Molecular Nanotechnology Laboratory

The dielectric behavior of a linear cluster of two or more living cells connected by tight junctions is analyzed using a spectral method. The polarizability of this system is obtained as an expansion over the eigenmodes of the linear response operator, showing a clear separation of geometry from electric parameters. The eigenmode with the second largest eigenvalue dominates the expansion as the junction between particles tightens, but only when the applied field is aligned with the cluster axis. This effect explains a distinct low-frequency relaxation observed in the impedance spectrum of a suspension of linear clusters.

We use a fully relativistic layer Green's functions approach to investigate spin-dependent tunneling through a symmetric indirect band gap barrier like GaAs/AlAs/GaAs heterostructure along [100] direction. The method is based on Linear Muffin Tin Orbitals and it is within the Density Functional Theory (DFT) in the Local Density Approximation (LDA). We find that the results of our ab-initio calculations are in good agreement with the predictions of our previous empirical tight binding model [Phys. Rev. B, 075313 (2006)]. In addition we show the k || -dependence of the spin polarization which we did not previously include in the model. The ab-initio calculations indicate a strong k || -dependence of the transmission and the spin polarization due to band non-parabolicity. A large window of 25-50% spin polarization was found for a barrier of 8 AlAs monolayers at k || = 0.03 2π/ a. Our calculations show clearly that the appearance of energy windows with significant spin polarization depends mostly on the location of transmission resonances and their corresponding zeros and not on the magnitude of the spin splitting in the barrier.

Coherent transport through resonant tunneling diodes at high bias is determined by the transfer of carriers described by momentum and energy from a bath in the emitter through a central resonance to the collector. Simplified treatments of coherent carrier transport assume the transverse carrier dispersions to be identical and parabolic in the emitter and central device. This results in a carrier transport that is dominated by carriers at the ⌫ zone center. Other work has shown that more realistic dispersions result in off-zone center current flow. Incoherent scattering adds more degrees of freedom to match the emitter energy and momentum (E em ,k em ) with the resonance energy and momentum (E res ,k res ) with (⌬E scatt ,⌬k scatt ). It is shown analytically and numerically that this additional degree of freedom redirects carriers in energy and momentum space resulting in an off-zone center current for large voltage regions. Interface roughness, polar optical phonon, and acoustic phonon scatterings are explicitly considered.

The bandgap and band bowing parameter of semiconductor alloys are calculated with a fast and realistic approach. The method is a dielectric scaling approximation that is based on a scissor approximation. It adds an energy shift to the bandgap provided by the local density approximation (LDA) of the density functional theory (DFT). The energy shift consists of a material-independent constant weighted by the inverse of the high-frequency dielectric constant. The salient feature of the approach is the fast calculation of the dielectric constant of alloys via the Green function (GF) of the TB-LMTO (tight-binding linear muffin-tin orbitals) in the atomic sphere approximation (ASA). When it is applied to highly mismatched semiconductor alloys (HMAs) like Zn Te x Se 1−x , this method provides a band bowing parameter that is different from the band bowing parameter calculated with the LDA due to the bowing exhibited also by the high-frequency dielectric constant.

The dielectric behavior of a linear cluster of two or more living cells connected by tight junctions is analyzed using a spectral method. The polarizability of this system is obtained as an expansion over the eigenmodes of the linear response operator, showing a clear separation of geometry from electric parameters. The eigenmode with the second largest eigenvalue dominates the expansion as the junction between particles tightens, but only when the applied field is aligned with the cluster axis. This effect explains a distinct low-frequency relaxation observed in the impedance spectrum of a suspension of linear clusters.

The improvement of light absorption in Si/BeSe$_{0.41}$Te$_{0.59}$ heterostructures for solar cell applications is studied theoretically. First, using simple approaches we found that light absorption could be improved in a single (uncoupled) quantum well with a thickness up to 20 {\AA}. Second, by semiempirical tight-binding methods we calculated the electronic structure and optical properties of various (Si$_{2})_{n}$/(BeSe$_{0.41}$Te$_{0.59})_{m}$ [001] superlattices. Two bands of interface states were found in the band gap of bulk Si. Our calculations indicate that the optical edges are close to the fundamental band gap of bulk Si and the transitions are optically allowed.

Plasmonic properties of rod-shaped nanoantenna dimers in a touching configuration are studied with an electrostatic method. First, a scaling law of rod nanoantennas is given in the electrostatic approximation. Furthermore, we have found that the dimer plasmon resonance corresponding to the dipolar mode of a single antenna is only slightly shifted, while a new resonance emerges in infrared. This new resonance moves deeper to larger wavelengths as the dimer junction becomes tighter. The maximum of the field enhancement is at the ends of a single nanoantenna. The dimer still has the same field enhancement at its ends, but a much larger enhancement is obtained in the dimer gap. The field enhancement of the dimer outperforms also the enhancement of nanoantennas of the same length.

The Rabi Hamiltonian is studied in the dispersive regime and ultra-strong coupling. We employ a recent unitary transformation to obtain not only the approximate Hamiltonian and its energy levels but also its eigenfunctions. The relationship of the approximation with other regimes and their approximations are also discussed.

Using a boundary integral equation method we analyze the extinction spectrum and the near-field enhancement induced by surface plasmon resonances with respect to the shape modifications of metallic nanospheres. We found that the shape effects are greater on near-field enhancement than on extinction. The far-field properties are affected by global geometric changes like the volume modifications. The near-field, however, is rather affected by targeted local shape variations like the curvature variation of nanosphere surface.

The effect of monolayer roughness on the peak-to-valley ratio of Si/SiO 2 resonant tunnel diodes is numerically modeled. Atomic scale roughness is shown to be acceptable. As the roughness, that is the island size increases above the atomic scale, the peak-to-valley ratio quickly degrades to less than 5 for 1 nm roughness and less than 2 for 2 nm roughness.

The influence of SDS upon the molecular properties of proflavine (3,6-diaminoacridine), acridine yellow (2,7-dimethyl-3,6-diaminoacridine) and methylene blue (3,7-bis-dimethylamino-phenothiazine) was studied comparatively to their properties in that of aqueous media. The absorption and emission spectra of the three dyes in SDS aqueous solution (1-100 mmol/l) were recorded. The spectroscopic data also allowed the evaluation of the critical micellization concentration (CMC), acidity constants in fundamental (pK a ) and excited (pK * a ) states, and lifetimes of excited singlet states.

We report a new model for highly mismatched semiconductor (HMS) alloys. Based on the Anderson impurity Hamiltonian, the model generalizes the recent band anti-crossing (BAC) model, which successfully explains the band bowing in highly mismatched semiconductors. Our model is formulated in empirical tight-binding (ETB) theory and uses the so called sp 3 s* parameterization. It does not need extra parameters other than bulk ones. The model has been applied to BeSe x Te 1−x alloy. BeTe and BeSe are wide-band gap and highly mismatched semiconductors. Calculations show large band bowing, larger on the Se rich side than on the Te rich side. Linear interpolation is used for an arbitrary concentration x. The results are applied to calculation of electronic and optical properties of BeSe 0.41 Te 0.59 lattice matched to Si in a superlattice configuration.

The time-dependent behavior of a two-level system interacting with a quantum oscillator system is analyzed in the case of a coupling larger than both the energy separation between the two levels and the energy of quantum oscillator (Ω < ω < λ, where Ω is the frequency of the transition between the two levels, ω is the frequency of the oscillator, and λ is the coupling between the twolevel system and the oscillator). Our calculations show that the amplitude of the expectation value of the oscillator coordinate decreases as the two-level system undergoes the transition from one level to the other, while the transfer probability between the levels is staircase-like. This behavior is explained by the interplay between the adiabatic and the non-adiabatic regimes encountered during the dynamics with the system acting as a quantum counterpart of the Landau-Zener model.

Usually, numerical self-consistent calculations predict a much larger intrinsic bistability region than actually is measured in resonant tunneling diodes (RTDs). In addition, numerical calculations have shown that scattering in the well reduces bistability. We used a uniÿed treatment of current owing from continuum states and emitter quasi-bound states to show numerically and analytically that not only the scattering in the quantum well but also the scattering in the emitter reduces bistability. Moreover, within the Hartree approximation, bistability occurs by tunneling resonantly between emitter quasi-bound state and well quasi-bound state as a pitchfork bifurcation. ?

The effect of smooth shape changes of metallic nanoparticles on localized surface plasmon resonances is assessed with a boundary integral equation method. The boundary integral equation method allows compact expressions of nanoparticle polarizability which is expressed as an eigenmode sum of terms that depends on the eigenvalues and eigenfunctions of the integral operator associated to the boundary integral equation method. Shape variations change not only the eigenvalues but also their coupling weights to the electromagnetic field. Thus rather small changes in the shape may induce large variations of the coupling weights. It has been found that shape changes that bring volume variations greater than 12% induce structural changes in the extinction spectrum of metallic nanoparticles. Also the largest variations in eigenvalues and their coupling weights are encountered by shape changes along the smallest cross-sections of nanoparticles. These results are useful as guiding rules in the process of designing plasmonic nanostrucrures.