Surface, Optical and Thermal Characterization of Nanoparticle Systems

Transcription

1 Surface, Optical and Thermal Characterization of Nanoparticle Systems Francesca Pignatelli Introduction, aim Wettability Optical properties Thermal properties Practical Example, current research

2 Materials Characterization: Test of the synthezided nanocrystal Well defined properties capture the useful characteristics of materials The caracterization constitue an essential background for... Processing Designing the structure to achieve specific properties of materials Control of the performance

3 Polymers Polymers are attractive because they are usually lightweight and inexpensive to make, and usually very easy to process, either in molds, as sheets, or as coatings. Most are very resistant to the environment. They are poor conductors of heat and electricity, and tend to be easy to bend, which makes them very useful as insulation for electrical wires. Polymers offer often a suitable matrix to insert nanoparticles Nanoparticle Systems Doping polymers by nanoparticles in order to modify, enhance properties of materials.

4 Wetting and Adhesion Properties of materials Surface tension (or surface energy) is the ammount of work necessary to form a new part of surface. The surface tension is due to the different integrated molecular or atomic forces in the material interior and on the surface. The surface of a material has a free energy larger then in the bulk material: the area of the material is therefore minimized.

5 The surface tension of solids (γ s ) is related to their wettability and is an usefull parameter for the study of the interactions between the solid surface and the biological surround. Wetting is the phenomenon of the adherent contact of liquids to the surface of solids (or other liquids). This can be evaluated by the measure of the contact-angle between a drop of liquid and a solid surface (Θ). This is the static contact-angle, in this case the liquid should give rise to a symmetric drop on the solid surface.

6 θ = 0 totally wetting Hexane on polyethylene Water on gold θ < 90 partially wetting Water on glass (10 to 15 ) Water on PMMA (74 ) θ 90 non wetting θ = 180 totally non wetting Water on teflon Water on polypropilene 180 not possible with gravity Solid surfaces with contact angle above 150 are considered as super hydrophobic.

7 We refer to a dynamic contact-angle in case of a liquid drop mouving on a solid surface. In particular we can define an advancing contact angle (Θ A ) and a receding contact angle (Θ R ). The difference between these two angles (Θ A - Θ R ) is called hysteresis of the contact-angle and determines the pinning force of the drop on the surface. The measure of the hysteresis of the contact-angle is a measure of the morphological properties of the surface.

8 Drop Shape Analysis Method Takes images of the droplet at hight magnification Analyse the immage to determine the droplet profile Fits the experimental profile with the theoretical profile Determines the surface tension γ and the contact angle Θ as fitting parameters

9 Printing: the ink must not spread out Large contact-angle the ink has to be absorbed time dependence of the contact-angle Coating and varnishes: coating/varnish should be continous small contact angle Contact adhesives: the knoledge of the surface tension of both adhering components and their polarities is important for a correct choice of the adhesive Biocompatibility: hydrofillicity of a surface is an important characteristic of biocompatibles materials

10 Self-cleaning surfaces, What we can learn from Nature Chemical modification of smooth solid surfaces can tipically lead to water contact angle up to 120 by using fluore polymeric or silane layes, but not more. How to produce a super-hydrophobic surface (θ >150 )? The equilibrium shape of the drops on solid surface is only half of the story - to clean the surface the material (dirt) has to be transported along the surface and best off it. Lotus self-cleaning effect Water drops rolling down the lotus leaf surface pick up the dirt particles and effectively clean the leaf surface.

11 Lotus plant-nelumbo Nucifera Some plant leafs have contact angles approaching 180 (Lotus effect) To reach the extreme values of the contact angle near 180 the surface must be not only hydrophobic but also must have a special structure

12 Super hydrophobic surfaces, such as lotus leaves, with micro/nano combined structures found in nature have attracted many researchers interest because of their importance in fundamental research and practical applications such as self cleaning, anti-fogging/snowing, drag reduction effect etc. In this regard, diverse methods have been proposed to produce such surfaces. Nelumbo Nucifera the rough surface is characterized by papillose epidermal cells and aditional layer of epicuticular waxes Mutisia Decurrens the petal surface is characterized by cuticular folds For low- energy surface roughness promotes drying: It becomes energetically expensive for the liquid to follow the surface corrugations. For hydrophobic materials surface nanostructuring enhance hydrophobicity.

13 Hydrophilic surfaces If the surface has a high interfacial free energy, roughness promotes wetting and liquid will accumulate within the corrugation. Gnetum Gnemon the smooth leaves are almost lacking microstructures Magnolia Denudata the leaves are characterized by sunken and raised nervatures Summarizing: wettability is improved by roughness for hydrophilic surfaces (θ* < θ for θ < 90 ) but gets worse for hydrophobic surfaces (θ* > θ for θ > 90 ).

14 Optical Properties of materials Refractive index Refractive index (n) measures how fast light propagates through a medium. This describe how different particles affect light propagation, measured with respect to air. The refractive index n of a dielectric medium is: n = c v where c is the speed of light in vacuum, and v is the speed in the medium.

15 Due to dielectric loss in real material the refractive index can be complex, with a real and imaginary part. ñ(λ) = n(λ) + ik(λ) Value of the imaginary part is a measure of the absorption strength. For most media, refractive index varies with wavelength. This gives the familiar rainbow spectrum with white light in glass or water.

16 Ellipsometry Overview Technique that determines the change in polarization state of light reflected from a sample. Typically used for characterizing thin films. Used in a wide range of applications. Non contact, non destructive method.

17 Thickness CAPABILITIES Characterization of thin films, surface and interface - Accurate thin film measurement from a few angstroms to several microns - Characterization of single layer and complex multilayer stacks - Characterization of surface and interface Optical properties - Refractive index (n) and extinction coefficient (k) from the far- UV to near-ir for complex materials, graded and anisotropic layers Material properties - Composition / crystallinity - Microstructure - Film uniformity by area and depth

18 Technique Measurement of the polarization change of light reflected from a sample in terms of two parameters and Ψ. P 1 - Linearly polarized light 3 - Elliptically polarized light Φ 2 Reflection on the surface

19 ANALYSIS OF DATA Ellipsometry does not measure film thicknesses or optical constants, it measures Ψ and Psi ( ) TiO2 on SiO2 substrate Delta ( ) E (ev) To extract these informations from a sample, it is necessary to perform a model dependant analysis of the ellipsometric angles In this step, the number of layers is fixed, and the basic structure concerning the contents of each layer is set.

20 SE DATA ANALYSIS FLOWCHART Measurement Model Fit Results Psi ( ) EXPERIMENTAL DATA E (ev) Delta ( ) Psi ( ) roughness TiO 2 SiO 2 substrate 2 GENERATED DATA E (ev) Delta ( ) Psi ( ) FIT RESULTS E (ev) Delta ( ) Re(Index) χ 2 = 1.6 d TiO2 = 4200 Å d rough = 20 Å (n,k) = f(lambda) for the TiO2 layer lambda (nm) Im(Index)

21 Thermal Properties of materials Well defined properties/characteristics which allow us to answer questions like How does an engineering component change size when heated? How well does it transmit heat? How will its temperature change when heated? At what temperatures can I use it? Thermal Analysis Differential Scanning Calorimetry (DSC) Measure difference of heat absorbed or liberated during heating or cooling in respect to a reference

22 Qualitative analysis What Can You Measure with DSC? Fingerprinting of minerals, clays, polymers Sample purity Melting points Heat capacity, c p Glass transition temperature, T g Crystallization temperature, T c Phase diagrams Where Used? Pharmaceutical industry Purity Food industry Characterization of fats and oils Polymer industry Synthetic blends

23 Schematic of DSC Instrument Sample Reference T 1 T 2 Pt thermopile heater Pt thermopile heater N 2 flow W

24 Output of DSC Thermogram dh/d dt, mj/s glass transition crystallization melting endo exo Temperature, C

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26 Phase Diagrams Graphical representation of relationship between phase composition and a state variable (often temperature). The phase diagram allows to determine phase compositions and phase amounts at any point on a binary diagram. Phase a homogeneous, physically distinct, region of matter (solid, liquid).

27 Completelly soluble mixture, same shape so can substitute each other

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29 The example: PEMMA+SP The idea enhance the properties of the copolymer (PEMMA) by mixing it with photocromic molecule (SP). Poly(ethyl methacrylate)co-poly(methyl acrylate), shortly called here PEMMA + 1,3 -dihydro-1,3,3 -trimethyl-6-nitrospiro[2h-1-benzopyran- 2,2 -(2H)-indole, shortly called here SP The propertes of the resulting material can be furtherly enhanced by surface nanostructuring.

30 The embedded non-polar spiropyran molecules can be converted to their polar merocyanine, upon UV laser irradiation; the process can also be reversed upon green laser irradiation. Insering the photocromic molecules in a polymeric matrix and eventually nanostructuring the surface of the film can allow for Enhancing the different properties of the sample. Optical control of the properties.

31 Photocontrolled variations in the wetting capability of photocromic polymer PEMMA + 5% SP Due to the dipolar moment of the MC isomers the surface of the sample becomes more hydrophillic

32 Enhancement by surface nanostructuring Nanoimprinted grating Cassie-Baxter model The surface roughness give rise to enhanced contact angle photo-induced differences.

33 Photocontrolled changes in the optical properties of the photocromic polymer Polyethylmethacrylate-comethylacrylate (PEMMA) 1,56 χ 2 = n PEMMA Film 0,08 1,4940 0,020 1,55 n Fit 0,07 t=(914 ± 4) nm k PEMMA Film 0,018 1,54 0,06 1,4935 k Fit 1,53 0,05 0,016 1,52 0,04 1,4930 0,014 0,03 n k 1,51 n k 0,02 1,4925 0,012 1,50 0,01 0,010 1,49 1,4920 0,00 1,48 0,008-0,01 1,47 1,4915 0,006-0,02 1,46-0,03 0,004 1, Wavelenght (nm) Wavelenght (nm)

34 PEMMA + photochromic Spiropyran (10%) 1,54 χ 2 = ,53 1,52 1,51 n 1,50 1,49 1,48 t=(453 ± 2) nm n PEMMA+SP Film n Fit k PEMMA+SP Film k Fit Wavelenght (nm) 0,035 0,028 0,021 0,014 k 0,007 0,000-0,007 0,040 1,52 PEMMA SP(10%) n= 1.509, λ = 822 0,036 0,032 1,51 0,028 n 0,024 1,50 PEMMA n= 1.491, λ = 822 k 0,020 1,49 0,016 PEMMA SP(10%) 0,012 1,48 PEMMA 0,008 0, Wavelenght (nm) Doping the polymer the refractive index is slightly increased.

35 1,55 1,54 1,53 1,52 n 1,51 1,50 1,49 1,48 After cw He-Cd laser irradiation (325nm) the film shows an absorption peak around 550nm. PEMMA SP(10%) on Glass irradiated for 20'' at 325nm χ 2 = 0.22 t=(462 ± 2) nm 0,045 0,040 0,035 0,030 0,025 k 0,020 0,015 0,010 0,005 0, W avelenght (nm) 1,60 1,58 1,56 1,54 n 1,52 1,50 1,48 1,46 1,60 1,58 1,56 1,54 n 1,52 1,50 1,48 PEMMA SP(10%) on Glass irradiated for 60'' at 325nm 0,10 χ 2 = 0.18 t=(458 ± 2) nm Wavelenght (nm) PEMMA SP(10%) on Glass irradiated for 40'' at 325nm 0,09 χ 2 = ,08 t=(458 ± 2) nm 0,07 0,06 0,05 k 0,04 0,03 0,02 0,01 0, ,09 0,08 0,07 0,06 0,05 k 0,04 0,03 0,02 0,01 0,00 Wavelenght (nm)

36 1,56 1,55 1,54 1,53 1,52 40 sec 60 sec 60 sec n=1, sec n=1, sec n=1,516 0,11 0,10 0,09 0,08 0,07 n 1,51 1,50 0 sec n=1,509 0,06 0,05 k 1,49 0,04 1,48 0,03 1,47 1,46 0 sec 20 sec 0,02 0,01 1,45 0, Wavelenght (nm) 822 nm Increasing of the index of refraction and is absorption coefficient in function of the laser exposure time of the film. After 60 sec it reaches the saturation value. The sample change from transparent to red coloured.

37 Thermal characterization Addition of small amount of SP in the polymeric matrix (5%, 10%, 15% and 20%) does not affect the T g of the sample (39 C) while at higher concentrations the increase of the SP concentration causes the decrease of the T g (for 30% T g =38 C and 37 C for 50%)

38 Possible Applications? Microfluidic devices The polymetric matrix doped with photocromic SP molecules can alter its macroscopic volume reversibily upon UV irradiation and green laser. Controlable drug delivery Controllable surface absorption can allow for controllable release of drug. Self cleaning surfaces Control droplet positioning and motion on a prepatterned substrate. Optical switch The photocromic transformation between the SP and MC,olecules can alter the macroscopic volume of their host polymer matrices.