J. Phys. Chem. B 2005, 109, 1663-1668
1663
ARTICLES Emission Properties of Manganese-Doped ZnS Nanocrystals Sameer Sapra,† Ankita Prakash,† Ajit Ghangrekar,‡ N. Periasamy,‡ and D. D. Sarma*,†,§ Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India, and Department of Chemical Sciences, Tata Institute of Fundamental Research, Mumbai 400 005, India ReceiVed: January 1, 2004; In Final Form: NoVember 4, 2004
We have performed steady-state and time-resolved fluorescence studies on undoped and Mn-doped ZnS nanocrystals with approximately 16 Å diameter. While there is no band-edge emission, the intensity of the steady-state blue fluorescence from ZnS surface states decreases upon Mn incorporation, which gives rise to an orange emission. These results show that Mn incorporation competes very effectively with the donoracceptor surface states for the energy transfer from the electron-hole pair excited across the band gap. In both undoped and doped samples, the time-resolved fluorescence studies establish the presence of a distribution of decay lifetimes possibly due to a number of emission centers in the nanocrystals. A faster short-time decay of the blue emission in the Mn-doped samples compared to that in the undoped sample suggests an additional decay channel for the surface states via an energy transfer from these states to the dopant levels.
1. Introduction Doped semiconductors have been studied extensively in the past few decades;1 recent years have seen a resurgence in the research activity in this field. The interest in doped semiconductors is mainly due to the luminescence properties. One can achieve photoluminescence and electroluminescence at different wavelengths upon doping different ions in the same host semiconductor.2 The other major application of such doped systems is in the area of magnetism. In this case, the doped magnetic ions are responsible for the magnetic behavior; it can even give rise to half-metallic ferromagnetic systems such as Mn-doped GaAs.3,4 A new dimension can be added to these magnetically doped semiconductors by controlling the size of the semiconductor particles in the nanometer range. Primarily, Ga1-xMnxAs,5 Zn1-xMnxS 6-20 and Cd1-xMnxS 21-24 have been extensively investigated. Also, other systems such as Zn1-xEuxS,25,26 Zn1-xCuxS,2 and Zn1-xMnxSe 27 have been studied. In doped nanocrystals, the luminescence quantum efficiency is expected to increase as a result of a greater overlap between the electron and the hole of the host semiconductor material with the localized dopant levels. Another advantage is that the band gap of the semiconductor nanocrystals can be tuned between the bulk band gap value and the molecular HOMOLUMO gap, resulting in a nearly continuous tunability of the band gap over as much as a 2 eV range. This allows tuning of the excitation energy given to the host semiconductor to emit through the dopant levels. Also, it is possible to change the * To whom correspondence should be addressed. E-mail: sarma@ sscu.iisc.ernet.in. † Indian Institute of Science. ‡ Tata Institute of Fundamental Research. § Also at Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560 064, India, and the Centre for Condensed Matter Theory, Indian Institute of Science.
dopant-emission wavelength by changing the size of the nanocrystals as discussed in the case of Zn1-xEuxS.26 Specifically, Mn-doped ZnS is used as a phosphor material for electroluminescent displays.1 The greater overlap between the host and dopant wave functions in a nanocrystal compared to the bulk material leads to an enhancement in the luminescence intensity due to an enhanced transfer between the host and dopant levels. Furthermore, it has been claimed8 that the increase in this overlap also leads to a drastic reduction of the radiative lifetime of the Mn orange emission. Subsequent time-dependent fluorescence studies of Zn1-xMnxS and Cd1-xMnxS nanocrystals have, however, shown that the lifetime of emission through the Mn d levels is hardly affected due to quantum confinement.13,24 These later studies suggest that the short lifetime of Mn d-d emission reported in ref 8 is in fact an experimental artifact. Recently, it has been shown19 that the overlap between the semiconductor states and the Mn d levels plays an important role in determining the band gap shifts with the Mn concentration in Zn1-xMnxS; thus it appears reasonable to expect an effect of doping on the luminescence properties too. To probe the effect of Mn doping on the luminescence properties, we prepared high-quality, nearly monodispersed, Zn1-xMnxS samples with different Mn doping concentrations. 2. Methodology The synthesis procedure is adapted from Nanda et al.28 who originally used it for the synthesis of undoped ZnS nanocrystals. The Zn1-xMnxS nanocrystals were prepared by adding the nominal amounts of Zn(OAc)2‚2H2O and Mn(OAc)2‚4H2O in dimethylformamide along with the capping agent 1-thioglycerol. The sulfides were precipitated upon addition of aqueous sodium sulfide solution slowly. Then the reaction mixture was heated at 100 °C for 12 h. The nanocrystals were precipitated from the clear solution thus obtained by addition of acetone. The nanocrystals were then washed several times with methanol and
10.1021/jp049976e CCC: $30.25 © 2005 American Chemical Society Published on Web 01/14/2005
1664 J. Phys. Chem. B, Vol. 109, No. 5, 2005 dried in vacuo for 6 h. The dried powders were used for all the measurements. X-ray diffraction (XRD) patterns were recorded on a Siemens D5005 diffractometer using Cu KR radiation. Since the diffracted intensities from these nanocrystals are generally weak, all patterns were recorded at a slow scan rate (10 s/0.1°) in order to get a reasonable signal-to-noise ratio. UV-vis absorption spectra were recorded with powders redissolved in deionized water on a Perkin-Elmer double beam SP8-100 spectrophotometer, and the steady-state fluorescence measurements were carried out with the same solutions on a Shimadzu RF510 spectrofluorophotometer. Time-resolved fluorescence measurements were carried out using time-correlated single-photon counting (TCSPC) technique and a high repetition rate picosecond laser source.29 The nanocrystalline powder sample was taken in a quartz capillary. In some experiments, the sample was dissolved in deionized water and taken in a standard quartz cuvette. The sample was excited by 10 ps, 310 nm laser pulses at a repetition rate of 800 or 400 kHz. The PL from the sample was passed through a cutoff filter (Schott WG 345), a quartz lens, a polarizer oriented at the magic angle (54.7°) with respect to the excitation polarization, and a monochromator. The PL at a selected wavelength was detected at the single-photon level by a microchannel plate photomultiplier (Hamamatsu R2809). In some experiments, the PL decay of “total fluorescence” in the blue emission band (370-520 nm) was obtained by scanning the monochromator (30 nm/min) while detecting photons. The detected photons were time correlated with respect to the excitation laser pulses that are detected by a fast photodiode (IBH, U.K.). The electronic signals from the photodiode and photomultiplier were processed through constant fraction discriminators (CFDs) for generating timing signals, a time-toamplitude converter (TAC) to generate a voltage pulse that is proportional to the difference in time, and histogrammed in a multichannel analyzer. The instrument response function (IRF) was obtained using a scatterer (MgO powder) instead of the sample. The fwhm of the instrument response function was 40 ps. The detected PL photon rate is less than 0.5% of the excitation rate (i.e.
