ups applications-2 - washington state universityscudiero/documents/571-ups-lecture2_003.pdf · how...
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UPS Applications-2
Louis Scudiero
http://www.wsu.edu/~scudiero; 5-2669
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MoS2 photoelectron energy distribution spectra intensity as a functionof "binding” energy and photon energy.
Photon energies: Ne (I) 16.8 eV, He (I)21.2 eV, Ne(II) 26.9 eV, He (II) 40.8eV, and He (IIβ) 48.4 eV.
Note that the relative intensity inpeaks changes in the distributions withphoton energy. This is due to cross-section σ changes for the “d” and “p”region of the VB.
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How is this related to the electronic structure of the material, what is thesignificance of the intensity variations?
To answer these questions, let us again look at the 3-step model that was firstproposed by Berglund and Spicer (C. N. Berglund and W. E. Spicer, Phys. Rev. A136,1030, 1964).
Initial Block state with energy Ei(allowed state with a periodicitydetermined by the periodic potential ofthe crystal lattice). These electrons arethen excited (1) by the photon into aconduction band state at Ej (2). Theexcited electron finally escape from thesurface (matching at the vacuum interfacebetween the excited state and somefree electron state in vacuum). This statecan be represented by a single plan wavee ip.r≈ (3).
ik
jk
jk
'jk
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The contribution to the photocurrent I (hν,ε) with some initial stateenergy Ei = ε (analyzer set to ε) can be expressed as follows:
Where T (k) is some function expressing the probability of transportand escape (steps 2 and 3) and where the dipole matrix element
expresses the optical excitation (step 1). A is the vector potential, ∇gradient and the δ functions express the fact that Ei and Ej areseparated by hν and the second that the energy analyzer is “tuned “ topick out the initial state Ei.Illustration of several importance effects:
1. Density of states 2. Matrix elements 3. Transport effects
( ) ) )dkE(εhEδ(EkAkT(k),hIiij
2
fi−−−∇•∞∫ δνεν
2
ijkAk ∇•
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Theoretical calculations versus photoemission spectrum
Let us assume that we are slowly varying transport functions and matrix elements sothat it can be given by:
The two δ functions define 2 intersecting surfaces in reciprocal space and if theintegral is transformed to a line integral around the line of intersection thephotocurrent is expressed as:
The integral diverges when both gradient terms go to zero (band becomes essentially“flat”). But (1/∇(E)) dE is the density of states in an energy interval dE.
So, if we neglect the singularities, the photoemission measures the overall density ofstates in the valence band of the solid and can be compared directly with calculatedband diagram.
Density of States
) )dkE(εhEδ(EIiij
−−−∞∫ δν
∫ ∇∇∞
)()(dlI
ikjkEE
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Very good agreement betweentheory and experiment in the “p”region. No contribution of the “s”states is seen in the He II data. Thisis due to a very small value of thedipole matrix element.
No “s” contribution because
02≈∇• ij kAk
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Dipole Matrix ElementsPhotoionization Cross-section
The absence of the ‘s’ state in the SnSe2 He (II) spectrum can beexplained by looking at the matrix element
At a given photon energy hν and for a fixed final state
the matrix element will vary depending on whether is s, p, d orf like.
In practice, the ‘s’ cross-section is extremely low above about 10 eVuntil we reach soft x-ray energies. Therefore only p, d and felectrons will be detected in the energy range of 0 - 50 eV (UPS).
2
ijkAk ∇•
fk
ik
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Calculated cross-section
for certain rare-gas shells
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So, back to molybdenum disulfide, the VB statesare formed from Mo 4d and S 3p electrons. Theband near the EF is purely ‘d’ like and theremaining structure comes from covalent bondingband based on the S 3p electrons. Some mixingp/d character is also expected.
Spectra between 5 and 40 eVBased on the plot cross-section vs. PE energyshown in the previous slide the 3p states dominatesbelow 25 eV and the 4d states above 25 eV.Bands 2 and 4 are purely S 3p and finally bands 3and 5, expected to have a mixed character p/d.
Atomic character of the states causeintensity changes
54
3
21
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Energy loss introduced by electron-electron interactions (scattering andsecondary emission).If these electrons escape thespectrum will display peaks that willmove as the photon energy ischanged.
Structure which ‘moves’ betweenNeI and HeI spectra is assigned tosecondary electron emission(identified by bars)
Transport Effects
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Band Theory and PE Spectrum(electron energy distributions)
He II
“s”
2s 1s forbidden but 2p 1s allowed
“p”
XPS
Because the C – C bond distance ingraphite ~ 1.4 Å strong interatomicinteractions > than in diamond. The VBelectron distribution is importanttheoretically and experimentally.
UPS spectrum is closer to the DOS dataP. M. Williams, Handbook of x-ray and ultraviolet photoelectron spectroscopy, edited by D. Briggs, Heyden&Son Ltd, 1977.
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C. Bandis, L. Scudiero et al, Surface Science, 442 (1999) 413-419
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Dispersion MeasurementsFar more useful is the direct E ---> k
(dispersion) measurement obtained
by angular photoemission studies.
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By measuring the energy of the peak position with respect to theangle θ and using the expression k// α E sinθ. The dispersionrelation graph that plots the binding energy-wave vector dependencefor graphite can be obtained.
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The data obtained here (BE vsk// ) for the first Brillouin zoneagrees well with the calculatedbands of Painter and Ellis (G.S.Painter and D. E. Ellis, Phys. Rev.B1, 4747, 1970) shown here.
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Adsorption on solidsAdsorption of a monolayer of CO onpolycrystalline Pt and Ni.From this kind of data the followingschematic of molecular orbitals could beobtained:
5σ1π 4σ5σ, 1π and 4σ
contribute to the PE spectrum
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Polymer (P3HT) thermally deposited on clean Ag
18 16 14 12 10 8 6 4 2 0
2 1 0
2 1 0
Inte
nsity
(abi
trar
y un
its)
Binding Energy (eV)
P3HT thickness Ag foil 0.25 nm 0.32 nm 0.44 nm 0.75 nm 1.0 nm 1.9 nm 3.1 nm
0.56 eVEF
Clean Ag
P3HT 0.44 nmHOMO cutoff
0.8 eV
4 3 2 1 0
0.55 eV
Δ = 0.56 eV (interface dipole moment) and HOMO (VBM) = 0.55 eV
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L. Scudiero & al. applied Surface Sciences 255 (2009) 8593-8597
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Hybrid Nanocomposites for Electronics Applications(P3HT-Ag nanoparticles)
4.5 4 3.5 3 2.5 2 1.5 1 0.5 0Binding Energy (eV)
18 16 14 12 10 8 6 4 2 0Binding Energy (eV)
P3HT
Clean Ag foil
Δ
Δ3
P3HT
(c)
Δ2
Δ1
3:1
1:1
1:3
3:1
1:1
1:3
Ag
(b)(a)
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Blend film 3:1Blend film 1:1Blend film 1:3
0.55 eVHOB edge
Fermi‐level
Δ3(3:1)
ΦBlend film
(3.77 eV)
0.79 eV
Δ2(1:1)
ΦBlend film
(3.47 eV)
1.36 eV
Δ1(1:3)
ΦBlend film
(3.37 eV)
ΦP3HT film
(3.71 eV)
P3HT film
0.60 eV
0.56 eV
EF
Ag
Evac
ΦAg
(4.27 eV)
It is possible to tune the value of the barrier height ( ) from 0.55 to 1.36 eV by simply varying the composition of the blend film (P3HT/Ag ratio ranging from 3:1 to 1:3) and changing the electronic properties of nanocomposite materials.
L. Scudiero & al. ACS Applied Materials & Interfaces
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Combining UV-Vis measurements and/or computational chemistry to measure or calculate band gap of materials and UPS measurements of the HOMO (VB energy) a energy diagram can be easily drawn.
Example: C60(CN)2 deposited on semiconductor by spin coating for photovoltaic applications.UV-Vis measurements give a band gap energy = 2.18 eV, Calculation (TD-DFT) gives 1.99 eV (isolated molecule, energy difference between HOMO-LUMO).UPS measurements yield HOMO (valence band max) position at 1.1 eV.
~ 1.1 eV
~ 1.82 eV
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2nd Edition E. H. Rhoderick and R. H. Williams Oxford Science Publications
Metal- Semiconductor Contacts
Electron states in solids and at surfaces
Density of states for n-type Si (a)and band structure as a function ofdistance (b).
The important surface parameter forsemi-conductor is the electronaffinity χs defined as the differencein energy between an electron at restoutside the surface and an electron atthe bottom of the conduction bandjust inside the surface
I is the minimum energy needed to remove an electron from the VB.gs
EI += χ
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Band bending near the surfacecould be cause by either surfacestates on a free surface or ametal overlayer.
Typical PE spectrum
for n-type SC
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(Theoretical variation of cross-section).
Likelihood of ionization of anelectron from a given orbital in anatom.
Photoionization Cross-section of C
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1. Elemental dependency of the cross-section on photon energy for C, N and O.
2. Difference in cross-section for different orbitals for Phosphorus.
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References
• Handbook of x-ray and ultraviolet photoelectron spectroscopy, edited by D. Briggs, Heyden&Son Ltd, 1977.
• P.M. Williams same Handbook. p.313 – 340
• M. Thompson, P.A. Hewitt and D. S. Wooliscroft same Handbook, p341 -379
• B. L. Sharma, Plenum Press, New York