9 research outputs found
Minimum Line Width of Surface Plasmon Resonance in Doped ZnO Nanocrystals
The
optical response of ZnO nanocrystals (NCs) doped with Al (Ga)
impurities is calculated using a model that incorporates the effects
of quantum confinement, dielectric mismatch, surface, and ionized
impurity scattering. For dopant concentrations of a few percent, the
NC polarizability is dominated by a localized surface plasmon resonance
(LSPR) in the infrared (IR) which follows the Drude-Lorentz law for
NC diameter above ā¼10 nm but is strongly blue-shifted for smaller
diameters due to quantum confinement effects. The intrinsic width
of the LSPR peak is calculated in order to characterize plasmon losses
induced by ionized impurity scattering. Widths below 80 meV are found
in the best cases, in agreement with the lowest values recently measured
on single NCs. These results confirm that doped ZnO NCs are very promising
for the development of IR plasmonics. The width of the LSPR peak strongly
increases when dopants are placed near the surface of the NCs or when
additional fixed charges are present
Effects of Strain on the Carrier Mobility in Silicon Nanowires
We investigate electron and hole mobilities in strained
silicon
nanowires (Si NWs) within an atomistic tight-binding framework. We
show that the carrier mobilities in Si NWs are very responsive to
strain and can be enhanced or reduced by a factor >2 (up to 5Ć)
for moderate strains in the Ā±2% range. The effects of strain
on the transport properties are, however, very dependent on the orientation
of the nanowires. Stretched āØ100ā© Si NWs are found to
be the best compromise for the transport of both electrons and holes
in ā10 nm diameter Si NWs. Our results demonstrate that strain
engineering can be used as a very efficient booster for NW technologies
and that due care must be given to process-induced strains in NW devices
to achieve reproducible performances
Mercury Telluride Colloidal Quantum Dots: Electronic Structure, Size-Dependent Spectra, and Photocurrent Detection up to 12 Ī¼m
HgTe colloidal quantum dots are synthesized with high monodispersivity with sizes up to ā¼15 nm corresponding to a room temperature absorption edge at ā¼5 Ī¼m. The shape is tetrahedral for larger sizes and up to five peaks are seen in the absorption spectra with a clear size dependence. The size range of the HgTe quantum dots is extended to ā¼20 nm using regrowth. The corresponding room temperature photoluminescence and absorption edge reach into the long-wave infrared, past 8 Ī¼m. Upon cooling to liquid nitrogen temperature, a photoconductive response is obtained in the long-wave infrared region up to 12 Ī¼m. Configuration-interaction tight-binding calculations successfully explain the spectra and the size dependence. The five optical features can be assigned to sets of single hole to single electron transitions whose strengths are strongly influenced by the multiband/multiorbital character of the quantum-dot electronic states
Size Dependence of the Exciton Transitions in Colloidal CdTe Quantum Dots
In this paper, we present a detailed investigation of
the size
dependence of the optical transitions of colloidal CdTe QDs ranging
in diameter from 2.9 to 14.8 nm. The energy integrated absorption
cross section per CdTe unit is investigated in detail for the lowest
two exciton transitions (1S<sub>3/2(h)</sub>ā1S<sub>(e)</sub> and 2S<sub>3/2(h)</sub>ā1S<sub>(e)</sub>) and shown to increase
with decreasing size, although the size dependence of the 2S<sub>3/2(h)</sub>ā1S<sub>(e)</sub> is less pronounced. The experimental absorption
spectra are compared to spectra calculated by using a tight-binding
approach. The calculations were carried out with electronāhole
configuration interaction (CI) and without (single-particle, SP).
The optical absorption spectra calculated by using the CI approach
are in excellent agreement with the experiment, as well as the evolution
of the optical gap and the optical transitions with nanocrystal size
Nanoscale Carrier Multiplication Mapping in a Si Diode
Carrier multiplication (CM), the
creation of electronāhole
pairs from an excited electron, has been investigated in a silicon
pān junction by multiple probe scanning tunneling microscopy.
The technique enables an unambiguous determination of the quantum
yield based on the direct measurement of both electron and hole currents
that are generated by hot tunneling electrons. The combined effect
of impact ionization, carrier diffusion, and recombination is directly
visualized from the spatial mapping of the CM efficiency. Atomically
well-ordered areas of the pān junction surface sustain the
highest CM rate, demonstrating the key role of the surface in reaching
high yield
Comparative Study on the Localized Surface Plasmon Resonance of Boron- and Phosphorus-Doped Silicon Nanocrystals
Localized surface plasmon resonance (LSPR) of doped Si nanocrystals (NCs) is critical to the development of Si-based plasmonics. We now experimentally show that LSPR can be obtained from both B- and P-doped Si NCs in the mid-infrared region. Both experiments and calculations demonstrate that the Drude model can be used to describe the LSPR of Si NCs if the dielectric screening and carrier effective mass of Si NCs are considered. When the doping levels of B and P are similar, the LSPR energy of B-doped Si NCs is higher than that of P-doped Si NCs because B is more efficiently activated to produce free carriers than P in Si NCs. We find that the plasmonic coupling between Si NCs is effectively blocked by oxide at the NC surface. The LSPR quality factors of B- and P-doped Si NCs approach those of traditional noble metal NCs. We demonstrate that LSPR is an effective means to gain physical insights on the electronic properties of doped Si NCs. The current work on the model semiconductor NCs, <i>i.e.</i>, Si NCs has important implication for the physical understanding and practical use of semiconductor NC plasmonics
Asymmetric Optical Transitions Determine the Onset of Carrier Multiplication in Lead Chalcogenide Quantum Confined and Bulk Crystals
Carrier
multiplication is a process in which one absorbed photon
excites two or more electrons. This is of great promise to increase
the efficiency of photovoltaic devices. Until now, the factors that
determine the onset energy of carrier multiplication have not been
convincingly explained. We show experimentally that the onset of carrier
multiplication in lead chalcogenide quantum confined and bulk crystals
is due to asymmetric optical transitions. In such transitions most
of the photon energy in excess of the band gap is given to either
the hole or the electron. The results are confirmed and explained
by theoretical tight-binding calculations of the competition between
impact ionization and carrier cooling. These results are a large step
forward in understanding carrier multiplication and allow for a screening
of materials with an onset of carrier multiplication close to twice
the band gap energy. Such materials are of great interest for development
of highly efficient photovoltaic devices
Crystal Facet Engineering in Ga-Doped ZnO Nanowires for Mid-Infrared Plasmonics
The metalāorganic
chemical vapor deposition growth of various
Ga-doped ZnO nanostructures for plasmonics is investigated, with a
particular focus on the nanowire facet transformations induced by
the addition of trimethylgallium in the gas phase. For nonintentionally
doped spontaneous ZnO nanowires, the aspect ratio is strongly decreased
due to residual Ga in the reactor, and the shape evolves rapidly toward
Christmas-tree-like and hierarchical structures upon intentional Ga
doping. Regarding ZnO/ZnO:Ga coreāshell structures, a change
of the smooth initial M-oriented facets occurs, with the development
of {202Ģ
1} surfaces, and further {101Ģ
1} and {0001} surfaces.
Interestingly, a similar evolution of the lateral roughness is observed
in Au-catalyzed doped nanowires. High concentrations of Ga in the
grown nanostructures are revealed by photoluminescence and confirmed
by Rutherford backscattering spectrometry. First photoacoustic measurements
show an optical absorption at 6 Ī¼m, evidencing that the degenerated
material is suitable for plasmonics applications in the infrared range.
The influence of Ga doping on the facet transformations and the occurrence
of unexpected {0001} polar surfaces are discussed. The results can
be mainly understood by a Ga surfactant effect (at least partial)
responsible for the modification of the surface energies and kinetics.
Density functional calculations support the floating behavior of the
negatively charged Ga<sup>ā</sup> ion on the growing surface
Broadband and Picosecond Intraband Absorption in Lead-Based Colloidal Quantum Dots
Using femtosecond transient absorption spectroscopy, we demonstrate that lead chalcogenide nanocrystals show considerable photoinduced absorption (PA) in a broad wavelength range just below the band gap. The time-dependent decay of the PA signal correlates with the recovery of the band gap absorption, indicating that the same carriers are involved. On this basis, we assign this PA signal to intraband absorption, that is, the excitation of photogenerated carriers from the bottom of the conduction band or the top of the valence band to higher energy levels in the conduction and valence band continuum. We confirm our experiments with tight-binding calculations. This broadband response in the commercially interesting near- to mid-infrared range is very relevant for ultra-high-speed all-optical signal processing. We benchmark the performance with bulk Si and Si nanocrystals