纳米材料和纳米结构第八讲

纳米材料和纳米结构

米材料和纳米结构

Laser Ablation 激光烧蚀法

第八

纳米材料和纳米结构

IntroductionIdeal Properties of an Nanomaterial Preparing Technique – High collection rate – Exact conversion of the composition of bulk source materials into nanocrystalline materials – Clean and uncontaminated products – Ease of control of processing variables – Potential to produce nanocrystalline materials from any composition and with any microscopic state such as non-equilibrium, multi equilibrium, multi-component alloys, and ceramics – Narrow crystalline size distribution – Crystal structure manipulation

纳米材料和纳米结构

Advantages of Laser in Material Preparations – High energy density – Pulsed beam with an easily modifiable profile, both temporally and spatially – Wide range of available wavelengths – High transmission in the atmosphere – Monochromatic and coherent beam – Easy handling with relatively inexpensive optics Advantage of Laser Ablation Technique – Create a much lager flux of evaporated material – Produce plumes of evaporated materials with stoichiometry congruent to the target even for complex chemical systems – Be fast and easily controlled by altering the laser pulse energy, repetition rate and focus spot size

纳米材料和纳米结构

A brief description on the physics of laser ablation A high energy laser beam interacts with a target, converting part of the electromagnetic energy of the beam into thermal energy sufficient to produce melting and evaporation of the target. The evaporated material typically is partially ionized to form a plasma. The plasma expands, cools and condenses into clusters, which continue to migrate away from the target until they are collected on a cooled substrate.

纳米材料和纳米结构

Principles of Laser AblationFundamental Process In laser ablation, the energy of the laser beam is transferred to the target material. There three identifying pathways, which are primarily determined by the laser power, wavelength, pulse duration and the surface nature, by which the laser energy gives rise to ablation and removes materials from the target: – Photothermal ablation – Plasma-assisted ablation – Photochemical ablation Photothermal Ablation – Occurred at lower laser power (100-500 MW/cm2) and longer pulse length ( > 10 ns)

纳米材料和纳米结构

– Energy transfer from laser beam to target through the photons being absorbed by the free or bo electrons on target surface, which results in electron oscillation rise to a temperature increase of target material structure dissipate their energy and their respective volume increases rapidly – Subsurface heated atoms are then forcibly ejected along with the cooler surface atoms due to rapid volume expansion – Threshold laser energy is required in this mode

– The excited electrons relax through interacting with other electrons and lattice phonons, and giv

– The energy transferred in a very short pulse time, and is limited in the first few atomic layers of

– The surface atoms are allowed to cool via evaporative cooling w

hile the subsurface atoms can

纳米材料和纳米结构

Plasma-assisted Ablation – Occurred at high laser power ( >> 500 MW/cm2) – The laser pulse produce a temporary plasma above the target surface – The produced high-energy ions bombard the target surface and giving up their energy, and the target atoms are then heated and thermally ejected from the surface Photochemical (Photolytic) Ablation – The laser photon energy be of the same order of the bonding energy of the target molecules – The photon energy is absorbed and accumulated by the bond, leading to the dissociation of the bond, and causing the ejection of the material

纳米材料和纳米结构

光热烧蚀和光化学烧蚀原理比较示意图

纳米材料和纳米结构

Theoretical Model – Solid target: heat transfer be assumed to occur by conduction in the axial direction only linearly across the film and the liquid expulsion is neglected properties such as pressure, temperature, density, and velocity incoming laser beam distribution in the plasma

– Liquid film: the liquid film above the solid phase is thin enough so that the temperature changes

– Kundsen layer: be assumed to be with zero thickness, creating a discontinuity in the state

– Plasma: be assumed to be optically thin so that no coupling occurs between the plasma and the

– Particle formation: be determined by the droplet droplet-growth theory, including the particle size

纳米材料和纳米结构

激光烧蚀的物理过程示意图(理论模型) 激光烧蚀的物理过程示意图(理论模型)

纳米材料和纳米结构

Experimental Conclusion-1

The amount of the melted material increases with the laser intensity, or with the ablation time fo given laser intensity

Variation of the solid-liquid interface depth with ablation time liquid

纳米材料和纳米结构

Experimental Conclusion-2

The vapor flux increases with the laser intensity, and it increases rapidly during the first seve nanoseconds to eventually reach a constant value

Variation of the flux of vapor at the surface of the liquid with ablation time

纳米材料和纳米结构

Experimental Conclusion-3 The particle size increases with the height in the plasma and the ablation time

Distribution of particle size at various heights in the plasma phase

纳米材料和纳米结构

3 Processing ExperimentsProcess Chamber – High-vacuum ablation chamberPowder-collector Rotating target holder Load lock Stepping motor Monitoring devices for temperature and pressure

– Cooling system – Vacuum system – Optical system, including processing excimer laser and diagnostic dye laser

纳米材料和纳米结构