Indian Journal of ChemistryVol.32A, February 1993.,pp. 92-95

Unusual LEED datterns from mica surfacest

Kflthna G Bhattacharyya

Department of Chemistry, Ga ati University, Guwahati 781 014, Assam, India

Received 23 March 1992; evised 2 July 1992;accepted 17 August 1992

An air-cleaved mica surface does not yield any low energy electron diffraction (LEED) pattern. A surfacecleaved in argon shows LEED patterns with beam energies of more than 70V. The patterns show unusualspot ..•splitting with different orientations an separations. The vacuum-cleaved surface produces diffusespots with streaks which degenerate into tripl ts or triangles on prolonged heating of the sample. All patternsdisappear after two or three days. Comple electrical fields at the mica surface have been found to beresponsible for the unusual LEED pattern .

Fig. 1(a) . Edge on view of ideal muscovite mica structure

t Theexperimentalpart of this work wasdone at the Departmentof Chemistry. Imperial College. London SW7 2AY.

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SILICON ATOMS WITH ALUMINIUM NEIGHBOURS

OXYGEN ATOMS

SITES PREVIOUSLY OCCUPIED BY K+ IONS

SILICON ATOMS WITHOUT ALUMINIUM NEIGHBOURS

TETRAHEDRAL ALUMINIUM ATOMS

SITES FILLED WITH K+ IONS

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EBoo~

o

major one is the presence of tetrahedral Si- and

AI-atoms in different planes, the Si-plane is 0.12Acloser to the octahedral plane. An edge-on view of themuscovite structure is shown in Fig. 1(a). Muscoviteshows perfect cleavage along the (001) plane in thelayer of potassium ions. The cleaved faces share the

potassium ions equally, but Tolansky6 has shown theexistence of steps of thickness lOA or its multiples onthe cleaved faces. The overall picture of the top threelayers of a cleaved face is shown in Fig. 1(b) afterMuller and Chang7•8 and Lewis and Anderson9.

Fig. I(b)--Surface sites on ideal muscovite mica structure. Theunit meshand the distorted oxygenhexagon areoutiined.Halfthe

potassium sites are empty after cleaving

.3'IOAoo()oo

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Sl,.I\l

51,AI

$1,1\1

Al

O,OH

a,oH

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Natural muscovite mica, KAhSi3AIOIO(OHh has a

composite three layered structurel - 3, consist ng oftwo identical tetrahedral layers of compositi n (Si"AlhOs. with their vertices pointing inward a d anoctahedral layer. in between, of mostly alum niumatoms. The 2: I sheet has a net negative charge ue to

substitution of approximately one-fourth f thetetravalent silicons by trivalent aluminium n thetetrahedral layer. The charge is balanced by a I yer of

potassium ions. The plane between the tetra edralland the octahedral layers contains un haredhydroxyl groups which lie at the centre 0 eachsix-fold ring formed by the basal oxygens but r mainin the same plane as the apical oxygens. Gatine u andlMermg4, and Radoslovich5 have pointed out v riousdistortions in the muscovite structure, of whi h tht:

BHATTACHARYYA: LEED PATTERNS FROM MICA SURFACES 93

Fig. 2-LEED patterns from a mica surface cleaved in I atmosphere argon at beam energy (a) 90Y, (b) IOOY,(c) I I5Y and (d) l35Y

LEED has not been much used to study the micasurface because of the problem of surface-chargingassociated with mica. However, Muller10 and Devilleet a/.II did observe the hexagonal unit mesh of micasurface with LEED. Muller and Chang7,8 foundnormal round spots for the air-cleaved mica surfacewhich deteriorated and blanked out after some time.For the vacuum-cleaved surface, these authorsnoticed unusual patterns of triangular spots,three-winged stars, triple or double spots or streaks,all having similar orientation. The presentinvestigation was aimed at having a closer look at theunusual LEED patterns of mica.

Materials and Methods

LEED patterns from mica surfaces were studied inan ESCALAB MK II system (Vacuum Generators,U.K.) fitted with LEED facility and a high precisionsample manipulator in the preparation chamber. Thechamber was routinely maintained at a pressure of1O-9-1O-10torr(l torr = 133.3 Nm-Z).Onecmzmica samples (0.25 mm thick) were made frommuscovite mica of highest purity (grade 5) and were

mounted on nickel stubs with tantalum clips at thefour corners. The top face of the sample could becleaved in situ with a wabble stick and a small silverwire loop attached to the sample. The sample could beannealed by placing it in the P8 heating probe insidethe URV chamber.

Results and Discussion

Because of surface charging, stable LEED patternscould be seen only with an electron beam energy onoVOl' more. Good patterns were obtained with 90-150V beam energy. The air-cleaved mica surfaceproduced very faint patterns which could not bephotographed. Annealing the surface to atemperature of 700 K did not improve the LEEDpatterns. The carbonaceous overlayers of theair-cleaved mica surface 12-14 could be responsiblefor this.

Best LEED patterns were seen when mica wascleaved in flowing argon at atmospheric pressure andthe cleaved facewas introduced into the LEED chamber

immediately. The patterns obtained are shown in Fig.2(a), (b), (c) and (d) for beam energies of90, 100, 115

94 INDIAN J CHEM, SEe. A, FEBRUARY 1993

due to the neutralisation of surface electrical fields bypresence of contamination.

An interesting feature of the LEED patterns in thepresent investigation is that they vanished after themica sample was kept in the system under UHVconditions for a few days. Good patterns were againseen after cleaving the sample at :2 x 109 torr. Twosuch patterns at 90 and 120 V respectively are shownin Fig. 3(a) and (b). The patterns were similarexcepting that the one at 90 V had more diffuse spotsand very pronounced streaking. The patternsappeared to be distorted hexagonal arrays in whichthe spots on certain azimuths were shifted relative tothe others. Measurement of the diffraction patternindicated that the first order array of six spots musthave been obscured by the sample manipulator andthe spots in the photographs were second order spots,some of which were still obscured. The electrical fieldson the vacuum-cleaved mica surface appeared muchmore complex causing different effects in differentdir~ctions.

Although Muller and ChangS observed LEEOpatterns for the vacuum-cleaved mica surface, theregular hexagonal array of spots were split intotriplets. In the present work, triplets were seen onlyafter the vacuum-cleaved surface was annealed at a

temperature of 500 K for 19hr or more. The patternswere blurred with a high background intensity. Onesuch pattern is shown in Fig. 4(a) where the tripletsappear as triangles with similar orientations. Thepattern deteriorated with time, and two days aftercleaving only faint triangles with changed orientationwere seen [Fig. 4(b)). The pattern completelydisappeared after three days. It is likely to be due todeposition of layers of contamination on the micasurface. Similar observations were earlier made forthe air-cleaved surface 7.S.

Fii 3-LEED patterns from a mica surface freshly cleaved in UHV with beam energy (a) 90V and (b) 120V

and 135 ~respectivelY. The hexagonal structure ofthe patter s can be explained on the basis of thesurface 0 ;Ygenhexagons in the basal plane of the

tetrahedr~'llaYer of mica. The spots, which appeared

as doubl ts, showed different separations andorientatio s with changes in beam energy. The spotsplitting i generally associated either with antiphasedomains or with surface steps. Such an explanation

can be ru,d out in the present case as the orientation

of the d ublets was not constant. The splittingappeared 0 be connected with the electrical fields at

the surf<}ce which caused deflection of theback-scattered electrons.

The cl~aved surface of mica contains half amonolayer. of potassium ions which neutralise thecharge-imbalance due to the random presence oftrivalent ajIuminium atoms, replacing approximatelyone-fourth of the silicon atoms in the tetrahedral

layer. Th2 non-uniform, random distribution ofpositive potassium ions) and negative(trivalentaluminiu atoms) charge centres leads to surfaceelectrical flelds15 which may be dipolar in nature7,s.

These fielps will be particularly strong immediately

after cle~ing and may attain some stabilization,

however mporary, through cleaving in argon. Thesplitting the LEED spots was most likely to be dueto the infl ence of these electrical fields. The splitting

occurred rn the same direction for anyone patternand thus~the electrical fields at the surface appear to

be unidij;ctional across the area covered by the

electron earn.Norma circular LEED spots were seen in this work

at 135 V, ,he highest beam energy used. The increasein energ definitely diminished surface charging,resulting in the disappearance of spot splitting. Theair-cleavdd mica surface has been known to exhibitsuch splitting-free circular spots7,Jo,J 1 apparently

--~~---.-. ~-~~-~_-·'w-=lIl"',,"".tl:=- "".1

BHATTACHARYYA: LEED PATTERNS FROM MICA SURFACES 95

Fig. 4-Lead patterns for the UHV-cleaved mica surface at 120V beam energy [(a) After annealing at 500K for II} hours, and (b) twodays after clea ving]

This work has thrown some light on the nature ofthe surface electrical forces on mica. These forces are,however, extremely complex and much more workneeds to be done for a better understanding of theirinfluence. Cleaving mica in argon had some definiteeffects. The surface charging due to the influence ofthe electron beam on insulating mica surface and theelectrical fields due to the non-uniform distributionof potassium ions on cleaving may be togetherresponsible for the unusual LEED patterns. Only halfa monolayer of potassium ions remains on the micasurface after cleaving and thus half the oxygenhexagons remain empty. The positions of thepotassium ions correspond to the positions of thealuminium atoms in the tetrahedral layer and becauseof lack of ordering of these aluminium atoms, thepotassium ions remain randomly distributed,making them unlikely to contribute to the LEEDpatterns. The electrical fields, apparently andinherently non-uniform after cleaving, are the mostlikely influence responsible for producing theunusual patterns.

AcknowledgementEncouragement and assistance received from Dr.

David O. Hayward of the Department of Chemistry,

Imperial College of Science, Technology andMedicine, London in the interpretation of observedresults is gratefully acknowledged.

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