J. Phys. Chem. B 1999, 103, 7839-7845
`
`7839
`
`Colloidal Synthesis and Electroluminescence Properties of Nanoporous MnIIZnS Films
`
`J. Leeb,† V. Gebhardt,‡ G. Mu1ller,† 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 photolu-
`minescence 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 corre-
`sponding nanoporous layers. The EL-testing device is a single
`Mn2+:ZnS layer placed between ITO and Al electrodes, free of
`
`* Corresponding author. E-mail: spanhel@silchem.uni-wuerzburg.de.
`† 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-bu-
`tylphosphine (97%, Fluka), chloroform (99%, Allied Signal),
`and heptane (99%, Aldrich) were used without further purifica-
`tion.
`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 tri-
`n-butylposphine (TBP) was added to this suspension, which
`dissolved after refluxing at 130 (cid:176)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
`
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`Published on Web 08/27/1999
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`

`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 byprod-
`ucts). The resulting white “powder” was redissolved in chlo-
`roform, 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 char-
`acterized as single-layer devices. Here, semitransparent indium-
`tin oxide (ITO)-coated glass substrates with a sheet resistance
`below 30 ¿/0 (Balzers company) served as the hole injectors.
`After dipping in and withdrawing ((cid:25)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 (cid:176)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 measure-
`ments was varied between 100 and 300 nm. The test components
`were completed by evaporating Al metal electrodes (pressure,
`1 (cid:2) 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 (cid:176)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 signal-
`to-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 measure-
`ments 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, (cid:236) ) 1.5406 nm, Bragg-
`Brentano 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 mi-
`croscopic 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
`(cid:15)535 of 4 (cid:2) 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 (cid:237)4-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” repre-
`sented by an asymmetric dimer cluster. A third possible
`explanation of the above absorption spectrum would take into
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`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 single-
`crystal 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 absorp-
`tion edge around 300 nm is detected, reflecting the presence of
`ZnS particles. Plotting on a (Rh(cid:238))2 vs h(cid:238) scale (R ) (cid:15)c ln 10,
`c ) molar concentration, (cid:15) ) 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 XRD-
`and 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
`((cid:30)) 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 (cid:30) 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 concentra-
`tion quenching15 (short-range energy transfer between neighbor-
`ing 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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`7842 J. Phys. Chem. B, Vol. 103, No. 37, 1999
`
`Leeb et al.
`
`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(cid:176)
`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
`before infiltration
`after infiltration
`
`Si
`0.06
`0.05
`
`P
`0.15
`0.3
`
`Zn
`1
`1
`
`S
`0.6
`0.2
`
`Mn
`0.009
`0.008
`
`a The values given represent the molar percentage with respect to
`Zn.
`
`top and middle part) after the solvent evaporation. The dimen-
`sion 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 spec-
`trum), 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 character-
`istic 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(cid:176)
`(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 electrolumines-
`cence 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
`
`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 prepara-
`tions. We have found a remarkable tendency of the chloroform
`colloids to yield narrowly sized flake-shaped aggregates (the
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`Nanoporous MnIIZnS Films
`
`J. Phys. Chem. B, Vol. 103, No. 37, 1999 7843
`
`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.
`
`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 con-
`centration 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 inor-
`ganic. 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 (cid:176)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 elec-
`troluminescence 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.
`
`active EL region
`TBP-Mn:ZnS
`
`TBP-Mn/CdZnS
`
`TABLE 2: Effect of Infiltration on the Electroluminescence
`Characteristics of Nanoporous ITO/Mn:ZnS/Al and ITO/
`Mn:CdZnS/Al Test Components
`infiltrated
`species
`
`luminance
`(cd/m2)
`0.1
`1
`
`EL efficiency
`(10-3 %)
`0.001
`0.0014
`
`none
`(Zn4O)OAc6,
`(Zn10O4)OAc12
`TBP-ZnCl2
`0.002
`1
`TBP-ZnI2
`1
`10
`1
`10
`none
`TBP-ZnI2
`10
`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 nanocrys-
`talline 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 electrolumi-
`nescence, 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 TBP-
`complexed zinc chloride (see Experimental Section),
`the
`luminance increased by 1 order of magnitude. The most
`significant improvement was achieved with ZnI2-TBP com-
`plexes 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 current-
`voltage characteristics under forward bias (x on ITO) along
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`7844 J. Phys. Chem. B, Vol. 103, No. 37, 1999
`
`Leeb et al.
`
`Figure 8. Energy level diagram of the investigated Mn:ZnS and Mn:
`ZnCdS devices from Figures 6 and 7.
`
`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 How-
`ever, the fit of our data to this model (ln(I/V)2 (cid:24) 1/V) was
`unsatisfactory also. An alternative description of charge transport
`would take a hopping-like conductivity into account, character-
`ized 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 dc-
`field 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 corre-
`sponding 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
`
`Figure 7. Current-voltage (circles) and EL-intensity-voltage char-
`acteristics (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 “cross-
`over” values of the corresponding current-voltage curves
`possessing a I (cid:24) Vx power law character.
`At first sight, this nonohmic relationship indicates a bulk-
`limited 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 (cid:24) V7.5 in
`ZnS and I (cid:24) 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
`
`Samsung Ex. 1096
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`Nanoporous MnIIZnS Films
`
`J. Phys. Chem. B, Vol. 103, No. 37, 1999 7845
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`our