Two-step Growth Process Research Articles

Process of in situ forming well-aligned Zinc Oxide nanorod arrays on wood substrate using a two-step bottom-up method

A good nanocrystal covering layer on wood can serve as a protective coating and present some new surface properties. In this study, well-aligned ZnO nanorods (NRs) arrays were successfully grown on wood surface through a two-step bottom-up growth process. The process involved pre-sow seeds and subsequently their growing into NRs under hydrothermal environment. The interface incorporation between wood and ZnO colloid particles in the precursor solution during the seeding process was analyzed and demonstrated through a schematic. The growth process of forming well-aligned ZnO NRs was analyzed by field-emission scanning electron microscopy and X-ray diffraction, which showed that the NRs elongated with increased reaction time. The effects of ZnO crystal form and capping agent on the growth process were studied through different viewpoints.

Controlled Ti Seed Layer Assisted Growth and Field Emission Properties of Pb(Zr0.52Ti0.48)O3 Nanowire Arrays

We report on the directed upright growth of ferroelectric (FE) Pb(Zr0.52Ti0.48)O3 (PZT) nanowire (NW) arrays with large aspect ratios of >60 using a Ti seed layer assisted hydrothermal process over large surface areas on ITO/glass substrates. In a two-step growth process, Ti seed layer of low surface roughness with a thickness of ~500 nm and grain size of ~100 nm was first deposited by radio frequency (RF) sputtering which was subsequently used as substrates for the growth of highly dense, single crystalline PZT NWs by controlled nucleation. The electron emission properties of the PZT NWs were investigated using the as-grown NWs as FE cathodes. A low turn-on field of ~3.4 V/μm was obtained from the NW arrays, which is impressively lower than that from other reported values. The results reported in this work give direction to the development of a facile growth technique for PZT NWs over large surfaces and also are of interest to the generation of high current electron beam from FE NW based cathodes for field emitter applications.

Light Output Improvement of GaN-Based Light-Emitting Diodes Using Hydrothermally Grown ZnO Nanotapers

A new two-step hydrothermal growth (HTG) process with a shorter processing time and better growth control is proposed for the synthesis of ZnO nanotapers (NTs). The application of HTG ZnO NTs as surface roughening nanostructures to improve the light output power (Lop) of GaN-based LEDs is demonstrated. Compared with that of ZnO nanowires, the use of ZnO NTs leads to an improvement in Lop by 24.5% at 350 mA, which could be attributed to the fact that tapered ends of ZnO NTs offer more constructive photon scattering to maximize light extraction.

Hierarchical Oriented Anatase TiO2 Nanostructure arrays on Flexible Substrate for Efficient Dye-sensitized Solar Cells

The vertically oriented anatase single crystalline TiO2 nanostructure arrays (TNAs) consisting of TiO2 truncated octahedrons with exposed {001} facets or hierarchical TiO2 nanotubes (HNTs) consisting of numerous nanocrystals on Ti-foil substrate were synthesized via a two-step hydrothermal growth process. The first step hydrothermal reaction of Ti foil and NaOH leads to the formation of H-titanate nanowire arrays, which is further performed the second step hydrothermal reaction to obtain the oriented anatase single crystalline TiO2 nanostructures such as TiO2 nanoarrays assembly with truncated octahedral TiO2 nanocrystals in the presence of NH4F aqueous or hierarchical TiO2 nanotubes with walls made of nanocrystals in the presence of pure water. Subsequently, these TiO2 nanostructures were utilized to produce dye-sensitized solar cells in a backside illumination pattern, yielding a significant high power conversion efficiency (PCE) of 4.66% (TNAs, JSC = 7.46 mA cm−2, VOC = 839 mV, FF = 0.75) and 5.84% (HNTs, JSC = 10.02 mA cm−2, VOC = 817 mV, FF = 0.72), respectively.

Effect of high-temperature buffer thickness on quality of AlN epilayer grown on sapphire substrate by metalorganic chemical vapor deposition

The effect of an initially grown high-temperature AlN buffer (HT-AlN) layer's thickness on the quality of an AlN epilayer grown on sapphire substrate by metalorganic chemical vapor deposition (MOCVD) in a two-step growth process is investigated. The characteristics of AlN epilayers are analyzed by using triple-axis crystal X-ray diffraction (XRD) and atomic force microscopy (AFM). It is shown that the crystal quality of the AlN epilayer is closely related to its correlation length. The correlation length is determined by the thickness of the initially grown HT-AlN buffer layer. We find that the optimal HT-AlN buffer thickness for obtaining a high-quality AlN epilayer grown on sapphire substrate is about 20 nm.

Structural and photoluminescence studies on catalytic growth of silicon/zinc oxide heterostructure nanowires

Silicon/zinc oxide (Si/ZnO) core-shell nanowires (NWs) were prepared on a p-type Si(111) substrate using a two-step growth process. First, indium seed-coated Si NWs (In/Si NWs) were synthesized using a plasma-assisted hot-wire chemical vapor deposition technique. This was then followed by the growth of a ZnO nanostructure shell layer using a vapor transport and condensation method. By varying the ZnO growth time from 0.5 to 2 h, different morphologies of ZnO nanostructures, such as ZnO nanoparticles, ZnO shell layer, and ZnO nanorods were grown on the In/Si NWs. The In seeds were believed to act as centers to attract the ZnO molecule vapors, further inducing the lateral growth of ZnO nanorods from the Si/ZnO core-shell NWs via a vapor-liquid-solid mechanism. The ZnO nanorods had a tendency to grow in the direction of [0001] as indicated by X-ray diffraction and high resolution transmission electron microscopy analyses. We showed that the Si/ZnO core-shell NWs exhibit a broad visible emission ranging from 400 to 750 nm due to the combination of emissions from oxygen vacancies in ZnO and In2O3 structures and nanocrystallite Si on the Si NWs. The hierarchical growth of straight ZnO nanorods on the core-shell NWs eventually reduced the defect (green) emission and enhanced the near band edge (ultraviolet) emission of the ZnO.

Diameter and location control of ZnO nanowires using electrodeposition and sodium citrate

We report single-step growth of spatially localized ZnO nanowires of controlled diameter to enable improved performance of piezoelectric devices such as nanogenerators. This study is the first to demonstrate the combination of electrodeposition with zinc nitrate and sodium citrate in the growth solution. Electrodeposition through a thermally-grown silicon oxide mask results in localization, while the growth voltage and solution chemistry are tuned to control the nanowire geometry. We observe a competition between lateral (relative to the (0001) axis) citrate-related morphology and voltage-driven vertical growth which enables this control. High aspect ratios result with either pure nitrate or nitrate-citrate mixtures if large voltages are used, but low growth voltages permit the growth of large diameter nanowires in solution with citrate. Measurements of the current density suggest a two-step growth process. An oxide mask blocks the electrodeposition, and suppresses nucleation of thermally driven growth, permitting single-step lithography on low cost p-type silicon substrates.

Protein-modified silver nanoplates for the complementary analytical method of localised surface plasmon resonance and matrix assisted laser desorption/ionisation mass spectrometry

A complementary analytical methodology combining localised surface plasmon resonance (LSPR) sensing and matrix assisted laser desorption/ionisation-mass spectrometry (MALDI-MS) has been developed by using triangular silver nanoplates (Ag NPLs). Ag NPLs with near-IR LSPR absorbance were prepared via a two-step photo-mediated growth process from Ag nanoparticles. These could be utilised as a common platform for LSPR/MALDI-MS, as they were suitable for LSPR sensing via the analysis of surface plasmon absorption bands and as the assisting material for LDI-MS. For practical use, the detection of analytes by LSPR sensing can be achieved by using specific biomolecular recognition techniques that involve the surface modification of metal NPs with large molecules such as proteins. We investigated the effect on LSPR sensitivity (i.e. refractive index unit, RIU) of modifying the Ag surface with molecules of different sizes, and found that the RIU values were proportional to the amplitude of the cube root of the molecular weight, Mw1/3. We demonstrated the utility of this complementary LSPR and MALDI-MS analysis methodology by evaluating the binding of a soybean trypsin inhibitor to the Ag NPL substrate covalently modified with trypsin. This specific biomolecule recognition phenomenon was first detected using the LSPR technique, and then successfully identified by MALDI-MS.

Vertically p–n-junctioned GaN nano-wire array diode fabricated on Si(111) using MOCVD

We demonstrate the fabrication of n-GaN:Si/p-GaN:Mg nanowire arrays on (111) silicon substrate by metal organic chemical vapor deposition (MOCVD) method .The nanowires were grown by a newly developed two-step growth process. The diameter of as-grown nanowires ranges from 300-400 nm with a density of 6-7 × 10(7) cm(-2). The p- and n-type doping of the nanowires is achieved with Mg and Si dopant species. Structural characterization by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) indicates that the nanowires are relatively defect-free. The room-temperature photoluminescence emission with a strong peak at 370 nm indicates that the n-GaN:Si/p-GaN:Mg nanowire arrays have potential application in light-emitting nanodevices. The cathodoluminscence (CL) spectrum clearly shows a distinct optical transition of GaN nanodiodes. The nano-n-GaN:Si/p-GaN:Mg diodes were further completed using a sputter coating approach to deposit Au/Ni metal contacts. The polysilazane filler has been etched by a wet chemical etching process. The n-GaN:Si/p-GaN:Mg nanowire diode was fabricated for different Mg source flow rates. The current-voltage (I-V) measurements reveal excellent rectifying properties with an obvious turn-on voltage at 1.6 V for a Mg flow rate of 5 sccm (standard cubic centimeters per minute).

Antiphase domain annihilation during growth of GaP on Si by molecular beam epitaxy

Growing high-quality GaP on Si typically requires a two-step growth process in which the nucleation and overgrowth processes are decoupled. However, even under ideal circumstances, antiphase domains inevitably form during the nucleation step and thus need to be annihilated during the overgrowth step to achieve smooth, single-orientation GaP films. We investigated the effects of V/III flux ratio and growth rate on the propagation and annihilation of antiphase boundaries during the overgrowth step. By enhancing Ga-adatom orientation-dependent surface diffusion, the growth of existing antiphase domains was suppressed by the surrounding film such that the antiphase domains annihilated and yielded a single-orientation film. Characterization of GaP films by atomic force microscopy, transmission electron microscopy, and reflection high-energy electron diffraction shows that certain conditions promoted the annihilation of antiphase boundaries and resulted in smoother films.

Ultraclean Emission from InAsP Quantum Dots in Defect-Free Wurtzite InP Nanowires

We report on the ultraclean emission from single quantum dots embedded in pure wurtzite nanowires. Using a two-step growth process combining selective-area and vapor-liquid-solid epitaxy, we grow defect-free wurtzite InP nanowires with embedded InAsP quantum dots, which are clad to diameters sufficient for waveguiding at λ ~ 950 nm. The absence of nearby traps, at both the nanowire surface and along its length in the vicinity of the quantum dot, manifests in excitonic transitions of high spectral purity. Narrow emission line widths (30 μeV) and very-pure single photon emission with a probability of multiphoton emission below 1% are achieved, both of which were not possible in previous work where stacking fault densities were significantly higher.

One-step graphene coating of heteroepitaxial GaN films

Today, state-of-the-art III-Ns technology has been focused on the growth of c-plane nitrides by metal–organic chemical vapor deposition (MOCVD) using a conventional two-step growth process. Here we show that the use of graphene as a coating layer allows the one-step growth of heteroepitaxial GaN films on sapphire in a MOCVD reactor, simplifying the GaN growth process. It is found that the graphene coating improves the wetting between GaN and sapphire, and, with as little as ∼0.6 nm of graphene coating, the overgrown GaN layer on sapphire becomes continuous and flat. With increasing thickness of the graphene coating, the structural and optical properties of one-step grown GaN films gradually transition towards those of GaN films grown by a conventional two-step growth method. The InGaN/GaN multiple quantum well structure grown on a GaN/graphene/sapphire heterosystem shows a high internal quantum efficiency, allowing the use of one-step grown GaN films as ‘pseudo-substrates’ in optoelectronic devices. The introduction of graphene as a coating layer provides an atomic playground for metal adatoms and simplifies the III-Ns growth process, making it potentially very useful as a means to grow other heteroepitaxial films on arbitrary substrates with lattice and thermal mismatch.

Stress Evaluation on Hetero-Epitaxial 3C-SiC Film on (100) Si Substrates

SiC is a candidate material for micro- and nano-electromechanical systems (MEMS and NEMS). In order to understand the impact that the growth rate has on the residual stress of CVD-grown 3C-SiC hetero-epitaxial films on Si substrates, growth experiments were performed and the resulting stress was evaluated. Film growth was performed using a two-step growth process with propane and silane as the C and Si precursors in hydrogen carrier gas. The film thickness was held constant at ~2.5 µm independent of the growth rate so as to allow for direct films comparison as a function of the growth rate. Supported by profilometry, Raman and micro-machined free-standing structures, this study shows that the growth rate is a fundamental parameter for low-defect and low-stress hetero-epitaxial growth process of 3C-SiC on Si substrates. Stress analysis performed by modify Stoney’s equation trough optical curvature measurement, Raman shift analysis and micro-machining of free-standing structures that shows apparent disagreement about the nature of the stress. These odds between the experimental data can be explained assuming a strong stress field located in the substrate and related to defects generated in the silicon during the growth process.

N2-plasma nitridation on Si(111): Its effect on crystalline silicon nitride growth

This study uses a combination of atom-resolved scanning tunneling microscopic and spectroscopic (STM/STS) techniques to investigate the effects of N2-plasma nitridation on a crystalline β-Si3N4 ultrathin film grown on the Si(111) substrate. The proposed two-step growth process, including N2-plasma nitridation followed by vacuum annealing both at high temperature (~900°C), can substantially improve the atomic and electronic structures of the β-Si3N4 having an atomically uniform morphology and stoichiometry. The effects of nitridation and post-annealing temperatures were examined by monitoring the morphological and electronic–structural evolutions of a non-stoichiometric surface. Moreover, the two-step N2-plasma nitridation process has extremely low activation energy and thus minimizes the thermal energy from the substrate for practical growth of β-Si3N4 ultrathin film.

The production of large bilayer hexagonal graphene domains by a two-step growth process of segregation and surface-catalytic chemical vapor deposition

A novel two-step process combining surface catalyzed process with segregation growth was used to prepare single crystal hexagonal bilayer graphene domains on Cu metal substrates by ambient pressure chemical vapor deposition. Carbon atoms are first dissolved into the quasi-melting Cu metal at 1080°C and then segregated on the Cu surface to form nucleation centers of single-layer graphene during cooling. The graphene crystallites spontaneously act as templates to induce the carbon atoms to form hexagonal bilayer graphene domains. The bilayer graphene domains are size-tunable by controlling the growth conditions. The yield of the bilayer graphene is over 90% and the defect-free domains reach ∼100μm in size, greater than the reported single-layer domains.

Wire-on-Wire Growth of Fluorescent Organic Heterojunctions

Dendritic organic heterojunctions with aluminum tris(8-hydroxyquinoline) (Alq(3)) microwire trunks and 1,5-diaminoanthraquinone (DAAQ) nanowire branches were prepared by a two-step growth process. The prefabricated Alq(3) microwires act as nucleation centers for site-specific secondary vapor growth of DAAQ nanowires, resulting in the unique dendritic heterostructures. When the trunk was excited with a focused laser beam, emitted light of various colors was simultaneously channeled from the branched nanowires via both waveguiding and energy transfer. The intensity of the out-coupled emissions was modulated effectively by changing the polarization of the incident light.

Controlling growth of self-propagating molecular assemblies

A series of six self-propagating molecular assemblies (SPMAs) were generated by alternative solution-deposition of ruthenium polypyridyl complexes and d8palladium and platinum salts on glass and silicon substrates. The d6 polypyridyl complexes have three pyridine units available for forming networks by coordination to the metal salts. This two-step film growth process is fast (15 min/step) and can be carried out conveniently under ambient conditions in air. The reactivity of the common metal salts (ML2X2: M = Pd, X = Cl, L = PhCN, ½ 1,5-cyclooctadiene (COD), SMe2 and M = Pt, X = Cl, Br, I, L = PhCN) is a dominant factor in the film growth. Although the assembly structures are comparable, their exponential growth can be controlled by varying the metals salts. The co-ligands, halides, and metal centers can be used to control the film thicknesses and light absorption intensities of the metal-to-ligand charge transfer (MLCT) bands by a factor of ∼3.5 for 13 deposition steps, whereas the surface morphologies and molecular densities of the SPMAs are similar. The surface-confined assemblies have been characterized using a combination of optical (UV/Vis, ellipsometry) spectroscopy, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and synchrotron X-ray reflectivity (XRR).

Oriented hierarchical single crystalline anatase TiO2nanowire arrays on Ti-foil substrate for efficient flexible dye-sensitized solar cells

Hierarchical anatase TiO2 nanowire (HNW) arrays consisting of long single crystalline nanowire trunks and short single crystalline nanorod branches have been synthesized on Ti-foil substrate via a two-step hydrothermal growth process. The formation of the HNW arrays based on anatase TiO2 nanowire (NW) arrays can be ascribed to the crystallographic relationship between trunk and branch. The power conversion efficiency of dye-sensitized solar cells (DSSCs) based on such a HNW photoelectrode (4.51%) shows a significant enhancement compared to TiO2 nanowire (NW) array photoelectrode (3.12%) with similar thickness (∼15 μm in nanowire length), which can be attributed to more dye loading, superior light scattering ability and comparable electron transport rate for the former. Furthermore, flexible DSSC using TiO2 HNW arrays on Ti substrate as working electrode and transparent PEDOT/ITO-PET prepared via in situelectrodeposition as counter electrode shows a comparable photovoltaic performance to the rigid Pt/FTO-glass cell. A power conversion efficiency as high as 4.32% (Jsc = 7.91 mA cm−2, Voc = 796 mV, FF = 0.69) is obtained for the first time for fully flexible DSSC based on hierarchical TiO2 nanowire arrays and Pt-free counter electrode.

Self-assembly of amorphous calcium carbonate microlens arrays

Biological materials are often based on simple constituents and grown by the principle of self-assembly under ambient conditions. In particular, biomineralization approaches exploit efficient pathways of inorganic material synthesis. There is still a large gap between the complexity of natural systems and the practical utilization of bioinspired formation mechanisms. Here we describe a simple self-assembly route leading to a CaCO3 microlens array, somewhat reminiscent of the brittlestars' microlenses, with uniform size and focal length, by using a minimum number of components and equipment at ambient conditions. The formation mechanism of the amorphous CaCO3 microlens arrays was elucidated by confocal Raman spectroscopic imaging to be a two-step growth process mediated by the organic surfactant. CaCO3 microlens arrays are easy to fabricate, biocompatible and functional in amorphous or more stable crystalline forms. This shows that advanced optical materials can be generated by a simple mineral precipitation.

Growth characteristics of uniaxial InGaN/GaN MQW/n-GaN nanowires on Si(111) using MOCVD

We report on the growth of the uniaxial InGaN/GaN multiple quantum well (MQW) on n-GaN nanowires (NWs) on Si(111) substrates by dynamically adjusting the growth parameters using the pulsed flow metalorganic chemical vapor deposition (MOCVD) technique. We carried out a two-step growth process to grow the uniaxial InGaN/GaN MQW/n-GaN NWs structure. In the first step, the n-GaN NWs were grown at 950 °C and in the second stage, we suitably decreased the growth temperature to 630 and 710 °C, respectively, to grow InGaN/GaN MQW NWs. The surface morphology, structural and optical characterization of the grown InGaN/GaN MQW/n-GaN NWs were studied by field-emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), photoluminescence (PL) and cathodoluminescence (CL) measurements. The CL spectrum recorded on an individual InGaN/GaN MQW/n-GaN NW is dominated by the band-edge emission at 366.7 and 413.5 nm corresponding to n-GaN and MQW structures, respectively. The resultant NWs were found to be free of strain. These results indicate that the pulsed MOCVD technique is an effective method to grow uniaxial InGaN/GaN MQW on n-GaN NWs which is advantageous to other growth techniques. These kinds of uniaxial NWs are promising to allow flat band quantum structures that are shown to improve the efficiency of light-emitting diodes.