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Electrosynthesized CdS/HgS Heterojunctions Anthony Gichuhi, B. Edward Boone, and Curtis Shannon* Department of Chemistry, Auburn University, Auburn, Alabama 36849-5312 Received June 29, 1998 We report the results of a study of electrosynthesized CdS-HgS heterojunctions using scanning tunneling microscopy (STM), photoluminescence spectroscopy (PL), and electrochemistry. CdS thin films were grown by electrochemical atomic-layer epitaxy (EC-ALE) onto Au(111) substrates and were terminated with a single HgS monolayer using one of two methods. The first method, which involved the chemical exchange of the terminal Cd layer with Hg2+, produced a disordered, highly polycrystalline film, as evidenced by the PL spectrum, which was dominated by CdS trap luminescence. The second method, in which the HgS monolayer was grown by EC-ALE, resulted in the formation of a high-quality heterojunction. PL measurements indicate a high degree of electronic coupling between the CdS substrate and the electrochemically deposited HgS layer in this case. These findings were confirmed by atom-resolved and micron-scale STM images.
Introduction Electrosynthesized thin films of group II-VI compound semiconductors display rich and interesting physics and chemistry.1 A wealth of new information on the atomiclevel details of electrodeposition has been revealed by studies of these systems,2 as have some new and unexpected photophysics.3 In addition to these fundamental issues, the ability to fabricate high-quality thin films and layered structures under ambient conditions makes electrosynthesis an attractive route to materials for use in solar cells, photon detectors, and optoelectronic devices. Layered structures can be produced in a variety of ways from the electrochemical environment. Perhaps the most common approach is to apply an oscillating current or potential function to the working electrode during deposition.4 Recently, Switzer has reported a variation on this general theme in which electrochemically driven selfassembly was used to produce periodic Cu2O superlattices.5 Modulation lengths on the order of tens of nanometers are possible using either of these two approaches. A completely different strategy involves building the target structures from the “ground up” using surface-limited reactions such as underpotential deposition (UPD) to control the structural integrity of each monolayer as it is deposited. An elegant example of this approach, which is * To whom correspondence should be addressed. Tel.: (334) 8446964. Fax: (334) 844-6959. E-mail:
[email protected]. (1) (a) Rajeshwar, K. Adv. Mater. (Weinheim, Ger.) 1992, 4, 23. (b) Loizos, Z.; Mitsis, A.; Spyrellis, N.; Froment, M.; Maurin, G. Thin Solid Films 1993, 235, 51. (c) Hodes, G. Isr. J. Chem. 1993, 33, 95. (2) (a) Aloisi, G. D.; Cavallini, M.; Innocenti, M.; Forest, M. L.; Pezzatini, G.; Guidelli, R. J. Phys. Chem. B 1997, 101, 4774. (b) Demir, U.; Shannon, C. Langmuir 1996, 12, 594. (c) Lister, T. E.; Colletti, L. P.; Stickney, J. L. Isr. J. Chem. 1997, 37, 287. (d) Colletti, L. P.; Teklay, D.; Stickney, J. L. J. Electroanal. Chem. 1994, 369, 145. (3) (a) Boone, B. E.; Shannon, C. J. Phys. Chem. 1996, 100, 9480. (b) Boone, B. E.; Gichuhi, A.; Shannon, C. J. Phys. Chem. B, submitted for publication. (4) Phillips, R. J.; Golden, T. D.; Shumsky, M. G.; Bohannan, E. W.; Switzer, J. A. Chem. Mater. 1997, 9, 1670. (5) Switzer, J. A.; Hung, C.-J.; Huang, C.-Y.; Switzer, E. R.; Kammler, D. R.; Golden, T. D.; Bohannan, E. W. J. Am. Chem. Soc. 1998, 120, 3530. (6) (a) Gregory, B. W.; Suggs, D. W.; Stickney, J. L. J. Electrochem. Soc. 1991, 138, 1279. (b) Suggs, D. W.; Villegas, I.; Gregory, B. W.; Stickney, J. L. J. Vac. Sci. Technol., A 1992, 10, 886. (c) Villegas, I.; Stickney, J. L. J. Electrochem. Soc. 1992, 139, 686. (d) Suggs, D. W.; Stickney, J. L. Surf. Sci. 1993, 290, 375. (7) Demir, U.; Shannon, C. Langmuir 1994, 10, 2794.
becoming increasingly widespread as a synthetic route to the II-VIs, is electrochemical atomic-layer epitaxy (ECALE).6 Previous work in our laboratory has focused on the synthesis and characterization of ultrathin films of the II-VI sulfides using EC-ALE.7 More specifically, we have recently characterized the structure and optical properties of CdS films 2-10 monolayers in thickness by scanning tunneling microscopy (STM), photoluminescence (PL), and resonance Raman spectroscopy (RRS).8 These films were found to be atomically flat on the micrometer scale with a low density of structural defects. For example, the root mean square (rms) roughness of these films, on the micron scale, is typically 0.4 nm or less, which is on the order of the d spacing of wurtzite CdS along the {111} direction (0.335 nm).9 PL data indicate weak charge-carrier confinement along the surface normal and bulk optical properties in the surface plane, as expected for a thin film displaying one-dimensional quantum confinement. Using RRS detuning experiments, the onset of quantum confinement was found to occur at a CdS coverage of 7 ML. In this study, we were interested in using these highquality monolayer films as platforms for the growth of more complex layered structures such as heterojunctions. Demonstration of the ability to form a stable heterojunction electrochemically would be the first step toward the fabrication of multiple quantum well structures, for example. Here, we report on our initial experiments in this areasthe growth of CdS/HgS heterojunctions in an electrochemical cell. This system was chosen because of the excellent lattice match between the two materials (CdS, 0.4135 nm; HgS, 0.4149 nm) and because the band gap of HgS (2.1 eV) is smaller than that of CdS (2.47 eV). Experimental Section Preparation of Au (111) Substrates. Au micro-bead electrodes were prepared as previously described.10 These polycrystalline substrates contain numerous large elliptical (111) facets with major and minor axis lengths of approximately 1000 and (8) Boone, B. A.; Gichuhi, A.; Shannon, C. J. Phys. Chem. B 1998, 102, 6499. (9) Landolt-Bornstein Numerical Data and Functional Relationships In Science and Technology; Helwege, K. H., Ed. New Series III; Springer: Berlin, 1989; Vols. 22a and 23a. (10) Hsu, T. Ultramicroscopy 1988, 11, 167.
10.1021/la980780d CCC: $18.00 © 1999 American Chemical Society Published on Web 12/12/1998
764 Langmuir, Vol. 15, No. 3, 1999 500 µm, respectively. These substrates can be aligned for STM imaging and PL measurements using a low-magnification optical microscope. Chemicals. 3CdSO4‚8H2O, Na2S‚9H2O, NaClO4‚H2O, HClO4, and H2SO4 (Certified ACS Plus) were used as received. Millipore-Q purified water was used to make all solutions. Electrochemistry. Electrochemistry experiments were carried out in a single-compartment, three-electrode Teflon cell. In addition to the Au microbead working electrodes, we used Ag/ AgCl (3 M NaCl) reference electrodes and refer all electrode potentials to this reference; the counterelectrode was a platinum wire. Cyclic voltammetry was performed using a Pine AFRDE-4 bipotentiostat and an HP-7055B X-Y recorder; the bipotentiostat was modified in-house to improve the signal-to-noise ratio. The electrochemical cell was directly connected to a solution-handling manifold that allowed the electrolyte to be changed without the working electrode being exposed to the ambient. The solution reservoirs and all components that came into contact with the solutions were made of Teflon or Kel-F. All solutions were purged for 20 min with ultrahigh-purity (UHP) Ar to remove O2. Electrochemical measurements were performed using unmasked Au microbead electrodes. Therefore, cyclic voltammograms are characteristic of a polycrystalline Au surface. The substrate was thoroughly rinsed with blank electrolyte solution following the deposition of each element. After the final deposition cycle, the substrates were emersed under potential control, dried in a stream of UHP Ar, and stored under UHP Ar prior to analysis by STM or PL. Cadmium deposition was carried out from a 1 mM CdSO4 solution (pH 2.9) prepared from 3CdSO4‚8H2O in 0.1 M H2SO4. To deposit the first monolayer of Cd, the electrode potential was scanned from +0.800 to -0.560 V at a scan rate of 0.100 V s-1. The second and third monolayers of Cd were deposited at a fixed potential of -0.500 V. Sulfur was deposited from a solution prepared from 1 mM Na2S‚9H2O in 0.1 M NaClO4‚H2O/0.01 M HClO4 supporting electrolyte. Sulfide solutions were prepared fresh prior to each experiment because of the evolution of H2S at this acidic pH (2.5-2.9). The first monolayer of S was deposited by sweeping the electrode potential from -0.630 to -0.300 V at 0.100 V s-1. The second and third monolayers of S were formed at fixed-electrode potentials of -0.200 and -0.350 V, respectively. Mercury deposition was effected from a 1 mM solution of Hg(OOCCH3)2 in 0.10 M NaOOCCH3/0.010 M CH3COOH. Chemical exchange of Cd for Hg was carried out in a 1 µM Hg(OOCCH3)2 solution. Scanning Tunneling Microscopy. All scanning tunneling microscopy (STM) work was performed in air using a model SA-1 ambient STM (Park Scientific Instruments, Sunnyvale, CA). Atomic- and micron-scale images were acquired using constant current (atomic resolution) and constant height (micron-resolution) modes, respectively. In all experiments, W tips, prepared by etching a 0.5-mm diameter wire in 1 M KOH solution using a commercial (Park Scientific) etching circuit, were used for imaging. The tip was biased positive relative to the sample. The calibration of the piezoelectric scanner in the x-y plane was performed using highly oriented pyrolytic graphite (HOPG, donated by Dr. Arthur Moore, Union Carbide, Parma, OH) and a Au (111) single crystal. The standard deviation for all the lateral dimensions reported here is (0.011 nm. The z calibration (perpendicular to the plane of the surface) of the piezo was carried out using the Au atomic step height (0.235 nm). The rms roughness calculations were carried out using image analysis software from Park Scientific Instruments (Sunnyvale, CA). The rms roughness of a line scan is defined as the standard deviation of the experimental data from a linear least-squares fit of the data set. Photoluminescence. The PL spectrometer used for this work has been described previously.3a Briefly, PL was excited using the 457.6-nm line of a Coherent Innova-70 Ar+ laser. CW power levels at the samples were maintained at ca. 100 mW. The incident light was p-polarized (p, in the plane of incidence) and was focused to a spot approximately 100 µm in size on the surface of a (111) facet using a spherical lens. The angle of incidence was approximately 40-50° with respect to the surface normal. The scattered light was collected at an angle of about 45° to the surface
Gichuhi et al. Scheme 1. Growth of CdS-HgS Heterostructures by (A) Chemical Exchange and (B) Electrochemical Deposition
normal using an f-matched f/1.4 camera lens (Canon). The acquisition time per spectrum was typically 250 s.
Results and Discussion Chemical and Electrochemical Growth of HgS monolayers on CdS/Au(111). In all experiments, the substrates were Au(111) onto which three monolayers of CdS had been deposited by EC-ALE; S was the final element deposited in all cases. Heterojunctions were prepared by both chemical and electrochemical methods, Scheme 1. The chemical deposition method involved an initial electrochemical step in which the three-monolayer CdS thin film was capped with an atomic layer of Cd by UPD. The Cd capping layer was then exchanged with Hg by exposing the surface to a 1 µM Hg2+ solution. On the basis of the vastly different solubilities of the two materials (pKsp(HgS) ) 53; pKsp(CdS) ) 27), the outermost layer of Cd is expected to exchange for Hg on exposure to an Hg2+ solution, resulting in the precipitation of a monolayer of HgS on top of the CdS thin film. This chemistry is based on the method first described by Weller and co-workers for the synthesis of CdS/HgS quantum dot-quantum wells.11 The electrochemical deposition method consisted of a single HgS EC-ALE deposition cycle. That is, a single monolayer of Hg and S, each, was deposited at their respective underpotentials onto the CdS/Au(111) substrate. Cyclic voltammograms for the deposition and stripping of an atomic layer of Hg on CdS/Au(111) are shown in Figure 1A. The cathodic wave centered at 0.434 V corresponds to the UPD of Hg and the anodic wave at 0.896 V to the stripping of this atomic layer. This behavior, particularly the large peak separation between the cathodic and anodic waves, is characteristic of the UPD (11) (a) Eychmu¨ller, A.; Ha¨sselbarth, A.; Weller, H. J. Lumin. 1992, 53, 113. (b) Ha¨sselbarth, A.; Eychmu¨ller, A.; Eichberger, R.; Giersig, M.; Mews, A.; Weller, A. J. Phys. Chem. 1993, 97, 5333. (c) Mews, A.; Eychmu¨ller, A.; Giersig, M.; Schooss, D.; Weller, H. J. Phys. Chem. 1994, 98, 934. (d) Mews, A.; Kadavanich, A. V.; Banin, U.; Alivisatos, A. P. Phys. Rev. B: Condens Matter 1996, 53, R13242.
Electrosynthesized CdS/HgS Heterojunctions
(a)
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(b)
Figure 1. Cyclic voltammetric behavior of Hg and S at CdS/ Au surfaces in the underpotential deposition region. The data were used to determine the deposition conditions for the growth of the HgS monolayer. The scan rate was 0.100 V s-1 in both cases. (A) Underpotential deposition of Hg from 1 mM Hg(OOCCH3)2 in 0.10 M CH3COONa/0.01 M CH3COOH electrolyte. (B) Atomic-layer deposition of S from 1 mM Na2S in 0.10 M NaClO4/0.01 M HClO4 supporting electrolyte.
of Cd, Zn, and Hg on S-terminated surfaces. The voltammetric behavior of HS- at the Hg-terminated surface is shown in Figure 1B. The anodic wave corresponds to the deposition of an S atomic layer and the cathodic wave to the stripping of this layer from the surface. The E0′ for this process is -0.332 V, somewhat positive of its value on Cd-terminated surfaces at the same pH, reflecting the stronger Hg-S interaction. The voltammogram consists of a single wave, which is very similar to what we previously have observed on Cd-terminated surfaces in acidic media. In basic media, two peaks are always observed. Structure of the HgS monolayer. In the case of the electrochemically deposited monolayer, we were able to obtain high quality STM images that illustrate both the atomic-level and the micron-scale structure of the HgS surface, Figure 2. Atom-resolved images, Figure 2A, show that the HgS monolayer is closest packed and 6-fold symmetric. The center-to-center distance between adjacent atoms in this image is 0.42 nm, which is close to the interatomic spacing (0.42 nm) we observed for the terminal CdS layer, and is also consistent with the interatomic spacing in the hexagonal form of HgS (0.42 nm). The micron scale image (Figure 2B) shows an atomically smooth surface in which the roughness is 0.069 ( 0.010 nm. The bright band in the image corresponds to an atomically flat gold terrace. The terrace height (0.24 nm) is characteristic of Au(111). This region of the image was excluded when calculating the rms roughness value. The rms roughness value is actually less than what we observed for the CdS surface itself (0.368 ( 0.032 nm) and indicates that the deposition of Hg does not lead to significant structural disordering of the CdS thin film. In the case of the HgS monolayer deposited chemically, however, we were not successful in obtaining images at either the atomic or the micron level. We believe this has to do with the dissolution and reprecipitation of CdS
Figure 2. Scanning tunneling microscopy of an electrochemically deposited HgS monolayer on CdS/Au(111). The film is S-terminated in both cases. (A) 4.0 × 4.0 nm atom-resolved image showing the formation of a hexagonal HgS phase on CdS/Au(111). (B) 0.25 × 0.25 µm micron-scale image of the same surface. Imaging conditions are given in the Experimental Section. Images have been tilt corrected but are otherwise unmanipulated.
associated with the exchange chemistry, and it will be discussed more fully below. Such a chemical treatment may lead to the formation of an unstable tunnel junction. We are not suggesting that chemical approaches are not viable routes to high-quality thin film materials. Indeed, we note that other than varying the Hg concentration, no systematic attempts were made to optimize the Hgexchange chemistry. In the case of ZnS thin films, we find that both electrochemical and chemical routes lead to materials with identical atomic-level structures. Photoluminescence. PL spectra obtained from the chemically and electrochemically deposited HgS monolayers are shown in Figure 3. The upper trace in this figure is the PL response from the chemically grown HgS layer. This spectrum is characterized by three major peaks at approximately 505, 595, and 710 nm, which are assigned as the CdS band edge luminescence, the HgS band edge luminescence, and the CdS mid gap or trap luminescence, respectively.12 The PL spectrum for the electrochemically (12) (a) Weller, H. Angew. Chem., Int. Ed. Engl. 1993, 32, 41. (b) Ha¨sselbarth, A.; Eychmu¨ller, A.; Eichberger, R.; Giersig, M.; Mews, A.; Weller, H. J. Phys. Chem. 1993, 97, 5333. (c) Simpson, C. T.; Imaino, W. I.; Becker, W. M.; Faile, S. P. Solid State Commun. 1978, 28, 39.
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Gichuhi et al. Scheme 2. Energy Level Diagram Illustrating the Dominant Photophysical Pathwaysa
a For simplicity, only transitions involving electrons are shown. Radiative transitions are indicated by solid lines and nonradiative processes by dashed lines. Band offsets were not measured directly; however, the diagram is consistent with the photoluminescence data shown in Figure 3.
Figure 3. Photoluminescence spectra of CdS-HgS heterojunctions excited using p-polarized 457.9-nm radiation. The incident power was ca. 100 mW at the sample. Growth conditions are described in the text.
deposited monolayer is shown in the lower trace. The band assignments for this spectrum are the same as those given for the upper trace. Note, however, that the CdS band edge luminescence peak occurs at 499 nm, blue shifted by about 6 nm from the bulk value of 505 nm. For comparison, the exciton absorption maximum of CdS colloids of a similar critical dimension (ca. 1.0 nm) is shifted by over 100 nm from the bulk λmax, indicating that we are observing a weak quantum confinement effect associated with onedimensional confinement of the charge carriers.13 In addition, the position of this peak is identical to what we observed for the as-grown three-monolayer CdS films.3b This finding is significant because it indicates that the electrodeposition of the HgS capping layer does not alter the optical properties of the underlying CdS quantum well. Furthermore, this result corroborates the STM data in Figure 2, which showed a well-ordered monolayer at the atomic level and an rms roughness comparable to that of the substrate itself. In contrast, when the HgS capping layer is deposited chemically, CdS band edge luminescence, characteristic of bulk CdS, is observed at 505 nm. This result, taken together with the negative STM results discussed previously, indicates that the structure of the CdS thin film has been significantly altered by the chemical treatment. Evidently, both Cd and Hg are dissolved and reprecipitated on the Au surface during the chemical treatment, resulting in a film that is probably composed of large polycrystalline deposits of both CdS and HgS. There is a second significant difference in the appearance of the PL spectra of the two films. The intensity of the HgS band edge luminescence feature as compared with the CdS trap luminescence feature is much higher in the electrochemically deposited film than in the chemically treated film. In fact, trap luminescence is (13) Alivisatos, A. P. Science 1996, 271, 933.
extremely weak in this spectrum. On the other hand, the PL spectrum of the chemically deposited HgS layer is dominated by CdS trap luminescence. These findings confirm our earlier assertion regarding the dissolution and reprecipitation of CdS and HgS crystallites on the Au surface during the chemical treatment. In the case of the electrodeposited monolayer, the PL data suggests that the HgS monolayer forms an excellent passivating film. That is, there is efficient electronic coupling between the two materials, facilitating nonradiative decay of the CdS charge carriers into the HgS layer. Furthermore, we observe only luminescence characteristic of CdS and HgS; there is no evidence of the formation of an alloy. This suggests the presence of an abrupt interface between the two materials. In contrast, in the chemically deposited monolayer, there is poor contact between the CdS and the HgS and essentially no coupling between the two materials; consequently, the nonradiative trapping of charge carriers at CdS surface states dominates the photophysics. In essence then, two competing nonradiative pathways operate in these systems: charge-carrier trapping to CdS surface states (in regions where there is no HgS capping layer) and charge transfer across the CdS-HgS interface, Scheme 2. When random precipitation of HgS occurs, the former process is more facile; however, when a robust CdS/HgS interface forms, the latter branch dominates. Conclusions We have presented electrochemical, STM, and PL evidence that high-quality CdS/HgS heterojunctions can be synthesized in an electrochemical cell at room temperature. STM images of these materials indicate that the electrodeposited HgS layer is well-ordered from the micron scale down to the atomic level. PL indicates that efficient electronic coupling across the CdS-HgS interface occurs and suggests that interdiffusion and alloying do not occur to a significant extent, resulting in the formation of an abrupt interface. These results suggest that it is possible to grow high-quality layered structures on electrode surfaces using a ground up approach such as EC-ALE. Characterization of simple electrochemically fabricated quantum well structures is ongoing in our laboratory and will be reported on soon. Acknowledgment. The financial support of this work by the National Science Foundation and Auburn University is gratefully acknowledged. LA980780D
