O. Contreras

Synthesis and optelectronic characterization of gallium doped zinc oxide transparent electrodes

Abstract In this work we report on pulsed laser deposition of gallium-doped zinc oxide (ZnO:Ga) transparent-conducting thin films grown on glass at different substrate temperatures. A widening in the optical bandgap and a good gallium-doping efficiency were observed in the films when the substrate temperature was raised from 150 °C to 300 °C, as determined from optical and electrical measurements. X-ray diffraction measurements revealed that the films grow preferentially oriented in the [002] crystallographic direction of the ZnO grains. The crystallinity of the films was also found to be strongly dependent on the substrate deposition temperature. The ZnO:Ga transparent films had excellent transmittance (85%) in the visible spectrum and a low electrical resistivity value (7 × 10 −4 Ω cm) in 200 nm thickness samples deposited on glass by laser ablation at 300 °C.

Dislocation annihilation by silicon delta-doping in GaN epitaxy on Si

The addition of bursts of silicon has been observed to correlate with the reduction of threading screw dislocations during epitaxial growth of GaN on silicon by metalorganic chemical vapor deposition. The reduction is associated with bending of screw dislocations and “pairing” with equivalent neighboring dislocations with opposite Burgers vectors. This results in the formation of square dislocation loops. When the right type of dislocation is not available, the dislocation continues propagating in the original direction, leaving behind a kink at the silicon-rich position. These observations apply only to dislocations with a screw component. Edge dislocations are not affected by silicon delta-doping. A mechanism for the termination of threading screw dislocation is proposed, which involves pinning by the silicon impurities of the surface lattice steps associated with screw dislocations.

Atomic Arrangement at the AlN/Si(110) Interface

GaN has been grown on Si(110) substrates by metalorganic vapor phase epitaxy using a low-temperature AlN nucleation layer. The atomic arrangement at the AlN/substrate interface has been investigated by high-resolution transmission electron microscopy. Lattice images of the AlN/Si interface taken along the AlN∥ Si and AlN∥ Si projections show an abrupt crystalline interface. A highly coherent epitaxial relationship between (1100)AlN and (001)Si planes is observed. The atomic bonding configuration at the AlN/Si interface is analyzed taking into consideration the chemical coordination, lattice mismatch, and net charge balance. A structure model of the bonding at the interface is presented.

Microstructure of gallium nitride films grown on silicon (110)

The microstructure of GaN layers grown on Si(110) is studied by transmission electron microscopy. The GaN layers were grown by metal-organic vapor phase epitaxy using low-temperature AlN interlayers and a high-temperature AlN seed layer. Anisotropic misfit strain originating at the AlN/Si(110) interface is notably reflected in the microstructure of the GaN layers. The stress produced in GaN/Si(110) films is relieved by bending of edge type threading dislocations over the basal plane, generating horizontal segments aligned all along the closely lattice matched direction [11¯00]. It is proposed that the horizontal defects are generated by a driven force with glide- and climb-components manifested on some of the prismatic slip planes of GaN. The general mechanism of the change in the propagation direction of dislocations is discussed.

Single ZnO Nanowire-Based Gas Sensors to Detect Low Concentrations of Hydrogen

Low concentrations of hazardous gases are difficult to detect with common gas sensors. Using semiconductor nanostructures as a sensor element is an alternative. Single ZnO nanowire gas sensor devices were fabricated by manipulation and connection of a single nanowire into a four-electrode aluminum probe in situ in a dual-beam scanning electron microscope-focused ion beam with a manipulator and a gas injection system in/column. The electrical response of the manufactured devices shows response times up to 29 s for a 121 ppm of H2 pulse, with a variation in the nanowire resistance appreciable at room temperature and at 373.15 K of approximately 8% and 14% respectively, showing that ZnO nanowires are good candidates to detect low concentrations of H2.

Synthesis and Upconversion Luminescence of Nanoparticles Y2O3 and Gd2O3 Co-doped with Yb3+ and Er3+

Upconversion nanoparticles (UCNs) find application in nanomedicine as biolabels to identify cancer cells. In this work, Y2O3: Er3+/Yb3+ and Gd2O3: Er3+/Yb3+ systems were fabricated by combustion synthesis (CS) and sol-gel (SG) methods to obtain nanoparticles with different shapes and sizes after post-annealing treatments. The crystallinity and morphology were analysed by x-ray diffraction (XRD) and transmission electron microscopy (TEM), respectively, and the luminescent properties by spectrofluorescence measurements. The luminescence varied depending on the doping level. In this study, for the host Y2O3, with Er3+/Yb3+ (1%/1% mol), the green emission was more strongly attributed to the 2H 11/2→4I 15/2, 4S3/2→4I 15/2 transitions of Er3+ ions. Red emission was present in Yb3+ (5%/10% mol) due to the 4F9/2→4I15/2 transition for both hosts. For the host Gd2O3, green emission was present in Er3+ (2% mol) and Yb3+ (3% mol). The surface of some UCNs was coated with a thin silica layer and analysed in order to compare the luminescence with the uncoated UCNs. The purpose of the coating was to prepare the samples for further functionalization for biolabel use. Results show that the emission intensity of all samples strongly depends on the synthesis method. SG was more reliable than CS synthesis in producing UCNs with better morphology and luminescent properties.

Interface analysis of CVD diamond on TiN surfaces

Abstract We prepared polycrystalline diamond thin films on smooth silicon substrates with the help of a titanium nitride (TiN) buffer layer. TiN layers of different thickness were deposited first on crystalline silicon substrates with mirror finish. The TiN layers were placed by physical vapor deposition (PVD) assisted by direct current reactive magnetron sputtering. Later, diamond thin films were grown by hot filament chemical vapor deposition (HF-CVD). Scanning electron microscopy observations show a notable increase in the size of diamond particles on the substrates with the TiN buffer layer, as opposed to the plain, only scratched substrates. The diamond films were characterized by high-resolution transmission electron microscopy (HRTEM), electron energy loss (EELS) and energy dispersive spectroscopies (EDS). A buffer layer ∼0.8 nm thick is observed between the diamond particles and the TiN layer. EDS experiments reveal a carbon nitride compound at the interphase. There is no evidence of degradation (cracking, delamination, etc.) of the TiN layers for thickness below 0.5 μm.

A biosensor based on Coriolopsis gallica laccase immobilized on nitrogen-doped multiwalled carbon nanotubes and graphene oxide for polyphenol detection.

Abstract The use of nanomaterials allows the design of ultrasensitive biosensors with advantages in the detection of organic molecules. Catechol and catechin are molecules that occur naturally in fruits, and their presence in products like dyes and wines affects quality standards. In this study, catechol and catechin were measured at the nanoscale by means of cyclic voltammetry. The oxidation of Coriolopsis gallica laccase immobilized on nitrogen-doped multiwalled carbon nanotubes (Lac/CNx-MWCNT) and on graphene oxide (Lac/GO) was used to measure the concentrations of catechol and catechin. Nitrogen-doped multiwalled carbon nanotubes (CNx-MWCNT) were synthesized by spray pyrolysis and characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and x-ray photoelectron spectroscopy (XPS). Covalently bonded hybrids with laccase (Lac/CNx-MWCNT and Lac/GO) were generated. Catalytic activity of free enzymes determined with syringaldazine yielded 14 584 UmL−1. With Lac/CNx-MWCNT at concentrations of 6.4 mmol L−1 activity was 9326 U mL−1, while enzyme activity measured with Lac/GO at concentration of 6.4 mmol L−1 was 9 234 U mL−1. The Lac/CNx-MWCNT hybrid showed higher stability than Lac/GO at different ethyl alcohol concentrations. The Lac/CNx-MWCNT hybrid can measure concentrations, not previously reported, as low as 1 × 10−8 mol L−1 by measuring the electric current responses.

Low resistivity-highly transparent ZnO:Ga TCO`s grown by laser ablation

Electrically conductive and optically transparent thin film oxides, most commonly known as TCO`s are materials of vital importance due to their use for the operation and testing of many opto-electronic devices such as electroluminescent systems, vacuum fluorescent displays, flat panel displays, and also for solar cells. Highly transparent with low electrical resistivity gallium-doped zinc oxide (ZnO:Ga) thin films were prepared on glass by Pulsed Laser Deposition at different substrate temperatures. ZnO:Ga evaporation targets were prepared by pressing and sintering powder materials obtained with a novel combustion synthesis technique. An excellent transmittance of more than 85% in the visible range and a low sheet resistance value as low as 13 {Omega}/sq. were measured on a 200 nm thickness film grown at Ts = 300 C. From optical measurements the authors observed an increase in the ZnO:Ga bandgap when the substrate temperature is raised from 150 C to 300 C. With this, a remarkable improvement in the blue-green region transmittance is obtained. X-Ray diffraction patterns indicate that the films grow highly oriented in the basal plane direction (c-axis) of the hexagonal ZnO grains.

Magnetic-luminescent cerium-doped gadolinium aluminum garnet nanoparticles for simultaneous imaging and photodynamic therapy of cancer cells

Nanoparticle (NP) and photosensitizer (PS) conjugates capable of X-ray photodynamic therapy (X-PDT) are a research focus due to their potential applications in cancer treatment. Combined with X-PDT, appropriate imaging properties of the nanocomposite will make it suitable for theranostics of deep lying tumors. In this work, we describe the development of magnetic-luminescent Gd2.98Ce0.02Al5O12 nanoparticles (GAG) coated with mesoporous silica (mSiO2) and loaded with rose bengal (RB) to yield a nanocomposite GAG@mSiO2@RB capable of X-PDT. GAG nanoparticles were synthesized using the sol-gel method. The synthesized GAG nanoparticles showed a strong visible yellow emission with a quantum yield of ∼32%. Moreover, the broad emission spectra of GAG nanoparticles centered at 585 nm showed a good overlap with the absorption of RB. Upon irradiation with X-rays (55 KV), the GAG@mSiO2@RB nanocomposite produced significantly higher singlet oxygen compared with RB alone, as confirmed by the 1,2-diphenylisobenzofuran (DPBF) assay. The developed GAG@mSiO2@RB nanocomposite significantly reduced the viability of human breast cancer (MDA-MB-231) cells upon irradiation with blue light (λ = 470 nm). The calculated LC50 of GAG@mSiO2@RB nanocomposites were 26.69, 11.2, and 6.56 µg/mL at a dose of ∼0.16, 0.33 and 0.5 J/cm2, respectively. Moreover, the nanocomposite showed paramagnetic properties with high magnetic mass susceptibility which are useful for high contrast T1 weighted magnetic resonance imaging (MRI). Together with X-PDT, the paramagnetic properties of the proposed GAG@mSiO2@RB nanocomposite system are promising for their future application in simultaneous detection and treatment of deep-lying tumors.