Ultrafast exciton dynamics in CdxHg(1-x)Te alloy quantum dots

Ultrafast transient absorption spectroscopy is used to investigate sub-nanosecond exciton dynamics in CdxHg(1−x)Te alloy colloidal quantum dots. A bleach was observed at the band gap due to state-filling, the mono-exponential decay of which had a characteristic lifetime of 91 ± 1 ps and was attributed to biexciton recombination; no evidence of surface-related trapping was observed. The rise time of the bleach, which is determined by the rate at which hot electrons cool to the band-edge, ranged between 1 and 5 ps depending on the pump photon energy. Measuring the magnitude of the bleach decay for different pump fluences and wavelengths allowed the quantum yield of multiple exciton generation to be determined, and was 115 ± 1% for pump photons with energy equivalent to 2.6 times the band gap

Influence of elevated radiative lifetime on efficiency of CdSe/CdTe Type II colloidal quantum dot based solar cells

Colloidal quantum dots (CQDs) are promising materials for solar cells because their optoelectronic properties are easily adjusted by control of their size, structure and composition. We present calculations of the band gap and radiative lifetime for varying core diameter and shell thickness of CdSe/CdTe core/shell Type II CQDs using a combination of single particle (2,6)-band k·pk·p and many-electron configuration interaction (CI) Hamiltonians. These calculations are validated by comparison with experimental absorption spectra and photoluminescence decay data. The results are then incorporated into a model of photovoltaic efficiency which demonstrates how the overall performance of a solar cell based on Type II CQDs is affected by changes in the core/shell geometry. The largest effect on photovoltaic efficiency is found to be due to the longer radiative lifetime produced by increasing the shell thickness

Spin Dynamics and Spin Control in Narrow Gap Semiconductors.

We report a spin dynamic investigation with the use of the optical polarisation pump-probe technique in bulk and quantum well narrow gap semiconductors (NGSs) of the III-V and IV-VI families. By taking advantage of their zinc-blende (III-Vs) and rock-salt (IV-VIs) crystal symmetries, their direct energy band-gap, small effective mass, high electron effective g-value, strong relativistic effects, and with the use of the Surrey Ultrafast Laser system and the Free Electron Laser system (FELIX) for interband excitation process we prove that they are attractive for high speed electronic and new proposed spintronic concepts. The electron spin relaxation times have been measured in InSb and InAs epilayers in Faraday and Voigt configurations. A strong and opposite field dependence of the spin lifetime has been observed. We report the existence of a new spin relaxation process, which was theoretically predicted but not observed before. We demonstrate that for these NGSs, and specifically for the III-Vs, the electron spin lifetime can be modified by more than one order of magnitude simply by changing the direction of a moderate, externally applied magnetic field. We used circular two-photon absorption (CTPA) and investigated the “allowed-forbidden” and “allowed-allowed” transitions in n-InSb, the theory of which has not been investigated experimentally until now. The detailed CTPA spectrum presented here shows that the sign of photo-excited spin polarisation can in principle be controlled by the excitation wavelength in the two-photon absorption processes. We report the analysis of optical measurement of the spin dynamics at elevated temperatures and in zero magnetic field, for degenerately n-doped InSb/InAlSb QWs, one asymmetric and one symmetric. For the asymmetric QW, by making use of directly determined experimental parameters, we have made a direct measurement of the zero field spin splitting without the influence of the large Zeeman effect. The extracted Rashba parameter is more than an order of magnitude larger than that measured earlier for GaAs QWs. Pump-pump photoconductivity experiments have been performed and we successfully measured the recombination time in InSb-based devices of different geometries, allowing new investigation methods of low dimensional structures and manipulation of samples of weak transmittance signals. We investigated the dependence of spin lifetime on controlled asymmetry in IV-VI Multi-QWs. We show for the first time experimentally that lead-chalcogenide semiconductor heterostructures can exhibit long spin lifetimes by virtue of their centro-symmetric crystal structure, and a tuning of the lifetime of over one order of magnitude after appropriate structure control corresponding to a large zero field spin splitting. The results imply that this system can be the material of choice for certain semiconductor spintronic applications requiring control of spins. We report a significant temperature dependence of the transverse electron g*-factor in symmetric IV-VI Multi-QWs. A second oscillation frequency has been observed, suggesting a possible method for distinguishing the actual excited states and the activation of an electron-hole mechanism. Although temperature tuning of lead salt laser emission wavelengths has been the method of choice in these systems for many years, we demonstrate that temperature can also be used to modulate g*, and hence the spin lifetime in lead salt QW spintronic devices

Spin dynamics and spin control in narrow gap semiconductors

We report a spin dynamic investigation with the use of the optical polarisation pump-probe technique in bulk and quantum well narrow gap semiconductors (NGSs) of the Ill-V and IV-VI families. By taking advantage of their zinc-blende (Ill-Vs) and rock-salt (IV-VIs) crystal symmetries, their direct energy band-gap, small effective mass, high electron effective g-value, strong relativistic effects, and with the use of the Surrey Ultrafast Laser system and the Free Electron Laser system (FELIX) for interband excitation process we prove that they are attractive for high speed electronic and new proposed spintronic concepts. The electron spin relaxation times have been measured in InSb and InAs epilayers in Faraday and Voigt configurations. A strong and opposite field dependence of the spin lifetime has been observed. We report the existence of a new spin relaxation process, which was theoretically predicted but not observed before. We demonstrate that for these NGSs, and specifically for the Ill-Vs, the electron spin lifetime can be modified by more than one order of magnitude simply by changing the direction of a moderate, externally applied magnetic field. We used circular two-photon absorption (CTPA) and investigated the "allowed- forbidden" and "allowed-allowed" transitions in n-InSb, the theory of which has not been investigated experimentally until now. The detailed CTPA spectrum presented here shows that the sign of photo-excited spin polarisation can in principle be controlled by the excitation wavelength in the two-photon absorption processes. We report the analysis of optical measurement of the spin dynamics at elevated temperatures and in zero magnetic field, for degenerately n-doped InSb/lnAISb QWs, one asymmetric and one symmetric. For the asymmetric QW, by making use of directly determined experimental parameters, we have made a direct measurement of the zero field spin splitting without the influence of the large Zeeman effect. The extracted Rashba parameter is more than an order of magnitude larger than that measured earlier for GaAs QW s. Pump-pump photoconductivity experiments have been performed and we successfully measured the recombination time in InSb-based devices of different geometries, allowing new investigation methods of low dimensional structures and manipulation of samples of weak transmittance signals. We investigated the dependence of spin lifetime on controlled asymmetry in IV- VI Multi-QWs. We show for the first time experimentally that lead-cha1cogenide semiconductor heterostructures can exhibit long spin lifetimes by virtue of their centro-symmetric crystal structure, and a tuning of the lifetime of over one order of magnitude after appropriate structure control corresponding to a large zero field spin splitting. The results imply that this system can be the material' of choice for certain , semiconductor spintronic applications requiring control of spins. We report a significant temperature dependence of the transverse electron g*- factor in symmetric IV-VI Multi-QWs. A second oscillation frequency has been observed, suggesting a possible method for distinguishing the actual excited states and the activation of an electron-hole mechanism. Although temperature tuning of lead salt laser emission wavelengths has been the method of choice in these systems for many years, we demonstrate that temperature can also be used to modulate g*, and hence the spin lifetime in lead salt QW spintronic devices.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Ultrafast Charge Dynamics in Dispersions of Monolayer MoS₂ Nanosheets

Ultrafast charge dynamics in dispersions of MoS₂ nanosheets in <i>N</i>-methyl-2-pyrrolidone are reported. Samples were prepared by the ultrasonication-assisted exfoliation of MoS₂ powder, resulting in nanosheets that were predominantly monolayer and had an average sheet size of 32.4 ± 0.1 nm. These dispersions were characterized using absorption and photoluminescence spectroscopy, transient photoluminescence measurements, and atomic force microscopy before the ultrafast charge dynamics were studied via transient absorption spectroscopy. The transient absorption spectra exhibited bleach peaks and photoinduced absorption peaks in spectral regions corresponding to both the A and B excitons of MoS₂. The growth and decay of the features in the B exciton region were determined largely by the dynamics of the exciton population, while the features in the A exciton region depend on the dynamics of both excitons and trions

Ultrafast Charge Dynamics in Dispersions of Monolayer MoS₂ Nanosheets

Ultrafast charge dynamics in dispersions of MoS₂ nanosheets in <i>N</i>-methyl-2-pyrrolidone are reported. Samples were prepared by the ultrasonication-assisted exfoliation of MoS₂ powder, resulting in nanosheets that were predominantly monolayer and had an average sheet size of 32.4 ± 0.1 nm. These dispersions were characterized using absorption and photoluminescence spectroscopy, transient photoluminescence measurements, and atomic force microscopy before the ultrafast charge dynamics were studied via transient absorption spectroscopy. The transient absorption spectra exhibited bleach peaks and photoinduced absorption peaks in spectral regions corresponding to both the A and B excitons of MoS₂. The growth and decay of the features in the B exciton region were determined largely by the dynamics of the exciton population, while the features in the A exciton region depend on the dynamics of both excitons and trions

Energy Structure of CdSe/CdTe Type II Colloidal Quantum Dots - Do Phonon Bottlenecks Remain for Thick Shells?

AbstractThe electronic structures of CdSe/CdTe type II colloidal quantum dots are predicted using a model based on k·p theory and the many-particle configuration interaction method. The separation of energy levels in the conduction band is examined and used to identify phonon bottlenecks, and how these evolve as the shell thickness is increased. Bottlenecks are found to persist both above and below the threshold for multiple exciton generation for all the shell thicknesses investigated. The overall electron cooling rate is thus expected to fall as the shell thickness is increased and Auger cooling suppressed, and this is confirmed experimentally using ultrafast transient absorption measurements. A reduced overall rate of electron cooling will enhance the quantum yield of multiple exciton generation with which it competes. Using a detailed-balance model, we have thus calculated that with proper design of core/and shell structures the efficiency of a solar cell based on CdSe/CdTe quantum dots can be enhanced to 36.5% by multiple exciton generation

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