Post-growth Oxidation Research Articles

Improved flux and anti-fouling performance of a photocatalytic ZnO membrane on porous stainless steel substrate for microalgae harvesting

Photocatalytic self-cleansing ZnO membranes were developed and used for pre-concentrating microalgae solution prior to harvesting. The inorganic membranes were fabricated via two different methods: (i) direct deposition of ZnO onto a porous stainless steel substrate and (ii) post-growth oxidation of a deposited metallic Zn layer. Systematic surface characterisation revealed a thin layer of homogeneous, crystalline ZnO on the porous membrane. Direct ZnO deposition resulted in a thicker layer with higher UV light absorption capability compared with the post-growth oxidization method. The ZnO coating made the membrane surface highly hydrophilic, resulting in two-fold increase in water permeance compared to the base stainless steel substrate. The high hydrophilicity of the ZnO-coated membrane also led to an increase in the permeate flux of the microalgae solution by up to 100%, making it suitable for microalgae pre-concentration. Upon UV light irradiation, the ZnO membrane demonstrated self-cleansing capability due to the photocatalytic activity of the ZnO coating layer. After 30 min of UV irradiation, the ZnO membrane could achieve 60% permeance recovery after complete fouling. No recovery was observed with the base stainless steel membrane, which was used as the control.

Enhanced solar-driven water splitting performance using oxygen vacancy rich ZnO photoanodes

This work reports a facile two-step method of producing highly efficient ZnO photoanodes for photoelectrochemical (PEC) water splitting under solar light conditions and describes the role of surface oxygen vacancies (VO) in their enhanced PEC performance. The photoanode fabrication involves post-growth oxidation of a metallic Zn layer, which produces a nanostructured ZnO film consisting of ∼50 nm diameter nanorods containing a high concentration of VO defects. The PEC activity of the ZnO films is investigated by studying water oxidation in an aqueous electrolyte under simulated solar illumination. The relationship of PEC and charge transfer characteristics of the ZnO photoanodes with ionized surface VO defects is established using cathodoluminescence, X-ray photoemission, voltammetry, electrochemical impedance spectroscopy and chronoamperometry. The combined results show that the photoanode fabricated in this work possesses a high surface density of ionized VO states that facilitate the effective transportation of holes for water oxidation. It is found that the photoanode exhibits an exceptional photocurrent density of 1.14 mA/cm2 at 1.23 VRHE, being one of the best performances reported in the literature for ZnO-based photoelectrodes so far. Our results demonstrate a simple, low-cost method for fabricating highly efficient VO rich ZnO-based PEC photoanodes that is suitable for large scale production.

Compositional analysis of oxide-embedded III–V nanostructures

Nanowire growth enables creation of embedded heterostructures, where one material is completely surrounded by another. Through materials-selective post-growth oxidation it is also possible to combine amorphous oxides and crystalline, e.g. III–V materials. Such oxide-embedded structures pose a challenge for compositional characterization through transmission electron microscopy since the materials will overlap in projection. Furthermore, materials electrically isolated by an embedding oxide are more sensitive to electron beam-induced alterations. Methods that can directly isolate the embedded material, preferably at reduced electron doses, will be required in this situation. Here, we analyse the performance of two such techniques—local lattice parameter measurements from high resolution micrographs and bulk plasmon energy measurements from electron energy loss spectra—by applying them to analyse InP-AlInP segments embedded in amorphous aluminium oxide. We demonstrate the complementarity of the two methods, which show an overall excellent agreement. However, in regions with residual strain, which we analyse through molecular dynamics simulations, the two techniques diverge from the true value in opposite directions.

Investigation of the cross-sectional morphology of epitaxial Si nanowires grown by chemical vapor deposition for the fabrication of vertical devices

The interfacial structure of Si nanowire transistors is decisively important to their electronic properties. Smooth sidewall surface of Si nanowires is therefore a prerequisite for the fabrication of vertical transistors. In this study, the axial cross-sectional morphology of epitaxial Si nanowires, which are grown vertically on Si[111] substrates by chemical vapor deposition via the vapour-liquid-solid mechanism, are analyzed by transmission electron microscopy to understand their surface morphology. It is noted that the cross-sections do not exhibit sharp facets. The roughness of the sidewall surface is attributed to precipitation of Si from AuSi liquid droplets. The cross-sectional shape of the Si nanowires is truncated-triangular, but not exactly symmetric. A larger distortion of the cross-sectional shape appears in tilted nanowires. It is also demonstrated that after removing the surface Au, post-growth oxidation of Si nanowires is helpful to rebuild smooth sidewall surface.

Vanadium and Manganese Carbonyls as Precursors in Electron-Induced and Thermal Deposition Processes.

The material composition and electrical properties of nanostructures obtained from focused electron beam-induced deposition (FEBID) using manganese and vanadium carbonyl precursors have been investigated. The composition of the FEBID deposits has been compared with thin films derived by the thermal decomposition of the same precursors in chemical vapor deposition (CVD). FEBID of V(CO)6 gives access to a material with a V/C ratio of 0.63–0.86, while in CVD a lower carbon content with V/C ratios of 1.1–1.3 is obtained. Microstructural characterization reveals for V-based materials derived from both deposition techniques crystallites of a cubic phase that can be associated with VC1−xOx. In addition, the electrical transport measurements of direct-write VC1−xOx show moderate resistivity values of 0.8–1.2 × 103 µΩ·cm, a negligible influence of contact resistances and signatures of a granular metal in the temperature-dependent conductivity. Mn-based deposits obtained from Mn2(CO)10 contain ~40 at% Mn for FEBID and a slightly higher metal percentage for CVD. Exclusively insulating material has been observed in FEBID deposits as deduced from electrical conductivity measurements. In addition, strong tendencies for postgrowth oxidation have to be considered.

Synthesis and Characterization of SrFexMn1-x(O,F)3-δ Oxide (δ = 0 and 0.5) and Oxyfluoride Perovskite Films.

We report the synthesis and characterization of as-grown SrFexMn1-xO2.5 epitaxial films, which were also subjected to postgrowth oxidation and topotactic fluorination to obtain SrFexMn1-xO3 and SrFexMn1-xO2.5-δFγ films. We show how both the B-site cation and anion composition influence the structural, electronic, and optical properties of this family of perovskite materials. The Fe substitution of Mn in SrMnO2.5 gradually expands the c-axis parameter, as indicated by X-ray diffraction. With increasing x, the F content incorporated under identical fluorination conditions increases, reaching its maximum in SrFeO2.5-δFγ. In the compounds with mixed B-site occupation, the Fe 2p photoemission peaks are shifted upon fluorination, while the Mn 2p peaks are not, suggesting inductive effects lead to asymmetric responses in how F alters the Mn and Fe bonds. Electronic transport measurements reveal all compounds are insulators, with the exception of SrFeO3, and demonstrate that fluorination increases resistivity for all values of x. Optical absorption spectra in the SrFexMn1-xO2.5 and SrFexMn1-xO3 films evolve systematically as a function of x, consistent with a physical scenario in which optical changes with Fe substitution arise from a linear combination of Mn and Fe 3d bands within the electronic structure. In contrast, the F incorporation induces nonlinear changes to the optical response, suggesting a more complex impact on the electronic structure in materials with concurrent B-site and anion site substitution.

Grain Boundaries and Gas Barrier Property of Graphene Revealed by Dark-Field Optical Microscopy

We demonstrate that dark-field (DF) optical microscopy is a powerful tool to visualize grain boundaries (GBs) and grain structure of graphene grown by chemical vapor deposition (CVD). Copper oxide nanoparticles sparsely formed along the graphene GBs by postgrowth mild oxidation allow one to determine the position and structure of the GBs by the DF microscope. As DF imaging offers a much higher sensitivity than bright-field (BF) microscopy, some GBs were observed even without the postgrowth oxidation. We found that periodic Cu steps formed below graphene can be also used to visualize the grain structure of the as-grown graphene by DF microscopy. Moreover, DF imaging is applicable to study of the gas barrier property of CVD graphene. Interestingly, the dissolved oxygen inside Cu foil enhanced oxidation of the Cu surface below graphene in spite of the fact that the graphene protects the underlying Cu from the exterior gas. Our work highlights the wide availability of DF optical microscopy in characterizing g...

Oxygen vacancy induced magnetization switching in Fe3O4 epitaxial ultrathin films on GaAs(100)

The magnetic and transport properties of half metallic Fe3O4, which are sensitive to the stoichiometry, are the key issue for applications in spintronics. An anomalous enlargement of the saturation magnetic moment is found in a relatively thick sample of epitaxial Fe3O4 film by post-growth oxidation method. The investigation of the thickness dependence of magnetic moment suggests that the enhanced magnetism moment may come from the existence of oxygen vacancies. First-principles calculations reveal that with oxygen vacancies in Fe3O4 crystal the spin of Fe ions in the tetrahedron site near the vacancy is much easier to switch parallel to the Fe ions in the octahedron site by temperature disturbance, supported by the temperature dependence of magnetic moment of Fe3O4 films in experiment.

Semiconductor-Oxide Heterostructured Nanowires Using Postgrowth Oxidation

Semiconductor-oxide heterointerfaces have several electron volts high-charge carrier potential barriers, which may enable devices utilizing quantum confinement at room temperature. While a single heterointerface is easily formed by oxide deposition on a crystalline semiconductor, as in MOS transistors, the amorphous structure of most oxides inhibits epitaxy of a second semiconductor layer. Here, we overcome this limitation by separating epitaxy from oxidation, using postgrowth oxidation of AlP segments to create axial and core-shell semiconductor-oxide heterostructured nanowires. Complete epitaxial AlP-InP nanowire structures were first grown in an oxygen-free environment. Subsequent exposure to air converted the AlP segments into amorphous aluminum oxide segments, leaving isolated InP segments in an oxide matrix. InP quantum dots formed on the nanowire sidewalls exhibit room temperature photoluminescence with small line widths (down to 15 meV) and high intensity. This optical performance, together with the control of heterostructure segment length, diameter, and position, opens up for optoelectrical applications at room temperature.

Layer-by-layer oxidation of InN(0001) thin films into body-center cubic In2O3(111) by cycle rapid thermal annealing

We report on a study of In2O3 thin films obtained by post-growth thermal oxidation of InN(0001) using rapid thermal annealing (RTA) and cycle-RTA (CRTA). The crystal qualities of both the resultant In2O3 and the remaining InN were significantly improved by using CRTA instead of RTA. Body-center cubic (bcc) In2O3, consisting of two In2O3(111) variants in-plane rotated 60° with respect to each other, was epitaxially grown on InN(0001). The in-plane orientations were determined as In2O3[01]//InN[110] and In2O3[01]//InN[20] for the two In2O3(111) variants. Microscopic and spectroscopic analyses, together with the effect of Si3N4 encapsulations, provide evidence that the oxidation of InN is realized by oxygen inward diffusion. The oxygen inward diffusion is slowed down and the thermal decomposition of InN is suppressed by using CRTA, which in turn leads to a layer-by-layer oxidation of InN.

Role of synthesis for oxygen defect incorporation in crystalline rubrene

Using photoluminescence characteristics at room temperature, we analyze incorporation of an oxygen-related defect, most likely rubrene peroxide, which produces an acceptor state in crystalline rubrene. Incorporation of rubrene-peroxide molecules can occur during the crystal growth as well as a result of the postgrowth oxidation, which depends on the presence of structural disorder.

Integrated silicon nanowire diodes and the effects of gold doping from the growth catalyst

We report on integrated, silicon single-nanowire diodes. Gold catalyst templates, defined by lithography, controlled the location of nanowires grown with a vapor-liquid-solid mechanism. The nanowire growth, by atmospheric-pressure chemical vapor deposition, used SiCl4 diluted in H2 on (100) n-type silicon substrates. Postgrowth oxidation and wet etching reduced the nanowire diameters and removed unintentional small diameter nanowires. Spin-on glass isolated the nanowire tips from the substrate, which were then contacted with aluminum. Current-voltage measurements show rectification and ideality factors consistent with pn junction diodes. However, the gold catalyzed nanowires have much higher than expected hole concentrations that cannot be explained by behaviors reported for gold diffused into silicon.

Oxygen diffusion and reactions in Hf-based dielectrics

Oxygen transport in and reactions with thin hafnium oxide and hafnium silicate films have been investigated using medium energy ion scattering in combination with O218 isotopic tracing methods. Postgrowth oxidation of Hf-based films in an O218 atmosphere at 490–950°C results in O exchange in the film. The exchange rate is faster for pure hafnium oxides than for silicates. The amount of exchanged oxygen increases with temperature and is suppressed by the SiO2 component. Films annealed prior to oxygen isotope exposure show complex incorporation behavior, which may be attributed to grain boundary defects, and SiO2 phase segregation.

Characterization of Ga 1− x Mn x As/(001)GaAs epilayers grown by low-temperature molecular beam epitaxy

Ga1− x Mn x As/(001) GaAs (x = 2.2, 5.6, 9 at.%) epitaxial layers grown by low-temperature MBE, exhibiting anisotropic magnetic properties, have been characterized using a range of transmission electron microscopy techniques and secondary-ion mass spectrometry. Banded contrast features on inclined planes are observed for the [110] projection of micron-thick samples with high Mn content, and are attributed to a compositional fluctuation in the interstitial Mn content. Precipitates attributed to tetragonally distorted MnAs are associated with the onset of stacking-fault formation during layer growth. The formation of an Mn–O layer at the surface of the samples is also observed, attributed to post-growth oxidation of an Mn surfactant layer.

XPS and XMCD study of Fe/sub 3/O/sub 4//GaAs interface

Ultrathin Fe oxide films of various thicknesses prepared by post-growth oxidation on GaAs(100) surface have been investigated with X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and X-ray magnetic circular dichroism (XMCD). The XPS confirms that the surfaces of the oxide are Fe/sub 3/O/sub 4/ rather than Fe/sub 2/O/sub 3/. XAS and XMCD measurements indicate the presence of insulating Fe divalent oxide phases (FeO) beneath the surface Fe/sub 3/O/sub 4/ layer with the sample thickness above 4 nm. This FeO might act as a barrier for the spin injection into the GaAs.

On the morphological instability of silicon/silicon dioxide nanowires

Silicon–silicon dioxide core-shell nanowires grown on gold-coated silicon wafers by thermal evaporation of silicon monoxide sometimes show an oscillation in diameter. The two possible causes for this behaviour are a self-oscillation process during the growth or the so-called Rayleigh instability. By analyzing the thickness distribution of the nanowires, we will show that a self-oscillation process is responsible for the periodic instability during growth. In contrast, during post-growth etching and oxidation the nanowires can develop Rayleigh instabilities, leading to silicon nanocrystals embedded in silicon dioxide nanowire.

Structure and microstructure of epitaxial Sr n Fe n O3 n −1 films

Thin films of SrFeO3 − x (0 ≤ x ≤ 0.5) (SFO) grown on a (LaAlO3)0.3 (SrAl0.5Ta0.5O3)0.7 (LSAT) substrate by pulsed laser deposition have been structurally investigated by electron diffraction and high resolution transmission electron microscopy for different post-deposition oxygen treatments. During the deposition and post-growth oxidation, the oxygen-reduced SFO films accept extra oxygen along the tetrahedral layers to minimize the elastic strain energy. The oxidation process stops at a concentration SFO2.875 and/or SFO2.75 because a zero misfit with the LSAT substrate is reached. A possible growth mechanism and phase transition mechanism are suggested. The non-oxidized films exhibit twin boundaries having a local perovskite-type structure with a nominal composition close to SFO3.

All-epitaxial current- and mode-confined AlGaAs∕GaAs VCSEL

An all-epitaxial current- and mode-confining vertical-cavity surface-emitting laser is demonstrated that eliminates the need for any post-growth oxidation step. The current and optical mode are self-aligned and confined to the same lithographically defined area in an AlGaAs cavity.

Oxidized porous diamond/pyrocarbon nanocomposites as improved field electron emitters

Porous diamond/sp2-bonded carbon nanocomposites were studied for field electron emission properties and electroconductivity depending on composition and post-growth oxidation. It is found that the oxidation in nitric acid can sufficiently improve the field emission from the diamond samples. The oxidized composites exhibit lower emission threshold fields Eth=1–2 V/μm compared to Eth=4–6 V/μm for non-oxidized samples, and the emission uniformity (the number of emission sites per unit area) is also improved. More porous composites typically show better results. A possible reason of the emission improvement by oxidation in this two-phase carbon system is the lowering of the tunneling barrier due to formation of the dipole layers on the oxidized carbon surfaces.

Oxidized porous diamond/pyrocarbon nanocomposites as improved field electron emitters

Porous diamond/sp 2-bonded carbon nanocomposites were studied for field electron emission properties and electroconductivity depending on composition and post-growth oxidation. It is found that the oxidation in nitric acid can sufficiently improve the field emission from the diamond samples. The oxidized composites exhibit lower emission threshold fields E th=1–2 V/μm compared to E th=4–6 V/μm for non-oxidized samples, and the emission uniformity (the number of emission sites per unit area) is also improved. More porous composites typically show better results. A possible reason of the emission improvement by oxidation in this two-phase carbon system is the lowering of the tunneling barrier due to formation of the dipole layers on the oxidized carbon surfaces.