Physical methods for the synthesis of low-dimensional nanomaterials & Tuning properties of PLD Magnetic Oxide Thin Films NGUYEN Hoa Hong Which materials should be classified as Nano (Low dimensional)? Normal: 3D (bulk) Below 3D (i.e. 2D; 1D; OD): low-dimensional • Definition: Nanomaterials: • At least size in 1D should be less than 100 nm (now ideally 10 nm) 2D: in 1 dimension, size <100 nm (ultra-thin films) 1D: in 2 dimensions, size <100 nm (nano-fiber: tubes or rods) OD: in all 3 dimensions, size <100 nm (nanoparticles-dots) Why are they important and interesting? Matters in nanosize behave differently Therefore, nanomaterials may have interesting properties that indeed do not exist in bulks of the same compounds Usually can be used widely in devices & modern applications including environment, energy & healthcare How could they own remarkable properties? • Size, shape, specific surface area, aspect ratio • Size distribution • Surface morphology/topography • Structure, including crystallinity & defects • Solubility Quantum confinement: can be observed once the diameter of a material is of the same magnitude as the de Broglie wavelength of the electron wave function. When materials are this small, their electronic & optical properties become different from those of bulk materials Principles of synthesis The goal of any method intended for the synthesis of nanomaterials is to obtain a material that exhibits properties that originate from their characteristic length scale being in the nanometer range (1-100 nm). Accordingly, the synthetic method should exhibit control of size in this range. Physical methods of preparation often include top-down & bottom-up approaches. \vMiiviih from Atomv Molrculc\ TOP-DOWN BOTTOM-IT Synthesis Of Nanomaterials Synthesis r l'h>*iral r Mi-chinirol I I i lii'ltikjl Colled?, sol-gel, L-B film-*, [morse micelles 1 Yipaur I T 1 IMhi IM--I.: 1 I Using. Noinemhfan«, hl&lriKfKmicdJ. DNA, enzymes und Chemical ^r**«- intern or^aniuru Jcp^Jtion. Piiniklc arresting m glass or Ar1>lilL-> ur [••nls liters, Micnxinukiou- A.'tslllL- lljfjlt rtvfjis hall L ii-Hf r nN.'ittnn. Snulter Pulsed Laser Deposition (PLD) method to fabricate thin films (mostly functional oxides) Laser is an electronic-optical device that produces coherent radiation. The term is acronym for Light Amplification by Stimulated Emission of Radiation. Pulsed Laser Deposition (PLD) is a physical vapor deposition technique where a high-power pulsed laser beam is focused to shoot a target of the desired composition. Material is then vaporized & deposited as a thin film on a heated substrate facing the target. This process can occur in ultra high vacuum or in the presence of a background gas, such as 02 when depositing films of oxides. One can alter the properties of functional materials by controlling the external parameters such as temperature (T) , electric & magnetic fields (E & H), pressure (P02) etc.! PLD stages: * laser ablation of target * dynamic of plasma * film nucleation and growth Concept of PLD I.!--:' ::-.ri !.!•;:■■ vacuum gauge sutstratc holder rotating toiget Advantage: -film's stoichiometry is near to that of the target. -versatile: many materials can be deposited in a wide variety of gases over a broad range of gas pressures. -cost-effective: one laser source can serve many chambers, -fast: high quality films can be grown in 10-15 minutes. Important: Type of Eximer Laser; Beam alignement; Energy density; Pulse repetition rate;Time of shooting; Vacuum-02 Pressure; Type of substrate; Substrate temperature; Distance between target & substrate Growth of a film Laser beam Target Cairo use Rotating target -W -20 -Id 0 10 » » Fabrication Conditions (T, P, substrate, etc) play key roles in shaping up desired properties! Laser ablation for synthesis of carbon nanotubes In laser ablation, a pulsed laser vaporizes a graphite target in a high-temperature reactor under the stream of an inert gas inside the chamber. Nanotubes develop on the cooler surfaces of the reactor as the vaporized carbon condenses. A water-cooled surface may be included in the system to collect the nanotubes. Diagram+Result Fabrication Conditions play key roles in shaping up desired properties! Laser ablation in liquid for the synthesis of nanoparticles Laser ablation usually generates the nanoparticles from target placed inside the liquid. Concentration of the ablated material (CM) decreases, while the concentration of solution species (Cs) increases. Figure from V. Amendola etal./ Phys. Chem. Chem. Phys. 2013, 15, 3027-3046 Laser ablation in liquid is useful for the generation of metal & metal oxide nanoparticles. Typical parameters that should be controlled: pulse energy; repetition rate; wavelength; solution type; existence of other solutions, depth of target placement; purity of target. £1 1 Solvent 1 Water Ethanol Acetonitrile -= N Au 2 nm Metal Au MetalAu Metal Au Ag 10 m - 5 nrti 5 nm Metal Ag/ Oxide AgO Metal Ag Metal Ag In general, laser ablation in liquid generates quasi-spherical nanoparticles of metals! The following video demonstrates the laser ablation process: https://www.youtube.com/watch?v=kOy0yu WpUzU Figure from V. Amendola et ail Phys. Chem. Chem. Phys. 2013, 15, 3027-3046 Flame combustion method In this method, a spray of precursor solution mixed with a gas carrier is burned in flame. In general, the method is suitable for fabrication of spherical metal oxide nanoparticles (undoped or doped). Metal salt solution + Oil: Kerosene + Surfactant to vacuum pump Atomizer Spray Flame Particles During the Flame combustion process, size of nanoparticles can be controlled by means of: ^Salt concentration (lower concentration form smaller nanoparticles) ^Addition of surfactants (restrict the growth of nanoparticles) ^Precursor & gas supply rate, flame temperature, viscosity, etc. ^Nanoparticles > N a nop article Growth Precursor High Tempera hire Flame ^Secondary Droplets * Primary Spray Air. 1 F it el + Precursor Mechanical method to make nanoparticles Mechanical Methods High Energy Ball Milling Bulk Nanoparticles in the form of Powder. Melt Mixing Form/Arrest Nanoparticles in Glass. High Energy Ball Milling Some of the materials like Co, Cr, W, Ni, Ti, Al-Fe and Ag-Fe are made nanocrystalline using ball mill. Few milligrams to several kilograms of nanoparticles can be synthesized in a short time of a few minutes to a few hours. One of the simplest ways of making nanoparticles of some metals and alloys in the form of powder. Usually 2:1 mass ratio of balls to material is advisable. If the container is more than half filled, the efficiency of milling is reduced. Heavy milling balls increase the impact energy on collision. To avoid any impurities from balls, the container may be filled with air or inert gas. However this can be an additional source of impurity, if proper precaution to use high purity gases is not taken. A temperature rise in the range of 100-1,100°C is expected to take place during the collisions. Lower temperatures favour amorphous particle formation. The gases like 02 N2 etc. can be the source of impurities as constantly new, active surfaces are generated. Cryo-cooling Is used sometimes to dissipate the heat generated. By controlling the speed of rotation of the central axis and container as well as duration of milling it is possible to ground the material to fine powder (few nm to few tens of nm) whose size can be quite uniform. Physical methods of preparation can be faster & more energy efficient compared to conventional chemical methods. Therefore, these methods can be easily scaled- up for industrial production & commercialization. Nano -v Devices Nanobio-technology Medicine & Optics Defense & Safety Innovative Applications Nano-technology Bio- ^ Engineering ^ Cosmetics Textiles, Fabrics Nanoparticles for biomedical applications Therapeutic s Drug Delivery Permeation enhancer Fluorophore —-, Therapeutic agent Polymer/Silica Shell Diagnosis fluorescent dyes silica shall NH2-groups ľ* •.u i r I IM IM V ' ••Ä r . ' 1 V c) Tuning Properties of PLD Magnetic Oxide Films • Changing parameters of deposition (type of substrate, heating temperature, 02 pressure, repetition rate, energy density distance between target & substrate) • Varying thickness (control by fixing time of deposition) • Extra-annealing • Multilayers How could fabrication conditions influence films' properties? • Type of substrates: 20 30 40 50 60 70 80 29 O Figure 1. XRD pattern for a Co: TiO; film deposited at 700'C (a) on a Si substrate, under an oxygen pressure of 10~5 Torr: (b) on a LAO substrate under an oxygen pressure of 10~6Torr and (c) on a STO substrate under an oxygen pressure of 10~5 Torr. Figure 6. The topographic («) and MFM (/>) images obtained at room temperature on an area 2x2 ftnr of the V : TiO: film grown on Si at 650"C The tip was applied perpendicular to the film's plane. I iim <> 31 nm -0.085° +0.085° Figure II). The topographic (rt) and MFM (b) images taken at room temperature on the area 5x5 fim2 of the V : TiO: film grown on LAO at 650"C. The tip was applied perpendicular to the film's plane (This figure is in colour only in the electronic version) Figure 2. Magnetization versus magnetic field at 300 K for the Co: TÍO2 film fabricated at 700"C. under an oxygen pressure of 10~6 Torr on Si. LAO and STO substrates. How heating temperature could change properties of films? 0.0 0.2 Field (T) Kijiiin- 5. Magnetization versus magnetic field at 300 K forCriTiOj films grown on an Si substrate at 650nC and 700C The inset shows the AŕííTŕ turn: taken under 0.2 T tor the film fabricated at 700T. (c) 2 0 1 5 I 10 -ID 3 0.5 I o.o ■s -°-5 I -10 -1.5 -2.0 (d) (e) —F©650 -^FeľOO --1-1-1-'- -0.4 -0.2 0.0 0.2 Field (T) 0.4 O o 4 -5 1 c 0 1 o N 1 -1 O) co 2 -2 -3 ' -«-Co650 -o-CoľOO i-o-o o-o-o-o-o-o-W1 ..J 1 ' 1 < -0.4 -0.2 0.0 0.2 Field (T) 0.4 ? 2 c O O) CO 2 -■— NÍ650 - -o— NÍ700 ^ . » ■ ■ 1 ' 1 ' 1 1 -0.4 -0.2 0.0 Field Figure 7. Magnetization versus magnetic field at 300 K for TM : TiO: films grown on LAO substrates at 650'C and 70O"C: i i Second Derivative ^ A i i 1 i i ii —Tv———tn—1—1—1— i /1 iTu i \ I n^tt" i i i i 1 i i i i 1 n G 3 -1-!-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1- (b) Annealed 220 nr i 4.6 eV H 1 1 1 1 I 1 1 1 1 (c) 220 nm - -\—1 -*\~*~r \ I I I I 1 i i i i i i i i i i i i i i ii i i * \ 4.6 eV t Kf^' \ 1 1 T[££">L-—f ' \ * | | 1 | | | | 1 i | | 1 ^ 1 1 1 1 1 1 | ■ i . i r i i i i i 1 1 J r T n 1 1 1 J 1 J 1 1 1 1 1 ~~i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—r (d) 10 nm ■M-t-rf i i i i | i i 515 520 525 530 535 540 Emission/Excitation Energy (eV) FIG. 3. (Color online )0 tf-edge x-ray emission (O KorXESfand absorption (O \s XAS) spectra for bulk SnO^ (a), the 200-nm-thick SnO: thin film with postannealing treatment (b). the 200-nm-thick as-grown film (c). and the 10-nm-thick as-grown film.