Quantum-confined Nanostructures Research Articles

Room temperature ferromagnetism in magic-sized Cr-doped CdS diluted magnetic semiconducting quantum dots

Manipulation of carrier spins in semiconductors for spintronics applications has received great attention driven by improved functionalities and higher speed operation. Doping of semiconductor nanocrystals by transition-metal ions pronounced as diluted magnetic semiconductors (DMS) has attracted tremendous attention. Such doping is, however, difficult to achieve in low-dimensional strongly quantum-confined nanostructures by conventional growth procedures. In the present case, magic-sized, pure, and Cr-doped CdS DM-QDs have been synthesized by solution phase chemistry (lyothermal method). Structural, optical, and magnetic investigation suggest an intrinsic nature of ferromagnetism with highly quantum-confined system. Optical and magnetic results of pure and doped QDs reveal major physical consequences of dopant localization within the capacity to engineer dopant-carrier exchange interactions introducing magnetic functionalities within the host semiconductor lattice. Unpaired Cr ions in Cd substitutional sites could create spin ordering and ferromagnetic coupling. The results presented herein illustrate some of the remarkable and unexpected complexities that can arise in doped QDs.

Publisher’s Note: Engineered valley-orbit splittings in quantum-confined nanostructures in silicon [Phys. Rev. B83, 195323 (2011)

Publisher’s Note: Engineered valley-orbit splittings in quantum-confined nanostructures in silicon [Phys. Rev. B<b>83</b>, 195323 (2011)

Engineered valley-orbit splittings in quantum-confined nanostructures in silicon

An important challenge in silicon quantum electronics in the few electron regime is the potentially small energy gap between the ground and excited orbital states in 3D quantum confined nanostructures due to the multiple valley degeneracies of the conduction band present in silicon. Understanding the "valley-orbit" (VO) gap is essential for silicon qubits, as a large VO gap prevents leakage of the qubit states into a higher dimensional Hilbert space. The VO gap varies considerably depending on quantum confinement, and can be engineered by external electric fields. In this work we investigate VO splitting experimentally and theoretically in a range of confinement regimes. We report measurements of the VO splitting in silicon quantum dot and donor devices through excited state transport spectroscopy. These results are underpinned by large-scale atomistic tight-binding calculations involving over 1 million atoms to compute VO splittings as functions of electric fields, donor depths, and surface disorder. The results provide a comprehensive picture of the range of VO splittings that can be achieved through quantum engineering.

Investigation of Optical Effects in Silicon Quantum Dots by Using an Empirical Pseudopotential Method

A computer simulation using a pseudopotential approach has been carried out to investigate the band gap as a function of the size and the shape of small silicon (Si) dots having 3 to 44 atoms per dot with and without surface passivation. We used an empirical pseudo-potential Hamiltonian, a plane-wave basis expansion and a basic tetrahedral structure with undistorted local bonding configurations. In our simulation, the structures of the quantum dots were relaxed and optimized before and after passivation. We found that the gap increased more for an oxygenated surface than a hydrogenated one. Thus, both quantum confinement and surface passivation determined the optical and the electronic properties of Si quantum dots. Visible luminescence was probably due to radiative recombination of electrons and holes in the quantum-confined nanostructures. The effect of passivation of the surface dangling bonds by hydrogen and oxygen atoms and the role of surface states on the gap energy was also examined. We investigated the entire energy spectrum starting from the very low-lying ground state to the very high-lying excited states. The results for the sizes of the gap, the density of states, the oscillator strength and the absorption coefficient as functions of the size are presented. The importance of the confinement and the role of surface passivation on the optical effects are also discussed.

A perspective on solar-driven water splitting with all-oxide hetero-nanostructures

A perspective on the design of all-oxide heterostructures for application in photoelectrochemical cells for solar water splitting is provided. Particular attention is paid to those structures which possess nanoscale feature dimensionality, as structures of this type are most likely to utilize the benefits afforded by the formation of oxide heterojunctions and likely to show functional behavior relating to the interfacial region. In the context of this discussion, a novel hetero-nanostructure array, based on quantum-confined and visible light-active iron(III) oxide nanostructures and their surface modification with tungsten(VI) oxide, is introduced. The heterostructure architecture is designed to combine the functionality of the consituent phases to address the primary requirements for electrodes enabling the efficient generation of hydrogen using solar energy: visible light activity, chemical stability, appropriate bandedge characteristics, and potential for low-cost fabrication. Photoelectrochemical characterization for solar hydrogen/oxygen generation indicates the presence of unexpected minority carrier transfer dynamics within the oxide hetero-nanostructures, as observed additionally by ultrafast transient absorption spectroscopy.

Femtosecond all-optical devices for ultrafast communication and signal processing

Future bandwidth demand in optical communication and signal processing systems will soon exceed 100 Gb s−1 as is commonly forecasted from a throughput experience curve for communication systems. However, such systems cannot be realized without introducing ultrafast, all-optical devices, since existing optoelectronic and electronic devices and integrated circuits would not be able to function at a bit rate exceeding 100 Gb s−1, because of the speed limit intrinsic to conventional semiconductor materials and devices. All-optical devices based on completely new principles, not being restricted by properties of existing materials and device principles, must be developed for the realization of ultrafast communication and signal processing systems. This paper reviews requirements of ultrafast all-optical devices and recent progress in ultrafast light sources and all-optical switches based on either novel device principles or ultrafast phenomena in novel materials such as quantum-confined nanostructures. Recent developments described here include mode-locked lasers and a variety of all-optical switches based on different phenomena including Mach–Zehnder interferometer structures, spin relaxation, intersubband transition, and ultrafast absorption recovery in organic thin films and semiconductor quantum dots. Some of the recent developments have already shown capability of basic functions such as ultrafast pulse generation and signal processing at the bit rate of 500 Gb s−1 to 1 Tb s−1. Technical challenges expected for the future are discussed in view of their applications in real systems.

Mechanisms of self-ordering in nonplanar epitaxy of semiconductor nanostructures

We have developed an analytic model that describes in detail the establishment of self-ordered profiles during semiconductor epitaxy on corrugated surfaces. Lateral, self-ordered epitaxy derives from surface gradients in the chemical potential due to the nonplanarity of the profile (capillarity). The growth rate variation on the different facets composing the profile determines whether the profile sharpens up at the bottom of the grooves or at the apex of the corrugations. For alloy growth, additional entropy of mixing effects affect the profile shape and composition. The predictions of the model were applied to explain the self-limiting surface profiles obtained by organometallic chemical vapor deposition on $[011\ifmmode\bar\else\textasciimacron\fi{}]$-oriented grooves. Finally, this model is used to design a variety of low-dimensional quantum-confined nanostructures.

Ab initio study of quantum confined unpassivated ultrathin Si films

An ab initio study of the electronic structure of the one-dimensional quantum-confined crystalline ultrathin Si (100) films both with the unreconstructed p(1×1) and the reconstructed c(2×2) film surfaces in a large thickness range of 2.7–29.7 Å has been made after employing a self-consistent full potential linear muffin tin orbital method along with the density functional theory in local approximation. A relatively large unit cell of 28 layers has been employed. In the case of the unreconstructed p(1×1) surface, the dangling bond states fill the fundamental energy gap completely and these states are weakly localized, i.e., the wave functions of these states die out in the inner region of the film slowly. After the c(2×2) surface reconstruction, the bonding and the antibonding dangling bond states are seen to lie near the valence band maximum and the conduction band minimum in the fundamental energy gap, respectively, and are again weakly localized. There remain dangling bond states within the gap after reconstruction. The separation between the two surface Si atoms in a dimer shows quite an appreciable change of about 36–40%. The band gaps of the various films remain practically unchanged after surface reconstruction; the changes lie well within 6%. The present results give convincing theoretical evidence for the occurrence of the weakly localized dangling bond states and the absence of too much localized states in quantum-confined systems, which may be responsible for the occurrence of luminescence observed in a comparatively low energy region in the quantum-confined nanostructures, e.g., in the infrared region in porous silicon. The presently predicted exponential rise of the band gap with the decrease in the quantum-confined size in the ultrathin film region is in sharp contrast to an approximate linear rise in the band gap observed in the spectroellipsometric measurements.

Spintronics and quantum computing: switching mechanisms for qubits

Quantum computing and quantum communication are remarkable examples of new information processing technologies that arise from the coherent manipulation of spins in nanostructures. We review our theoretical proposal for using electron spins in quantum-confined nanostructures as qubits. We present single- and two-qubit gate mechanisms in laterally as well as vertically coupled quantum dots and discuss the possibility to couple spins in quantum dots via exchange or superexchange. In addition, we propose a new stationary wave switch, which allows to perform quantum operations with quantum dots or spin- 1 2 molecules placed on a 1D or 2D lattice.

First-principles study of one-dimensional quantum-confined H-passivated ultrathin Si films

An ab initio study of the electronic structure of the one-dimensional quantum-confined H-passivated crystalline ultrathin Si in large thickness range of 2.7–27.0 Å has been made, employing a self-consistent full potential linear muffin-tin orbital (FPLMTO) method along with the density functional theory in local approximation. Calculations have been performed for a quite big unit cell with sufficient separation between the unit cells to avoid interference effects between the two surfaces of a unit cell as well as between the two successive unit cells. The present results show the absence of overlocalized states in quantum-confined systems and the occurrence of the extended states responsible for the luminescence observed in quantum-confined nanostructures. The presently calculated values of the band gap show an exponential rise with a decrease in the quantum-confined size in the ultrathin film region. The present results have been obtained for a more extended film thickness region as compared to earlier ab initio calculations.