Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications

The optical properties of metal nanoparticles have long been of interest in physical chemistry, starting with Faraday's investigations of colloidal gold in the middle 1800s. More recently, new lithographic techniques as well as improvements to classical wet chemistry methods have made it possible to synthesize noble metal nanoparticles with a wide range of sizes, shapes, and dielectric environments. In this feature article, we describe recent progress in the theory of nanoparticle optical properties, particularly methods for solving Maxwell's equations for light scattering from particles of arbitrary shape in a complex environment. Included is a description of the qualitative features of dipole and quadrupole plasmon resonances for spherical particles; a discussion of analytical and numerical methods for calculating extinction and scattering cross-sections, local fields, and other optical properties for nonspherical particles; and a survey of applications to problems of recent interest involving triangula...

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The Optical Properties of Metal Nanoparticles: The Influence of Size, Shape, and Dielectric Environment

Novel physical phenomena can emerge in low-dimensional nanomaterials. Bulk MoS2, a prototypical metal dichalcogenide, is an indirect bandgap semiconductor with negligible photoluminescence. When the MoS2 crystal is thinned to monolayer, however, a strong photoluminescence emerges, indicating an indirect to direct bandgap transition in this d-electron system. This observation shows that quantum confinement in layered d-electron materials like MoS2 provides new opportunities for engineering the electronic structure of matter at the nanoscale.

Emerging Photoluminescence in Monolayer MoS2

The interest in nanoscale materials stems from the fact that new properties are acquired at this length scale and, equally important, that these properties * To whom correspondence should be addressed. Phone, 404-8940292; fax, 404-894-0294; e-mail, mostafa.el-sayed@ chemistry.gatech.edu. † Case Western Reserve UniversitysMillis 2258. ‡ Phone, 216-368-5918; fax, 216-368-3006; e-mail, burda@case.edu. § Georgia Institute of Technology. 1025 Chem. Rev. 2005, 105, 1025−1102

Chemistry and properties of nanocrystals of different shapes.

Localized surface plasmon resonance (LSPR) spectroscopy of metallic nanoparticles is a powerful technique for chemical and biological sensing experiments. Moreover, the LSPR is responsible for the electromagnetic-field enhancement that leads to surface-enhanced Raman scattering (SERS) and other surface-enhanced spectroscopic processes. This review describes recent fundamental spectroscopic studies that reveal key relationships governing the LSPR spectral location and its sensitivity to the local environment, including nanoparticle shape and size. We also describe studies on the distance dependence of the enhanced electromagnetic field and the relationship between the plasmon resonance and the Raman excitation energy. Lastly, we introduce a new form of LSPR spectroscopy, involving the coupling between nanoparticle plasmon resonances and adsorbate molecular resonances. The results from these fundamental studies guide the design of new sensing experiments, illustrated through applications in which researchers use both LSPR wavelength-shift sensing and SERS to detect molecules of chemical and biological relevance.

https://www.photonics.ethz.ch/fileadmin/user_upload/Courses/NanoOptics/willets.pdf

Localized Surface Plasmon Resonance Spectroscopy and Sensing

To explore the relationship between local electromagnetic field enhancement and the large SERS (surface enhanced Raman scattering) enhancement that enables the observation of single molecule Raman spectra, we measure both resonant Rayleigh scattering spectra and rhodamine 6G Raman spectra from single Ag particles. Our apparatus combines the techniques of dark-field optical microscopy for resonant Rayleigh measurements, and grazing incidence Raman spectroscopy. The Rayleigh spectra show that the citrate-reduced Ag colloid is extremely heterogeneous. Only the larger particles, in part created by salt induced aggregation, show a large SERS effect. In agreement with the work of Nie and Emory, we find that a few nanocrystals show huge single molecule R6G SERS intensities. While all SERS active particles have some resonant Rayleigh scattering at the 514.5 nm laser wavelength, there is no correlation between the resonant Rayleigh spectra and the SERS intensity. We discuss a model in which huge SERS intensities r...

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Surface Enhanced Raman Spectroscopy of Individual Rhodamine 6G Molecules on Large Ag Nanocrystals

Introduction Sp3-hybridized semiconductors (including InP, GaAs, CdSe, and Si) are remarkable from the perspective of physical chemistry. A single electron created by HOMOLUMO promotion moves rapidly in response to an applied electric field, because there is little lattice distortion (i.e., small Franck-Condon factors) accompanying its creation. According to Marcus-Hush electron transfer theory, electron motion is resonant in the limit of vanishing reorganization energy. Franck-Condon factors are also small around an electron-hole pair, which is an electronically excited state. As a consequence, radiationless internal conversion (unimolecular decay converting electronic energy into heat) is extremely slow. Excited states decay radiatively in a defect-free, direct gap semiconductor such as CdSe. These simple spectroscopic facts have important practical consequences. Semiconductor lightemitting diodes have narrow emission bands and can show near 30% efficiency in converting electrical power into light. Semiconductor lasers and diodes also show excellent long-term stability against photochemical and current-induced degradation, when compared with many organic materials. All these properties reflect the strong chemical bonding, and the extremely delocalized nature of the electronic wave functions. Semiconductor nanocrystals lie between the traditional regimes of chemistry and solid-state physics.1 Nanocrystal research was initially motivated by an effort to understand the evolution of bulk structural and electronic properties from the molecular scale.2 Presently, technological interest in nanocrystals stems from the prospect of creating novel materials with distinct physical properties. Nanocrystals act like molecules as they interact with light via their electronic transition dipoles. Yet, their delocalized solid-state parentage causes them to display unusual photophysics relative to molecules. In many molecules vibronic interaction in the excited state is strong as the wave function is localized on just one or a few bonds. The molecular excited state has a different structure which promotes fast nonradiative deactivation into the ground state. Emission quantum yields can be low, and often sensitive to quenching by the local environment. The situation is different in nanocrystals. In a 23 A diameter nanocrystal, for example, the wave function is delocalized over ∼100 unit cells with little probability density at the surface. This suggests that, in the absence of defects, internal or surface, a nanocrystal should exhibit near unity fluorescence quantum yield, and partial protection from quenching. The emission spectrum should be sharp as the Franck-Condon factors are small. At room temperature nanocrystals can be better photoemitters than bulk semiconductors because in nanocrystals the electron and hole remain superimposed due to quantum confinement. Nanocrystals have the potential to serve as ideal chromophores if their surface chemistry can be understood and controlled. In this Account we describe nanocrystal photophysics and make comparisons between inorganic solid-state materials and organic dye molecules.

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Luminescence Photophysics in Semiconductor Nanocrystals

We review three areas where significant progress has recently occurred in our understanding of semiconductor nanocrystals. The first two involve luminescence properties of single and ensembles of Cadmium Selenide (CdSe) nanocrystallites (Quantum Dots) between 10 and 50 A in radius. The size, magnetic field, and temporal dependence of emission from ensembles of nanocrystallites at cryogenic temperatures uncovers the fundamental mechanism of radiative recombination in these nanocrystals. Effective mass models that take into account the electron-hole exchange interaction can quantitatively account for observed luminescence Stokes shifts. Furthermore, the magnetic field dependence of luminescence lifetimes and longitudinal-optical (LO) phonon ratios demonstrate that the exciton ground state in these nanocrystals is optically passive (“dark exciton”) with spin projection ±2. Picosecond time resolved measurements probe exciton relaxation into this level. Recent results on the spectroscopy of single CdSe nanocrystals at room temperature are also presented. Remarkably, emission from a single CdSe nanocrystal under C.W illumination is observed to turn on and off discretely (fluorescence intermittency) on a ~0.5s timescale. The excitation intensity dependence, and the influence of a passivating high band gap shell of Zinc Sulfide (ZnS) encapsulating the CdSe nanocrystal on the on/off times, suggest that this phenomenon is caused by photoionization. Finally, the third area originates in diamond anvil studies of the solid-solid phase transitions of nanocrystals under pressure. These studies show that a single nucleation event occurs per nanocrystal, and that as a consequence the nanocrystals change shape. The kinetic activation barrier increases with increasing size. Under suitable conditions nanocrystals in dense, six-coordinate high pressure phases may be metastable at STP.

M. Nirmal

Papers

Surface Enhanced Raman Spectroscopy of Individual Rhodamine 6G Molecules on Large Ag Nanocrystals

Luminescence Photophysics in Semiconductor Nanocrystals

Luminescence Photophysics in Semiconductor Nanocrystals

Semiconductor Nanocrystals: Exciton Quantum Mechanics, Single Nanocrsytal Luminescence, and Metastable High Pressure Phases