Vibrational HREELS spectra of atomic nitrogen and oxygen on Cu(100)

Surface Science 185 (1987) L467-L474 North-Holland, Amsterdam

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S U R F A C E SCIENCE LETTERS V I B R A T I O N A L H R E E L S SPECTRA O F A T O M I C N I T R O G E N AND OXYGEN O N Cu(100) Mohamed H. M O H A M E D and L.L. K E S M O D E L Department of Physics, Indiana University, Bloomington, IN 47405, USA

Received 28 November 1986; accepted for publication 25 February 1987

Vibrational HREELS spectra for a c(2x2) overlayer of atomic nitrogen on Cu(100) are reported for a nominal energy resolution of 5 meV at low primary beam energy(0 < E0 < 20 eV). The observed loss peaks are ascribed to surface resonances and the perpendicular and parallel modes of the Cu-N stretching vibration. The spectra of the Cu(100)-N system differ substantially from those of the related Cu(lll)-N system which was believed to reconstruct to Cu(100)c(2 × 2)-N domains. Vibrational spectra of atomic oxygenadsorbed on Cu(100) are also presented. In addition to the loss peak corresponding to the Cu-O perpendicular stretching vibration, a new loss peak is observed and is attributed to a surface resonance. Like the peak assigned to the Cu-O stretching vibration, the energy of this new peak is also found to be coveragedependent.

High resolution electron energy loss spectroscopy (HREELS) has over the past several years been shown to provide valuable information for the characterization of the adsorption of atoms and molecules on single crystals. Such information may be related to, for instance, adsorption sites, adsorption geometries and the strength of chemical bonding [1]. In this Letter the experimental results obtained for the room temperature adsorption of atomic nitrogen and oxygen on Cu(100) will be briefly presented. The adsorbate structures are already known from L E E D studies for both nitrogen [2] and oxygen [3]. In the case of oxygen adsorption on the Cu(100) surface limited H R E E L S experimental results are also available [4], whereas only the vibrational spectra of nitrogen adsorbed on C u ( l l l ) have been reported thus far [5]. The latter report suggested that, when atomic nitrogen adsorbs on the C u ( l l l ) surface, the surface reconstructs to give overlayers characterized by a complicated LEED pattern consisting of domains of Cu(100)c(2 × 2)-N. The reported vibrational spectra [5] are obtained from such a reconstructed C u ( l l l ) surface. In view of this, it is of interest to obtain the vibrational spectra of atomic nitrogen adsorbed on the Cu(100) surfaces. This will not only be interesting in its own right but will also provide an interesting comparison with those spectra obtained for the nitrided Cu(111). The study of the adsorption of oxygen on Cu(100) was motivated to a large extent by the results obtained following nitrogen adsorption. 0039-6028/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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M.H. Moharned, L.L. Kesmodel / HREELS of N and 0 on Cu(lO0)

The measurements were performed in an ultrahigh vacuum system with a base pressure of 6 × 10-11 Torr. The Cu(100) surface was cleaned by cycles of Ar + ion b o m b a r d m e n t followed by annealing at 400°C. Excellent L E E D patterns were obtained for the annealed Cu(100) surface, and Auger measurements revealed only trace amounts of carbon which, however, did not give rise to loss features in the vibrational spectra of the clean surface. The scattering plane is very nearly defined by the surface normal of the (100) face and by the {110} direction. The vibrational spectra to be presented were obtained using the H R E E L S spectrometer described previously [6] at an energy resolution of 5 meV (40 c m - a). Atomic nitrogen was adsorbed on an annealed Cu(100) surface by electron dissociation of highly pure molecular nitrogen. This was accomplished by operating an ion gun in an ambient nitrogen gas pressure of 4 × 10 5 Torr. The voltages of the ion gun and the crystal were +500 V and +700 V, respectively, thus exposing the Cu(100) surface to predominantly neutral nitrogen species [7]. The resulting L E E D pattern is c(2 × 2) but exhibits a high background. Annealing of the Cu(100)c(2 × 2)-N system to 2 5 0 ° C for 1 / 2 hour greatly improves the contrast in the LEED pattern and results in sharp diffraction spots. Adsorption of atomic oxygen on annealed Cu(100) was accomplished by exposing the crystal to oxygen at 2.0 x 10 - 7 Torr.

Adsorption of nitrogen on Cu(lO0). Tile vibrational spectra of the Cu(100)c(2 x 2)-N system exhibits loss peaks at 155, 207, 324, and 750 cm -1. Spectra showing these losses along with the scattering conditions are presented in figs. 1-3. As is evident from the figures, the losses at 207 and 750 cm -1 are observed only for q, 4:0 (off-specular) whereas those at 155 and 324 cm 1 are seen when qH = 0 (specular) as well. Fig. 4 shows the dependence of the intensities of the elastic and the 324 cm -1 peaks as a function of the angle from specular scattering. The decrease of the intensity of the 324 cm 1 peak approximately parallels the decrease of the intensity of the elastic peak and is indicative of a predominantly dipolar scattering. The feature at 155 cm -1 appears to have appreciable cross section only in the incident energy range 8.4 < E 0 < 14 eV while that at 207 cm-1 appears to be strong for E 0 between 8.4 and 20 eV. The loss features given above are observed after annealing the Cu(100)c(2 x 2)-N system obtained by adsorbing atomic nitrogen onto the annealed Cu(100) surface. Somewhat different vibrational spectra are observed, however, when the adsorption occurs on a sputtered Cu(100) surface, or if the Cu(100)c(2 x 2)-N system is not annealed. The vibrational losses obtained from these systems are presented in table 1. For these systems the c(2 x 2) L E E D pattern is still evident although the diffraction spots are considerably more diffuse. Loss features similar to those observed when nitrogen is adsorbed on annealed Cu(100), but not followed by post-adsorption annealing,

M.H. Mohamed, L.L. Kesmodel / HREELS I

I

I

o f N a n d 0 on C u ( l O 0 )

I

L469

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317

Cu (100) -N Eo = 4.54eV

if)

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x40~

0

\

\,

q II : O.OOA

I

I

400

2OO

600

I

800

ENERGY LOSS(cm q) Fig. 1. Specular and off-specular HREELS spectra for Cu(100)c(2 x 2)-N at E o = 4.54 eV. The elastic peak shown is for specular scattering.

i

i

i

~ j°5

i

Cu ( I 0 0 1 - N

.5 322

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I.-z >rr

x200

I

r,~ rn rr < >lZ Z

I

o-I

5A

i4 0

200

400

600

ENERGY LOSS (cm-I) Fig. 2. Specular and off-specular HREELS spectra for Cu(100)c(2 × 2)-N at E 0 = 10 eV. The elastic peak shown is for specular scattering.

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M.H. Mohamed, L.L. Kesmodel / HREELS of N and 0 on Cu(lO0) i

i

Cu ( I O 0 ) - N

u) I--

E o = 16eV

x40

515

Z >FF

r

= 0 3 /~

207

,<

58

>kZ m Z

\ I

1

l

200

400

~.

0

ENERGY LOSS (cm -~)

Fig. 3. Off-specular H R E E L S spectrum of Cu(100)c(2 x 2)-N at E 0 = 16 eV.

are also observed following Auger measurements (incident energy = 3 keV, emission current approximately equal to 100 #A) on annealed Cu(100)c(2 x 2)-N, and we have indicated them also in table 1. The vibrational spectra presented in figs. 1-3 are different from those reported by Higgs et al. [5] who observed peaks at 266, 403, 650, and 750 i

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I

ioi Cu(lOO)c(2x 2)-N

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A0 Fig. 4. Comparison of the intensities of the 324 cm ~ loss and the elastic peak as a function of the angle A0 (degree) from specular scattering for an incident beam energy of 4.54 eV.

M.H. Moharnea[ L.L. Kesmodel / HREELS of N and 0 on Cu(lO0)

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Table 1 Vibrational losses (cm-1) observed for nitrogen adsorption on differently prepared Cu(100) surfaces at E0 = 10 eV Surface preparation a)

Loss number 1

2

3

Q, (,~- 1 )

A B C D

b) b) 155 155

324 324 324 324

422 431 O 420

0.00 0.00 0.00 0.00

adsorption on sputtered Cu(100); B: adsorption on annealed Cu(100); C: adsorption on annealed Cu(100) followedby annealing of the Cu-N system; D: after AES measurementson a surface prepared according to C. b) Not resolved. ¢) Not observed. a) A:

cm -1. Although we also observe a loss at 750 cm -1 the two peaks are dissimilar since the peak observed by Higgs et al. [5] is observed in specular measurements whereas ours is apparent only in off-specular scattering. The peaks at 324 and 750 c m - 1 in fig. 1 are interpreted as the perpendicular and parallel modes of the C u - N stretching vibration. The peaks at 155 and 207 cm-1 which are shown in figs. 2 and 3, respectively, cannot be so readily assigned, since their energies are not higher than those expected for the substrate surface phonons and resonances [8]. The M point of the substrate surface Brillouin zone (SBZ) and the F point of the c(2 × 2) SBZ are equivalent which means that the surface phonons at M can be excited at the F point. But the slab calculation performed by Castiel et al. [9] shows at M only an S 1 phonon having an energy of approximately 114 cm -1, assuming correspondence between the maximum phonon energy of the Cu(100) slab calculation and that of the bulk phonon energy. Hence, the energy of the S 1 phonon is too low to be related to the loss peaks in question. Because the energy of the 155 and 207 cm -1 features are close to the energy of the surface projected bulk bands, we tentatively attribute them to surface resonances. The loss peak at - 425 c m - 1 is likely due to nitrogen adsorbing at defect sites. Adsorption of oxygen on Cu(lO0). T h e vibrational spectra for oxygen adsorbed on Cu(100) surface are presented in figs. 5 and 6. Fig. 5 shows two loss features at 339 and 287 c m - 1 while fig. 6 shows additional loss features at 169 and 155 cm -1. The peaks at 169 and 339 cm -1 are seen for a disordered oxygen overlayer while the peaks at 155 and 287 cm -1 are seen for a (¢~ × 2v~-)R45 ° oxygen overlayer. All of these features are observed in specular measurements. The peaks at 155 and 169 c m - ~ are being reported for the first time, whereas the loss peaks at 339 and 287 cm -~ have been reported previously by Sexton [4]. We also observe the same dependence of the energy values of the loss peaks on oxygen exposure as was discussed earlier [4].

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M.H. Mohamed, L.L. Kesmodel / HREELS of N and 0

1

i

1

x 700

on

Cu(lO0)

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Cu ( I 0 0 )

- 0

E o : 52eV qll =0

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<

>-

287

FZ uJ F-

z_

xl

/

/ 0

oo/1 ~

, 200

OROERED 400

600

ENERGY L O S S ( c m -j)

Fig. 5. Specular H R E E L S spectra of Cu(100)-O

at

E o = 5.2 eV.

Sexton [4] has attributed the peaks at 339 and 287 cm t to the C u - O stretching vibration corresponding to (v~- × v~-)R45 ° and (v~ × 2V~-)R45 o overlayers, respectively. While we agree with the interpretation given to the losses in question we found that the 339 cm -1 loss peak is seen only for the disordered oxygen overlayer, as evidenced by the lack of any LEED pattern. As the oxygen exposure is increased the 339 c m - I loss peak moves to lower energy loss values until its loss energy reaches 287 cm-1. The LEED pattern corresponding to the latter loss peak is found to be (v/2 × 2v~-)R45 °, an oxygen overlayer structure which has been reported by several investigators [3,10]. The 169 and 155 cm -1 peaks are thought to be related in a manner similar to that of 339 and 287 cm -1 peaks as the 169 cm -t peak is seen for the disordered oxygen overlayer while the peak at 155 cm -1 is seen for the ordered one and as the energy of the 169 cm-1 feature shifts to lower energy to become the 155 c m - 1 peak in the ( ~ - x 2v~-)R45 ° overlayer structure, just like the 339 cm -~ loss. According to the available substrate surface calcu-

M.H. Mohamea~ L.IL Kesmodel / HREELS of N and 0 on Cu(lO0)

L473

i

Cu (100)-0 EO= 6 e V qll = O~-I

(90

I--

3~)2

>nr" n" Fn." <( >.-

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Z LI.I F-Z

xlO0

/'-~

DS I ORDERED

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( J"~x 2 J2 )R 45*

\ 0

200

4O0

ENERGY LOSS (cm -I) Fig. 6. Specular HREELS spectra of Cu(100)-O at E o = 6 eV.

lations there exists no localized mode at - 1 6 0 cm -a. For this reason and because the energy of this feature is close to that of the surface projected bulk bands, we tentatively assign this loss to a surface resonance. This is analogous to our assignment of the 155 and 207 cm -1 loss features seen for nitrogen adsorption on Cu(100). It is clear, however, that these assignments can only be confirmed through calculations. We have presented the vibrational spectra resulting from the adsorption of atomic nitrogen and oxygen on Cu(100). For nitrogen adsorption, loss features arising from the C u - N stretching vibration as well as from surface resonances have been observed. These features are not in accord with those obtained after nitriding C u ( l l l ) which was reported to reconstruct to domains of Cu(100)c(2 × 2)-N [5]. For oxygen adsorption, we observe, in addition to the feature originating from the C u - O stretching vibration reported previously, a new feature which is ascribed to a surface resonance. The peak positions of both features are also found to depend on oxygen exposure.

L474

M.H. Mohamed, L.L. Kesmodel / H R E E L S of N and 0 on Cu(lO0)

This work was supported by the US Department of Energy. We are grateful to Dr. John Noonan of the Oak Ridge National Laboratory for providing the Cu(100) sample and to G.D. Waddill and M.K. Hosek for technical assistance.

References [1] See, for example, C.R. Brundle and M. Morowitz, Eds., Proc. of the 1982 Third Intern. Conf. on Vibrations at Surfaces (Elsevier, Amsterdam, 1983). [2] J.M. Burkstrand, G.G. Kleiman, G.G. Tibbetts and J.C. Tracy, J. Vacuum Sci. Technol. 13 (1976) 291. [3] P. Hofmann, R. Unwin, W. Wyrobisch and A.M. Bradshaw, Surface Sci. 72 (1978) 635. [4] B.A. Sexton, Surface Sci. 88 (1979) 299. [5] V. Higgs, P. Hollins, M.E. Pemple and J. Pritchard, J. Electron Spectrosc. Related Phenomena 39 (1986) 137. [6] L.L. Kesmodel, J. Vacuum Sci. Technol. A1 (1983) 1456. [7] R.N. Lee and H.E. Farnsworth, Surface Sci. 3 (1965) 461. [8] R.E. Allen, G.P. Alldredge and F.W. DeWette, Phys. Rev. B4 (1971) 1661. [9] D. Castiel, L. Dobrzynski and D. Spanjaard, Surface Sci. 59 (1976) 252. [10] J.H. Onuferko and D.P. Woodruff, Surface Sci. 95 (1980) 555.