16424
J. Phys. Chem. C 2009, 113, 16424–16431
Spectrally Resolved Resonance Energy Transfer from ZnO:MgO Nanocrystals Sabyasachi Rakshit and Sukumaran Vasudevan* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore-560012, India ReceiVed: July 2, 2009; ReVised Manuscript ReceiVed: July 31, 2009
Resonance energy transfer (RET) from the visible emission of core-shell ZnO:MgO nanocrystals to Nile Red chromophores, following band gap excitation in the UV, has been investigated for four different nanocrystal sizes. With use of steady state and time-resolved fluorescence spectroscopic measurements the wavelength dependent RET efficiencies have been determined. The RET process in ZnO:MgO nanocrystals occurs from emissions involving trap state recombination. There are two such processes with different RET efficiencies for the same particle size. This is shown to be a consequence of the fact that the recombination processes giving rise to the two emissions are located at different distances from the center of the particle so that the donor-acceptor distances for the two are different, even for the same particle size. Introduction The advent of inorganic semiconductor nanocrystals with size tunable photoluminescence properties has led to a renaissance in the use of fluorescent probes in biolabeling,1 sensing,2 and assay.3 Their potential applications can be expanded by exploiting their ability to function as resonant energy transfer (RET) donors; semiconductor nanocrystal-protein sensors have already shown the way.4 There is strong motivation for using semiconductor nanocrystals as donors in RET experiments in place of conventional organic dye molecules because of the unparalleled ability to size-tune fluorescent emission as a function of crystallite size so as to match those of an acceptor molecule.5 In addition, the nanocrystals have narrow emission spectral profiles with high quantum yields and a broad excitation spectra enabling the effective separation of donor and acceptor fluorescence thus circumventing one of the main problems of conventional RET, namely crosstalk between the donor and acceptor emission. The RET process involves nonradiative transfer from an excited donor to an acceptor. The efficiency of which is defined as Q ) 1 + τ/kDA, where kDA is the RET rate and τ is the fluorescence lifetime of the donor. For molecules within the weak coupling limit, Fo¨rster6 derived an expression for the rate constant of dipole-dipole-induced RET and showed that the efficiency scales with the sixth power of the center-to-center separation, r, between the donor and acceptor, Q ) R06/(R06 + r6).7 The critical Fo¨rster distance, R0, is the distance at which the efficiency equals 50%, i.e., the distance at which an equal probability exists for the excited chromophore to relax to the ground state via emission of a photon or to undergo RET. R0 depends critically on the spectral overlap of the donor emission and acceptor absorption as well as the relative orientation of the donor and acceptor transition dipoles. The distance dependencies of RET efficiencies have been used extensively as spectroscopic “rulers” to obtain distance information in realtime on single biomolecules, especially conformational changes of proteins and peptides.7,8 In principle, semiconductor nanocrystals offer an additional advantage over organic dyes: their sizes can be tuned and thus * To whom correspondence should be addressed. E-mail: svipc@ ipc.iisc.ernet.in.
different “rulers” can be obtained, ranging from several angstroms to several nanometers. Obtaining distance information from RET experiments by using semiconductor nanocrystal donors is, however, contentious because application of Fo¨rster theory to the case where the probes involve semiconductor nanocrystals is not clearly established. The Fo¨rster theory treats the donor and acceptor as points in the interaction space, whereas the nanocrystals have finite size with extended transition densities. There have been reports in the literature of deviations from the Fo¨rster R-6 dependence of RET efficiencies involving Au as well as semiconductor nanocrytsals.9-11 Most theoretical calculations have, however, recovered the R-6 dependence except at very short distances.12-14 A possible reason for the observation of non-Fo¨rster behavior, especially in semiconductor nanocrystals, is the involvement of surface states. A recent RET study with CdSe nanocrystals found that the transfer efficiency does not follow a linear dependence on spectral overlaps as predicted by Fo¨rster theory due to the involvement of surface states in the energy transfer process.11,15,16 The role of surface states and/or defects in trapping electron and hole carriers generated following band gap excitation in semiconductors is well-known. Trap state emission, i.e., emission arising from recombination of trapped carriers, has characteristics different from those of the exciton or band gap emission including longer lifetimes.17 The trap states are usually localized so that emission may reasonably be described by a point-dipole transition. They could, however, be localized anywhere within the nanocrystal and therefore in RET studies where donor emission involves trap states, measuring donoracceptor distances, rD-A, from the center of the nanocrystal may give rise to apparent non-Fo¨rster-like behavior. Here we report RET studies using visible light emitting ZnO nanocrystals as donors and the organic dye, Nile Red, as acceptor. Unlike in the quantum dots the energy transfer here is not from the excitonic emission but from emission originating from recombination of photogenerated electron and hole carriers that are trapped in surface states. The visible emission in the ZnO nanocrystals is a pure trap state emission. It has no corresponding “absorption” in the optical spectra since it arises from recombination of carriers that are generated by band gap excitation in the UV but subsequently trapped in shallow states that lie just below the conduction band or just above the valence
10.1021/jp906202e CCC: $40.75 2009 American Chemical Society Published on Web 08/19/2009
Resonance Energy Transfer from ZnO:MgO Nanocrystals band.18 There are in fact two spectral features in the visible emission of ZnO nanocrystals.19 We have investigated how the RET efficiencies from these two feature vary with the nanocrystal dimensions in an attempt to decipher whether the recombination emission processes originate from different locations within the nanocrystal. ZnO nanocrystals as donors are particularly attractive for the present study because the spectral profile of the trap state emission, unlike the band gap excitonic emission, does not change with particle size; the only variable is the distance separating the center of the nanocrystal and the Nile Red acceptor molecule. ZnO is an II-VI semiconductor with a direct dipole allowed band gap of 3.4 eV in the near-UV and a large exciton binding energy of 60 meV. The emission spectra of ZnO has been extensively studied; the as-prepared ZnO nanocrystals exhibit a sharp UV excitonic emission at ∼380 nm (3.3 eV) and a broad intense visible emission between 450 and 600 nm (eV) that has been attributed to recombination of shallow trapped photogenerated carriers with localized deep trap states that have been identified as single positively charged oxygen vacancies, Vo+, that lie 2.0 eV below the conduction band and are an inherent defect of the ZnO structure.20,21 From time-resolved emission spectroscopy studies it had been shown that there are in fact two emission bands in the visible, a band at 480-520 nm and a band at 570 nm.19 These features fit within the broad (450-600 nm) envelope of the steady state photoluminescence spectra. The two features had been ascribed to recombination involving photogenerated electrons and holes in shallow traps, respectively, with the deep trap states. The ratio of the intensities of the visible trap emission to the UV excitonic emission is strongly dependent on particle size being larger the smaller the nanocrystal; for ZnO nanocrystals larger than 12 nm the visible emission is usually absent.22 Dispersions of the nanoscale ZnO tend to aggregate or undergo Ostwald ripening because of high surface free energy resulting in the disappearance of the visible emission. It has been shown that small additions of Mg (
