Light harvesting in silicon(111) surfaces using covalently attached protoporphyrin IX dyes

Article abstract of DOI:10.1039/c7cc04767c

We report the photosensitization of crystalline silicon via energy transfer using covalently attached protoporphyrin IX (PpIX) derivative molecules at different distances via changing the diol linker to the surface. The diol linker molecule chain length was varied from 2 carbon to 10 carbon lengths in order to change the distance of PpIX to the Si(111) surface between 6 ? and 18 ?. Fluorescence quenching as a function of the PpIX-Si surface distance showed a decrease in the fluorescence lifetime by almost two orders of magnitude at the closest separation. The experimental fluorescence lifetimes are explained theoretically by a classical Chance-Prock-Silbey model. At a separation below 2 nm, we observe for the first time, a F?rster-like dipole-dipole energy transfer with a characteristic distance of R₀ = 2.7 nm.

Full text of DOI:10.1039/c7cc04767c

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Reactivity differences of Sc
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3
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Light harvesting in silicon (111) surfaces using covalently attached
protoporphyrin IX dyes
Received 00th January 20xx,
Accepted 00th January 20xx
Nicholas Alderman,a,b,♮ * Lefteris Danos, * Liping Fang, Martin C. Grossel, and Tom Markvart
,
c ,
d
b
a,e
DOI: 10.1039/x0xx00000x
www.rsc.org/
9
–12
13
We report the photosensitization of crystalline silicon via energy Blodgett (LB) films
or zeolite structures
but at a greater
transfer using covalently attached protoporphyrin IX (PpIX) distance of the emitter from the surface of silicon (>2nm) than
derivative molecules at different distances via changing the diol the work presented in this paper where we focus on emitter-
linker to the surface. The diol linker molecule chain length was surface separations of less than 2 nm. The native oxide
varied from 2 carbon to 10 carbon lengths in order to change the present on the surface of silicon then needs to be removed
distance of PpIX to the Si(111) surface between 6Å and 18Å. and the silicon surface subsequently functionalised. This
Fluorescence quenching as a function of the PpIX-Si surface results in a well-prepared surface with careful control of the
distance showed a decrease in the fluorescence lifetime by almost emitter-surface distance. Viewed more generally, the
two orders of magnitude at the closest separation. The modification of the silicon surface via the direct covalent
experimental fluorescence lifetimes are explained theoretically by attachment of organic molecules has seen an intense
1
4–16
a classical Chance-Prock-Silbey model. At a separation below 2 activity
resulting in oxide free silicon surfaces (with
nm, we observe for the first time, a Förster-like dipole-dipole molecules immobilised) with good stability and electronic
energy transfer with a characteristic distance of R
o
= 2.7 nm.
(passivation) properties. Alkyl-modified silicon surfaces have
received significant interest due to their potential applications
1
7,18
19,20
21,22
The term “light harvesting” is often used to describe a process in solar cells,
which enhances the absorption cross section of the and catalysis.
microelectronics,
biochemical sensing
2
3
photosynthetic reaction complex by energy transfer, often In the present letter we report what we believe is the first
1
called Förster resonance energy transfer (FRET). It has been demonstration of energy transfer at a controlled sub
suggested on several occasions that a similar approach could nanometer separation between a chromophore and silicon,
2
–5
be applied to semiconductors. In the case of an indirect which can be unequivocally attributed to near-field dipole-
band gap material such as crystalline silicon, dipole interaction. dipolar emitter placed within
A
a
photosensitisation has the potential to substantially enhance subwavelength distance from the silicon surface “sees” an
the photoexcitation rate for electron-hole pairs, resulting in increased density of states into which radiation can be
significant savings in material, and bringing about an exciting emitted, on account of coupling directly to the trapped modes
new paradigm for future solar cells and other optoelectronic in silicon via the evanescent field. Sometimes called “photon
6
devices. Previous experimental studies have
reported tunnelling” , this mechanism explains well the increase of
quenching of molecular fluorescence in proximity to silicon quenching rate at dipole separations of the order of a few tens
4
,5,7
8
24
from evaporated dyes layers,
quantum dots, Langmuir- of nanometers. In contrast, the interaction at emitter-surface
separations of the order of 1 nm which are under investigation
in this paper is dominated by the near field interaction
between the emitter dipole moment and the transition dipole
moment of the electron-hole pair excited in silicon. This
interaction is similar to the near field dipole interaction
between molecules which gives rise to the Förster energy
transfer. Also known resonance energy transfer, this
mechanism is generally accepted as underpinning the light
harvesting energy collection in photosynthesis. By changing
the distance from the dye to silicon via the length of a linker
molecule, allows the dependence of this energy transfer from
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coverages up to 30%. Subsequent functionalisation with PpIX
molecules was verified with coverage of about 1-2%. The
infrared spectrum of a 1,4 butanediol treated silicon surface
after functionalisation with PpIX is shown in Fig. S3 and S5
(ESI†). The O-H peak of the functionalised surface is clearly
visible, showing that unreacted alcohol groups remain. This is
due to the much larger porphyrin group blocking unreacted
surface sites. The various C-H groups of the porphyrin are
OH
OH
PCl5 saturated
chlorobenzene,
benzyl peroxide
OH OH OH
Cl
Si
Si
Cl
Si
Si
Cl
Si
Si
OH
n
n
NH4F
solution
H
Si
Si
H
Si
Si
H
Si
Si
O
O
HO
Si
n
Cl
Si
Si
Si
Si
O
Si
Si
O
Si
O
O
Si Si
O
O
O
Si
Si
Si
Si
Si
O
O
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Cl
Cl
Cl
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
H3C
HN
H
H
H
O
H3C
H
H
H3C
H
O
Fig. 1 A molecular model of the silicon (111) surface with diols and a PpIX molecule
H3C
H
Cl
H2C
O- Na+
H
H
H
H
H2C
covalently attached.
HN
H
H
H
H
N
N
H
H
H
H
H
N
O
chromophore to semiconductor to be investigated in detail,
and to separate the Förster-like energy transfer from photon
tunnelling through the evanescent field.
N
O
O-
Oxalyl Chloride, DCM
2 hours, room temperature
NH
NH
H
H
Cl
H3C
CH3
H
H
Na+
H
H3
C
CH3
H
H
CH2
H
CH2
The attachment of porphyrins to silicon surfaces has been well
Methanol, DCM, Pyridine
16 hours, room temperature
25,26
documented
and perhaps the closest example found for
2
H3C
H
H
7
H
O
sensitizing silicon is reported in ref; simpler experiments
report the attachment of benzene or anthracene terminal
H3C
H2C
HN
N
OMe
H
H
H
H
H
H
O
2
groups to a silicon alkyl monolayer by an esterification
8
H
N
NH
H
H
OMe
H3C
CH3
reaction, resulting in coverage of at least 70%. For the
H
H
CH2
experiments performed in this paper, Protoporphyrin IX was
Scheme 1 Synthesis and attachment of the Protoporphyrin IX acyl chloride (PpIX-
chosen due to the commercial availability, ease of
Cl) to the Si(111) surface.
functionalisation and shifting of the absorption and emission
maxima by coordination with metal ions. An example of a clearly visible in the spectrum, as well as the CH peak of the
2
molecular model of a silicon (111) surface with ethane-diol butanol-terminated surface and the ester C=O peak (Fig. S5,
linkers and a PpIX molecule attached is shown in Fig. 1. The ESI†).
presence of the large pi-ring will most likely force the molecule The presence of the porphyrin was further confirmed from XPS
to adopt a parallel orientation with respect to the surface of data. The nitrogen 1s binding region showed three distinct
nitrogen binding energies; one for iminic nitrogen, another for
silicon.
The first step towards attachment of PpIX was the addition of pyrrolic nitrogen, and finally a smaller peak for possible
the appropriate diol linker (Table S1, ESI†) on the chlorine- physisorbed nitrogen, (Fig. S6, ESI†). The two peaks in the
terminated Si(111) surface (Scheme 1). By adding a small nitrogen 1s binding energy matched other literature data for
28
amount of base (such as pyridine), an increase in the porphyrins. Interestingly, a 1:1 ratio of the integrals of the
uniformity of the siloxane monolayer is observed as well as a pyrrolic and iminic nitrogen peaks is not obtained as would be
reduction in the required reaction temperature. FT-IR expected. This could suggest that a proportion of the surface-
measurements on the resulting alcohol-terminated Si(111) attached porphyrin has bound a transition metal ion within the
surfaces confirmed the formation of the required monolayer, porphyrin ring, reducing the number of pyrrolic nitrogen
allowing subsequent functionalization of the alcohol chains atoms observed. This metal could be incorporated during the
(
Fig. S4, ESI†). It was found empirically that to attach PpIX to porphyrin processing steps, arising from trace metal impurities
the modified silicon surface successfully, pyridine should be found in all solvents. To confirm the correct assignment of the
added to the solution in order to catalyse and reduce the physisorbed nitrogen, a nitrogen 1s XPS spectrum was
reaction temperature between the porphyrin and alcohol obtained for a 1,4-butanediol surface (Fig. S7, ESI†).
surface. This allowed a more gentle heating temperature of Variable
Angle
Spectroscopic
Ellipsometry
(VASE)
o
8
0 C, reducing the chance of porphyrin decomposition. The measurement of the diol linker silicon surfaces and analysis
resulting silicon surfaces with PpIX molecules attached were confirmed the attachment of the diols linkers with different
then washed with DCM to remove any physisorbed porphyrin, carbon chain lengths on the surface of silicon and were found
and allow further characterisation with FT-IR, XPS and in good agreement with DFT calculations (Table 1). The
fluorescence measurements. Reflection FT-IR and XPS verified ellipsometric measurements were fitted to a simple Cauchy
the diol linker terminated Si(111) surfaces with estimated model (Fig S8, ESI†). Examples of steady state fluorescence
2
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spectra obtained from PpIX-functionalised (propane, butane transfer becomes greater and the fluorescence lifetime decay
and decane diol linker) silicon surfaces are shown in Fig. S9 shortens significantly. When the chromophore-surface
(ESI†). To confirm that the spectrum was indeed due to the distance becomes very small (such as for the ethanediol-linked
attachment of the porphyrin, a fluorescence spectrum of a 1,2- PpIX), the decay almost matches the profile of the laser pulse
ethanenediol terminated only surface was also obtained and reaching the resolution limit of the instrument (250 ps). As the
showed no fluorescence activity. All PpIX attached silicon linker chain-length is increased, the fluorescence decay time
surfaces with different linkers showed similar steady state increases, although the largest differences in decay rates were
spectra and only the intensity of emission changed. The between those for different shorter linker groups (Table S2,
spectra do not show the typical two peaks for a PpIX in ESI†).
2
9
solution (Dimethyl ester) occurring at 633 nm and 700 nm To describe the fluorescence quenching we have modified the
3
0
but instead show a broad emission peak starting from 610 nm classical theory by Chance, Prock and Silbey (CPS) developed
to 800 nm with a maximum at 670 nm. This is to be expected originally for the observation of fluorescence quenching of dye
for the broad bandwidth (16 nm) employed in order to layers on the surface of metals. As can be seen in Table S2
measure sub-monolayer dye coverages. Normalised (ESI†), the fluorescence lifetime increases with increasing
fluorescence decay curves of the various porphyrin-linked distance to the surface. The fluorescence lifetime change is
surfaces, together with decay fits are shown in Fig. 2.
Table 1 Variable Angle Spectroscopic Ellipsometry analysis results for the
different Diol linkers on Silicon with different carbon chain lengths
considerable, changing by almost two orders of magnitude
when moving from ethanediol to decanediol-terminated
surfaces, and the PpIX-silicon separation decreases from just
below 2 nm to 0.6 nm. The fluorescence lifetime for the diol-
Carbon Sample
Chain
Measured
Calculated
Thickness (Å)
Thickness (Å) linked PpIX-terminated surfaces is shown in Fig. 3. The red
Length
2
3
4
6
8
1
Si(111)-O-CH
CH -OH
Si(111)-O-CH
CH -CH -OH
Si(111)-O-(CH
CH -OH
Si(111)-O-(CH
CH -OH
Si(111)-O-(CH
CH -OH
Si(111)-O-(CH
CH -OH
2
-
-
5.5 ± 0.6
9.5 ± 0.6
14.4 ± 0.5
15.0 ± 0.4
17.2 ± 0.4
18.5 ± 0.4
6.0
7.3
2
2
2
2
2
2
2
2
-
-
-
-
8.6
2 2
)
11.1
13.6
16.3
2 3
)
2 4
)
0
2 5
)
The fluorescence decay curves show that as the fluorophore is
moved closer to the silicon surface, the fluorescence decay
lifetimes become shorter indicating a greater degree of
fluorescence quenching. The reason for the quenching of the
fluorescence is likely to arise from energy transfer from the
excited PpIX to silicon. As the distance between PpIX and the
surface is reduced from 15 Å down to 5 Å the rate of energy
Fig. 3 The fluorescence lifetime for various diol linked porphyrin surfaces.
Experimental points are fitted by Chance-Prock-Sibley theory (CPS, full blue line).
Also shown, by dashed and dotted lines, are separate contributions from different
mechanisms.
solid line in Fig. 3 was obtained by fitting the well-known CPS
3
theory using a fluorescence quantum yield of 0.2 which was
0
3
1
close to reported value in literature. We assumed, in keeping
with theoretical understanding of electron transition in
porphyrins, that the electric transition dipole moment is
oriented in the plane of the molecule, parallel to the silicon
surface. Other orientations, however, could also be used with
little difference observed (Fig. S10, ESI†).
Fig. 3 shows different lifetime components for a dipole near
the silicon surface, as modelled by the CPS theory. The far field
contribution is simply the dipole emission in free space as
modified by interference with a wave reflected from the
surface. Photon tunnelling (studied in detail in ref. 16) is an
optical effect describing energy transfer, through the
evanescent field, from the excited molecular state directly to
the photon states in silicon trapped by total internal reflection.
Of principal interest is the near field component which
Fig. 2 The fluorescence decay curves of various chain-length terminated surfaces
functionalised with protoporphyrin IX.
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corresponds to the interaction between the transition dipole
moment at the molecule and at the excited electron-hole pair
in silicon. The Förster-like energy transfer between an emitter
dipole and an array of acceptors filling a semi-infinite half
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ꢇ
ꢀ
ꢁ
ꢀ
ꢄ
ꢂ
= ꢁ
ꢃ
ꢆ
(Eq.1)
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where
τ
o
is the fluorescence lifetime in the absence of silicon
is the
0
1
2
3
interface, d is the chromophore-silicon separation and R
o
5
equivalent of the Förster radius between molecules. We
believe that the present work demonstrates for the first time,
the Förster-like energy transfer between a molecule and
semiconductor and allows the determination of the Förster
1
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o
radius R = 2.7 nm. Further work is needed to elucidate finer
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In conclusion, we have successfully attached a porphyrin dye
to the silicon (111) surface by a siloxane/alcohol technique, as
confirmed by infrared spectroscopy, XPS and fluorescence
spectroscopy. By using terminal dihydroxyalkanes as the linker
chain we have prepared a number of samples with controlled
separations (ranging from 6 to 18Ắ) between the chromophore
and the silicon surface. The silicon-chromophore separations
determined by ellipsometry agree well with the calculated
valued determined by DFT. The key quantity of interest -
fluorescence quenching - agrees well with the predictions of
the CPS model. In particular, at dipole separation from the
silicon surface below 2 nm, we have identified unequivocally
the dominant role of Förster-like resonance energy transfer,
with a “Förster radius” of 2.7 nm. We believe that direct
sensitisation of indirect bandgap semiconductors such as
silicon has potentially a huge technological significance,
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This work was funded by the UK EPSRC SUPERGEN program
“
Photovoltaic materials for the 21st century” (Reference:
2
2, 2146–55.
EP/F029624/1 and EP/F029624/2). Centre for Advanced
Photovoltaics is supported by Czech Ministry of Education,
Youth and Sport. CZ.02.1.01/0.0/0.0/15_003/0000464. Liping
Fang thanks for financial support from National Natural
Science Foundation of China (item No. 61604138). We would
like to thank Dr M. Coogan for helpful suggestions on the
manuscript and Dr A. Kerridge for assistance with molecular
modelling.
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