Excitation energy transfer in poroussilicon/laser dye composites

Vytenis Pranculis*, Irena Šimkien _e, Marius Treideris, and Vidmantas Gulbinas

Center for Physical Sciences and Technology, Savanoriu Ave. 231, 02300 Vilnius, Lithuania

Received 3 May 2013, accepted 12 August 2013Published online 11 September 2013

Keywords energy transfer, laser dyes, porous silicon, time resolved fluorescence

* Corresponding author: e-mail vytenis.pranculis@ftmc.lt, Phone: þ370 61494992, Fax: þ370 52627123

Excited state relaxation and energy transfer in porous silicon(PS)/laser dye [oxazine 1 (Ox1), rhodamine 6G (Rh6G)]composites have been studied by means of steady-state andtime-resolved fluorescence. Fluorescence decay kineticsreveals the nonradiative energy transfer from PS to the dye.Increased decay rate of the dye luminescence in the composite

indicates that opposite energy transfer is also likely. Analysis ofthe time-resolved fluorescence of the PS/R6G composite alsoshows that there is no energy transfer from silicon oxideresponsible for the “blue” fluorescence band to siliconnanocrystalites, and that interaction between Si nanocrystalsresponsible for the “red” PS fluorescence is absent or weak.

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1 Introduction Since its discovery, porous silicon(PS) proved to be a non-trivial fluorescent system. Havingmany different energy levels, each with its own relaxationtime and at least two distinct fluorescence mechanisms(surface oxide and Si nanocrystallites) PS is still not fullyunderstood.

PS/organic material composites were first investigatedfew years after the discovery of the visible luminescence ofPS. Polymers were spin-coated onto PS in order to producehybrid LEDs with variable fluorescence spectra [1–3], whilelaser dye impregnation into the pores was motivated bypossible application of such composites in laser physics[4–7]. One of the key processes in such systems – energytransfer, attracted most of the researcher’s attention becauseof its natural complexity resulting from intricate fluorescenceof PS.

In 1993 Canham [8] reported impregnation of PS withlaser dye in order to get a solid state host for optically activemedia – a goal of great interest still. In his work Canhamexamined possible energy transfer in such composites. Sincethen, driven by possible applications of such materials inoptoelectronics, a number of researchers investigated PS/laser dye composites using different techniques and reportedenergy transfer from PS to the dye as their main findings[9–13].

Time-gated fluorescence of PS reveals two bands – highenergy “blue” fluorescence, which is intensive but decays inless than 5 ns, and low energy “red” fluorescence, which is

weaker but stays measurable for over 20ms [14]. However,the red band dominates in stationary PS fluorescencespectrum since several thousand times longer life time of thered fluorescence overcompensates the few times lowerintensity.

The blue fluorescence in most cases is attributed to theoxidized surface states [15–18], although extremely small Sicrystals – nanocrystallites could recreate the samebehavior [14]. Red fluorescence band usually attributed tonanometer sized Si nanocrystallites [19–23] is alsoinhomogeneous: fluorescence at different wavelengthsdecays with different rates and is attributed to nano-crystallites of different dimensions. Such behavior is alsoambiguous – increasing relaxation times with longerdetection wavelength could also be explained as a resultof energy migration between nanocrystallites [14].

In our recent work we investigated PS/polymercomposites and showed that polymers are responsible forthe quenching of the “blue” PS fluorescence [24]. In thecurrent publication we address PS/laser dye composites andenergy transfer in these systems. Our work is not focused ona single process in these complex nanocomposites, but ratheron getting the entire relaxation picture.

Oxazine 1 (Ox1) and rhodamine 6G (R6G) were chosenfor our PS/dye composites because of their high quantumyield, fast relaxation times and convenient absorption andfluorescence spectra. In both composites we confirmnonradiative energy transfer from PS to the dye, and also

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present indications of the reverse energy transfer, from thedye to PS, which, to the best of our knowledge, has not beeninvestigated.

2 Materials and methods Formation of the PS wascarried out on a commercial p-type Si(B) wafer. The waferwas electrochemically etched for 20min in a mixture of HFsolution (48%) and ethanol of a ratio 1:1. Applied currentdensity was 25mAcm�2. PS layers were formed at roomtemperature in ambient atmosphere of 80% humidity.

PS/oxazine 1 (PS/Ox1) and PS/rhodamine 6G (PS/R6G)composites were prepared by immersing PS in dye solution inethanol with dye concentrations of 6.4� 10�3 and4.2� 10�2mol L�1, respectively. For better impregnationof the pores by dyes, immersed samples were heated to 60 8Cand exposed to ultrasound for 1 h. Dye molecules, which didnot enter the pores, were rinsed of the surface with pureethanol. After preparation and during all measurementssamples were kept in sealed glass containers with saturatedvapor pressure of ethanol to prevent solvent evaporation fromthe pores. PS samples used as references in fluorescencemeasurements were processed with pure ethanol in the sameway to avoid any solvent related systematic errors.

Fluorescence spectra and decay kinetics were measuredwith the Edinburgh Instruments time-correlated singlephoton counting fluorimeter. Diode laser generating 70 psduration pulses at 375 nm wavelength was used for thesample excitation. Time resolution of the fluorimeter wasabout 100 ps after deconvolution of the apparatus function.

3 Results and discussion3.1 PS/Ox1 Figure 1 presents spectroscopic properties

of Ox1, PS, and PS/Ox1 composite. The black line showsstationary fluorescence spectrum of PS. The major part of thefluorescence is located in the “red” band, centered at 750 nm,while the “blue” band (shown in the inset) is much weaker.Fluorescence and absorption spectra of Ox1 in ethanolsolution reveal the motivation for choosing this dye. Very

low absorption of Ox1 at excitation wavelength (375 nm)ensures selective excitation of PS in the composite, whilegood spectral overlap of Ox1 absorption band with thefluorescence of PS indicates that efficient energy transferfrom PS to the dye is expectable. Although absorption of PSis not known, we can infer from the “red” fluorescence band,that there is some PS absorption in the Ox1 fluorescencespectral region. Thus, energy transfer from laser dye to PSmay also take place in such composites. As stated previously,the red PS fluorescence band has a lifetime in the ms timedomain indicating low oscillator strength of the fluorescencetransition. Therefore, according to Förster energy transfermechanism, the non-radiative energy transfer from the dye toPS in PS/Ox1 composites is also expected to be slow.

Fluorescence spectra of both, Ox1 and PS may be easilydistinguished in the PS/Ox1 composite spectrum (red line inFig. 1). Interestingly, fluorescence intensity of the compositein the 570–620 nm region is much lower than that of the purePS.

Time-resolved fluorescence measurements were employedin order to get more insight into the processes behind thesespectral peculiarities. Their results are presented in Fig. 2.

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Figure 1 Absorption and fluorescence spectra of Ox1 andfluorescence spectra of PS and PS/Ox1 composite. The insetshows a magnified short wavelength part of PS fluorescence. Thearrow marks the excitation wavelength.

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Figure 2 Fluorescence decay kinetics of PS and PS/Ox1 atdifferent detection wavelengths.

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Since Ox1 has no absorption in the 400–500 nm region, thereshould be no energy transfer from oxidized surface statesresponsible for the “blue” PS fluorescence to Ox1. Indeed,fluorescence decay kinetics at the center of the blue PSfluorescence band (450 nm) are identical for the compositeand for the dye-free PS (Fig. 2A).

The decrease of the composite’s fluorescence in the 570–620 nm region could be due to two types of interactionsbetween PS and the dye. Ox1 could simply absorbfluorescence of PS (radiative energy transfer) becauseOx1 has strong absorption in this spectral region. Thefluorescence decrease could also be a result of thenonradiative excitation energy transfer from PS to thedye. To distinguish between the two energy transfer types,fluorescence decay kinetics of the composite and of the dye-free PS were measured at 610 nm (Fig. 2B). One wouldexpect lower fluorescence intensity in the PS/Ox1 andunaffected decay rate in the case of the radiative energytransfer. However, Fig. 2B reveals equal initial intensitiesand faster decay rate for the PS/Ox1 than for the dye-free PSpointing to the nonradiative energy transfer.

Assuming energy transfer to the dye is the solemechanism affecting the fluorescence decay rate in PS/Ox1 composite, we can express the transfer rate kET and thetransfer efficiency EET as

kET ¼ 1t� 1t0

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hET ¼ kETkET þ 1=t0

; ð2Þ

where t0 and t are fluorescence decay rates of dye-free PS, ofPS/Ox1, respectively. For the rough estimate of the energytransfer rate we neglect the fast relaxation component.Exponential approximation of the slow decay component at610 nm gives fluorescence relaxation times of 330 and158 ns for PS and for composite respectively. Using Eqs. (1)and (2), we calculate the energy transfer rate and efficiencybeing equal to kET¼ 3.30� 106 s�1 and hET¼ 52%.

The composite fluorescence shows a clear Ox1 bandwith maximum slightly shifted to the long wavelength siderelatively to the Ox1 fluorescence in solution. Since Ox1 hasweak absorbance at the excitation wavelength, this fluores-cence should be attributed to the energy transfer from PS tothe dye. On the other hand, direct dye excitation cannot becompletely ruled out. Indeed, fluorescence decay kinetics ofthe composite measured at 670 nm (Fig. 2C) shows as anintense and rapidly decaying fluorescence component, whichshould be attributed to the direct excitation of Ox1.However, this component is too small to account for the670 nm peak in the steady-state PS/Ox1 spectrum. More-over, the fast relaxation component reveals faster decay ofthe Ox1 fluorescence in the composite (tPS/Ox1¼ 420 ps)compared to Ox1 solution in ethanol (tOx1¼ 920 ps). Thissignificant difference is most likely a result of nonradiativeenergy transfer from the Ox1 to PS. Assuming intrinsic

excited state relaxation rates of Ox1 in solution and in PSbeing equal, this gives the Ox1 to PS energy transfer ratebeing equal to k

0ET ¼ 1.29� 109 s�1. To the best of our

knowledge the dye to PS energy transfer in such compositeshas not been considered before. Several hundred times fasterOx1 to PS energy transfer rate in comparison with the PSto Ox1 energy transfer is not a straightforward result.Therefore, other dye fluorescence quenching mechanismscannot be completely excluded.

Fluorescence decay at 670 nm may be also affected bythe direct energy transfer from PS to the dye. The influenceof this process becomes clear at longer delay times whendirectly excited Ox1 fluorescence has already decayed. Thecomposite fluorescence then still remains stronger than thatof pure PS, but its decay is faster. This is due to thecontribution of the Ox1 fluorescence created by the energytransfer from PS. Dye molecules, quenching the PSfluorescence in the 570–620 nm region, reradiate it in theOx1 fluorescence region with the time dependence deter-mined by the Si nanocrystals fluorescing in the 600 nmregion. Thus, dye molecules make a kind of bridge for thefluorescence transfer to longer wavelength retaining its timedependence at short wavelength.

In conclusion, we do not observe energy transfer fromsilicon oxide to Ox1. However, there are indications of atwo-way energy transfer between silicon crystallites andOx1: fluorescence of small silicon crystallites at shortwavelengths is quenched by Ox1, while Ox1 moleculestransfer their excitation energy to larger silicon crystallitesfluorescing at longer wavelength than Ox1.

3.2 PS/R6G R6G has absorption and fluorescencebands at shorter wavelength compared to Ox1 (seeFig. 3). Therefore, energy transfer conditions are expectedto be different. The absorption band of R6G overlapswith the blue fluorescence band of PS, allowing theinvestigation of the energy transfer between oxidized Siand dye. Fluorescence spectrum of PS/R6G composite

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reveals bands of both PS and the dye (red line in Fig. 3),similarly to that of PS/Ox1.

Although the “blue” fluorescence is usually attributed tooxidized surface states [15–18], extremely small nano-crystallites could also radiate in this spectral region, andprobably may show similar short fluorescence lifetime [14].Excitation energy transfer in PS/R6G composite gives usadditional information about the properties of the bluefluorescence. Let us examine fluorescence decay at 420 nmwhere R6G has very weak absorption (see Fig. 4). Decaykinetics of the PS/R6G measured at this wavelength issignificantly faster than that of the pure PS indicating that thedye quenches the fluorescence despite the fact that directenergy transfer to the dye from species radiating at 420 nm isnot possible. It shows that R6G molecules, absorbing on thelong wavelength side of the blue fluorescence band quenchthe entire blue fluorescence band. It leads to the conclusionthat the blue fluorescence band is homogeneously broad-ened. Such short wavelength fluorescence quenchingwould not be possible if the blue band was composed offluorescence of separate Si nanocrystallites radiating atdifferent wavelengths.

Figure 4C shows fluorescence kinetics of PS andPS/R6G measured at 550 nm, at the center of the R6Gfluorescence band. Similarly to PS/Ox1, intense initial signalcorresponding to direct excitation of R6G can be clearlydistinguished by its fast relaxation. Significantly faster decayof the R6G fluorescence in the composite (tPS/R6G¼ 0.7 ns)as compared to that in ethanol solution (tR6G¼ 4 ns) shouldalso be attributed to the fluorescence quenching bynonradiative energy transfer from the dye to PS thussupporting the above suggested bi-directional energytransfer model.

Figure 4C also shows that similarly to fluorescence ofPS/Ox1, fluorescence of PS/R6G at 550 nm has also astrong long-living component, significantly stronger thanthat of pure PS. As already discussed, such a strong long-living fluorescence in the dye fluorescence region is anindication of the nonradiative energy transfer from PS tothe dye.

Although the main “red” fluorescence band of PS isknown to originate from silicon nanocrystallites [19–23], themechanism behind slower relaxation at longer detectionwavelength in this band is still debatable. The wavelengthdependent relaxation rate could be attributed to the sizedistribution of nanocrystallites, because fluorescence wave-length and probably its lifetime increase with the nano-crystallite size [25]. Yet, similar slow relaxation would bealso expected if excitation in PS could migrate betweendifferent nanocrystallites [14]. In the latter case, one wouldexpect a strong impact of R6G on the long wavelength PSfluorescence because dye would quench the short wave-length PS fluorescence before energy transfer to the largenanocrystallites with low energy states. Indeed, we observeinfluence of R6G on the fluorescence kinetics at 775 nmwhere R6G fluorescence is absent (Fig. 5). The initialfluorescence decay is faster in the composite and conse-quently fluorescence at longer times is weaker. This is mostlikely the consequence of the above described blockingof the excitation energy pathway from small to large Sinanocrystallites.

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4 Summary We used stationary and time-resolvedfluorescence measurements to investigate energy transferprocesses in PS/laser dye composites. Fluorescence decaykinetics in both PS/Ox1 and PS/R6G composites confirmenergy transfer from PS to the dye and indicate possibleopposite process.

Analysis of the composite fluorescence gives alsoadditional information about the properties of the PSfluorescence. Quenching of the entire SiO2 fluorescenceband of PS by R6G dye indicates that SiO2 fluorescenceband is homogeneously broadened. Weakening of the longwavelength part of the PS fluorescence by dyes fluorescing atshorter wavelength indicates that large nanocrystalites arepartly excited by energy transfer from small crystallites.

Acknowledgements The work has been partially financedby the Lithuanian-Latvian-Taiwan project TAP-LLT-12-003.

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