to PhysOrg.com. “[Erwin et al.] show that, changing their solution to an anion-rich (negatively-charged) environment, they could put more impurities into the nanocrystals. Their argument was that there is a shape change in their nanocrystal that increases the binding energy of the impurity into the surface. We show that the difficulty of nanocrystal doping can also be explained through energetic arguments: when you change the solution to an anion-rich environment, the formation energy of the defects is decreased. Explore further ‘Nanocrystal doping’ enhances semiconductor nanocrystals Citation: Nanocrystals’ ‘self-purification’ mechanisms explained by energetics (2006, June 21) retrieved 18 August 2019 from https://phys.org/news/2006-06-nanocrystals-self-purification-mechanisms-energetics.htmllast_img read more