1 d structure materials

1. Supervisor: prof . Nguyen Huu Lam Group : Hoàng Văn Tiến Nguyễn Đình Trung Phạm Đức Thịnh Class: MSE –K54 1-D structure materials HA NOI UNIVERSITY OF SCIENCE AND TECHNOLOGY **** AdVANCEd training PROGRAM ****

2. Content Introduction about nano materials 1 D materials Fabrication Application

3. Introduction Materials with nano structure has attracted a great attention of the scientific commune in over the world because of their promise properties with quantum effects Three kinds of nano structure: 0D (quantum dots), 2D (thin films), 1D ( nanowires , nanorods ,…) Many ways to fabricate the nanostructure: CVD, PVD, Spin-coating, Sol-gel, sputtering …

4. Some 1-D nanostructures Nanorods Nanowires Nanotubes

5. Synthesis methods Spontaneous growth Evaporation ( dissolation ) condensation Vapor –liquid –solid (VLS) growth Stress- induced recrystallization Templated – based synthesis Electroplating and electrophoretic deposition Colloid dispersion , melt or solution filling Conversion with chemical reaction Electrospinning

6. Spontaneous Growth A growth driven by reduction of Gibbs free energy or chemical potential. This can be from either recrystallization or a decrease in supersaturation . Growth along a certain orientation faster than other direction – anisotropic growth. For nanowire, growth occurs only along one direction, but no growth along other directions .

7. Evaporation (Dissolution) – Condensation Growth

8. Disadvantages of Evaporation – Condensation Deposition Nanowire grown by EC most likely have faceted morphology and are generally short in length with relatively small aspect ratios, particular when grown in liquid medium . Anisotropic growth induced by axial imperfections, such as screw dislocation, microtwins and stacking faults, or by impurity poisoning, can result in the growth of nanowires with large aspect ratios.

9. Vapor-Liquid-solid (VLS) Growth VLS is basically a Chemical Vapor Deposition ( CVD) method driven in the presence of a Catalyst   that can accelerate the rate of a reaction, without itself taking any part in the reaction . Most semiconductor nanomaterials of the group III-Nitrides class are synthesized by this technique . - Basic  principle : heat a foil or powder of the group III  metal such as Ga , In, or Al ( source) in the presence of Nitrogen (N 2 )   or NH 3 at temperatures suitable for vaporization of the source and dissociation of the nitriding gas . -Catalysts : Transition metal and their oxides - The easiest form of catalyst used is a thin film of the transition metal (Fe, Ni, Co) or Noble metals like Gold (Au ).

10. Steps of VLS growth A thin (~1-10 nm) film of catalyst(Au in our case) is deposited onto a wafer substrate(Si) by sputter deposition or thermal evaporation or any other means suitable. Then the precursor   ( group III material : Ga ) is heated to evaporate to take part in the reaction. The vapour is absorbed by the molten catalytic droplet which becomes supersatured and Ga gets precipitated to the bottom where it take part in reactionto form GaN . As the melting point of GaN is >2500◦C   which is much higher than the reaction temp (around 900◦C   ) so it grows on the Si substrate in a Hexagonal crystal structure.

11. A schematic of the VLS process

12. Growth mechanism The shape of a catalyst particle at  the surface of a crystalline substrate is determined by a balance of the forces of  surface tension  and the liquid-solid interface tension .  The radius of the droplet varies with the contact angle as: R= -R is the radius of the contact area - β   is defined by a modified Youngs equation: σ 1 cos(β   ) =   σ s – σ ls - It is dependent on the surface( σs ) and liquid-solid interface ( σ ls ) tensions, as well as an additional line tension (   ) which comes into effect when the initial radius of the droplet is small ( nanosized ).   Schematic illustration of metal-alloy catalyzed whisker growth

13. VLS Setup ( GaN nanowires )

14. Wagner summarized the requirements for VLS growth The catalyst or impurity must form a liquid solution with the crystalline material to be growth at the deposition temperature The distribution coefficient of the catalyst or impurity must be less than unity at the deposition temperature The equilibrium vapor pressure of the catalyst or impurity over the liquid droplet must be very small because it can reduce the total volume of the liquid droplet The catalyst or impurity must be inert chemically ( not react with the chemical species ) The interfacial energy plays a very important role For a compound nanowire growth , one of the constituents can serve as the catalyst For controlled unidirectional growth, the solid –liquid interface must be well defined crystallographycally .

15. Advantages of the VLS method Greatly lowered reaction energy  compared to normal vapor-solid growth Wires grow only in the areas activated by the metal catalysts and the size and position of the wires are determined by that of the metal catalysts . This growth mechanism can also produce highly anisotropic nanowire arrays from a variety of materials

16. SUMMARY Types of one - dimensional nanostructures : nanorod , nanowires, nanotubes. Synthesis methods Growth mechanism Applications

17. THANKS FOR YOUR ATTENTION

18. QUESTIONS