Tansel Karabacak

Physical self-assembly and the nucleation of three-dimensional nanostructures by oblique angle deposition

Growth front morphology of a thin film formed by physical vapor deposition is controlled by many factors including surface diffusion and shadowing effects. Instabilities can occur if shadowing is more dominant compared to other surface effects and can lead to many diverse physically self-assembled three-dimensional nano-size structures. In this article, we explore the fundamental nucleation and growth mechanisms of the nanostructures during oblique angle deposition. Monte Carlo simulations were carried out to predict the island density, island size distribution, and island–island correlation during the initial stages of growth. The results were compared to that obtained by the oblique angle sputter deposited tungsten films imaged by atomic force microscopy and scanning electron microscopy. Isolated islands with quasiperiodic distribution were formed as a natural consequence of the shadowing effect. Isolated columnar structures are shown to grow on these islands and the width W of the columns is predicted ...

β-phase tungsten nanorod formation by oblique-angle sputter deposition

We report the creation of an unusual simple cubic β-phase W(100) nanorods with a pyramidal tip having four (110) facets using an oblique-angle sputter deposition technique with substrate rotation (also known as glancing-angle deposition). During the oblique-angle deposition, both β-phase W(100) and α-phase W(110) islands exist at the initial stages of growth. The β-phase W(100) islands grow taller due to the lower adatom mobility on these islands. The taller islands survive in the competition and form isolated nanorods in the later stages of growth. This is in contrast to the sputter deposition at normal incidence, where only the thermodynamically stable bcc α-phase W(110) polycrystalline films were formed when the film grows to a certain thickness.

Uniform Si nanostructures grown by oblique angle deposition with substrate swing rotation

Slanted nano-columns and square nano-springs made of amorphous silicon (a-Si) were fabricated on bare Si and patterned substrates by oblique angle deposition with a back–forth substrate swing rotation mode. Scanning electron microscopy was used to characterize the grown nanostructures. The tilt angle of slanted nano-rods is determined by the incident angle of deposition flux and the azimuthal swing rotation angle of a substrate. The controlled substrate rotation affects the uniformity and the shape of the nanostructures. On the patterned substrate, the broadening of the size of individual nano-columns is greatly reduced and the nano-columns are not connected as they grow. A simple model based on decomposing the deposition flux is used to describe the effect of substrate rotation on tilt angle, uniformity, and the top-end shape of nanostructures. The feasibility of fabricating separated and well aligned nanostructures by our swing rotation method provides an effective and controllable way to fabricate nano-devices.

Low temperature melting of copper nanorod arrays

We report the melting of nanorod arrays of copper at temperatures much lower than its bulk melting point (1083°C). The Cu nanorods were produced by an oblique angle sputter deposition technique through a physical self-assembly mechanism due to the shadowing effect. The as-deposited nanorods were ∼2300nm in length, ∼100nm in diameter, and separated from each other with gaps varying between ∼10 and ∼30nm. The melting process was investigated through the analysis of scanning electron microscopy, transmission electron microscopy, and x-ray diffraction measurements. Start of premelting (or surface melting) has been observed to occur at annealing temperature ∼400°C under vacuum (10−6Torr). As the temperature was raised the arrays of Cu nanorods started to coalesce and formed a dense continuous film with a (111) texture at ∼550°C. The results of this work may be useful for low temperature soldering applications.

Quasi-periodic nanostructures grown by oblique angle deposition

We report that tungsten nanocolumns grown by oblique angle sputter deposition develop a quasi-periodic morphology which is not observed for continuous films deposited at normal incidence. The maximum position in power spectral density of the quasi-periodic nanostructures decreases exponentially as a function of thickness. We explain the formation of the quasi-periodic nature by a “shadowing length” concept which plays a similar role to conventional surface diffusion length. Also, we show that the change of the spatial frequency of the periodicity is a result of the elimination of shorter columns due to the shadowing effect during growth.

Stress reduction in tungsten films using nanostructured compliant layers

The residual stress in thin films is a major limiting factor for obtaining high quality films. We present a strategy for stress reduction in sputter deposited films by using a nanostructured compliant layer obtained by the oblique angle deposition technique, sandwiched between the film and the substrate. The technique is all in situ, does not require any lithography steps, and the nanostructured layer is made from the same material as the deposited thin film. By using this approach we were able to reduce stress values by approximately one order of magnitude in sputter deposited tungsten films. These lower stress thin films also exhibit stronger adhesion to the substrate, which retards delamination buckling. This technique allows the growth of much thicker films and has enhanced structural stability. A model is developed to explain the stress relief mechanism and the stronger adhesion associated with the presence of the nanostructured compliant layer.

Thin-film growth dynamics with shadowing and re-emission effects

Growth dynamics of thin-films involves both shadowing and re-emission effects. Shadowing can originate from obliquely incident atoms being preferentially deposited on hills of the surface, which leads to a long range geometrical effect, as well as from an atomic shadowing process that can occur even during normal angle deposition. Re-emission effect is a result of nonsticking atoms, which can bounce off from hills and deposit on valleys of the surface. In the case of an energetic incident flux, re-emission can also originate from a resputtering process that includes a surface atom being knocked off by an incident ion/atom followed by redeposition to another surface point. Due to their long-range nonlocal nature, both the shadowing effect (which tries to roughen the surface) and re-emission effect (which has a smoothening effect) have been shown to be more dominant over local effects such as surface diffusion, and have been proven to be critical processes in accurately determining the dynamic evolution of surface roughness. Recent Monte Carlo simulation methods that involve shadowing, re-emission, surface diffusion, and noise effects successfully predicted many experimentally relevant surface roughness evolution results reported in the literature. For example, root-mean-square surface roughness (ω) of Monte Carlo simulated thin-films have evolved with time t according to a power law behavior ω ∼ t β , with β values ranging from about 0 to 1 for a growth with strong re-emission effects (i.e., low sticking coefficients) and a growth with dominant shadowing effects (i.e., with high sticking coefficients), respectively. Potential future thin-film growth modeling studies are also discussed. These include advanced simulation approaches that can incorporate atomistic details of physical and chemical processes and a recently developed network growth model that can potentially capture some universal aspects of thin-film growth dynamics independent of the details of growth process. C 2011

Glancing angle sputter deposited nanostructures on rotating substrates: Experiments and simulations

Ordered arrays of Si nanorods and nanospirals have been produced by ion beam sputter glancing angle deposition of Si on rotating substrates. The substrates were prepatterned with honeycomb and hexagonal-closed-packed arranged Au dots obtained by nanosphere lithography. The effects of template type, substrate rotational speed, height of the artificial Au seeds, and deposition angle θ of the incident flux on the growth of the Si nanostructures is examined. Especially for the deposition of Si on honeycomb templates at different deposition angles, it is shown that the structure of the growing film changes drastically. A continuous film with honeycomblike arranged hillocks on top is deposited at normal incidence. With increased θ, the structure shifts to almost dense films with a mesh of hexagonally arranged pores (θ=70°). Finally, separated rodlike structures with triangular cross section are obtained under glancing angle conditions (θ=85°). In addition, the structural evolution of the glancing angle deposite...

High optical absorption of indium sulfide nanorod arrays formed by glancing angle deposition.

Indium(III) sulfide has recently attracted much attention due to its potential in optical sensors as a photoconducting material and in photovoltaic applications as a wide band gap material. On the other hand, optical absorption properties are key parameters in developing photosensitive photodetectors and efficient solar cells. In this work, we show that indium sulfide nanorod arrays produced by the glancing angle deposition technique have superior absorption and low reflectance properties compared to conventional flat thin film counterparts. We observed an optical absorption value of approximately 96% for nanorods at wavelengths <500 nm in contrast to 79% for conventional thin films of indium sulfide. A superior photoconductivity (PC) response as high as about 40% (change in resistance upon illumination) was also observed in nanorod samples. This is mainly believed to be due to their high optical absorption, whereas only less than 1% PC change was detected in conventional thin films. We give a preliminary description of the enhanced light absorption properties of the nanorods by using the Shirley-George model, which predicts diffusion of light as a function of the roughness of the surface.

Oxygen Reduction Reaction Electrocatalytic Activity of Glancing Angle Deposited Platinum Nanorod Arrays

The electrocatalytic oxygen reduction reaction (ORR) activity of vertically-aligned Pt nanorods has been evaluated utilizing cyclic voltammetry (CV) and rotating-disk electrode (RDE) techniques in a 0.1 M HClO4 solution at temperatures ranging from 20 to 60 � C. A glancing angle deposition (GLAD) technique was used to fabricate Pt nanorod arrays on glassy carbon (GC) electrodes. GLAD catalyst nanorods, without any carbon support, have been produced at different lengths varying between 50 and 400 nm, corresponding to 0.04–0.32 mg/cm 2 Pt loadings, with diameter and spacing values ranging from about 5 up to 100 nm. The scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) results reveal that Pt nanorods are well-isolated, vertically aligned, and single-crystal. Crystal orientation analysis demonstrates that large surface area Pt nanorod sidewalls are mainly dominated by Pt(110) planes, which is known to be the most active crystal plane of Pt for the ORR. Compared to a commercial high-surface-area-supported Pt (Pt/C) catalyst, the CV results show that the Pt-nanorod electrocatalyst exhibits a more positive oxide reduction peak potential, indicating that GLAD Pt nanorods are less oxophilic. Moreover, the nanorods exhibit enhanced stability against loss of electrochemically-active surface area as a result of potential cycling in acidic electrolyte as compared to the Pt/C catalyst. Specific ORR activities determined by the RDE technique for GLAD Pt nanorods of different lengths are analyzed and compared to literature values for polycrystalline Pt, nano-structured thin film Pt (3M NSTF Pt), and to those measured for Pt/C. RDE results reveal that Pt-nanorod electrocatalysts exhibit higher area-specific activity, higher electron-transfer rate constant, and comparable activation energy for ORR than those of Pt/C due to their larger crystallite size, single-crystal property, and dominance of the preferred crystal orientations for ORR. However, Pt nanorods show lower mass specific activity than that of Pt/C electrocatalyst due to the large diameter of nanorods. V C 2011 The Electrochemical Society. [DOI: 10.1149/1.3599901] All rights reserved.