Physical Review B Condensed Matter and Materials Physics, 2006
In the impact ionization process, a hot carrier relaxes by generating an exciton. We present tigh... more In the impact ionization process, a hot carrier relaxes by generating an exciton. We present tight binding calculations showing that the rate of this process in PbSe nanocrystals has a strong energy dependence and that the relaxation energy increases linearly with the carrier excess energy. The impact ionization can be extremely fast (˜fs) but it is not enhanced by the
Colloidal quantum dots (QDs) raise more and more interest as solution-processable and tunable opt... more Colloidal quantum dots (QDs) raise more and more interest as solution-processable and tunable optical gain materials. However, especially for infrared active QDs, optical gain remains inefficient. Since stimulated emission involves multifold degenerate band-edge states, population inversion can be attained only at high pump power and must compete with efficient multi-exciton recombination. Here, we show that mercury telluride (HgTe) QDs exhibit size-tunable stimulated emission throughout the near-infrared telecom window at thresholds unmatched by any QD studied before. We attribute this unique behaviour to surface-localized states in the bandgap that turn HgTe QDs into 4-level systems. The resulting long-lived population inversion induces amplified spontaneous emission under continuous-wave optical pumping at power levels compatible with solar irradiation and direct current electrical pumping. These results introduce an alternative approach for low-threshold QD-based gain media base...
One of the important factors limiting solar-cell efficiency is that incident photons generate one... more One of the important factors limiting solar-cell efficiency is that incident photons generate one electron-hole pair, irrespective of the photon energy. Any excess photon energy is lost as heat. The possible generation of multiple charge carriers per photon (carrier multiplication) is therefore of great interest for future solar cells 1 . Carrier multiplication is known to occur in bulk semiconductors, but has been thought to be enhanced significantly in nanocrystalline materials such as quantum dots, owing to their discrete energy levels and enhanced Coulomb interactions 1-3 . Contrary to this expectation, we demonstrate here that, for a given photon energy, carrier multiplication occurs more efficiently in bulk PbS and PbSe than in quantum dots of the same materials. Measured carriermultiplication efficiencies in bulk materials are reproduced quantitatively using tight-binding calculations, which indicate that the reduced carrier-multiplication efficiency in quantum dots can be ascribed to the reduced density of states in these structures.
The electronic structure of CdTe and CdSe nanocrystals has been calculated as a function of size ... more The electronic structure of CdTe and CdSe nanocrystals has been calculated as a function of size using the tight-binding approximation with an sp3d5s* basis including spin-orbit coupling and taking into account d core atomic functions on the cation atoms. Two particle electron-hole interaction is also incorporated in the calculation of the photoemission decay rate. For CdSe, we compare the results for the two zinc blende and wurtzite atomic structures. Comparison is made with recent experimental results.
Ultraviolet irradiation of pyrimidine and 4-and 5methylpyrimidine in methanol at 113 K gave the c... more Ultraviolet irradiation of pyrimidine and 4-and 5methylpyrimidine in methanol at 113 K gave the corresponding semiquinone radicals in a monophotonic process. It is likely that this process involves an na* triplet state of the photoexcited pyrimidines.
ABSTRACT The irradiation of 9-phenylacridine in methanol, ethanol, or ether leads to the formatio... more ABSTRACT The irradiation of 9-phenylacridine in methanol, ethanol, or ether leads to the formation of the 9-phenylacridinyl radical. In methanol-d4, the radical formed is the singly deuterated analog. The structure of these radicals, which exhibit a hyperfine structure in their e.s.r. spectra measured at 233 °K, is supported by simulation of the spectra using calculated spin densities. The agreement between the experimental and theoretical second moment values of the deuterated radical indicates that the parameters previously chosen for the calculation of the theoretical second moments of pyridinyl-type radicals derived from six-membered ring azaaromatics are suitable and that the second moment method can be used in studies of radicals of this type. We conclude that acridine irradiated in methanol-d4 leads to the formation of the singly deuterated acridinyl radical.
In a self-consistent free-electron scheme, the authors calculate the charge oscillations and the ... more In a self-consistent free-electron scheme, the authors calculate the charge oscillations and the interface energy of two metals they show that the Friedel oscillations are small. Exchange and correlations reduce the interface energy calculated in the Hartree approximation. Application is made to the Ag-Al interface. The interface energy is of the same order of magnitude as the surface energies.
In degenerate narrow bands studied in the tight binding limit, Wannier functions of partial bands... more In degenerate narrow bands studied in the tight binding limit, Wannier functions of partial bands cannot be used for computing intraatomic Coulomb interactions, because each of them involves several atomic states, on different sites. In the (Hartree-Fock) approximation using one determinant made of Bloch functions, the change in intraatomic Coulomb energy from free atoms to paramagnetic metal thus varies parabolically with the numbers of occupied atomic states, and not with the number of occupied states in each partial band. ...
Depending on the size, the photoluminescence (PL) of silicon quantum dots present in porous silic... more Depending on the size, the photoluminescence (PL) of silicon quantum dots present in porous silicon can be tuned from the near infrared to the ultraviolet when the surface is passivated with Si-H bonds. After exposure to oxygen, the PL shifts to the red by as much as 1 eV. This shift and the changes in PL intensity and decay time, show that both quantum confinement and surface passivation determine the electronic states of silicon quantum dots. A theoretical model in which new electronic states appear in the band gap of the smaller quantum dots when a Si=O bond is formed, is in good agreement with experiments. This result clarifies the controversy regarding the PL mechanisms in porous silicon.
Physical Review B Condensed Matter and Materials Physics, May 25, 2009
We present calculations predicting that defects at the surface of semiconductor nanocrystals have... more We present calculations predicting that defects at the surface of semiconductor nanocrystals have a strong influence on the dynamics of hot carriers after photoexcitation. The relaxation of excited carriers by impact ionization of carriers trapped on deep levels is efficient, in particular in the presence of a band of defect states in the gap. Impact ionization of defects can also induce single-carrier multiplication but carriers generated in this way have a 1-100 ps lifetime due to multiphonon capture by the defects. These results are used to discuss recent experimental studies on carrier relaxation and multiplication in nanocrystals.
Recent works on semiconductor quantum dots charged with electrons reported optical transitions be... more Recent works on semiconductor quantum dots charged with electrons reported optical transitions between the lowest states in the conduction band, the analog of Sarrow P transitions in atoms. All these works concerned semiconductors characterized by a direct gap. In contrast, almost nothing is known about the intraband transitions in indirect gap semiconductors, in particular in Si in spite of an intense activity on the optical properties of this material. We presents tight binding calculations of no-phonon and phonon-assisted intraband optical transitions of electrons in Si nanocrystals. We predict the existence of new types of transitions between states originating from the six conduction band minima whose the degeneracy is lifted by the quantum confinement. Our work suggests that study of these transitions should provide rich information on the electronic structure of the nanocrystals and on the inter-valley scattering of the electrons.
Physical Review B Condensed Matter and Materials Physics, 2006
In the impact ionization process, a hot carrier relaxes by generating an exciton. We present tigh... more In the impact ionization process, a hot carrier relaxes by generating an exciton. We present tight binding calculations showing that the rate of this process in PbSe nanocrystals has a strong energy dependence and that the relaxation energy increases linearly with the carrier excess energy. The impact ionization can be extremely fast (˜fs) but it is not enhanced by the
Colloidal quantum dots (QDs) raise more and more interest as solution-processable and tunable opt... more Colloidal quantum dots (QDs) raise more and more interest as solution-processable and tunable optical gain materials. However, especially for infrared active QDs, optical gain remains inefficient. Since stimulated emission involves multifold degenerate band-edge states, population inversion can be attained only at high pump power and must compete with efficient multi-exciton recombination. Here, we show that mercury telluride (HgTe) QDs exhibit size-tunable stimulated emission throughout the near-infrared telecom window at thresholds unmatched by any QD studied before. We attribute this unique behaviour to surface-localized states in the bandgap that turn HgTe QDs into 4-level systems. The resulting long-lived population inversion induces amplified spontaneous emission under continuous-wave optical pumping at power levels compatible with solar irradiation and direct current electrical pumping. These results introduce an alternative approach for low-threshold QD-based gain media base...
One of the important factors limiting solar-cell efficiency is that incident photons generate one... more One of the important factors limiting solar-cell efficiency is that incident photons generate one electron-hole pair, irrespective of the photon energy. Any excess photon energy is lost as heat. The possible generation of multiple charge carriers per photon (carrier multiplication) is therefore of great interest for future solar cells 1 . Carrier multiplication is known to occur in bulk semiconductors, but has been thought to be enhanced significantly in nanocrystalline materials such as quantum dots, owing to their discrete energy levels and enhanced Coulomb interactions 1-3 . Contrary to this expectation, we demonstrate here that, for a given photon energy, carrier multiplication occurs more efficiently in bulk PbS and PbSe than in quantum dots of the same materials. Measured carriermultiplication efficiencies in bulk materials are reproduced quantitatively using tight-binding calculations, which indicate that the reduced carrier-multiplication efficiency in quantum dots can be ascribed to the reduced density of states in these structures.
The electronic structure of CdTe and CdSe nanocrystals has been calculated as a function of size ... more The electronic structure of CdTe and CdSe nanocrystals has been calculated as a function of size using the tight-binding approximation with an sp3d5s* basis including spin-orbit coupling and taking into account d core atomic functions on the cation atoms. Two particle electron-hole interaction is also incorporated in the calculation of the photoemission decay rate. For CdSe, we compare the results for the two zinc blende and wurtzite atomic structures. Comparison is made with recent experimental results.
Ultraviolet irradiation of pyrimidine and 4-and 5methylpyrimidine in methanol at 113 K gave the c... more Ultraviolet irradiation of pyrimidine and 4-and 5methylpyrimidine in methanol at 113 K gave the corresponding semiquinone radicals in a monophotonic process. It is likely that this process involves an na* triplet state of the photoexcited pyrimidines.
ABSTRACT The irradiation of 9-phenylacridine in methanol, ethanol, or ether leads to the formatio... more ABSTRACT The irradiation of 9-phenylacridine in methanol, ethanol, or ether leads to the formation of the 9-phenylacridinyl radical. In methanol-d4, the radical formed is the singly deuterated analog. The structure of these radicals, which exhibit a hyperfine structure in their e.s.r. spectra measured at 233 °K, is supported by simulation of the spectra using calculated spin densities. The agreement between the experimental and theoretical second moment values of the deuterated radical indicates that the parameters previously chosen for the calculation of the theoretical second moments of pyridinyl-type radicals derived from six-membered ring azaaromatics are suitable and that the second moment method can be used in studies of radicals of this type. We conclude that acridine irradiated in methanol-d4 leads to the formation of the singly deuterated acridinyl radical.
In a self-consistent free-electron scheme, the authors calculate the charge oscillations and the ... more In a self-consistent free-electron scheme, the authors calculate the charge oscillations and the interface energy of two metals they show that the Friedel oscillations are small. Exchange and correlations reduce the interface energy calculated in the Hartree approximation. Application is made to the Ag-Al interface. The interface energy is of the same order of magnitude as the surface energies.
In degenerate narrow bands studied in the tight binding limit, Wannier functions of partial bands... more In degenerate narrow bands studied in the tight binding limit, Wannier functions of partial bands cannot be used for computing intraatomic Coulomb interactions, because each of them involves several atomic states, on different sites. In the (Hartree-Fock) approximation using one determinant made of Bloch functions, the change in intraatomic Coulomb energy from free atoms to paramagnetic metal thus varies parabolically with the numbers of occupied atomic states, and not with the number of occupied states in each partial band. ...
Depending on the size, the photoluminescence (PL) of silicon quantum dots present in porous silic... more Depending on the size, the photoluminescence (PL) of silicon quantum dots present in porous silicon can be tuned from the near infrared to the ultraviolet when the surface is passivated with Si-H bonds. After exposure to oxygen, the PL shifts to the red by as much as 1 eV. This shift and the changes in PL intensity and decay time, show that both quantum confinement and surface passivation determine the electronic states of silicon quantum dots. A theoretical model in which new electronic states appear in the band gap of the smaller quantum dots when a Si=O bond is formed, is in good agreement with experiments. This result clarifies the controversy regarding the PL mechanisms in porous silicon.
Physical Review B Condensed Matter and Materials Physics, May 25, 2009
We present calculations predicting that defects at the surface of semiconductor nanocrystals have... more We present calculations predicting that defects at the surface of semiconductor nanocrystals have a strong influence on the dynamics of hot carriers after photoexcitation. The relaxation of excited carriers by impact ionization of carriers trapped on deep levels is efficient, in particular in the presence of a band of defect states in the gap. Impact ionization of defects can also induce single-carrier multiplication but carriers generated in this way have a 1-100 ps lifetime due to multiphonon capture by the defects. These results are used to discuss recent experimental studies on carrier relaxation and multiplication in nanocrystals.
Recent works on semiconductor quantum dots charged with electrons reported optical transitions be... more Recent works on semiconductor quantum dots charged with electrons reported optical transitions between the lowest states in the conduction band, the analog of Sarrow P transitions in atoms. All these works concerned semiconductors characterized by a direct gap. In contrast, almost nothing is known about the intraband transitions in indirect gap semiconductors, in particular in Si in spite of an intense activity on the optical properties of this material. We presents tight binding calculations of no-phonon and phonon-assisted intraband optical transitions of electrons in Si nanocrystals. We predict the existence of new types of transitions between states originating from the six conduction band minima whose the degeneracy is lifted by the quantum confinement. Our work suggests that study of these transitions should provide rich information on the electronic structure of the nanocrystals and on the inter-valley scattering of the electrons.
Uploads
Papers by Guy Allan