Tzu-Liang Chan

Controlling diffusion of lithium in silicon nanostructures.

The ability to control the diffusion of dopants or impurities is a controlling factor in the functionalization of materials used in devices both at the macro- and nanoscales. At the nanoscale, manipulating diffusion of dopants is complicated by a number of factors such as the role of quantum confinement and the large surface to volume ratio. Here we examine Li in Si nanostructures, as atoms with low atomic mass such as Li can be used as a carrier for energy storage with high specific energy capacity. Specifically, Li-ion batteries with specific energy capacity as high as 4200 mA h g(-1) using Si nanowires as anodes have been achieved. Using ab initio calculations, we determine how the factors of size and dimensionality can be used to achieve an optimal diffusion of Li atoms in Si nanostructures.

Computational studies of doped nanostructures

One of the most challenging issues in materials physics is to predict the properties of defects in matter. Such defects play an important role in functionalizing materials for use in electronic and optical devices. As the length scale for such devices approaches the nano-regime, the interplay of dimensionality, quantum confinement and defects can be complex. In particular, the usual rules for describing defects in bulk may be inoperative, i.e. a shallow defect level in bulk may become a deep level at the nanoscale. The development of computational methods to describe the properties of nanoscale defects is a formidable challenge. Nanoscale systems may contain numerous electronic and nuclear degrees of freedom, and often possess little symmetry. In this review, we focus on new computational methods, which allow one to predict the role of quantum confinement on the electronic, magnetic and structural properties of functionalized nanostructures. We illustrate how these methods can be applied to nanoscale systems, and present calculations for the electronic, magnetic and structural properties of dopants in semiconductor nanocrystals and nanowires.

Morphology and texture evolution of nanostructured CaF2 films on amorphous substrates under oblique incidence flux

The morphology and biaxial texture of vacuum evaporated CaF(2) films on amorphous substrates as a function of vapour incident angle, substrate temperature and film thickness were investigated by scanning electron microscopy, x-ray pole figure and reflection high energy electron diffraction surface pole figure analyses. Results show that an anomalous [220] out-of-plane texture was preferred in CaF(2) films deposited on Si substrates at < 200 °C with normal vapour incidence. With an increase of the vapour incident angle, the out-of-plane orientation changed from [220] to [111] at a substrate temperature of 100 °C. In films deposited with normal vapour incidence, the out-of-plane orientation changed from [220] at 100 °C to [111] at 400 °C. In films deposited with an oblique vapour incidence at 100 °C, the texture changed from random at small thickness (5 nm) to biaxial at larger thickness (20 nm or more). Using first principles density functional theory calculation, it was shown that [220] texture formation is a consequence of energetically favourable adsorption of CaF(2) molecules onto the CaF(2)(110) facet.

Large photonic band gaps in certain periodic and quasiperiodic networks in two and three dimensions

The photonic band structures in certain two- and three-dimensional periodic networks made of one-dimensional waveguides are studied by using the Floquet-Bloch theorem. We find that photonic band gaps exist only in those structures where the fundamental loop exhibits antiresonant transmission. This is also true for quasiperiodic networks in two and three dimensions, where the photonic band structures are calculated from the spectra of total transmission arising from a source inside the samples. In all the cases we have studied, it is also found that the gap positions in a network are dictated by the frequencies at which the antiresonance occurs.

Algorithms for the electronic and vibrational properties of nanocrystals

Solving the electronic structure problem for nanoscale systems remains a computationally challenging problem. The numerous degrees of freedom, both electronic and nuclear, make the problem impossible to solve without some effective approximations. Here we illustrate some advances in algorithm developments to solve the Kohn-Sham eigenvalue problem, i.e. we solve the electronic structure problem within density functional theory using pseudopotentials expressed in real space. Our algorithms are based on a nonlinear Chebyshev filtered subspace iteration method, which avoids computing explicit eigenvectors except at the first self-consistent-field iteration. Our method may be viewed as an approach to solve the original nonlinear Kohn-Sham equation by a nonlinear subspace iteration technique, without emphasizing the intermediate linearized Kohn-Sham eigenvalue problems. Replacing the standard iterative diagonalization at each self-consistent-field iteration by a Chebyshev subspace filtering step results in a significant speed-up, often an order of magnitude or more, over methods based on standard diagonalization. We illustrate this method by predicting the electronic and vibrational states for silicon nanocrystals.

An effective one-particle theory for formation energies in doping Si nanostructures

By examining the formation energy (Eform) of P, As, and Al-doped Si nanostructures, we find that the many-body interactions related to the chemistry of the dopant are short ranged and hence size-insensitive when the dopant is at least a few bond length away from the surfaces. As a result, the size evolution of Eform can be understood remarkably well by an effective one-particle picture, and is found to follow two universal curves; one for donors and one for acceptors. Only for nanostructures smaller than ∼2 nm in diameter, different dopants may exhibit different Eform trend due to many-body interactions.

Interaction Range of P-Dopants in Si[110] Nanowires: Determining the Nondegenerate Limit

It is well known that the activation energy of dopants in semiconducting nanomaterials is higher than in bulk materials owing to dielectric mismatch and quantum confinement. This quenches the number of free charge carriers in nanomaterials. Though higher doping concentration can compensate for this effect, there is no clear criterion on what the doping concentration should be. Using P-doped Si[110] nanowires as the prototypical system, we address this issue by establishing a doping limit by first-principles electronic structure calculations. We examine how the doped nanowires respond to charging using an effective capacitance approach. As the nanowire gets thinner, the interaction range of the P dopants shortens and the doping concentration can increase concurrently. Hence, heavier doping can remain nondegenerate for thin nanowires.

Quasi-single Crystal Semiconductors on Glass Substrates Through Biaxially Oriented Buffer Layers

High efficiency photovoltaic devices are normally fabricated on single crystalline substrates. These single crystalline substrates are expensive and volume production for widespread usage has not been realistic. To date, large volume production of solar cells is on less expensive noncrystalline substrates such as glass. Typically the films grown on glass are polycrystalline with less than ideal efficiency. It was proposed that a dramatic gain in the efficiency may be achieved if one uses a biaxially oriented buffer layer on glass to grow biaxial semiconductor films to fabricate solar devices compared to that of films grown directly on glass. Biaxial films are not exactly single crystal but have strongly preferred crystallographic orientations in both the out-ofplane and in-plane directions. Typically the misorientation between grains can be small (within a few degrees) and may possess low carrier recombination rate. In this paper we shall discuss growth techniques that would allow one to produce biaxial buffer layers on glass. A specific strategy using an atomic shadowing mechanism in an oblique angle deposition configuration that allows one to grow biaxial buffer layers such as CaF2 on glass substrate will be discussed in detail. Results of heteroepitaxy of semiconductor materials such as CdTe and Ge on these biaxial buffer/glass substrates characterized by x-ray pole figure, reflection high energy electron diffraction (RHEED) pole figure and transmission electron microscopy (TEM) will be presented.

Doping Nanocrystals and the Role of Quantum Confinement

Recent progress in developing algorithms for solving the electronic structure problem for nanostructures is illustrated. Key ingredients in this approach include pseudopotentials implemented on a real space grid and the use of density functional theory. This procedure allows one to predict electronic properties for many materials across the nano‐regime, i.e., from atoms to nanocrystals of sufficient size to replicate bulk properties. We will illustrate this method for doping silicon nanocrystals with phosphorous.