Editing Heterojunction (section)

== Nanoscale heterojunctions ==
[[Image:Fe3O4-CdS Nano Heterojunction.JPG|400px|thumb|right|Image of a nanoscale heterojunction between iron oxide (Fe<sub>3</sub>O<sub>4</sub>&nbsp;— sphere) and cadmium sulfide (CdS&nbsp;— rod) taken with a [[Transmission electron microscopy|TEM]]. This staggered gap (type II) offset junction was synthesized by Hunter McDaniel and Dr. Moonsub Shim at the University of Illinois in Urbana-Champaign in 2007.]] 
In [[quantum dot]]s the band energies are dependent on crystal size due to the [[quantum size effects]]. This enables band offset engineering in nanoscale heterostructures. It is possible<ref>{{cite journal|doi=10.1021/ja068351m|title=Type-II Core/Shell CdS/ZnSe Nanocrystals: Synthesis, Electronic Structures, and Spectroscopic Properties|year=2007|last1=Ivanov|first1=Sergei A.|last2=Piryatinski|first2=Andrei|last3=Nanda|first3=Jagjit|last4=Tretiak|first4=Sergei|last5=Zavadil|first5=Kevin R.|last6=Wallace|first6=William O.|last7=Werder|first7=Don|last8=Klimov|first8=Victor I.|journal=Journal of the American Chemical Society|volume=129|issue=38|pages=11708–19|pmid=17727285}}</ref> to use the same materials but change the type of junction, say from straddling (type I) to staggered (type II), by changing the size or thickness of the crystals involved. The most common nanoscale heterostructure system is [[ZnS]] on [[CdSe]] (CdSe@ZnS) which has a straddling gap (type I) offset. In this system the much larger [[band gap]] ZnS [[Passivation (chemistry)|passivates]] the surface of the [[fluorescent]] CdSe core thereby increasing the [[quantum efficiency]] of the [[luminescence]]. There is an added bonus of increased [[thermal stability]] due to the stronger [[chemical bond|bonds]] in the ZnS shell as suggested by its larger band gap. Since CdSe and ZnS both grow in the [[zincblende (crystal structure)|zincblende]] crystal phase and are closely lattice matched, core shell growth is preferred. In other systems or under different growth conditions it may be possible to grow [[anisotropic]] structures such as the one seen in the image on the right.

The driving force for [[Intervalence charge transfer|charge transfer]] between [[conduction band]]s in these structures is the conduction band offset.<ref name="Robel">{{cite journal|doi=10.1021/ja070099a|title=Size-Dependent Electron Injection from Excited CdSe Quantum Dots into TiO2Nanoparticles|year=2007|last1=Robel|first1=István|last2=Kuno|first2=Masaru|last3=Kamat|first3=Prashant V.|journal=Journal of the American Chemical Society|volume=129|issue=14|pages=4136–7|pmid=17373799}}</ref> By decreasing the size of CdSe nanocrystals grown on [[Titanium dioxide|TiO<sub>2</sub>]], Robel et al.<ref name="Robel" /> found that electrons transferred faster from the higher CdSe conduction band into TiO<sub>2</sub>. In CdSe the quantum size effect is much more pronounced in the conduction band due to the smaller effective mass than in the valence band, and this is the case with most semiconductors. Consequently, engineering the conduction band offset is typically much easier with nanoscale heterojunctions. For staggered (type II) offset nanoscale heterojunctions, [[photoinduced charge separation]] can occur since there the lowest energy state for [[electron hole|holes]] may be on one side of the junction whereas the lowest energy for electrons is on the opposite side. It has been suggested<ref name="Robel" /> that anisotropic staggered gap (type II) nanoscale heterojunctions may be used for [[photocatalysis]], specifically for [[water splitting]] with solar energy.