Ultrathin CdSe in Plasmonic Nanogaps for Enhanced Photocatalytic Water Splitting.

Enhanced plasmonic fields are a promising way to increase the efficiency of photocatalytic water splitting. The availability of atomically thin materials opens up completely new opportunities. We report photocatalytic water splitting on ultrathin CdSe nanoplatelets placed in plasmonic nanogaps formed by a flat gold surface and a gold nanoparticle. The extreme field intensity created in these gaps increases the electron–hole pair production in the CdSe nanoplatelets and enhances the plasmon-mediated interfacial electron transfer. Compared to individual nanoparticles commonly used to enhance photocatalytic processes, gap-plasmons produce several orders of magnitude higher field enhancement, strongly localized inside the semiconductor sheet thus utilizing the entire photocatalyst efficiently.This work was supported by the U.K. EPSRC grant EP/G060649/1 and EPSRC grant EP/L027151/1, Defence Science and Technology Laboratory (DSTL), a Marie Curie Intra-European Fellowship (FP7-PEOPLE-2011-IEF 298012 to L.Z.) and ERC grant 320503 LINASS.This is the final version of the article. It first appeared from ACS via http://dx.doi.org/10.1021/acs.jpclett.5b0027

Probing confined phonon modes in individual CdSe nanoplatelets using surface-enhanced Raman scattering.

The phonon modes of individual ultrathin CdSe nanoplatelets are investigated using surface-enhanced Raman scattering in a tightly confined plasmonic geometry. The surface-enhanced Raman scattering spectra, taken on single nanoplatelets sandwiched between a gold nanoparticle and a gold surface, reveal a phonon doublet arising from oscillations perpendicular to and within the platelet plane. The out-of-plane mode cannot be observed with conventional Raman spectroscopy. The resulting strong electric field enhancements and the field vector reorientation within such nanometer-sized plasmonic gaps reveal otherwise hidden information deep into the Brillouin zone illuminating the vibrational properties of ultrathin materials.EPSRCThis is the Author Accepted Manuscript of Daniel O. Sigle, James T. Hugall, Sandrine Ithurria, Benoit Dubertret, and Jeremy J. Baumberg which was published in Physical review letters (https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.113.087402). "Copyright 2014 by the American Physical Society.

Surface spin magnetism controls the polarized exciton emission from CdSe nanoplatelets

The surface of nominally diamagnetic colloidal CdSe nanoplatelets can demonstrate paramagnetism owing to the uncompensated spins of dangling bonds (DBSs). We reveal that by optical spectroscopy in high magnetic fields up to 15 Tesla using the exciton spin as probe of the surface magnetism. The strongly nonlinear magnetic field dependence of the circular polarization of the exciton emission is determined by the DBS and exciton spin polarization as well as by the spin-dependent recombination of dark excitons. The sign of the exciton-DBS exchange interaction can be adjusted by the nanoplatelet growth conditions

Spatially uniform enhancement of single quantum dot emission using plasmonic grating decoupler

International audience1 We demonstrate a spatially uniform enhancement of individual quantum dot (QD) fluorescence emission using plasmonic grating decouplers on thin gold or silver films. Individual QDs are deposited within the grating in a controlled way to investigate the position dependency on both the radiation pattern and emission enhancement. We also describe the optimization of the grating decoupler. We achieve a fluorescence enhancement ~3 times higher than using flat plasmon film, for any QD position in the grating. Future optical quantum devices require the development of photonic sources with control of light down to the single photon limit. Excellent examples of single photon emitters are the colloidal nanocrystal quantum dots (QDs) which are considered as the building blocks for future quantum devices such as quantum qubits and quantum cryptographic devices 1,2. The application area of quantum emitters is wide and these applications require control of their emission such as emission rate, polarization, spectral properties, collection efficiency etc. Integration of single molecule or nanocrystals into plasmonic structures has recently proved to be one of the most promising yet challenging ways to control the emission properties at the single photon level 3,

Electron and hole g-factors and spin dynamics of negatively charged excitons in CdSe/CdS colloidal nanoplatelets with thick shells

We address spin properties and spin dynamics of carriers and charged excitons in CdSe/CdS colloidal nanoplatelets with thick shells. Magneto-optical studies are performed by time-resolved and polarization-resolved photoluminescence, spin-flip Raman scattering and picosecond pump-probe Faraday rotation in magnetic fields up to 30 T. We show that at low temperatures the nanoplatelets are negatively charged so that their photoluminescence is dominated by radiative recombination of negatively charged excitons (trions). Electron g-factor of 1.68 is measured and heavy-hole g-factor varying with increasing magnetic field from -0.4 to -0.7 is evaluated. Hole g-factors for two-dimensional structures are calculated for various hole confining potentials for cubic- and wurtzite lattice in CdSe core. These calculations are extended for various quantum dots and nanoplatelets based on II-VI semiconductors. We developed a magneto-optical technique for the quantitative evaluation of the nanoplatelets orientation in ensemble

Monitoring morphological changes in 2D monolayer semiconductors using atom-thick plasmonic nanocavities.

Nanometer-sized gaps between plasmonically coupled adjacent metal nanoparticles enclose extremely localized optical fields, which are strongly enhanced. This enables the dynamic investigation of nanoscopic amounts of material in the gap using optical interrogation. Here we use impinging light to directly tune the optical resonances inside the plasmonic nanocavity formed between single gold nanoparticles and a gold surface, filled with only yoctograms of semiconductor. The gold faces are separated by either monolayers of molybdenum disulfide (MoS2) or two-unit-cell thick cadmium selenide (CdSe) nanoplatelets. This extreme confinement produces modes with 100-fold compressed wavelength, which are exquisitely sensitive to morphology. Infrared scattering spectroscopy reveals how such nanoparticle-on-mirror modes directly trace atomic-scale changes in real time. Instabilities observed in the facets are crucial for applications such as heat-assisted magnetic recording that demand long-lifetime nanoscale plasmonic structures, but the spectral sensitivity also allows directly tracking photochemical reactions in these 2-dimensional solids.This work was supported by the UK EPSRC grant EP/G060649/1, Defence Science and Technology Laboratory (DSTL), and ERC grant 320503 LINASS.This is the final version of the article. It first appeared from ACS via http://dx.doi.org/10.1021/nn5064198

Addressing the exciton fine structure in colloidal nanocrystals: the case of CdSe nanoplatelets

We study the band-edge exciton fine structure and in particular its bright-dark splitting in colloidal semiconductor nanocrystals by four different optical methods based on fluorescence line narrowing and time-resolved measurements at various temperatures down to 2 K. We demonstrate that all these methods provide consistent splitting values and discuss their advances and limitations. Colloidal CdSe nanoplatelets with thicknesses of 3, 4 and 5 monolayers are chosen for experimental demonstrations. The bright-dark splitting of excitons varies from 3.2 to 6.0 meV and is inversely proportional to the nanoplatelet thickness. Good agreement between experimental and theoretically calculated size dependence of the bright-dark exciton slitting is achieved. The recombination rates of the bright and dark excitons and the bright to dark relaxation rate are measured by time-resolved techniques

Investigating the n- and p-Type Electrolytic Charging of Colloidal Nanoplatelets

We investigate the ion gel gating of 2D colloidal nanoplatelets. We propose a simple, versatile, and air-operable strategy to build electrolyte-gated transistors. We provide evidence that the charges are injected in the quantum states of the nanocrystals. The gating is made possible by the presence of large voids into the NPL films and is sensitive to the availability of the nanocrystals surface