Narrow-band-gap IV-VI semiconductors offer promising optoelectronic properties for integration as light-absorbing components in field-effect transistors, photodetectors, and photovoltaic devices. Importantly, colloidal nanostructures of these materials have the potential to substantially decrease the fabrication cost of solar cells because of their ability to be solution-processed. While colloidal nanomaterials formed from IV-VI lead chalcogenides such as PbS and PbSe have been extensively investigated, those of the layered semiconductors SnS, SnSe, GeS, and GeSe have only recently been considered. In particular, there have been very few studies of the germanium chalcogenides, which have band-gap energies that overlap well with the solar spectrum. Here we report the first synthesis of colloidal GeS and GeSe nanostructures obtained by heating GeI(4), hexamethyldisilazane, oleylamine, oleic acid, and dodecanethiol or trioctylphosphine selenide to 320 °C for 24 h. These materials, which were characterized by TEM, SAED, SEM, AFM, XRD, diffuse reflectance spectroscopy, and I-V conductivity measurements, preferentially adopt a two-dimensional single-crystal nanosheet morphology that produces fully [100]-oriented films upon drop-casting. Optical measurements indicated indirect band gaps of 1.58 and 1.14 eV for GeS and GeSe, respectively, and electrical measurements showed that drop-cast films of GeSe exhibit p-type conductivity.

The availability of high-quality colloidal nanosheets underpins a diverse range of applications and investigations into dimension-dependent physical properties. To facilitate this, synthetic methods that yield single-crystal colloidal nanosheets with regular shapes, uniform lateral dimensions, and tunable thicknesses are critically important. Most strategies that yield colloidal nanosheets achieve some, but not all, of these morphological characteristics. Here, we describe a synthetic pathway that generates colloidal nanosheets of SnSe with uniform lateral dimensions and tunable thicknesses. SnSe represents an excellent prototype system for studying the formation of colloidal nanosheets because of its layered crystal structure and the growing interest in its potential application as an absorption layer in low-cost photovoltaic devices. Freestanding colloidal SnSe nanosheets were synthesized by slowly heating a one-pot reaction mixture of SnCl(2), oleylamine, trioctylphosphine selenide (TOP-Se), and hexamethyldisilazane (HMDS) to 240 °C. The SnSe nanostructures adopt a uniform square-like morphology with lateral dimensions of approximately 500 nm × 500 nm, and the average nanosheet thicknesses can be tuned from approximately 10 to 40 nm by adjusting the concentrations of the SnCl(2) and TOP-Se reagents. Aliquot studies reveal fundamental insights into how the nanosheets form: they first "grow out" laterally via coalescence of individual nanoparticle building blocks to yield a single-crystal nanosheet template and then "grow up" vertically (through nanoparticle attachment to the nanosheet template) in a pseudo layer-by-layer fashion. Vertical growth is therefore limited, and can be controlled, by reagent concentration. Drop-cast films of the SnSe nanosheets are photoactive and have a bandgap of approximately 1 eV. These studies, demonstrated for SnSe but potentially applicable to other systems, establish a straightforward pathway for tuning the thicknesses of colloidal nanosheets while maintaining lateral uniformity.

Germanium nanoparticles have excited scientists and engineers because of their size-dependent optical properties and their potential applications in optoelectronics, biological imaging and therapeutics, flash memories, and lithium-ion batteries. In order to further develop these applications and to gain deeper insights into their size-dependent properties, robust and facile synthetic methods are needed to controllably synthesize Ge nanoparticles. However, when compared to other II-VI, IV-VI, and III-V semiconductor systems, colloidal routes to Ge NPs with uniform sizes and shapes are much less mature. In this Review Article, we highlight the progress that has been made in this field and provide insights into the strategies used for the colloidal synthesis of size and shape-controlled germanium nanomaterials. We also survey some of the potential applications of these materials in optoelectronics, biological imaging, and energy conversion and storage. Finally, we discuss the colloidal synthesis of other germanium-containing compounds, emphasizing technologically relevant germanium chalcogenides that include GeS, GeSe, and GeTe.