On the use of Kumakhov Polycapillaries to improve laboratory

Size: px

Start display at page:

Download "On the use of Kumakhov Polycapillaries to improve laboratory"

Carol Shelton
5 years ago
Views:

1 ICXOM Frascati (INFN - LNF) September 2005 On the use of Kumakhov Polycapillaries to improve laboratory Energy Dispersive X-ray X Diffractometry and Reflectometry B. Paci 1, V. Rossi Albertini 1, A. Generosi 1, S. Dabagov 2,O. Mikhin 3 1 ISM-C.N.R., Area della Ricerca di Roma Tor Vergata (Rome, Italy) 2 INFN - LNF Frascati, Italy 3 Institute for Roentgen Optics (Moscow, Russia)

2 Sammary of the presentation: General : What is the Energy Dispersive Mode? Why using the Energy Dispersive Mode? Applications of polycapillary lenses in EDXD : Why using polycapillary lenses in EDXD Use of polycapillary optics as collection/collimation devices in EDXD: advantages and problems Test experiments Conclusions

3 What is the Energy Dispersive Mode? A diffraction pattern represents the intensity of an X-ray radiation elastically scattered by a sample as a function of the momentum transfer Δp. If the system under measurement is isotropic, the scattering depends no longer on the direction of Δp but on its magnitude only and the relevant parameter is: Scattering parameter q (E, θ) = a E sin θ (a=1.014 Å -1 /kev) (E = energy of an electromagnetic radiation, 2θ=scattering angle) To scan a certain q range, two possibilities are available: 1. to make an angular scan, keeping E fixed (Angular Dispersive ADXD). 2. to use an X-ray white beam, fixing the angle θ (Energy Dispersive EDXD)

$5000 Machine and spectrum 4000 Sketch of the ED X-ray diffractometer.$

4 5000 Machine and spectrum 4000 Sketch of the ED X-ray diffractometer. 1) X-ray source (W anode tube) 2) collimation slits 3) sample holder 4) Ge single-crystal energy sensitive detector. counts channels

5 Why using the Energy Dispersive Mode? 1. The simpler geometric arrangement: no movement is required during the measurement Prevention of systematic errors due to movement 2. The higher energy of the incident beam (typically up to kev) reduces the X-ray absorption thick samples, sample environment allows the q-scan of a wider region of reciprocal space, since q E 3. The integrated intensity of the white beam used in EDXD is an order of magnitude higher than that of the fluorescence lines (ADXD) acquisition time reduction 4. Parallel collection of the experimental points at the various q-values Time-resolved in situ measurements Combined EDXD and EDXR measurements on thin films

6 Drawbacks 1. Complication of the experimental data processing 2.The q-resolution q in ED is lower than in AD Δ q/q = ΔE/E + ctgϑ Δϑ The energy term, that in ADXD is negligible, in EDXD is relevant because of the finite energy resolution of the Solid State Detector The EDXD is usually devoted to the wide class of samples not requiring high resolution since the diffraction peaks they produce are so wide that the further broadening due to the use of EDXD is negligible Amorphous solids, liquids, semi-crystalline materials, nanocrystalline powders

7 Immobility of the experimental apparatus during data collection: In the EDXD: the whole pattern is obtained in a single measurement, a fast recording of film Bragg peaks (of their rocking curves). This is particularly important when thin films are analyzed, since the scarceness of the material available for the scattering may make the measurements of diffraction patterns with acceptable signal to noise ratio difficult and time consuming. In the EDXR: In Reflectometry, in the grazing geometry required minimal misalignments of the sample may induce relevant relative errors during the angular scan Moreover, if many scans have to be carried out consecutively, reproducibility problems may arise because of the mechanical movements of the diffractometer arms. Moreover, when an X-ray reflectivity experiment is performed, the decrease in q resolution due to the uncertainties on both the angle and the energy, which is the main drawback of the ED technique, does not remarkably affect the Reflectometry spectra, which are characterized by broad peaks or long period oscillations.

8 Why using polycapillary lenses in EDXD A limit of laboratory X-ray instruments is their low photon flux Need: Increase the photon flux by concentrating the beam in a small transversal section Usual optical devices able to guide and focus X-rays : Fresnel zone plates (soft X-rays) Refractive/diffractive optics Batteries of hollow metal blocks (hard X-rays) Problems connected to ED techniques using polychromatic X-ray beams unavoidable chromatic aberration effect producing an energy-dispersed focal spot A solution: Use of polycapillary lenses based on X-rays reflection and not on refraction/diffraction

9 Being Kumakhov optics based on the total external reflection of X-rays at the air-capillary wall interface, the focal distance is (almost) independent of the energy and, therefore, no relevant chromatic aberration is expected If the spot of an X-ray tube anode is placed in the polycapillary half-lens focus the half-lens will collect all the photons emitted within its acceptance solid angle A polychromatic parallel beam is obtained to be used to perform EDXD measurements Flux increase of 2-3 orders of magnitude at the typical laboratory X-ray energy

10 Problems connected with the use of Kumakhov polycapillary in EDXD Consequences on the spectral distribution of the polychromatic beam: 1) absorption of the polycapillary material cannot be neglected and will act in a selective way as a function of the photon energy, removing more effectively the softer energetic components 2) The harder components, which are characterised by smaller critical angles for total external reflection, have a lower probability to be in reflection conditions when they impinge on the capillary wall (and a higher probability to be transmitted out of the lens side, therefore leaving the main beam) The photons energy distribution is modulated by the passage in the polycapillary, acting as a bandpass filter that cuts the lateral parts of the spectral profile.

11 Experimental evaluation of the effect of polycapillaries on the spectral intensity of the X-ray X beam to be used for ED diffraction measurements. Measurements of the primary beam were carried out using polycapillary optics (halflens and cylindrical capillary ) collecting/parallelising the X-ray beam from the anode. Sequence of measurements: White beam directly from the anode (at 30 KeV and 100 μa working conditions) White beam after collection and guiding of a polycapillary half-lens White beam after collimation of a cylindrical capillary

12 Experimental Results: cylindrical polycapillary primary beam half-lens

13 absorption transmission out of the lens side Preliminary test showed that the X-ray intensity decreases with the photon energy of the white beam: this effect-due to transmission out of lens sideis more dramatic for the half-lens.

14 Conclusions It would be very interesting to use Polycapillary optics in EDXD : To collect the X-rays emitted on a wide solid angle by the anode spot As collimation devices to parallelise the X-ray beam Preliminary test allowed to evaluate the effect of the use of such optics on the spectral distribution of the polychromatic beam to be used in EDXD experiments The X-ray intensity decreases with the increasing photon energy This effect is relevant in particular when a half lens is used The actual devices are optimized to work in the low energy range It would be very interesting to have polycapillary optics optimized to work also in the high energy range to be used to perform EDXD (use of new materials).

16 Possibilities offered by the Energy Dispersive Mode -X-Ray Diffraction: -structural characterization of bulk samples (monocrystals, polycrystals, microcrystals, nanocrystals, semi crystals and amorphous) -structural characterization of films analysis of the degree of epitaxy (Rocking curves), of the growth characteristics (mismatch with the substrate) and of the induced stress -Time resolved in situ measurements of the structural evolution of samples: during the growth, experiencing diffusion processes, chemical transformations, phases transitions -Measurement using different in-situ devices (vacuum chambers, controlled atmosphere, high pressure / temperature cells, cryogenic chambers) - X-ray reflectometry (morphological study of thin and ultra-thin films) -Time resolved in situ measurements of the morphological evolution of samples

17 Possibilities offered by the Energy Dispersive Mode - X-ray small angle scattering: study of the long range correlation in disordered systems (biological samples, polymers, liquid crystals) - X-ray spectroscopy: chemical analysis and detection of impurities -Xray absorption: transmission measurements to detect chemical / structural inomogenities -X-ray tomographystudy of the macroscopic structure inside the sample and detection of dislocations, fluctuations of density /composition

18 EDXR Snell Law: n 1 cos θ 1 = n 2 cos θ 2 TR: θ 2 = 0, θ 1 = θ c, cos θ c = n 2 =n X-Rays: refraction index n = (1 - ρλ2r 0 Ζ 2 / 2π) < 1 TR occurs at very grazing angle passing from the less dense to the more dense material 1000 n 1 θ 1 θ 1 n θ 2 2 Reflection at an interface. (θ c / λ) = constant and, since q = a E sin θ =4 π sin θ/λ the reflected intensity can be studied as a function of q,around its critical value q c θ c /λ. I r Reflectivity profile of the reflection at an interface q < qc, the X-ray radiation is totally reflected when q qc the radiation starts penetrating the sample as q>qc, the reflectivity decays more than exponentially. The position of the critical edge (q=qc) is determined by the material density and the reflectivity profile of the threshold by the surface roughness (defined as the variance from the average thickness), respectively q c q (Å -1 )

19 10 RuPh 400 Å nominali Dati sperimentali Fit log R 1 1E-1 1E-2 1E-3 Reflection at two interfaces (thin film model) the reflected waves interfere producing a sinusoidal modulated signal. 1E-4 1E-5 0,00 0,02 0,04 0,06 0,08 0,10 0,12 q (Å -1 ) The period of the modulation is connected to the film thickness the inclination of the threshold and the damping of the oscillations are related to both the surface and the interface roughness.

20 Example 1 In situ Energy Dispersive X-ray Reflectometry Measurements on organic solar cells upon working Organic Active layer Al Reflected Intensity Log scale (a.u.) 0.08 Reflected Intensity (a.u.) scattering parameter (Å -1 ) scattering parameter (Å -1 ) σ (Å) d (Å) a) b) Au contact ITO Glass time (hours) In situ is EDXR: a powerful non-destructive tool to investigate the aging effects at the interface of polymer PV cells in working conditions. B. Paci et al. APL in press

21 E-3 Logarithmic Scale 1E-4 1E-5 1E-6 1E-7 1E-8 1E-9 1E-10 1E-11 1E-12 1E-13 Sample 7 θ = Sample 8 θ = Sample 9 θ = Example 2: Cr/Pt bilayer films σ 4 Å scattering parameter( Å -1 ) Sample q ( Å -1 ) q ( Å -1 ) Sample 9 Relative Area Cr film acts as a structural template for the epitaxial growth of the magnetic Co-based film application: magnetic recording hard disks Substrates: Si, MgO and amorphous SiO 2 T dep = C MgO Cr Rocking Angle (degee) 1 2 RC [0.001deg] α0~10 (FWHM)~50 EDXD EDXR Structural properties lattice parameters Rocking curves, degree of epitaxy Morphological characteristics films thickness and roughness α(degree) scattering parameter (Å -1 ) Δq /q B. Paci et al. Chem. Mat. 16; (2003)

Data collection Strategy. Apurva Mehta

Data collection Strategy. Apurva Mehta Data collection Strategy Apurva Mehta Outline Before.. Resolution, Aberrations and detectors During.. What is the scientific question? How will probing the structure help? Is there an alternative method?