Colloidal Synthesis and Electroluminescence Properties of

J. Phys. Chem. B 1999, 103, 7839-7845

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Colloidal Synthesis and Electroluminescence Properties of Nanoporous MnIIZnS Films J. Leeb,† V. Gebhardt,‡ G. Mu1 ller,† D. Haarer,‡ D. Su,§ M. Giersig,§ G. McMahon,| and L. Spanhel*,† Institut fu¨ r Silicatchemie der UniVersita¨ t Wu¨ rzburg, Ro¨ ntgenring 10, D-97070 Wu¨ rzburg, Germany, Physikalisches Institut der UniVersita¨ t Bayreuth, D-95440 Bayreuth, Germany, Hahn-Meitner-Institut Berlin GmbH, Glienicker Strasse 100, D-14109 Berlin, Germany, and CANMET, Materials Technology Laboratory, 568 Booth Street, Ottawa, Ontario K1A0G1, Canada ReceiVed: May 7, 1999; In Final Form: July 20, 1999

An organometallic colloidal route was employed to prepare orange fluorescing Mn2+ functionalized ZnS and CdS particles. For this purpose, tributylphosphine-capped manganese oxide clusters were first synthesized, which served as heterogeneous nucleation centers in the ZnS condensation process. An exposure of the resulting Mn:ZnS particles to Cd2+ initiates a metal replacement yielding Mn:CdS. This process broadens the characteristic internal Mn2+-emission band peaking at 600 nm and enhances the fluorescence quantum yield from 3% to 6%. The size of the metal sulfide particles was determined to be 3-4 nm using HRTEM and XRD measurements. The colloids were further used to prepare thin, crack-free layers placed between ITO and Al contact electrodes, which allowed us to investigate their electroluminescence properties. We have found that an infiltration of ZnI2 or other molecular cluster species dramatically improves the air stability and efficiency of the EL device. The most efficient samples have shown a luminance of about 10 cd/m2 at 100 mA/cm2 and 9 V. The corresponding EL efficiency was about 0.001%. Current-voltage data collected on the EL-test components indicate a mobility controlled transport of the injected charge carriers. Finally, an energy level scheme is proposed to describe the excitation mechanism of the electroluminescence in the Mn2+-containing inorganic-organic nanostructures.

Introduction The first observation of a size dependent electroluminescence (EL) in nanoparticulate II-VI-semiconductor aggregates1,2 opened an interesting opportunity to study light emission processes induced via an external electron-hole pair injection into differently organized zero-dimensional quantum structures.3-5 In these studies, three EL-cell layouts were shown to emit light under external electric field conditions without using liquid electrolytes: (1) p-n-diode-like p-paraphenylenevinylene (PPV)CdSe bilayers,1 (2) inorganic-organic composite layers where CdSe2,5 or ZnS3 particles are combined with an electron transport species (oxadiazole derivatives) and/or a hole transport species (conjugated organic polymers), or (3) single layers with closely packed thioglycerate-capped CdS clusters,4 respectively. The preliminary results collected on the above nanostructures indicate that for high EL efficiency and stability, the control of the cluster-“core/shell”-interface chemistry (i.e., the photoluminescence quantum yield) as well as the nature of packing the nanoparticles are decisive factors. To our knowledge, there are no comparable studies on nanoparticle aggregates carrying entrapped fluorescing foreign atoms. In this paper, we explore such a system presenting a new synthesis of tri-n-butylphosphine-stabilized Mn2+:ZnS colloids and the electroluminescence properties of the corresponding nanoporous layers. The EL-testing device is a single Mn2+:ZnS layer placed between ITO and Al electrodes, free of * Corresponding author. E-mail: [email protected]. † Institut fu ¨ r Silicatchemie der Universita¨t Wu¨rzburg. ‡ Physikalisches Institut der Universita ¨ t Bayreuth. § Hahn-Meitner-Institut Berlin GmbH. | CANMET, Materials Technology Laboratory.

additional organic charge transfer species. With this concept, we attempted to see whether the externally injected electrons and holes can reach the active Mn2+ fluorescence sites located within the ZnS particle aggregates. In addition, we infiltrated into these nanoporous layers several kinds of foreign cluster species and molecules to monitor changes in EL efficiency and stability of the device. In this contribution, we first discuss the spectroscopic changes accompanying the activation of the internal yellow Mn2+ fluorescence in Mn2+:ZnS as well as in Mn2+:ZnCdS colloids. Thereafter, we discuss the structural and electroluminescence data along with the current-voltage characteristics of the nanoporous films. Finally, some remarks to the possible electroluminescence mechanisms are made. Experimental Section 1. Synthesis and Preparation of the Colloids for Coatings. General Information. All reactions were carried out under ambient atmosphere conditions. Zinc chloride (>98%, Fluka), manganese chloride (98%, Fluka), cadmium chloride (99.99%, Aldrich), bis(trimethylsilyl) sulfide (98%, Aldrich), tri-n-butylphosphine (97%, Fluka), chloroform (99%, Allied Signal), and heptane (99%, Aldrich) were used without further purification. Mn2+:ZnS. ZnCl2 (1.36 g, 10 mmol) and MnCl2 (13 mg, 0.10 mmol) were placed into a 250 mL round flask containing 92 mL of chloroform. In the next step, 8.5 mL (40 mmol) of trin-butylposphine (TBP) was added to this suspension, which dissolved after refluxing at 130 °C for 1 h. To the resulting dark violet solution, 1.05 mL (5 mmol) of (TMS)2S was added under room-temperature conditions. Stirring for 12 h and final

10.1021/jp991514r CCC: $18.00 © 1999 American Chemical Society Published on Web 08/27/1999

7840 J. Phys. Chem. B, Vol. 103, No. 37, 1999 refluxing for 0.5 h produced a clear colorless Mn2+:ZnS nanocolloid (Zn/S ) 2, 1 mol % Mn2+, 0.1 M with respect to Zn2+). The condensed TBP-Zn-terminated particles were purified by precipitating and repeatedly washing them with heptane (removal of the excess TBP and (CH3)3SiCl byproducts). The resulting white “powder” was redissolved in chloroform, again yielding 0.1-0.3 M sols, which were finally pressed through a micropore filter (100 nm pore size) prior to dip coating. Mn2+:ZnCdS. For the preparation of Mn2+:ZnCdS colloids, 550 mg (3 mmol) of CdCl2 was added to 30 mL of the above as-prepared 0.1 M Mn2+:ZnS solution. After 1 h of magnetic stirring and 15 min of final refluxing, a yellow CdS sol was obtained containing 200% Zn and 200% Cd with respect to S (i.e., Zn2Mn0.01Cd1.99S assuming a complete replacement of Zn against Cd). The fraction of the excess metal ions present in the solution and on the cluster surface was experimentally not determined. In further text we will use the term “Mn2+:ZnCdS” for samples prepared via this metal replacement route. To obtain crack-free homogeneous layers, heptane washing of this colloid was not necessary. Prior to dip coating, the fresh sols were pressed through a micropore filter and directly used for the film preparations. 2. Electroluminescence Investigations. Test Components. Nanocrystalline Mn2+:ZnS and Mn2+:ZnCdS films were characterized as single-layer devices. Here, semitransparent indiumtin oxide (ITO)-coated glass substrates with a sheet resistance below 30 Ω/0 (Balzers company) served as the hole injectors. After dipping in and withdrawing (≈20 cm/min) the ITO substrates from the 0.1 M colloids, the wet films were dried in an evacuated oven (1 mbar) for 10 min at 200 °C and cooled to room temperature under vacuum. Repeated coating and thermal curing allowed is to vary the thickness of the films. Typically, 20-50 nm thick films per single dip coating step were obtained. The final thickness of the samples employed in EL measurements was varied between 100 and 300 nm. The test components were completed by evaporating Al metal electrodes (pressure, 1 × 10-5 mbar; rate, 0.2 nm/s) onto the nanocrystalline layers to form the top contacts for electron injection. The active areas of the light-emitting regions were approximately 15 mm2. Infiltration Procedures. As will be shown in the Results, infiltration of the nanoporous Mn2+:ZnS films with certain clusters and complexes prior to evaporating the Al top contacts dramatically enhances the stability and EL efficiency of the test devices. For this purpose, we used ethanolic 0.1 M solutions containing either TBP-complexed ZnI2 and ZnCl2 (TBP/Zn ) 2) or thermally dissolved zinc acetate dihydrate (obtained by refluxing the ethanolic salt suspension for 3 h6). In the last case, the products of the thermal dissolution probably are acetate (Ac)capped tetrahedral ZnxOy clusters of the general formulas (Zn4O)Ac6 and (Zn10O4)Ac12, respectively.7 For infiltration, the dry films were dipped for 2 min into the above-mentioned homogeneous dust free solutions and dried at 200 °C for 5 min under vacuum conditions (1 mbar). Thereafter, the top contacts were established as described above. Electroluminescence Measurements. The current-voltage data were taken with a Keithley 237 source-measure unit (SMU). Simultaneously, the spectrally integrated electroluminescence was obtained with a Hamamatsu R928 photomultiplier using lock-in techniques (Stanford SR850 DSP) to improve the signalto-noise ratio. Photo- and electroluminescence spectra were recorded with a Perkin-Elmer LS50 and a Shimadzu RF 5301 PC spectrofluorometer, respectively. The external quantum efficiency of the investigated samples was measured with a

Leeb et al. calibrated optical power meter (Newport 1830-C), neglecting reflection phenomena inside the substrate. The luminance was detected with a Minolta Luminance Meter LS 100. All measurements were made under ambient atmospheric conditions. 3. Optical Characterizations. Optical absorption spectra of the colloidal solutions and layers were collected at room temperature with a Hitachi U 3000 UV/vis spectrophotometer. 4. XRD Data Collection. X-ray diffraction measurements were carried out at room temperature with a STOE STADI P diffractometer (Cu KR1-radiation, λ ) 1.5406 nm, BraggBrentano geometry). Prior to the reflection measurements under ambient conditions, the concentrated colloids were cast on a Mylar membrane and allowed to dry. 5. SIMS Investigations. Secondary ion mass spectrometry (SIMS) was used to determine the elemental distribution of the elements in electroluminescing Mn(II):ZnS layers. All analyses were performed using a Cameca ims4f double-focusing magnetic sector SIMS. After the analyses, depths of the sputter craters were measured using a Tencor profilometer. 6. HRTEM Measurements. High-resolution electron microscopic measurements were performed on a 120 kV Phillips CM 12 microscope equipped with a super-twin lens (Cs ) 1.2 mm) and a 9800 EDX analyzer. For these investigations, a small amount of the diluted Mn2+:ZnS colloid was deposited on a carbon-activated gold mesh grid and transferred in an air-free holder into the microscope. Results and Discussion 1. From Manganese Oxide Clusters to Fluorescing Mn2+: ZnS Nanocrystals. There exist several preparation routes to Mn2+-functionalized ZnS and CdS nanocrystals.8-10 Typically, the colloidal synthesis is based on coprecipitation of CdS (or ZnS) and MnS in liquid homogeneous media or reversed micelles followed by washing or extraction of the orange fluorescing particles. In this paper we present a new approach based on the use of manganese oxide clusters as heterogeneous nucleation centers of the II-VI-semiconductor condensation process. As stated in the Experimental Section, the reaction of a mixture of MnCl2 and ZnCl2 with tri-n-butylphosphine (TBP) in chloroform prior to addition of the sulfide source yields strongly violet solutions. It should be pointed out that the same intense coloration and its spectral response are also produced in the absence of ZnCl2. Figure 1a shows the optical absorption spectrum of this precursor, displaying two bands centered at 400 and 535 nm with the maximum molar extinction coefficient 535 of 4 × 104 M-1 cm-1. We recall the results of McAuliffe et al.11 who first reported about the synthesis of similar MnX2(TBP)2 complexes (X: Cl, Br, I) that become intensely colored due to the coordination of molecular oxygen. In their later studies,12 the same group succeeded in growing single crystals from the MnI2(TBP)2 “complexes” exposed to oxygen and found that these “complexes” are actually adamantanoid-derived Mn4I6O(TBP)4 clusters with entrapped µ4-coordinated O2-. In view of these tetrahedral structures, the optical absorption spectrum in Figure 1a could be interpreted in at least three ways going beyond the classical assignment to a “charge transfer band”. On the basis of the size quantization theory of low dimensional inorganic solids, the two absorption peaks might be understood in terms of electronic transitions into the first and second “excitonic” state in strongly “quantized” Mn4Cl6O(TBP)4 clusters. The two bands peaking at 400 and 535 nm could also originate from a “two frequency oscillator” represented by an asymmetric dimer cluster. A third possible explanation of the above absorption spectrum would take into

Nanoporous MnIIZnS Films

J. Phys. Chem. B, Vol. 103, No. 37, 1999 7841

Figure 1. Development of the optical absorption (a) and fluorescence spectra (b) of growing TBP-capped Mn2+:ZnS colloids in chloroform (0.1 M ZnS, 1 at. % Mn2+ with respect to Zn, Zn/S ) 2, TBP/Zn ) 4). 0 min: spectra of the manganese oxide clusters prior to ZnS condensation process and 20-120 min after addition of the sulfide source (for details see Experimental Section and text below). (c) Quantum yield of the 600 nm fluorescence as a function of the Mn2+ content in ZnS.

account an existence of two differently sized tetrahedron clusters with “Mn4O” (400 nm peak) and larger “Mn10O4” cores (535 nm peak). Nevertheless, to proof which of the structural assignments is correct would require additional optical singlecrystal data and quantum mechanical calculations. The most striking feature of the nonfluorescent manganese oxide clusters is their ability to serve as heterogeneous nucleation “seeds” to synthesize yellow fluorescing Mn2+:ZnS nanocrystals. The violet manganese oxide cluster solutions containing Zn2+ start to lose their intense color upon the addition of a sulfide source. The sols become completely colorless after 120 min. Consequently, clearly seen in Figure 1a, the optical absorption in the visible disappears whereas a new UV absorption edge around 300 nm is detected, reflecting the presence of ZnS particles. Plotting on a (Rhν)2 vs hν scale (R ) c ln 10, c ) molar concentration,  ) molar extinction coefficient) and extrapolating to R ) 0, we determined the band gap energy to be 4 eV, which is characteristic for weakly quantized ZnS particles, in contrast to 3.7 eV known for macroscopic bulk crystals. From the band gap energy difference and by using the tight-binding-model-derived band gap/dimension-correlation diagrams,13 we determined the ZnS crystallite size to be 3 ( 0.2 nm, which agrees fairly well with the experimental XRDand HRTEM- derived sizes (see section 3). Concomitantly to the growth of small ZnS crystallites, the characteristic internal Mn2+-fluorescence band14 at 590 nm is activated (see Figure 1b), indicating that Mn2+ ions are attached to ZnS. After 120 min, a maximum fluorescence quantum yield (φ) of about 3% is reached, which is strongly dependent on the Mn2+ content employed in the ZnS condensation process. In Figure 1c, one recognizes that φ approaches the maximum value at 1 at. % Mn2+ (with respect to Zn) and drops down at higher concentrations. The decrease in fluorescence quantum yield with increasing Mn2+ content coincides with the observations made by other groups, which can be explained in terms of concentration quenching15 (short-range energy transfer between neighboring Mn2+ centers). 2. Effect of Cd2+ Addition on the Mn:ZnS Fluorescence. The room-temperature quantum yield of the Mn2+ fluorescence can be increased from 3% to 6% by reacting the Mn:ZnS particles with a stoichiometric amount of CdCl2 salt (see Experimental Section). Due to the lower solubility of CdS, the Zn2+ ions are substituted by Cd2+ ions, yielding yellowish Mn2+: CdS colloids with an excess Zn2+. Consequently, as can be seen

Figure 2. Fluorescence excitation and emission spectra of Mn:ZnS before (solid lines) and after the reaction with a stoichiometric amount of Cd2+ (with respect to sulfide). The spectra are normalized to maximum intensity. See also the corresponding changes in the XRD patterns in Figure 4.

in Figure 2, the initial fluorescence excitation peak at 290 nm (solid line) shifts toward 420 nm (dashed line) whereas the Mn2+-fluorescence band is broadened after the reaction with CdCl2. Using the tight binding model13 or the finite potential well model,16 we determined the CdS particle size (taking the excitation maximum at 420 nm) to be 3 ( 0.4 nm. This size is comparable to that of the starting ZnS particles, indicating a complete metal replacement without significant change in the primary cluster size. In addition, the observation of the electronic transitions at 290 and 420 nm indicates that in the semiconductor particles, an “exciton” is initially formed which relaxes directly to Mn2+ centers or decays in a cooperative energy transfer process. Furthermore, the CdS-growth-induced fluorescence band broadening and the fluorescence intensity doubling deserve to be discussed, too. A broader energetic distribution of the surface trap sites (related to the existing anion vacancies due to a large metal excess) might explain the detected broadening. It is also likely that the radiative transitions occur in CdS clusters without assistance of the Mn2+-fluorescence centers. However, this process seems less probable considering the previous studies where the fluorescence maximum located at 700 nm was found for similarly sized Mn2+-free CdS clusters.17 Finally, it appears that after the complete metal replacement, the expelled Zn2+

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Figure 4. Experimental XRD patterns of Mn:ZnS nanocrystallites before (top) and after the reaction with a stoichiometric amount of Cd2+ (with respect to sulfide). The data were taken in Bragg-Brentano geometry (the Fe reflection around 45° is due to the sample holder). For comparison, the JCPDS reference data are also included.

TABLE 1: Results of the ICP-AES Analysis Performed before and after the Infiltration of the Mn:ZnS Films with Zn(TBP)2I2 Complexesa TBP-(Mn:ZnS) layers

Zn

S

Mn

Si

P

before infiltration after infiltration

1 1

0.6 0.2

0.009 0.008

0.06 0.05

0.15 0.3

a

The values given represent the molar percentage with respect to

Zn.

Figure 3. HRTEM images of the aggregated (top and middle) and primary Mn:ZnS particles (bottom). The single particle diffraction spectrum (inset in the bottom part) shows the typical reflexes of cubic ZnS of 1/0.31 nm-1. For comparison, see also the XRD data displayed in Figure 4 and the discussion in the text.

ions are partly passivating the surface of the new CdS core, which might explain the increase in the fluorescence intensity. 3. Structural and Elemental Analysis. Figure 3 shows the HRTEM images of the Mn:ZnS particles prior to film preparations. We have found a remarkable tendency of the chloroform colloids to yield narrowly sized flake-shaped aggregates (the

top and middle part) after the solvent evaporation. The dimension of these aggregates was determined to 20-30 nm, whereas the size of the primary particles ranges between 3 and 4 nm (bottom part). One also recognizes the well-resolved lattice planes with spacings around 0.31 nm (inset diffraction spectrum), indicating the presence of (111) planes of the cubic phase. The corresponding XRD pattern of these primary cubic Mn: ZnS nanocrystallites is shown in Figure 4. From the characteristic broad diffraction peaks and using the Warren-Averbach formula, we calculated the mean crystallite size to be 3 nm, which is consistent with sizes observed in the HRTEM images. Furthermore, the above-described metal replacement after an exposure of Mn:ZnS to Cd2+ suppresses completely the ZnS diffraction peaks, producing a new pattern characteristic for CdS nanocrystallites (Figure 4). In addition, by comparing the experimental XRD pattern of the Mn:ZnCdS particles with the JCPDS reference line spectra, one notices a weak diffraction peak at 47° (103 plane) in the experiment. Hence, the starting ZnS cubic phase seems to be converted into the hexagonal one after the metal replacement. Nevertheless, we do not attempt to further discuss the phase transitions in these nanostructures. Of main interest for the discussion of the electroluminescence data was an elucidation of the structural and chemical properties of the films composed of the above consolidated nanoparticles. Their electroluminescence efficiency and the electric field stability in air are greatly enhanced if these layers are dipped into an acetonitrile solution containing Zn(TBP)2I2 complexes (for details see Experimental Section). Table 1 shows the inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analytical results collected on Mn:ZnS layers before and after this treatment. Before the

Nanoporous MnIIZnS Films

Figure 5. SIMS depth profile analysis data of the ZnI2-infiltrated Mn: ZnS films. The anion and cation data were taken using a Cs and O beam, respectively.

exposure to Zn(TBP)2I2, the S and Mn concentrations nearly correspond to the conditions employed in the colloid synthesis (S/Zn ) 0.5, 1 at. % Mn2+). Moreover, the initial TBP/Zn ratio decreased from 4 to 0.15, indicating that the heptane washing necessary to prepare crack-free homogeneous layers removes the excess TBP. This finding also indicates that the Mn:ZnS particles in thermally cured layers are still carrying the TBP shell. The detected Si additionally shows that even a repeated heptane washing does not remove completely the trimethylsilyl ligands of the (TMS)2S precursor. As expected, after the infiltration of Zn(TBP)2I2, the overall sulfur content largely decreased with respect to Zn while the phosphorus content increased. The layers are entirely infiltrated and not simply overcoated. This was proved in secondary ionic mass spectroscopic measurements (SIMS) delivering the concentration depth profiles. Figure 5 shows the result of this investigation performed on 60 nm thin layers deposited on a silicon wafer. All expected elements of interest are found to be homogeneously distributed across the entire layer, which confirms that the Zn(TBP)2I2 moieties are completely infiltrated. The detected minor amount of oxygen is not surprising considering the fact that our colloid and film synthesis takes place under ambient laboratory conditions. The elemental analysis data mentioned above clearly shows that the layers employed in electroluminescence studies are not purely inorganic. They are composed of aggregated nanoparticles with covalently bound TMSi and TBP ligands, presumably via TMS-S and TBP-Zn bridges, the latter acting as spacers between the metal-terminated particles. SEM and AFM images of the layers thermally cured at 200 °C revealed the presence 40-60 nm large aggregates (composed of 3-4 nm primary crystallites), in contrast to 20-30 nm ones seen in HRTEM images of the as-prepared colloids. The surface roughness of these layers was found to be comparable to that of the ITO substrates used. In the next section, we demonstrate the electroluminescence properties of the above inorganic-organic nanostructures. 4. Electroluminescence Investigations. Figure 6 shows the electroluminescence spectra collected on ITO/Mn:ZnS/A and ITO/Mn:ZnCdS/Al films. One recognizes two bands positioned around 600 nm that resemble the internal Mn(II) fluorescence, also produced via photoexcitation of the ZnS or CdS carriers, respectively (see Figure 2). The very similar spectral response of the electro- and photoluminescence underlines the fact that both electronic and optical excitations generate electron-hole pairs that recombine in d-orbitals of the localized Mn2+ centers.

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Figure 6. Electroluminescence spectra of nanoporous 100-120 nm thick Mn:ZnS and Mn:ZnCdS films placed between ITO and Al electrodes. For comparison, see the fluorescence spectra in Figure 2.

TABLE 2: Effect of Infiltration on the Electroluminescence Characteristics of Nanoporous ITO/Mn:ZnS/Al and ITO/ Mn:CdZnS/Al Test Components active EL region TBP-Mn:ZnS

TBP-Mn/CdZnS

infiltrated species

luminance (cd/m2)

EL efficiency (10-3 %)

none (Zn4O)OAc6, (Zn10O4)OAc12 TBP-ZnCl2 TBP-ZnI2 none TBP-ZnI2

0.1 1

0.001 0.0014

1 10 10 10

0.002 1 1 2

The performance of the nanoporous test components is strongly limited by the electrical stability of the samples under investigation. Current densities higher than 1 mA/cm2 across these nanostructures cause fast degradation after a few minutes. Due to the internal rough structure of the films, the Al electrode evaporation causes the metal to diffuse into the interior of the films. The consequence are extremely high local electric fields when a bias is applied to the device, giving rise to the formation of pinholes and shorts. However, the porosity of the nanocrystalline systems can be significantly reduced by infiltrating the films with certain foreign clusters and complexes. This internal surface modification step dramatically improves the electrical stability as well as the quantum efficiency of the electroluminescence, without changes in optical absorption and the spectral response. Table 2 summarizes the results of our investigations on differently infiltrated films. Without performed infiltration, the maximum luminance in Mn:ZnS of 0.1 cd/m2 and the quantum efficiency of 10-6 photons per electron are very low. The electronic stability of such devices is weak, too. By infiltrating the nanoporous films with zinc oxide clusters or TBPcomplexed zinc chloride (see Experimental Section), the luminance increased by 1 order of magnitude. The most significant improvement was achieved with ZnI2-TBP complexes delivering a luminance of 10 cd/m2 with a quantum efficiency of 10-3 photons per electron. Furthermore, Mn2+containing CdS carriers with a large metal excess (100% Cd and 200% Zn with respect to CdS) possess luminance and the EL-efficiency values comparable to those determined on the best infiltrated Mn:ZnS samples. An additional infiltration of Mn:ZnCdS films with ZnI2-TBP complexes further doubles the EL efficiency (see Table 2). The achieved air stability of the nanoporous devices under external electric fields allowed a more detailed investigation of their electrical properties. Figure 7 shows the typical currentvoltage characteristics under forward bias (x on ITO) along

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Figure 8. Energy level diagram of the investigated Mn:ZnS and Mn: ZnCdS devices from Figures 6 and 7.

Figure 7. Current-voltage (circles) and EL-intensity-voltage characteristics (squares) of ZnI2-infiltrated Al/ZnS:Mn/ITO and Al/Mn: ZnCdS/ITO devices plotted on a double logarithmic scale. All data were taken under air. The corresponding EL spectra are shown in Figure 6.

with the EL-intensity-Voltage data of the ZnI2-infiltrated Mn: ZnS and Mn:ZnCdS films, all plotted on a double-logarithmic scale. Obviously, the onset voltage value of 10 V needed to start to produce light in ZnS is substantially higher than the 2 V needed in the CdS-derived device. Both systems reached a similar maximum luminance of about 10 cd/m2 at 100 mA/cm2 and 4 V (CdS) or 19 V (ZnS). In addition, the Mn:CdZnS system reached a higher quantum efficiency at lower current densities in comparison to Mn:ZnS (see Discussion). For both samples, the onset voltages of the electroluminescence represent “crossover” values of the corresponding current-voltage curves possessing a I ∼ Vx power law character. At first sight, this nonohmic relationship indicates a bulklimited conduction in the presence of traps as known from the extended theory of trapped-charge-limited currents (TCLC).18-20 In accordance with this classical model, the current density at low fields depends quadratically on the applied voltage in both Mn:ZnS and Mn:ZnCdS samples (see Figure 7). Under these conditions, the electron mobility is very low due to “shallow” trapping. Above the “crossover” voltage, where the films start to produce light, the traps are increasingly filled, which raises the mobility of the injected charge carriers, causing a higher power-law increase in current. Taking our results (I ∼ V7.5 in ZnS and I ∼ V10.6 in CdS) and employing the TCLC model, we calculated characteristic trap depth values of 160 meV in Mn:ZnS and 210 meV in Mn:CdZnS samples, respectively. Indeed, the calculated values demonstrate the presence of deep traps as required by the TCLC model (>kT ) 25 meV). However, the question arises what significance the calculated trap energy values and the assumed band conductivity have for

the understanding of the electroluminescence mechanism in our nanostructures. From previous studies on II-VI-semiconductor colloids and layers it is well established that much deeper midgap states exist in the nanoparticles.4,17 It should be noted that the classical TCLC model requires the existence of energy bands with an exponential distribution of trap,18-20 which is not the case in our material. Also, in the case of Mn:ZnCdS, the deviation from the power law at high voltages between 4 and 10 V is evident (see Figure 7). Another interpretation of the current-voltage characteristics would consider the tunneling process of charge carriers through the barrier at the contact electrodes and the injection into existing “trap” states, as is believed for porous electroluminescent silicon sponges.21 However, the fit of our data to this model (ln(I/V)2 ∼ 1/V) was unsatisfactory also. An alternative description of charge transport would take a hopping-like conductivity into account, characterized by random incoherent jumps between and along the nanograin boundaries.4,22 This kind of mobility limitation is intimately related to the structural origin of the material under investigation.23 To resolve the multiple trapping versus random walk controversy requires additional temperature dependent as well as time-resolved EL measurements, which will be published elsewhere.24 5. Remarks on the Electroluminescence Mechanism in Mn2+:Zn(Cd)S Nanostructures. As stressed in the Structural section, the Mn:Zn(Cd)S nanostructures are composed of aggregated individual “core/organic shell” particles with an excess of metal and infiltrated halogenides. Thus, there is no doubt that these films possess a large number of “traps” located in cores and shells as well as between the individual particles. In this last section, we would like to discuss the chemical nature of the traps and the related excitation energy scheme of the dcfield driven Mn:ZnS and Mn:CdZnS electroluminescence. Figure 8 depicts a schematic energy level diagram of the investigated devices on a standard electrochemical scale (0 V vs NHE corresponds to 4.5 eV on the vacuum energy scale). The energy levels of the electrode contacts were taken from the literature. The HOMO-LUMO gaps of the size-quantized ZnS and CdS nanoparticles were extrapolated from the corresponding bulk values using the photoluminescence excitation peaks from Figure 2. Additionally, drawn in Figure 8 are the energy levels of the “deep trap states”, the one-electron oxidation potentials of the infiltrated halogenides, and the internal d-orbital transition of the yellow Mn2+ fluorescence centers. The energetic position of the Mn2+ level in ZnS is still the subject of an unresolved controversy. There are reports suggesting that the Mn2+ ground state (6A1g) is located 0.9 V below the top of the ZnS valence band.14,15 Other authors consider the Mn2+ to be strongly localized; e.g., both the ground state and the first excitation level (4T1g) are positioned within the band gap.25 In

Nanoporous MnIIZnS Films our energy scheme we assume the second option, taking into account the standard potential E(Mn2+/Mn3+) ) +1.5 V of the one-electron oxidation of the Mn2+ to Mn3+ by holes injected through ITO. On the basis of the scheme shown in Figure 8, we first examine the ZnI2(TBP)2-infiltrated Mn:ZnS device at low fields (at voltages between 0.5 and 10 V), where a minor dark current flows across the biased nanostructure without producing orange light (see Figure 7). Under these conditions, the injected electrons (at Al contacts) and holes (at ITO contact) survive in or migrate along deep trap states located within the 6A1g-4T1g energy gap. Thus, from the thermodynamical point of view, they do not reach the Mn2+ orbitals. Chemically, the trapped electrons could be represented by “Znn(2n-1)+”-cluster species in accordance with the structure of “Mn4O” clusters discussed above in the Optical section. On the other hand, the trapped holes could be understood in terms of the existing radical anions centered at oxygen, phosphorus, or iodine atoms. At higher bias, around 10 V, the cathodic and anodic shift of the energy levels of the contact electrodes (plus on ITO) allows an additional injection of electrons and holes into the LUMO and HOMO levels of ZnS nanoparticles. Under these conditions, electron-hole pairs are now thermodynamically allowed to relax to Mn2+ to recombine there under emission of 600 nm light (see Figure 8). As pointed out earlier, the infiltrated zinc iodide complexes dramatically improve our EL performance. Iodine/iodide electrolytes are known to be important hole transport media in photovoltaic dye-sensitized titania membranes.26 Apparently, the iodides act as an efficient hole transport species in our device, too. This process is thermodynamically allowed due to almost equal energetic positions of the oxidation potentials of I- and Mn2+. Contrary to our results, the energy level of the infiltrated chloride (by 1.3 V more positive than that of the iodide) should deliver a larger thermodynamic driving force for the holes to reach the Mn2+ ions. At least two possibilities exist to explain the higher EL efficiency of the iodide system. First, the shuttle of the holes from ITO or S•- toward the Mn2+ centers via deep iodide levels appears more favorable to bypass the competitive deep trapping. Second, the higher EL efficiency of the iodideinfiltrated system could also arise from a more favorable d-orbital overlap between Mn2+ and the iodide species (e.g., I•-, I2•-, I3•-, etc.), whose structure is still the subject of intense research.27 Nevertheless, to better understand this complex issue of pore filling in nanoparticulate aggregates requires further investigations. At this point we refer to our recent study on Al: ZnO systems, where it has been shown that pore filling via infiltrations might become an interesting tool to tune the charge carrier mobility in nanocrystalline electrodes.28 Beyond the Mn:ZnS, the energy scheme in Figure 8 also allows us to interpret the detected lower EL-voltage onset around 2 V in the CdS-derived device. Due to the substantially narrower energy gap in CdS, the energy levels of the CdS particles are closer to those of the contact electrodes. Hence, lower dc fields are required to inject the charge carriers into the HOMO and LUMO levels of the CdS carriers. In addition, the energetic position of the CdS HOMO level still enables the injected holes to be shuttled toward Mn2+ via iodide levels, which explains the iodide induced increase in EL efficiency of the Mn:CdZnS films, too. Conclusion and Outlook The presented experimental data show that “trapping” and/ or an incoherent migration responsible for the nonradiative recombinations still dominate in both Mn:ZnS and Mn:ZnCdS

J. Phys. Chem. B, Vol. 103, No. 37, 1999 7845 systems. Consequently, the achieved EL efficiency is lower than in p-n-diode-like PPV/CdSe/CdS core/shell composites.1 Nevertheless, the above single-layer concept shows the way to air stable devices based on infiltrated nanoparticle aggregates with entrapped atomic fluorescence centers. Introduction of the wellestablished core/shell strategy to increase the fluorescence quantum yield of the Mn2+ particle carriers and further optimization of the pore-filling procedure by introducing new charge carrier transporting moieties are the next steps of our electroluminescence studies on Mn2+-derived devices. Acknowledgment. This work was supported by the Bundesministerium fu¨r Bildung und Forschung BMBF. The authors thank Dr. W. Hopp from the Fraunhofer Institute for Silicate Research in Wu¨rzburg for the ICP-AES analytical measurements and PD Dr. M. Lerch for providing us with the XRD data. References and Notes (1) (a) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (b) Schlamp, M. C.; Peng, X.; Alivisatos, A. P. J. Appl. Phys. 1997, 82, 5837. (2) Dabbousi, B. O.; Bawendi, M. G.; Onitsuka, O.; Rubner, M. F. Appl. Phys. Lett. 1995, 66, 1316. (3) (a) Yang, Y.; Xue, S.; Liu, S.; Huang, J.; Shen, J. Appl. Phys. Lett. 1996, 69, 377. (b) Huang, J.; Yang, Y.; Xue, S.; Yang, B.; Liu, S.; Shen, J. Appl. Phys. Lett. 1997, 70, 2335. (c) Yang, Y.; Huang, J.; Liu, S.; Shen, J. J. Mater. Chem. 1997, 7, 131. (4) Artemyev, M. V.; Sperling, V.; Woggon, U. J. Appl. Phys. 1997, 81, 6975. (5) ] Gao, M., Richter, B.; Kirstein, S.; Mo¨hwald, H. J. Phys. Chem. B 1998, 102, 4096. (6) Spanhel, L.; Anderson, M. J. Am. Chem. Soc. 1991, 113, 2826. (7) Bonnani, M. Dissertation Thesis, University of Wu¨rzburg, 1998. (8) Wang, Y.; Herron, N.; Mo¨ller, K.; Bein, T. Solid State Commun. 1991, 77, 33. (9) (a) Bhargava, R.; Gallagher, D.; Hong, X.; Nurmikko, A. Phys. ReV. Lett. 1994, 72 (3), 416. (b) Khosravi, A. A.; Kundu, M.; Kuruvilla, B. A.; Shekhawat, G. S.; Gupta, R. P.; Sharma, A. K.; Vyas, P. D.; Kulkarni, S. K. Appl. Phys. Lett. 1995, 67 (17), 506. (c) Counio, G.; Esnouf, S.; Gacoin, T.; Boilot, J. P. J. Phys. Chem. 1996, 100, 20021; 1998, 102, 5257. (10) Levy, L.; Hochepied, J. F.; Pileni, M. P. J. Phys. Chem. 1996, 100, 18322; 1997, 101, 9153. (11) (a) McAuliffe, C. A.; Al-Khateeb, H. F.; Jones, M. H.; Levason, W.; Minten, K.; McCulluough, F. P. J. Chem. Soc., Chem. Commun. 1979, 736. (b) McAuliffe, C. A.; Al-Khateeb, H. F.; Barrat, D. S.; Briggs, J. C.; Challita, A.; Hossainy, A.; Little, M. G.; Minten, K. J. Chem. Soc., Dalton Trans. 1983, 2147. (c) McAuliffe, C. A.; Little, M. G.; Raynor, J. B. J. Chem. Soc., Chem. Commun. 1982, 68. (12) Beagley, B.; McAuliffe, C. A.; Rory, P. P. M.; Ndifon, P. T.; Pritchard, R. J. Chem. Soc., Chem. Commun. 1990, 309. (13) Lippens, P. E.; Lannoo, M. MRS Symp. Proc. 1992, 272, 217. (14) Dreyhsig, J.; Allen, J. W. J. Phys. Condens. Matter 1989, 1, 1087. (15) Bringuier, E. J. Appl. Phys. 1994, 75 (9), 4292. (16) Vossmeyer, T.; Katsikas, L.; Giersig, M.; Popovic, I. G.; Diesner, K.; Chemseddine, A.; Eychmu¨ller, A.; Weller, H. J. Phys. Chem. 1994, 98, 7665. (17) (a) Resch, U.; Eychmu¨ller, A.; Haase, M.; Weller, H. Langmuir 1992, 8, 2215. (b) Spanhel, L.; Anderson, M. J. Am. Chem. Soc. 1991, 112, 2279. (18) Rose, A. Phys. ReV. 1955, 97, 1538. (19) Lampert, M. A. Phys. ReV. 1956, 203, 1648. (20) Burrows, P. E.; Shen, Z.; Bulovic, V.; McCarty, D. M.; Forrest, S. R.; Cronin, J. A.; Thompson, M. E. Appl. Phys. 1996, 79 (10), 7991. (21) (a) Hirschmann, K. D.; Tsybeskov, L.; Duttagupta, S. P.; Fauchet, P. M. Mater. Res. Soc. Symp. Proc. 1997, 452, 705. (b) Ueno, K.; Ozaki, T.; Koyama, H.; Koshida, N. Mater. Res. Soc. Symp. Proc. 1997, 452, 699. (22) Yakimov, A. I.; Stepina, N. P.; Dvurechenskii, A. V.; Scherbakova, L. A. Physica B 1995, 205, 298. (23) Ben-Chorin M.; Moller, F.; Koch, F.; Schirmacher, W.; Eberhard, M. Phys. ReV. 1995, B5, 2199. (24) Gebhardt, V.; Spanhel, L.; M.; Leeb, J.; Mu¨ller, G.; Haarer, D. Manuscript in preparation. (25) (a) Gumlich, H. E. J. Lumin. 1981, 23, 73. (b) Sooklal, K.; Cullum, B. S.; Angel, S. M.; Murphy, C. J. J. Phys. Chem. 1996, 100, 4551. (26) Smestad, G. P.; Gra¨tzel, M. J. Chem. Educ. 1998, 75, 752. (27) Janata, E. Private communication. (28) Hilgendorff, M.; Spanhel, L.; Rothenha¨usler, CH.; Mu¨ller, G. J. Electrochem. Soc. 1998, 145 (10), 3632.