Emergent Optoelectronic Properties of Mixed-Dimensional Heterojunctions

Conspectus The electronic dimensionality of a material is defined by the number of spatial degrees of confinement of its electronic wave function. Low-dimensional semiconductor nanomaterials with at least one degree of spatial confinement have optoelectronic properties that are tunable with size and environment (dielectric and chemical) and are of particular interest for optoelectronic applications such as light detection, light harvesting, and photocatalysis. By combining nanomaterials of differing dimensionalities, mixed-dimensional heterojunctions (MDHJs) exploit the desirable characteristics of their components. For example, the strong optical absorption of zero-dimensional (0D) materials combined with the high charge carrier mobilities of two-dimensional (2D) materials widens the spectral response and enhances the responsivity of mixed-dimensional photodetectors, which has implications for ultrathin, flexible optoelectronic devices. MDHJs are highly sensitive to (i) interfacial chemistry because of large surface area-to-volume ratios and (ii) electric fields, which are incompletely screened because of the ultrathin nature of MDHJs. This sensitivity presents opportunities for control of physical phenomena in MDHJs through chemical modification, optical excitation, externally applied electric fields, and other environmental parameters. Since this fast-moving research area is beginning to pose and answer fundamental questions that underlie the fundamental optoelectronic behavior of MDHJs, it is an opportune time to assess progress and suggest future directions in this field. In this Account, we first outline the characteristic properties, advantages, and challenges for low-dimensional materials, many of which arise as a result of quantum confinement effects. The optoelectronic properties and performance of MDHJs are primarily determined by dynamics of excitons and charge carriers at their interfaces, where these particles tunnel, trap, scatter, and/or recombine on the time scales of tens of femtoseconds to hundreds of nanoseconds. We discuss several photophysical phenomena that deviate from those observed in bulk heterojunctions, as well as factors that can be used to vary, probe, and ultimately control the behavior of excitons and charge carriers in MDHJ systems. We then discuss optoelectronic applications of MDHJs, namely, photodetectors, photovoltaics, and photocatalysts, and identify current performance limits compared to state-of-the-art benchmarks.