Electrodeposition of Ru by atomic layer deposition (ALD)

Studies are presented describing attempts to form a cycle for the growth of Ru nanofilms using the electrochemical form of atomic layer deposition (ALD). Au substrates have been used to form Ru nanofilms, based on layer by layer growth of deposits, using surface limited reactions. These deposits were formed using surface limited redox replacement (SLRR), where an atomic layer of a sacrificial element is first deposited by underpotential deposition (UPD), and is then exchanged for the element of interest. The use of the UPD atomic layer limits subsequent growth by limiting the number of electrons available for deposition. In the present study, Pb atomic layers were used, and exposed to solutions of Ru³⁺ ions at open circuit. This process can then be repeated to grow films of the desired thickness. It was shown that less than an at.% of Pb was evident in the deposits, using electron probe microanalysis (EPMA), and even that could be removed if a stripping step was added to the ALD cycle. The deposits displayed the expected Ru voltammetry, as well as the Ru hcp XRD pattern. There were some differences in the first 20 cycles, compared with subsequent, suggesting some nucleation process that must be investigated. However, after 20 cycles, the deposit showed the linear growth with the number of cycles expected for an ALD process. The morphology of Ru films, deposited on template-stripped Au was studied using ex situ scanning tunneling microscopy (STM), and showed no evidence of 3D growth.     

Quantum confinement in PbSe thin films electrodeposited by electrochemical atomic layer epitaxy (EC-ALE)

PbSe is a IV-VI compound semiconductor with a narrow band gap (0.26 eV), used in photodetectors, photoresistors and photoemitters in the infrared (IR). This paper reports the first instance of PbSe formation by electrochemical atomic layer epitaxy (EC-ALE). The film’s composition and structure were characterized using electron probe microanalysis (EPMA) and X-ray diffraction (XRD), respectively. The optical properties were studied via infrared absorption measurements. Films were stoichiometric via EPMA, a Pb/Se ratio of one. XRD indicated the expected rock salt structure for PbSe, and a preferred (2 0 0) orientation. IR adsorption studies, of films grown with 10-50 cycles, showed strong blue shifts of the fundamental absorption edge, which was believed to result from quantum confinement in the thin films.     

Effect of potential on bismuth telluride thin film growth by electrochemical atomic layer epitaxy

In the present study, bismuth telluride compound thin film was grown by means of electrochemical atomic layer epitaxy (ECALE) with an automated thin layer flow cell deposition system. The dependence of the Bi and Te deposition potentials on Pt electrode was studied. Because developing a contact potential between the substrate and the growing semiconductor, the deposition potential adjustment is necessary for the first 30 or more cycles of each component. The dependence of the deposit as a function of the deposition potential adjustment slope has been investigated. The results show that an excess elemental Bi existed at a slope of −2 mV/p (p indicates per cycle), indicating that this is a lack of deposition at the potential. Single-phase Bi₂Te₃ compound could be obtained between −4 and −6 mV/p. Bi₂Te₃ and Bi₄Te₃ coexistence is observed at a slope of −10 mV/p. The EDS data indicates that the stoichiometry of compound is consistent with XRD result. SEM studies show that the deposits are inhomogeneous and have an micron sized particles morphology.     

Optimization studies of HgSe thin film deposition by electrochemical atomic layer epitaxy (EC-ALE)

Studies of the optimization of HgSe thin film deposition using electrochemical atomic layer epitaxy (EC-ALE) are reported. Cyclic voltammetry was used to obtain approximate deposition potentials for each element. These potentials were then coupled with their respective solutions to deposit atomic layers of the elements, in a cycle. The cycle, used with an automated flow deposition system, was then repeated to form thin films, the number of cycles performed determining the thickness of the deposit. In the formation of HgSe, the effect of Hg and Se deposition potentials, and a Se stripping potential, were adjusted to optimize the deposition program. Electron probe microanalysis (EPMA) of 100 cycle deposits, grown using the optimized program, showed a Se/Hg ratio of 1.08. Ellipsometric measurements of the deposit indicated a thickness of 19 nm, where 35 nm was expected. X-ray diffraction displayed a pattern consistent with the formation of a zinc blende structure, with a strong (1 1 1) preferred orientation. Glancing angle fourier transform infrared spectroscopy (FTIR) absorption measurements of the deposit suggested a negative gap of 0.60 eV.     

Atomic layer deposition (ALD) of nanoscale coatings on SrAl2O4-based phosphor powders to prevent aqueous degradation

The aqueous degradation of Eu²⁺-activated and Dy³⁺-codoped strontium aluminate (SrAl₂O₄:Eu²⁺, Dy³⁺, SA2-Green) long afterglow phosphors synthesized from solid-state reaction and coated with nanoscale metal oxide protective layers (≤12 nm) via atomic layer deposition (ALD) is investigated. Uncoated phosphor powders degrade rapidly upon water immersion and lose their green phosphorescence within 48 hours of water exposure. Postmortem investigations reveal hydration and decomposition of the SrAl₂O₄ phase. ALD of ~10 nm Al₂O₃ or ~12 nm TiO₂ is found to significantly improve the powder's resistance to aqueous degradation. All ALD-coated powders show minimal structural and chemical degradation and retain phosphoresence after 48 hours of water immersion. This enhanced durability offers a new pathway for applying long afterglow phosphors to outdoor applications like roadway markings or safety signage and for their incorporation into more eco-friendly waterborne coatings.     

Effect of inert ambient annealing on structural and defect characteristics of coaxial N-CNTs@ZnO nanotubes coated by atomic layer deposition

The structure and intrinsic defects of ZnO determine the photogeneration of electron-hole pairs. The control of these characteristics allows for improving the activity of photocatalytic nanostructures. The intrinsic defects of ZnO atomic layer deposited around carbon nanotubes, coaxial N-CNT@ZnO, and its evolution after annealing in a nitrogen atmosphere are studied. TEM showed that the coaxial N-CNT@ZnO structure is preserved up to 500 °C; some damage is seen at 600 and 700 °C. Raman and XPS reveal a low surface defect concentration for as-grown ZnO, increasing to a maximum for the 600 °C annealed sample. Cathodoluminescence shows a broad low-intensity green emission associated with oxygen vacancies of ZnO. The surface defects improved the photocatalytic degradation of amaranth dye, however, damage to the coaxial structure lowers the catalytic activity.     

Structural,electrical, and optical properties of Ti-doped ZnO films fabricated by atomic layer deposition

High-quality Ti-doped ZnO films were grown on Si, thermally grown SiO₂, and quartz substrates by atomic layer deposition (ALD) at 200°C with various Ti doping concentrations. Titanium isopropoxide, diethyl zinc, and deionized water were sources for Ti, Zn, and O, respectively. The Ti doping was then achieved by growing ZnO and TiO₂ alternately. A hampered growth mode of ZnO on TiO₂ layer was confirmed by comparing the thicknesses measured by spectroscopic ellipsometry with the expected. It was also found that the locations of the (100) diffraction peaks shift towards lower diffraction angles as Ti concentration increased. For all samples, optical transmittance over 80% was obtained in the visible region. The sample with ALD cycle ratio of ZnO/TiO₂ being 20 had the lowest resistivity of 8.874 × 10⁻⁴ Ω cm. In addition, carrier concentration of the prepared films underwent an evident increase and then decreased with the increase of Ti doping concentration.     

Optical and microstructural properties of ZnO/TiO2 nanolaminates prepared by atomic layer deposition

ZnO/TiO₂ nanolaminates were grown on Si (100) and quartz substrates by atomic layer deposition at 200°C using diethylzinc, titanium isopropoxide, and deionized water as precursors. All prepared multilayers are nominally 50 nm thick with a varying number of alternating TiO₂ and ZnO layers. Sample thickness and ellipsometric spectra were measured using a spectroscopic ellipsometer, and the parameters determined by computer simulation matched with the experimental results well. The effect of nanolaminate structure on the optical transmittance is investigated using an ultraviolet–visible-near-infrared spectrometer. The data from X-ray diffraction spectra suggest that layer growth appears to be substrate sensitive and film thickness also has an influence on the crystallization of films. High-resolution transmission electron microscopy images show clear lattice spacing of ZnO in nanolaminates, indicating that ZnO layers are polycrystalline with preferred (002) orientation while TiO₂ layers are amorphous.     

Electrodeposition and characterization of Bi2Se3 thin films by electrochemical atomic layer epitaxy (ECALE)

Bismuth selenide thin films were grown on Pt substrate via the route of electrochemical atomic layer epitaxy (ECALE) in this work for the first time. The electrochemical behaviors of Bi and Se on bare Pt, Se on Bi-covered Pt, and Bi on Se-covered Pt were studied by cyclic voltammetry and coulometry. A steady deposition of Bi₂Se₃ could be attained after negatively stepped adjusting of underpotential deposition (UPD) potentials of Bi and Se on Pt in the first 40 deposition cycles. X-ray diffraction (XRD) analysis indicated that the films were single phase Bi₂Se₃ compound with orthorhombic structure, and the 2:3 stoichiometric ratio of Bi to Se was verified by EDX quantitative analysis. The optical band gap of the as-deposited Bi₂Se₃ film was determined as 0.35 eV by Fourier transform infrared spectroscopy (FTIR), which is consistent with that of bulk Bi₂Se₃ compound.     

Low temperature atomic layer deposition of nickel sulfide and nickel oxide thin films using Ni(dmamb)2 as Ni precursor

Nickel(II) 1-dimethylamino-2-methyl-2-butoxide (Ni(dmamb)₂) with water and hydrogen sulfide as oxygen and sulfur sources was employed in atomic layer deposition (ALD) of nickel oxide (NiO) and nickel sulfide (NiS) thin films. Both NiO and NiS thin films demonstrate temperature-independent growth rates per cycle of 0.128?nm/cycle and 0.0765?nm/cycle, at 130–150?°C and 80–160?°C, respectively. Comparison of two nickel-based thin film materials demonstrates dissimilar deposition features depending on the reactivity of the Ni precursor, i.e., Ni(dmamb)₂ with anion sources provided by the water and hydrogen sulfide reactants. Difference in reactivity observed for NiO and NiS ALD processes is further investigated by density functional theory (DFT) simulations of surface reactions, which indicated that H₂S demonstrate higher reactivity with surface-adsorbed Ni precursor than H₂O. The material properties of ALD NiO and NiS thin films including stoichiometry, crystallinity, band structure, and electronic properties were analyzed by multiple experimental techniques, showing potential of ALD NiS as electrode or catalyst for energy-oriented devices.     

Optimization of temperature coefficient of resistance of Al-doped vanadium oxide thin film prepared by atomic layer deposition for uncooled microbolometer

Vanadium oxide (VO_X) is an excellent thermal sensitive candidate for uncooled microbolometers. However, undoped VO_X prepared by atomic layer deposition (ALD) has a temperature coefficient of resistance (TCR) of ca. ?2 ～ ?3%/K. For improving its TCR, our deposition approach based on the combination of ALD and rapid post-deposition annealing (RTA) is proposed. Besides, aluminum-doping into the VO_X films is performed via that approach, and the number of Al₂O₃ cycles is adjusted for varying the dopant loadings. Changes in physical, chemical, and electrical characteristics of the VO_X films due to Al-doping are discussed in detail. The advantage of introducing Al³⁺ dopants is to hinder the thermally activated phase transition of VO₂ phases, leading to an improvement in TCR of Al-doped VO_X up to ?4.2%/K, which remains stable over a wide temperature range of 298–328 K. However, the excessive Al doping also carries an adverse effect on TCR. The reason for that is discussed for further understandings of doping effects.     

Template-directed synthesis of porous alumina particles with precise wall thickness control via atomic layer deposition

Highly porous alumina particles with precise wall thickness control were synthesized by atomic layer deposition (ALD) of alumina on highly porous poly(styrene-divinylbenzene) (PS-DVB) particle templates. Alumina ALD was carried out using alternating reactions of trimethylaluminum and water at 33 °C. The growth rate of alumina was ∼0.3 nm per coating cycle. The wall thickness can be precisely controlled by adjusting the number of ALD coating cycles. Thermo-gravimetric analysis, X-ray diffraction, nitrogen adsorption, scanning electron microscopy, and transmission electron microscopy were used to characterize the fabricated porous alumina particles. The effect of number of ALD coating cycles and calcination temperature on the mesoporous structure of the alumina particles was investigated. γ-Alumina was formed at temperature above 600 °C. Porous alumina particles with a surface area of 80-100 m²/g were obtained and thermally stable at 800 °C. The pore volume of the porous particles can be as high as 1 cm³/g after calcination at 800 °C. Such porous alumina particles may find wide application in nanotechnology and catalysis.     

Optimization of CdTe nanofilm formation by electrochemical atomic layer epitaxy (EC-ALE)

This paper concerns optimization studies of the growth of cadmium telluride, an important II–VI compound semiconductor, using electrochemical atomic layer epitaxy (EC-ALE). The importance of the potentials used to deposit atomic layers of Cd and Te, as well as the potential used to strip excess Te, were investigated. These potentials were used in a cycle, an EC-ALE cycle, to form deposits one atomic layer at a time, using a sequence of surface limited reactions. The optimal potentials for the CdTe EC-ALE cycle included Cd deposition at − 0.65 V, Te deposition at − 0.35 V and bulk Te stripping at − 0.70 V. The deposits obtained were stoichiometric, with a Te/Cd atomic ratio of 1.01 from electron probe microanalysis (EPMA). Electrochemical quartz crystal microbalance (EQCM) studies of the optimal condition indicated that about a third of the deposited Cd was oxidatively stripped at the potential used to deposit Te. Glancing angle X-ray diffraction studies showed a (111) preferred orientation for the deposit, while room temperature near infrared absorption measurements indicated a direct band gap of 1.5 eV.     

Supported iridium catalysts prepared by atomic layer deposition: effect of reduction and calcination on activity in toluene hydrogenation

Ligand removal from supported iridium catalysts prepared by atomic layer deposition from Ir(acac)₃ was studied by direct reduction in hydrogen flow and by calcination in oxygen flow followed by reduction in hydrogen flow, as well as by thermogravimetric analysis. Thermal decomposition of acac ligand residuals required high temperatures, and in samples containing no iridium the removal of carbonaceous species was not complete. Metallic iridium particles less than 2 nm in size were formed during direct reduction and larger particles upon calcination followed by reduction. The activity of the catalysts in toluene hydrogenation in most cases depended on particle size.     

PbTe微晶的水热法合成及表征

文章以Te粉和Pb(NO3)2为原料,在NaOH和KBH4的作用下通过水热法制备PbTe微晶,并用X射线衍射(XRD)、扫描电镜(SEM)以及光电子能谱(XPS)对所制备的PbTe样品进行了表征。以本方法合成的PbTe在碱性溶液中结晶良好。此外还讨论了PbTe微晶的形成机理。     

Development of growth cycle for antimony telluride film on Au (1 1 1) disk by electrochemical atomic layer epitaxy

Electrochemical deposition of Sb₂Te₃ thin film on Au (1 1 1) disk via the route of electrochemical atomic layer epitaxy (ECALE) is described in this paper. Electrochemical aspects of Te and Sb on Au, Te on Sb-covered Au, and Sb on Te-covered Au were studied by means of cyclic voltammetry and coulometry. The apparent variation of coverage for Te or Sb on hetero-covered substrate is explained by considering the thermodynamic process of compound formation. A steady ECALE deposition for Sb₂Te₃ compound could be attained after negatively adjusting the underpotential deposition (UPD) potentials of Sb and Te on Au in steps over the initial 40 cycles, and the potentials could be kept constant for the following deposition. A 200-cycle deposit, which was grown with the steady deposition potentials, was proved to be a single phase Sb₂Te₃ compound by X-ray diffraction analysis. The 2:3 stoichiometric ratio of the deposit was further verified by energy dispersive X-ray (EDX) quantitative analysis. The p-type semiconductive property was demonstrated by measurements of the Seebeck coefficient and the electrical resistivity with a value of 145 μV/K and 9.37 μΩm, respectively. The morphologies of deposits with various growth cycle numbers were observed with FE-SEM. The evolvement mechanism of the morphology was investigated. The results show that the morphology of deposit has changed after initial potential adjustment and numberless thin sheets appeared and grew uprightly during the continuous cycle process. Fourier transform infrared spectroscopy (FTIR) absorption measurements suggested a band gap of 0.26 eV in very good agreement with literature reports for Sb₂Te₃ single crystals.     

Enhancing the thermoelectric properties of super-lattice Al2O3/ZnO atomic film via interface confinement

Aluminum oxide (Al₂O₃)/zinc oxide (ZnO) thin films deposited via atomic layer deposition (ALD) are demonstrated to enhance their thermoelectric properties by manipulating them with a nano-thick Al₂O₃ interface. The overall superlattice structure is tuned by varying the ZnO ALD sequence and the Al₂O₃ ALD sequence while maintaining the same composition. An aluminum-doped zinc oxide (AZO) thin film is deposited at 250 °C, and the Al₂O₃ thickness in the superlattice is gradually increased from 0.13 nm to 1.23 nm. The total film composition is fixed at 2% AZO. We observe that an efficient superlattice structure is made with a specific Al₂O₃ thickness. The thermal conductivity is significantly decreased from 0.57 W/mK to 0.26 W/mK as the thickness of the Al₂O₃ layer is increased. Additionally, the absolute Seebeck coefficient is increased from 14 μV/K to 65 μV/K. This may be caused by the interface confinement effect and interface scattering between the ZnO layer and the Al₂O₃ layer. The figure of merit ZT value is 0.14 for the most efficient structure.     

Photocurrent detection of chemically tuned hierarchical ZnO nanostructures grown on seed layers formed by atomic layer deposition

We demonstrate the morphological control method of ZnO nanostructures by atomic layer deposition (ALD) on an Al₂O₃/ZnO seed layer surface and the application of a hierarchical ZnO nanostructure for a photodetector. Two layers of ZnO and Al₂O₃ prepared using ALD with different pH values in solution coexisted on the alloy film surface, leading to deactivation of the surface hydroxyl groups. This surface complex decreased the ZnO nucleation on the seed layer surface, and thereby effectively screened the inherent surface polarity of ZnO. As a result, a 2-D zinc hydroxyl compound nanosheet was produced. With increasing ALD cycles of ZnO in the seed layer, the nanostructure morphology changes from 2-D nanosheet to 1-D nanorod due to the recovery of the natural crystallinity and polarity of ZnO. The thin ALD ZnO seed layer conformally covers the complex nanosheet structure to produce a nanorod, then a 3-D, hierarchical ZnO nanostructure was synthesized using a combined hydrothermal and ALD method. During the deposition of the ALD ZnO seed layer, the zinc hydroxyl compound nanosheets underwent a self-annealing process at 150 °C, resulting in structural transformation to pure ZnO 3-D nanosheets without collapse of the intrinsic morphology. The investigation on band electronic properties of ZnO 2-D nanosheet and 3-D hierarchical structure revealed noticeable variations depending on the richness of Zn-OH in each morphology. The improved visible and ultraviolet photocurrent characteristics of a photodetector with the active region using 3-D hierarchical structure against those of 2-D nanosheet structure were achieved.     

Preparation of PbS thin films: A new electrochemical route for underpotential deposition

Nanostructured thin films of lead sulfide have been synthesized by a new electrochemical approach based on the underpotential deposition (UPD) of Pb and S from the saturated solution of PbS containing excess of PbS particles as a source of Pb²⁺ and S²⁻ at various temperatures.We have demonstrated that this new electrochemical route is a simple method with several advantages, including better control of the growth conditions and a one-step process to obtain the nanostructures of PbS. Scanning probe microscopy studies indicate that the growth of PbS nanofilms follows a two-dimensional layer-by-layer growth kinetics at the beginning of electrodeposition but a three-dimensional growth dominates after the formation of the first few layers. The results of morphological and structural investigations reveal that PbS nanostructures grown by this method are single-crystalline in cubic structure and have a preferential orientation along the [2 0 0] direction. The optical absorption spectra of PbS nanostructures show the blue-shift with respect to those of the bulk counterpart, which are attributed as quantum-size effect.     

Failure mechanism of thin Al2O3 coatings grown by atomic layer deposition for corrosion protection of carbon steel

Combined analysis by electrochemical impedance spectroscopy (EIS), time-of-flight secondary ion mass spectrometry (ToF-SIMS) and field emission scanning electron microscopy (FESEM) of the corrosion protection provided to carbon steel by thin (50 nm) Al₂O₃ coatings grown by atomic layer deposition (ALD) and its failure mechanism is reported. In spite of excellent sealing properties, the results show an average dissolution rate of the alumina coating of ∼7 nm h⁻¹ in neutral 0.2 M NaCl and increasing porosity of the remaining layers with increasing immersion time. Alumina dissolution is triggered by the penetration of the solution via cracks/pinholes through the coating to the substrate surface where oxygen reduction takes place, raising the pH. At defective substrate surface sites of high aspect ratio and concentrated residual mechanical stress (along scratches) presumably exposing a higher steel surface fraction, localized dissolution of the coating is promoted by a more facile access of the solution to the substrate surface enhancing oxygen reduction. De-adhesion of the coating is also promoted in these sites by the ingress of the anodic dissolution trenching the steel surface. Localized corrosion of the alloy (i.e. pitting) is triggered prior to complete dissolution of the alumina film on the elsewhere still coated surface matrix.