13.3 Summary: AZO/C 60 interface and the UV influence
17.1.1 Valence band and gap state analysis of different titania samples
17.1 Identification of gap states in different titania samples
In Section 8.1 and Section 8.2 different titania samples were characterized by AFM, XRD/Laue diffraction and XPS. In this section the valence band and band gap region has been studied in detail, focusing on the appearance of gap states in amorphous ALD prepared TiOx(substrate temperature 80 °C), at 500 °C at air post annealed ALD prepared TiOx, nanocrystalline TiO2and cleaved anatase sc-(101) surface. Up to this work PES results were only published of surfaces prepared by sputter anneal cycling. The preparation of clean anatase single crystal surfaces under UHV conditions by sputter anneal cycling is e.g. facing the problem of iron segregation at the surface or a roughening of the TiO2surface.[238]Therefore a new method for in situ anatase preparation was developed in this work. The natural grown single crystal was cleavedin situ with pliers and by this a clean anatase surface was obtained under UHV conditions without any post treatment like sputtering and annealing. For details of the preparation process see Section 4.1.
Figure 17.1 shows various valence band (left) and band gap spectra (right) ofin situandex situanatase sc-(101) surfaces (spectra 1-14), nanocrystalline TiO2out of slurry (15), ALD prepared 500 °C annealed TiOx (16) and as deposited 80 °C ALD TiOx (17). Some cleavage surfaces had residues of carbon on the surfaces. The structure of the valence band varies at the different cleavage surfaces. In the band gap region two different band gap emission are observed, deep gap states (DGS) at around 1.2 eV and shallow gap states (SGS) arising just below EF and extending to about 0.6 eV binding energy. Sample 1 displays only SGS (see Fig. 17.8 spectrum B), while sample 17 displays only DGS (see Fig. 17.2 spectrum D). All other samples display DGS and SGS emissions.
Intensity [a.u.]
10 5 0
1 2 3 4 9
5 6 7 8 10 12 13 14 15
11 16 17
sc-(101) anatase Valence band
90eV norm to max
2.5 0.0
1 2 3 4 9
5 6 7 8 12 13 14 15
11 16 17
10
DGS SGS
sc-(101) anatase Band gap
90eV norm to max
ex-situ; TGM-7 in-situ 300K in-situ 300K in-situ 300K in-situ 300K in-situ 300K ex-situ 300K
ex-situ 300K in-situ90K in-situ 90K
in-situ 90K in-situ 300K in-situ 300K
in-situ 90K nc TiO2 300K
nc TiO2 300K 500°C ALD TiOx 300K 80°C ALD TiOx 300K
Binding energy [eV]
Figure 17.1.– Different valence band (left) and band gap (right) spectra ofin situandex situcleaved anatase sc-(101) surfaces compared to thin film TiO2prepared under different conditions. Spectrum1was recorded at the beamline TGM-7. All other spectra were recorded at the beamline U49-2/PGM-2. Spectrum15is nanocrystalline TiO2out of a slurry,16is ALD prepared at 80 °C and17is ALD prepared and annealed to 500 °C. Spectra were taken at 300 K and 90 K as indicated. All blue spectra are discussed later in more detail.
Intensity (arb.units)
2.5 0
Band gap region hn = 90eV
Intensity [a.u.]
12 8 4 0
VB Region hn = 90eV
B C D
nc ALD
ALD annealed
A
sc-(101)
Binding energy [a.u.]
= EF = EF
D
B A C D
(a) (b)
SGS DGS
VBM 1 = ~3eV VBM 2 = ~3.5eV
Figure 17.2.– Valence band region (a) and band gap (b) spectra of D: ALD prepared amorphous TiOx; C:
500 °C post annealed ALD TiOx; B: nanocrystalline anatase out of a slurry and A:in situcleaved anatase sc-(101) surface. Depending on the method of the VBM determination, the VBM is between 3 and 3.5 eV.
Amorphous ALD TiOxshows only DGS emission. All other titania show DGS and SGS emission.
176 17. Synchrotron induced study of gap states in TiO2
Intensity [a.u.]
3.0 2.0 1.0 0.0
Binding energy [eV]
Intensity [a.u.]
3.0 2.0 1.0 0.0
Binding energy [eV]
In-situ Anatase radiation influence
on gap states
A
A(I) TiOx ALD
radiation influence on gap states normal
photon flux
reduced photon flux
D
D(I)
time time
Figure 17.3. – Left: Amorphous ALD TiOxspectra affected by synchrotron radiation. The gray spectra (D(I)) is recorded with a reduced photon flux and increased step width. Black spectra (D) are recorded with standard source conditions. Initially no DGS are observed and they increase drastically with synchrotron irradiation. Right: Influence on thein situcleaved sc-(101) surface. DGS increase with synchrotron irradiation time, while SGS seem not to increase.
naturally grown anatase single crystals.[139] A survey scan of the single crystal ranging from 550 eV to -2 eV did not reveal any emission of species besides Ti and O and hence contaminations are not existent or below the detection level of PES (which is in the range of 1 vol %). In Fig. 17.2 B and C the valence band spectra of the nanocrystalline and post annealed ALD TiOx are displayed. Both are very similar to the single crystal anatase spectra, which suggest a complete transformation of the amorphous, 40 nm thick, ALD film to anatase after annealing. This is in contrast to the partial transformation evidenced by XRD on a 150 nm thick film. The reason could be a complete transformation of the 40 nm film or the presence of an amorphous phase deeper in the layer of the 150 nm film, which cannot be detected by the surface sensitive method of photoemission.
In Fig. 17.2b) a magnification of the band gap region of the four titania samples is displayed. The amorphous as prepared ALD TiOx sample (D) shows a strong emission from deep gap states (DGS) around 1.2 eV binding. The in air annealed ALD TiOxshows much smaller DGS emission, but addition-ally an emission of shallow gap states (SGS), which arises just below EF and extends to about 0.6 eV binding energy. The nanocrystalline sample (B) and the anatase single crystal (A) show similar gap state emissions of DGS and SGS as the post annealed ALD sample. On all ex situsamples (80 °C ALD, 500 °C ALD and nc-anatase) the increase of the O 2p derived valence band is less compared to thein situcleaved anatase crystal.
All PES spectra of the four investigated titania samples show an increase of the DGS emission with exposure time to synchrotron radiation as shown for non-annealed ALD titania (Fig. 17.3 left) and the sc-(101) cleavage plane (Fig. 17.3 right). In contrast the SGS emission of the sc-(101) surface is not or only slightly increasing upon synchrotron irradiation (see Fig. 17.3 right). Apparently the origin of the DGS emission does increase with synchrotron exposure, while the origin of SGS does not. The light gray spectra in Fig. 17.3 D(1) and A(1) represent initial spectra, where the influence of synchrotron irradiation was kept to a minimum. D(I) (Fig. 17.3 left) has been taken at the TGM-7 beamline with increased step
Intensity (arbit. unit)
462 460 458 456 454 452
Binding energy [eV]
Ti2p3/2 hv = 600 eV increase of Ti3+
Ti 3+
10s 450s
Figure 17.4.– Ti3+increase of an as deposited ALD prepared TiOxsample upon synchrotron irradiation. All spectra were recorded under same source conditions. First spectrum (black) was recorded after about 10 s irradiation time. The measurement was performed on a different sample, than displayed in Fig. 17.3 left.
width and reduced photon flux, while the other, exposure time dependent spectra, have then been taken with a smaller step width and the standard photon flux. With exposure to synchrotron light the DGS emission first grows fast till its intensity saturates after 3000 s of synchrotron radiation (spectrum D).
Along with the DGS emission, the Ti3+emission of the Ti 2p3/2core level line increases as well (Fig. 17.4).
Hence the DGS emission of the amorphous ALD titania layer can be assigned to occupied Ti 3d orbitals of TiOx due to missing oxygen as shown for crystalline anatase.[85] The results are in contrast to the interpretation of the results in Section 11.2, where it was assumed that oxygen vacancies are always present at the surface and not induced by electromagnetic radiation. One interpretation of the results in this chapter and Section 11.2 is that oxygen vacancies at the surface of ALD prepared TiOx are present, but adsorbed oxygen from the atmosphere is disguising the vacancies. Synchrotron radiation then causes the desorption of the attached oxygen. At 0 eV (=EF) and up to 0.5 eV below no emission is observed and no SGS are observed.
Thein situcleaved sc-(101) surface shows initially already a SGS emission, while the DGS emission is comparably low (see Fig. 17.3). The first spectrum in Fig. 17.3 right, A(I), was recorded with standard exposure conditions. To reach the binding energy region of the gap state region, the crystal was exposed to synchrotron radiation for about 30 s. After an additional synchrotron exposure time of 30 s the DGS intensity is strongly increased (see spectrum A). Thus the DGS emission in the initial spectrum A(I) is already influenced by the measurement and the initial DGS density of the cleaved sc-(101) surface may be zero or close to zero before the exposure to synchrotron radiation. The DGS emission of the sc-(101) surface is also very low compared to surfaces prepared by sputter anneal cycling.[85] STM measurements on sputter annealed sc-(101) surface did not show VO at the surface.[240,241]In situcleavage also avoids additional pitfalls of clean surface preparation, as e.g. surface segregation of contaminations present in natural crystals.[238] The increase of DGS on sc and nc anatase is generally attributed to oxygen vacancies caused by beam damage due to the synchrotron radiation.[86] Summarizing it is observed that SGS emission of the sc-(101) surface is not or only slightly increasing with synchrotron irradiation time, while DGS are strongly influenced by synchrotron light on amorphous and crystalline TiO2. SGS
178 17. Synchrotron induced study of gap states in TiO2
Intensity [a.u.]
1.2 0.8 0.4 0.0 -0.4 -0.8
Binding energy [eV]
B
A* (90 K) single crystal A (298 K) single crystal B (298 K) nanocrystalline
A*
A
exp. data of SGS Gauss fit Fermi fit Fermi-Gauss fit
Figure 17.5– Fit of the SGS emission (black dots) of thein situcleaved sc-(101) surface at 90 K sample temperature (A*), at 298 K (A) and a nc-TiO2at 298 K. Spectra were
subtracted by a polynomial background and a Gaussian fit of the DGS (see Fig. 17.6). The Fermi-Gauss fit (red) is composed of a Gaussian fit (blue) of the SGS emission and Fermi fit (gray) of the Fermi-edge. The fwhmF of the Gaussian like derivative of the Fermi part in the fits are 0.18, 0.23 and 0.27 eV for A*, A and B spectra.
are present only on crystalline anatase, be it the single crystal (101) cleavage plane or nanocrystalline anatase (also 500 °C post annealed anatase), while SGS occur not on amorphous ALD titania and cannot be induced by the photoemission light source.
Intensity (arb. unit)
3.0 2.0 1.0 0.0
Binding energy [eV]
sc-TiO2: Background substraction in-situ measured spectra
polynomial background
polynomial background + fitted DGS
Intensity [a.u.]
3.0 2.0 1.0 0.0
Binding energy [eV]
nc-TiO2: Background substraction measured spectra polynomial background
polynomial background + fitted DGS
Figure 17.6. – Background treatment of the nc-TiO2(left) and the sc-(101) anatase band gap region (right).
Background was subtracted by a polynomial fit. DGS were subtracted by a Gaussian-Lorentzian fit.
In addition to the performed experiments at room temperature, the in situ cleaved sc-(101) surface was also measured at 90 K temperature. In Fig. 17.5 the SGS emissions of sc-(101) taken at 90 K (A*) and at room temperature (RT) (A), and for nc anatase at RT (B) are displayed for further analysis. To facilitate a comparison of the SGS emission of the three measurements, DGS emission and secondary electron background have been subtracted from the experimental spectra by a Gaussian peak and a polynomial base line, respectively, thus the SGS emission is only left. The respective spectra of the background removal are shown in Fig. 17.6. The SGS emission is fitted using a Gaussian peak line multiplied by the Fermi distribution of state occupation. The maximum of the Gaussian is at around 0.13 eV binding energy and its fwhmG is 0.47 eV and 0.58 eV for the single crystal and nanocrystalline film respectively. ThefwhmF of the Gaussian like derivative of the Fermi edge fit broadens for the single
crystal from 0.17 eV at 90 K to 0.23 eV at 300 K. ThisfwhmF is composed (square root of sum of squares) of thermal broadening and overall photoemission resolution of around 0.1 eV. The mathematical fitting treatment and the fit parameters are given in the appendix in equation Eq. (1.3) and table Table B.1.
The temperature dependence of the low energy onset clearly demonstrates that the occupied SGS DOS and thus its appearance in the spectra is limited by a Fermi edge. The Gaussian part in the SGS fit is weakened as only the occupied part of the DOS appears in photoemission and the signal to noise ratio is low. Therefore it is not claimed that the fitted Gaussian distribution is the actual DOS of the SGS states, but merely is an approximation in accord to our measurements. The high binding energy part was modeled using an exponential tail as a trial function as well, but the resulting fit had less compliance, than the modeled Gaussian distribution.