Ultralow Growth Rate Research Articles

Gold nanostructures growth in HAuCl4-CTAB-NaOH aqueous solution without any reducing agent

In chemical synthesis of gold nanostructures, reducing agents are always needed in the preparation process. Meanwhile, NaOH acts as accelerator also playing an important role. Even though previous calculations have shown that NaOH and HAuCl4 with a certain chemical ratio can be used for the preparation of gold nanomaterials, but there is no experimental confirmation. Here, we investigated and demonstrated the gold nanostructures growth process in HAuCl4-CTAB-NaOH system without any reductant in details. All results can be assigned that the reaction system can be carried out at room temperature with an ultralow growth rate (more than two weeks) and long-term stability. UV–vis, transmission electron microscopy (TEM) and PH monitoring were used characterized the as-prepared gold nanostructures which corresponding to (111) plane’s gold nanoclusters and (200) plane’s gold nanoparticles obtained. It is expected to be applied to the accurate preparation of gold nanostructures.

Power density and temperature dependent multi-excited states in InAs/GaAs quantum dots

Self-assembled InAs/GaAs (001) quantum dots (QDs) were grown by molecular beam epitaxy using ultra low-growth rate. A typical dot diameter of around 28 ± 2 nm and a typical height of 5 ± 1 nm are observed based on atomic force microscopy image. The photoluminescence (PL) spectra, their power and temperature dependences have been studied for ground (GS) and three excited states (1–3ES) in InAs QDs. By changing the excitation power density, we can significantly influence the distribution of excitons within the QD ensemble. The PL peak energy positions of GS and ES emissions bands depend on an excitation light power. With increasing excitation power, the GS emission energy was red-shifted, while the 1–3ES emission energies were blue-shifted. It is found that the full width at half maximum of the PL spectra has unusual relationship with increasing temperature from 9 to 300 K. The temperature dependence of QD PL spectra shown the existence of two stages of PL thermal quenching and two distinct activation energies corresponding to the temperature ranges I (9–100 K) and II (100–300 K).

Single-dot optical emission from ultralow density well-isolated InP quantum dots

We demonstrate a straightforward way to obtain single well-isolated quantum dots emitting in the visible part of the spectrum and characterize the optical emission from single quantum dots using this method. Self-assembled InP quantum dots are grown using gas-source molecular-beam epitaxy over a wide range of InP deposition rates, using an ultralow growth rate of about 0.01 atomic monolayers/s, a quantum-dot density of 1 dot/μm2 is realized. The resulting isolated InP quantum dots embedded in an InGaP matrix are individually characterized without the need for lithographical patterning and masks on the substrate. Such low-density quantum dots show excitonic emission at around 670 nm with a linewidth limited by instrument resolution. This system is applicable as a single-photon source for applications such as quantum cryptography.

Low temperature growth of Bi 2(Sr,Ca) 2CuO x thin films by ion beam sputtering and effects of oxygen plasma

Thin films of Bi–Sr–Ca–Cu–O were co-deposited on MgO by ion beam sputtering at substrate temperatures ( T s) of 400°C–700°C with supply of either oxygen molecules or plasma at around 1 mTorr. High-quality Ca-doped Bi2201 phases can be grown with c-orientation having extremely small XRD linewidths ( Δ=0.1°–0.2°) in the region of 550< T s<650°C for both supplies. The Δs of both films increase with decreasing T s for T s<550°C, however they are considerably reduced by the plasma then a quasi high-quality film can be grown at a ultralow T s of 450°C. The c-parameters of all the films are extremely short, indicating elongated a/ b lattice constants due to the ultralow growth rate of 10 −2 Å/s. Bi compositions of both films decrease drastically with increasing T s for T s>550°C, due to extensive re-evaporation of Bi 2O 3 formed in the gas phase. This result supports the proposal of self-limited Bi sticking coefficient. Film thickness increases with increasing T s in T s<550°C for the plasma and in T s<600°C for the molecular oxygen supply, and then decreases rapidly because of the Bi 2O 3 re-evaporation. SEM images show 3D grain-like surface morphologies for T s>550°C while 2D-like smooth morphologies for T s<550°C. The rms surface roughness is reduced by the plasma, and it is 128 Å for the film with the thickness of 1730 Å deposited at 450°C whereas it is only 27 Å for the film with the thickness of 600 Å deposited at 500°C. These films show 1–2 unit cell steps, suggesting the 2D growth mode. All the characteristics indicate that the growth is governed by the thermal process for T s>550°C and the plasma effects are prominent for T s<550°C. The proposed origins for the high-quality film growth are: (1) moderate thermal energy, (2) the ultralow growth rate, (3) collision-induced moderate energy of the sputtered particles and (4) suitably excited oxygen, (5) plasma-induced multiple nucleation, and (6) ordering of Sr and Ca, and reduction of the surface energy of the growing film by the plasma. The last one is proposed to be an important mechanism for the enhancement of film growth and the improvement of crystallinity by the plasma at the low T s.

Comparison between Bi-superconductor thin films fabricated via co-deposition and layer-by-layer deposition by ion beam sputtering method

Bi 2Sr 2Ca nCu n+1O y (n≥0; BSCCO) thin film is fabricated via two different processes using an ion beam sputtering method i.e. co-deposition and layer-by-layer deposition. A single phase of Bi2212 can be fabricated via the co-deposition process, while it cannot be obtained by the layer-by-layer process. Ultra-low growth rate in our ion beam sputtering system brings out the difference in Bi element adsorption between the two processes and results in only 30% adsorption against total incident Bi amount by layer-by-layer deposition, in contrast to enough Bi adsorption by co-deposition.

The influence of Bi-sticking coefficient in the growth of Bi(2212) thin film by ion beam sputtering

BSCCO thin films are fabricated via a co-deposition process by an ion beam sputtering method with an ultra-low growth rate, and sticking coefficients of the respective elements are evaluated. The sticking coefficient of Bi element exhibits a characteristic temperature dependence; almost a constant value of 0.49 below 730°C and decreases linearly with temperature over 730°C. This temperature dependence can be elucidated from the evaporation and sublimation rates of bismuth oxide, Bi 2O 3, from the film surface. It is considered that the liquid phase of the bismuth oxide plays an important role in the Bi(2212) phase formation in the co-deposition process.

Extremely smooth YBa2Cu3Oy thin films grown using the reactive coevaporation technique in radical oxygen at an ultralow growth rate

The effect of using drastically reduced growth rates on the surface morphology of YBa2Cu3Oy (YBCO) thin films grown on lattice-matched (110) NdGaO3 substrates using the reactive coevaporation technique in a radical oxygen atmosphere is reported. The surface of films grown at an ultralow growth rate (∼0.0065 nm/s) appears to be very smooth without any precipitates, holes, crevasses, or pits. The film surface roughness is on the order of the unit-cell constant of superconducting YBCO, about 1.2 nm. This shows that decreasing the growth rate enhances the lateral spreading of nucleated islands, with a height of one unit cell, in the Stranski–Krastanov growth mode [Ber. Akad. Wiss, Wien 146, 797 (1938)].

Near-Threshold Fatigue Crack Growth in 2 1/4 Cr-1Mo Pressure Vessel Steel in Air and Hydrogen

As part of an ongoing program to examine subcritical flaw growth in candidate steels for proposed coal gasifier pressure vessels, an initial study is made of characteristics of ultralow growth rate fatigue crack propagation in thick-section, normalized 2 1/4 Cr-1Mo pressure vessel steel (ASTM A387, Class 2 Grade 22). Crack propagation data are generated over a wide range of growth rates, from 10−8 to 10−2 mm/cycle, for load ratios between 0.05 and 0.80 at ambient temperatures in low pressure environments of moist air, dry hydrogen gas and dry argon gas. Particular emphasis is placed on behavior at near-threshold growth rates, below 10−6 mm/cycle, approaching the so-called threshold stress intensity for fatigue crack growth, ΔK0. Near-threshold growth rates, in addition to showing a marked sensitivity to load ratio, are found to be significantly enhanced in gaseous hydrogen compared to air. Similar environmentally-enhanced growth is observed in argon gas. To account for such results, previous models of threshold behavior based on environmental factors (e.g., hydrogen embrittlement) are questioned, and a new approach is presented in terms of the role of oxide debris from moist environments in promoting crack closure. This oxide-induced closure model is found to be consistent with most experimental observations of near-threshold fatigue crack propagation behavior and is proposed as a mechanism for environmental effects at ultra-low growth rates.

Near-threshold fatigue-crack propagation in steels

AbstractThe characteristics of fatigue-crack propagation in metals and alloys have been the subject of several extensive reviews in recent years, but in very few instances have the details of ultralow growth rate, nearthreshold fatigue-crack propagation been similarly discussed. In this review the effects are examined of various mechanical, microstructural, and environmental factors which influence fatigue-crack propagation in steels at growth rates less than 10-6 mm/cycle, where the alternating stress intensity ∆K approaches the so-called threshold stress intensity ∆K0 below which crack growth cannot be experimentally detected. The marked influences of load ratio, material strength, and microstructure on such near-threshold growth are analysed in detail and rationalized in terms of possible environmental contributions and crackclosure concepts. These effects are contrasted with crack-propagation behaviour in other engineering materials and at higher growth rates.