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.

Thermal doping by vacancy formation in copper sulfide nanocrystal arrays.

A new approach for doping of Cu2S nanocrystal arrays using thermal treatment at moderate temperatures (T < 400 K) is presented. This thermal doping process yields conductance enhancement by 6 orders of magnitude. Local probe measurements prove this doping is an intraparticle effect and, moreover, tunneling spectroscopy data signify p-type doping. The doping mechanism is attributed to Cu vacancy formation, resulting in free holes. Thermal-doping temperature dependence exhibits an Arrhenius-like behavior, providing the vacancy formation energy of 1.6 eV. The moderate temperature conditions for thermal doping unique to these nanocrystals allow patterned doping of nanocrystal films through local heating by a focused laser beam, toward fabrication of nanocrystal-based electronic devices.

Collective effects in charge transfer within a hybrid organic-inorganic system.

A collective electron transfer (ET) process was discovered by studying the current noise in a field effect transistor with light-sensitive gate formed by nanocrystals linked by organic molecules to its surface. Fluctuations in the ET through the organic linker are reflected in the fluctuations of the transistor conductivity. The current noise has an avalanche character. Critical exponents obtained from the noise power spectra, avalanche distributions, and the dependence of the average avalanche size on avalanche duration are consistent with each other. A plausible model is proposed for this phenomenon.

Folate-mediated tumor cell uptake of quantum dots entrapped in lipid nanoparticles.

Quantum dots (QDs) are fluorescent semiconductor nanocrystals with superior optical properties compared to organic dyes currently undergoing rapid development for biological applications, particularly in fluorescence imaging. The folate receptor, overexpressed in a broad spectrum of malignant tumors, is an attractive target for selective delivery of imaging agents to tumor cells. This study examines nanoparticles containing QDs entrapped in a lipid shell, and post-loaded with a folate-lipid conjugate for targeting to mouse and human tumor cells expressing the folate receptor. Hydrophobic QDs were mixed with 1,2 dipalmitoyl-sn-glycero-3 phosphocholine and methoxy-polyethylene-glycol-distearoyl-phosphatidyl-ethanolamine (mPEG-DSPE) generating a nanoparticle referred to as lipodot, with a mean diameter size of approximately 100 nm. Folate-derivatized PEG-DSPE was post-loaded into the lipodots at 0.5% lipid molar concentration. Mouse J6456 lymphoma cells (J6456-FR) and human head and neck KB cancer cells (KB-FR), up-regulated for their folate receptors, were incubated with folate-targeted and non-targeted lipodots in vitro. Using fluorescence microscopy, it was found that only folate-targeted lipodots were taken up by tumor cells. Confocal depth scanning showed substantial internalization. Confirming the specificity of folate-targeted lipodots, binding and internalization were inhibited by free folate, and no uptake was found in a folate-receptor negative cell line. Selective binding and uptake of folate-targeted lipodots by J6456-FR cells was also observed in vivo after intra-peritoneal injection in mice bearing ascitic J6456-FR tumors based on FACS analysis and confocal imaging of harvested cells from the peritoneal cavity. Folate-targeted lipodots represent an attractive approach for tumor cell labeling both in vitro and in vivo.

Cavity QED with semiconductor nanocrystals.

We report on a strongly coupled cavity quantum electrodynamic (CQED) system consisting of a CdSe nanocrystal coupled to a single photon mode of a polymer microsphere. The strong exciton-photon coupling is manifested by the observation of a cavity mode splitting variant Planck's over 2piOmega(exp) between 30 und 45 microeV and photon lifetime measurements of the coupled exciton-photon state. The single photon mode is isolated by lifting the mode degeneracy in a slightly deformed microsphere cavity and addressing it by high-resolution imaging spectroscopy. This cavity mode is coupled to a localized exciton of an anisotropically shaped CdSe nanocrystal that emits highly polarized light in resonance to the cavity mode and that was placed in the maximum electromagnetic field close to the microsphere surface. The exciton confined in the CdSe nanorod exhibits an optical transition dipole moment much larger than that of atoms, the standard system for CQED experiments, and a low-temperature homogeneous line width much narrower than the high-Q cavity mode width. The observation of strong coupling in a colloidal semiconductor nanocrystal-cavity system opens the way to study fundamental quantum-optics phenomena and to implement quantum information processing concepts that work in the visible spectral range and are based on solid-state nanomaterials.

Electronic structure of metal-semiconductor nanojunctions in gold CdSe nanodumbbells.

The electronic properties of metal-semiconductor nanojunctions are investigated by scanning tunneling spectroscopy of gold-tipped CdSe rods. A gap similar to that in bare CdSe nanorods is observed near the nanodumbbell center, while subgap structure emerges near the metal-semiconductor nanocontact. This behavior is attributed to the formation of subgap interface states that vanish rapidly towards the center of the rod, consistent with theoretical predictions. These states lead also to modified Coulomb staircase, and in some cases to negative differential conductance, on the gold tips.

Size-dependent optical spectroscopy of a homologous series of CdSe cluster molecules.

The optical properties and electronic structure of a homologous series of CdSe cluster molecules covering a size range between 0.7 and 2 nm are investigated. CdSe cluster molecules with 4, 8 10, 17, and 32 Cd atoms, capped by selenophenol ligands, were crystallized from solution and their structures determined by single-crystal X-ray diffraction. The cluster molecules are composed of a combination of adamanthane and barylene-like cages, the building blocks of the zinc blende and the wurtzite structures of the bulk CdSe. The onset of the room temperature absorption and low-temperature photoluminescence excitation spectra exhibit a systematic blue shift with reduced cluster size manifesting the quantum confinement effect down to the molecular limit of the bulk semiconductor. Blue-green emission, shifted substantially to lower energy from the absorption onset, is observed only at low temperature and its position is nearly independent of cluster size. The wavelength dependence of both photoluminescence and photoluminescence excitation was measured. The emission is assigned to forbidden transitions involving the cluster-molecule surface-capping ligands. This assignment is supported by the emission decay which exhibits distributed kinetics with microsecond time scale. The temperature dependence of the emission intensity is quantitatively explained by multiphonon-induced nonradiative relaxation mediated by low-frequency vibrations of the selenophenol capping ligands. Upon irradiation, the emission of all cluster molecules is quenched. Warming up and recooling leads to recovery of the emission (partial or complete) for all but the cluster molecule with 10 Cd atoms. This temporary darkening is assigned to the photoinduced charging of the cluster-molecule surface ligands, resembling the reversible on-off blinking of the emission observed for larger CdSe nanocrystals.

Imaging and spectroscopy of artificial-atom states in core/shell nanocrystal quantum dots.

Current imaging scanning tunneling microscopy is used to observe the electronic wave functions in InAs/ZnSe core/shell nanocrystals. Images taken at a bias corresponding to the s conduction band state show that it is localized in the central core region, while images at higher bias probing the p state reveal that it extends to the shell. This is supported by optical and tunneling spectroscopy data demonstrating that the s-p gap closes upon shell growth. Shapes of the current images resemble atomlike envelope wave functions of the quantum dot calculated within a particle in a box model.

Synthesis and Characterization of InAs/InP and InAs/CdSe Core/Shell Nanocrystals.

The effect of the outer surface of core/shell nanocrystals on the fluorescence quantum yield was observed for InAs/InP and InAs/CdSe core/shells (see picture). For InAs/CdSe we observed substantial enhancement of the fluorescence quantum yield compared to the InAs core, and up to two times larger than the laser dye IR-140. Such core/shell nanocrystals have potential use as biological fluorescent markers in the near IR spectral range.