Structural, optical and electrical studies of several hybrids of organic and inorganic nanostructures as well as core shell
nanocrystalline structures will be presented. The effects of thermal annealing on the morphological and photoconductive
properties of cadmium selenide quantum dots coreshell quantum dots together with indicate that there are collective
events happening due to annealing. Two different types of hybrid structures will be discussed. Optical and electrical
experimental results in semiconductor nanostructures in conductive polymers as well as those that were integrated into
the organic photosystem I (PS1), as part of an artificial light harvesting complex (LHC) will be presented.
Novel optoelectronic systems based on ensembles of semiconductor are described. We will present here the optical and electronic properties of organic-inorganic hybrid structures that enable integration of useful organic and inorganic characteristics for novel sensing applications. Several semiconductor nanostructures with both direct band gap and indirect band gap will be discussed in a few different polymer and biological matrices. A number of these colloidal
semiconductor quantum dots and related quantum-wire structures have been characterized using absorption, photoluminescence, and Raman measurements; these Raman measurements include those made on self-assembled monolayers of DNA molecules terminated on one end with a common substrate and on the other end with TiO2 quantum
dots. The electronic properties of these structures are modeled and compared with experiment. Devices fabricated with these materials as well as their potential for sensing will be discussed.
Nanoscale functionalization of semiconductor quantum dots (SQDs) with biomedical structures is promising for many
applications and novel studies of intrinsic properties of both constituent systems. Results of our study of structural
properties of the nanoscale functionalized SQDs such as CdS, and ZnS-capped CdSe SQDs, conjugated with
biomolecules such as short peptides and cells are presented. We study CdS SQDs functionalized with peptides specially
composed of the following amino acid chains: CGGGRGDS, CGGGRVDS, CGGIKVAV, and CGGGLDV, where R is
arginine, D - aspartic acid, S - serine, V - valine, K - lysine and L is Levine. As will be seen the cysteine (C) amino acid
links to CdS SQDs via the thiol link, the GGG sequences of glycine (G) amino acid, provide a spacer in the amino acid
chain. At the same time the RGDS, RVDS, IKAV, and LDV sequences have selective bonding affinities to specialized
transmembrane cellular structures known as integrins of neurons and MDA-MB-435 cancer cells, respectively. We found
that the quantum confinement and functionalizing in biomedical environments plays in altering and determining the
electronic, optical, and vibrational properties of these nanostructures as well as demonstrated the effectiveness to use
semiconductor quantum dots as integrin sensitive biotags.
Spontaneous polarizations of GaN nanostructures and quantum dots are calculated for different surface terminations. In
addition, dimensionally-confined phonons in GaN-based nanostructures are discussed. GaN-based nanostructures have
applications in a variety of systems and concepts including: non-charge-transfer-based devices and single-photon
detectors based on GaN-based double-barrier quantum-well injectors, conductive-polymer collectors, and colloidal
quantum dot recombination regions. In this paper, application of photodetectors is considered along with the related
application of using colloidal quantum-dot-based ensembles for solar cell applications.
Novel optoelectronic systems based on ensembles of semiconductor nanocrystals are addressed in this paper. Colloidal
semiconductor quantum dots and related quantum-wire structures have been characterized optically; these optical
measurements include those made on self-assembled monolayers of DNA molecules terminated on one end with a
common substrate and on the other end with TiO2 quantum dots. The electronic properties of these structures are
modeled and compared with experiment. The characterization and application of ensembles of colloidal quantum dots
with molecular interconnects are considered. The chemically-directed assembly of ensembles of colloidal quantum dots
with biomolecular interconnects is demonstrated with quantum dot densities in excess of 10+17 cm-3. A number of novel
photodetectors have been designed based on the combined use of double-barrier quantum-well injectors, colloidal
quantum dots, and conductive polymers. Optoelectronic devices including photodetectors and solar cells based on threedimensional
ensembles of quantum dots are considered along with underlying phenomena such as miniband formation
and the robustness of minibands to displacements of quantum dots in the ensemble.
This paper focuses on understanding the THz-phonon mediated transport of polarons in biomolecules, with particular attention on polaron transport in DNA. In order to exploit biology-based approaches to realizing new electronic systems, it is necessary to understand the electrical transport properties and THz-phonon interactions of biomolecules that portend applications both as electrically conductive wires and as structures that facilitate the chemically-directed assembly of massively integrated ensembles of nanoscale semiconducting elements into terascale integrated networks. Special attention is given to charge transport in biomolecules using indirect-bandgap colloidal nanocrystals linked with biomolecules.
A variety of colloidal semiconductor quantum dots and related quantum-wire structures are characterized using absorption and photoluminescence measurements. The electronic properties of these structures are modeled and compared with experiment. The characterization and application of ensembles of colloidal quantum dots with molecular interconnects are considered. The chemically-directed assembly of ensembles of colloidal quantum dots with biomolecular interconnects is demonstrated with quantum dot densities in excess of 10+17 cm-3. Non-charge transfer processes for switching based on dipole-dipole interactions - Forester interactions - are examined for colloidal quantum dots linked with biomolecules. Charge transport in biomolecules is studied using indirect-bandgap colloidal nanocrystals linked with biomolecules.
Self-assembled GaN quantum dots are characterized using Raman techniques. The electrical and optical properties of these GaN quantum dots are modeled in light of optoelectronic applications. Strain-induced changes in the phononic properties of these nanostructures are modeled and the strain-induced frequency shifts are compared with Raman measurements. Acoustic phonons in colloidal GaN quantum dots are modeled using a quantized elastic continuum model. Shifts observed in the Raman signatures for different excitation wavelengths provide evidence the Raman signatures of GaN quantum dots are observed.
It is well known for bulk semiconductors that amplification (generation) of a phonon mode can be achieved via the Cerenkov effect when the electron drift velocity exceeds the phonon phase velocity. The following three requirements are necessary for practical use of this effect: high electron mobilities, large electron densities, and strong coupling between electrons and phonons. In this report we show that in quantum well heterostructures these requirements can be met and both confined acoustic and confined optical phonon modes can be efficiently generated (amplified) by the drift of two-dimensional carriers. General formulae for the gain coefficient as a function of the acoustic phonon frequency and structure parameters as well as for the confined phonon increment are derived. Taking into account the electron-acoustic-phonon interaction through the deformation potential as well as the piezoelectric interaction, we found that amplification coefficient can reach hundreds of 1/cm for the AlGaAs-based heterostructures and thousands of 1/cm for the SiGe-based heterostructures in the terahertz phonon frequency range. Amplification takes place in a spectrally separated and relatively narrow amplification bands. We show that the optical phonon increment depends critically on the electron drift velocity. Detailed analysis of the optical phonon increment as a function of phonon wavevector, electron-phonon coupling strength, electron temperature and drift velocity indicates that the electron drift in selectively doped AlAs/GaAs/AlAs and GaSb/InSb/GaAs quantum wells can generate coherent confined optical modes. Finally, we discuss nonlinear mechanisms which stabilize the increase of phonon population and lead to the steady state phonon generation.
Fundamental properties of phonons in III-V nitrides are examined with a view toward understanding processes important in the operation of III-V nitride devices. Firstly, confined, interface and propagating modes in wurtzite quantum wells are described in terms of Loudon's model for uniaxial semiconductors and the dielectric continuum model. Basic properties of the phonon modes and carrier-phonon interactions are considered in the basis of this treatment of dimensionally-confined phonons in wurtzite structures. A key feature of these phonon modes is their enhanced dispersion and its origin from the non-isotropic nature of the wurtzites. As will be discussed, this dispersion has important consequences for phonon propagation and phonon energy spectra. Secondly, the second-order phonon decay process of combined point defect scattering and anharmonic decay is examined as a means of estimating line broadening associated with the decay of phonons in III-V nitrides of wurtzite structure containing point defects. Thirdly, an analysis of Raman linewidths measured for AlN and GaN wurtzites is made to estimate phonon lifetimes.
The (Gamma) -X scattering rate of electrons in type-II superlattices by optical-phonon emission is calculated. The tight binding method for electronic band structure and the dielectric continuum model for phonons are used. The relative strength of scattering due to different phonon modes is examined for varying superlattice dimensions. The scattering rate is highest when the energy separation between the (Gamma) and X levels is smallest, and decreases quickly as the separation increases. It is found that the strongest scattering rate is due to the emission of AlAs confined modes. Changing of parity with layer thickness and its effect on scattering are discussed.
The Hamiltonian describing the deformation potential interaction of confined acoustic phonons with carriers is derived by quantizing the appropriate, experimentally verified approximate compressional acoustic phonon modes in a rectangular quantum wire. The scattering rate due to the deformation potential interaction is calculated for a range of quantum wire dimensions.
As device dimensions in nanoscale structures and mesoscopic devices are reduced, the characteristics and interactions of dimensionally-confined longitudinal-optical (LO) phonons deviate substantially from those of bulk polar semiconductors. This account emphasizes the properties of LO-phonon modes arising in polar-semiconductor quantum wells and quantum wires. In particular, this review highlights recent results of both microscopic and macroscopic models of LO phonons in polar-semiconductor quantum wells and quantum wires with a variety of cross sectional geometries. Emphasis is placed on the dielectric continuum model of confined and interface phonons. In addition, this review provides brief discussions of how carrier-LO-phonon interactions change in the presence of dynamical screening. Finally, the use of metal-semiconductor heterointerfaces to reduce unwanted inelastic scattering in nanoscale electronic and optoelectronic structures is discussed.
Experimental results on GaAs-AlAs multiple quantum wells where the confined electron level is initially delocalized due to the mixing between the (Gamma) and X levels are presented. The applied electric field reduces this coupling and reconfines the electron in the GaAs layer. This causes an increase in oscillator strength and a blue shift of the heavy hole to (Gamma) - electron transition. Reduction of the charge transfer from narrow wells to a wide well has also been observed.
In this paper, it is demonstrated that establishing metal-semiconductor interfaces at the heterojunctions of polar semiconductor quantum wells introduces a set of boundary conditions that dramatically reduces or eliminates unwanted carrier energy loss caused by interactions with interface longitudinal-optical (LO) phonon modes.
We report the first photoreflectance measurement of strain-induced piezoelectric field in a (111)B InGaAs/GaAs structure. The InGaAs quantum well was pseudomorphically grown in the undoped regions of a GaAs undoped-heavily doped structure. Four structures, two each with the same layer structures but different orientation, (111)B and (100), were used in this study. The electric fields in the undoped GaAs region were measured by Franz-Keldysh oscillations in photoreflectance. All the samples have a surface barrier height of about 0.7 eV. However, the measured electric field is 30% stronger in the (111)B sample compared to the (100) sample. We attribute this difference to the strain induced electric field in the (111)B sample. The piezoelectric field in (111)B strained In0.15Ga0.85As obtained in this measurement is 2.2 +/- 0.5 X 105 V/cm, which agrees very well with theory.
The Hamiltonian describing the iteraction of both confined longitudinal-optical and surface-optical photons with charge carriers is derived from the macroscopic dielectric continuum model for the case of a rectangular quantum wire where photon confinement occurs in two of the three spatial dimensions. The full interaction Hamiltonian is used to calculate the total scattering rate for electron-optical-phonon scattering of electrons traversing a GaAs square quantum wire. The results demonstrate that the interaction by the surface-optical phonon modes is very strong and may dominate over other scattering processes, especially with dimensions of about 100 A or less. A considerable decrease in the total scattering rate for optical phonons as a result of simple reduction in dimensionality is not observed in this study.
The theories describing the dispersion of longitudinal-optical (LO) phonon
modes and electron-LO-phonon interactions are generalized to include the effects
of both strain and confinement in semiconductor superlattices and semiconductor
microstructures. In particular, the effects of strain and confinement on LO
phonon frequencies are analyzed for short-period strained-layer superlattices as
well as for semiconductor microstructures where effective force constants are
altered near heterojunction interfaces.
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