Nanocrystal solids: A modular approach to materials design
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uction An interesting chemical discrepancy was noticed that was both amusing and inspiring. On one hand, solid-state chemists widely use binary and ternary phase diagrams when planning their syntheses.1 On the other hand, I asked many organic chemists if they ever used C-H and C-H-O phase diagrams. The general answer was no. Chemists rarely use such phase diagrams when they work with molecular species (e.g., with organometallic and coordination compounds). What makes solid-state chemistry so different from molecular chemistry? This can be understood if we take into account that the majority of bulk solids are thermodynamically stable phases corresponding to the global minimum of free energy. In contrast, the absolute majority of molecules does not represent the most stable atomic arrangements and can be converted into more stable species (typically CO2, H2O, N2, and other small molecules with strong bonds). This applies to almost all organic molecules. As a result, organic synthesis requires a step-by-step assembly rather than heating of carbon, hydrogen, and oxygen until they react with each other. To build complex molecules, organic chemists do not allow
reactants to fully equilibrate and form the most stable products. Instead, they rely on a well-defined hierarchy of energy scales that allow driving from one local energy minimum to the other local minimum following the reaction pathway.2 This approach is the heart of the enormous compositional and structural diversity of molecular compounds. Our research tries to bring these ideas from molecular chemistry into the arena of solids. We were not the first to do so: a good example of non-equilibrium solid is a stack of GaAsAlAs quantum wells shown in Figure 1. Such a structure can exist only far below the melting points of GaAs and AlAs; if heated, it will equilibrate into a Ga1–xAlxAs alloy. The discovery of semiconductor heterostructures has introduced entirely new ways for electronic structure engineering, enabling exciting device applications and scientific breakthroughs recognized by several Nobel prizes (e.g., Alferov, Kroemer, and Kilby).3 At the same time, traditional semiconductor heterostructures, grown by vacuum deposition techniques such as molecular beam epitaxy, also have limitations, one being that the electronic structure can be precisely modulated only along one direction, which is
Dmitri V. Talapin, Department of Chemistry at the University of Chicago; [email protected] DOI: 10.1557/mrs.2011.337
© 2012 Materials Research Society
MRS BULLETIN • VOLUME 37 • JANUARY 2012 • www.mrs.org/bulletin
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NANOCRYSTAL SOLIDS: A MODULAR APPROACH TO MATERIALS DESIGN
Figure 1. (a) Stack of quantum wells made of a semiconductor with the bandgap energy Egb showed as dark regions separated with barrier layers made of a semiconductor with the bandgap energy Ega—semiconductor superlattice that allows precise tuning of the electronic structure along the stack direction as shown in panel (b).
the growth direction. As yet, we are unable to provide the same level
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