Observation of protrusions and ring structures on MoS2 by scanning tunnelingmicroscopyHaryani Permana, Sangho Lee, and K. Y. Simon Ng

Citation: Journal of Vacuum Science & Technology B 10, 2297 (1992); doi: 10.1116/1.586057 View online: http://dx.doi.org/10.1116/1.586057 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/10/5?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Epitaxy and scanning tunneling microscopy image contrast of copper–phthalocyanine on graphite and MoS2 J. Vac. Sci. Technol. B 12, 1963 (1994); 10.1116/1.587680 Observation of negative differential resistance in tunneling spectroscopy of MoS2 with a scanning tunnelingmicroscope J. Vac. Sci. Technol. B 9, 1083 (1991); 10.1116/1.585265 Scanning tunneling microscopy observation of MoS2 surface and gold clusters deposited on MoS2 surface J. Vac. Sci. Technol. A 8, 500 (1990); 10.1116/1.576374 Probing of basal planes of MoS2 by scanning tunneling microscopy Appl. Phys. Lett. 52, 2252 (1988); 10.1063/1.99769 Imaging of MoS2 by scanning tunneling microscopy Appl. Phys. Lett. 51, 1560 (1987); 10.1063/1.98635

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Observation of protrusions and ring structures on MoS2 by scanning tunneling microscopy

Haryani Permana, Sangho Lee, and K. Y. Simon Nga) Department of Chemical Engineering, Wayne State University, Detroit, Michigan 48202

(Received 12 December 1991; accepted 9 June 1992)

Natural MoS2 was investigated by scanning tunneling microscopy operated in air. The atomic resolution image yields the hexagonal symmetry of the surface of the crystal with lattice constant of 3.1 A, in agreement with the reported x-ray crystallographic value. Corrugations along the [110] cell diagonal show major and secondary peaks which have been reported to correspond to molybdenum atoms and sulfur atoms, respectively. A large-area image of MoS2

reveals protrusions and ring-type structures of similar sizes on the surface of the sample. The islands and ring-type structures may be due to vanadium and fluoride ions, which are found in low concentrations (0.03 and 0.5 at. %, respectively) in the MoS2 sample when investigated by secondary ion mass spectroscopy. The protrusions and ring-type structures were observed only at a higher bias voltage (-0.084 V), suggesting that the impurities are found in the molybdenum layer only. Low bias voltage (-0.003 V) does not show any indications of islands or ring structures, which suggests that there are no impurities on the sulfur layer.

I. INTRODUCTION

Recent surface studies using scanning tunneling micros­copy (STM) have focused on a number of semiconductors with layered structures. In the past several years, a few attempts have been made to study cluster growth and or­ganic adsorbate on MoSz, 1-3 and also the electronic struc­ture and surface topography of MOS2,4-8 Uozumi et al.I

investigated vacuum-deposited gold film on MoS2 surface. They were able to obtain atomic corrugations which re­flected the Au ( 111) surface. In addition, the gold islands were observed to move and change their shape during mi­croscopy. The movement of the gold islands was believed to be induced by the electrostatic effect from the tip during scanning. Ichinokawa et al. 2 studied vacuum-deposited gold clusters on MoSb reporting the formation of gold fcc structure independent of the MoS2 lattice. They attributed this to weak atomic interaction between the sulfur plane and the gold. These studies have shown that the basal plane of MoS2 is inert, as demonstrated by the movement of gold on MoS2, indicating a weak interaction between the clusters and the substrate. Hara et al. 3 have used MoS2 as a substrate to anchor organic molecules, and they compare the results with those from graphite substrate. They ob­served strong molecule-surface interaction on MoS2 sur­face, and also the sulfur lattice of MoS2 and the carbon lattice of graphite, have distinct differences in their adsorbate ordering due to different lattice constants and differences in epitaxial growth on the two substrates.

STM studies of the topography of natural MoS2 have been carried out by several groups. Stupian et ai. 4 reported atomically resolved images of MoS2 at negative sample bias. They were able to observe the hexagonal symmetry of MoS2 with a lattice constant of about 0,31 nm. These au­thors suggested that the STM image is produced by elec­trons originating from the filled molybdenum d; orbital. Weimer et aL 5 have obtained atomic resolution images of MoS2 with both positive and negative sample bias, and

they observed a lattice constant of 0.316 nm. These authors have resolved two distinct atomic sites, one corresponding to the top sulfur layer and the other to the molybdenum layer below the sulfur plane. The dominant STM image may be due to the 4d orbital of the molybdenum. Sarid et aZ. 6 also observed the hexagonal symmetry of MoS2 using both positive and negative sample bias. They measured the semiconductor band gap and reported a value of 0.75 V. An indirect band gap of 1.2 eV (Ref. 7) and an optical band gap of 1.9 eV (Ref. 8) were also reported. Clearly, these studies have shown that STM is capable of imaging a MoS2 surface at the atomic level. The results reported are in general agreement with the values obtained from x-ray crystallography.

Interesting topographical features were also reported on MoS2 surfaces using STM. Kramar9 observed the appear­ance of islands or mounds on a relatively large scanning area of MoS2• The islands range in diameter from 15 to 30 nm. Kramar9 proposed that these mounds might have been due to interstitial contaminants between the sulfur layers, or to subsurface interstitial impurities. Recently, Heckl et ai. 10 reported ringlike structures on the surface of MoS2•

The ring diameters range from 1.5 to 4 nm, with a height of about 0.3 nm, Heckl et al. 10 suggested that the rings could be organic or biological remnants included when the mineral was formed, or they could be the result of purely electronic effects caused by impurities. The nature of the rings and/or islands i.s intriguing, because this may con­tribute to our understanding of the effect of impurities on the sensitivity of topographical images, In this article, we report the first STM observation of both protrusions and ringlike structures on the surface of MoS2. The results of our study will be discussed along with one of the hypoth­eses proposed by Heckl et al.1O

II. EXPERIMENTAL

The STM used in this study is a commercial ultrahigh vacuum (UHV) -compatible Omicron unit operated under

2291 J, Vac, Sci. Techno!. B 10(5}, Sep/Oct 1992 0734-211 X/92/052297-05$01.00 @1992 American Vacuum Society 2297

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2298 Permana, lee, and IIIg: Protrusions and ring structures on MoS2 2298

ambient conditions. STM tips for these studies were pre­pared from 0.508 mm Pt-Ir (80%-20%) wire (99.9% grade from Johnson-Matthey). The tips were electrochem­ically etched in a mixture of 3 N NaCN and 1 N KOH, using the ac drop-off method. II After etching, the tips were thoroughly rinsed, first with de-ionized water, then with acetone. The tips were then clamped into the tip holder and used without further preparation.

Our samples were natural 2H-MoS2 (molybdenite) mined near Otter Lake, Quebec and purchased from Ward's Natural Science Establishment. These materials appear shiny grey and metallic. The samples used in this study were all from the same batch. The samples were mounted on stainless steel sample holders with silver paint or tantalum foil to provide good electrical contact. Essen­tially no differences in sample conductivity were observed with either mounting method. The purity of the sample was examined using x-ray diffraction (XRD), scanning Auger spectroscopy, and secondary ion mass spectroscopy (SIMS) to determine possible contaminants. The samples were cleaved in air by simply peeling off a few of the top­most layers with adhesive tape to expose a fresh surface before imaging.

m. RESULTS AND DISCUSSION

In some cases, the approach of the STM tip to the sur­face gave indications of a contaminated tip or sample. In this condition, no tunneling is obtained, or if there is tun­neling, measurement of the surface topography is impossi­ble because the tunneling current becomes very sensitive to tip-sample spacing. The tip or the sample must then be cleaned again by the methods outlined in the experimental section, or a different area of the sample must be found. Once a successful approach is made, rough and irregular surfaces are often found. These may be due to cleavages or imperfections in the crystal. The tunneling characteristics lack stability in this region. Therefore it is often necessary to scan a number of areas and to optimize the voltage or the current to obtain an atomic resolution image of the surface. Different voltages and tunneling currents ranging from -1.9 V and 10 nA (Ref. 4) to +0.14 V and 2 nA (Ref. 5) were used i.n previous studies to obtain STM im­ages. The bias voltage of +0.14 V used in (Ref. 5) was below the reported band gap values,6--8 and so was the bias voltage used in our study. We found that the optimal bias voltages and tunneling currents used to obtain good images vary from one sample to another, and are highly dependent on the resistance of the sample. Coley et al. 12 have pro­posed that, based on electronic effects, second-atom-Iayer molybdenum gives rise to the largest current in negative sample bias. Therefore, the dominant STM image is due to tunneling between the tip and the molybdenum d;. orbitals. However, at low bias voltage, and thus at very small gap distances, sulfur p orbitals may dominate in the image due to the presence of roughly 27% sulfur p character in the highest occupied molecular orbital. Thus a particular volt­age and current set may reveal atoms in the molybdenum layer or the sulfur layer. However, it is not clear whether the transition between Mo dominance and S dominance

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F[G. 1. A 50 A x 50 A area of a MoSz surface taken in air. The image was obtained under the constant current mode, with a current of 1.0 nA and a sample bias of --0.084 V. The complete scan was acquired in 72 s. The lattice constants between neighboring atoms are about 3.1 A, in agree­ment with the x-ray crystallographic value (Ref. 10). Note that second­ary sites (corresponding to sulfur atoms) are also observed.

will occur before the breakdown of gap resistance. 12 Figure 1 is a scan of a 50 A X 50 A area of a MOS2 surface, taken in air. The STM was operated in the constant current mode with a current of 1.0 nA and a sample bias of -0.084 V. The typical scanning time for our STM image is 75 s per frame. In addition to the dominant image of molybdenum atoms, it should be noted that secondary sites (correspond­ing to sulfur atoms) are also observed. This particular area is not perfectly fiat, with regions that are slightly higher (-0.5 A) than adjacent ones. The nature of this phenom­ena is not clear but is believed to be inherent in the sample itself. Figures 2(a) and 2(b) show simple sinusoidal mod­ulation along the [100] and [010] cell edge, with a peak-to­hollow corrugation amplitude of roughly 2 A. The lattice constants in the [100} and [010] directions (between neigh­boring atoms) are about 3.1 A, which agrees well with those determined by x-ray crystallography.13 Kramar9 has observed two distinct atomic sites at atomic resolution im­aging. This is consistent with our observation of the cor­rugations along the [110] cell diagonal [Fig. 2ec)], which indicates a major peak, a secondary peak, and a hollow. The peak-to-hollow corrugation amplitude is about 2 A, and the height of the secondary peak is about 0.75 A. The corrugation is higher than the reported x-ray crystallogra­phy value (1.59 A). Kramar9 suggested that a sharper tip would result in larger corrugations, since it is scanning a smaller area of the sample plane, thus allowing it to resolve better the detailed corrugations of the surface wave func­tion. However, Tersoff et al. 14 have reported that di.fferent atoms have different tunneling probabilities. Thus as the tip is scanned across the sample surface, it will experience the tunneling probabilities of molybdenum and sulfur, re­sulting in different gap spacings under constant current. In

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2299 Permana, lee, and I\Ig: Protrusions and ring structures on MoS2 2299

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FIG. 2. Cross-sectional profiles of the surface in Fig. 1. (a) Aloug the [100] cell edge, and (b) corrugations along the [010] cell edge of the surface. Note that the peak-to-hollow corrugation amplitude for (a) and (b) is about 2 A, higher than the reported x-ray crystallographic value (Ref. 10). (c) Along the [110J cell diagonal, major peaks, secondary peaks, and holloware observed. The height of the secondary peak is about 0.75 A.

addition, charge densities from both sulfur and molybde­num may contribute to the total density of states, and thus the corrugation depths obtained from STM should not be used to represent the distance between layers.

On natural MoS2 samples, Kramar9 observed islands ranging from 15 to 30 nm, with heights of about 50 nm, while Heckl et al. 10 observed ring-type structures with radii of 1.5 and 4 nm, and heights of about 0.3 nm. Interestingly, in our investigations we observed both the islands and the ring structures. Figures 3 and 4 are images of MoSz over a larger area, 100 A X 100 A and 200 A X 200 A, respec­tively. The scans were acquired at 1.0 nA constant current and -0.084 V sample bias. In both large-area scans, which were obtained at different times and locations, we observed protrusions (islands) and ring structures on the MoS2 sur­face. The islands are about 15-25 A in diameter and 30 A high, while the rings are about 30-40 A in diameter and about 24--28 A high. The heights of the islands are about

J. Vac. Sci. Technol. B. Vol. 10, No.5, Sep/Oct 1992

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FIG. 3. An image of 100 A X 100 A area of MoS2• The image was acquired at 1.0 nA constant current and ~O.084 V sample bias. The complete scan was obtained in 75 s. In this image, both protrusions and ring-type struc­tures were observed. The protrusions are about 15-25 A in diameter and 30 A high, while the ring is about 3(,\.40 A in diameter and 20-28 A high.

the same as those of the ring structures; even the diameters are quite close. It is interesting to note that the diameters of the ring structures that we observed are about the same as those observed by Heckl et aI., 10 but that the protrusions are ten times smaner than those observed by Kramar.9

Kramar suggested that these islands may be due to inter­stitial contaminants between the sulfur layer or subsurface interstitial impurities included in the mOlybdenum sulfide

FIG. 4. A 200 AX200 A image of MoS2• The image was acquired at the same voltage and current setting as in Fig. 3, but at a different time and a different scanning area. This image also shows protrusions and ring-type structures on a MoSz surface. Note that the diameter and the height of the protrusions are about the same as those of the ring structures.

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2300 Permana, lee, and Ng: Protrusions and ring structures on MoS2 2300

layers. However, no impurity information was given on his sample. It should be noted that protrusions on MoS2 were observed even at 10- 10 Torr,9 suggesting that they are not likely to be due to molecular adsorbate from the ambient atmosphere. Heckl et al. 10 have proposed two possible or­igins to explain the occurrence of the ring structures. One is that the ring structure is due purely to an electronic effect, and the appearance of the ring structures is the re­sult of dopant molecules which function as negatively charged acceptors. These negatively charged acceptors can induce band bending, so that tunneling in this area leads to indentations in the image. If the dopant molecules are ex­cessively charged with positive acceptor, this will induce protrusions which are seen as islands or protrusions in the image. If there are both negatively and positively charged acceptors in one local area, then both effects would add up to yield a ringlike structure. In our investigation, we ob­served both islands and rings, which indicates that, besides the positively charged acceptors (protrusions) on our sam­ple, negatively charged acceptors also exist together with positively charged acceptors to form rings. When the sam­ple bias was switched to positive (+0.084 V) without changing the scanning area, the protrusions appeared as depressions, and mirror images of ring structures were ob­served. This is in agreement with Stroscio et al.'s 15 obser­vation of adsorbed oxygen on GaAs, thus supporting our interpretation that the protrusions and ring structures are the results of charged impurities. Due to the detection limit, both XRD and Auger spectroscopy cannot detect any impurity. The positive ion spectrum of SIMS indicates that our samples contain vanadium at concentrations of 0.03 at. %, while the negative ion spectrum shows a peak for fluoride ions at a concentration of 0.5 at. %. These results lead us to believe that in our sample the protrusions are due to vanadium, while the indentations are due to fluoride ions. We observed that the impurity level in our sample is lower than that reported by Heckl et al., 10 who found that the impurities in their natural sample (mined near Snarum, Norway) include 12C and 55Mn at concen­tration of 0.5% each, and 27 Al and 40Ca at concentrations of about 0.4% and 1.5%, respectively. No negative ions were reported. We did not detect any of these elements as impurities in our sample.

The appearance of the protrusions and rings was found to be voltage-dependent. When the bias voltage decreases successively from -0.084 V, the quality of the image con­trast decreases, then the images disappear, and then reap­pear again at -0.003 V. Interestingly, the same sample surface appears flat at this low voltage, suggesting sulfur atoms as the dominant image (Fig. 5). The very low bias voltage may suggest the occurrence of a point contact image. 16 However, preliminary examination of the I-V characteristic does not show a linear relationship. There­fore, the tip and the sample are not likely to be in ohmic contact mode. The observation at the low voltage (-0.003 V) implies that the protrusions and rings are probably not due to organic or biological remnants, the other possibility proposed by Heck!. 10 This may also explain why no ring structure was observed on synthetic MOS2•

1O Thus, we be-

J. Vac. ScI. Techno!. B, Vol. 10, No.5, Sep/Oct 1992

FIG. 5. An image of 100 Ax 100 A area of MoS2 surface. The scan was obtained at 1.0 nA constant current and - 0.003 V sample bias. The image obtained at low voltage appears quite flat compared to those ob­tained at high voltage (Figs. 3 and 4).

lieve the impurities (vanadium and fluoride ions) that lead to the observation of protrusions and rings at higher volt­ages are interstitially located in the molybdenum layer but not the sulfur layer. The result from our observation par­allels the structure proposed by Tops0e et al., 17 which sug­gests that the location of Co promoter ions in MoS2 cata­lysts is in the molybdenum plane.

IV. SUMMARY

The atomic resolution image of natural MoS2 obtained by STM shows agreement with the x-ray crystal structure. Two distinct atomic sites, corresponding to molybdenum and sulfur atoms, were observed. The distance between the molybdenum layer and the sulfur layer was higher than the value reported from the x-ray crystal structure, due to the contribution from the charge density waves of both molyb­denum and sulfur.

Rings and protrusions of similar sizes were observed simultaneously on large-area images of the sample. These structures are attributed to the electronic effects of vana­dium and fluoride ion impurities in the natural MoS2 sam­ple. OUf results suggest that the impurities are intercalated in the molybdenum layer. The image of the sulfur layer indicated sulfur atoms only, and no other types of struc­tures were observed. The impurities may have been in­cluded in the Mo layer during the geological formation of this material. Also, the origin of the MoS2 will determine the type of impurities, and thus observations of different structures.

ACKNOWLEDGMENTS

The authors would like to thank W. P. Zhu and S. L. Kaberline of Ford Motor Company for providing the Au­ger spectroscopy and SIMS results. Financial support of

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2301 Permana, Lee, and Ng: Protrusions and ring structures on MoS2 2301

this project from the National Science Foundation (Grant No. CTS-8909382) and from the Institute for Manufactur­ing Research of Wayne State University is gratefully ac" knowledged.

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