Chemical analysis of surfaces and organic thin films by means of XPS
X-ray photoelectron spectroscopy The photoelectric effect Wilhelm Hallwachs (1886), Albert Einstein (1905) und Ernest Rutherford (1914) K after 500 000 s Wilhelm Hallwachs and Philipp Lenard, around 1900: flux density: D = 10-5 Wm -2 flux density per K atom: DA K = 10-5 Wm -2 8.610-20 m 2 = 8.610-25 W energy at 1 s irradiation: E prim = 8.610-25 Ws energy to remove one electron: E i = 2.24 ev = 3.610-19 Ws During irradiation only the 1/2,000,000 part of energy is supplied which is necessary to remove one electron.
X-ray photoelectron spectroscopy The photoelectric effect Wilhelm Hallwachs (1886), Albert Einstein (1905) und Ernest Rutherford (1914) h. n E kin core level valence band Einstein s Photochemical Equivalence Law (1905) h n = E bind + E kin ii [ev] to determine experimentally value specific for each element and level primary energy
X-ray photoelectron spectroscopy The photoelectric effect Wilhelm Hallwachs (1886), Albert Einstein (1905) und Ernest Rutherford (1914) K after 500 000 s Wilhelm Hallwachs and Philipp Lenard, around 1900: flux density: D = 10-5 Wm -2 flux density per K atom: DA K = 10-5 Wm -2 8.610-20 m 2 = 8.610-25 W energy at 1 s irradiation: E prim = 8.610-25 Ws energy to remove one electron: E i = 2.24 ev = 3.610-19 Ws During irradiation only the 1/2,000,000 part of energy is supplied which is necessary to remove one electron.
X-ray photoelectron spectrometer Kai Manne Börje Siegbahn (1954/1955) the first spectrum was recorded from common salt Born in Lund (Sweden), April 20, 1918 Son of K.M.G. Siegbahn Studies at the Univ. of Uppsala 1944: PhD (Univ. of Stockholm) 1951:Professor in Stockholm at the Royal Institute of Technology since 1954: University of Uppsala Nobel Prize of Physics 1981 hn = E bind + E kin Siegbahn, K.: Beta-ray-spectrometer theory and design. High resolution spectroscopy. In: Beta- and gamma-ray spectroscopy. (ed. by K. Siegbahn). Nort-Holland Publ. Co., Amsterdam (1955), S. 52
Modern X-ray photoelectron spectrometer Axis Ultra (Kratos Analytical, Manchester, United Kingdom) Excellent vacuum: ca. 10-12 mbar, at 20 C, one collision each 30 years (at 1000 mbar: ca. 10 9 collisions/s)
Modern X-ray photoelectron spectrometer X-ray source hn = E bind + E kin mirror analyzer S i h. n i filament trajectories of image mode outer hemisphere slit plate + 20 kv cooling water I K a 3...6 K b l min D E 0.2 ev K a 1,2 characteristic radiation L l anode (Mg, Al or Ag) block of copper Bremsstrahlung electrostatic lenses slit plate X-ray sample on sample holder inner hemisphere photoelectrons mono- chromator detector system screen CCD camera deflection plates charge compensation unit magnetic lense analysis chamber E max = e. U E vacuum pumps
Measuring the kinetic energy of the photoelectrons (E kin ) e.g. 180 hemispheric analyzer mirror analyzer trajectories of image mode outer hemisphere slit plate E E E E outer hemisphere detection system electrostatic lenses slit plate inner hemisphere photoelectrons mono- chromator detector system screen CCD camera deflection plates charge compensation unit inner hemisphere E - de E E + 3dE E + 2dE E + de X-ray sample on sample holder magnetic lense analysis chamber vacuum pumps
High surface sensitivity vs. X-rays S i h. n i + 20 kv cooling water filament anode Mg, Al or Ag block of Cu XPS A surface sensitive technique How depth we do analyze? di A N exp x = - l dx Middle free path of photoelectrons In the sample material l (C 1s, Al a ) = 1.2 nm I d = I - - d 1 exp l X-rays Photoelectron contributes to the spectrum inelastically scattered photoelectrons only contribute to the spectrum background n l d 1.0 0.8 0.6 0.4 0.2 0 probability W 60 Å at W = 0.01 10 nm at W = 0.0001 0 20 40 information depth [nm]
Interpretation of XPS spectra Wide-scan spectrum of poly(3-hydroxybutyrate) I [kcounts/s] 350 300 250 200 150 100 50 0 KLL series 1s C 1s Si 2s Si 2p 2s 1000 800 600 400 200 0 binding energy [ev] qualitative surface anlysis all elements were detected (without hydrogen and helium) elements with higher numbers in the periodic table show a couple of peaks. background element peaks relaxation phenomena (Auger peaks) CH 3 CH CH 2 C n
Cu 2p 3/2 Interpretation of XPS spectra Designation of the peaks Coupling of orbit quantum number and electron spin: j = l + s s = ± ½ l total orbit quantum number j orbit quantum number l main quantum number s chemical symbol D BE = 19.9 ev Cu 2p 3/2 Cu 2p 1/2 E M L 3 2 d (l = 2) p (l = 1) s (l = 0) p (l = 1) s (l = 0) 5/2 3/2 3/2 1/2 1/2 3/2 1/2 1/2 3d 3d 3p 3p 3s 2p 2p 2s 5/2 3/2 3/2 1/2 3/2 1/2 970 960 950 940 930 binding energy [ev] K 1 s (l = 0) 1/2 1s n l j BE [ev]
Interpretation of XPS spectra Quantification I [kcounts/s] 350 300 250 200 150 100 50 0 KLL series 1s C 1s C 1s Si 2s Si 2p 2s 1000 800 600 400 200 0 Binding energy [ev] qualitative surface analysis (showing all elements), quantitative surface analysis [C] [] [Si] = 68.39 At-% = 31.22 At-% = 0.39 At-% CH 3 CH CH 2 C n []:[C] = 0.456 [Si]:[C] = 0.005 []:[C] should be = 0.5
Interpretation of XPS spectra Quantification I(X) = [n(x),, (X,i,h (E, ] photon n), l kin ), A,T(Ekin ) Usually only relative values are the matter of interest: e.g.si transmission area of 2 means elemental or atomic ratio of [Si]:[] = 1:2 acceptance take-off angle middle free path of Hence, electrons results are given in atom percents [At-%] cross section of the photon-electron interaction I ( X ) n ( X ) ( X flux ) density ( E [ X of ]) the T photons ( E [ X ]) c ( X ) T ( E [ X ]) S ( X ) 1 1 1 l = 1 kin 1 kin 1 number of kin atoms = 1 1 I ( X ) n ( X ) ( X ) l ( E [ X ]) T ( E [ X ]) c ( X ) T ( E [ X ]) S ( X ) 2 2 2 kin 2 kin 2 2 kin 2 2 measured intensity Relative sensitivity factors are experimentally determined Peak S C 1s 0.278 1s 1.000 1s 0.549 2s 0.033 Ag 3d 5/2 3.592 Different elements have a different sensitivity and will be analyzed with different minimum concentration limits. However, all elements having a minimum concentration of ca. 0.1 at-% will be trustworthy analyzed.
Analyzing the chemical structure and binding states Chemical shifts in the C 1s and 1s spectra of poly(3-hydroxybutyate) C C CH 3 CH 3 CH CH 1 : 1 1 : 1 C C CH CH 3 3 CH CH 1 : 1:1:1 CH 3 C H C H 2 C qualitative surface analysis (showing all elements), quantitative surface analysis C C H 3 chemistry of the element s neighbourhood controls the electron density chemical shift (types of functional groups, degree of oxidation), 535 535530 530 - CH 3 - H2C C CH 290 280 Bindungsenergie [ev] - CH 3 - H2C C CH CH 3 CH CH 2 C n
Chemical shifts and binding states Qualitative and quantitative detection of functional groups binding energy [ev] 285 286 287 288 289 290 291 292 C C ; C H C = C C = C C C (ether) C H C C= (ester) C = (keto group) xirane C C = (ester) H C = (carbonic acid) anhydride carbonate C Cl C 2 C 3 + C NR 2 ; C NR 3 0 1 2 3 4 5 6 7 D BE [ev] primary shift CH 2 CH 2 CH C 2 - H b-shift by strongly electronegative groups CH 2 C H H C C 0.05...0.2 ev C() C 0.40...0.6 ev C C Cl C C DBE 0.5...0.7 ev 0.4 ev
300 295 290 285 280 Binding energy [ev] C 1s spectra to analyze chemical structures Polymethacrylates with different alcohol rests A CH 2 A CH 2 A CH 2 ACH 3 B C EC C1 CH 2 GC 2 HC 3 ACH 3 B C EC C1 CH 2 HC 3 ACH 3 B C EC C CH 3 n n n 300 295 290 285 280 Binding energy [ev] A CH 2 R C CH 2 G G G G G R C CH 2 B1 CH 2 G1C 2 GC 2 HC 3 A CH 3 B C E C C1 CH 2 G C 2 G C 2 H C 3 4 n n n
C 1s spectra to analyze surface reactions Hydrophobic fluorocarbon films should get hydrophilic properties PE-dimethacrylate UV-grafted onto an oxygen plasma treated C film PE-monoacrylate melt-grafted onto an oxygen plasma treated C film C film with PE-PP-PE cross-linked in an argon plasma PE-monoacrylate UV-grafted onto an oxygen plasma treated C film unmodified fluoro carbon film (C film) PE-monoacrylate grafted onto an oxygen plasma treated C film
Derivatization to analyze of functional surface groups Double bonds and carbonic acids Double bonds Carbonic acids + Br 2 Br Br + H H 3 C CH 3 Si N CH 3 N CH 3 Si CH 3 CH 3 + HN N + s 4 s + H R 3 N R 3 NH + H NaH in MeH Na + H AgN 3 in H 2 + Ag HN 3
Derivatization to analyze of functional surface groups Alcohol, oxiran and amino groups Alcohol groups xiran groups H + 3 C C H 3 + HCl 3 C H C Cl 3 3 C + Amino groups 3 C NR2 + Cl CH 2 C 3 C 3 R N CH 2 NH 2 + CS 2 H N S SH
Chemical shift in the Si 2p spectrum Detection of silanol groups and the condensation of silanes on glass surfaces No significant chemical shift at BE ca. 102 ev. Separation of component peaks of Si( ) 3, Si H und Si C seemed to be impossible. C 2 H 5 H 2 N CH 2 CH 2 CH 2 Si C 2 H 5 C 2 H 5 CH 3 H 2 N CH 2 CH 2 CH 2 Si CH 3 CH 3 Si 2p 1/2 Si 0 Si 2p 3/2 H 2 N CH 2 CH 2 HN CH 2 CH 3 CH 2 CH 2 Si CH 3 CH 3 CH 3 Si Cl CH 3 Si _ 110 106 102 98 binding energy [ev] 106 102 98 106 102 98 binding energy [ev] binding energy [ev]
Chemical shift in the Si 2p spectrum Detection of silanol groups and the condensation of silanes on glass surfaces Ag L Al Ka Si 1s Si 2p 1s Si 2s C 1s N 1s Si KLL KLL C KVV 0 500 1000 1500 2000 2500 Bindungsenergie [ev] Pleul, D.; renzel, R.; Eschner, M.; Simon,.: X-ray photoelectron spectroscopy for the detection of different Si bonding states of silicon. Analytical and Bioanalytical Chemistry, 375 (2003) 1276-1281.
Chemical shift in the Si 2p spectrum Detection of silanol groups and the condensation of silanes on glass surfaces Si C Si Si C C Si Si H 1840 1844 1848 1836 1840 1844 1848
Angle-resolved X-ray photoelectron spectroscopy (ARXPS) Depth profiles in the range of a few nanometers Spectrometer C 3 C 2 C 3 ED = ID = 5l = 0 75 (C 2 ) 5 C 2 H N hydrophobic end-group sample 60 0 ID = 5 l. cos () 300 290 280 binding energy [ev] ED = 5 l [X]:[C] 0.6 0.4 0.2 0 information depth []:[C] []:[C] CH 3 n
XPS employed to study biological surfaces XPS spectra recorded at low temperature (Cryo-XPS) gel Na 1s Zn 2p KLL 1s Na KLL Ca 2p N 1s C 1s Cl 2p S 2p P 2p skin S 2p gel 10 µm Baum, C.; Simon,.; Meyer, W.; leischer, L.G.; Siebers, D.: Surface properties of the skin of Delphinids. Biofouling 19 (2003) 181-186 skin 170 166 162 Si 2p 300 1000 800 600 400 200 0 binding energy [ev] K 2p 290 280
Ag 4d 3/2 Imaging XPS Y Y Y colloidal silver Ag 3d polyelectrolyte layer N 1s 800 400 0 binding energy [ev] semi-fluorinated silane 1s S S S silicone wafer ø 3.05 mm reactive anchor HS C 10 H 20 CH S 2p laterally structured gold dots Au 4f metal oxide Si 2p silicon wafer laterally structured gold dots gilded surface
Imaging XPS Laterally structured samples a) b) Eschner, M.; Simon,.; Pleul, D.; Spange, S.: In: "Dresdener Beiträge zur Sensorik", Band 16, w.e.b. Universitätsverlag, Dresden (2002) 149-153, ISBN 3-935712-71-5 100 µm 100 µm Si 2p S 2s c) d) e) N 1s before silanisation N 1s after silanisation N 1s + Ag 3d after silver deposition
Small spot XPS real XPS-imaging 50 µm 1250 27 55 µm 110 µm 60 µm 27 µm spot size hollow fibre of an artificial kidney cut along its length axis KLL 1s N 1s C 1s S 2s S 2p 0 1000 800 600 400 200 0 binding energy [ev]
Summary XPS XPS is a method oriented to detect elements in the surface region of samples qualitative surface analysis: identification of all elements (H and He are excluded), quantitative surface analysis relative amounts of the elements, e.g. [X]:[Y], peak deconvolution allows to analyze (qualitatively and quantitatively) functional surface groups or the oxidation states, labelling can be helpful to identify the species, depth profile analysis on molecular scale without destroying the sample, sputtering is also able, special features: cryo-xps, thermo- XPS, imaging XPS, small spot XPS
Auger electron spectroscopy (AES) Relaxation: Competition between Auger process and X-ray fluorescence Small atomic number: High atomic number: I [kcounts/s] 350 300 250 200 150 100 50 0 KLL Serie E Auger = hn E kin Auger process is preferred X-ray fluorescence takes place 1s C 1s Si 2s Si 2p 2s 1000 800 600 400 200 0 Bindungsenergie [ev] E e - h. n photo ionisation relaxation h. n' e - h. n' Auger emission
sample X-ray source Auger electron spectroscopy (AES) Auger electron spectrometer electron gun detection system qualitative surface analysis quantitative surface analysis chemistry of the element s neigh- bourhood controls the electron density chemical shift (types of functional groups, degree of oxidation) depth profiling combined with sputtering imaging AES in nanometer resolution cylindrical analyzer trajectories SEM Al Si 10 µm
Auger electron spectroscopy (AES) Spectral information KL 1 L 1 KL 1 L 23 KL 23 L 23 KL 1 L 1 KL 1 L 23 KL 23 L 23 2p 2s 1000 900 800 Bindungsenergie binding energy [ev] 1st derivation 1s 2p 2s 2s 0 2p 6 KL 1 L ( 1 1 S) 2s 1 2p 5 2s 1 2p 5 KL 1 L 23 ( 1 3 P) KL 1 L 23 ( P) 0 1s 2s 2 2p 4 KL 23 L ( 1 23 S) 2s 2 2p 4 2s 2 2p 4 KL ( 3 P) ( 1 23 L 23 KL 23 L 23 D) 500 600 700 kinetische kinetic Energie energy [ev]
Summary AES AES is also a method oriented to detect elements in the surface region of samples qualitative surface analysis: identification of all elements (H and He are excluded), quantitative surface analysis relative amounts of the elements, e.g. [X]:[Y], peak deconvolution allows to analyze (qualitatively and quantitatively) functional surface groups or the oxidation states, labelling can be helpful to identify the species, depth profile analysis by sputtering techniques imaging of the lateral distribution of elements in nanometer resolution