Nanotechnology for catalysis and solar energy conversion

This roadmap on Nanotechnology for Catalysis and Solar Energy Conversion focuses on the application of nanotechnology in addressing the current challenges of energy conversion: 'high efficiency, stability, safety, and the potential for low-cost/scalable manufacturing' to quote from the contributed article by Nathan Lewis. This roadmap focuses on solar-to-fuel conversion, solar water splitting, solar photovoltaics and bio-catalysis. It includes dye-sensitized solar cells (DSSCs), perovskite solar cells, and organic photovoltaics. Smart engineering of colloidal quantum materials and nanostructured electrodes will improve solar-to-fuel conversion efficiency, as described in the articles by Waiskopf and Banin and Meyer. Semiconductor nanoparticles will also improve solar energy conversion efficiency, as discussed by Boschloo et al in their article on DSSCs. Perovskite solar cells have advanced rapidly in recent years, including new ideas on 2D and 3D hybrid halide perovskites, as described by Spanopoulos et al 'Next generation' solar cells using multiple exciton generation (MEG) from hot carriers, described in the article by Nozik and Beard, could lead to remarkable improvement in photovoltaic efficiency by using quantization effects in semiconductor nanostructures (quantum dots, wires or wells). These challenges will not be met without simultaneous improvement in nanoscale characterization methods. Terahertz spectroscopy, discussed in the article by Milot et al is one example of a method that is overcoming the difficulties associated with nanoscale materials characterization by avoiding electrical contacts to nanoparticles, allowing characterization during device operation, and enabling characterization of a single nanoparticle. Besides experimental advances, computational science is also meeting the challenges of nanomaterials synthesis. The article by Kohlstedt and Schatz discusses the computational frameworks being used to predict structure-property relationships in materials and devices, including machine learning methods, with an emphasis on organic photovoltaics. The contribution by Megarity and Armstrong presents the 'electrochemical leaf' for improvements in electrochemistry and beyond. In addition, biohybrid approaches can take advantage of efficient and specific enzyme catalysts. These articles present the nanoscience and technology at the forefront of renewable energy development that will have significant benefits to society.

In situ spectroscopic characterization of a solution-phase X-type ligand exchange at colloidal lead sulphide quantum dot surfaces

We employed quantitative NMR spectroscopy and spectrophotometric absorbance titration to study a quantum dot X-type ligand exchange reaction.

Electron and hole transfer from indium phosphide quantum dots

Electron- and hole-transfer reactions are studied in colloidal InP quantum dots (QDs). Photoluminescence quenching and time-resolved transient absorption (TA) measurements are utilized to examine hole transfer from photoexcited InP QDs to the hole acceptor N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) and electron transfer to nanocrystalline titanium dioxide (TiO2) films. Core-confined holes are effectively quenched by TMPD, resulting in a new approximately 4-ps component in the TA decay. It is found that electron transfer to TiO2 is primarily mediated through surface-localized states on the InP QDs.

Multiexciton generation by a single photon in nanocrystals

We have theoretically shown that efficient generation of multi-electron-hole pairs by a single photon observed recently in semiconductor nanocrystals1-4 is caused by breaking the single electron approximation for carriers with kinetic energy above the effective energy gap. Due to strong Coulomb interaction, these states form a coherent superposition with charged excitons of the same energy. This concept allows us to define the conditions for dominant two-exciton generations by a single photon: the thermalization rate of a single exciton, initiated by light, should be lower than both the two-exciton state thermalization rate and the rate of Coulomb coupling between single and two exciton states. Possible experimental manifestations of our model are discussed.

Exciton Multiplication and Relaxation Dynamics in Quantum Dots: Applications to Ultrahigh-Efficiency Solar Photon Conversion

Huge amounts of carbon-free energy will be required during the coming decades in order to stabilize atmospheric CO2 to acceptable levels. Solar energy is the largest source of non-carbonaceous energy and can be used to produce both electricity and fuel. However, the ratio of the areal cost to the conversion efficiency for devices converting solar photons to electricity or fuel must be reduced by at least 1 order of magnitude from the present values; this requires large increases in the cell efficiency and large reductions in the cost per unit area. We have shown how semiconductor quantum dots may greatly increase photon conversion efficiencies by producing multiple excitons from a single photon. This is possible because quantization of energy levels in quantum dots slows the cooling of hot excitons, promotes multiple exciton generation, and lowers the photon energy threshold for this process. Quantum yields of 300% for exciton formation in PbSe quantum dots have been reported at photon energies 3.8 times the HOMO-LUMO transition energy, indicating the formation of three excitons/photon for all photoexcited quantum dots. Similar high quantum yields have also been reported for PbS quantum dots. A new model for this effect that is based on a coherent superposition of multiple excitonic states has been proposed.

Spectroscopy and hot electron relaxation dynamics in semiconductor quantum wells and quantum dots

Photoexcitation of a semiconductor with photons above the semiconductor band gap creates electrons and holes that are out of equilibrium. The rates at which the photogenerated charge carriers return to equilibrium via thermalization through carrier scattering, cooling by phonon emission, and radiative and nonradiative recombination are important issues. The relaxation processes can be greatly affected by quantization effects that arise when the carriers are confined to regions of space that are small compared with their deBroglie wavelength or the Bohr radius of bulk excitons. The effects of size quantization in semiconductor quantum wells (carrier confinement in one dimension) and quantum dots (carrier confinement in three dimensions) on the respective carrier relaxation processes are reviewed, with emphasis on electron cooling dynamics. The implications of these effects for applications involving radiant energy conversion are also discussed.

Three-dimensional confinement in the conduction band structure of InP

Strong quantum confinement in InP is observed to significantly reduce the separation between the direct and indirect conduction band states. The effects of three-dimensional confinement are investigated by tailoring the initial separation between conduction band states using quantum dots (QDs) of different sizes and hydrostatic pressure. Analyses of the QD emission spectra show that the X(1c) states are lowest in energy at pressures of approximately 6 GPa, much lower than in the bulk. The transition to the X(1c) states can be explained by either a sequence of gamma-L and L-X crossings, or by the crossover between strongly coupled gamma and X states.