AVS2012 Session NS-MoM: Nanoparticles and Quantum Structures

As-grown light emitting self-assembled Ge nanocrystals (Ge-NCs) embedded in a SiO₂ matrix were produced by a sequential deposition process of SiO₂/Ge/SiO₂ layers employing reactive radio frequency sputtering technique. Obtained Ge-NCs shown a crystallographic phase whose proportion, size, quality and specific orientation are determined by the oxygen partial pressure. Photoluminescence (PL) spectra indicate that the size distribution of Ge-NCs is reduced and centered at around 8 nm when higher oxygen partial pressure is employed; the formation of Ge-NCs is corroborated by transmission electron microscopy (TEM) measurements, their sizes are consistent with estimates from PL measurements. After vacuum annealing it is observed the elimination of an instable high pressure tetragonal phase of germanium present in as-grown samples. It is possible that this phase is related to the SiO₂ matrix stress on the Ge-NCs. In addition, the PL peaks shifted to higher energies indicating the formation of Ge-NCs probably from Ge dispersed within SiO₂ matrix. It was also found that the PL intensity increases drastically after annealing process. The strong size dependence of the PL spectra indicates that the observed PL originates from the recombination of electron-hole pairs confined in Ge-NCs.

^† : partially funded by CONACyT-Mexico and ICYT-DF.

Over the past decade we have developed a radical new strategy for the fabrication of atomic-scale devices in silicon [1]. Using this process we have demonstrated few electron, single crystal quantum dots [2], conducting nanoscale wires with widths down to ~1.5nm [3] and most recently a single atom transistor [4]. We will present atomic-scale images and electronic characteristics of these atomically precise devices and demonstrate the impact of strong vertical and lateral confinement on electron transport. We will also discuss the opportunities ahead for atomic-scale quantum computing architectures and some of the challenges to achieving truly atomically precise devices in all three spatial dimensions.

[1] F.J. Rueß et al., Nano Letters 4, 1969 (2004).

[2] M. Fuchsle et al., Nature Nanotechnology 5, 502 (2010).

[3] B. Weber et al., Science 335, 64 (2012).

[4] M. Fuchsle et al., accepted for Nature Nanotechnology (2012).

Silicon nanoparticles (NPs) are of interest in a variety of optoelectronic applications due to the observation of multiple exciton generation, room temperature photoluminescence, size tunable band gap, and optical gain. Design of any device employing Si NPs requires control over both the size and interfacial passivation as these parameters dictate the electronic properties of the NPs. Our synthesis process employs a capacitively-coupled tubular Ar/SiH₄ plasma to produce H-terminated Si NPs. The surface composition and functionalization of the Si NPs was characterized via in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. Organic passivation of the H-terminated Si NPs was achieved through a two-stage gas-phase hydrosilylation process using 1-alkynes with different numbers of C atoms that led to surface coverages comparable to the thermodynamic limit for alkenyl monolayers on bulk H:Si(111) surfaces. The hydrosilylation reaction requires abstraction of a surface hydride to stabilize an intermediate surface radical formed upon absorption of the 1-alkyne. Injection of H₂ into the afterglow region of the synthesis plasma was employed to manipulate the surface hydride composition, which led to an increase in the relative fraction of SiH_x(x = 2, 3) species on the surface compared to SiH. The impact of these higher hydrides on the hydrosilylation reaction of 1-alkynes was investigated through in situ ATR-FTIR spectroscopy. Finally, using in situ photoluminescence measurements, we also determined the effect of the various hydrosilylation processes on the relative quantum yield from these Si NPs.

Iron oxide nanoparticles, Fe₃O₄ and γ-Fe₂O₃, are of great interest for applications in high-density magnetic recording media, sensor technology, and biomedicine. These systems are also excellent candidates for probing fundamental properties due to well-established synthesis methods that yield uniform, high quality particles with good control over size and shape. It has recently been shown by polarized small angle neutron scattering that Fe₃O₄ nanoparticles possess a chemically uniform, but magnetically distinct, core and canted-spin shell structure [1]. While many similarities exist in the magnetic and microstructural properties of γ-Fe₂O₃ and Fe₃O₄, the exchange bias that can be observed in nanoparticles of γ-Fe₂O₃ [2] is not present in similarly sized Fe₃O₄ [3]. Therefore, an investigation of the evolution of spin canting angle and shell thickness with temperature in γ-Fe₂O₃ is expected to yield valuable information about subtly altered spin geometries that can be correlated with the presence of exchange bias in this nanoparticle system.

In order to compare results for both chemically distinct and chemically uniform exchange-biased systems, we report the synthesis and characterization of γ-Fe₂O₃ and core/shell Fe/γ-Fe₂O₃ nanoparticles. The p articles used in the present study were synthesized by high temperature decomposition of iron organometallic compounds. X-ray diffraction and transmission electron microscopy techniques were used to study the structural and microstructural properties of the nanoparticles, which confirmed the presence of bcc iron and fcc γ-Fe₂O₃ in these particles. High-resolution TEM images evidenced monodisperse products with particle diameters of 9±0.8 nm in the pure γ-Fe₂O₃, while the core/shell particles showed 9.8±0.7 nm Fe cores surrounded by a shell of γ-Fe₂O₃ with 2±0.4 thickness. The DC magnetic properties of the samples were characterized using a vibrating sample magnetometer over a temperature range of 5-300 K, revealing a superparamagnetic behavior. Pronounced exchange bias of up to ~4100 Oe was confirmed in these particles using cooling fields of up to 5T. While the spin-glass-like phase associated with disordered surface spins in γ-Fe₂O₃ plays an important role as a fixed phase in both systems, providing the pinning force to the reversible spins, the frozen spins at the interface between Fe and γ-Fe₂O₃ are also shown to contribute to EB in the core/shell Fe/γ-Fe₂O₃ nanoparticles .

nanoparticle fillers.

This work was supported in part by DOE grant DE-FG02-07ER46354

[1] B. Roldán Cuenya, M. Alcántara Ortigoza, L. K. Ono, F. Behafarid, S. Mostafa, J. R. Croy, K. Paredis, G. Shafai, T. S. Rahman, L. Li, Z. Zhang, and J. C. Yang, PRB 84, 245438 (2011)

[2] G. S. Shafai, M. Alcántara Ortigoza, and T. S. Rahman; J. Phys.: Condens. Matter 24, 104026 (2012)

In recent years, sol–gel chemistry has been a subject of intense investigations in the fields of chemical sensors and biosensors as it offers a cost effective route the fabrication of high surface area plate forms for sensitive biosensors. The inherent low temperature process of the technology, provide an attractive way for the immobilization of heat-sensitive biological entities (enzyme, protein, and antibody). This class of sol–gel silica (TiO₂) matrix possesses chemical inertness, non-toxic, physical rigidity, tunable porosity, high photochemical and thermal stability, and optical transparency.

Nanostructured nature of the metal-oxides films have been extensively reported, we have utilized this property of the material for effective enzyme adsorption per unit area to develop high sensitivity biosensors. We have further improved the matrix property to alter the surface properties (microstructure and charge transport) by Fe doping in the TiO ₂ film. The modified matrix has been characterized by XRD, XPS, FTIR, SEM. The Fe³⁺ ion doped TiO₂ matrix successfully introduces redox property in the electrode and provides enhanced electron communication features. Sensing response obtained by the bioelectrocatalytic oxidation of cholesterol by cholesterol oxidase. Chox/ Fe³⁺ ion doped TiO₂ /ITO electrode was studied using cyclic voltammetry (CV). These studies reveal that the Chox/ Fe³⁺ ion doped TiO₂ /ITO bio-electrode exhibits improved biosensing response as compared to Chox/ Undoped TiO₂ /ITO electrode. The results confirm promising application of the Fe³⁺ ion doped TiO₂ thin film matrix for the realization of efficient cholesterol biosensor.