Microstructure-Dependent Nucleation in Atomic Layer Deposition of Pt on TiO₂

The effects of TiO₂ microstructure on Pt nucleation and on formation of continuous ultrathin Pt films by atomic layer deposition (ALD) were investigated. Pt was deposited by a metalorganic Pt precursor ((methylcyclopentadienyl)­trimethylplatinum) and air as a counter reactant on in situ grown ALD TiO₂ surfaces. For the same number of Pt ALD cycles, the Pt surface coverage was found to depend on the thickness of the underlying TiO₂. From X-ray diffraction (XRD) analysis, it was found that the amorphous microstructure of as-deposited TiO₂ transforms into anatase microstructure because of an annealing effect at the elevated Pt ALD temperature, and that this effect is a function of TiO₂ thickness. Transmission electron microscopy revealed that continuous growth of ALD Pt occurs on anatase TiO₂ whereas island growth occurs on amorphous TiO₂. These results indicate that Pt nucleation is significantly affected by the microstructure of TiO₂. Effects beyond surface hydrophilicity, such as a catalytic effect, are needed to explain the different nucleation properties of ALD Pt on TiO₂. These results provide insight into initial growth during metal ALD and the effects of surface structural properties on ALD nucleation

Formation of Continuous Pt Films on the Graphite Surface by Atomic Layer Deposition with Reactive O₃

Because graphite surfaces are chemically stable, it is difficult to form a uniform layer on graphite by atomic layer deposition (ALD), which is a surface reaction-based deposition method. In this work, reactive O₃ is employed for Pt ALD as a counter reactant, and a continuous Pt film is achieved on the graphite surface. The growth morphology of the O₃-based Pt ALD process differs significantly from that using an O₂ reactant, in which selective growth occurs on step edges of graphite. Pretreatment of the graphite with O₃ prior to Pt ALD using an O₂ reactant shows a continuous Pt film morphology similar to that obtained from the full O₃-based ALD process. The analysis indicates that O₃ etches the graphite surface and generates pits containing additional step edges, resulting in an increase in the extent of Pt nucleation. The nucleation of Pt is less active at lower deposition temperatures because the generation of additional step edges is dependent on temperature. This Pt ALD process using a reactive O₃ reactant can be an effective route for fabricating a uniform and continuous Pt catalyst on three-dimensional carbon electrodes for highly efficient fuel cells

Effect of O₃ on Growth of Pt by Atomic Layer Deposition

The growth characteristics of Pt deposited by atomic layer deposition (ALD) with methylcyclopentadienyltrimethylplatinum (MeCpPtMe₃) and O₃ are studied both experimentally and by modeling. The growth rate of Pt ALD using O₃ is higher than that using either air or O₂ counter reactants. In addition, a low deposition temperature of 150 °C for the deposition of metallic Pt using O₃ is obtained. To investigate the role of O₃ during initial growth, Pt is deposited on O₃-pretreated SiO₂ using air as the counter reactant. Pt deposited in this way on O₃-pretreated SiO₂ shows a rapid increase of surface coverage, which is similar to Pt ALD using O₃ and different from Pt ALD using air on untreated SiO₂. From the modeling study, it is found that pretreating the surface with O₃ increases the steady state nucleation rate and decreases the nucleation incubation time on the SiO₂ surface, the same phenomena which are believed to occur during the initial growth of ALD Pt using O₃ counter reactant

Internal and External Atomic Steps in Graphite Exhibit Dramatically Different Physical and Chemical Properties

We report on the physical and chemical properties of atomic steps on the surface of highly oriented pyrolytic graphite (HOPG) investigated using atomic force microscopy. Two types of step edges are identified: internal (formed during crystal growth) and external (formed by mechanical cleavage of bulk HOPG). The external steps exhibit higher friction than the internal steps due to the broken bonds of the exposed edge C atoms, while carbon atoms in the internal steps are not exposed. The reactivity of the atomic steps is manifested in a variety of ways, including the preferential attachment of Pt nanoparticles deposited on HOPG when using atomic layer deposition and KOH clusters formed during drop casting from aqueous solutions. These phenomena imply that only external atomic steps can be used for selective electrodeposition for nanoscale electronic devices

Organosulfide Inhibitor Instigated Passivation of Multiple Substrates for Area-Selective Atomic Layer Deposition of HfO₂

With recent advancements in semiconductor technology, continuous efforts are being made to meet the requirements for further reductions in the feature sizes of electronic interconnects in semiconductor devices. Efforts to improve area-selective deposition (ASD) processes have led to researchers manipulating deposition surfaces using surface inhibitors as tools for area-selective atomic layer deposition (AS-ALD). In this study, organosulfide small-molecule inhibitors (SMIs) were utilized for AS-ALD on metal, oxide, and nitride surfaces such as Cu, SiO2, and TiN, respectively. Upon high-temperature exposure, the organosulfide SMI decomposes to assist the adsorption of its fragmentation products on the Cu and SiO2 substrates, thereby simultaneously adsorbing and passivating the two surfaces upon SMI exposure. The surface chemistry and reactivity were explained by calculations using density functional theory with the slab approach and Monte Carlo simulations. Furthermore, the blocking potential of the SMIs was evaluated using atomic layer deposition (ALD) of HfO2. The SMI-covered Cu substrate showed inhibition against ALD growth of HfO2 with a selectivity of approximately 98% over 25 growth cycles compared to the uncovered Cu substrate successfully blocking approximately 3 nm of HfO2 ALD. The SMI-covered SiO2 substrate showed a lowered selectivity compared to the SMI-covered Cu substrate but still, a substantial selectivity was present compared to bare SiO2 and TiN substrates where no blocking was observed. These results agree with the theoretical findings. This possibility to block two important surfaces in semiconductor manufacturing (Cu and SiO2) while leaving a third one (TiN) unblocked for ALD growth is an important step for the future application of ASD in the production of ever smaller semiconductor devices

Growth of Pt Nanowires by Atomic Layer Deposition on Highly Ordered Pyrolytic Graphite

The formation of Pt nanowires (NWs) by atomic layer deposition on highly ordered pyrolytic graphite (HOPG) is investigated. Pt is deposited only at the step edges of HOPG and not on the basal planes, leading to the formation of laterally aligned Pt NWs. A growth model involving a morphological transition from 0-D to 1-D structures via coalescence is presented. The width of the NWs grows at a rate greater than twice the vertical growth rate. This asymmetry is ascribed to the wetting properties of Pt on HOPG as influenced by the formation of graphene oxide. A difference in Pt growth kinetics based on crystallographic orientation may also contribute

Nucleation-Controlled Growth of Nanoparticles by Atomic Layer Deposition

We demonstrate a method for growing metal nanoparticles (NPs) by atomic layer deposition (ALD) with the ability to vary aerial density and NP size using nucleation control. Self-assembled monolayers (SAMs) preadsorbed on the substrate serve as a template for subsequent growth of the NPs by ALD. Defects in the SAM resulting from incomplete formation time in solution are shown to act as nucleation sites for Pt. The strategy is demonstrated experimentally using ALD of Pt from a metal organic Pt precursor and O₂ counter reactant on silicon dioxide surfaces pretreated with octadecyltrichlorosilane SAMs. The aerial density and mean diameter of the Pt NPs are controlled by changing the SAM dip time and the number of ALD cycles. An isothermal nucleation model was developed in which several nucleation behaviors were considered in comparison with experimental data. A model incorporating nucleation incubation provided the best fit to the data

Effects of Cl-Based Ligand Structures on Atomic Layer Deposited HfO₂

Atomic layer deposition (ALD) of HfO₂ is a key technology for the application of high dielectric constant gate dielectrics ranging from conventional Si devices to novel nanodevices. The effects of the precursor on the growth characteristics and film properties of ALD HfO₂ were investigated by using hafnium tetrachloride (HfCl₄) and bis­(ethylcyclopentadienyl)hafnium dichloride (Hf­(EtCp)₂Cl_2, Hf­(C₂H₅C₅H₄)₂Cl₂) with O₂ plasma reactant. The growth characteristics were significantly affected even by simply changing the precursor. Theoretical calculations utilizing geometrical information on the precursor and density functional theory revealed that the steric demands of the precursor ligands have a dominant effect on the different growth characteristics rather than the reaction probability of the precursor on the surface. The chemical compositional analysis results showed that the Cl residue in the HfO₂ films was reduced by using Hf­(EtCp)₂Cl₂ due to the lower number of Cl atoms in each Hf precursor molecule and the relieved bridge formation of Hf–Cl–Hf bridge on the surface compared to HfCl₄. The electrical property measurement results showed significantly improved insulating properties in HfO₂ using Hf­(EtCp)₂Cl₂ compared to HfCl₄ due to the low concentration of Cl residue in the film. These results provide broad insights to researchers who are interested in the fabrication of high quality dielectric layers to achieve better device performance and overcome physical limitations in the nanoscale regime

Reversible Liquid Adhesion Switching of Superamphiphobic Pd-Decorated Ag Dendrites via Gas-Induced Structural Changes

Adhesion control of various liquid droplets on a liquid-repellent surface is a fundamental technique in novel open channel microfluidic systems. Herein, we demonstrate reversible liquid droplet adhesion switching on superamphiphobic Pd-decorated Ag dendrites (Pd/Ag dendrites). Although adhesion between liquids and the superamphiphobic surfaces was extremely low under ambient air, high adhesion was instantly achieved by exposure of the dendrites to 8% hydrogen gas. Transition from low to high adhesion and the reverse case were successfully repeated more than 10 times by switching from atmospheric ambient air to 8% hydrogen gas. This is the first technique that allows real-time reversible adhesion change with various liquid droplets to a surface using gas-induced structural changes and can potentially be used to realize various functions for droplet-based microfluidics

Self-Assembly Based Plasmonic Arrays Tuned by Atomic Layer Deposition for Extreme Visible Light Absorption

Achieving complete absorption of visible light with a minimal amount of material is highly desirable for many applications, including solar energy conversion to fuel and electricity, where benefits in conversion efficiency and economy can be obtained. On a fundamental level, it is of great interest to explore whether the ultimate limits in light absorption per unit volume can be achieved by capitalizing on the advances in metamaterial science and nanosynthesis. Here, we combine block copolymer lithography and atomic layer deposition to tune the effective optical properties of a plasmonic array at the atomic scale. Critical coupling to the resulting nanocomposite layer is accomplished through guidance by a simple analytical model and measurements by spectroscopic ellipsometry. Thereby, a maximized absorption of light exceeding 99% is accomplished, of which up to about 93% occurs in a volume-equivalent thickness of gold of only 1.6 nm. This corresponds to a record effective absorption coefficient of 1.7 × 10⁷ cm^–1 in the visible region, far exceeding those of solid metals, graphene, dye monolayers, and thin film solar cell materials. It is more than a factor of 2 higher than that previously obtained using a critically coupled dye J-aggregate, with a peak width exceeding the latter by 1 order of magnitude. These results thereby substantially push the limits for light harvesting in ultrathin, nanoengineered systems