© Copyright 1998 American Chemical Society
JULY 7, 1998 VOLUME 14, NUMBER 14
Letters Electrode Potential-Dependent Acceptor Images in p-MoSe2 Observed by Electrochemical Scanning Tunneling Microscopy Takashi Ohmori† and Carlos R. Cabrera* Department of Chemistry and Materials Research Center, P.O. Box 23346, University of Puerto Rico, Rı´o Piedras Campus, San Juan, Puerto Rico 00931-3346 Received January 27, 1998. In Final Form: May 1, 1998 The surface of a p-MoSe2 electrode has been observed in 0.05 M HNO3, in situ, by electrochemical scanning tunneling microscopy (ECSTM). Randomly oriented nanometer size spots have been observed on the surface. The STM images of the p-MoSe2 surface have been found to change with the electrode potential. That is, the nanometer size spots appeared as a dark contrast under a smaller band bending condition (0.2 V versus SCE) while the spots disappeared or further appeared as bright contrast under a larger band bending condition (0 and -0.2 V versus SCE). This is presumed to result from whether the acceptor level, within the band gap of the p-MoSe2, is occupied or not. In addition, it will depend on the electrode potential, which determines the amount of the band bending.
Introduction Layered transition metal dichalcogenides such as WX2 and MoX2 (X ) S, Se, Te) have been extensively studied for photoelectrochemical solar cells.1 Not only their electronic but also topographic properties play a vital role for their performance. Originally, these compounds have been introduced with the advantages that the d-character energy bands provide fitting band gaps for the solar spectrum absorption and also favorable optical transitions between nonbonding electronic orbitals.2 The latter could avoid the rupture of chemical bonds of the semiconductor surface, enhancing the stability against photocorrosion. From the topographic viewpoint, structural defects such as grain boundaries, steps, and dislocations or impurities * To whom correspondence should be addressed. Phone: 787764-0000 x-4807. Fax: 787-756-8242. E-mail:
[email protected]. † Present address: Catalysis Science Laboratory, Research Institute of Innovative Technology for the Earth (RITE), 9-2, Kizugawadai, Kizu-cho, Soraku-gun, Kyoto 619-0292, Japan. (1) Photoelectrochemistry and Photovoltaics of Layered Semiconductors; Aruchamy, A., Ed.; Physics and Chemistry of Materials with LowDimensional Structures 14; Kluwer Academic Publishers: Dordrecht, 1992. (2) Tributsch, H.; Bennett, J. C. J. Electroanal. Chem. 1977, 81, 97.
in the crystal lattice bring detrimental effects in the efficiency by enhancing surface recombination. Despite these facts, the knowledge of the interface of these materials, with respect to the electronic and topographic structures under the reaction processes, has been quite scarce,3,4 especially on the microscopic and molecular levels. Recently, we have been studying (photo)electrochemical processes at p-MoSe2.5,6 Copper5 and silver6 (photo)electrodepositions at p-MoSe2 were investigated by means of using techniques such as Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and photovoltammetric methods. Currently, we are studying these processes by using scanning tunneling microscopy (STM)7 in situ. At first, we performed in situ observation of the p-MoSe2 surface in electrolyte solutions which do not contain metallic ions for deposition, that is, only in supporting electrolytes. Here, we have found that the STM images of the p-MoSe2 change with the electrode (3) Kautek, W.; Gerischer, H. Surf. Sci. 1982, 119, 46. (4) Sakamaki, K.; Hinokuma, K.; Hashimoto, K.; Fujishima, A. Surf. Sci. 1990, 237, L383. (5) Castro, R. J.; Cabrera, C. R. J. Electrochem. Soc. 1992, 139, 3385. (6) Castro, R. J.; Cabrera, C. R. Langmuir 1995, 11, 1375.
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Figure 1. Cyclic voltammograms of (a) p-MoSe2 and (b) the Pt/Ir STM tip in 0.05 M HNO3. The sweep rates were 50 mV/s.
potential, in which the nanometer size spots appeared, changing the contrast from dark to bright. This is considered to result from whether the acceptor level within the band gap of the p-MoSe2 is occupied or not, depending on the electrode potential, similarly to the results performed in vacuum8 or air.9 To our knowledge, this is the first report which demonstrates the STM image change of semiconductor impurity under potentiostatic conditions. Experimental Section The p-MoSe2 used in our experiments was synthesized by chemical vapor transport techniques, using selenium as the transporting agent. Details on the synthesis of the p-MoSe2 crystals have been reported in a previous paper.5 For the in situ STM studies the p-MoSe2 was cleaved immediately before each experiment was performed. The STM apparatus used was a Nanoscope III from Digital Instruments. The STM electrochemical cell was composed of Pt wires as reference and counter electrodes, and the p-MoSe2 was the working electrode. The Pt/Ir STM tip, purchased from Digital Instruments, was used as received, that is, without any precleaning procedures. The STM tips used in the electrochemical STM measurements were coated with Apiezon wax except at its apex for imaging. The tunneling currents were all set at 1 nA. The cyclic voltammograms (CVs) were obtained with a Princeton Applied Research (PAR) potentiostat 173, a PAR universal programmer 175, and a Soltec x-y recorder VP-64243. The electrolyte solution of 0.05 M HNO3 was prepared from concentrated HNO3 (Fisher Scientific) and pure water which was served through a Nanopure system from Barnstead. All electrode potentials are reported with respect to the saturated calomel electrode (SCE).
Results and Discussion Prior to STM observation, the p-MoSe2 and the STM tip were electrochemically characterized by measuring CVs. In Figure 1 we show the respective CVs which were (7) Manivannan, A.; Santiago, Y.; Cabrera, C. R. J. Vac. Sci. Technol. 1994, B12, 2111. (8) Zheng, Z. F.; Salmeron, M. B.; Weber, E. R. Appl. Phys. Lett. 1994, 64, 1836. (9) Whangbo, M.-H.; Ren, J.; Magonov, S. N.; Bengel, H.; Parkinson, B. A.; Suna, A. Surf. Sci. 1995, 326, 311.
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obtained in 0.05 M HNO3. The anodic reaction of surface oxidation of the p-MoSe2 is found to start from approximately 0.4 V versus SCE while the cathodic reaction is not recognized in the present potential region. It is found in Figure 1b that the double-layer potential region of the STM tip used here ranges between 0.5 and 1.2 V versus SCE. From both cyclic voltammograms, the experimental condition under which we are able to acquire a stable image of the p-MoSe2 surface is limited to the negative sample bias polarity to the STM tip standard (refer to the double-layer potential regions of both, STM tip and p-MoSe2, electrodes). Figure 2 shows the STM images of the p-MoSe2 surface which were observed at different electrode potentials of the p-MoSe2. We see in the image obtained at 0.2 V versus SCE (Figure 2a) that a large amount of dark spots is scattered together with some surface roughness. The surface roughness depended on the electrode potential, which implies that it derives from the electronic property as well as from the geometrical structure of the p-MoSe2 surface. These dark spots disappeared and some bright spots appeared when the surface was imaged at -0.2 V versus SCE (Figure 2b). In Figure 2c and d, which were observed with a higher magnification scale, it is evident that the two dark spots (arrows) changed into slightly bright contrasts, although the imaging position has somewhat drifted between the two images. Beside these two spots, we can also see that some bright spots appeared in Figure 2d. These contrast changes were reversible to some extent with the electrode potential; however, they were not perfectly reversible with increasing the number of potential cycles. The sizes of these spots were measured to be approximately 100 Å in diameter and 2-4 Å in depth or height, respectively, which are somewhat larger when compared to those reported for the Zn acceptor in GaAs.8 Similar STM image changes have been observed for MoSe2 in air by changing the polarity of the bias voltage.9 For explaining the phenomenon shown in Figure 2, we assume that the half-filled acceptor levels above the valence band of the p-MoSe2 have trapped holes around the acceptor anion. This is following the paper by Whangbo et al.,9 although the p-type semiconducting property of the MoSe2 used here is considered to derive from its nonstoichiometric nature rather than the Se substitution with the other acceptor dopant. We used Se as the transporting material for the synthesis of the p-MoSe2. At negative sample bias voltages, two factors could make the hole-trapped area appear as a dark spot in STM image. One is that electron tunneling will tend to occur to a lesser extent from the hole-trapped area than from its surrounding because the area is deficient of electrons. The other is the contribution (competition) of electron tunneling from the valence band. When higher negative sample bias voltages are applied, the opposite factor must be considered. That is, the acceptor level becomes occupied completely with the large band bending, making the vicinity of the acceptor site negatively charged. This enhances electron tunneling from this region, which will result in the disappearance of a dark spot or further the appearance of a bright spot in STM image. In Figure 3a we show the energy band diagram of the p-MoSe2 studied here. The valence and conduction band edges and the Fermi level are located at 0.9 V, -0.5 V, and 0.8 V versus SCE, respectively.5,10 From the effect of the acceptor level, it is considered that the STM image change observed in Figure 2 was caused by the acceptor level which was half filled at the electrode potential 0.2 V versus (10) Castro, R. J. Ph.D. Thesis, University of Puerto Rico, 1996.
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Figure 2. In situ STM images of p-MoSe2 which were observed in 0.05 M HNO3 with low (500 nm × 500 nm, a and b) and high (97 nm × 97 nm, c and d) magnification scales. The p-MoSe2 sample (Es) and STM tip (Et) potentials versus SCE were as follows: (a) Es, 0.2 V; Et, 0.65 V; (b) Es, -0.2 V; Et, 0.65 V; (c) Es, 0.2 V; Et, 1.0 V; (d) Es, 0 V; Et, 1.0 V. The tunneling currents were all at 1 nA.
electron tunneling from the valence band was checked by comparing the STM image changes obtained with the STM tip potential at 0.65 V and at 1.0 V versus SCE. The STM tip potential change from 0.65 to 1.0 V versus SCE increases the overlap of the valence band of the p-MoSe2 with the vacant levels of the STM tip. Thus, the contribution of the valence band in electron tunneling will become larger at 1.0 V than at 0.65 V versus SCE. As would be expected, the number of spots exhibiting the contrast change was observed to be smaller for 1.0 V than for 0.65 V versus SCE.
Figure 3. Energy band diagram of p-MoSe2 (a), and its changes with the band bending and the occupation situation of the acceptor level with the electrode potentials of (b) 0.2 V and (c) -0.2 V versus SCE.
SCE becoming completely occupied at 0 or -0.2 V versus SCE. We show this feature schematically in Figure 3b and c. Here only one representative acceptor level is drawn arbitrarily. It is supposed that the same distribution exists in all of the acceptor levels as that within the band gap. This implies that there will exist spots which have disappeared already at 0.2 V versus SCE. The effect of
For comparison, we also measured the p-MoSe2 surface in air, H2O, and 0.05 M HNO3, respectively, without potentiostatic conditions. The sample bias voltage was changed between -0.8 V and -1.2 V, that is, approximately the same bias voltage range as that studied in Figure 2. The band bending under the 0 bias condition is expected to be nearly 0, assuming for example 5.5 eV as the work function of the Pt/Ir11 STM tip and 4.4 eV as the potential difference between the vacuum level and (11) CRC Handbook of Chemistry and Physics, 72nd ed.; CRC Press: Boca Raton, FL, 1991; pp 12-97.
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the normal hydrogen electrode potential.12 In none of these experiments was the STM image changed clearly, as was observed in Figure 2. This is expected because, without potentiostatic conditions, the band bending does not vary significantly with the bias voltage change due to the relatively high carrier concentration13 of the p-MoSe2 used here (3 × 1019 cm-3). On the other hand, under potentiostatic conditions, the voltage will be applied mostly within the space charge region of the p-MoSe2,14 making a large band bending. Finally, we should note the important advantage for experiments performed under potentiostatic conditions compared to those under vacuum or air. That is, to study (12) Parsons, R. In Standard Potentials in Aqueous Solution; Bard, A. J., Parsons, R., Jordan, J., Eds.; Marcel Dekker Inc.: Dordrecht, 1985; p 13. (13) Fan, F.-R.; Bard, A. J. J. Phys. Chem. 1991, 95, 1969. (14) Scholz, G. A.; Gerischer, H. J. Electrochem. Soc. 1992, 139, 165.
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the phenomenon, we can vary the sample and STM tip potentials independently with respect to a reference electrode although the variable potential ranges are limited within the double-layer regions. As was mentioned above, it is possible to do such experiments as we vary the STM tip potential while keeping the sample band bending (i.e., the sample potential) constant or vice versa. In addition, mapping of the energy levels of the acceptor states and estimation of their densities could be done with the present imaging technique. Acknowledgment. The authors would like to acknowledge the financial support of NSF-EPSCoR grant number OSR-9452893, DOD-EPSCoR grant number P-35825-CH-DPS, and the University of Puerto Rico. LA980106W
