Emergent Properties Resulting from Type‐II Band Alignment in Semiconductor Nanoheterostructures

Abstract

The development of elegant synthetic methodologies for the preparation of monocomponent nanocrystalline particles has opened many possibilities for the preparation of heterostructured semiconductor nanostructures. Each of the integrated nanodomains is characterized by its individual physical properties, surface chemistry, and morphology, yet, these multicomponent hybrid particles present ideal systems for the investigation of the synergetic properties that arise from the material combination in a non-additive fashion. Of particular interest are type-II heterostructures, where the relative band alignment of their constituent semiconductor materials promotes a spatial separation of the electron and hole following photoexcitation, a highly desirable property for photovoltaic applications. This article highlights recent progress in both synthetic strategies, which allow for material and architectural modulation of novel nanoheterostructures, as well as the experimental work that provides insight into the photophysical properties of type-II heterostructures. The effects of external factors, such as electric fields, temperature, and solvent are explored in conjunction with exciton and multiexciton dynamics and charge transfer processes typical for type-II semiconductor heterostructures.

Type-II Colloidal Quantum Wells: CdSe/CdTe Core/Crown Heteronanoplatelets

“…35 Finally, it can be concluded that the red-shifted emission originates from excitons formed in both the CdSe core region and the CdTe crown region, which is expected for Type-II materials. 17,21 Moreover, by using transmission electron microscopy (TEM), quantitative EDX analysis, and X-ray diffraction (XRD), structural characterization of CdSe/CdTe core/crown NPLs was performed. HAADF-TEM images of CdSe/CdTe core/crown NPLs having a higher quantum yield are shown in Figures 3a and 3b.…”

“…16 In contrast to Type-I semiconductor nanocrystals, semiconductor nanocrystals having Type-II electronic structure, where there is a significantly reduced overlap between electron and hole wave functions, show different and interesting properties. 12,17 First, they exhibit strongly red-shifted emission originating from the radiative recombination of electron and hole across the core−shell interface, which would otherwise not be possible to achieve from either only core or only shell materials. Second, as a result of the separation of electron and hole wave functions, a reduced level of oscillator strength is observed with significantly increased radiative lifetime.…”

“…On the other hand, while asymmetric core/shell heterostructures such as barbells and tetrapods enable more efficient charge extraction with respect to spherical ones, their inferior film formation limits the device performance. 17 Therefore, new architectures of colloidal semiconductor nanocrystals enabling efficient charge extraction and high quality film formation are welcomed for enhanced light harvesting purposes.…”

Controlling the spatial location of photoexcited electrons in semiconductor CdSe/CdS core/shell nanorods

“…For example, applications in photovoltaic devices and photocatalysis are improved by rapid separation of photoexcited electrons and holes into the different regions of the heterostructure. 2 By contrast, for applications where the nanocrystals emit light, including light-emitting diodes, 3 lasers, 4 and luminescent solar concentrators, 5 localization of electrons and holes in a single region is generally desirable, since it leads to high recombination rates and high luminescence quantum yields. 6 In order to determine the spatial location of electrons and holes in semiconductor heterostructures, it is often thought to be sufficient to consider only the bulk band-edge alignments of the different materials involved.…”

Amphiphilic self-assembly of semiconductor nanocrystals with heterogeneous compositions

“…-The controlled synthesis of nanoparticle materials has advanced over the last decades, which enables us to access to nanoparticles of tailored sizes, shapes, compositions and properties with high uniformity [1][2][3]. Along with such developments, nanoparticle materials have found applications in diverse technologies including optelectronic devices [4][5][6]. Meanwhile, recent interest has focused on these nanoparticle materials as building blocks for self-assembly [6][7][8][9][10].…”