Applications of transmission channeling for ADATOM location on metal surfaces

Nuclear Instruments and Methods in Physics Research B48 (1990) 334-338 North-Holland

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APPLICATIONS OF TRANSMISSION ON METAL SURFACES F. JENSEN,

I. STENSGAARD,

CHANNELING

F. BESENBACHER

FOR ADATOM

LOCATION

and K. MORTENSEN

Institute of Physics, University of Anrhus, DK-8ooO Aarhus C, Denmark

The present status of transmission channeling (TC) as a method for the determination of adsorption positions on crystal surfaces is assessed. Specific experimental problems (thin crystals, adsorption and detection) as well as problems related to the interpretation are discussed.

1. Introduction Over the past few years transmission channeling (TC) through thin crystals has been used successfully to study a number of adsorption geometries on metal surfaces. The following systems have been investigated: Ni(lOO)-H and Ni(lOO)-Te [l], Pd(lOO)-H [2], Ni(lll)-H [3], Pt(lll)-H [4], Pt(lll)-0 [5] and Ni(lOO)-0 [6]. It has been possible in each case to determine the detailed adsorption position as a function of coverage and to get information about the rms displacement amplitude of the adsorbate parallel to the surface. Table 1 summarizes the results. It should be noted that the determination of adsorption positions, especially for hydrogen, is a difficult challenge in surface science. Conventional techniques like low-energy electron diffraction (LEED), surface-ex-

tended X-ray absorption fine structure (SEXAFS) and medium-energy ion scattering (MEIS) have very little sensitivity to adsorbed hydrogen. Other techniques like electron energy loss spectroscopy (EELS), He diffraction and angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) derive the position information in a very indirect way. Since hydrogen adsorption is of extreme technological importance and constitutes the simplest adsorption system for tests of theoretical chemisorption schemes, there is an obvious need for a direct method like transmission channeling, where detailed position information may be obtained. The purpose of the present paper is to assess the present status of the technique with particular emphasis on the problems involved in TC experiments, whether of purely practical nature or of a more fundamental character.

Table 1 Summary of results Surface

Adsorb.

Ni(100) Ni(100) Pd(100)

D Te D D D D D 0 0 0 0

Ni(ll1) Pt(ll1) Ni(100)

Coverage a) 0.5 0.5 1.0 0.5 1.0 1.0 0.25 0.25 0.5 < 0.2

LEED

Site b,

Height [A]

P =’

(1X1) @2X2) 42x21

4FH 4FH 4FH 4FH 3FH *) FCC-3FH FCC-3FH FCC-3FH 4FH 4FH 4FH

0.5(l) 2.0(l) 0.45(10) 0.30(5) 0.80(10) 0.80(10) 0.58(4) O.SS(lO) O.SS(lO) 0.80(10) O.SS(lO)

0.23 0.17 0.25 0.25 0.25 0.25 0.29 0.16 0.15 0.15

P(1 x I) (2X2) (1 XI) (1 x 1) P(2 x 2) P(2 x 2) c(2X2) (1 X 1)

[Al

Ref.

[II 111 121 121 [31 [31 [41 [51 PI WI WI

a) Nominal coverage in monolayers. b, 4FH = fourfold hollow, 3FH = threefold hollow. c, The two-dimensional vibration amplitudes (parallel to the surface) are measured at = 120 K, except for Te on Ni(lOO), which was measured at room temperature. The uncertainties are 0.03-0.05 A, based on the assumption that the adsorbate is in the perfectly symmetric position. (It is difficult to distinguish between static displacement of 0.05-0.1 A and an enhanced vibration amplitude.) *) An equal distribution between FCC and HCP threefold sites. 0168-583X/90/$03.50 (North-Holland)

0 Elsevier Science Publishers B.V.

F: Jensen et al. / Applications

2. Transmission

channeling

of transmission

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3. Thin crystals In order to utilize the TC technique, free-standing single crystals of high quality and thicknesses af typiCally = Xl00 A must be produced. Although this is no trivial matter, it is fortunately possible in several cases. Excellent thin crystals of AI, Ni, Cu, Pd, Pt, Ag, Si and W have been produced [7]. Apart from the case of Si, where the thin crystal is made by an etch technique exploiting the different etch rates of pure and B doped material [S], the production process always involves epitaxial growth. In the very simplest case, e.g, Ni(lOO), the cryst+l is grown on radiation-hardened, cleaved rock-salt (100) surfaces and will therefore display a number of cleavage steps. In other cases the growth mode is mare intricate. Excellent Ni(ll1) crystals can be grown by sequential epitaxy of CaF, on Si(lll), N&l on CaF, and finally Ni on N&l f7]_ Following @taxi& growth, the thin crystal is separated from the substrate by a suitable chemical process and floated onto a ring made of the same material as the crystal. This leaves a free-standing area of = 10 mm*, sufficient for the TC experiments. The crystal is subsequently placed in a UHV goniometer and cleaned by standard procedures, i.e. sputtering-annealing and/ or oxidation-reduction cycles. There is of course a limit to how much sputtering is possible for such thin crystals.

In TC experiments a beam of light, energetic particles (e.g. 2 MeV Hei ions) is made incident on a thin crystal along a major axis. The particles will now be channeled via a series of correlated coll.isions with the atomic rows. At a certain depth the flux distribution in the channels has reached an equilibrium which can be calculated with some approximations. At the exit side of the crystal the particle flux therefore constitutes a probe of known periodicity and atomic-scale resolution. It is evident that the positions of adsorbates placed on the beam-exit side of the crystal can be determined by analyzing the angular dependence of the ion-adsorbate interaction yields for different directions of incidence (cf. fig. I)_ Important qualitative information, e.g. the distinction between possible types of sites, can be derived from the data without any calculations. Detailed information on the adsorption geometry requires comparison with calculated yields. The TC technique is quite analogous to lattice-location experiments performed with charmeling for bulk impurities in single crystals, but in general without the complications caused by populations of equivalent sites. There remains, however, a number of problems in the measurement and interpretation of the experimental angular scans.

113

CIOY

Detector

w

-2

-1

0

1

2

ANGLE

Thin CryStat

Fig. 1. Schematic two-dimens~onal iilustratian of how a particular adsorption site may be shadowed [S) or exposed (El for two different incident-beam direetkns in a transmission channeling experiment. Also shown are representative adsorbate angular yields together with the host dips for the two directions.

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In practice this does not pose a problem since the absolute amount of impurities diffusing/segregating out of the bulk is also small. Although the efforts involved are larger than for similar bulk crystals, the thin-crystal quality, as evidenced by mosaic spread and channeling minimum yields, is quite comparable to the quality of bulk crystals.

4. Adsorption In some cases the adsorbate is detected by a process which makes distinction between adsorbates placed on the beam-entrance and beam-exit sides difficult or even impossible. It is thus desirable to be able to adsorb on only one side of the thin crystal. For metal adsorbates, this is easily achieved, but for adsorption from the gas phase special precautions must be taken. In practice the crystal is placed close to a capillary-tube inlet system. It is possible in this way to achieve impingement rates at least two orders of magnitude higher than those caused by the genera1 increase in background pressure. Additionally the beam-entrance side can be passivated by pre-adsorption of a suitable adsorbate. Adsorption of CO will, e.g., totally passivate the Pt(ll1) surface for subsequent H adsorption. In general the demand for single-side adsorption is not a serious obstacle.

5. Detection Incident ions channeled through the crystal will have an energy loss which is smaller than the random energy loss. This means that special care has to be taken when the detection scheme (Rutherford backscattering (RBS), nuclear reaction analysis (NRA) or elastic recoil detection (ERD)) has an energy-dependent cross section. As an example, consider RBS detection of 1 MeV He+ ions transmitted through a 2000 A thick Ni crystal. The energy loss is about 140 keV in a random and maybe only 90 keV in one particular channeling direction. A perfectly random distribution of adsorbates would therefore give rise to an artifact in the form of a dip in the angular scan while an adsorbate situated in the centre of the channels would be detected with a fluxpeak yield about 10% too small. If the adsorbate detection has to be based on a cross section with a narrow resonance the effects may become even worse. It is thus preferable to work under conditions where the energy dependence is minimized. In the case of hydrogen adsorption it is convenient to work with deuterium instead of hydrogen and to employ ERD. The D (4He, D)4He cross section [9] has a huge resonance for forward recoils around 2130 keV but is virtually energy-independent around 1800 keV, where the cross

I

I

I

I

NI (100)-O

600 -

1 l

s w >

[llll

AXIS

0 RANDOM

= x 7j

500-

g

400-

,0 300 -

ENERGY(keV)

Fig. 2. Yield from the “O(p, u)‘~N nuclear reaction as a function of incident energy for oxygen adsorbed on the beamexit side of a = 800 A thick Ni(100) crystal. The reaction yield peaks at different energies because of the different energy losses for random and aligned incidence.

section is still = 500 mb. The resonance has therefore been used to determine absolute coverages (at direct incidence) while the flat portion of the resonance curve has been used for the angular scans in the transmission mode. If it is unavoidable to use a resonance (as in the case of r*O(p, a)r5N), energy scans at different angular positions may be necessary. This is illustrated in fig. 2, which shows the measured shift in necessary incident energy for transmitted particles to stay at the resonance.

6. Calculations The detailed adsorption height and rms displacements must be derived from comparisons with calculated angular scans. The channeling calculations have been performed within the continuum mode1 [lo] with the effects of dechanneling due to electronic and nuclear multiple scattering included via a diffusion-type equation [ll]. The program was written by Beth Nielsen [12]. To make the comparison as realistic as possible, the experimental angular scans are obtained as azimuthal averages for each tilt angle. It is difficult to investigate in detail how realistic the continuum-model program predicts the shape of the angular scan for a particular adsorption geometry, but a few tests can be done. The particle distribution for large transverse energies determines the bulk yield recorded at the exit side of the crystal. Fig. 3 shows the experimental and calculated angular scans for 857 keV protons incident along the [lOO] direction of a thin Ni(100) crystal. It is evident that there is good agreement in the “widths” (half-angles) of the scans but significant devi-

F. Jensen et al. / Applications of transmission channeling

1.2

I

I

I

1

I

I

NI (100)

ANGLE (degrees)

Fig. 3. Measured Ni yields (points) and calculated values (curve) for normal incidence on a =1800 A Ni(100) crystal. The Ni yields correspond to a depth equal to the crystal thickness.

at small tilt angles and in the shoulder region. (In some cases the deviations have been larger than in the present example.) The discrepancy in the region close to perfect alignment has been studied by Barrett [13] and found to be due to focusing effects and thermal-fluctuation-induced large-angle scatterings which are not properly accounted for in the continuum model. The effect is important only for sites which look substitutional along the direction considered. The deviations in the shoulder region are probably due to a lack of isotropy in transverse momentum space caused by planar effects [12]. In order to improve the agreement in these tilt angle intervals Monte Carlo type calculations could be considered. This type of calculation requires much more computer power but has recently become more feasible with more efficient machines. In fact, it has been possible to compare results from the above mentioned continuum model and a Monte Carlo calculation for lattice location experiments of B in Si [14]. Although some differences were observed, the agreement in the derived B lattice positions was surprisingly good, lending further confidence in the continuum model calculations.

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many cases because the adsorption of, e.g., hydrogen changes the surface relaxation by negligible amounts relative to the experimental uncertainties in the determined height (typically about or less than 0.1 A). In fact, the adsorption of hydrogen tends to diminish the clean-crystal relaxation in many cases. The problem is more serious in other respects. Since the position is determined relative to the bulk lattice we cannot distinguish between surface and subsurface (or even bulk) sites from the TC experiments alone. This is a general problem in surface crystallography. In most cases the bulk position of the adsorbate is known (and different from the derived position). It is then possible to estimate an upper limit on the fraction of the coverage which may be located subsurface. In a few cases where complicated, unsolved surface reconstructions exist (as on certain semiconductor surfaces) the position of the adsorbate relative to the bulk lattice may actually be an important parameter in the solution of the geometry, but in genera1 the different basis is a definite drawback inherent to TC.

ations

8. Conclusions This paper has focused attention on the difficulties involved in employing transmission channeling as a tool for the determination of adsorption geometries. As discussed above, most of these difficulties are of a purely practical nature and can be overcome with some experience. The strength of the method is that it works in real space (in contrast to diffraction techniques), that quite detailed position information can be derived from the angular scans without any calculations, that light adsorbates such as, e.g., hydrogen can be investigated and, finally, that information about vibrational properties can be derived. Further developments in applications, such as studies of molecular adsorbates, are envisaged. We are grateful the thin crystals.

to Jacques

Chevallier

for growing

References 7. Adsorption position

In surface crystallography the major interest is to determine positions of adsorbates relative to the first substrate layer(s). Unfortunately, the TC experiments yield the position relative to the bulk lattice because the flux distribution is determined by the bulk. If the interlayer distances in the surface differ from the bulk value, the relaxation must be determined separately in order to pinpoint the height relative to the surface layer. Fortunately, such a distinction becomes u~mportant in

[I] 1. Stensgaard and F. Jakobsen, Phys. Rev. Lett. 54 (1985) 711; I. Stensgaard, Nucl. Instr. and Meth. B15 (1986) 300. [2] F. Besenbacher, I. Stensgaard and K. Mortensen, Surf. Sci. 191 (1987) 288. [3] K. Mortensen, F. Besenbacher, I. Stensgaard and W.R. Wampler, Surf. Sci. 205 (1988) 433. 141 K. Mortensen, F. Besenbacher, I. Stensgaard and C. Klink, Surf. Sci. 211/212 (1989) 813. [5] K. Mortensen, C. Klink, F. Jensen, F. Besenbacher and I. Stensgaard, Surf. Sci. 220 (1989) L701. v. SURFACE

PHENOMENA

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[6] F. Jensen, I. Stensgaard, (71 @] [9] [IO] [ll]

F. Besenbacber and C. Khnk, to be published AB crystals used have been produced by J. Chevallier, Inst. of Physics, University of Aarhus. R.M. Finne and D.L. Klein, J. Electrochem. Sot. 114 (1967) 965. F. Besenbacher, I. Stensgaard and P. Vase, Nucl. Instr. and Meth. B15 (1986) 459. J. Lindhard, K. Dan. Vidensk. Selsk. Mat. Fys. Medd. 34, no. 14 (1965). E. Bonderup, H. Esbensen, J.U. Andersen and H. Schiott, Radiat. Eff. 12 (1972) 261.

1121 B. Beth Nielsen, Phys. Rev. B37 (1988) 6353; B. Beth Nielsen, private communication; J.U. Andersen and U. Uguzzoni, these Proceedings (13th Int. Conf. on Atomic Co&ions in Solids, Aarhus, Denmark, 1989) Nucl. Instr. and Meth. B48 (1990) 181. [13] J.H. Barrett, F. Fujimoto, K. Komaki and Y. Hashimoto, Radiat. Eff. 28 (1976) 119. [14] P.J.M. Smuiders, D.O. Boerma, B. Beth Nielsen and M.L. Swanson, to be published.