J. Phys. Chem. 1994, 98, 934-941
934
Preparation, Characterization, and Photophysics of the Quantum Dot Quantum Well System CdS/HgS/CdS A. Mews, A. Eychmiiller,’ M. Giersig, D. Schooss, and H. Weller Hahn- Meitner- Institut Berlin GmbH, Abteilung Photochemie, Glienickerstrasse 100, D- 14109 Berlin, Germany Received: July 15, 1993; In Final Form: October 26, 1993”
The synthetic procedure, the characterization, and some photophysical properties of a quantum dot quantum well (QDQW) system are described in detail. The novel structures prepared via wet chemical methods consist of a core of size-quantized C d S and a well of 1-3 monolayers of H g S capped by 1-5 monolayers of C d S acting as the outermost shell. Additionally, theoretical calculations based on the effective mass approximation appropriate to describe the 1s-1s electronic transition of the composite particles are presented.
Experimental Section
Introduction Over the past ten years extensive research has been carried out in the field of nanometer-sized semiconductor particles (for reviews see, e.g., refs 1-9). It is this very size regime in which solids are gradually losing their bulk properties, approaching more and more molecularlike behavior. Due to the confinement of charge carriers to the restricted volume of the small particles, quantum mechanical phenomena are observable for which the range of sizes under investigation is frequently called the size-quantization regime. Enormous progress has been made in the preparation and characterization of almost all 11-VI compounds. Recently, it was proven possible to prepare 111-V semiconductor colloids.l”I2 Even the elemental IV-IV semiconductors have been synthesized in solution13-l5 as well as in the gas phase.1621 Besides new preparative techniques to make accessible the up to now scarcely available semiconductor particles such as InP and Gap, current directions in this field of research include the synthesis of monodisperse semiconductor species, i.e. the chemical synthesis of large molecules with a given agglomeration number (up to 300 or 400) in a given structure.22-24 Furthermore, the deposition of size-quantized particles onto substrates has been achieved in which the current interest is to study the optical and electrochemical properties of the evolving transparent films.25-29 In this paper, the progress made in yet another direction in small-particle research which has been followed over the past few years is reported. This is the modification of a given size-quantized semiconductor particle by means of surface chemistry. First attempts in this direction were made as early as 1984 with the generation of CdS islands on the surface of ZnS parti~les.~O Following this work, thegrowth of islands or complete layers of AgzS on CdS,31 Cd(0H)z on CdS,32 ZnS on CdS,33 Ag2S on AgI,34 CdSe on Z ~ I SCdSe , ~ ~on ZnSe,36 HgS on CdS,3’ PbS on CdS,38 and CdS on HgS39 has been achieved. Earlier this year we reported on the successful syntheses of the first quantum dot quantum well (QDQW): this is a three-layered structure consisting of a size-quantized CdS particle acting as the core and a complete layer of HgS on the surface of this core covered by, again, CdS as the outermost shell.40 Here, we describe in detail the synthetic procedure to build structures of this kind with variable thicknesses of the HgS and CdS layers, and it is demonstrated how the structures are characterized. A few experiments on the photophysics of the particles are presented. At the end of the article, computational results on the energetics and the wave functions of the charge carriers are presented. The calculations are based on the effective mass approximation. They represent an extension of previous work by B r d 5 and Haus et aL41 a Abstract
published in Advance ACS Abstracts, December 15, 1993.
0022-3654/94/2098-0934%04.50/0
Apparatus. Absorption spectra were measured with an Omega 10spectrophotometer (Bruins Instruments). UV-vis fluorescence
spectra were recorded with a Fluoromax Spectrometer (Spex). NIR fluorescence spectra were taken with the aim of a self-built spectrometer equipped with a germanium detector. All fluorescence spectra were corrected for the spectral response of the emission monochromators and detectors, the excitation intensities, and the optical densities of the colloidal solutions at the excitation wavelength. Fluorescence quantum yields were calculated by comparison with a Rhodamin 6 G dye solution emitting with a quantum yield close to 100%. The excitation source for the time-resolved fluorescence measurements was a cavity-dumped Rhodamine 6 G dye laser synchronously pumped by an argon ion laser (Spectra Physics) producing 6-ps-short pulses at a repetition rate of 80 kHz. The decay curves were recorded with a single photon counting system using a Hamamatsu R 2809 microchannelplate photomultiplier allowing an instrumental time resolution of about 50 ps. Temperature-dependent experiments were carried out in a Leybold VSK 4-300 continuous flow H e cryostat. For electron microscopy, a small drop of the colloidal solution was adsorbed on a copper grid coated with a carbon support film of 5-nmthickness. The dried grids were then examined with a Phillips C M 12 electron microscope at an acceleration voltage of 120 kV. For the histograms a t each stage of preparation the sizes of approximately 200 particles were measured with a total magnification of 4 000 OOOX. Subsequently, Gaussian size distribution functions were calculated using least-squares fits. The high-resolution electron microscopy (HREM) images were made under conditions of minimum phase-contrast artifack48 Axial illumination as well as a “nanoprobe mode” were used for imaging. The latter mode allows the reduction of the beam spot size to 1 nm, enabling electron diffraction of individual particles. The HREM micrographs were digitized with an electronic camera (Data Copy Corp., Model 610F)supported by an IBMcomputer. The scan step size was 13 pm which is equivalent to 0.23 A at the sample level. The calculations of the image processing were done on a pVax workstation. The particles of interest were extracted in image fields of 256 X 256 or 512 X 512 pixels depending on the size of the particles. The image processing technique is described by Giorgio et al.49 Ion detection in the colloidal solution was carried out with a polarographic analyzer Model 364 (EG&G, Princeton Applied Research) combined with a dropping mercury electrode. As supporting electrolyte, 0.05 M NaC104 was used. One polarographic scan took about 5 min. For ion detection of the free ions in solution after precipitation of the colloids an ICP-MS (Inductive Coupled Plasma Mass 0 1994 American Chemical Society
The QDQW System CdS/HgS/CdS Spectroscopy) Perkin ElmerSciex Elan 5000 was used. Precipitation of the colloids was induced by adding 0.01 M BaC104 and 0.01 M NaS04: by adsorbing to the colloids, Bas04 forms a precipitate which readily settles out and thereby extracts the particles from the solution. The ions remaining in the supernatant transparent solution above the sediment are then detected via ICP-MS. The sedimentation is carried out quickly (10 s) after each preparative step for which the ion concentration is to be determined. Preparation. CdS Core. The synthesesof thecolloidal solutions were carried out in a 2-L three-necked round bottom flask with a pH electrode, septum, and gas inlet system. The basic CdS colloids were prepared following a method described by Spanhel et al.? 2 L of deionized water were purged with argon for 20 min. After the addition of 4 mL of 0.1 M sodium polyphosphate (Riedel de Haen, M, = 609 g/mol) as the stabilizing agent and 4 mL of 0.1 M Cd(C104)2) (Alfa), the pH was brought to 9.4 using 0.1 M NaOH. Under vigorous stirring, 9 mL of H2S was injected into the gas volume above the solution. During the next 10 min the pH decreased to 4.2. It was then adjusted to pH = 7.0 by again using 0.1 M NaOH. Subsequently, the emerging yellow colloidal solution was purged with argon for 30 min. CdSIHgSICdS Composites. To 1 L of the standard CdS colloid, 80 mL of the neutral le3 M Hg(C10& (Alfa) was added under stirring all a t once followed by readjustment of the pH to 7.0. This procedure results in the substitution of the surface Cd2+ions by Hg2+ ions, forming a monolayer of HgS on CdS (see below). The next preparative step was the reprecipitation of the Cd2+ ions released into the solution due to the substitution by Hg2+: the colloidal solution was treated by dropwise addition of 200 mL of a 5 X 10-4 M HzSIwater solution (25% excess S2-over the released Cd2+ ions) within 30 min. During the reaction, the pH decreased to 4.5 and was then readjusted to a neutral value with 0.1 M NaOH. Subsequent purging with argon and rotary evaporation of 200 mL of solvent removed the excess HzS and corrected the volume. The emerging colloidal particles consisted of a CdS core surrounded by a monolayer of HgS and almost one monolayer of CdS as the outermost shell (see below). From this point the preparation divides into two branches: either increasing the HgS layer thickness or increasing the thickness of the outermost CdS layer. The thickening of the HgS layer was simply achieved by repeating the substitution and reprecipitation steps described above: to the colloidal solution prepared so far was added 80 mL of l e 3 M Hg(C104)2 followed by the sulfidization with 200 mL of 5 X 10-4 M HzS/water solution. This procedure may be repeated up to three times. An increase in the CdS layer thickness was performed independent of the thickness of the formerly prepared HgS layer in the following manner: to 1 L of a given solution was added 2 mL of 0.1 M Cd(C104)2 a t pH = 7.0. During the dropwise addition of 200 mL of 1.2 X l e 3 M HzS/water solution, within 30 min, the pH decreases to 4.0. The solutions were then purged with argon at pH = 7.0 for 15 min, and the solvent was rotary evaporated until the starting volume was reached again. This procedure might be repeated up to five times. Fluorescence "activation" was performed as described earlier32 irrespective of the composition of the particles: to 1 L of a neutral colloidal solution was added 10 mL of 0.1 M Cd(C10& and the pH was brought to the alkaline pH = 11.5 using 1 M NaOH.
Results and Discussion Experiment. Figure 1, spectrum a shows the absorption of the starting CdS colloid (analytical concentration of CdS 2 X 10-4 M). The Is-1s (HOMO-LUMO or "excitonic") transition defined as the point where the second derivative of the absorption spectrum has its minimum lies a t 470 nm (2.65 eV). Figure 2a shows a transmission electron micrograph of this sample revealing
The Journal of Physical Chemistry, Vol. 98, No. 3, 1994 935
0.5
w
t 400 500 600 700 800 wavelength lnml
0.1 I-
400
500
600
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wavelength [nml Figure 1. Absorption spectra of the CdS starting colloid (a) and those M Hg2+(al-bz). Inset: CdS after the addition of portions of 2 X 8X M Hg2+after the times indicated togetherwith an intentionally
+
prepared mixed binary phase colloid of the same composition. spherical particles with cubic crystal structure. In Figure 3a the corresponding particle size histogram is seen which shows that particles with an average diameter of 5.3 nm and a fairly narrow distribution of sizes (f0.9 nm or 17% standard deviation) have been prepared. These results are in agreement with earlier work on nanometer-sized CdS particles (see e.g., refs 32 and 42). The shift of the transition energy from 2.5 eV (the bulk bandgap of CdS) to 2.65 eV is understood in terms of the confinement of the charge carriers (or the exciton) to the restricted volume of the 5.3-nm CdS spheres. The amount of the shift is consistent with theoretical calculations based on various approaches such as the use of the effective mass approximation,43." tight binding,45 and pseudopotential calc~lations,46~~~ the results of which do not differ significantly in this very size regime. The effect of stepwise addition of portions of 2 X 10-5 M Hg(C104)2to thestarting colloid is reflected in theabsorption spectra of Figure 1, al-b2: the first addition (spectrum al) leads to an increased absorption over the entire spectrum together with a shift of the onset of absorption to longer wavelengths as well as a shift of the ls-1s transition. This trend proceeds up to the addition of 8 X l t 5M Hgz+(spectra a2,a3,and b). At this point, the long-wavelength wing of the absorption is still pretty steep and the ls-1s transition is clearly detectable at 500 nm (2.48 eV). Further addition of Hg2+leaves the absorption spectra almost unaffected (spectra bl and b2, Figure 1): neither on the longwavelength side nor around the ls-1s transition region aredistinct spectral changes detectable. From Figure 2b it is seen that the particles with absorption spectrum b in Figure 1 are still almost spherical and crystalline. Within the experimental error, they have the same average diameter as the starting colloid (5.4 nm) and the same particle size distribution (20% standard deviation, cf. Figure 3, graph b). The course of the reaction indicated by the evolution of the absorption spectra of Figure 1 has been monitored polarographically in the very colloidal solutions as well as via ICP-MS measurements after precipitation of the colloids. In Figure 4a, the results of the polarographic experiments are plotted as the Cd2+ions detected in the colloidal solutions as a function of the concentration of the added Hg2+ ions. Up to the addition of 8 X l t 5 M Hg2+, the Cd2+ ion concentration increases linearly with a slope of 1, Le. each Hg2+ion added to the solution releases one CdZ+ ion into the solution. Upon further addition of Hg2+, the slope decreases to about 0.2. (It is not possible to detect Hg2+ ions polarographically in the colloidal solutions without additional complexing agents.) In Figure 4b the results of the ICP-MS experiments are plotted. The evolution of the Cd2+ ion concentration upon the addition of Hg2+ to the CdS solution detected with ICP-MS resembles very closely what has been observed with polarography: up to the addition of 8 X 10-5 M Hg2+ the Cd2+ concentration increases
Mews et al.
936 The Journal of Physical Chemistry, Vol. 98, No. 3, 1994
+Cd2+
+Cd2'
+Cd2' +Hg2' __c
CdS a)
1 layer o f HgS b)
Cl
2 layers of HgS d)
3 layers o f HgS
fl
e)
9)
Figure 2. TEM micrographs of CdS/HgS/CdS composites at various stages of the preparation (cf. text).
stages of syntheses, presumably due to an adsorptive interaction between the free ions and the BaS04 precipitate.) The results of these experiments are explained straightforwardly by a substitution reaction (1) taking place up to the addition of 8 X 1C5M Hg2+. The assumption of a substitution-type reaction (CdS),
I
20
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00
la0
120
diameter [nml Figure 3. Gaussian distribution functions of CdS/HgS/CdS composites at various stages of the preparation (cf. text).
10
$8 a
a)
-0
2 4 6 8 10 12 14 16 18 20 added Hg2* [lo-' mole/ll
Figure 4. Concentration of Cd2+and Hg2+ions in solution in the course of the preparation detected by polarography (a) and ICP-MS (b).
linearly whereas further addition of Hg2+ does not change the Cd2+ concentration any more. The Hg2+ ion concentration behaves inversely, Le. from the up to 10 X lW5 M of added Hg2+ almost none is detectable in solution whereas further addition yields a strong increase of free Hg2+ ions in the solution. (The absolute concentration of Cd2+detected with ICP-MS is somewhat lower than the polarographically determined ones in the respective
+ nHg2+
-
(CdS),,(HgS),
+ nCd2+
(1)
is feasible, bearing in mind that the solubility product of HgS is at least 22 orders of magnitude smaller compared to that of CdS. After 8 X 10-5 M Hg2+ has been added, the reaction (1) is terminated, as is seen from the absorption spectra, the constant Cd2+ concentration (ICP-MS), and the simple linear increase of the Hg2+concentration in solution upon further addition of Hg2+. The substitution reaction may very well be facilitated by the fact that HgS precipitating in the colloidal phase as ~ u b i c @ - H g S ~ ~ has almost the same lattice parameter (5.851 A) as cubic CdS (5.818 A). Since in 5.3-nm CdS particles the surface to volume ratio is approximately 0.42 and the reaction is completed after replacing 40% of the Cd2+ in the CdS colloids by Hg2+,we conclude that one surface layer of CdS has been substituted by HgS, which then causes a passivation against further Hg2+ attack. Another kind of growth would be the formation of small and separated islands of HgS in the surface of the CdS particles. Indeed, this type of CdS/HgS heterostructure has been prepared and characterized earlier in our laborat0ry.3~J~Here, we exclude this possibility because of the termination of the substitution reaction as detected optically as well as polarographically and by ICP-MS experiments, the results of which are inconsistent with the conception of islandlike HgS growth. What remains to be explained from the experiments described so far is the slight but definite increase in the Cd2+ concentration beyond the addition of 8 X 10-5 M Hg2+ detected in the polarographic experiments (cf. Figure 4a). During the preparative work it has been recognized that quick handling of the colloidal solutions is essential in all cases in which the HgS formed is not capped by CdS (see below). Obviously, some kind of solid-state reaction takes place within minutes to hours. This process can be accelerated by heating, and it presumably leads finally to a particle consisting of an isotropically mixed binary phase. This is concluded by a comparison of the evolving unstructured absorption spectrum extending far into the red spectral region with the one of an intentionally prepared mixed binary phase colloid c(.! inset in Figure 1). If this is true principally, the
The QDQW System CdS/HgS/CdS
The Journal of Physical Chemistry, Vol. 98, No. 3, 1994 937
wavelength [nml
450 500 600
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1 energy IeVl Figure 5. Absorption spectra of the colloidal solutions of (a) CdS, (b) a + 8 X 10-5 M Hg2+, (c) b + H2S, (d) c + 8 X lk5M Hg2+, (e) d + HzS, (f) e + 8 X M Hg2+,and (8) f + HB.
TABLE 1 sample
b
C
d
f
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4.7 x 10-7 6X1P5 8 x m5
5.5 x 10-7 7Xlod 8x
3.5 x 10-7 4.4X10-s 1.6 x 10-4
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CHg
ccd
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polarographic experiments are easily understood: since one polarographic scan takes about 5 min, the solid-state reaction plays a role in the sense that an increasing number of Cd2+ions are exposed to the solution from where they might be replaced by the added excess Hg2+. A contribution of this solid-state reaction is excluded in the ICP-MS experiments since the Bas04 precipitate is formed within seconds after the respective procedure. Furthermore, this reaction is supressed in the course of the preparative steps described below by working quickly. In Figure 5 the spectral evolution in the course of the further preparation is depicted. Spectra a and bare already known from Figure 1, spectra a and b, i.e. the basic CdS colloid before and after the addition of 8 X M Hg2+. Spectrum c, Figure 5, evolves after slow sulfidization of sample b with H2S as described in the Experimental Section: together with an overall increase of absorbance, the spectrum loses structure and extends to about 1.7 eV (730 nm). Figure 2c shows a micrograph of this sample, and Figure 3c, the corresponding size histogram. The size distribution function is centered at 6 nm and has a standard deviation of 15%. Thus, the sample still contains uniform particles whose diameters are about 0.6 nm larger than those of the precursors, samples a and b. This preparative step is interpreted as the reprecipitation of the Cd2+ions (which formerly have been released into the solution by the addition of the Hg*+ ions) by H2S on the surface of the CdS/HgS heterostructures. As a consequence, almost no Cd2+ is found in the solution after the reprecipitation (cf. Table 1, second row, b c). The increase of the mean diameter of the particles is in agreement with this concept: the amount of the free Cd2+ ions together with the excess H2S is sufficient to form (almost) one monolayer of CdS on the CdS/HgS composites which (having in mind a lattice parameter of 5.8 A) corresponds nicely to the measured increase of the diameter of 0.6 nm. The so formed CdS/HgS/CdS structure has earlier been named the first quantum dot quantum well (QDQW).@ Spectrum d, Figure 5, is obtained after the addition of 8 X l t 5 M Hg2+ to sample c: the general trend is the same as observed before, i.e. an increase of absorption over the whole spectral range together with an extension to lower energies, although the effect is less dramatic than the steps a band b c. The particle sizes are left unaffected by the addition of Hg2+, as is seen from Figures 2d and 3d. The mean diameter remains at 6.0 nm; the size distribution is a little broader (19% standard deviation) than that of sample c. Sulfidization of sample d yields spectrum e, Figure 5. The
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onset of absorption shifts even more into the red spectral region. The first distinct transition is now located a t 1.75 eV, and the absorption is enhanced over the entire spectrum between 1.5 and 3.5 eV. Again, the sulfidization step leads to bigger particles as is seen from Figures 2e and 3e. Still, spherical and crystalline particles are observed in TEM. They have a mean diameter of 7.05 nm and a still narrow distribution of sizes (18% standard deviation). Thus, homogeneous growth is observed from samples d to e. The addition of yet another portion of 8 X 10-5 M Hgz+ to sample e yields spectrum f, Figure 5, featuring the meanwhile well-known characteristics of an increased absorption at all energies and an extension of absorption further into the red spectral range. Again, the particle sizes are not affected by the addition of Hg2+, as is seen from Figure 3f (mean diameter of 7.1 nm, standard deviation of 17%) and the TEM picture Figure 2f. The last step in this preparative series yields spectrum g, Figure 5: sulfidization enhances the absorption over the entire spectrum as well as extends the transition energies to as far as 1.3 eV (950 nm). Again, sulfidization leads to homogeneous particle growth as is seen from Figure 3g (TEM micrograph, Figure 2g): the mean diameter of the particles after this final step is 7.7 nm, and the size distribution remains narrow with a standard deviation of 16%. At the reaction stages corresponding to spectra b-f in Figure 5, the concentrations of the free metal ions in solution were determined by ICP-MS after precipitation of the colloidal particles as described above. The results are listed in Table 1. The main findings from these experiments are the following: (1) from the added Hg2+, less than 1% is found in the solutions, Le. all Hg2+ ions have reacted with CdS (first row, samples b, d, f), (2) after the addition of Hg2+,the concentration of CdZ+ in the solution is high (second row, samples b, d, f ) and rises as a consequence of the addition (second row, c d), and (3) sulfidization lowers the Cd2+ concentration (second row, b c) (as described above). Following the arguments given above in the context of the preparation of the first layer of HgS and the reprecipitation of CdS as the outermost shell, the interpretation of the preparative steps leading from sample d to sample g is an easy task. Step c d is again the substitution of the surface Cd2+ by Hg2+, increasing the thickness of the HgS layer to about 2 monolayers. Step d e consists of the reprecipitation of Cd2+ by H2S proceeding with an increase in the particle size. Steps e f and f-garesimplyrepetitionsoftheformer twosteps, i.e. substitution of Cd2+by Hg2+and reprecipitation of the released Cd2+. Thus, the final product emerging from this, rather demanding but concerning the basic ideas, quite simple preparation is an ensemble of particles which have a CdS core of roughly 4.7 nm surrounded by about three layers of HgS and about a monolayer of CdS totaling theoretically 7.1 nm. The difference from the measured final diameter of 0.6 nm seems to be tolerable, considering the multistep preparative route. In Figure 6a, a high-resolution image of a single QDQW particle is shown. For this investigation extremely large particles were synthesized by capping particles from solution g with 1 X 10-3 M CdS. This was achieved in five steps using Cd(C104)2 and H2S asdescribed above. In this way the thicknessoftheoutermost CdS shell was increased. Following the considerations above, this procedure should result in particles having total diameters of approximately 115 A. The picture in Figure 6a has been recieved after image processing, as described in the Experimental Section. As a result of the technique used, inverted contrasts are observed, i.e. areas of strong absorption of the electron beam appear white, and areas of less absorption appear darker. Thus, the particle shown in Figure 6a reveals nicely what has already been deduced from the experiments described above, mainly, the occurrence of a ring of stronger absorption between the core and
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Mews et al.
938 The Journal of Physical Chemistry, Vol. 98, No. 3, 1994 wavelength Inml
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Figure 6. (a) HREM image of an individual QDQW particle, (b) power spectrum from the area marked with an arrow in a, and (c) equatorial line scan from theory and from experiment.
the less absorbing CdS shell. In Figure 6c we compare the theoretically expected variation of the contrast of a QDQW particle shown as a horizontal-line scan through the middle of the particle with the measured one of the very particle of Figure 6a. The agreement between experiment and theory seems to be sufficiently good to indicate that, indeed, spherically layered particles have been synthesized. Another indication in this direction is gained from Figure 6b, where the power spectrum received from the area marked with an arrow in Figure 6a is shown. In this area where CdS and HgS are expected, indeed, the periodical information from both subunits is detectable. Upon moving further to the edge of the particle, only one pair of reflexes is observed, which also is consistent with the expectation. In Figure 7a yet another set of absorption spectra are depicted. These spectra have been received after treating samples c, e, and g with 2 X 10-4M Cd2+ and H2S as described in the Experimental Section. As outlined in detail in ref 40, this results in the increase of the thickness of the outermost CdS layer. This is accompanied by the Occurrence of fluorescence from the particles, which becomes stronger as the outermost CdS layer becomes thicker. Comparison of the absorption spectra of Figure 7 with spectra c, e, and g from Figure 5 shows little but measurable differences: the first electronictransition of sample c becomes more pronounced at about 2 eV, the shape of spectra e and g remains almost unaffected, but the 1s-1s transitions are slightly shifted to lower energy (by about 100 meV). Theoretical calculations treating these QDQW’s in the framework of the effective mass approximation are presented in the next paragraph. “Fluorescence activation” (i.e. the addition of 1 X M Cd(C104)2 and raising of the pH to 11.Y2 has no influence on the absorption spectra (apart from a slight increase of absorption over the entire spectrum) but leads to an enhancement of fluorescence intensity by about a factor of 20 compared to the ones without “activation”. (A luminescence enhancement of this kind was recently reported also for the solid-state system G a s on
Figure 7. (a) Absorption spectra of the “activated” samples of CdS and CdS/HgS/CdS QDQW’s with 1-3 monolayers of HgS in between the CdS segments and (b) fluorescence spectra of the same samples (bXc = 450 nm, normalized intensities).
GaAs.50) The fluorescence spectra of the “activated” samples a, c, e, and g are shown in Figure 7b (bXc = 450 nm (2.75 ev)). The fluorescence excitation spectra resemble the absorption spectra very well, i.e. the choice of the excitation wavelength influences neither the position nor the shape of the fluorescence bands. Four features in this set of spectra are of interest: (1) All spectra consist of a single band, (2) all fluorescence bands occur close to the onset of absorption of the respective samples, a feature which in former publications has led to name this fluorescence a “bandgap” or “excitonic” fluorescence, (3) related to this, the thicker the HgS layer in between the CdS segments the more the fluorescence maximum is shifted to lower energy (from 1.74 eV or 713 nm to 1.29 eV or 961 nm), and (4) the fluorescence bands remain narrow (between 170 and 200 meV), independent of the HgS layer thickness. The fluorescence intensities have been normalized to the CdS bandgap fluorescence intensity. The room-temperature fluorescence quantum yield decreases from approximately 10% for pure CdS to 5% for sample c, to 4% for sample e, and ends at approximately 3% for sample g. A feature similar to this has been observed by Zajicek et al.51 for the photoluminescence from ultrathin ZnSe/CdSe quantum wells. By these authors, the reduction of the photoluminescence intensity is attributed to the generation of misfit dislocations when the thickness of the CdSe layers exceeds a critical thickness of 4 f 1 monolayers. With the current status of the investigations, we shall not go this far in our interpretation. We simply accept the fact that with increasing HgS layer thickness radiationless transitions either in the interface between CdS and HgS or within the HgS shells become more and more the dominating relaxation process. Temperature-dependent measurements have been carried out with sample c (including the outer CdS shell and “fluorescence activation”). With decreasing temperature the fluorescence intensity increases, reaching a maximum at about 80 K. At this temperature the fluorescence quantum yield is between 20 and 30%, depending on the specific sample. Below about 50 K the fluorescence intensity begins to decrease again, terminating at 4 K with a quantum yield of between 10 and 20%. Over the whole temperature range, the shape of the fluorescence band is not affected except that in some samples a slight shoulder on the red wing of the fluorescence band emerges at low temperatures. Together with this, a slight red shift of the fluorescence maximum is detectable. In Figure 8 a few temperature-dependent fluorescence decay
The QDQW System CdS/HgS/CdS
jt
The Journal of Physical Chemistry, Vol. 98, No. 3, 1994 939
t"
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ro r l r2 Figure 9. Potential applied in the calculations for a particle (electron or hole) in a spherical QDQW. 80 120 160 200 time Ins1 Figure 8. Time-resolvedfluorescence decay signals of a CdS/HgS/CdS QDQW with one monolayer of HgS in between the CdS segments (tact.) at the temperatures indicated. Inset: Same as main figure but on a shorter time scale.
0
40
curves are plotted, resembling a quite complex photophysical behavior. The overall decay is multiexponential with at least two components the amplitudes of which may be temperature dependent. From Figure 8 (& = 600 nm, &bs = 700 f 25 nm) it is seen that at elevated temperatures the fluorescence decay is completeafter about 200ns. At 25 Kthedecayisclearlyseparated into a t least two components, a fast decay with a lifetime of about 1 ns and a slow decay with a lifetime of more than 200 ns. The inset of Figure 8 shows the evolution of the fluorescence decay on a short time scale. At first sight an acceleration of the fluorescence decay is observed with decreasing temperature. A careful kinetical analysis of these decay signals might equally well unravel a decreasing amplitude of the slow process and a temperature-independent fast process. To avoid misinterpretations, we shall not go into a detailed description of these timeand temperature-dependent fluorescence experiments. Thus far, we conclude that, besides nonradiative transitions, at least two kinetically distinguishable processes act as channels for the removal of the excitation energy. It has been observed in the past that the surfaces of very small particles play an important role in the dynamics of the relaxation processes, giving rise to, e.g., delayed fluorescence from shallow surface In our even more complex QDQW's, with the introduction of two semiconductor boundaries inside the particles, the dynamics might be even more complicated. Perhaps, temperature-dependent charge carrier localization of the photogenerated electrons and holes within the particles and activation barriers between different areas of the particles as well as charge separation phenomena depending on the thickness of the inner layer may be of importance. We shall postpone the introduction of a fluorescence model until a QDQW system (e.g. ZnS/CdS/ZnS) can be prepared which might luminesce in a spectral range more conveniently accessible to our experimental setup. Temperature-dependent experiments in this direction should at least include the use of different excitation wavelengths as well as the observation of the kinetics of the fluorescence at the high- and low-energy wing of the fluorescence band. Theory. In the first part of this section the theoretical approach is outlined briefly. Essentially, the calculation follows the ideas of BrusS5and Haus et a1.,4l who earlier treated spherically layered quantum dots in the framework of the effective mass approximation. The latter authors presented a procedure with the aim of which, in principal, it is possible to calculate the eigenenergies and overlap integrals for the charge carrier wave functions for semiconductor spheres consisting of N layers of up to N materials with variable layer thicknesses. It turns out that the energy of the 1s-1s transition (the new "bandgap") even in the simplest case of a core with one shell of another material is not easily
predictable: it dependson the radius of thecore, the shell thickness, the conduction band offset of the materials (i.e. the energy difference between the conduction bands of the respective bulk materials), and the effective masses of the charge carriers in the respective materials. Depending on these parameters, an increase or a decrease of the Is-1s transition energy compared to the one of the core may be obtained. In certain combinations even charge carrier separation may take place. The most important extension of our calculations compared to former ones consists of the numerical execution for three-layered structures. In the second part of this section a few results for the CdS/HgS/CdS structures are depicted and compared with the experimental data. The Model. The stationary Schriidinger equation reads
HQ = EQ with the Hamiltonian r 2
H = - LV
2m
+ V(r)
where V(r) is a spherosymmetric potential. Solutions of the Schriidinger equation are the wave functions Qn,/,m(r,8,q)= RpJr)Y/,m(8,~) with R&) the radial eigenfunction and Y,,,(S,cp)the spherical harmonic. We shall restrict the calculations to 1s states, i.e. the main quantum number n equals 1 and the angular momentum quantum number 1 and the magnetic quantum number m equal 0. This leads to the Schriidinger equation
1
z As the potential V(r) we take (cf. Figure 9)
V(r) =
{
Vofor 0 I r < ro and for rl Ir < r2 VI for ro 5 r for r = r2
< rl
03
This is a potential appropriate to describe a particular (electron or hole) in a spherically three-layered structure consisting of two different materials. The material with the smaller bulk bandgap is embedded between the core and the outermost shell, which both consist of the same material. For simplicity, the potential is assumed to be infinity a t r = r2. To calculate the energy eigenvalues, Bessel functions, linear combinations of Bessel and Neumann functions, and Hankel functions are taken for the regions 0, I, and I1 of the potential, respectively. Together with the boundary conditions (a) for r 0, the wave functions have to remain regular and (b) for r m, the wave
--
Mews et al.
The Journal of Physical Chemistry, Vol. 98, No. 3, 1994
940
31
F’ 2.5 aJ
v) I
2
’
1
HgS
.\ 0
HgS
t’
0.2 0.4 0.6 0.8 1 HgS Layer thickness Inml
5nm
Figure 10. Energy of the 1s-1s transition in CdS/HgS/CdS particles as a function of the HgS layer thickness: theory (-) and experiment (0).
functions have to vanish, and the fitting condition56957
(mq= effective mass of the electron or the hole), a homogeneous system of linear equations evolves which is solved numerically. The calculations are performed independently for electrons and holes. The sum of the confinement energies (with the correct signs) and the bulk bandgap of the larger bandgap material gives the new bandgap for the composite material. Comparison Experiment-Theory. For the calculations of the specific system CdS/HgS/CdS, the following numbers are used (me and mh are the effective masses of the electrons and holes, respectively, and Eg is the bulk bandgap):
CdS HgS
CdS
me
mh
0.2 0.036
0.7 0.044
4 2.5 eV58 0.5 eV9*@
And the band offset, i.e. the energy difference between the conduction band of CdS and the conduction band of HgS, is calculated after NethercotP to be 1.35 eV. In Figure 10 we plotted the energy of the 1s-1s transition for the QDQW’s described in the Experimental Section, i.e. for the composites consisting of a core of 4.7 nm CdS, variable HgS layer thickness, and 0.6-0.8 nm CdS (Le. the deposition of 2 X 10-4 M CdS on the CdS/HgS particles of given size) as the outermost shell. The solid line represents the results of the calculation. It starts at 2.75 eV for pure CdS and decreases gradually with increasing HgS layer thickness in between the CdS segments. Together with these results, the experimentally determined 1s-1s transition energies of the species c, e, and g are plotted (solid circles). It is recognized that the general trend observed in the experiments is nicely reflected in the calculations but the difference between experimentally and theoretically received data amounts to 280-140 meV, depending on the thickness of the HgS layer. Preliminary results of calculations, including a finite potential barrier instead of the infinite potential at r = r2 in the present calculations and including the Coulomb interaction between electron and hole, show that the discrepancy observed between experiment and theory can be eliminated.62 Thus, it is concluded that the effectivemass approximation seems to be a surprisingly good description for the composite particles even in cases where the structures under investigationare as small as one monolayer. In Figure 11 a visualization of the energetic situation in the particles is given together with the wave functions in the respective QDQW’s. On the left hand side, pure size-quantized CdS is displayed: the confinement energies (the differences between the band positions (solid) and the eigenenergies (dotted)) amount to 50 meV for the holes and 200 meV for the electrons. The resulting 1s-1s transition energy amounts to 2.75 eV. The wave functions
Figure 11. Potentials, eigenenergies,and wave functions of electrons and holes in CdS (left) and CdS/HgS/CdS compositesof various compositions (middle and right).
are symmetrical and exhibit their maxima in the center of the potential wells. On the right hand side, a QDQW consisting of a 4.7-nm CdS core, a 0.9-nm HgS well, and a 0.6-nm CdS surface layer is depicted. Both electron and hole eigenergies have merged into the former CdS bandgap, resulting in a new, red-shifted 1s-1s transition energy for the whole QDQW of 1.5 eV. For both charge carriers the wave functions have maxima within the HgS well and minima in the CdS segments. Thus, the overlap integral
(and therefore the transition probability) is large. The sketch in the middle of Figure 11 reflects a situation in between the two extremes on the left and right hand sides. Here, we plotted the energies and wave functions for a particle with just one monolayer of HgS in between the CdS segments. It is seen that the relaxation of the confinement energies is only slight and that the localization of the charge carriers in the HgS well is not as pronounced as in the case of the thicker HgS well. Due to the larger effective mass of the hole compared to the electron, the localization in the valence band is stronger than in the conduction band. As a consequence, the overlap integral is smaller in these particles than in both the pure CdS and the QDQW’s having thicker HgS wells. Conclusions
The wet chemical route of synthesis of the CdS/HgS/CdS QDQW’s described in this article seems to have the potential to be of more general character. It is believed that the method developed opens up the way to the generation of colloidal QDQW’s consisting of more than three layers as well as of more than two materials. The evolving particles may act as prototypic structures for investigations of semiconductor boundaries, inner surfaces, and charge carrier separation and localization. The optical properties of these new composite materials, the understanding of which is still poor, at this early stage of the experiments may unravel interesting phenomena for applications such as tunable optical switches. Thermal or laser-optical annealing might increase the fluorescence quantum yields, holding out hopes for possible laser applications. Acknowledgment. Thanks are due to Mrs. U. Bloeck for her skillful work at the electron microscope, to P. Dulski for carrying out the ICP-MS measurements, and to Dr. R. Eichberger for help with the time-resolved fluorescence experiments. References and Notes (1) Henglein, A. Pure Appl. Chem. 1984,56, 1215. (2) Brus, L. E. ZEEE J . Quantum Electron. 1986, 22, 1909. (3) Brus, L. E. J . Phys. Chem. 1986, 86, 2555. (4) Henglein, A. Top. Curr. Chem. 1988, 143, 115. (5) Henglein, A. Chem. Rev. 1989, 89, 1861.
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