0 Copyright 1992 American Chemical Society
MARCH 1992 VOLUME 8, NUMBER 3
Letters Epitaxial Electrodeposition of CdSe Nanocrystals on Gold Yuval Golan, Lev Margulis, Israel Rubinstein,' and Gary Hodes* Department of Materials and Interfaces] The Weizmann Institute of Science, Rehovot 76100, Israel Received November 4, 1991. In Final Form: January 2, 1992 Epitaxially-orientedCdSe quantum dots (ca. 5 nm diameter) with controllablespatial distribution and narrow size distributionhave been electrodepositedon evaporated gold substrates. The degree of aggregation of the nanocrystals can be controlled by the deposition temperature and current density. All the nanocrystals deposited on a single gold crystallite are of the wurtzite structure with the basal plane parallel to the gold (Ill] plane and with the same azimuthal orientation. This simple and convenient method of preparing oriented quantum dots provides an effective means for studying the transition between molecular and bulk properties, as well as a new surface architecture on the nanometer scale. There has been increasing interest in materials which are intermediate in size between singlemolecules and bulk, i.e. in the size regime where the optoelectronic properties are changing with s i ~ e . l - ~Such nanoscale groupings include clusters, quantum dots, and colloids (small scale in three-dimensions), quantum wires (two-dimensions), and superlattices (one-dimension). Part of this interest is fundamental, i.e. understanding how bulk properties evolve from molecular properties, and part is the great potential offered by a group of materials which are novel by virtue of their physical, rather than their chemical] structure. Nanocrystalline semiconductors have been previously reported, which exhibit a blue shift in the absorption spectrum, attributed to charge localization in the individual nano~rystals.*'~ Recently, nanocrystalline films of CdSe and CdS have been prepared by both chemical15
* To whom correspondence should be addressed.
(l)Brue, L.; Siegel, R. W.; et al. J. Mater. Res. 1989, 4, 704, and references cited therein. (2) Henglein, A. Chem. Reu. 1989,89, 1861. (3) Fojtik, A.; Weller, H.; Koch, U.; Henglein, A. Eer. Bunsen-Ges. Phys. Chem. 1984,88,969. (4) Gritzel, M. Nature 1991, 349, 740. (5) Brus, L.J. Chem. Phys. 1983, 79,5566. Brus, L.J. Chem. Phys. 1984,80, 4403. (6) Rossetti, R.;Ellison, J. L.; Gibson, J. M.; Brus, L. J. Chem. Phys. 1984,80, 4464. (7) Brus, L. J. Chem. Phys. 1986,90, 2555.
and electrochemical deposition, the latter using a method first described by Baranski et al.16J7and which was later shown by Hodes et al.18J9to produce films exhibiting size quantization. This paper describes for the first time arrays of CdSe quantum dots electrodeposited onto gold substrates. These nanocrystals are epitaxially oriented]with the basal plane parallel to the gold { 111)surface and with a uniform azimuthal orientation on the gold (111)single crystals. The distribution of these quantum dots-either as isolated nan0crystaJ.s or aggregates of varying surface coverage-can be controlled by a combination of time, current density, (8)Meyer, M.; Walberg, C.; Kurihara, K.;Fendler,J. H. J. Chem. SOC., Chem. Commun. 1984,W. (9) Chang, A.; Pfeiffer, W. F.; Guillaume, B.; Baral, S.; Fendler, J. H. J. Phys. Chem. 1990, 94, 4284. (10) Yi, K. C.; Fendler, J. H. Langmuir 1990, 6, 1519. (11) Zhao, X.K.; McCormick, L. D.; Fendler, J. H. Langmuir 1991,7, 1255. ~~. . (12) Wang, Y. Acc. Chem. Res. 1991,24, 133. (13) Zen, J.; Fan, F. F.; Chen, G.; Bard, A. J. Langmuir 1989,5,1355. (14) Herron, H.; Wang, Y.; Eckert, H. J. Am. Chem. SOC.1990, 112, 1933
(15) Hodes, G.; Albu-Yaron, A.; Decker, F.; Motisuke, P. Phys. Reu. E . 1987,36,4215. (16) Baranski, A. S.;Fawcett, W. R.; McDonald, A. C.; de Nobriga, R. M.; McDonald, J. R. J. Electrochem. SOC.1981, 128, 963. (17) Baranski. A. S.:Bennet. M. S.; Fawcett, W. R. J. Appl. .. Phys. 1983,54, 6390. (18) Hodes,G.;Engelhard,T.;Albu-Yaron,A.;Pettford-Long,A.Mater. Res. SOC.Symp. h o c . 1990, 164, 81. (19) Hodes, G.; Albu-Yaron, A. h o c . Electrochem. SOC. 1988,88,298.
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Figure 1. TEM BF images of CdSe nanocrystals electrodeposited at 120 "C under the following conditions: (a) 10 pA*cm-2X 30 s (300 pC-cm-2);(b) 50 pA-cm-2X 6 s (300 pC-cm-2); (c) 700 pA-cm-2 X 0.5 s (350 pC-cm-2); (d) 3000 pA*cm-2X 0.1 s (300 pC*cm-2).
and temperature of deposition, and this distribution is stable at ambient temperature for over a year after their deposition. The substrates used for nanocrystal deposition were gold films, 35 nm thick, evaporated on glass slides at room temperature and then annealed in air for 3 h at 250 "C 2o The CdSe nanocrystals were deposited from an unstirred solution of 50 mM Cd(C10&6H20 and 15mM elemental selenium in dimethyl sulfoxide CdSe was electrodeposited at a constant current in a two-electrode cell, with the gold substrate used as the cathode (1.0 cm2area) and a glassy carbon rod as the anode. The deposition temperature was usually 120 "C. The working electrode was then washed in pure hot DMSO, rinsed thoroughly with triple-distilled water, and dried under a stream of argon. Samples for transmission electron microscopy (TEM) were prepared by carefully floating the gold films in 5% aqueous HF and lifting onto TEM grids.20p22 Nanometer-size particles are characterized by a very high surface-to-volumeratio and, thus, a very high surface energy. Aggregation of the nanocrystals would lower the surface energy and, hence, is energetically preferred. It is therefore expected that, given appropriate conditions (e.g. long deposition time or high temperature), the nanocrystals would tend to aggregate. The effect of current density vs time is demonstrated in the TEM bright field (BF) images of samples electrodeposited at 120 "C by passing approximately the same amount of charge (300-350 pC-cm-2),as shown in Figure 1. While CdSe nanoparticles deposited at 10 pA*cm-2for 30 s (Figure la) form large aggregates of several hundreds of nanocrystals each, the same amount of charge passed during 6 s results in a mixture of isolated nanocrystals and smaller aggregates of up to several tens of nanocrystals (Figure lb). A t a short deposition time of 0.5 s (Figure IC),one obtains mainly isolated nanocrystals with some small aggregates; at an even shorter deposition time (0.1 ~~
(20) Golan, Y.;Margulis, L.; Rubinstein, I. Surf. Sci. 1992,264,312.
(21) The depositionsolutionis preparedat 140'Cwithvigorous stirring. (22) The samples were examined at room temperature using a Phillips analytical EM-400T TEM operating at 120 kV. Bright-field and darkfield images were taken at amagnificationof XlOO OOO ( 5 % magnification error). Electron diffraction patterns were generated at a camera length of 800 mm and calibrated according to the gold substrate.
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Figure 2. TEM BF images of CdSe nanocrystals electrodeposited by passing 100 pA-cm-2X 7 s at temperatures of (a) 150 OC, (b) 120 "C, and (c) 90 "C. (d) Same temperature as b but at 700 pA*cm-2X 1 s. s, Figure Id) a low density of nanocrystals is obtained, mostly as aggregates of 2-6.23 The different nanocrystal distribution patterns may either be formed at the nucleation stage or result from surface mobility of nanocrystals (i.e. time-dependent aggregation), or both. This is being studied in more detail. The effect of the deposition temperature is demonstrated in a series of samples deposited at different temperatures by passing 100pA-cm-2 for 7 s (Figure 2a-c). As the deposition temperature is lowered from 150 to 90 "C, the tendency to aggregate is diminished, apparently as a result of restricted thermal mobility. Figure 2d is similar to Figure 2b in deposition temperature and the amount of charge passed, but the deposition time is substantially shortened, demonstrating the combined effect of deposition temperature and time in promoting aggregation. It should be noted that while the different conditions in Figures 1 and 2 result in very different nanocrystal spatial distribution, the nanocrystal size remains unaffected in all cases, i.e. an average diameter of 5.1 nm with a standard deviation of 1.5 nm. Figure 3b shows a transmission electron diffraction pattern obtained from the (111)textured gold surface covered with CdSe nanocrystals in Figure 3a. A typical polycrystallinering pattern is obtained, containinghtense rings correspondingto the gold (111)substrate9 Although most of the diffraction rings corresponding to the CdSe nanocrystals are concealed by the intense gold reflections, the ring corresponding to the (lOi0) planes in hexagonal (wurtzite) CdSe24can be observed (the ring of smallest radius, highlighted in Figure 3b by the objective aperture image); the corresponding d-spacing is 3.72 A, a region where neither gold nor cubic CdSe reflections exist. Hence, the detection of the 3.72-A ring serves as an unequivocal indication for the presence of hexagonal (wurtzite) CdSe on the gold. Figure 4b presents a dark-field (DF) image taken with the objective aperture positioned on the CdSe ring, as shown in Figure 3b. The bright contrast of the (23) Figure Id indicatesthat the faradaicefficiencyfor CdSedeposition decreases substantially a t high current densities. (24) We use the four-axis hexagonal system a1,02,03, c to define the indices of directions and planes in the hexagonal CdSe lattice (see ate, H.M.; Crocker, A. G. Phys. Status Solidi 1965,9,441).
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Figure 4. TEM BF (a)and DF (b) images of CdSe nanocrystals electrodeposited by passing 100 pA*cm-2X 7 s at 120 "C. The position of the objective aperture for the DF image is shown in Figure 3b. The proximityof the objectiveaperture to the primary beam results in reduced contrast in the DF image.
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Figure 3. (a) TEM BF image showing Au substrate crystals with CdSe nanocrystals electrodeposited as in Figure 2c. Note the selected area aperture image on the large gold crystal in the center. (b) Electron diffraction pattern obtained from part a. Note the objective aperture position on the inner ring (correspondingto the DF image in Figure 4b). (c) Selectedarea electron diffraction pattern obtained with the selected area aperture positionedas in part a. Note that although over a hundred nanocrystals are imaged within the selected area aperture, the spots corresponding to hexagonal (wurtzite)CdSe give a typical singlecrystal diffraction pattern.
particles establishes explicit correlation of the nanocrystals with the wurtzite CdSe diffraction pattern.25 Figure 3c shows a selected area electron diffraction pattern obtained from a single gold grain (note the selected
Figure 5. (a)CdSewurtziteunit cell.% (b)Schematicillustration of a hexagonal (wurtzite) CdSe basal plane on a (111) section of the gold lattice, emphasizing the 2:3 lattice match. Note the (lll)AU11(0001)(y~orientation, with the CdSe a-directionsaligned along the ( 11o)Au. The outlined rhombus indicatesthe projection of a CdSe unit cell.
area aperture in Figure 3a) with the (111)axis oriented along the electron beam. The gold reflections construct a hexagonal single-crystal spot pattern, as only one gold grain is imaged within the selected area aperture.20 A prominent observation in Figure 3c is the perfect singlecrystal spot pattern obtained from the numerous CdSe (25)The varying intensitiesof the nanocry&ds in the DF image (Figure 4b) are likely to result from either variation in nanocrystal thickness (also evident in the BF image) or small deviations from perfect planarity of the gold substrate.
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The epitaxial growth of wurtzite CdSe on (111)Au seems to result from the notable numerical fit between the relevant lattice spacings d{lIo)Au(the smallest interatomic distance in the (111)plane of the face-centered cubic ) smallest interatomic distance in lattice) and U O ( ~ S (the the basal plane of the hexagonal lattice). The distances are d{llOJAu= 2.884 A and aO(C&) = 4.299 A, with a ratio of 2:3 and a very small misfit of 0.6 % The epitaxial orientation of CdSe nanocrystalsis further demonstrated in Figure 6. When part of a neighboring Au substrate grain (at a different azimuthal orientation) is imaged within the selected area aperture (Figure 6a), a second set of diffraction spots is obtained for both the Au substrate and the CdSe nanocrystals, as expected (Figure6b). Figure 6c,d presents selected-areadiffraction of a thicker (two to three monolayersof nanocrystals) CdSe film; the ring patterns indicate loss of epitaxy as CdSe nanocrystals begin to deposit onto CdSe and not directly onto the gold substrate2' In conclusion, a novel method for simple and efficient deposition of CdSe quantum dots has been introduced. The nanocrystals are epitaxially deposited on Au surfaces using electrochemicaldeposition. The spatial distribution of the nanocrystals can be controlled by variation of the deposition temperature and the ratio of deposition time to current density, allowing the formation of aggregated or isolated nanocrystals, stable for a period of at least 1 year. The absolute single-crystalalignment of the CdSe nanocrystals can be accounted for by the numerical fit of the (111)gold and the wurtzite CdSe network spacings, with a 2:3 ratio (0.6% misfit). Hence, the existence of such a small misfit seems to be the reason for the epitaxial growth of hexagonal CdSe nanocrystals on (111)gold. The superior control over the distribution and crystallographic orientation of the new quantum dot arrays holds considerable promise in fundamental studies concerning well-defined molecular-to-bulk transformations. Furthermore, it may provide the basis for new nanolithographic procedures.
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nanocrystals included in the selected area aperture, indicating that they are all aligned in exactly the same azimuthal orientation, which can only be explained by epitaxy with the underlying substrate. The diffraction pattern with a 6-fold symmetryobtained from the CdSe and the fact that only reflections from are planes parallel to the c-axis (i.e. of the form (h,k,(i),O)) obtained clearly indicate that the basal plane of the hexagonal CdSe26 (see Figure 5a) is parallel to the gold (Ill)plane, i.e. a (lll)Aul((OOO1)cds,orientationrelationship reflections are perfectly exists. Moreover, the (1120)cd~ aligned with the (220)Au reflections, meaning that in the real lattice the CdSe a-directions are aligned along the ( 1 1 o ) A u directions, as illustrated in Figure 5b. (26) Wyckoff, R. W. G. Crystal Structures; Wiley: New York, 1963; VOl. 1, pp 111-112.
Acknowledgment. We wish to thank L. Leiserowitz for a critical review of the material and L. Addadi for helpful suggestions. Registry No. DMSO,67-68-5; CdSe, 1306-24-7; Au, 744057-5; Cd(C104)2-6Hz0,10326-28-0; Se, 7782-49-2. (27) The secondary ring reflections in Figure 6d are due to double diffraction, i.e. beams diffracted by the Au film are diffracted again by the CdSe layer.
