JOURNAL OF APPLIED PHYSICSVOLUME 95, NUMBER 7

1 APRIL 2004

Chemical reaction mediated by excited states of Si nanocrystals—Singlet
oxygen formation in solution
Minoru Fujii,a) Motofumi Usui, and Shinji Hayashi
Department of Electrical and Electronics Engineering, Faculty of Engineering, Kobe University, Rokkodai,
Nada, Kobe 657-8501, Japan

Egon Gross, Dmitri Kovalev, Nicolei Künzner, and Joachim Diener
Technische Universität München, Physik-Department E16, D-85747 Garching, Germany

Victor Yu. Timoshenko
Faculty of Physics, Moscow State M. V. Lomonosov University, 119992 Moscow, Russia

共Received 21 November 2003; accepted 16 January 2004兲
Formation of singlet oxygen in solution by using Si nanocrystals as photosensitizers has been
demonstrated. It has been shown that the absorption band of 1,3-diphenylisobenzofuran 共DPBF兲 inbenzene centered at 416 nm decreases by irradiating green 共514.5 nm兲 or red 共632.8 nm兲 light if
fresh porous Si powder is dispersed in the solution. The decomposition of DPBF was observed only
when fresh porous Si was irradiated by light, i.e., without light irradiation no effects were observed.
Furthermore, the effect was drastically suppressed if porous Si powder was annealed and amonolayer of oxide was formed on the surface of nanocrystals. The rate of the decomposition of
DPBF was accelerated when the solution was bubbled by oxygen gas. These results indicate thatelectronic excitation of Si nanocrystals is transferred to molecular oxygen dissolved in solution,
resulting in the formation of singlet oxygen. Generated singlet oxygen reacts with DPBF
共1,4-cycloaddition reaction兲, forming endoperoxides, which in turn decompose to yield irreversible
products. In addition to the singlet-oxygen-mediated decomposition of DPBF, the possibility of
direct reaction between triplet excited states of Si nanocrystals and DPBF is discussed. © 2004
American Institute of Physics. 关DOI: 10.1063/1.1664021兴

I. INTRODUCTION

found that a molecule-like electronic structure of excitons in
Si nanocrystals, i.e., excitonic states are split into an optically active spin-singlet state and an optically inactive spintriplet state which is lowered in energy, is essential in realizing a function as a photosensitizer. Despite a small singlettriplet splitting being in the milli-electron-volt range, 75% of
excitons reside in the threefold degenerated triplet state evenat elevated temperatures. The efficient formation of triplet
excitons makes triplet–triplet-annihilation type energy transfer to oxygen molecules possible.
The fundamental mechanism of the energy transfer from
Si nanocrystals to oxygen molecules is now almost clarified.
The next step is to demonstrate the practical usefulness of
porous Si as a photosensitizer for singlet oxygen formation.
For many applications in chemical and biological fields, formation of singlet oxygen in solution is required, because
singlet oxygen mediated reactions proceed usually in solution. The standard method to detect singlet oxygen in solution is to use biochemical traps 共singlet oxygen acceptors兲
and analyze a specific reaction product or monitor the decrease in the amount of acceptor materials. Typical biochemical traps are cholesterol, 1,3-diphenylisobenzofuran
共DPBF兲, p-nitrosodimethylalanine, sodium azide, etc.8 In thiswork, we employed DPBF as a singlet oxygen acceptor.
DPBF readily undergoes a 1,4-cycloaddition reaction withsinglet oxygen forming endoperoxides, which in turn decompose to yield irreversible products 共1,2-dibenzoylbenzene兲.
This process can be monitored by the decrease in the inten-

Molecular oxygen is one of the most important substances on Earth. The ground state of molecular oxygen is
the triplet state ( 3 ⌺), with the two lowest-energy excitedstates being singlets ( 1 ⌬ and 1 ⌺). The excited state molecular oxygen is called singlet oxygen.1 The singlet oxygen mediates important processes in chemistry and biology.2– 4 It
reacts with many organic compounds including aromatic,steroids, vitamins, amino acids, proteins, etc. It also is involved in modification of biological structures. Photodynamic cancer therapy is an example of its medical
application.4
Formation of singlet oxygen requires the presence of alight-absorbing photosensitizer with subsequent energy
transfer from electronic excitation to oxygen molecules. Theenergy transfer process is explained by the triplet–triplet annihilation of photosensitizers and oxygen molecules by electron exchange 共Dexter transfer兲5 coupling; annihilation of a
triplet excited state of a photosensitizer is accompanied byspin-flip excitation of an oxygen molecule. Strongly lightabsorbing dye molecules 共Rose Bengal, methylene blue, etc.兲
are usually employed as a photosensitizer.1 Recently, we
have demonstrated that Si nanocrystal assemblies, especiallyporous Si, act as an efficient photosensitizer for singlet oxygen formation.6,7 From detailed spectroscopic studies, wea兲Electronic mail: fujii@eedept.kobe-u.ac.jp

0021-8979/2004/95(7)/3689/5/$22.00

3689© 2004 American Institute of Physics

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J. Appl. Phys., Vol. 95, No. 7, 1 April 2004

sity of the absorption band of DPBF centered at 416 nm.9,10
We will study the absorption spectra of DPBF-dissolved solution containing porous Si powder under light irradiationand demonstrate that porous Si acts as a photosensitizer for
singlet oxygen generation in solution.
II. EXPERIMENTAL DETAILSDPBF can be dissolved in variety of organic solvents. In
this work, benzene 共99.8%兲 was employed as a solvent because the lifetime of singlet oxygen in it is relatively long
共the lifetime of 1 ⌬ state is 24 ␮s兲1 and thus a larger probability of the interaction with DPBF is expected. Furthermore, compared to other organic solvents, e.g., methanol,benzene does not quench luminescence of porous Si somuch, leading to a smaller probability of nonradiative recombination of excitons before transferring energy to oxygen
molecules and higher efficiency of singlet oxygen formation.
DPBF was dissolved into benzene with the concentration of
about 40 ␮mol/l. The maximum absorbance关 ⫺ln(transmittance) 兴 of the solution was about 4.8. Oxygen
was not intentionally dissolved but benzene was exposed toair for a long period before DPBF was dissolved.
Porous Si was prepared by electrochemical etching of a
共100兲 oriented boron-doped bulk Si wafer with a resistivityof 11–15 ⍀ cm in a 55:45 by volume mixture of hydrofluoric
acid 共46 wt % in water兲 and ethanol.11 The current densityand etching time were 54 mA/cm2 and 3 h, respectively.
After the electrochemical etching process, porous Si layers
were rinsed with ethanol several times to reduce the amountof residual fluorine as small as possible. With this preparation condition, porous Si layers are cracked and become
powder during drying. The powder was then corrected and
10 mg of it was put into a quartz cell filled with DPBFdissolved benzene. Since the powder is not ground down, thesize of the powder was distributed in a wide size range.
As has been discussed in previous papers,6,7 the abilityof singlet oxygen formation depends strongly on the band
gap energy of Si nanocrystals comprising porous Si. The
efficiency of singlet oxygen formation is the best for nanocrystals having a luminescence 共band gap兲 energy at around1.63 eV, which corresponds to the energy separation of thesecond excited singlet state of oxygen molecules ( 1 ⌺) andthe triplet ground state ( 3 ⌺); these nanocrystals can resonantly transfer their electronic excitation to oxygen molecules staying on the surface. On the other hand, if the band
gap energy is far from 1.63 eV, emission or absorption ofphonons are required during the energy transfer process tosatisfy the energy conservation rule. The requirement of thephonon participation drastically reduces the energy transferrate, and efficient formation of singlet oxygen is not expected. The band gap energy of porous Si can be controlled
by the resistivity of Si wafers, etching current density, concentration of hydrofluoric acid in etching solution, etc. We
have tuned the sample preparation condition so that porous
Si powder exhibits photoluminescence 共PL兲 maximum ataround 1.63 eV.
The schematic illustration of the quartz cell (1⫻1
⫻4.5 cm3 ) containing porous Si powder is shown in the in-

FIG. 1. Absorption spectra of DPBF-dissolved benzene as a function ofirradiation time. The wavelength and power of the irradiation light are 514.5
nm and 40 mW, respectively, with the spot size of about 7 mm in diameter,
共a兲 with porous Si powder, and 共b兲 without porous Si powder. Inset in 共a兲 isthe configuration of experiments.

set of Fig. 1共a兲. Large powder which can be seen easily by
naked eyes sink down to the bottom, and only a portion ofsmall powder wanders in the solution. Existence of small
powder in solution can easily be recognized by the red luminescence of powder if we irradiate the cell with green light.
Light transmittance measurements of the cell were made by adouble-beam spectrophotometer equipped with a double
monochromator 共Shimadzu UV-3101PC兲 by using a quartz
cell filled with pure benzene as a reference. DPBF easily
degrades by absorbing blue light. To make the degradation
during the measurements as small as possible, a fast scanmode was employed, i.e., the total measurement time wasabout 30 s for each scan, and samples were treated in a darkroom. No degradation of DPBF was observed during measurements.
After an initial transmittance measurement, the solution
was irradiated by a 514.5 nm line of an Ar ion laser or a632.8 nm line of a He–Ne laser for 10 min from the bottomof a quartz cell. The laser power was 40 mW with the spot
size of about 7 mm in diameter. The cycle of light irradiationand transmittance measurements was repeated until total irradiation time becomes 60 min.
III. RESULTS AND DISCUSSION

Figure 1共a兲 shows absorption spectra of DPBF-dissolved
benzene containing fresh porous Si powder. A strong absorption band centered at 416 nm is due to DPBF. Porous Si

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J. Appl. Phys., Vol. 95, No. 7, 1 April 2004

Fujii et al.

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FIG. 3. Infrared absorption spectra of fresh 共solid curve兲 and annealed
共dashed curve兲 porous Si.
FIG. 2. Change of absorbance at 416 nm as a function of laser irradiation
time for DPBF-dissolved benzene solution containing different amount ofporous Si powder. 䊐 is the data obtained without light irradiation, whileothers are obtained under light irradiation.

shows a broad absorption band starting from a red region.12
However, the absorption by porous Si does not appear on the
spectra in Fig. 1共a兲 because of a small amount of powder ina light pass during the transmittance measurements 关see theinset of Fig. 1共a兲兴.
After an initial transmittance measurement, the solution
was irradiated by a 514.5 nm line of an Ar ion laser. In Fig.
1共a兲, by irradiating the solution with 514.5 nm light, the absorbance decreases, implying that DPBF is decomposed. It
should be stressed here that the change in the absorbance wasobserved only when porous Si powder is added to the solution; without porous Si, the absorbance change is negligibly
small 关see Fig. 1共b兲兴.
In Fig. 2, the degree of the absorbance change is plotted
as a function of irradiation time for cells containing different
amount of porous Si. The data plotted by 䊐 are obtained
without light irradiation. Without irradiation, porous Si powder does not exert any effects on DPBF. This implies that theobserved decomposition of DPBF is not due to chemical
reaction with porous Si or with impurities in porous Si. Porous Si in the photoexcited states is crucial for the observed
phenomena.
Under irradiation, the change in the absorbance is larger
for the solution containing a larger amount of porous Si powder. However, the degree of the change is not proportional tothe amount. This may due to the fact that all Si nanocrystals
are not uniformly excited; the bottom of the cell is coveredby large powder and they prevent excitation light to illuminate the cell uniformly.
The degree of absorbance change depends also on the
wavelength of excitation light. Under 632.8 nm irradiation,
the change was smaller than that under 514.5 nm light irradiation. The absorption coefficient of porous Si at 632.8 nm
is about one order of magnitude smaller than that at 514.5
nm.12 The difference in the absorption coefficient may resultin the different rate of DPBF decomposition. The excitation
wavelength dependence of the decomposition rate is anotherevidence that photoexcited states of porous Si is responsible
for the decomposition of DPBF.
To understand the role of porous Si more in detail, we
annealed porous Si at 450° for 1 min in air. The infraredabsorption spectra before and after the annealing are shownin Fig. 3. As-prepared 共fresh兲 porous Si has a hydrogenterminated surface with characteristic absorption lines of thevibrational modes of the H–Si bonds at around 2100 cm⫺1
共Fig. 3, solid curve兲. Annealing the porous Si introduces amonolayer of backbonded oxygen on the nanocrystal surface, while the hydrogen passivation of the surface is partially preserved.13 The chemically modified surrounding of
the H–Si bonds shifts the infrared absorption lines to higher
wave numbers 共Fig. 3, dashed curve兲. The shift is accompanied by the decrease in the integrated intensity of the H–Si
vibration modes and strong growth of the Si–O–Si mode ataround 1080 cm⫺1, suggesting that a part of surface hydrogen atoms are effused, and the surface is partly oxidized.
The change in the absorbance of DPBF is studied for theannealed porous Si powder. The result is shown in Fig. 4. In
contrast to fresh porous Si powder, the annealed powder exerts no effects on DPBF. This is considered to be due to apoor ability of singlet oxygen formation for the annealed

FIG. 4. Comparison of absorbance change between fresh and annealed porous Si powder under the same experimental condition.

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Fujii et al.

FIG. 6. PL quenching strength, i.e., the ratio of a PL spectrum in vacuum tothat in oxygen ambient, of fresh 共solid curve兲 and annealed 共dashed curve兲porous Si. The spectral position of the excited state of an oxygen molecule
is indicated ( 3 ⌺→ 1 ⌺). Electron-spin configurations and spectroscopic labeling of molecular oxygen states are shown in the inset.

FIG. 5. PL spectra of 共a兲 fresh and 共b兲 annealed porous Si measured in
vacuum 共solid curve兲 and in O2 gas 共1 atm兲 共dashed curve兲 at room temperature.

powder.6,7 The ability for the formation of singlet oxygen canbe examined by PL spectroscopy. Figure 5 compares PL
spectra of fresh and annealed porous Si measured in vacuum
and in oxygen gas ambient. By annealing, PL intensity issignificantly dropped due probably to the formation of dangling bonds. In addition, the effect of ambient on the PL
spectrum is different between the fresh and annealed porous
Si. In the case of fresh porous Si, PL is quenched by introducing oxygen gas, while in annealed porous Si, PL spectrum does not depend on the ambient. Figure 6 shows theratio of PL spectra measured in vacuum to that in oxygen gas
ambient 共strength of PL quenching兲. In fresh porous Si, thestrength of PL quenching has a maximum at around 1.63 eV,
while the annealed porous Si does not have any structure.
Since the energy of 1.63 eV corresponds to the energy separation between the second excited state ( 1 ⌺) of oxygen molecules and the ground state ( 3 ⌺), the PL quenching observed
for the fresh porous Si is due to the energy transfer from Si
nanocrystals to oxygen molecules, i.e., the singlet oxygen
formation. On the other hand, singlet oxygen is not generated
by annealed porous Si, and thus the decomposition of DPBF
with the interaction of singlet oxygen does not occur.
If singlet oxygen generated by the energy transfer from
Si nanocrystals is involved in the decomposition of DPBF,
the rate of the absorbance change should depend on the concentration of oxygen dissolved in benzene. To see the effectof oxygen concentration, we bubbled DPBF-dissolved benzene solution containing porous Si powder with nitrogen andoxygen gases and compared the degree of the absorbancechange. In Fig. 7共a兲, we first bubbled the solution with oxygen gas, put a cap on the cell and irradiated the cell for 30
min, then bubbled the same solution with nitrogen gas andagain irradiated the cell for 30 min. In spite of the very roughprocedure, we can clearly see the change in slope dependingon the kind of gases used for bubbling; the slope becomes
smaller after the nitrogen gas bubbling. In Fig. 7共b兲, the procedure is vice versa, i.e., first nitrogen gas bubbling and thenoxygen gas bubbling. In this case, we can see an enhancement of the slope after the oxygen gas bubbling. These observations are the evidence that oxygen is involved in thedecomposition of DPBF. By combining all the results ob-

FIG. 7. Change of absorbance of DPBF as a function of time. 共a兲 共solid line兲DPBF-dissolved benzene solution was bubbled by oxygen gas before starting the measurement/irradiation cycles. After 30 min of irradiation, the solution was bubbled by nitrogen gas. 共b兲 共dashed line兲 The solution was firstbubbled by nitrogen gas, and after 30 min irradiation bubbled byoxygen gas.

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Fujii et al.

J. Appl. Phys., Vol. 95, No. 7, 1 April 2004

tained, the whole process can be summarized as follows.
First, Si nanocrystals are excited by light irradiation and excitons are formed. Part of them stay in the triplet states. The
triplet exciton transfers energy to a ground state triplet oxygen molecule dissolved in the solution by the triplet–triplet
annihilation process, resulting in the formation of singlet
oxygen. Generated singlet oxygen reacts with DPBF, undergoes a 1,4-cycloaddition reaction forming endoperoxides,
which in turn decompose to yield irreversible products.
Although it is clear that singlet oxygen is involved in thedecomposition of DPBF in the present experiments, we cannot completely rule out the possibility that triplet excitedstates of Si nanocrystals interact directly with DPBF. In organic photosensitizers, two types of pathways 共types I and II兲
are generally considered in the chemical reaction assisted byexcited triplet states.8 The type I pathway is a direct reaction
with acceptor materials by donating and accepting hydrogenor electrons and producing free radicals or free-radical ions.
The type II pathway is the process we have discussed so far;excited triplet sensitizer reacts with ground state triplet oxygen to form singlet oxygen by triplet–triplet annihilation.
The rate of the type II pathway is mostly dependent on theconcentration of oxygen molecules in solution. As the oxygen in a system becomes depleted, the shift from type II to I
mechanism is favored. In the present work, relatively small
difference in the slope of the absorbance change between
oxygen gas bubbled and nitrogen gas bubbled solution 共Fig.
7兲 might suggest that, in addition to the type II mechanism,type-I-like direct interaction between triplet excitons andDPBF exists.
IV. CONCLUSION

We have succeeded in demonstrating singlet oxygen formation in solution by using porous Si as a photosensitizer.
Porous Si has extremely large accessible internal surface area
共up to 1000 m2/cm3兲 that permits a direct contact between thewhole Si nanocrystals and substances in the pore. The structure is thus ideal in realizing efficient energy transfer fromexcitons in Si nanocrystals to oxygen molecules. Furthermore, a very broad absorption band of porous Si covering all
the visible range implies that porous Si can utilize light in all
visible range for singlet oxygen formation. This is a goodadvantage from the point of view of practical application asa photosensitizer. The nontoxicity of porous Si could allow it

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to be used in biological and medical fields. Therefore, thepresent results offer further application of the most commercially used semiconductors.
However, in the present work, quantitative discussion on
the formation rate of singlet oxygen was not made. The
quantitative discussion is difficult because of the inhomogeneity of the present system. Since pores of porous Si are very
narrow and have a very high aspect ratio, the availability ofoxygen molecules and DPBF is not equal for all nanocrystals, and the reaction speed is considered to be quite different
between nanocrystals on the surface of porous Si powder andthose deep inside narrow pores. In fact, if we intentionallycrashed porous Si powder, the reaction speed was changed
significantly. In order to make quantitative discussion, colloidal Si nanocrystals which can be uniformly dispersed in solution may be required.
ACKNOWLEDGMENTS

This work is supported by Industrial Technology Research Grant Program in ’02 from New Energy and Industrial Technology Development Organization 共NEDO兲 Japan,
and 2002 Corning Research Grand.
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