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Structures of the nitrogen monolayer adsorbed on MgO(100)
ELSEVIER
Surface Science 377-379
(1997) 38+l4
Structures of the nitrogen monolayer adsorbed on MgO( 100) M. Trabelsi a,*, J.P. Coulomb b, D. Degenhardt ‘, H. Lauter ’ aFacultk des Sciences de Bizerte, Dkpartement de Physique, 7021 Jarzoma Bizerte, Tunisia b Fact& des Sciences de Luminy, Dipartement de Physique, Case 901, 13288 Marseille Cedex 9, France ’Institut Laiie Langevin, B.P. 156 X, 38042 Grenoble Cedex, France Received
1 August
1996; accepted
for publication
15 October
1996
Abstract The monolayer structure of N2 molecules adsorbed upon a highly homogeneous MgO powder surface has been studied by means of neutron diffraction techniques within the lo-60 K temperature range. At T= 10 K the submonolayer undergoes a series of semicommensurate structures p(2 x n) followed by a highly ordered commensurate structure (V% x V?$)-rot.33.7” observed at the monolayer completion.
Keywords: Neutron
scattering;
Polar films; Surface
thermodynamics
1. Introduction
The natural (2d) arrangement for rare gases and spherical molecules is the hexagonal close packed one, as observed on graphite basal planes [ 1,2]. Adsorbed on substrates whose potential wells present a different symmetry, they will provide original (2d) structures. The square symmetry of the potential wells lattice of the MgO( 100) surface acts as an external “frustrating” field for the molecular positional order of adsorbed monolayer. Thus, a square commensurate c(2 x 2) (2d) solid and a rectangular centered commensurate p(2 x 3) (2d) solid are observed respectively for CH4 [3] and Ar [4,5]. To investigate the second original property of the MgO(100) surface, which is the periodic electrical field E, due to the ionic character of the surface ( Mg’ +, O”-), polar molecules are required. * Corresponding 0039-6028/97/$17.00
author. Copyright
PZI SOO39-6028(96)01324-6
With its simple form and appreciable quadrupole moment Q= 1.5 DA-‘, the N2 molecule was chosen. In addition to the van der Waals interaction and the quadrupole-quadrupole interaction, which are responsible for the different orientationally ordered nitrogen monolayer phases observed on the graphite basal plane at low temperature, a supplementary attractive potential U, must be considered. U,, due to the coupling of periodic electrical field gradient of the MgO( 100) surface with the permanent quadrupole moment Q of the N2 molecules, will disturb the low temperature natural orientational order of the nitrogen molecules. A previous thermodynamic study, by measuring adsorption isotherms [6], showed that the nitrogen film grows layer by layer at least up to the fourth layer. Structural investigations have been performed at the monolayer and the multilayer Glms. Evolution of the monolayer structures with the temperature was also studied. But we report here just the
0 1997 Elsevier Science B.V. All rights reserved
M. Trabelsi et al. / Surface Science.377-379
monolayer low temperature nitrogen (2d) structures since the allowed space is limited. The only experimental study of the nitrogen monolayers on MgO is that of Angot and Suzanne [7] where they show, by low energy electron diffraction (LEED), a poorly ordered phase at submonolayer coverage which becomes a more ordered (2 x n) commensurate (2d) solid at monolayer completion (n 2 3) indicating a uniaxial compression along the [Ol ] direction. Many theoretical [S-11] and experimental [1214] studies, dealing with the orientational order in nitrogen monolayers adsorbed on graphite at low temperature, have been carried out. To make comparison easy we present here the results for the NJgraphite system as summarized by Wang et al. [ 121. Three different orientational ordered monolayer phases have been observed at low temperature (TI 11 K). (1) a rectangular commensurate (C) ti x 3 herringbone structure with the N-N bonds lying on graphite surface at a coverage 19= 1 layer, (2) a uniaxial incommensurate herringbone structure (UI) observed at 8= 1.13 layers, (3) a triangular incommensurate structure (TI) determined at the fully compressed monolayer phase 8 = 1.67 layers.
2. Experimental
description
The neutron diffraction experiments were performed with a two-axis spectrometer DlB, equipped with a multidetector and a cryostate allowing temperature variation from 2 to 300 K, located at the Institut Latie Langevin, Grenoble, France. The multidetector subtended a scattering angle 26’of 80” and the incident neutron wavelength is ;1= 2.52 A. The sample consisted of MgO powders, prepared by burning Mg ribbons in a controlled atmosphere as described in [ 151. Such powders are essentially formed of micro-cubes delimited by the (100) faces. An isotherm of Kr or CH4 measured at 77 K is used firstly as a probe of the homogeneity of our substrate and secondly to calibrate our sample. The completion of the monolayer was
(1997) 38-44
39
taken to be at the inflexion point of the isotherm plateau (Qads= 19.3 cm3 STP for the used sample). In the figures the measured difference spectra (with and without adsorbate) are represented by crosses (+) and the solid curves represent the profile of the calculated relevant crystallographic structure factors. Such structure factors are calculated explicitly from the nuclear position of each atom in the unit cell. The thermal motion of the nuclei around their equilibrium position is assumed to be small enough at T= 10 K and is accounted for by a Debye-Waller factor exp (- I) where the mean square displacement (u”) =O.Ol A” is taken to be the same for all nuclei. The overlap of Bragg peaks results from the asymmetric Roland and Tompa line shape of the peaks, the dimension of the two dimensional (2d) crystallites L = 300 A and the finite Q resolution of the spectrometer.
3. Neutron diffraction as a technique of thin films structure study Neutron diffraction scattering and low energy electron diffraction (LEED) can be considered as two complementary techniques used in structural studies of physisorbed films, while LEED experiments give the (2d) unit cell and the epitaxial relation of the adsorbed films structures with the surface substrate. The neutrons, whose interaction with matter is so weak that it does not perturb the adsorbed film, allow investigation of positional and orientational order of adsorbed molecules in the unit cell. By measuring the neutron diffraction spectrum and the difference spectrum, one can deduce: (1) the position and orientation of molecules in the unit cell by peaks intensities comparison, (2) the range order along each peak direction from the full width at middle height of the peak (FWMH), (3) the parameters (a, b, y) of the (2d) unit cell from the scattering vectors position, which means that the accuracy of the unit cell parameters depends on the peaks number. Generally, more than one cell indexed the observed peaks and there the information given
40
M. Trabelsi et al. / Surface Science 377-379 Nz/ MgO
T=lO K
Cov. = 1.1 layer
i
z$-
L s 2
(1997) 38-44
1500
3 c g
1000
E
500
/
I
I
Cov.
400
300
200
/
I
= 0.8 layer
1
100
0
I
I
1.6
I
2.0
I
I
2.4
Q(-@)
Fig. 1. Neutron diffraction spectra from 0.8 and 1.1 layers of N, adsorbed on MgO at 10 K after subtraction The solid curves have been calculated for the cell parameters listed in Table 1.
by LEED experiments is of major interest to determine the unit cell. Other data like molecular size are also important.
4. Results and discussion: The structures of the N, (2d) film condensed on the MgO( 100) surface were investigated by means of neutron diffraction experiments. The spectra measured at T= 10 K are of two kinds: those observed at the submonolayer coverage (SC) (semi-commensurate solids with two principal peaks) and those measured at the monolayer completion (HC) (high order commensurate solid with
of the MgO background.
three peaks). As shown in Fig. 1, the peak at the wave vector Q= 2.11 A-’ is independent of the coverage. It corresponds to the distance (d= 2.98 A) between the Mg2+ channels of the MgO( 100) surface. It means that in the previous (2d) solids the center of mass of the N, molecules lies along the Mg2+ channels of the MgO(100) surface. For the spectra measured at the submonolayer regime, the peak observed at lower wave vector shifted upward in Q as the coverage increased (Fig. 2). We deduce that the (2d) nitrogen solid undergoes a uniaxial compression along the channels as the coverage increases. This effect is quantitatively expressed in Fig. 3 where the nearest
M. Tvabelsi et al. / Surface Science 377-379 NJ MgO
41
(1997) 38-44
52
T=lOK
5.0
‘P 4.4
\
P-
P P- ..-
4,2
4.0 -l0.6
I-“’
0.7
0.8
0.9
1,O
1.1
1.2
1.3
I,4
Coverage Fig. 3. Nearest neighbour distance 4, (A) of the N, molecules along the MgO( 100) surface channels versus the coverage.
8 =0.8 +
-++++-Jh
I
+ +**
+
+++
I
1.6
.I.+
+
++.*m++
7’
I
1.8
2.0
CICA-1)
Fig. 2. Low wave vectors Q position variation with the coverage range. From 0.8 to 1.0 layer the wave vector position is dependent on the coverage indicating a continuous compression along the [Ol] surface direction; at the monolayer completion 62 1.1 layers a high order (2d) commensurate structure is stabilized by the MgO(100) surface. The apparent broadened peaks observed on the spectrum respectively measured at 0.9 and 1.1 layers are due to the different Q resolution of the spectrometer since they have been measured in preliminary experiment. Nevertheless they show the reproducibility of the structures.
neighbour molecular distance variation d,, (A), along the channels direction, with the coverage range, is represented. The stability of the HC nitrogen (2d) solid in the coverage range 1.1 I 8 I 1.3 layer (Fig. 2) is a primary indication of a registered phase. The profiles which we have compared with the two low temperature neutron diffraction spectra
for 0, respectively equal to 0.8 and 1.2 layers are calculated with the unit cells whose parameters are given in Table 1. The molecule orientation in the unit cell, governed by the azimuthal and tilt out of plane angles 4 and /I respectively is given also in Table 1, and corresponds to the calculated fits which represent best the experimental spectra. Indeed, we systematically varied the orientational parameters 4 and p to obtain the best fit in the Q range from 1.5 to 2.5 A-’ (Fig. 4). The reproduction of the unit cells on the (100) MgO surface indicates that the SC and HC are (2d) p(2 x n) structures in accordance with LEED measurements [7]. Nevertheless, even at the monolayer completion, no locked structure is observed by LEED. This possibility induced the same orientation for all the N, molecules forming the adsorbed layer. Such a configuration, which seems to be incoherent with the quadrupole-quadrupole interaction, is evoked in the case of a strong electrical field at the surface of the substrate [ 161. We think that the potential, due to the interaction of the
42
M. Trabelsi et al. / Surface Science 377-379
(1997) 38-44
Table 1 Structural parameters (a, b, 7) of the SC and HC unit cells. The uncertainty of the orientational (2d) solid
Coverage
Unit cell parameters
SC HC 61(111)
0.8 layers 1.2 layers Bulk Bulk
a(&=3.84 a(&=3.78 a(A)=3.99 a(A)=4.04
8(111)
b(&=3.84 b(A)=3.58 b(A)=3.99 b(&=4.04
e = 00
yt”)=lOl.6 y(“)=108.3 y(“)=120 y(“)=120
cp= 00
parameters is about ,5”
P(“)
9(“)
SM(z@)
d,,(A)
Commensurability
0 60
60 30
14.45 12.84 13.80 14.20
4.86 4.30
P(2 x a) (fl x fi)-rot.33.7”
e = 300
ql =
e = 300
ql = 300
e=
CD =
900
I
I$/
MgO
Cov. = 1.2
T=
++
Q%
+
IOK
+
It+ %
+ +
layer
Y, ;; +“9 + + t*$ Aa+ +#+
+
++c$+:
++++ +tr,+ 4++
*+
H+++++
e=
1.6
900
@ = 00
2.4
2.0 Q ( A-’ )
8 1.6
900
90"
I
2.0
2.4 Q ( A-j )
Fig. 4. Calculated neutron diffraction spectra for several nitrogen molecular orientations in the HC unit mesh (a = 3.78 A; b = 3.58 A; y= 108.3”); 0 and 4 are respectively the polar and azimuthal angles (in text B=90”-0). Best fit to the experimental spectrum is obtained for the following angular values 0 = 30” and $ = 30”.
adsorbed molecules quadrupole moments with the gradient of the MgO( 100) surface electric field, is important enough to impose the molecular orientation.
The evolution with the temperature of the neutron diffraction spectra of the nitrogen (2d) Urns (not represented here) shows two important results:
M. Trabelsi et al. / Surface Science 377-379
(1) The low coverage (8=0.8 layers) spectra. The peak at Q = 1.71 A- 1 becomes broadened between 30 and 40 K indicating a loose order along the Mg2+ channels for the SC nitrogen (2d) solid phase, while the peak at &=2.11 A-’ is unchanged with the temperature increasing. We think that with increasing the temperature the p( 2 x n) semi-commensurate structures are characterized by two kinds of order, a short range order in the channel direction and a long range order in the perpendicular direction. (2) The second one relative to the monolayer completion coverage (0 = 1.2 layers), reveals the stability of the nitrogen (2d) condensed phase structure HC in the temperature range 10 I Ti 30 K, coupled with the primary indication of a registered phase, mentioned above. We can unambiguously conjecture that the HC (2d) nitrogen solid Glm presents a commensurate phase with the MgO( 100) surface: precisely the (a=b=10.73 A; y=90”) fl x ViXrot.33.7” which is represented in Fig. 5. While the potential channels of the MgO( 100) surface, manifested by the sharpness of the peak at Q=2.11 A-‘, impose the positional order in the adsorbed layer, the interaction of the electrical field of the MgO( 100) surface with the quadrupole moment of the N2 molecules seem to impose an orientational order which is different from the (111) and (100) bulk planes and equally different from the structures of the condensed films on graphite. In effect the nitrogen film condensed on MgO( 100) surface undergoes a series of uniaxial compressed phases to end in a locked structure at the monolayer completion. This behaviour is reversed on graphite where the N, film crystallize in a commensurate phase c(fi x 3) followed by a compression along the a axis with the coverage increasing. Additionally in the herringbone structures of the N, film condensed on graphite the molecules’ orientation is at least partially governed by the quadrupole-quadrupole interaction, whereas in the various (2d) structures observed on MgO ( 100) surface the orientation of the N, molecules is determined by the electrostatic interaction which seems to be more important than the quadrupole-quadrupole interaction.
43
(1997) 38-44
di2
x dC_X - rot. 33.7”
commensurate
a = 10.73
W
b = 10.73
a
structure
Y =900
Fig. 5. Schematic representation of the nitrogen (2d) Ehn in the coverage range 1.1-1.3 layer. The N2 (2d) solid is “locked” in the commensurate stucture: m x fl-rot.33.7” (a= 10.73 A; b=10.73A; ?=90”).
5. Summary and conclusion Neutron diffraction experiments on Nz molecules adsorbed onto the MgO( 100) surface have been performed at coverage range 0.6-1.2 layer and T= 10 K. The neutron spectra show two varieties of (2d) structures. In the submonolayer coverp(2 x n) structures, age, semi-commensurate stabilized by the potential channels of the MgO( 100) surface, have been observed and at the monolayer completion a high order fix flrot.33.7” commensurate structure is determined. The low temperature phase structures of N, and Ar on MgO bear some resemblance. Argon also has a p(2 x 3) commensurate structure at submonolayer coverages followed by an hcp one at the
44
M. Trabelsi et al. /Surface
monolayer completion. While the commensurate Ar (2d) structure is stable in a large coverage range 0.3 r010.8 layers [6], the N, (SC) one undergoes a uniaxial compression along the channels. The compression, which is about 11.5% for a coverage range going from 0.6 to 1.1 layers, is larger than that observed on the graphite, along the a direction, which is about 3.2% for a coverage ranging from 1.0 to 1.27 layer [12]. The influence of the MgO( 100) surface periodic electrical field on the thermodynamical properties of condensed phases has been well studied [17211. Structural investigations which are in progress, C,H,/MgO [21,22], CO/MgO c20], CO,/MgO [23] and recently H,O/MgO [ 24,251, must be continued at least for two reasons: firstly to complete the thermodynamical investigations and secondly in order to clarify the influence of the electrical field on the orientational order in physisorbed film structures.
References [I] J.K. Kjerns, L. Passel, H. Taub, J.G. Dash and A.D. Novaco, Phys. Rev. 13 (1976) 1446. [2] P. Vora, SK. Sinha and R.K. Crawford, Phys. Rev. Lett. 43 (1979) 704. [3] J.P. Coulomb, K. Madih, B. Croset and H. Lamer, Phys. Rev. Lett. 54 (1985) 1536. [4] D. Degenhardt, H. Lauter and J.P. Coulomb, to be published.
Science 377-379 (1997) 38-44
[5] J.P. Coulomb, in: Phase transitions in surface films II, NATO-AS& Series B 267, Plenum 113 (1991). [6] K. Madih, Thesis, University of Aix-Marseille II, France, 1986. [7] T. Angot and J. Suzanne, Surf. Sci. Vol. 24 (1991) 671. [8] B. Kuchta and R.D. Etters, Phys. Rev. B 36 (1987) 3400. [9] A.B. Harris and A.J. Berlinsky, Can. J. Phys. 57 (1979) 1852. [lo] C. Peters and M.L. Klein, Mol. Phys. 54 (1985) 895. [ll] C. Peters and M.L. Klein, Phys. Rev. B 32 (1985) 6077. [ 121 R. Wang, SK. Wang, H. Taub, J.C. Newton and H. Shechter, Phys. Rev. B 35 (1987) 5841. [13] J. Eckert, W.D. Ellenson, J.E. Hastings and L. Passel, Phys. Rev. Lett. 43 (1979) 1329. [14] H. You and SC. Fain, Jr., Faraday Disc. Chem. Sot. 81 (1985) 159. [15] J.P. Coulomb and O.E. Vilches, J. Phys. Paris 45 (1984) 1381. [ 161 H. You and SC. Fain, Phys. Rev. B 34 (1986) 2840. [17] T. Angot, Thesis, University of Aix-Marseille II, France, 1990. [18] M. Trabelsi, K. Madih and J.P. Coulomb, Phase Transition 30 (1991) 103. 1191M. Trabelsi, Thesis, University of Aix-Marseille II, France, 1991. 1201P. Audibert, M. Sidoumou and J. Suzanne, Surf. Sci. 467 (1992) L273. [21] D. Ferry and J. Suzanne, Surf. Sci. 19 (1996) L345. [22] J.P. Coulomb, Y. Larher, M. Trabelsi and I. Mirebeau, Mol. Phys. 81 (1994) 1259. [23] J. Suzanne, V. Panella, D. Ferry and M. Sidoumou, Surf. Sci. 912 (1993) L293. [24] D. Ferry, A. Glebov, V. Senz, J. Suzanne, J.P. Toe&es and H. Weiss, to be published. 1251M.H. Moulin, Thesis, University of Aix-Marseille II, France, in preparation.
Surface Science 377-379
(1997) 38+l4
Structures of the nitrogen monolayer adsorbed on MgO( 100) M. Trabelsi a,*, J.P. Coulomb b, D. Degenhardt ‘, H. Lauter ’ aFacultk des Sciences de Bizerte, Dkpartement de Physique, 7021 Jarzoma Bizerte, Tunisia b Fact& des Sciences de Luminy, Dipartement de Physique, Case 901, 13288 Marseille Cedex 9, France ’Institut Laiie Langevin, B.P. 156 X, 38042 Grenoble Cedex, France Received
1 August
1996; accepted
for publication
15 October
1996
Abstract The monolayer structure of N2 molecules adsorbed upon a highly homogeneous MgO powder surface has been studied by means of neutron diffraction techniques within the lo-60 K temperature range. At T= 10 K the submonolayer undergoes a series of semicommensurate structures p(2 x n) followed by a highly ordered commensurate structure (V% x V?$)-rot.33.7” observed at the monolayer completion.
Keywords: Neutron
scattering;
Polar films; Surface
thermodynamics
1. Introduction
The natural (2d) arrangement for rare gases and spherical molecules is the hexagonal close packed one, as observed on graphite basal planes [ 1,2]. Adsorbed on substrates whose potential wells present a different symmetry, they will provide original (2d) structures. The square symmetry of the potential wells lattice of the MgO( 100) surface acts as an external “frustrating” field for the molecular positional order of adsorbed monolayer. Thus, a square commensurate c(2 x 2) (2d) solid and a rectangular centered commensurate p(2 x 3) (2d) solid are observed respectively for CH4 [3] and Ar [4,5]. To investigate the second original property of the MgO(100) surface, which is the periodic electrical field E, due to the ionic character of the surface ( Mg’ +, O”-), polar molecules are required. * Corresponding 0039-6028/97/$17.00
author. Copyright
PZI SOO39-6028(96)01324-6
With its simple form and appreciable quadrupole moment Q= 1.5 DA-‘, the N2 molecule was chosen. In addition to the van der Waals interaction and the quadrupole-quadrupole interaction, which are responsible for the different orientationally ordered nitrogen monolayer phases observed on the graphite basal plane at low temperature, a supplementary attractive potential U, must be considered. U,, due to the coupling of periodic electrical field gradient of the MgO( 100) surface with the permanent quadrupole moment Q of the N2 molecules, will disturb the low temperature natural orientational order of the nitrogen molecules. A previous thermodynamic study, by measuring adsorption isotherms [6], showed that the nitrogen film grows layer by layer at least up to the fourth layer. Structural investigations have been performed at the monolayer and the multilayer Glms. Evolution of the monolayer structures with the temperature was also studied. But we report here just the
0 1997 Elsevier Science B.V. All rights reserved
M. Trabelsi et al. / Surface Science.377-379
monolayer low temperature nitrogen (2d) structures since the allowed space is limited. The only experimental study of the nitrogen monolayers on MgO is that of Angot and Suzanne [7] where they show, by low energy electron diffraction (LEED), a poorly ordered phase at submonolayer coverage which becomes a more ordered (2 x n) commensurate (2d) solid at monolayer completion (n 2 3) indicating a uniaxial compression along the [Ol ] direction. Many theoretical [S-11] and experimental [1214] studies, dealing with the orientational order in nitrogen monolayers adsorbed on graphite at low temperature, have been carried out. To make comparison easy we present here the results for the NJgraphite system as summarized by Wang et al. [ 121. Three different orientational ordered monolayer phases have been observed at low temperature (TI 11 K). (1) a rectangular commensurate (C) ti x 3 herringbone structure with the N-N bonds lying on graphite surface at a coverage 19= 1 layer, (2) a uniaxial incommensurate herringbone structure (UI) observed at 8= 1.13 layers, (3) a triangular incommensurate structure (TI) determined at the fully compressed monolayer phase 8 = 1.67 layers.
2. Experimental
description
The neutron diffraction experiments were performed with a two-axis spectrometer DlB, equipped with a multidetector and a cryostate allowing temperature variation from 2 to 300 K, located at the Institut Latie Langevin, Grenoble, France. The multidetector subtended a scattering angle 26’of 80” and the incident neutron wavelength is ;1= 2.52 A. The sample consisted of MgO powders, prepared by burning Mg ribbons in a controlled atmosphere as described in [ 151. Such powders are essentially formed of micro-cubes delimited by the (100) faces. An isotherm of Kr or CH4 measured at 77 K is used firstly as a probe of the homogeneity of our substrate and secondly to calibrate our sample. The completion of the monolayer was
(1997) 38-44
39
taken to be at the inflexion point of the isotherm plateau (Qads= 19.3 cm3 STP for the used sample). In the figures the measured difference spectra (with and without adsorbate) are represented by crosses (+) and the solid curves represent the profile of the calculated relevant crystallographic structure factors. Such structure factors are calculated explicitly from the nuclear position of each atom in the unit cell. The thermal motion of the nuclei around their equilibrium position is assumed to be small enough at T= 10 K and is accounted for by a Debye-Waller factor exp (- I) where the mean square displacement (u”) =O.Ol A” is taken to be the same for all nuclei. The overlap of Bragg peaks results from the asymmetric Roland and Tompa line shape of the peaks, the dimension of the two dimensional (2d) crystallites L = 300 A and the finite Q resolution of the spectrometer.
3. Neutron diffraction as a technique of thin films structure study Neutron diffraction scattering and low energy electron diffraction (LEED) can be considered as two complementary techniques used in structural studies of physisorbed films, while LEED experiments give the (2d) unit cell and the epitaxial relation of the adsorbed films structures with the surface substrate. The neutrons, whose interaction with matter is so weak that it does not perturb the adsorbed film, allow investigation of positional and orientational order of adsorbed molecules in the unit cell. By measuring the neutron diffraction spectrum and the difference spectrum, one can deduce: (1) the position and orientation of molecules in the unit cell by peaks intensities comparison, (2) the range order along each peak direction from the full width at middle height of the peak (FWMH), (3) the parameters (a, b, y) of the (2d) unit cell from the scattering vectors position, which means that the accuracy of the unit cell parameters depends on the peaks number. Generally, more than one cell indexed the observed peaks and there the information given
40
M. Trabelsi et al. / Surface Science 377-379 Nz/ MgO
T=lO K
Cov. = 1.1 layer
i
z$-
L s 2
(1997) 38-44
1500
3 c g
1000
E
500
/
I
I
Cov.
400
300
200
/
I
= 0.8 layer
1
100
0
I
I
1.6
I
2.0
I
I
2.4
Q(-@)
Fig. 1. Neutron diffraction spectra from 0.8 and 1.1 layers of N, adsorbed on MgO at 10 K after subtraction The solid curves have been calculated for the cell parameters listed in Table 1.
by LEED experiments is of major interest to determine the unit cell. Other data like molecular size are also important.
4. Results and discussion: The structures of the N, (2d) film condensed on the MgO( 100) surface were investigated by means of neutron diffraction experiments. The spectra measured at T= 10 K are of two kinds: those observed at the submonolayer coverage (SC) (semi-commensurate solids with two principal peaks) and those measured at the monolayer completion (HC) (high order commensurate solid with
of the MgO background.
three peaks). As shown in Fig. 1, the peak at the wave vector Q= 2.11 A-’ is independent of the coverage. It corresponds to the distance (d= 2.98 A) between the Mg2+ channels of the MgO( 100) surface. It means that in the previous (2d) solids the center of mass of the N, molecules lies along the Mg2+ channels of the MgO(100) surface. For the spectra measured at the submonolayer regime, the peak observed at lower wave vector shifted upward in Q as the coverage increased (Fig. 2). We deduce that the (2d) nitrogen solid undergoes a uniaxial compression along the channels as the coverage increases. This effect is quantitatively expressed in Fig. 3 where the nearest
M. Tvabelsi et al. / Surface Science 377-379 NJ MgO
41
(1997) 38-44
52
T=lOK
5.0
‘P 4.4
\
P-
P P- ..-
4,2
4.0 -l0.6
I-“’
0.7
0.8
0.9
1,O
1.1
1.2
1.3
I,4
Coverage Fig. 3. Nearest neighbour distance 4, (A) of the N, molecules along the MgO( 100) surface channels versus the coverage.
8 =0.8 +
-++++-Jh
I
+ +**
+
+++
I
1.6
.I.+
+
++.*m++
7’
I
1.8
2.0
CICA-1)
Fig. 2. Low wave vectors Q position variation with the coverage range. From 0.8 to 1.0 layer the wave vector position is dependent on the coverage indicating a continuous compression along the [Ol] surface direction; at the monolayer completion 62 1.1 layers a high order (2d) commensurate structure is stabilized by the MgO(100) surface. The apparent broadened peaks observed on the spectrum respectively measured at 0.9 and 1.1 layers are due to the different Q resolution of the spectrometer since they have been measured in preliminary experiment. Nevertheless they show the reproducibility of the structures.
neighbour molecular distance variation d,, (A), along the channels direction, with the coverage range, is represented. The stability of the HC nitrogen (2d) solid in the coverage range 1.1 I 8 I 1.3 layer (Fig. 2) is a primary indication of a registered phase. The profiles which we have compared with the two low temperature neutron diffraction spectra
for 0, respectively equal to 0.8 and 1.2 layers are calculated with the unit cells whose parameters are given in Table 1. The molecule orientation in the unit cell, governed by the azimuthal and tilt out of plane angles 4 and /I respectively is given also in Table 1, and corresponds to the calculated fits which represent best the experimental spectra. Indeed, we systematically varied the orientational parameters 4 and p to obtain the best fit in the Q range from 1.5 to 2.5 A-’ (Fig. 4). The reproduction of the unit cells on the (100) MgO surface indicates that the SC and HC are (2d) p(2 x n) structures in accordance with LEED measurements [7]. Nevertheless, even at the monolayer completion, no locked structure is observed by LEED. This possibility induced the same orientation for all the N, molecules forming the adsorbed layer. Such a configuration, which seems to be incoherent with the quadrupole-quadrupole interaction, is evoked in the case of a strong electrical field at the surface of the substrate [ 161. We think that the potential, due to the interaction of the
42
M. Trabelsi et al. / Surface Science 377-379
(1997) 38-44
Table 1 Structural parameters (a, b, 7) of the SC and HC unit cells. The uncertainty of the orientational (2d) solid
Coverage
Unit cell parameters
SC HC 61(111)
0.8 layers 1.2 layers Bulk Bulk
a(&=3.84 a(&=3.78 a(A)=3.99 a(A)=4.04
8(111)
b(&=3.84 b(A)=3.58 b(A)=3.99 b(&=4.04
e = 00
yt”)=lOl.6 y(“)=108.3 y(“)=120 y(“)=120
cp= 00
parameters is about ,5”
P(“)
9(“)
SM(z@)
d,,(A)
Commensurability
0 60
60 30
14.45 12.84 13.80 14.20
4.86 4.30
P(2 x a) (fl x fi)-rot.33.7”
e = 300
ql =
e = 300
ql = 300
e=
CD =
900
I
I$/
MgO
Cov. = 1.2
T=
++
Q%
+
IOK
+
It+ %
+ +
layer
Y, ;; +“9 + + t*$ Aa+ +#+
+
++c$+:
++++ +tr,+ 4++
*+
H+++++
e=
1.6
900
@ = 00
2.4
2.0 Q ( A-’ )
8 1.6
900
90"
I
2.0
2.4 Q ( A-j )
Fig. 4. Calculated neutron diffraction spectra for several nitrogen molecular orientations in the HC unit mesh (a = 3.78 A; b = 3.58 A; y= 108.3”); 0 and 4 are respectively the polar and azimuthal angles (in text B=90”-0). Best fit to the experimental spectrum is obtained for the following angular values 0 = 30” and $ = 30”.
adsorbed molecules quadrupole moments with the gradient of the MgO( 100) surface electric field, is important enough to impose the molecular orientation.
The evolution with the temperature of the neutron diffraction spectra of the nitrogen (2d) Urns (not represented here) shows two important results:
M. Trabelsi et al. / Surface Science 377-379
(1) The low coverage (8=0.8 layers) spectra. The peak at Q = 1.71 A- 1 becomes broadened between 30 and 40 K indicating a loose order along the Mg2+ channels for the SC nitrogen (2d) solid phase, while the peak at &=2.11 A-’ is unchanged with the temperature increasing. We think that with increasing the temperature the p( 2 x n) semi-commensurate structures are characterized by two kinds of order, a short range order in the channel direction and a long range order in the perpendicular direction. (2) The second one relative to the monolayer completion coverage (0 = 1.2 layers), reveals the stability of the nitrogen (2d) condensed phase structure HC in the temperature range 10 I Ti 30 K, coupled with the primary indication of a registered phase, mentioned above. We can unambiguously conjecture that the HC (2d) nitrogen solid Glm presents a commensurate phase with the MgO( 100) surface: precisely the (a=b=10.73 A; y=90”) fl x ViXrot.33.7” which is represented in Fig. 5. While the potential channels of the MgO( 100) surface, manifested by the sharpness of the peak at Q=2.11 A-‘, impose the positional order in the adsorbed layer, the interaction of the electrical field of the MgO( 100) surface with the quadrupole moment of the N2 molecules seem to impose an orientational order which is different from the (111) and (100) bulk planes and equally different from the structures of the condensed films on graphite. In effect the nitrogen film condensed on MgO( 100) surface undergoes a series of uniaxial compressed phases to end in a locked structure at the monolayer completion. This behaviour is reversed on graphite where the N, film crystallize in a commensurate phase c(fi x 3) followed by a compression along the a axis with the coverage increasing. Additionally in the herringbone structures of the N, film condensed on graphite the molecules’ orientation is at least partially governed by the quadrupole-quadrupole interaction, whereas in the various (2d) structures observed on MgO ( 100) surface the orientation of the N, molecules is determined by the electrostatic interaction which seems to be more important than the quadrupole-quadrupole interaction.
43
(1997) 38-44
di2
x dC_X - rot. 33.7”
commensurate
a = 10.73
W
b = 10.73
a
structure
Y =900
Fig. 5. Schematic representation of the nitrogen (2d) Ehn in the coverage range 1.1-1.3 layer. The N2 (2d) solid is “locked” in the commensurate stucture: m x fl-rot.33.7” (a= 10.73 A; b=10.73A; ?=90”).
5. Summary and conclusion Neutron diffraction experiments on Nz molecules adsorbed onto the MgO( 100) surface have been performed at coverage range 0.6-1.2 layer and T= 10 K. The neutron spectra show two varieties of (2d) structures. In the submonolayer coverp(2 x n) structures, age, semi-commensurate stabilized by the potential channels of the MgO( 100) surface, have been observed and at the monolayer completion a high order fix flrot.33.7” commensurate structure is determined. The low temperature phase structures of N, and Ar on MgO bear some resemblance. Argon also has a p(2 x 3) commensurate structure at submonolayer coverages followed by an hcp one at the
44
M. Trabelsi et al. /Surface
monolayer completion. While the commensurate Ar (2d) structure is stable in a large coverage range 0.3 r010.8 layers [6], the N, (SC) one undergoes a uniaxial compression along the channels. The compression, which is about 11.5% for a coverage range going from 0.6 to 1.1 layers, is larger than that observed on the graphite, along the a direction, which is about 3.2% for a coverage ranging from 1.0 to 1.27 layer [12]. The influence of the MgO( 100) surface periodic electrical field on the thermodynamical properties of condensed phases has been well studied [17211. Structural investigations which are in progress, C,H,/MgO [21,22], CO/MgO c20], CO,/MgO [23] and recently H,O/MgO [ 24,251, must be continued at least for two reasons: firstly to complete the thermodynamical investigations and secondly in order to clarify the influence of the electrical field on the orientational order in physisorbed film structures.
References [I] J.K. Kjerns, L. Passel, H. Taub, J.G. Dash and A.D. Novaco, Phys. Rev. 13 (1976) 1446. [2] P. Vora, SK. Sinha and R.K. Crawford, Phys. Rev. Lett. 43 (1979) 704. [3] J.P. Coulomb, K. Madih, B. Croset and H. Lamer, Phys. Rev. Lett. 54 (1985) 1536. [4] D. Degenhardt, H. Lauter and J.P. Coulomb, to be published.
Science 377-379 (1997) 38-44
[5] J.P. Coulomb, in: Phase transitions in surface films II, NATO-AS& Series B 267, Plenum 113 (1991). [6] K. Madih, Thesis, University of Aix-Marseille II, France, 1986. [7] T. Angot and J. Suzanne, Surf. Sci. Vol. 24 (1991) 671. [8] B. Kuchta and R.D. Etters, Phys. Rev. B 36 (1987) 3400. [9] A.B. Harris and A.J. Berlinsky, Can. J. Phys. 57 (1979) 1852. [lo] C. Peters and M.L. Klein, Mol. Phys. 54 (1985) 895. [ll] C. Peters and M.L. Klein, Phys. Rev. B 32 (1985) 6077. [ 121 R. Wang, SK. Wang, H. Taub, J.C. Newton and H. Shechter, Phys. Rev. B 35 (1987) 5841. [13] J. Eckert, W.D. Ellenson, J.E. Hastings and L. Passel, Phys. Rev. Lett. 43 (1979) 1329. [14] H. You and SC. Fain, Jr., Faraday Disc. Chem. Sot. 81 (1985) 159. [15] J.P. Coulomb and O.E. Vilches, J. Phys. Paris 45 (1984) 1381. [ 161 H. You and SC. Fain, Phys. Rev. B 34 (1986) 2840. [17] T. Angot, Thesis, University of Aix-Marseille II, France, 1990. [18] M. Trabelsi, K. Madih and J.P. Coulomb, Phase Transition 30 (1991) 103. 1191M. Trabelsi, Thesis, University of Aix-Marseille II, France, 1991. 1201P. Audibert, M. Sidoumou and J. Suzanne, Surf. Sci. 467 (1992) L273. [21] D. Ferry and J. Suzanne, Surf. Sci. 19 (1996) L345. [22] J.P. Coulomb, Y. Larher, M. Trabelsi and I. Mirebeau, Mol. Phys. 81 (1994) 1259. [23] J. Suzanne, V. Panella, D. Ferry and M. Sidoumou, Surf. Sci. 912 (1993) L293. [24] D. Ferry, A. Glebov, V. Senz, J. Suzanne, J.P. Toe&es and H. Weiss, to be published. 1251M.H. Moulin, Thesis, University of Aix-Marseille II, France, in preparation.