光電子能譜

1、XPS (X ray photoemission spectroscopy)/ESCA (Electron Spectroscopy for Chemical Analysis),XPS,Core-levels:electroniccore-levels are more atomic-like:elementchemical shifts from formal oxidation state of the at

2、om, the local chemical and physical environment :chemicallike Auger electron it has also short l:surface sensitive Angular dependence has diffraction effects:structure Magnetic dichroism magnetic,Surfac

3、e sensitivity,The short free path length of the electron at energies at tens to hundreds eV gives XPS/AES surface sensitivity,Theoretical calculation,,Check film thickness!,Theoretical consideration,As photoemission is m

4、uch more simple process than Auger process, conservation of energy then requires that : KE = hn - ( E(A+ ) - E(A) ) - FThe final term in brackets,,representing the difference in energy between the ionized and neutral a

5、toms, is generally called the binding energy (BE) of the electron . (F is the work function of the solid when KE is counted near surface, however, KE detected by analyzer then F is the work function of analyzer.),Z depen

6、dence,BE follows the energy levels: BE(1s)>BE(2s)>BE(2p)…BE with same orbital increase with Z: BE(Mg1s)>BE(Na1s),Elemental specific!,XPS data base,Main features of XPS,Three Step Model,,Absorption of the photon

7、 and excitation of electrons2.Transport of electrons to the surface3.The escape of the electrons from surface to the vacuum.,Inelastic scattering,In the step 2, inelastic scattering let XPS spectra consists of core-l

8、evel photo-emission peaks imposed by a step-like structure (background) due to the various mechanism to lose kinetic energy. Besides, there are also AES processes visible.,XPS peak fit,XPS peak identification,Photoelectr

9、on lines: core-level, valence bands, spin-orbit splittingAuger linesChemical shiftsX-ray satellitesX-ray “Ghost”Shake-up satelliteMultiplet satelliteEnergy loss lines,Spin-Orbit splitting:,Spin-orbit splitting is

10、an initial state effect. For any electron in orbital with orbital angular momentum, coupling between magnetic fields of spin (s) and angular momentum (l) occurs,Lower binding energy,Higher binding energy,Total angular mo

11、mentum j = |l ± s|, therefore for s electron there is no degeneracy, and other orbitals have two degeneracy:- s orbitals are not spin-orbit split - singlet in XPS- p, d, f… orbitals are spin-orbit split - doublets

12、 in XPS,- BE of lower j value in doublet is higher (BE 2p1/2 > BE 2p3/2)- Magnitude of spin-orbit splitting increases with Z- Magnitude of spin-orbit splitting decreases with distance from nucleus(increased nuclear

13、 shielding),,,Intensity ratio?,Core Level Chemical Shifts,Position of orbitals in atom is sensitive to chemical environment of atom. In solid all core levels for that atom shifted by approx. same amount (<10 eV). Chem

14、ical shift correlated with overall charge on atom (Reduced charge ®increased BE)For k-shell of an atom in a compound:EB(k) = EB(k,qA) +Vwhere EB(k,qA) is the binding energy of a free ion, A, qA is the net charg

15、e of A, and V is the potential at A due to all other atoms. V can be described as:V = e2qASqi/riA (i is any other atoms except A), riA is the distance between i and A, therefore chemical shift is:DEB(k)=EB(k,qA1)-

16、EB(k,qA2 )+V1 –V2qA1 and qA2 is the difference of charge in two states.,XPS spectra for Si and its compounds with F in a) and chemical shifts vs. the charge in b),Both S and Si binding energies increase with psitive ch

17、arge (the loss of negative charge of electron), and the same for C.,As the samples shown before, binding energies of Al3+ is higher than the metal atom, in the meanwhile, the binding energy of O atom (more positive charg

18、e) is higher than the O2- ion.,The chemical shifts due to the variation of the distribution of the charges at the atom site is the main reason for the other name of XPS: ESCA (Electron Spectroscopy for Chemical Analysis

19、),Shake-up and shake-off,Photoemission process can leave the ions in the ground state (main peak) and also possibly in an excited sate (shake-up/shake-off satellites), the latter makes the KE of photoelectron less: highe

20、r BE.- excitation of electron to bound state shake-up satellite- excitation of electron to unbound (continuum) state shake-off satellite- excitation of hole state shake-down satellite - rare,The shown is XPSspectra f

21、or Cu 2pphotoemission atdifferent chemicalstates. The shake-upLines does not existin Cu metal, and is unique for CuO And CuSO4,Some general rules,Shake-up features especially common in transition metal oxides asso

22、ciated with paramagnetic species. Generally, the shake-up/shake-off satellites have intensities and energy separations from the parent photoelectron line that are unique to each chemical state, which can be used to analy

23、ze the chemical state of the elements. Even Some Auger lines also exhibit changes due to these processes. With transition metal, the absence of these lines is the fingerprint for elemental or diamagnetic states. Prominen

24、t satellites occurs with paramagnetic states.,Multiplet splitting and shake-up/shake-off lines are generally expected in the paramagnetic states:,MnO XPS spectra,Chemical shifts are too small to distinguish the chemical

25、states of Mn in MnO from a). In b) the satellites are due to Mn2+, while for Mn3+ and Mn4+, although there should be satellites, they are with higher binding energies.,Shake-up/Shake-off satellites are another reason for

26、 the chemical sensitivity of XPS,Multiplet satellite,Following photoelectron emission, the remaining unpaired electron may couple with other unpaired electrons in the atom, resulting in an ion with several possible final

27、 state configurations with as many different energies. This produces a line which is split asymmetrically into several components. For s-type orbital with other unpaired electrons in the atom there are split lines like

28、 in the shown Figure for Mn 3s.For p or even higher orbital levels, is more complex and subtle,Energy loss lines,eph + esolid,,e*ph + e**solid,Photoelectrons travelling through the solid can interact with other electr

29、ons in the material. These interactions can result in the photoelectron exciting an electronic transition, thus losing some of its energy (inelastic scattering). Most common are due to interband or plasmons (bulk or sur

30、face).,,,Surface plasmon,(bulk plasmon),,,The plasmon loss satellites are rarely sharp in insulators but very prominent in the metals. The main peak is normally observed at higher binding energy with several lines with t

31、he same energy intervals and reduced intensity, and the interval can be not only single one due to different origins: bulk or surface plasmons, bulk one is more prominent and interval larger (21/2 factor of the surface o

32、ne).,Energy of Light,,,,,,,,Wavelength(?),106?m,103?m,1 ?m,10-3?m,10-6?m,Energy(E),,,,,,,Broad-cast,Short wave radio,,Infrared,UV,X-ray,Gamma Ray,Visible,1 MeV,1 KeV,1 eV,10-3eV,10-6eV,X-ray tube,Early x-ray source,Sta

33、ndard lab X-ray source is by very high energy e beam hitting the anode.,A common Dual anode X-ray tube,X-ray spectrum from x-ray tube,Characteristic lines from the X ray fluorescence process (XRF) and a broad background

34、(Bremsstrahlung), which is strongly depends on the energy of the electron,Typical X-ray anode material (Mg and Al),2p3/2 ® 1s and 2p1/2 ® 1s transitions produce soft x-raysKa1,2 radiation (unresolved doublet)

35、hn (eV) FWHM (eV)Mg 1253.6 0.7Al 1486.6 0.85Same transitions in doubly ionized Mg or Al produce Ka3,4 lines at hn ~ 9-10 eV higher…3p ® 1s transitions produce Kb x-rays,Energies and widths of ch

36、aracteristic soft X-ray lines of different materials,Mg K-shell X-ray emission spectrum.,The full line shows the characteristic line emissions after subtraction of a constant background as shown by the dashed line. Note

37、the logarithmic intensity scale.,X-ray satellites,Emission from non-monochromatic x-ray sources produces satellite peaks in XPS spectrum at lower BE.,“ghost peaks”,O Ka at 524.9 eV,Ghost peaks are due to contamination of

38、 the x-ray source, which produces x-ray emission at different wavelength and it can also due to contamination of the sample holder etc.,Monochromatic X-ray,Narrow peak widthReduced backgroundNo satellite & Ghost pe

39、aks,Goal to achieve,,Sample,X-ray Anode,Energy Analyzer,Quartz Crystal Disperser,e-,Rowland Circle,nl=2dsinq,For quartz (1010) surface, d=0.42 nm and 78.5 degree for Al Ka 0.93 nm,,Synchrotron Radiation,The synchrotro

40、n storage ring is a tubular vacuum chamber made to: Hold an electron beam travelling through it at nearly the speed of light. Maintain the high energy of the electron beam. As the accelerating electrons circle the ring

41、at relativistic velocities, they give off intense beams of light including x-rays. By using a monochromator the light will be Monochromatic.Key properties of synchrotron radiation:high intensitytunability in wide ran

42、genear-coherencepolarized. pulsedwell collimated,Sample charging effects,The light for XPS always charges surface positively (shifting of spectrum to higher binding energy) and leads to general instability (spectral

43、noise). For the metal sample, which can be grounded and the charges can be quickly gone. However, for insulator, this effects are serious and need to be treated.,C 1s shifts due to the charging,For XPS (even AES) never f

44、orget ground the sample !!!,,Inhomogeneous Surface Charging,Charging can even change the line shape due to Inhomogeneous Surface Charging, which have different positive voltage on the surface.,In a lot of cases, there is

45、 only spectral shift due to charging, which can be determined by comparison with known elemental XPS lines, for example C 1s.,Charge Compensation,When the spectra is distorted,electron flood gun mounted line of sight wit

46、h sample,electron flood gun mounted in analyzer axis + electromagnet,Which way is B field?,Other methods including make the sample very thin that is does not insulate, earthed metal mesh and very focused X-ray spot can a

47、lso help sometimes.,Quantitative analysis,X-ray penetrate much deep than the escape depth of electrons,,Can be found inhandbook,,,How to measure the intensity,Lorentzian or Gaussian functions plus a background (or even

48、more complicated functions) of E can be used to fit the peak to subtract the background. (More complicated Shirely background.),Instrumentation (analyzers),resolution,,,Acceptance angle,Analyzer: most essential part of a

49、ny electron spectroscopy, its characteristic are: energy range, energy resolution, sensitivity and acceptance angle. Normally its functions involve: retarding of the incoming electron, selection of the electrons with rig

50、ht kinetic energy (pass energy), detecting of the electrons (channeltron),How electron analyzer works?,Analyzer has certain pass energy (Ep), electrons with this energy in a small energy range (Ep±ΔE) can pass.Ener

51、gy resolution Δ E is proportional to Ep.As analyzer is works in small range of pass energies. To measure big energy range, electrons with different energy need to be retarded (or accelerated) by a potential to change t

52、he electron energy to be able to analyze. There are two methods to retard the energy of electrons:Constant pass energy mode: retard the electron energy to a fixed pass energy by varying the retarding voltage, theref

53、ore with fixed ΔE for whole spectrum.Constant retarding ratio mode: retard the electron energy with a fix ratio to a energy range that the analyzer can use corresponding pass energy to detect. Therefore the ΔE/E is fix

54、ed and not the ΔE is fixed.,,,,,E,RetardingV=(E-Ep),,Ep,,,Hemispherical Analyzer,X-raySource,ElectronOptics,Outer Sphere,Inner Sphere,Sample,Analyzer Control,Multi-Channel channeltron Electron Multiplier,Most widely u

55、sed for XPS,Hemispherical Analyzer,Pass energy: E = e U (b/a - a/b),DE/E = (x1+x2)/2r +a2,Resolution:,a=(a+b)/2,,U is the voltage difference between inner and outer sphere; a and b are radii of inner and outer spheres;

56、x1 and x2 are the radii of the entrance and exits apertures, respectively; a is the maximum deviation of the electron trajectories at the entrance with respect to the center line.,Angular resolved XPS,Photoemission is a

57、dipole interaction, its Hamilton can be write as:,Why?,The transition possibility is:,with,Obviously the experimental geometry (the directions of the incident light and electron emission) is crucial to the photoemission

58、process. Moreover, the electronic structure will be influenced by the presence of the surface, its possible influence will be present by the sample normal.,The change of emission angle with respect to the sample normal c

59、an also give different surface sensitivity.,The angular dependence of XPS is how the photoelectron diffraction (XPD) is done, which gives the structural information of the surface.,Angular resolved XPS,Various angular de

60、pendence,Surface sensitivity change due to angle and photon energy,More Surface Sensitive,less Surface Sensitive,Same path length but the depth different,,,,Can be done aslo with AES!,Sample for surface sensitivity chang

61、e due to angle,For photoemission,,Ekin = hn - EB,Change of photon energy can change photoelectron energy that also changes the free path length of the photoelectrons(surface sensitivity).,Cannot be done with AES!,AES vs.

62、 XPS,1. Common points,2. difference,Both elemental and chemical sensitiveBoth can be used to do quantitative analysis of chemical composition.Both are electron spectroscopy which have surface sensitivity.,AES: involved

63、 two electrons and one hole, due to coulomb interaction (no selection rule), complicated, peak broad, can be excited by many energetic particles including photon, no intrinsic angular dependence, commonly use CMA, AES pe

64、ak in XPS spectra is with fixed Ekin.XPS: involved in one electron, due to dipole interaction (selection rule), peak sharp, simple, only excited by photon, sensitive to angular geometry, often use angular resolved anal

65、yzer, XPS peak is with fixed binding energy.,Ultraviolet Photoelectron Spectroscopy (UPS),UV light ( hn = 5 to 100 eV) to excite photoelectron. From an analysis of the kinetic energy and angular distribution of the phot

66、oelectrons, information on the electronic structure (band structure) of the material under investigation can be extracted with surface sensitivity.,Can determine density of States D(Ei) (occupied),Angular Resolved UPS (A

67、RUPS),UPS spectra also have strong angular dependence for the excited electron, which can give information about band structure in the k space.,The light incidence angle and polarization and, the plane of incidence (with

68、 respect to the surface, crystal lattice) determine the A for the excitation process.,Three-step model,Full quantum-mechanicalAccurate but difficult,Less accurate but simpler and more instructive,Photoemission (UPS) the