dr. C. de Mello Donegá (Celso)
Chemistry and Photophysics of Colloidal Nanomaterials
Colloidal nanocrystals can be regarded as solution-grown inorganic-organic hybrid nanomaterials, since they consist of inorganic nanoparticles that are coated with a layer of organic ligand molecules. The hybrid nature of these nanostructures provides great flexibility and versatility in designing their properties. Their nanoscale dimensions give rise to remarkable size- and shape-dependent properties that can be further engineered by controlling their composition and compositional profile. The nanocrystal can consist of a single material (pure or doped) or be heterostructured, i.e., comprise two or more different materials joined together in the same particle by heterointerfaces (viz., a heteronanocrystal). The latter are particularly attractive since the spatial localization of photogenerated charge carriers in semiconductor heteronanocrystals can be manipulated by controlling the offsets between the energy levels of their different components. This has a dramatic impact on several properties. Moreover, the colloidal nature of these nanomaterials allows one to take full advantage of nanoscale effects to combine size-, shape- and composition-dependent properties with easy surface manipulation and solution processability. These features have turned colloidal nanocrystals into promising materials for several applications (viz., light-emitting devices, luminescent solar concentrators, biomedical imaging probes, photo- and electrocatalysts, photodetectors, transistors, solution-processed thin film solar cells). As such, they are enablers of sustainable and circular-by-design disruptive new technologies. Below, the research lines that we are currently pursuing are addressed in more detail.
Synthesis and optoelectronic properties of colloidal nanocrystals of III-V semiconductors
Many of the applications mentioned above have already been realized using Cd- and Pb-chalcogenide based NCs (e.g., CdSe quantum dots in displays, or PbS colloidal quantum dot solar cells), since control over these nanocrystals has reached a very mature level, owing to several decades of extensive research. However, the widespread deployment of these NCs is severely limited by the intrinsic toxicity of Cd and Pb. This has motivated a worldwide research effort on alternative materials based on non- (or less) toxic elements. Binary III-V semiconductors and their alloys (e.g., InP, GaP, InSb) are promising alternatives as they can cover the whole visible (InP) and NIR to mid-infrared range (InSb). The synthesis of colloidal nanocrystals of these materials is notoriously difficult and has remained underdeveloped with respect to that of II-VI and IV-VI semiconductors. In this research line, we have been developing methods to synthesize high-quality InP and InSb quantum dots and core/shell quantum dots, and investigating their optoelectronic properties.
(Left) Absorption and photoluminescence spectra of two sizes of colloidal InSb quantum dots. The insets show overview transmission electron microscopy (TEM) images of the samples. (right) TEM image of InSb quantum dots (inset shows a high-resolution TEM image of a selected QD).
Synthesis and Optoelectronic properties of Ultrathin One- and Two-Dimensional Colloidal Semiconductor Nanocrystals
Ultrathin 2-dimensional (2D) nanomaterials (nanosheets, NSs) are attracting increasing research efforts due to their extraordinary electronic, phononic, optical and mechanical properties. A well-example is graphene. Nanosheets offer compelling opportunities for fundamental studies in 2D physics and hold immense potential for spintronic devices, field-effect transistors, and nanoscale sensors. They are typically obtained by exfoliation of bulk materials or grown on substrates by MBE or CVD. These methods are however not suitable to produce large amounts of free-standing nanosheets and lack control over their shape and lateral dimensions. Solution-based “bottom-up” colloidal chemical methods offer an appealing alternative due to their versatility in terms of composition, size, shape and surface control, and at relatively low costs. Nevertheless, despite its potential, the field of ultrathin 1D and 2D colloidal nanocrystals is still in its infancy.
A. High-resolution transmission electron microscopy image of a single colloidal (Zn,Cd)Te/CdSe hetero-nanowire. B. TEM overview image of the same sample shown in (A). The inset shows colloidal dispersions of (Zn,Cd)Te/CdSe hetero-nnowires (2 nm diameter and ~100 nm long in all cases) with different compositions under UV illumination (E. Groeneveld et al., Nano Letters 12 (2012) 749-757).
Colloidal ultrathin Cu2-xS nanosheets of different shapes (W. van der Stam, et al. Chemistry of Materials 27 (2015) 283-291).
Colloidal Nanocrystals and Heteronanocrystals of Earth-Abundant and Non-toxic Compounds
Ternary I-III-VI2 (I= Cu+, Ag+; III= Al3+, Ga3+, In3+; VI= S2−, Se2−, Te2−) nanocrystals are emerging as promising alternatives to Cd- and Pb-chalcogenide based nanocrystals due to their inherently lower toxicity and wide spectral tunability. Moreover, they also offer characteristics that are unmatched by Cd- and Pb-chalcogenide nanocrystals, such as large global Stokes shifts and plasmonic properties. The goal of this research line is to develop high-quality (i.e., well-defined size, shape, and surface, high optoelectronic quality) colloidal nanocrystals and heteronanocrystals of I-III-VI2 semiconductors and gain a deeper understanding of their optoelectronic properties.
Artist’s impression of the synthesis of luminescent CuInS2/ZnS dot core/rod shell heteronanorods by seeded injection (image originally published as cover of the Journal of the American Chemical Society on occasion of the publication of our paper: C. Xia, et al., Journal of the American Chemical Society 140 (2018) 5755-5763. DOI: 10.1021/jacs.8b01412 ).
Artist’s impression of the excitation and luminescence of colloidal CuInS2 nanocrystals (image originally published as cover of the Journal of Physical Chemistry Letters on occasion of the publication of our invited Perspective paper: A. C. Berends, et al., Journal of Physical Chemistry Letters 10 (2019) 1600-1616.. DOI: 10.1021/acs.jpclett.8b03653.
Luminescent Solar Concentrators based on colloidal nanocrystals
Luminescent Solar Concentrators consist of a highly transparent plate, in which luminescent species are dispersed. These species absorb incident light and isotropically emit lower energy photons, with high efficiency. Internal reflection ensures collection of the emitted light in solar cells located at the side(s) of the plate. The present efficiency of LSCs is limited by several loss mechanisms. To minimize losses while making better use of the solar spectrum and improving the spectral matching between the LSC and the solar cell, we are investigating a variety of colloidal semiconductor nanocrystals. Sustainability, toxicity and circularity aspects are also taken into account in the development of the materials and devices.