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J. Phys. Chem. B 2002, 106, 3860-3863
Impact of Erbium-Doped Silicon Nanocrystals on the Properties of Polyphenylene Vinylene Films Junmin Ji and Jeffery L. Coffer* Department of Chemistry, Texas Christian UniVersity, Fort Worth, Texas 76129 ReceiVed: NoVember 6, 2001; In Final Form: January 17, 2002
This report describes the fabrication and properties of composite films composed of poly(phenylene vinylene) (PPV) and Er-doped Si nanocrystals. For these hybrid nanocrystal/polymer materials, preliminary results have focused on their characterization via atomic force microscopy (AFM), profilometry, absorption/fluorescence spectroscopy, and current-voltage measurements. In general, it is found that the presence of Er-doped Si nanoparticles in the film strongly impacts the steady-state photoluminescence of the PPV, whereas the presence or absence of solvent chosen for the nanocrystals influences the carrier transport and resultant visible electroluminescence of light-emitting diodes fabricated from these materials.
Introduction Interfacial phenomena play a key role in the fundamental and technologically relevant properties of luminescent organic semiconductors such as poly(phenylene vinylene) (PPV)1 and its related derivatives. Choice of solvent,2 precursor purity,3 and temperature,4 among other factors, can all impact the extent of interchain interactions and other structural properties of the desired polymer film. In principle, it should also be possible to alter such interactions with the incorporation of such threedimensionally confined semiconductor nanocrystals into the polymer matrix. The presence of such quantum dots should alter not only the film’s structure but also its electronic and photophysical characteristics as well. Thus, we report here studies of Er-doped Si nanocrystal5/PPV composites. Studies of the fundamental properties of three-dimensionally confined Si nanoparticles and related materials (such as those described in this article) have, in general, important ramifications on the future fabrication of ultra-small Si quantum dot devices relying on either carrier or photon transport for their function. As an optically active dopant in silicon, erbium is of particular interest because of the coincidental match of its 1540-nm fluorescence in the near-IR with the transmission maximum of SiO2. For these hybrid nanocrystal/polymer materials, preliminary results have focused on the characterization of these materials via atomic force microscopy (AFM), profilometry, absorption/ fluorescence spectroscopies, and current-voltage measurements. Previous studies of CdSe nanocrystal-doped PPV6-9 or rare earth-complex-doped CN-PPP10 focused on the feasibility of energy or electron transfer between the dopant species and the polymer. However, in contrast, the main points of emphasis of this report concern (1) the influence of Er-doped Si nanoparticles on the steady-state photoluminescence of the PPV and (2) the current-voltage behavior and resultant electroluminescence of light-emitting diodes fabricated from these materials.
He atmosphere at 1000 °C according to a previously published procedure.5 Er3+ was introduced into the He stream in the form of the β-dikeonate complex Er(tmhd)3. Nanocrystals were extracted, washed, and purified according to the procedure originally described in ref 5. These Er-doped Si nanoparticles prepared with a 6-cm length furnace have an average diameter of 5.9 nm (as evaluated by high-resolution electron microscopy (HREM)). For the preparation of nanocrystal-doped PPV, a modified version of the traditional three-step synthetic route was used.11 In a typical preparation, 2 mL of a MeOH solution of the polyelectrolyte PPV precursor (poly{p-phenylene[1-(tetrahydro- thiophen-1-io)ethylene chloride]}) (1% concentration) was mixed with 1 mL of a solution of Er-doped Si nanocrystals dissolved in ethylene glycol (for example, a 1.83 mg Si/cm3 solution is used for the 8% doped film). The mixture was then sonicated for 30 min and spin-coated on a patterned ITO/glass substrate at ∼1500 rpm. The substrate was heated under vacuum (∼10-6 Torr) at ∼220 °C for 12 h to convert it to the desired structure. For current-voltage and EL measurements, aluminum pads (approximately 200-nm thick) were thermally evaporated onto the desired film. The final LED structure has an active area of 4 mm × 4 mm. Instrumental Measurements. UV-visible absorption spectra were recorded at room temperature with an HP 8452A diode array spectrophotometer. Steady-state photoluminescence (PL) measurements were recorded using a Spex Fluorolog-2 0.22-m double spectrometer with a constant excitation wavelength provided via light from a 450 W Xe lamp focused into a single 0.22-m monochromator. Applied bias for the current-voltage and EL measurements was provided by a Keithley Instruments model 236 source-measure unit. Film thicknesses were measured with an Alpha-Step 2000 profilometer. AFM images were recorded in the intermittent contact mode using a Quesant scanning probe microscope.
Experimental Section
Results and Discussion
Nanocrystal and Film Preparation. Erbium-doped Si nanocrystals were formed from the pyrolysis of disilane in a
Use of the process outlined above produces PPV films typically on the order of 200 nm as evaluated by contact profilometry. The surface morphology of these Er-Si nanocrystal-doped films was evaluated by atomic force microscopy; a typical image is illustrated in Figure 1. In general, the surfaces
* To whom correspondence should be addressed. E-mail: J.Coffer@ tcu.edu.
10.1021/jp0140670 CCC: $22.00 © 2002 American Chemical Society Published on Web 03/20/2002
Properties of Polyphenylene Vinylene Films
J. Phys. Chem. B, Vol. 106, No. 15, 2002 3861
Figure 1. Atomic force microscopy (AFM) images of PPV films containing Er-doped Si nanocrystals. (a) Film containing crystallites randomly dispersed throughout the structure. (b) Layered film composed of PPV/doped Si nanocrystals/PPV.
of these films exhibit a topography that is common for spincast PPV or MEH-PPV materials,2 with nanometer-sized features detected on the surface. It is possible, however, that some of these features could arise from aggregates of the nanocrystals protruding from the film. Control experiments have been carried out whereby Er-Si nanocrystals are physically dispersed on a PPV-coated glass surface by adding drops of a dilute solution of the nanocrystals and allowing the solvent to evaporate, which is followed by deposition of a PPV precursor solution and subsequent conversion to PPV. A PPV/Er-Si nanocrystal/PPV sandwich structure is thus produced whereby
the surface is “spiked” with aggregated nanocrystals. The surface morphology of this type of doped polymer is quite different, with a markedly rougher surface and larger features observed (Figure 1b), which would suggest that in the original process it is unlikely that the majority of nanocrystals have extensively aggregated toward the bottom of the film. Spectroscopically, one can detect the presence of the erbiumdoped Si nanocrystals very sensitively by optical absorption. Examining the absorption spectrum for a given film on a glass substrate in the 500-700 nm region (beyond the point where the PPV absorbs), one finds a broad, featureless tail consistent
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Figure 2. Room-temperature visible photoluminescence (PL) spectra of PPV films doped with various amounts of Er-doped Si nanocrystals. λex ) 375 nm.
with the presence of the Si nanocrystals. In general, a weak absorption tail from 370-800 nm is responsible for the yellow color of the nanocrystals and is ascribed to the weak, indirect absorption at the indirect gap of Si.12 The presence of the Er-doped Si nanocrystals in the film during the conversion to PPV clearly affects the photoluminescence of the polymer, which can be seen in a series of roomtemperature PL spectra (Figure 2) where the effects of the increasing concentration of Er-Si are illustrated. For all samples, the strong S1 f S0 green fluorescence is observed, with the 0 f 0, 0 f 1, and 0 f 2 vibronic bands clearly resolved. Interestingly, we find that the relative intensity of the 0 f 0 transition at ∼511 nm to that of the 0 f 1 band at ∼546 nm increases as the percentage of Si nanocrystals in the film is increased (Figure 2). There is also a negligible blue shift of the emission maximum that accompanies this increase (
