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On the origin of large-scale periodicities observed during scanning tunneling microscopy studies of highly ordered pyrolytic graphite
Surface Science North-Holland
Letters
297 (1993) L119-L121
surface science letters
Surface Science Letters
On the origin of large-scale periodicities observed during scanning tunneling microscopy studies of highly ordered pyrolytic graphite David L. Patrick
and Thomas
P. Beebe,
Jr. *
Department of Chemistry, The University of Utah, Salt Lake City, UT 84112, USA Received
3 August
1993; accepted
for publication
2 September
1993
Scanning tunneling microscopy was used to investigate the origin of a large-scale periodicity observed on the basal plane of highly ordered pyrolytic graphite. Similar phenomena have previously been ascribed to a rotational misorientation between the uppermost graphite layers. We report the first direct measurement of graphite interlayer rotational alignment in the presence of a superlattice, and find that misorientation is not the cause.
Highly ordered pyrolytic graphite (HOPG) is frequently used as a substrate for scanning probe microscopy studies because of its large, atomically flat regions and ease of preparation. This widespread use has prompted a number of nanometer-scale investigations of its structural [1,2] and electronic [3,4] surface properties. Several of these studies have focused on anomalous large-scale periodicities, or superlattices, that are occasionally observed on the surface of HOPG terraces [5-91. It is generally believed that these periodicities are the result of a moire-type pattern arising from a rotational misorientation of one or more layers within the crystal [5,6], as shown schematically in fig. 1. Although the moire hypothesis is intuitively appealing, and has been shown to explain several key properties of graphite superlattices, it has never been directly tested by measuring the alignment between successive layers (refer to fig. 1). In this Letter, we report such a measurement for an HOPG terrace displaying the superlattice phenomenon. Our results indicate that, contrary to the moire hypothesis, interlayer rotational misalignment is not necessary to produce a superlattice.
* To whom correspondence 0039-6028/93/$06.00
should
be addressed.
0 1993 - El sevier Science
Publishers
The observations described here were made in the course of an unrelated investigation involving the liquid crystal 4-octyl-4’cyanobiphenyl (8CB) and monolayer-deep etch pits on HOPG using scanning tunneling microscopy @TM). During the investigation, a graphite terrace showing a superlattice was observed. Surrounding areas showed no evidence of theopattern. Also present on the terrace was a 325 A diameter etch pit [lo]. The depth o,f the etch pit measured by STM was 4.8 f. 1 A, somewhat greater th$n the ideal interlayer spacing of graphite (3.35 A). This difference probably arises from the presence of the superlattice; because of the way they are formed [lo], etch pits deeper than one layer are extremely uncommon. When they do occur, multilayer etch pits are easily discriminated from monolayer ones by either the presence of a second hole grown in the bottom of the first (and offset from center), or their extreme depth when formed at a screw dislocation. The repeat distcnce of the superlattice maxima in fig. 2 is 54,k 1 A, with a corrugation amplitude of 1.4 + 0.5 A. It exhibited a hexagonal symmetry superimposed on shorter period, smaller amplitude corrugations due to the atomic graphite lattice. The angle between the axes of the largescale periodicity and the atomic lattice of the
B.V. All rights reserved
D.L. Patrick, T.P. Beebe, Jr. / Origin of large-scale periodicities in STM of HOPG
uppermost graphite layer was 34 + 1” as determined by examination of atomic resolution images, both in real and reciprocal space. Inspection of fig. 2 shows that the large-scale periodicity is present only on the uppermost graphite layer - it is not observed on the next layer down, in the bottom of the pit. Therefore, if a rotational misorientation were the cause of the periodicity, only the uppermost layer could be rotated. Otherwise, the second layer would show the periodic&y as well [6]. The occurrence of an etch pit in a region containing a superlattice allowed for the direct observation of the rotational orientation between the first and second layers through comparison of atomic resolution images, thus providing an opportunity to test the moire-pattern hypothesis. The expected [5] periodicity P of a moire pattern arising from a rotational angle 8 between two hexagonal lattices of period a is P = a/2 sin(/3/2). From the observed period of the large-scale periodicity (P = 54 f 1 Ah and the lattice spacing for graphite (a = 2.46 A), the ex-
8 rotational mismatch angle between layers \I
superlatt&
maxima
upper graphite layer with hole /
lower graphite layer
Fig. 1. Schematic illustrating the origin of the superlattice according to the moire-pattern hypothesis and the angle defining the rotational misorientation. In this example, f = 5.6 and the spacing between superlattice maxima is 25 A.
Fig. 2. 800X800 A constant height (current mode) STM image of a superlattice surrounding a monolayer deep etch pit. Image has been median filtered to eliminate noise spikes. Note that the periodic&y is absent along the bottom of the hole. Image acquired with a mechanically cut Pt/Rh (90/10) tip in air at room temperature with a sample bias voltage of - 93 mV and an average tunneling current of 90 pA.
petted rotational angle is found to be Oexpected = 2.6 f 0.3”. Comparison of atomic resolution images [ll] acquired in the bottom of the pit to images acquired on the uppermost layer a few nanometers outside of the pit indicate, however, that the atomic lattices in the two layers are perfectly aligned (eobserved= 0 + 0.59. The difference between the expected and observed rotational angles is outside of the error in the measurements, suggesting that some mechanism other than a simple moire-type pattern is responsible for the large-scale periodic&y observed in this case. In addition to the moire-pattern hypothesis, other mechanisms have been proposed, including tip effects and perturbations arising from adsorbates or graphite defects. In the present case, the possibility that the large-scale periodicity arises from the liquid crystal molecules bears consideration, since these molecules are well known to form highly ordered crystalline arrays on graphite [12]. We believe this to be unlikely, however, for
D.L. Patrick, T.P. Beebe, Jr. / Origin of large-scale periodicities in STM of HOPG
three reasons: (1) The large-scale periodicity described here has a markedly different appearance than any known crystalline structure of 8CB. (2) Similar large scale periodicities have been reported on “clean” HOPG surfaces (samples prepared in air, but without purposeful application of an adsorbate), surfaces covered with high purity water, surfaces exposed to long chain hydrocarbons with different solvents, and several other liquid crystalline compounds. (3) At small image arising from the atomic sizes, corrugations graphite lattice are clearly imaged superimposed on the large scale periodic&. This characteristic is unique to the type of large-scale periodicities described here and is not known to occur when imaging molecular adsorbates, where it is necessary to vary imaging conditions to see the different periodic structures. We have reported to observation of a largescale, hexagonal periodicity on a terrace containing a monolayer-deep etch pit. Comparison of atomic resolution images in the bottom of the pit with those outside of the pit indicate that a rotational layer misorientation, moire-type explanation is not the cause of the superlattice. As such, this observation emphasizes the need for further study of the superlattice phenomenon and the development of a better understanding of its causes. The authors gratefully acknowledge Victor Cee for his contributions to this work. This work was supported by NSF grant CHE-9206802.
References [l] C.R. Clemmer and T.P. Beebe, Jr., Science 243 (1989) 370. [2] H. Chang and A.J. Bard, Langmuir 7 (1990) 1143. [3] H.A. Mizes and J.S. Foster, Science 244 (1989) 559. [4] D. Tom&trek, S.G. Lottie, H.J. Mamin, D.W. Abraham, R.E. Thomson, E. Ganz and J. Clarke, Phys. Rev. B 35 (1987) 7790. [51 M. Kuwabara, D.R. Clarke and D.A. Smith, Appl. Phys. Lett. 56 (1990) 2396. [61 C. Liu, H. Chang and A.J. Bard, Langmuir 7 (1991) 1138. 171 J.E. Buckley, J.L. Wragg, H.W. White, A. Bruckdorfer and D.L. Worcester, J. Vat. Sci. Technol. B 9 (1991) 1079. 181 P.I. Oden, L.A. Thundat, L.A. Nagahara, S.M. Lindsay, G.B. Adams and O.F. Sankey, Surf. Sci. Lett. 254 (1991) L454. [91 X. Yang, Ch. Bromm, Geyer and G. von Minnigerode, Ann. Phys. 1 (1992) 3. [lOI Monolayer deep etch pits have been described in detail elsewhere. They are approximately circular regions, one graphite layer deep, formed by oxidation when graphite is heated in air to above 550°C. The etching process does not measurably disturb the atomic structure outside of the pit or along its bottom; although we have examined hundreds of etch pits with STM, the superlattice effect has only been observed in two instances. For more details, refer to: (a) H. Chang and A.J. Bard, J. Am. Chem. Sot. 112 (1990) 4598; (b) H. Chang and A.J. Bard, J. Am. Chem. Sot. 113 (1991) 5588; (c) X. Chu and L.D. Schmidt, Carbon 29 (1991) 1251. was made using ten separate images, Dll The comparison five acquired within the hole, and five outside of it. HZ1 J. Frommer, Angew. Chem. Int. Ed. Engl. 31 (1992) 1298.
Letters
297 (1993) L119-L121
surface science letters
Surface Science Letters
On the origin of large-scale periodicities observed during scanning tunneling microscopy studies of highly ordered pyrolytic graphite David L. Patrick
and Thomas
P. Beebe,
Jr. *
Department of Chemistry, The University of Utah, Salt Lake City, UT 84112, USA Received
3 August
1993; accepted
for publication
2 September
1993
Scanning tunneling microscopy was used to investigate the origin of a large-scale periodicity observed on the basal plane of highly ordered pyrolytic graphite. Similar phenomena have previously been ascribed to a rotational misorientation between the uppermost graphite layers. We report the first direct measurement of graphite interlayer rotational alignment in the presence of a superlattice, and find that misorientation is not the cause.
Highly ordered pyrolytic graphite (HOPG) is frequently used as a substrate for scanning probe microscopy studies because of its large, atomically flat regions and ease of preparation. This widespread use has prompted a number of nanometer-scale investigations of its structural [1,2] and electronic [3,4] surface properties. Several of these studies have focused on anomalous large-scale periodicities, or superlattices, that are occasionally observed on the surface of HOPG terraces [5-91. It is generally believed that these periodicities are the result of a moire-type pattern arising from a rotational misorientation of one or more layers within the crystal [5,6], as shown schematically in fig. 1. Although the moire hypothesis is intuitively appealing, and has been shown to explain several key properties of graphite superlattices, it has never been directly tested by measuring the alignment between successive layers (refer to fig. 1). In this Letter, we report such a measurement for an HOPG terrace displaying the superlattice phenomenon. Our results indicate that, contrary to the moire hypothesis, interlayer rotational misalignment is not necessary to produce a superlattice.
* To whom correspondence 0039-6028/93/$06.00
should
be addressed.
0 1993 - El sevier Science
Publishers
The observations described here were made in the course of an unrelated investigation involving the liquid crystal 4-octyl-4’cyanobiphenyl (8CB) and monolayer-deep etch pits on HOPG using scanning tunneling microscopy @TM). During the investigation, a graphite terrace showing a superlattice was observed. Surrounding areas showed no evidence of theopattern. Also present on the terrace was a 325 A diameter etch pit [lo]. The depth o,f the etch pit measured by STM was 4.8 f. 1 A, somewhat greater th$n the ideal interlayer spacing of graphite (3.35 A). This difference probably arises from the presence of the superlattice; because of the way they are formed [lo], etch pits deeper than one layer are extremely uncommon. When they do occur, multilayer etch pits are easily discriminated from monolayer ones by either the presence of a second hole grown in the bottom of the first (and offset from center), or their extreme depth when formed at a screw dislocation. The repeat distcnce of the superlattice maxima in fig. 2 is 54,k 1 A, with a corrugation amplitude of 1.4 + 0.5 A. It exhibited a hexagonal symmetry superimposed on shorter period, smaller amplitude corrugations due to the atomic graphite lattice. The angle between the axes of the largescale periodicity and the atomic lattice of the
B.V. All rights reserved
D.L. Patrick, T.P. Beebe, Jr. / Origin of large-scale periodicities in STM of HOPG
uppermost graphite layer was 34 + 1” as determined by examination of atomic resolution images, both in real and reciprocal space. Inspection of fig. 2 shows that the large-scale periodicity is present only on the uppermost graphite layer - it is not observed on the next layer down, in the bottom of the pit. Therefore, if a rotational misorientation were the cause of the periodicity, only the uppermost layer could be rotated. Otherwise, the second layer would show the periodic&y as well [6]. The occurrence of an etch pit in a region containing a superlattice allowed for the direct observation of the rotational orientation between the first and second layers through comparison of atomic resolution images, thus providing an opportunity to test the moire-pattern hypothesis. The expected [5] periodicity P of a moire pattern arising from a rotational angle 8 between two hexagonal lattices of period a is P = a/2 sin(/3/2). From the observed period of the large-scale periodicity (P = 54 f 1 Ah and the lattice spacing for graphite (a = 2.46 A), the ex-
8 rotational mismatch angle between layers \I
superlatt&
maxima
upper graphite layer with hole /
lower graphite layer
Fig. 1. Schematic illustrating the origin of the superlattice according to the moire-pattern hypothesis and the angle defining the rotational misorientation. In this example, f = 5.6 and the spacing between superlattice maxima is 25 A.
Fig. 2. 800X800 A constant height (current mode) STM image of a superlattice surrounding a monolayer deep etch pit. Image has been median filtered to eliminate noise spikes. Note that the periodic&y is absent along the bottom of the hole. Image acquired with a mechanically cut Pt/Rh (90/10) tip in air at room temperature with a sample bias voltage of - 93 mV and an average tunneling current of 90 pA.
petted rotational angle is found to be Oexpected = 2.6 f 0.3”. Comparison of atomic resolution images [ll] acquired in the bottom of the pit to images acquired on the uppermost layer a few nanometers outside of the pit indicate, however, that the atomic lattices in the two layers are perfectly aligned (eobserved= 0 + 0.59. The difference between the expected and observed rotational angles is outside of the error in the measurements, suggesting that some mechanism other than a simple moire-type pattern is responsible for the large-scale periodic&y observed in this case. In addition to the moire-pattern hypothesis, other mechanisms have been proposed, including tip effects and perturbations arising from adsorbates or graphite defects. In the present case, the possibility that the large-scale periodicity arises from the liquid crystal molecules bears consideration, since these molecules are well known to form highly ordered crystalline arrays on graphite [12]. We believe this to be unlikely, however, for
D.L. Patrick, T.P. Beebe, Jr. / Origin of large-scale periodicities in STM of HOPG
three reasons: (1) The large-scale periodicity described here has a markedly different appearance than any known crystalline structure of 8CB. (2) Similar large scale periodicities have been reported on “clean” HOPG surfaces (samples prepared in air, but without purposeful application of an adsorbate), surfaces covered with high purity water, surfaces exposed to long chain hydrocarbons with different solvents, and several other liquid crystalline compounds. (3) At small image arising from the atomic sizes, corrugations graphite lattice are clearly imaged superimposed on the large scale periodic&. This characteristic is unique to the type of large-scale periodicities described here and is not known to occur when imaging molecular adsorbates, where it is necessary to vary imaging conditions to see the different periodic structures. We have reported to observation of a largescale, hexagonal periodicity on a terrace containing a monolayer-deep etch pit. Comparison of atomic resolution images in the bottom of the pit with those outside of the pit indicate that a rotational layer misorientation, moire-type explanation is not the cause of the superlattice. As such, this observation emphasizes the need for further study of the superlattice phenomenon and the development of a better understanding of its causes. The authors gratefully acknowledge Victor Cee for his contributions to this work. This work was supported by NSF grant CHE-9206802.
References [l] C.R. Clemmer and T.P. Beebe, Jr., Science 243 (1989) 370. [2] H. Chang and A.J. Bard, Langmuir 7 (1990) 1143. [3] H.A. Mizes and J.S. Foster, Science 244 (1989) 559. [4] D. Tom&trek, S.G. Lottie, H.J. Mamin, D.W. Abraham, R.E. Thomson, E. Ganz and J. Clarke, Phys. Rev. B 35 (1987) 7790. [51 M. Kuwabara, D.R. Clarke and D.A. Smith, Appl. Phys. Lett. 56 (1990) 2396. [61 C. Liu, H. Chang and A.J. Bard, Langmuir 7 (1991) 1138. 171 J.E. Buckley, J.L. Wragg, H.W. White, A. Bruckdorfer and D.L. Worcester, J. Vat. Sci. Technol. B 9 (1991) 1079. 181 P.I. Oden, L.A. Thundat, L.A. Nagahara, S.M. Lindsay, G.B. Adams and O.F. Sankey, Surf. Sci. Lett. 254 (1991) L454. [91 X. Yang, Ch. Bromm, Geyer and G. von Minnigerode, Ann. Phys. 1 (1992) 3. [lOI Monolayer deep etch pits have been described in detail elsewhere. They are approximately circular regions, one graphite layer deep, formed by oxidation when graphite is heated in air to above 550°C. The etching process does not measurably disturb the atomic structure outside of the pit or along its bottom; although we have examined hundreds of etch pits with STM, the superlattice effect has only been observed in two instances. For more details, refer to: (a) H. Chang and A.J. Bard, J. Am. Chem. Sot. 112 (1990) 4598; (b) H. Chang and A.J. Bard, J. Am. Chem. Sot. 113 (1991) 5588; (c) X. Chu and L.D. Schmidt, Carbon 29 (1991) 1251. was made using ten separate images, Dll The comparison five acquired within the hole, and five outside of it. HZ1 J. Frommer, Angew. Chem. Int. Ed. Engl. 31 (1992) 1298.