David J. Binks

Infrared emission and energy transfer in Tm 3+ , Tm 3+ -Ho 3+ and Tm 3+ -Yb 3+ -doped tellurite fibre

The infrared emission spectra and decay lifetimes of Tm(3+)-doped and Tm(3+)-Ho(3+), Tm(3+)-Yb(3+) co-doped tellurite fibres were measured using 808 nm and 978 nm diode laser pump sources in the range 1.35 mum to 2.2 mum. The spectra were compared with varying fibre lengths and core diameters. Tm(3+)-doped fibre shows strong emission at ~1.8 mum and when co-doped with Ho(3+), energy transfer results in strong Ho(3+) fluorescence at ~2.0 mum. These fibres show promise for compact mid-IR fibre laser sources.

A Yb3+/Tm3+/Ho3+ triply-doped tellurite fibre laser

Continuous wave laser emission at 2.1 microm from a Yb(3+)/Tm(3+)/Ho(3+) triply-doped tellurite fibre laser is reported. The fibre was pumped at 1.1 microm by a Yb(3+)-doped double-clad silica fibre laser. For a 17 cm fibre length and 99%???60% reflectance cavity, the threshold was 15 mW and the slope efficiency was 25%. A maximum output of 60 mW was observed for a launched pump power of 270 mW, corresponding to 22% optical-to-optical efficiency.

Electronic and surface properties of PbS nanoparticles exhibiting efficient multiple exciton generation

Ultrafast transient absorption measurements have been used to study multiple exciton generation in solutions of PbS nanoparticles vigorously stirred to avoid the effects of photocharging. The threshold and slope efficiency of multiple exciton generation are found to be 2.5 ± 0.2 ×E(g) and 0.34 ± 0.08, respectively. Photoemission measurements as a function of nanoparticle size and ageing show that the position of the valence band maximum is pinned by surface effects, and that a thick layer of surface oxide is rapidly formed at the nanoparticle surfaces on exposure to air.

Controlled Synthesis of Tuned Bandgap Nanodimensional Alloys of PbSxSe1−x

Truly alloyed PbS(x)Se(1-x) (x = 0-1) nanocrystals (∼5 nm in size) have been prepared, and their resulting optical properties are red-shifted systematically as the sulfur content of the materials increases. Their optical properties are discussed using a modified Vegards approach and the bowing parameter for these nanoalloys is reported for the first time. The alloyed structure of the nanocrystals is supported by the energy-filtered transmission electron microscope images of the samples, which show a homogeneous distribution of sulfur and selenium within the nanocrystals. X-ray photoelectron spectroscopy studies on ligand-exchanged nanocrystals confirmed the expected stoichiometry and various oxidized species.

Tm3+/Ho3+ codoped tellurite fiber laser

Continuous-wave and Q-switched lasing from a Tm(3+)/Ho(3+) codoped tellurite fiber is reported. An Yb(3+)/Er(3+)-doped silica fiber laser operating at 1.6 microm was used as an in-band pump source, exciting the Tm(3+) ions into the (3)F(4) level. Energy is then nonradiatively transferred to the upper laser level, the (5)I(7) state of Ho(3+). The laser transition is from the (5)I(7) level to the (5)I(8) level, and the resulting emission is at 2.1 microm. For continuous wave operation, the slope efficiency was 62% and the threshold 0.1 W; the maximum output demonstrated was 0.16 W. Mechanical Q switching resulted in a pulse of 0.65 microJ energy and 160 ns duration at a repetition rate of 19.4 kHz.

Multiple exciton generation in nanocrystal quantum dots – controversy, current status and future prospects

Multiple exciton generation is a process that can occur in quantum dots by which the energy of an absorbed photon in excess of the bandgap can be used to create one or more additional excitons instead of being wasted as heat. This effect has received considerable interest because it has the potential to significantly enhance the performance of solar cells, nanocrystal lasers, high speed electronic devices and photocatalysts. However, measuring the efficiency of multiple exciton generation is experimentally challenging and the results of these measurements have been the subject of some controversy. This Perspective describes the techniques used to determine the quantum yield of multiexcitons in nanocrystals and also details the experimental artefacts that can confuse these measurements and have been the source of much of the recent debate. The greater understanding of these artefacts that has emerged recently and the experimental techniques developed to eliminate their effects on quantum yield measurements will also be described. The efficiency of multiple exciton generation currently obtainable from nanocrystals and its potential impact on solar cell performance is assessed in the light of this improved experimental understanding. Whilst it is found the quantum yields thus far reported are insufficient to result in more than a modest increase in solar cell efficiency, an analysis of the expected performance of a nanocrystal engineered to maximise multiple exciton generation indicates that a significant improvement in solar cell performance is possible. Moreover, a nanocrystal design is proposed for optimised efficiency of multiple exciton generation which would allow its potential benefit to solar power production to be realised.

Multiple Exciton Generation in Colloidal Nanocrystals

In a conventional solar cell, the energy of an absorbed photon in excess of the band gap is rapidly lost as heat, and this is one of the main reasons that the theoretical efficiency is limited to ~33%. However, an alternative process, multiple exciton generation (MEG), can occur in colloidal quantum dots. Here, some or all of the excess energy is instead used to promote one or more additional electrons to the conduction band, potentially increasing the photocurrent of a solar cell and thereby its output efficiency. This review will describe the development of this field over the decade since the first experimental demonstration of multiple exciton generation, including the controversies over experimental artefacts, comparison with similar effects in bulk materials, and the underlying mechanisms. We will also describe the current state-of-the-art and outline promising directions for further development.

CdSe/CdS core/shell quantum dots as sensitizer of a photorefractive polymer composite

Abstract CdSe/CdS core/shell, tri-n-octylphosphine oxide passivated, quantum dots are used to sensitize a photorefractive polymer composite. The composite also consists of poly(N-vinylcarbazole) as the nominally charge transporting matrix and an electro-optic chromophore. The efficacy of sensitization and consequent photorefractive performance is investigated using transmission spectroscopy and ellipsometry, two-beam coupling and degenerate four-wave mixing experiments. The photorefractive nature of the photo-induced grating is confirmed by the observation of asymmetric two-beam coupling. Four-wave mixing reveals record diffraction efficiencies for a nano-particle-sensitized photorefractive polymer at the field levels applied (1.3% at 70 V.μm−1). A recently developed analytical technique is used to extract space-charge field rise time values from degenerate four-wave mixing transients. In turn, analysis of the dependence of the rise time on applied field is used to determine the zero-field charge dissociation efficiency to be 3.6 × 10−5 ± 0.5 × 10−5. It is further shown that the magnitude of this parameter accounts for most of the difference in photorefractive response rate between the present material and a similar C60 sensitized composite.

Near-unity quantum yields from chloride treated CdTe colloidal quantum dots.

Colloidal quantum dots (CQDs) are promising materials for novel light sources and solar energy conversion. However, trap states associated with the CQD surface can produce non-radiative charge recombination that significantly reduces device performance. Here a facile post-synthetic treatment of CdTe CQDs is demonstrated that uses chloride ions to achieve near-complete suppression of surface trapping, resulting in an increase of photoluminescence (PL) quantum yield (QY) from ca. 5% to up to 97.2 ± 2.5%. The effect of the treatment is characterised by absorption and PL spectroscopy, PL decay, scanning transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. This process also dramatically improves the air-stability of the CQDs: before treatment the PL is largely quenched after 1 hour of air-exposure, whilst the treated samples showed a PL QY of nearly 50% after more than 12 hours.

Multiple exciton generation and ultrafast exciton dynamics in HgTe colloidal quantum dots.

The investigation of sub-nanosecond exciton dynamics in HgTe colloidal quantum dots using ultrafast transient absorption spectroscopy is reported. The transmittance change spectrum acquired immediately after pumping is dominated by a bleach blue-shifted by ~200-300 nm from the photoluminescent emission band. Comparison with a tight-binding model of the electronic structure allows this feature to be attributed to the filling of band edge states. The form of the pump-induced transmittance transients is dependent on the excitation rate and the rate of sample stirring. For moderate pumping of stirred samples, the transmittance transients are well-described by a mono-exponential decay associated with biexciton recombination, with a lifetime of 49 ± 2 ps. For samples that are strongly-pumped or unstirred, the decay becomes bi-exponential in form, indicating that trap-related recombination has become significant. We also present a new analysis that enables fractional transmittance changes to be related to band edge occupation for samples with arbitrary optical density at the pump wavelength. This allows us to identify the occurrence of multiple exciton generation, which results in a quantum yield of 1.36 ± 0.04 for a photon energy equivalent to 3.1 times the band gap, in good agreement with the results of the model.