A. L. Rogach

Synthesis of surface-modified colloidal semiconductor nanocrystals and study of photoinduced charge separation and transport in nanocrystal-polymer composites

Abstract This paper provides an overview of semiconductor nanocrystals, which were synthesized either by an aqueous technique in the presence of different thiols as stabilizing agents (CdS, CdSe, CdTe, Cd x Hg 1− x Te and HgTe) or via organometallic reactions (CdSe, CdTe, InP and InAs). In all cases, successful wide-range tuning of the semiconductor band-gap energies was achieved by the control of the nanocrystal size. In addition, the surface properties of the nanocrystals can be controlled by the exchange of the capping ligand. Blends of nanocrystals and conducting polymers, were prepared using CdTe nanocrystals and polyaniline, polypyrrole or poly(3,4-ethylenedioxythiophene) (PEDT), and considered with respect to possible optoelectronic applications. The photoinduced charge separation and transport in the CdTe–PEDT composites was investigated by photoelectrochemical methods.

Anti‐Stokes Photoluminescence in II–VI Colloidal Nanocrystals

For the first time highly efficient photon energy up-conversion (anti-Stokes photoluminescence) has been observed in colloids of CdTe nanocrystals (NCs) and CdSe NCs covered by ZnS shell. The maximum magnitude of the up-converted blue shift ΔE max UC = 335 meV was obtained for CdSe/ ZnS NCs under excitation by a low-power He-Ne laser. The maximum efficiency for energy conversion was estimated to be about η max = 2.7 x 10 -2 % at room temperature. The intensity of the anti-Stokes photoluminescence increases with temperature and shows a linear dependence on the excitation intensity. We suggest that subgap surface states are involved as intermediate states in the up-conversion process rather than nonlinear two-photon absorption or Auger processes.

Confocal microscopy and spectroscopy of nanocrystals on a high-Q microsphere resonator

We report on experiments where we used a home-made confocal microscope to excite single nanocrystals on a high-Q microsphere resonator. In that way spectra of an individual quantum emitter could be recorded. The Q factor of the microspheres coated with nanocrystals was still up to 10 9 .W e also demonstrate the use of a prism coupler as a well-defined output port to collect the fluorescence of an ensemble of nanocrystals coupled to whispering-gallery modes.

Up-Conversion Luminescence in Colloidal CdTe Nanocrystals

We report on the efficient photoluminescence up-conversion in colloidally synthesized CdTe nanocrystals. We demonstrate that the efficiency of photon energy up-conversion and magnitude of spectral shift can be controlled: (i) by the size of the nanocrystals; (ii) by the temperature dependence of the excited state absorption coefficient; (iii) by the dependence on excitation intensity. From the analysis of the experimental data we can suggest that intrinsic gap states are involved as intermediate states in the photoluminescence up-conversion rather than nonlinear two-photon absorption or Auger processes.

Colloidal synthesis of monodisperse luminescent InP nanocrystals

Luminescent InP nanocrystals were prepared by organometallic approaches using long chain alkylamines, trioctylphosphine and trioctylphosphine oxide as stabilizing and size regulating agents. The nanocrystals were characterized by powder XRD, synchrotron XPS, HRTEM, absorption and luminescence spectroscopy. Nearly monodisperse fractions of InP nanocrystals were isolated by the size-selective precipitation technique. As prepared, InP nanocrystals exhibit very poor photoluminescent properties, however, their PL efficiency can be drastically enhanced by photoetching of the nanoparticle surface with certain fluorine compounds. A new technique of controllable size-selective photoetching of colloidal nanoparticles allowed the preparation of nearly monodisperse fractions of InP nanocrystals with room temperature PL quantum yields of 25-40%. The PL band was tunable from green to near-IR with increasing particle size.

Photonic Crystals Based on Two-Layer Opaline Heterostructures

Optical properties of several heterostructures representing two-layer opaline photonic crystals have been examined. Two separate stop-bands have been observed both in transmission and emission spectra. The effect of the interface disorder on the optical spectra was not observed, probably, due to the insufficient degree of order of the opaline layers.

Telluride colloidal quantum dots for telecommunication optical amplifiers

Several colloidally grown quantum dot systems have been demonstrated that show quantum confinement effects such as nanocrystallite size dependant absorption and fluorescence spectra. Bulk fluorescence spectra are blue shifted as the particle size is decreased and exciton-like features may be more apparent in the absorption and fluorescence spectra of individual dots or narrow size distributions of dots. The upper fluorescence wavelength limit is determined by the emission wavelength of the bulk material. For telecommunications applications at 1300 nm and 1550 nm the most popular colloidal systems, CdSe and CdS are therefore not appropriate. The paper shows our latest results with HgTe colloidal quantum dots where the fluorescence extends from around 1 /spl mu/m to almost 2 /spl mu/m, and with quantum efficiencies as high as 40% when pumped in the visible with an Argon ion laser running on all-lines. We have found that 3D quantum confinement effects in nanocrystallites may be used to size-tune the absorption and fluorescence so that the latter can be positioned to cover the 1300 nm and 1550 nm telecommunications windows.

Photobleaching and resonant mode shift in a microsphere-quantum dot system

Photobleaching of semiconductor quantum dots (QDs) strongly reduces the photoluminescence (PL) intensity of the emitted light generated by excitation with a pump laser beam. We investigated the photoluminescence (PL) spectrum of CdTe nanocrystals synthesised in an aqueous solution. Light emitted from the QDs is coupled into the microcavity and trapped inside by total internal reflection. The spectrum of the samples was taken with a RENISHAW micro-Raman system. A strong dependency between the shifting speed of the WGMs and the quality and quantity of the detected biomaterial could lead to a new highly sensitive biosensor application

Wavelength tunable optical mode lifetimes in photonic molecules: new concept for multiplexing and delaying an optical signal

In this paper, we report on the space selective optical switching and detailed lineshape analysis of m-resonances in a PM formed from two dielectric spherical microcavities with CdTe nanocrystals, which allow us to propose a new concept of a multi-channel, wavelength tunable delaying optical device. A layer-by-layer deposition technique provides controllable coating of the latex microspheres (3 mum in diameter) with a shell of close-packed nanocrystals of approximately 4 nm in size

Coupled cavity modes in photonic molecules

In this paper, we have studied the WGM (whispering-gallery-modes) structure in interacting spherical microcavities formed from melamine formaldehyde (MF) latex microspheres of 3.078 /spl mu/m average diameter (MicroParticles GmbH) with a thin shell of CdTe nanocrystals (NCs) on their surface. The micro-photoluminescence (PL) spectra were measured using a RENISHAW micro-Raman system (1800 mm/sup -1/ grating, /spl sim/1 cm/sup -1/ resolution).