RESEARCH

Chemistry and Photophysics of Colloidal Nanomaterials

 

The research efforts of Dr. Celso de Mello Donega have been directed toward the preparation, characterization and properties of three classes of materials: inorganic complexes, doped solids, and colloidal quantum dots and nanocrystals. In recent years his research work has been primarily focused on the latter. Colloidal nanocrystals (NCs) can be regarded as solution-grown inorganic-organic hybrid nanomaterials, since they consist of inorganic particles that are coated with a layer of organic ligand molecules. The hybrid nature of these nanostructures provides great flexibility in engineering their properties. The nanoscale dimensions of the NCs give rise to remarkable size- and shape-dependent properties that can be further engineered by controlling their composition. The NC can consist of a single material (pure or doped) or be heterostructured, i.e., comprise two or more different materials. The interaction between the inorganic component and the organic ligands allows the size and shape of the NCs to be controlled. Moreover, the organic layer opens up the possibility of surface chemistry manipulation, making it possible to tailor a number of properties. These features turn colloidal NCs into promising materials for a number of applications (e.g., optoelectronics, photonics, spintronics, catalysis, solar energy conversion, thermoelectrics, sensors and biomedical applications). The combined expertise on both the spectroscopy and the preparation of materials has been instrumental to the success of his efforts, leading to the successful development of new preparation methods for a number of colloidal quantum dots (QDs) and heteronanocrystals (HNCs), with improved or novel optoeletronic properties. The availability of such high-quality QDs and HNCs has in turn allowed his group to unravel new physical phenomena and to give important contributions toward a better understanding of the chemistry and physics of nanoscale materials.

1. Synthesis of high-quality colloidal quantum dots (QDs), quantum rods (QRs), and heteronanocrystals (HNCs). To realize the full potential of colloidal QDs and NCs their size, shape, surface, and composition must be strictly controlled. We have successfully developed new preparation methods for a number of colloidal QDs and HNCs, boosting photoluminescence efficiencies, narrowing size distributions, improving stability, and developing new compositions.

Self-Assembly of Colloidal Hexagonal Bipyramid and Bifrustum-shaped ZnS Nanocrystals into Two-Dimensional Superstructures, Nano Letters  (2014) 1032-1037.

Tailoring ZnSe-CdSe Colloidal Quantum Dots via Cation Exchange: From Core/Shell to Alloy Nanocrystals, ACS Nano  7 (2013) 7913-7930.

Synthesis and properties of colloidal heteronanocrystals, Chemical Society Reviews 40 (2011) 1512.

Semiconductor nanorod self-assembly at the liquid/air interface studied by in situ GISAXS and ex situ TEM, Nano Letters 12 (2012) 5515.

Highly luminescent (Zn,Cd)Te-CdSe colloidal heteronanowires with tunable electron-hole overlap, Nano Letters 12 (2012) 749.

 Highly Luminescent CdTe/CdSe Colloidal Heteronanocrystals with Temperature Dependent Emission Color, Journal of the American Chemical Society 129 (2007) 14880.

Single-step synthesis to control the photoluminescence quantum yield and size dispersion of CdSe nanocrystals, Journal of Physical Chemistry B 107  (2003) 489.

Highly Luminescent Water-Soluble CdTe quantum dots, Nano Letters 3 (2003) 503.

2. Size dependent optical properties and exciton fine-structure of colloidal quantum dots. The availability of high-quality quantum dots allows us to unravel new physical phenomena and to give important contributions towards a better understanding of the size-dependence of the optical properties and of the exciton fine structure and lifetimes of colloidal QDs. The optical properties are investigated by steady-state, time-resolved and time-domain optical spectroscopic techniques, at temperatures down to 1.2 K.

Size dependence of the exciton transitions in colloidal CdTe quantum dots, Journal of Physical Chemistry C 116 (2012) 23160.

High temperature luminescence quenching of quantum dots, ACS Nano 6 (2012) 9058.

Loosening quantum confinement: Observation of real conductivity in semiconductor nanoparticles smaller than the Bohr radius, Nano Letters 12 (2012) 4937.

Exciton Lifetimes of CdTe Nanocrystal Quantum Dots in High Magnetic Fields, Physical Review B 83 (2011) 035304.

Universal role of discrete acoustic phonons in the low-temperature optical emission of colloidal quantum dots, Physical Review Letters 102 (2009) 177402.

Direct observation of electron to hole energy transfer in CdSe quantum dots, Physical Review Letters, 96 (2006) 057408.

Size- and temperature-dependence of exciton lifetimes in CdSe quantum dots, Physical Review B 74 (2006) 085320.

3. Colloidal semiconductor heteronanocrystals (HNCs): new materials for tailoring nanoscale excitons. Controlling the formation of spatially indirect excitons is of great scientific interest, from both fundamental and applied viewpoints. Nevertheless, a comprehensive fundamental understanding of nanoscale spatially indirect excitons has yet to emerge. The availability of high-quality colloidal HNCs allows us to investigate the evolution of the optical properties of nanoscale excitons as a function of the size, shape, and composition of the HNC. The knowledge gained contributes towards a better understanding of nanoscale indirect excitons and will have an impact on the design of colloidal HNCs for optoelectronic applications (e.g., photovoltaic devices, optical switches, lasers, spintronic devices, biomedical imaging).

Enhanced Exciton-Phonon coupling in colloidal Type-II CdTe-CdSe heteronanocrystals, Journal of Physical Chemistry C 116 (2012) 16240.

Highly luminescent (Zn,Cd)Te-CdSe colloidal heteronanowires with tunable electron-hole overlap, Nano Letters 12 (2012) 749.

The different nature of band edge absorption and emission in colloidal PbSe/CdSe core/shell Quantum Dots, ACS Nano 5 (2011) 58.

Two-fold emission from the S-shell of PbSe(core)/CdSe(shell) quantum dots, Small 7 (2011) 3493.

Formation of nanoscale spatially indirect excitons: evolution of the type-II optical character of CdTe/CdSe heteronanocrystals, Physical Review B 81 (2010) 165303.

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6. Biolabels for multimodal imaging. Multimodal biolabels for molecular imaging are important medical diagnostic tools. In this research program we exploit colloidal QDs to construct bimodal biolabels detectable by both MRI and optical imaging techniques.

Near-Infrared Fluorescence Energy Transfer Imaging of Nanoparticle Accumulation and Dissociation Kinetics in Tumour-Bearing Mice, ACS Nano 7 (2013) 10362-10370.

Quantum dot and Cy5.5 labeled nanoparticles to investigate lipoprotein biointeractions via Förster resonance energy transfer, Nano Letters 10 (2010) 5131.

Quantum dots with a paramagnetic coating as a bimodal molecular imaging probe, Nano Letters 6 (2006) 1.

5. Luminescent Solar Concentrators. LSCs 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. Several problems limit the present efficiency, among which reabsorption by the spectral converter. To minimize reabsorption losses while improving the spectral matching between the LSC and the solar cell, we are exploring two different materials as spectral converters: type-II colloidal heteronanocrystals and doped nanocrystals.

Tackling self-absorption in Luminescent Solar Concentrators with type-II colloidal quantum dots,  Solar Energy Materials & Solar Cells 111 (2013) 57.

Exploration of parameters influencing the self-absorption losses luminescent solar concentrators with an experimentally validated combined ray-tracing/Monte-Carlo model, Proceedings of the SPIE, 8821 (2013) 882104, DOI: 10.1117/12.2023682.

Spectral Conversion for Thin Film Solar Cells and Luminescent Solar Concentrators, in: Advanced Concepts in Photovoltaics, Eds: A. J. Nozik, M. C. Beard, G. Conibeer (RSC, London, 2014) Ch. 14.

4. Colloidal Heteronanocrystals of Earth-Abundant and Non-toxic Compounds: New Materials for Tailoring and Harvesting Nanoscale Excitons. The goal of this project is to develop synthesis methodologies for high-quality (i.e., well-defined size, shape, and surface, high optoelectronic quality) colloidal HNCs of non-toxic and earth-abundant semiconductors (e.g., Cu2S, Cu2ZnSnS4) and to gain a deeper understanding of their optoelectronic properties. The availability of these novel materials will open up many applications possibilities (solar cells, luminescent solar concentrators, biomedical imaging, LEDs) .

Self-Assembly of Colloidal Hexagonal Bipyramid and Bifrustum-shaped ZnS Nanocrystals into Two-Dimensional Superstructures, Nano Letters  (2014) 1032-1037.