ARTICLE pubs.acs.org/JPCC
Ultrafast Excitation Energy Transfer in Vinylpyridine Terminated Silicon Quantum Dots Anja Sommer, Carla Cimpean, Michael Kunz, Christian Oelsner, Hans J. Kupka, and Carola Kryschi* Department of Chemistry and Pharmacy and ICMM, Friedrich-Alexander University of Erlangen-Nuremberg, Egerlandstr. 3, 91058 Erlangen, Germany ABSTRACT: Water-soluble 2- and 4-vinylpyridine terminated silicon quantum dots (SiQDs) with sizes between 2 and 3 nm were obtained by HF etching of oxidized SiQDs and subsequently passivating with vinylpyridine ligands via thermal hydrosilylation. The functionalized SiQDs emit photoluminescence in the blue-green spectrum with quantum yields around 30%. The photoluminescence is ascribed to radiative recombination of excitons confined to the SiQD core. Indication of efficient electronic interactions between excited ligand states and SiQD conduction band states has been achieved by real-time monitoring excitation relaxation dynamics on the subpicosecond time scale using timeresolved laser spectroscopy techniques. The observed ultrafast excitation relaxation dynamics involving decay and rise dynamics faster than 1 ps were ascribed to electronic excitation energy transfer from an initially photoexcited ligand state to SiQD conduction band states. These results were analyzed by performing a state population analysis on the basis of electron exchange interactions.
1. INTRODUCTION Silicon is an indirect band gap semiconductor and therefore an inefficient light emitter and absorber. In contrast, silicon quantum dots (SiQDs) with sizes smaller than free-exciton Bohr’s radius (4.3 nm) were observed to emit enhanced photoluminescence (PL) with a size-tunable spectrum in the visible.14 This is due to quantum confinement which does not only imply the widening of the energy gap with decreasing the SiQD size36 but also involves nonvanishing carrier wave function amplitudes at the QD surface. Natively passivated SiQDs are surrounded by an amorphous SiOx layer that contains a broad variety of optically inactive defect structures.79 Thus, the engineering of SiQDs for optoelectronic and photonic devices urgently requires surfaces that are modified, at least, by saturating all dangling bonds of the Si atoms. One fundamental objective for the realization of SiQD based nano-optoelectronics is to functionalize the QD surfaces with electronically coupled molecules which may act as antennae and transfer excitation energy to bulk states.1012 In the recent past, extensive research activities have been focused on the synthesis of monodisperse SiQDs with stable surface structures and welldefined sizes, shapes, and optical properties.1317 Monodisperse SiQDs were reported to be fabricated, by laser-driven SiH4 pyrolysis,18 by RF plasma-induced dissociation of SiH4,19 or through a liquid-phase reduction procedure of SiCl4 via the use of reverse micelles in nonpolar solvents.20 The reverse micelle wetchemistry procedure yielded aminopropyl-terminated SiQDs with sizes around 1.4 nm.20 On the other hand, gas-phase synthesis routes provided hydride-terminated SiQDs with narrow size distributions between 2 and 5 nm which were subsequently functionalized with 1-alkene or 1-alkyne derivatives via thermal hydrosilylation under total exclusion of oxygen.12,19,2123 Recent femtosecond (fs) laser transient absorption spectroscopy r 2011 American Chemical Society
studies provided an indication of efficient coupling between excited molecular ligand states and conduction band states in vinylthiophene and vinylpyridine terminated SiQDs with sizes between 2 and 3 nm.22 Recently, considerable research activities have been dedicated to photoinduced electron transfer (PET) and excitation energy transfer (EET) across the interface of a semiconductor substrate covered with adsorbed dye molecules.12,2427 One research objective was to gain in-depth insight into interfacial processes, which are essential for the optimization of light harvesting and energy conversion in dye-sensitized semiconductor solar cells.2432 Employing fs-resolved laser spectroscopy techniques, interfacial PET between a molecular electronic state of a surface-bound chromophore and conduction band states of TiO2 nanoparticles was observed to exhibit nonexponential dynamics on an ultrashort time scale down to a few tens of fs.2428 In comparison, recent fs spectroscopy studies on 3-vinylthiophene-terminated SiQDs yielded ultrafast nonexponential excitation dynamics which were ascribed to interfacial EET from initially excited ligand states to SiQD conduction band states.12 On the other hand, poly(9-vinylcarbozole)SiQD composites33 and SiQDs functionalized with a red emitting ruthenium complex34 were reported to exhibit fluorescence (or F€orster) resonance energy transfer (FRET).35 Poly(9-vinylcarbazole) films doped with SiQDs were observed to emit photoluminescence with lifetimes that become significantly shorter with increasing SiQD concentration. This dependency was analyzed in terms of the donor acceptor distance and F€orster radius. Energy transfer rates, kFRET, with values between 3.0 and 11.1 107 s1, were determined.33 Received: July 9, 2011 Revised: October 10, 2011 Published: October 14, 2011 22781
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Figure 1. Illustration of 2-vinylpyridine and 4-vinylpyridine terminated SiQDs.
Photoluminescence (PL) spectroscopy experiments on Ru complex functionalized SiQDs showed that shortening the alkyl linker between the complex and Si core caused the decrease of the PL intensity and lifetime of the SiQDs.34 These results were explained with FRET that takes place from photoexcited SiQDs to the covalently linked Ru complex. Remarkably high values of kFRET in the range of (0.22.2) 109 s1 were obtained for the undecyl, hexyl, and propyl linker group. However, ultrafast EET via distances shorter than 1 nm cannot be adequately theoretically described by F€orster's theory that is based on the pointdipole approximation and postulating thermally equilibrated donor and acceptor states.35,36 Moreover, FRET is assumed to take place between optically bright states, only.37 In this contribution, we present a spectroscopic and theoretical study of EET and exciton relaxation dynamics of luminescent 2- and 4-vinylpyridine-terminated SiQDs (Figure 1) dispersed in ethanol. The interplay between photoexcited ligand states and conduction band (CB) states of the functionalized SiQDs was examined by directly monitoring photoexcitation dynamics with femtosecond (fs) spectroscopy. Our experimental data indicated that photoinduced EET occurs on a time scale of several hundreds of femtoseconds. The ultrafast EET between a photoexcited ligand state and the manifold of electronic states of the SiQD is studied in the time domain employing an effective Hamiltonian that accounts for the electronic couplings between ligand and SiQD states.
2. EXPERIMENTAL SECTION Materials. The chemical reagents, 2-ethynylpyridine (Aldrich, 98%), 4-ethynylpyridine hydrochloride (Aldrich, 97%), o-xylene anhydrous (Aldrich, 97%), ethanol (Merck, for spectroscopy), and aqueous hydrofluoric acid (Riedel-de Ha€en, 40%), were used as received or were degassed by a series of freezepumpthaw cycles as described below. The reference compounds 2-vinylpyridine (Aldrich, 97%) and 4-vinylpyridine (Aldrich, 95%) were purified by distillation under reduced pressure (82 C/0.039 bar and 62 C/ 0.020 bar, respectively). SiQDs with a mean size of 5 nm were fabricated by Evonik DegussaCreavis Technologies & Innovation in a low-pressure microwave reactor using silane as a precursor.19 Functionalization of the SiQDs. An amount of 1.5 g of SiQDs was dispersed in 70 mL of ethanol and 10 mL of HF (40%) and stirred for 10 min. The ethanolic HF solution was removed by vacuum distillation. A solution of 2-ethynylpyridine or 4-ethynylpyridine in o-xylene (5 g in 70 mL) was degassed using at least three freezepumpthaw cycles and then added to freshly etched, hydride-terminated SiQDs. The suspension was refluxed at ∼140 C for 20 h. The functionalized SiQDs were
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washed over a polyvinylidene fluoride (PVDF) membrane filter (Millipore) with o-xylene and ethanol several times to remove the unreacted 2- or 4-ethynylpyridine molecules. High-Resolution Transmission Electron Microscopy (HRTEM). The functionalized SiQDs were prepared for HRTEM imaging by dispersing them in water and evaporating in air multiple drops of the suspension onto an ultrathin carbon-coated copper grid. The HRTEM images were recorded using a Phillips CM 300 UltraTwin microscope. The measurements were carried out at an accelerating voltage of 300 kV in the bright-field mode. Steady-State Spectroscopy. The FTIR spectra were measured using a Bruker Equinox 55 spectrometer. PL spectra were recorded on a Horiba Jobin-Yvon FluoroMax-3 spectrofluorometer using the magic-angle polarization configuration. All experiments were performed at room temperature employing either KBr (FTIR spectroscopy) or 10 mm quartz cuvettes. Functionalized SiQDs were dispersed in ethanol at a concentration of 100 μg per mL. Time-Resolved Spectroscopy. Femtosecond transient absorption spectroscopy experiments were conducted with a Clark MXR CPA 2101 laser system in conjunction with an Ultrafast TAPPS HELIOS detection system, consisting primarily of a glass fiber based spectrometer. The output pulses at 387 nm with a 150 fs pulse and a 1 kHz repetition rate were used as pump pulses. They were obtained by amplifying and frequency doubling the 775 nm seeding pulses of the Er3+-doped glass fiber oscillator in a regenerative chirped-pulse titaniumsapphire amplifier and with the frequency doubling BBO crystal in the nonlinear optical amplifier (NOPA), respectively. All samples were pumped at excitation densities between 1.14 109 and 1.90 109 W/cm2. The samples consisting of 1 mg of SiQDs dispersed in 0.8 mL of ethanol were irradiated in quartz cuvettes with a thickness of 2 mm. A fraction of the fundamental was simultaneously passed through a sapphire plate (3 mm) to generate the fs white-light continuum between 400 and 1400 nm. The chirp between 400 and 750 nm was approximately 350 fs. Transient absorption spectra of the functionalized SiQDs in ethanol were taken at delay times between 2 ps and 3 ns. They were recorded in the visible (VIS) between 420 and 750 nm. No photochemical degradation was observed after each experiment as the subsequently recorded PL spectrum of the sample showed. The transient absorption spectra were obtained as temporal evolution of the spectral changes in the optical density (ΔOD) of the sample. Therefore, a chopper wheel provided the blocking of each second pump pulse so that the probe pulse was alternately transmitted through a pumppulse excited and a ground-state sample. The intensity of the transmitted probe pulse after the pumppulse excited sample, I*(λ,τ), and that without pump pulse excitation, I0(λ) were measured as a function of the delay time τ. The ΔOD values were determined by ΔOD(λ,τ) = log(I0(λ)/I*(λ,τ)). For I0(λ) > I*(λ,τ), the ΔOD signal attains positive values and is assigned to absorption transitions of excited species, the so-called photoinduced absorption (PIA), whereas negative values of ΔOD result from I0(λ) < I*(λ,τ) and are ascribed to photoinduced bleaching (PIB) of the ground state population density.
3. THEORETICAL ANALYSIS OF THE STATE POPULATIONS In the following theoretical description of ultrafast EET, we consider the response of the donoracceptor system, that is, 22782
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a vinylpyridine-terminated SiQD, to an ultrashort laser pulse with the duration Δt applied at t0 = 0. The Hamiltonian of the vinylpyridineSiQD system, HS, is represented on the basis of the following states: the electronic ground state, the photoexcited ligand state, and a manifold of acceptor states. The full Hamiltonian H is split into the contribution due to the vinylpyridineSiQD system, HS, and a time-dependent term describing the interaction with the laser pulse H ¼ HS μE cos ωt
0 e t e Δt
H ¼ HS
t g Δt
ð1Þ
where E is the electric field of the incident pumppulse with the frequency ω and μ denotes the projection of dipole moment operator in the direction of the electric field. HS is the Hamiltonian of the system vinylpyridineSiQD that accounts for the temporal evolution of the excited states. After a time t evolved as t = Δt + τ the wave function of the system, ψ(t), is determined by the Hamiltonian H in eq 1 according to the expression ψðtÞ ¼ Uðt þ Δt, ΔtÞUðΔt, 0Þjψ0 æ
ð2Þ
where U(Δt, 0) is the unitary time evolution operator during the pumppulse excitation, while U(t + τ,Δt) is the evolution operator of the system after the pumppulse excitation. ψ0 = |ψSL0ψVB QD| is the wave function of the ground state of the system, consisting of the two subunits vinylpyridine ligand and SiQD. The superscripts S0 and VB denote the electronic ground-state and valence band (VB) wave functions of the ligand L and the SiQD, respectively. Under the condition of short time (Δt) excitation with an intense laser pulse, the time evolution of the subset of states, ψ0, ψ1, and ψ2, is
ground state is built up by normalized single electron real wave functions as VB VB VB VB VB S0 S0 S0 S0 ψ0 ¼ jjVB ̅ 1 :::ji j̅ i :::jm j̅ m j1 j̅ 1 :::jn j̅ n j 1 j
where the functions with the superscript “S0” are mainly localized in the ligand part and those functions with the superscript VB in the QD. In contrast to the ground state, where all single electron wave functions are doubly occupied, the wave function of the excited state is created by the transfer of an electron i from the highest occupied molecular orbital (HOMO) jSi 0 (as defined in the ground state configuration of the dot) into the unoccupied molecular orbital (LUMO) jCT p of the chargetransfer (CT) state 1 VB VB VB CT ψ1 ¼ pffiffiffi fjjS10 j̅ S1 :::jSi 0 :::jSm0 j̅ Sm0 jVB ̅ 1 :::jn j̅ n j̅ p j 1 j 2 VB VB VB CT jjS10 j̅ S10 :::j̅ Si 0 :::jSm0 j̅ Sm0 jVB ̅ 1 :::jn j̅ n jp jg 1 j
1 0 VB VB VB VB CB ψ2j ¼ pffiffiffifjjS10 j̅ S10 :::jSi 0 j̅ Si 0 :::jSm0 j̅ Sm0 jVB ̅ 1 :::jj :::jn j ̅ n j̅ p j 1 j 2
ð3Þ
where ωR = μE/h is the Rabi frequency and E0 is the ground-state energy. |ψ1æ is the initially populated ligand state denoted as charge-transfer (CT) state, and {ψ2k} represents the manifold of CB states acting as accepting states. Equation 3 suggests that after pumppulse excitation the populations of the ground and first excited states are cos2(ωRΔt/2) and sin2(ωRΔt/2), respectively. Since the pumppulse width (Δt ≈ 150 fs) is quite short, we may assume for our initially photoexcited system that any energy redistribution occurring via intramolecular vibrational relaxation will take place on a time scale larger than 500 fs.38,39 This justifies reducing the time scale to the interval (Δt,Δt + τ). The probability amplitudes after pumppulse excitation are given by the time evolution as Cαα0 ðτÞ ¼ Æψα jUðΔt þ τ, ΔtÞjψα0 æ,
ðα, α0 ¼ 1, 2kÞ
Our objective is now to evaluate the time evolution of the system at the time t = Δt + τ using the formalism of the effective Hamiltonian Heff = PH0P + PR(E)P4042
Hef f ¼
ð4Þ ð5Þ
Here Heff is an effective Hamiltonian yet to be determined. Analogously to the ground-state wave function, the wave functions of the excited states are composed of wave functions of the two subunits. In the presence of perturbation, the correct wave functions of the vinylpyridineSiQD system have to be antisymmetrized with respect to the exchange of electrons between the ligand and QD states. To illustrate this, it is worthwhile to specify the wave functions in terms of single electron molecular spin orbitals. The
ð7bÞ
VB VB VB VB CB jjS10 j̅ S10 :::jSi 0 j̅ Si 0 :::jSm0 j̅ Sm0 jVB ̅ 1 :::j̅ j :::jn j̅ n jp jg 1 j
j
i E1 Γ1 2 V1, 21
V1, 21
V1, 22
3
3
V1, 2n
i E21 Γ21 2
0
3
3
0
i E22 Γ22 2 0
0
0
0
3
3 3
3 3
3 3
0
3
3
V1, 22
0
3 3
3 3
V1, 2n
0
i E2n Γ2n 2
j
ð8Þ
with UðΔt þ τ, ΔtÞ ¼ expð iHef f τ=pÞ
ð7aÞ
This wave function has a hole “i” in the ligand subunit of the SiQD, and an additional electron “p” is placed in the CT state. The minimum energy configuration within all possible Slater determinants of the form (7a) corresponds to a CT state. The definition of such CT state is intuitively correct and favored by the electron flow between the positions “i” and “p”. Similarly, the wave functions of the final (excitonic) states ψ2k may be constructed as minimum energy configurations of Slater determinants defined by promoting the electron “p” from the CT state into CB levels of the SiQD, when a simultaneous transfer of an electron “j” takes place from the VB to the hole “i” of the ligand HOMO
ψðtÞ ¼ ψðΔt þ τÞ ¼ expð iE0 τÞexpðiωΔtÞcosðωR Δt=2Þjψ0 æ þ C1 ðτÞsinðωR Δt=2Þjψ1 æ þ C2 ðτÞsinðωR Δt=2Þjψ2 æ
ð6Þ
where the energy values are modified by the corresponding level shifts. The range of the matrix in eq 8 is defined by the number of excitonic states under consideration. Of primary interest are the widths Γα, being the imaginary part of the self-energy ΣS. The level width Γ1 is related to the lifetime of the excited state ψ1 and allows us to determine crudely the EET rate kEET = Γ1/p. The widths of the exciton states Γ2k are determined by damping processes such as radiative and nonradiative transitions. We now turn to the final problem that is the diagonalization of the time evolution operator exp(iHeffτ/p) and computation of the population amplitudes Cαα0 (τ) (α,α0 = 1,2k) by solving 22783
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j j C11 ðτÞ C1, 21 ðτÞ : C1, 2n ðτÞ
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j j
sinðωR Δt=2Þ 0 ¼ expð iHef f τ=pÞ 3 : 0
ð9Þ
where the column vector on the right side of eq 9 defines the initial condition at τ = 0. Thereupon, the time-dependent state populations are accessible by calculating P1(τ) = |C11(τ)|2 and P2(τ) = ∑nk|C1,2k(τ)|2. Employing eqs 8 and 9, Vα,α0 , Γα, and Eα are treated as adjustable parameters and determined by simulating transients using the same equations. The foregoing analysis was primarily intended to emphasize, through the example of a many-electron system, that any one-electron approximation is inadequate for a theoretical description of interfacial electron transfer phenomena in molecular ligandsemiconductor QD systems. It is clear from the foregoing discussion that any perturbation that couples the states (eqs 7a and 7b) is caused by exchange Coulomb forces G¼
mþn1 mþn
∑ μ¼1
mþn1 mþn
∑ gμν ¼ μ∑¼ 1 ν¼μ þ 1
e2
∑ ν ¼ μ þ 1 krμ
Figure 2. HRTEM image of 2-vinylpyridine terminated SiQDs (marked by red open circles).
ð10Þ
The required matrix elements for calculating the time evolution of the states in eq 7 become then a linear function of expressions of the type ~VB =jCB jCT Þ ðjS0 jCB =jVB jCT Þ ð11Þ G ¼ ðjS0 j ij, p
i
j
p
p
i
p
j
p
where the exchange Coulomb integrals are defined as ZZ
ðji jj =jk jl Þ ¼
ji ðrμ Þjj ðrμ Þgμν jk ðrν Þjl ðrν Þdrμ drν
ð12Þ
Figure 3. FTIR spectrum of 2-vinylpyridine terminated SiQDs.
The exchange Coulomb interaction represents, in general, only a part of the contribution to V1,2k. Coupled electronnuclear dynamics, both adiabatic and nonadiabatic in nature, are reasonably well justified.
4. RESULTS AND DISCUSSION Initially oxidized SiQDs with sizes around 5 nm were downsized and functionalized with 2- and 4-vinylpyridine ligands by carrying out a controlled HF etching procedure which is followed by thermal hydrosilylation reaction with 2- or 4-ethynylpyridine. The shape and sizes of 2-vinylpyridine terminated SiQDs were examined with high-resolution transmission electron microscopy (HRTEM) (Figure 2). The HRTEM image displays lattice fringes with a spacing of 0.31 nm, which is characteristic of Si {111} planes. Obviously, both spherical shape and single crystallinity of the SiQDs were maintained in the course of the HF etching and thermal hydrosilylation procedures, while their sizes were reduced to 23 nm of the HF etching and thermal hydrosilylation procedures, while their sizes were reduced to 23 nm. The success of the thermal hydrosilylation reaction, manifesting as complete saturation of the dangling bonds of surface Si atoms with 2-vinylpyridine or 4-vinylpyridine, could be proven by FTIR spectroscopy (Figures 3 and 4). The absence of any peak between 1000 and 1100 cm1, as being the characteristic spectral range for the ν(SiO) stretching mode, unambiguously indicates the complete removal of the SiOx shell. The very strong peak at 740 cm1 and the weak peak at 1250 cm1 are assigned to the δ(SiC) bending and
Figure 4. FTIR spectrum of 4-vinylpyridine terminated SiQDs.
ν(SiC) stretching vibration, respectively. The presence of these peaks substantiates the formation of covalent SiC bonds. The peaks at 1539 and 1466 cm1 are assigned to the ν(CdC) and ν(CN) modes which identify pyridine. FTIR spectra taken from 4 month old and 6 month old samples look exactly the same which verifies the chemical stability of this surface coverage. The photoluminescence emission (PL) and excitation (PLE) spectra of 2-vinylpyridine and 4-vinylpyridine terminated SiQDs dispersed in ethanol or water are depicted in Figures 5 and 6. The UV/vis absorption spectra of ethanol solutions of 2-vinylpyridine (red solid line) and 4-vinylpyridine (black solid line) are shown in Figure 7. As predicted by the quantum confinement effect, SiQDs 22784
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Figure 5. PL and PLE spectra of 2-vinylpyridine terminated SiQDs in ethanol and water.
Figure 6. PL and PLE spectra of 4-vinylpyridine terminated SiQDs in ethanol.
Figure 7. UV/vis absorption spectra of 2-vinylpyridine (red solid line) and 4-vinylpyridine (black solid line) dissolved in ethanol at a concentration of 1.5 104 M and 7 105 M, respectively.
with sizes between 2 and 3 nm emit PL in the visible between 420 and 750 nm. It has to be emphasized that neither the reagents used for thermal hydrosilylation of the SiQDs (i.e., 2-ethynylpyridine and 4-ethynylpyridine) nor the respective vinyl derivatives show innate fluorescence at wavelengths longer than 400 nm. The PL spectra recorded upon excitation at 387 nm exhibit nearly Gaussian-shaped emission bands with fwhm between 500 and 650 meV which peak at 509 nm (Figure 5) and 500 nm (Figure 6). These fwhm's are larger than those observed for Gaussian-shaped PL spectra of single SiQDs with values ranging from 120 to 152 meV,43 which indicates inhomogeneous broadening due to size inhomogeneity of the QDs. The structural
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Figure 8. Temporal evolution of the transient absorption spectra of 2-vinylpyridine terminated SiQDs monitored for distinct delay times between 0.4 ps and 3 ns.
features of the partially overlapping PLE spectra are more complex and fanoian shaped.44 For 4-vinylpyridine terminated SiQDs, the prominent maximum at 366 nm is assigned to the interband transition of the SiQD, while the shoulder at 280 nm presumably originates from the S0S1 absorption transition of the ligand. This assignment is in agreement with the weak absorption band around 280 nm for free 4-vinylpyridine (Figure 7: black solid line). In the case of 2-vinylpyridine terminated SiQDs in ethanol, the PLE spectrum (thick solid line) consists of a broad, weakly structured band at 351 nm and a rather sharp peak at 470 nm. The only match of the PLE spectrum with the spectral features of the UV/vis absorption spectrum of free 2-vinylpyridine (Figure 7: red solid line) is the weak shoulder at 295 nm which corresponds to the lowest-energy absorption of 2-vinylpyridine. The lack of spectral resemblance suggests that both electronic structure and geometry of the 2-vinylpyridine ligand covalently bound at the SiQD surface differ significantly from those of the free compound. In the PLE spectrum, the broad band at 351 nm is ascribed to interband absorption transitions of the SiQD, whereas the shoulders between 370 and 410 nm and, in particular, the prominent sharp peak at 470 nm are assigned to charge-transfer transitions, where the ground-state 2-vinylpyridine ligand (e.g., the nonbonding electron pair of the pyridine N atom) may act as an electron donor and surface-standing Si atoms function as an acceptor.45 In particular, the assignment of the PLE band at 470 nm to a charge-transfer transition was verified due to the red-shift of this prominent band in the PLE spectrum taken in higher polar solvent water (Figure 5, thin black line). Employing the comparative method by Williams et al.46 with rhodamine 6G as the fluorescence standard, the PL quantum yields of the hydrosilylated SiQDs in ethanol were determined to be around 30%. The rather high PL quantum yields are in agreement with those obtained from octadecyl-terminated SiQDs with sizes between 3 and 4 nm, which were reported to exhibit PL emission in the red (650790 nm) with PL quantum yields up to 60%.18 Furthermore, high PL quantum yields of quantum dots are in line with the quantum-confinement effect. The relaxation of the law of momentum conservation gives rise to enhanced probabilities of the optical zero-phonon and phonon-induced radiative transitions. This is confirmed by the drastic decrease of the Stokes shift between the respective PL and PLE spectra of 2-vinylpyridine and 4-vinylpyridine terminated SiQDs (Figures 5 and 6). Information of ultrafast interfacial EET and excitation relaxation processes was obtained upon conducting femtosecond (fs) transient absorption spectroscopy experiments. Therefore, ethanol 22785
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colloids of 2- and 4-vinylpyridine terminated SiQDs were excited with 150 fs pump pulses at 387 nm, while the time evolution of the excited states was monitored on the time scale from 0 ps to 3 ns using temporally delayed, white-light continuum probe pulses. Figure 8 depicts the time evolution of the transient absorption (TA) spectra of 2-vinylpyridine-terminated SiQDs. Initially an absorption maximum at 470 nm and a smaller one around 525 nm (2.3 eV) emerge within 0.1 ps, which is followed by the formation of a broad absorption band between 500 and 650 nm and a smaller one at 680 nm. The absorption bands gradually decay with slightly different decay dynamics until 3 ns. The temporally delayed formation of the broad absorption band between 500 and 650 nm partially occurs at the expense of the absorption band at 470 nm which is ascribed to the absorption of the initially photoexcited ligand state which also might be considered as a CT state that involves a surface-standing Si atom of the SiQD. While CT states with energies equal to and smaller than 3.2 eV may be directly photogenerated with pump pulses at 387 nm, pumppulse excitation densities of 1.14 109 W/cm2 are sufficiently large for the generation of electronically excited 2-vinylpyridine states (Figure 7) via two-photon absorption.12 On the other hand, the absorption occurring between 550 and 650 nm in the transient absorption spectra (Figure 8) is characteristic for absorption transitions of SiQD CB electrons.9 Since this absorption band originates from populated CB states, it provides a sensitive spectroscopic probe for directly monitoring interfacial electron transfer between ligandsurface states and excitonic CB states. The decay of the ligand absorption band at 470 nm with simultaneous formation of the absorption between
Figure 9. Short-time behavior of three transients detected at 540 nm (black circles), 600 nm (red circles), and 640 nm (blue circles); the respective calculated state populations of the SiQD CB states were obtained with the energy values listed in Table 1 and were depicted as solid lines in the corresponding color. The P2 scale is normalized according to the assumption that sin2(ωRΔt/2) = 1.
550 and 650 nm is explained to arise from ultrafast interfacial EET to SiQD CB states. Figure 9 shows the short-time behavior of transients detected at 540, 600, and 640 nm. The apparatus time resolution is about 270 fs, while the transients each show a delayed rise dynamics (ca. 450 fs) and a nonexponential decay behavior. Employing our theoretical approach for EET described in Section 3, the transients at 540, 600, and 640 nm were simulated by calculating the temporal evolutions of the population of the CB states, P2(t) = ∑i|C2k(t)|2. P2(t), depicted as black, red, and blue solid lines, respectively (Figure 9), were obtained upon solving Heff as a 3 3 matrix with the energy difference values Δ(E1 E21) and coupling matrix elements V1,21, V1,22, and level widths Γ1, Γ21, and Γ22, which are listed in Table 1.
Figure 10. Temporal evolution of the transient absorption spectra of 4-vinylpyridine terminated SiQDs monitored for distinct delay times between 0.4 ps and 2.3 ns.
Figure 11. Short-time behavior of three transients detected at 520 nm (black circles), 560 nm (red circles), and 600 nm (blue circles); the respective calculated state populations (solid lines) of the SiQD CB states were obtained under the same normalization as in Figure 9 and with the energy values listed in Table 2.
Table 1. Energy Differences Δ(E1 E21) and Δ(E21 E22) between the Interacting Energy States E1, E21, and E22, Coupling Matrix Elements V1,21 and V1,22, and Level Widths Γ1, Γ21, and Γ22 of 2-Vinylpyridine Terminated SiQDs Used in the Calculations of the State Populations P2(t) for Simulating the Transients at Different Detection Wavelengths detection wavelength
Δ(E1 E21) [eV]
Δ(E21 E22) [eV]
V1,21 [eV]
V1,22 [eV]
Γ1 [eV]
Γ21 [eV]
Γ22 [eV]
540 nm
0.91
0.2
0.029
0.0002
0.0042
0.000007
0.000001
560 nm
0.91
0.2
0.030
0.0002
0.0042
0.000007
0.000001
580 nm
0.91
0.2
0.030
0.0002
0.0043
0.000006
0.000001
600 nm
0.91
0.2
0.032
0.0002
0.0044
0.000005
0.000001
620 nm 640 nm
0.91 0.91
0.2 0.2
0.032 0.033
0.0002 0.0002
0.0042 0.0038
0.000005 0.000005
0.000001 0.000001
660 nm
0.91
0.2
0.033
0.0002
0.0038
0.000005
0.000001
680 nm
0.91
0.2
0.033
0.0002
0.0038
0.000005
0.000001
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Table 2. Energy Differences Δ(E1 E21) and Δ(E21 E22) between the Interacting Energy States E1, E21, and E22, Coupling Matrix Elements V1,21 and V1,22, and Level Widths Γ1, Γ21, and Γ22 of 4-Vinylpyridine Terminated SiQDs Used in the Calculations of the State Populations P2(t) for Simulating the Transients at Different Detection Wavelengths detection wavelength
Δ(E1 E21) [eV]
Δ(E21 E22) [eV]
V1,21 [eV]
V1,22 [eV]
Γ1 [eV]
Γ21 [eV]
Γ22 [eV]
500 nm
0.91
0.2
0.030
0.00022
0.006
0.000015
0.000001
520 nm
0.91
0.2
0.015
0.00020
0.006
0.000015
0.000001
540 nm
0.91
0.2
0.015
0.00020
0.004
0.000012
0.000001
560 nm
0.91
0.2
0.011
0.00020
0.002
0.000009
0.000001
580 nm 600 nm
0.91 0.91
0.2 0.2
0.011 0.009
0.00020 0.00018
0.002 0.002
0.000009 0.000010
0.000001 0.000001
620 nm
0.91
0.2
0.009
0.00018
0.002
0.000015
0.000001
650 nm
0.91
0.2
0.009
0.00018
0.002
0.000012
0.000001
In addition to the depicted simulations, other transients at detection wavelengths between 540 and 680 nm were calculated (Table 1). Calculations of eq 9 with higher-order matrices Heff show that besides the widths Γ1 and Γ2k (k = 1,2), the resonances described by Γ2k (k > 2) are negligibly small (Γ2k < 108 eV). The value of EET rate kEET can be approximately extracted from Γ1 = 40.9 meV, as obtained as a mean value from all simulated transients, with kEET = Γ1/p = 6.2 1012 s1, whereas the mean value of the level width Γ21 = 0.006 meV with τ21 = p/Γ21 = 110 ps ( 5 ps may be associated with fast decay through excitonphonon scattering or exciton trapping.9 The effect of EET from initially photoexcited ligand states to the SiQD CB appears even more pronounced in the time evolution of the TA spectra of 4-vinylpyridine terminated SiQDs (Figure 10). Pumppulse excitation primarily generates an absorption maximum around 480 nm which decays within 0.6 ps, while a new maximum at 560 nm emerges. In analogy with the TA spectra of 2-vinylpyridine terminated SiQDs, the photoinduced absorption transitions around 480 nm are attributed to an initially excited ligand or CT state, whereas the subsequently formed absorption band around 560 nm is ascribed to absorption transitions of CB electrons. The transient absorption band represented by the transients at 560 and 650 nm display each a relatively slow rise dynamics that arises from the population of CB states due to electron transfer from the CT state (Figure 11, red and blue open circles), whereas the steep rise and decay dynamics of the transient at 500 nm (Figure 11, black open circles) reflect the initial generation dynamics of the CT state and its decay by electron transfer. The time evolutions of the respective population P2(t) of the CB states (solid lines in Figure 11) were calculated using the energy parameter listed in Table 2. The transients detected between 520 and 650 nm exhibit a slower rise when compared with that of those observed for the 2-vinylpyridineSiQD system. The reason for this is presumably a larger reorganization energy dissipated during the EET from the excited 4-vinylpyridine ligand state ψ1 into the exciton states ψ2k. The value of the EET rate, kEET, with kEET = Γ1/p = 4.4 1012 s1, is smaller than that obtained for the 2-vinylpyridine ligand, and the strongest interacting exciton state (k = 1) decays significantly slower with τ21 = p/Γ21 = 177 ps + 7 ps. These results are explained with weaker electronic couplings between the excited ligand and SiQD states and less efficient exciton trapping, respectively.9 This substantiates the higher efficiency of the interactions between the lone pair of the pyridine N atom and the SiQD surface for the 2-vinylpyridine termination when compared with the 4-vinylpyridine ligand.
’ CONCLUSIONS In summary, we have studied ultrafast interfacial electron transfer occurring between molecular ligand and SiQD band states of 2-vinylpyridine and 4-vinylpyridine terminated SiQDs. Femtosecond transient absorption spectroscopy and photoluminescence spectroscopy studies provided an insight into the population dynamics of an initially photogenerated interfacial chargetransfer state and the interacting conduction band levels of the SiQD on a time scale between 270 fs and 3 ns. The spectroscopic data were analyzed in terms of a theoretical description based on an effective Hamiltonian which was developed for calculating time evolutions of the population of coupled excited electronic states. Explicit results were obtained for 2-vinylpyridine and 4-vinylpyridine terminated SiQDs, by calculating representative transients that display both the rise dynamics of population of CB states and their decay. These calculations were performed with parameters, as being the energy differences between the interacting energy states, the coupling matrix elements, and level widths. These parameters do not only comply with the spectroscopic data but also quantify the electronic interactions between ligand and QD states. Therewith, the observed EET could be confirmed, and the corresponding EET rates were determined. Moreover, information of ultrafast exciton decay dynamics due to phonon scattering and trapping processes was extracted as time constants from the relevant CB level widths. The differences in the temporal evolution of the transient absorption spectra of 2-vinylpyridine and 4-vinylpyridine terminated SiQDs were shown to arise from differences in the coupling energies and level widths between the interacting electronic ligand and SiQD band states and thereupon, from different EET rates. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT Support of the Deutsche Forschungsgemeinschaft (Graduiertenkollegs 1161/1) is gratefully acknowledged. We thank Dr. Andre Ebbers (Evonik-Degussa-Creavis Technologies & Innovation) for generously providing us with SiQDs. ’ DEDICATION Dedicated to the 65th birthday of Christian von Borczyskowski. 22787
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The Journal of Physical Chemistry C
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