Effect of the erbium dopant architecture on the femtosecond relaxation dynamics of silicon nanocrystals

Article abstract of DOI:10.1063/1.1695318

The Femtosecond time-resolved transient absorption spectroscopy was used to investigate the early relaxation dynamics in photoexcited Si crystals. The analysis of the transient absorption spectra showed the presence of additional deep trap states upon incorporation of erbium into the Si crystal. The addition of the electron acceptor, BQ, on the surface of the Si nanocrystals provided a method for the analysis of the relative depths of the trapping sites. The investigation results show that the charge transfer rise time and the back electron transfer to the nanocrystals supports the presence of low-lying trap states in the erbium-doped Si nanocrystals.

Full text of DOI:10.1063/1.1695318

Effect of the erbium dopant architecture on the femtosecond relaxation dynamics of
silicon nanocrystals
A. C. S. Samia, Y. Lou, C. Burda, R. A. Senter, and J. L. Coffer
Citation: The Journal of Chemical Physics 120, 8716 (2004); doi: 10.1063/1.1695318
View online: http://dx.doi.org/10.1063/1.1695318
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 120, NUMBER 18
8 MAY 2004
Effect of the erbium dopant architecture on the femtosecond relaxation
dynamics of silicon nanocrystals
A. C. S. Samia, Y. Lou, and C. Burdaa)
Center for Chemical Dynamics and Nanomaterials Research, Department of Chemistry,
Case Western Reserve University, Cleveland, Ohio 44106
R. A. Senter and J. L. Cofferb)
Department of Chemistry, Texas Christian University, Ft. Worth, Texas 76129
͑Received 17 September 2003; accepted 12 February 2004͒
Femtosecond pump–probe absorption spectroscopy is used to investigate the role of Er3ϩ dopants
in the early relaxation pathways of photoexcited Si nanocrystals. The fate of photoexcited electrons
in three different Si nanostructures was studied and correlated with the effect of Er-doping and the
nature of the dopant architecture. In Si nanocrystals without Er3ϩ dopant, a trapping component was
identified to be a major electron relaxation mechanism. Addition of Er3ϩ ions into the core or
surface shell of the nanocrystals was found to open up additional nonradiative relaxation pathways,
which is attributed to Er-induced trap states in the Si host. Analysis of the photodynamics of the Si
nanocrystal samples reveals an electron trapping mechanism involving trap-to-trap hopping in the
doped nanocrystals, whereby the density of deep traps seem to increase with the presence of erbium.
To gain additional insights on the relative depths of the trapping sites on the investigated
nanostructures, benzoquinone was used as a surface adsorbed electron acceptor to facilitate
photoinduced electron transfer across the nanocrystal surface and subsequently assist in back
electron transfer. The established reduction potential ͑Ϫ0.45 V versus SCE͒ of the electron acceptor
helped reveal that the erbium-doped nanocrystal samples have deeper trapping sites than the
undoped Si. Furthermore, the measurements indicate that internally Er-doped Si have relatively
deeper trapping sites than the erbium surface-enriched nanocrystals. The electron-shuttling
experiment also reveals that the back electron transfer seems not to recover completely to the ground
state in the doped Si nanocrystals, which is explained by a mechanism whereby the electrons are
captured by deep trapping sites induced by erbium addition in the Si lattice. © 2004 American
Institute of Physics. ͓DOI: 10.1063/1.1695318͔
I. INTRODUCTION
and the band structure is believed to become more ‘‘direct-
like’’ in nature. Thus, making the recombination process
more efficient and quantum efficiencies of up to 1%–10%
are achievable.9–14 Furthermore, it has been shown that when
selected active impurities are inserted in the Si lattice, room
temperature light emission can be observed.13,14 In the case
of Si nanoclusters embedded in bulk SiO2 , the excitation of
Er3ϩ using pump wavelengths that do not correspond to any
of the principal Er3ϩ bands is observed.3,15 This coupling
mechanism is very important since it relaxes the require-
ments on the Er3ϩ pump wavelength leading to the produc-
tion of pumpable broad-band optical amplifiers at the impor-
tant telecommunications wavelength of 1540 nm.
The implantation of Er3ϩ ions into silicon is an impor-
tant aspect in the fabrication of near IR light emitting diodes
and the associated optoelectronic components compatible
with available silicon technology.1–4 This combination is of
great interest because of the synergism between the 1540 nm
emission from the erbium centers and the maximum trans-
mission window of a silica-based waveguide.5,6 In the case
of erbium implanted into bulk crystalline Si, it appears that a
strongly competitive Auger pathway reduces the emission
intensity at room temperature,7,8 thereby diminishing its po-
tential usefulness in optoelectronic platforms. However, the
introduction of Er3ϩ ions into nanometer sized Si is one
possible route to suppress such competitive pathways.3,4,9–12
Since bulk Si is an indirect bandgap material, lattice vibra-
tions ͑phonons͒ are needed to moderate the radiative recom-
bination of an electron and hole in order to conserve momen-
tum. Therefore, this process is very inefficient and low
quantum efficiencies of 10Ϫ4% are achieved at room
temperature.13 However, when carriers are quantum con-
fined, their wave functions spread out in momentum space
Because of the great potential of nanometer sized Si for
optoelectronic applications, we have been extensively inves-
tigating the ‘‘bottom-up’’ synthesis of nanocrystals of this
material doped with the rare-earth element erbium (Er3ϩ
)
along with their resulting structure and photophysical
properties.16–21 In terms of dopant location, two different
types of particles can be prepared with the erbium centers in
two different coordination environment: Nanocrystals with
Er3ϩ randomly dispersed throughout the crystal,16,17,19–21
and nanocrystals whereby the Er3ϩ centers are preferentially
enriched at the surface.18,21 For the former, the characteristic
a͒
Electronic mail: burda@cwru.edu
Electronic mail: j.coffer@tcu.edu
b͒
0021-9606/2004/120(18)/8716/8/$22.00
8716
© 2004 American Institute of Physics
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J. Chem. Phys., Vol. 120, No. 18, 8 May 2004
Effect of erbium on relaxation dynamics in Si nanocrystals
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1540 nm emission associated with the rare earth ion is de-
tected via a pathway that involves energy transfer from the Si
exciton to the rare earth ligand field transition͑s͒.16 In the
latter case, it is significant to note that the as-formed erbium
surface-enriched Si nanocrystals are effectively nonlumines-
cent and only emit with a direct ligand field excitation
mechanism following a brief high temperature anneal at
800 °C.18
In both of the above systems, a crucial feature of their
photophysical properties relevant to emission, specifically
the initial dynamics of the photogenerated charge carriers, is
still lacking at present. Even in well passivated and disloca-
tion free Si nanocrystals, the luminescence lifetimes are very
long ͑␮s-ms at room temperature͒ thereby exposing the ra-
diative process to strong competition with fast nonradiative
recombination pathways.13 Thus, a better understanding of
the nonradiative carrier relaxation pathways in silicon nanoc-
rystals is important in optimizing the emission properties of
Si for optoelectronic applications.
duction of the erbium precursor the aerosol is again heated at
temperatures ranging from 750–900 °C with a small oven.
Similarly, the erbium surface-enriched nanocrystals were
collected as an ethylene glycol colloid and purified using the
same series of steps adapted for the other nanocrystals.
All of the Si nanocrystals were structurally characterized
by a combination of high-resolution transmission electron
microscopy ͑HRTEM͒, selected area electron diffraction
͑SAED͒, energy dispersive x-ray analysis ͑EDX͒, and ex-
tended x-ray absorption fine structure methods ͑EXAFS͒.21
B. Spectroscopic measurements
Femtosecond time-resolved transient absorption spectra
were recorded on a Clark MXR CPA 2001 femtosecond laser
system that entails an amplified erbium-doped fiber laser,
which is frequency doubled to 780 nm and amplified in a
regenerative amplifier. This laser set-up produces highly
stable laser pulses with 120 fs full width at half maximum
͑FWHM͒ duration and 800 ␮J output energy per pulse at a
repetition rate of 1 kHz. A small portion of the fundamental
output pulse train is used to generate white light in a 2 mm
sapphire crystal while the remaining laser light is used to
frequency-double the fundamental to achieve 390 nm excita-
tion light. The excitation beam is modulated by a chopper
with a 100 Hz frequency. On the other hand, the probe light
is guided into a spectroscope via reflective optics in order to
avoid the white light dispersion in quartz lenses. During
sample measurement, the excitation beam is focused to a
spot diameter of 500 ␮m and the probe beam to 100 ␮m. A
quartz cuvette of 2 mm path length is used for the nanocrys-
tal solutions and the sample is continuously stirred by a cell
stirrer to avoid permanent bleaching of a single volume ele-
ment in the solution. The pump–probe experiments are all
carried out at ambient temperature. LabView-assisted data
acquisition results in a ‘‘dynamics matrix’’ ͑spectra versus
delay time͒, which is analyzed by single-value decomposi-
tion method. Global analysis of the dynamics matrix also
facilitates the extraction of the significant dynamic compo-
nents with high reliability, as opposed to a kinetic fit at indi-
vidual wavelengths.
Studies of the ultrafast dynamics of optically generated
charge carriers have been previously shown to be informa-
tive with respect to a range of semiconducting silicon mate-
22,23
rials, including hydrogenated amorphous Si ͑a-Si:H͒
as
well as porous Si.24–26 We have employed femtosecond time-
resolved pump–probe spectroscopy to study the nonradiative
relaxation dynamics of the conduction band electrons in un-
doped silicon nanocrystals, randomly dispersed Er-doped
silicon nanocrystals, and erbium surface-enriched silicon
nanocrystals. Using ultrafast pump–probe spectroscopy we
investigated the influence of dopant architecture on the initial
carrier dynamics of erbium-doped silicon nanocrystals. To
gain insights on the relative depths of the trapping sites on
the investigated Si nanostructures, benzoquinone was used as
a surface adsorbed electron acceptor to facilitate the photo-
induced electron transfer across the nanocrystal surface and
subsequently assist in back electron transfer.
II. EXPERIMENT
A. Nanocrystal synthesis
Nanocrystals of randomly dispersed Er-doped Si were
prepared according to a previously reported procedure.16 Di-
luted 0.48% Si2H6 ͑He balanced, Praxair͒ was pyrolyzed in
the presence of the erbium CVD source compound,
Er͑tmhd)3 vapor ͑Strem͒, which is flowed through a small
pyrolysis oven ͑6 cm in length͒ that operates at 1000 °C.
After a reaction period of 24 hrs, the desired nanocrystals
were collected in an ethylene glycol bubbler and purified by
a series of centrifugation, washing, and precipitation steps.
As a control material, undoped Si nanocrystals were also
prepared by employing identical reaction conditions during
synthesis, except for the deliberate absence of the erbium
source compound. For the preparation of erbium surface-
enriched nanocrystals, the reactor was modified to decouple
the Si nucleation and growth steps from the erbium incorpo-
ration event.18 After the pyrolysis of the diluted disilane in a
6 cm pyrolysis oven ͑again operating at 1000 °C͒, the aerosol
is mixed with Er͑tmhd)3 vapor ͑Strem͒ in He carrier gas
͑Praxair, UHP grade͒. Approximately 4 cm from the intro-
III. RESULTS
A. Characterization of Si nanocrystals
A TEM image of an Er-doped Si nanocrystal sample is
shown in Fig. 1͑b͒. The SAED pattern ͓inset Fig. 1͑b͔͒ is
consistent with the diamond cubic phase of Si, and confirms
the absence of a second phase formation with the incorpora-
tion of Er3ϩ ions. Furthermore, previous structural analyses
by HRTEM confirm the presence of defects such as disloca-
tions and stacking faults in the nanocrystals.16 This suggests
that Er3ϩ inclusion in the Si lattice may be responsible for
some of the defect formation. It is interesting to note that the
control sample of homogenous Si nanocrystals ͓Fig. 1͑a͔͒,
which was synthesized by a similar method, was also found
to exhibit stacking faults and other defects. However, it is
important to stress, that there is a clear structural difference
induced by Er3ϩ in the doped nanocrystals, as the SAED
rings in the rare-earth doped Si sample are significantly
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J. Chem. Phys., Vol. 120, No. 18, 8 May 2004
Samia et al.
FIG. 1. HRTEM micrograph of a randomly dispersed Er-doped Si nanocrystal sample with an average size of 5 nm; the inset shows the SAED pattern of the
nanocrystals. ͑a͒ High resolution TEM image of undoped Si nanocrystals along with the associated selected area electron diffraction ͑SAED͒ pattern as an
inset, which is consistent with a cubic phase structure; ͑b͒ high resolution TEM image of randomly dispersed Er-doped Si nanocrystals with the inset showing
a more diffuse, less-resolved SAED pattern, but is still consistent with the cubic phase structure; ͑c͒ TEM image of Er surface-enriched Si nanocrystals where
a magnification is selected in which the higher Z contrast Er is evident along the surface of the nanocrystals.
broader, more diffused and less-resolved ͑Fig. 1͒. This obser-
vation supports the idea that Er3ϩ incorporation impacts the
nature of defect formation within the Si nanocrystals.16
In comparison to the bright-field HRTEM image of the
Er-doped Si nanoparticulate sample, the TEM image of the
erbium surface-enriched nanocrystals exhibit a contrast en-
hancement at the outer shell, which is attributed to the clus-
tering of erbium on the particle surface18 ͓Fig. 1͑c͔͒. For both
nanocrystal samples, EDX and EXAFS measurements con-
firm Er addition to the Si nanocrystal overall, with ϳ2%
erbium dopant in the doped nanocrystals, and about 4.6% in
the core-shell system.
In order to assess the relative depth of the trapping site
in the investigated Si nanocrystals, 1,4-benzoquinone ͑BQ͒
was added to the Si sample. From previous studies it has
been shown that quinones adsorbed on semiconductor nano-
crystals facilitate photoinduced electron transfer across the
NC surface and subsequently assist in back electron
transfer.28,29 This electron shuttling process is usually faster
than the inherent electron–hole recombination in the semi-
conductor alone.28 The quinone bypasses the semiconductor
core and surface trap states and shuttles the electron from the
conduction band to the hole in the NC.
The femtosecond transient spectrum of Si with BQ
shows the presence of an absorption between 476 and 588
nm, in addition to a bleach at 600 nm ͑Fig. 3͒. The transient
absorption fits to the published absorption for the benzo-
quinone anion30 and the transient bleach can be attributed to
the positively charged nanocrystals formed after the electron
transfer to the surface-adsorbed electron acceptor. By excit-
ing an electron from the valence band and quickly extracting
the excited electron with BQ, one introduces a transient
p-doping into the NC, which results in the observed transient
bleach.24,25,29 In effect, the transient spectra are superposi-
tions of the NC bleach and the BQ anion absorption.
B Femtosecond time-resolved spectroscopy
1. Undoped silicon nanocrystals (Si NCs)
Following light excitation at 390 nm with 120 fs laser
pulses, the undoped Si NCs show a broad, featureless tran-
sient absorption, which extends over the whole visible range.
This is indicative of the indirect band gap excitation of Si.27
The transient absorption, shown in Fig. 2, is extremely weak
and decays exponentially with a 134 ps time constant. This
rather long-lived transient is attributed to trap state absorp-
tion, which reflects the population and depopulation of defect
sites in the Si NCs.
FIG. 3. Femtosecond transient spectrum of undoped Si NCs with BQ ad-
sorbed on the surface after excitation with 390 nm pulses ͑120 fs duration͒.
The inset shows the kinetics observed for the transient absorption at 500 nm
and the transient bleach at 600 nm.
FIG. 2. Femtosecond transient absorption of Si NCs after excitation with
390 nm pulses ͑120 fs duration͒. The inset shows the kinetics observed for
the transient absorption at 600 nm.
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J. Chem. Phys., Vol. 120, No. 18, 8 May 2004
Effect of erbium on relaxation dynamics in Si nanocrystals
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FIG. 5. Femtosecond transient absorption of randomly dispersed ͑RD͒
Er3ϩ-doped Si NCs with BQ adsorbed on the surface after excitation with
390 nm pulses ͑120 fs duration͒. The inset shows the kinetics observed for
the transient absorption at 530 nm and the transient bleach at 650 nm.
FIG. 4. Femtosecond transient absorption of Si NCs with randomly dis-
persed Er3ϩ dopant centers after excitation with 390 nm pulses. The inset
shows the kinetics observed for the transient absorption at 610 nm.
Upon femtosecond laser excitation, the transient absorp-
tion of the BQϪ radical anion appears within 150 fs with a
maximum at 500 nm. At longer wavelengths a bleach is ini-
tially observed. It is interesting to note that the formation
time of the radical anion absorption was determined to be
similar to the formation time observed for the Si NC bleach
at 600 nm ͑inset, Fig. 3͒. The time average of the best fit for
the decay of the radical ion 75 ps matches the recovery of the
one for the bleach ͑␶ϭ58Ϯ10 ps͒. As the electron undergoes
back transfer, the 500 nm absorption decays and the bleach
does so as well. This suggests that the BQ acts as an electron
shuttle by facilitating the withdrawal of the electron from the
conduction band and the delivery to the hole in the valence
band.
tron transfer seems not to recover completely to the ground
state in contrast to the undoped Si NCs. Instead of the
electron–hole recombination, which would lead to the decay
of any transient spectrum, a long-lived transient absorption is
observed. This could be explained by a mechanism in which
the electrons are captured by Er3ϩ-related trapping sites hin-
dering the complete recovery of the NC ground state. How-
ever, it seems that the undoped Si NCs described above also
have low enough trapping sites available to intercept the
electron–hole recombination process, consistent with the
stacking faults pointed out in Fig. 1.
Overall, it appears that the internal Er3ϩ ions are more
efficient trapping sites for photoexcited conduction band
electrons. As one might expect, the Er3ϩ dopant gives rise to
additional deep traps, which may be responsible for the ob-
served ‘‘trap-to-trap hopping’’31,32 of the photoexcited con-
duction band electrons. This gives rise to a fast trapping
process on the 360 fs time scale and a slower trapping com-
ponent of 100 ps lifetime. Less obvious is the fact that the
trapping to dopant sites can be largely circumnavigated by
adding efficient electron acceptors on the nanocrystal inter-
face. This was accomplished with BQ, which removed the
photoexcited electron from the initially populated dopant-
induced defect sites. The back electron transfer from the
electron acceptor anion to the hole on the nanocrystal, is
intercepted by low-lying trap sites. This allows an estimate
of the redox potential of the trapping sites, which must be
lower than that of the electron acceptor ͑BQ: Ϫ0.45 V versus
SCE͒ in this system.
2. Randomly dispersed Er-doped Si NCs
Si NCs with randomly dispersed Er3ϩ dopants initially
show a similar broad absorption band, which is quickly re-
duced by a fast component with a lifetime of 360 fs ͑Fig. 4͒.
This spectral feature is assigned to a trapping component,
which is much faster than the trapping process in the un-
doped Si nanocrystals previously discussed. A second trap-
ping component is also observed with trap state lifetime of
100 ps. The transient absorption features of the trap state
differ from that of the undoped Si nanocrystals, with maxima
at 550 and 670 nm and the occurrence of two trapping pro-
cesses.
After the addition of BQ, laser excitation leads to fast
interfacial electron transfer forming the Er-doped SiϩBQϪ
ion pair. The corresponding charge transfer spectrum in Fig.
5 is characterized by the superposition of the features of the
BQ anion at 450–500 nm and the bleach, which is caused by
the missing electron ͑transient p-doping͒ on the NC. The
electron transfer time is determined to be 220 fs and the
weighted charge transfer state lifetime is found to be 82.5 ps
͑Fig. 5͒. A long-lived residual absorption is also observed
and can be assigned to the electrons that become localized in
the trapping sites caused by the randomly dispersed Er3ϩ
ions. In addition to the fast decay dynamics, the back elec-
3. Si NCs with an Er-enriched surface layer
The femtosecond spectroscopic measurement of the er-
bium surface-enriched system, presented in Fig. 6, shows
again the formation of a broad transient absorption with no
distinct features, as in the system with no erbium dopant.
But, as in the case of the randomly dispersed erbium-doped
nanocrystals, two trapping components with lifetimes of 2.4
and 182 ps were observed, indicative of a trap-to-trap hop-
ping mechanism.
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J. Chem. Phys., Vol. 120, No. 18, 8 May 2004
Samia et al.
360 fs in the doped nanocrystals. Similarly, the erbium
surface-enriched nanocrystals exhibit a faster electron trap-
ping time of 2.4 ps compared to the undoped Si NCs. It is
also noteworthy to mention the presence of an additional
trapping relaxation process with a lifetime of 100 ps for the
internally erbium-doped Si nanocrystal sample and 182 ps
for the surface-enriched sample. Addition of the BQ to the
nanocrystal samples reveals that the fastest electron transfer
occurs in the undoped Si nanocrystal sample, and is longer
for the erbium-doped samples. This observation indicates the
relative depth of the trapping sites in the erbium-doped
samples. Assuming that the excited-state electron transfer is
an activated process, the different rise times could result
from the different depths of the initial electron traps. Two
aspects of the early time relaxation dynamics of the investi-
gated Si nanocrystals is of considerable interest and will be
addressed in the following section: First, the effect of erbium
doping on the nonradiative relaxation pathways of the pho-
toexcited electrons in Si nanocrystals, and second, the influ-
ence of the erbium architecture on the carrier dynamics.
FIG. 6. Femtosecond transient absorption of Si NCs with an Er-enriched
surface layer after excitation with 390 nm laser pulses. The inset shows the
kinetics observed for the transient absorption at 550 nm.
Addition of BQ to the erbium surface-enriched nano-
crystal system leads to the formation of the charge transfer
state within 214 fs. The charge transfer state (Si NCϩBQϪ)
as in the previous cases is characterized by the superposition
of the transient spectra on BQϪ anion around 500 nm and the
partial bleach of the nanocrystal absorption at longer wave-
lengths ͑Fig. 7͒.
A. Influence of the erbium dopant
on the electron relaxation dynamics
The undoped Si nanocrystals show a sufficient density of
trapping sites such that trap state absorption is the dominant
transient feature after femtosecond laser excitation. In gen-
eral, the introduction of Er3ϩ centers adds other significant
trapping sites to the nanocrystal and is responsible for the
efficient trap state population and the corresponding transient
trap state absorption. Based on analogies with Er3ϩ ions im-
planted into bulk Si, it is likely that the electrons are not
actually trapped on Er3ϩ sites but rather that the dopant in-
duces other defect sites in the crystal that are populated. This
is consistent with the obtained HRTEM images obtained for
the Si nanocrystals, which indicates a great number of dislo-
cations and stacking fault defects in the crystal lattice of the
doped nanocrystals. This phenomenon may help explain the
additional electron trapping processes that are observed in
the doped nanocrystals.
IV. DISCUSSION
The parameters characterizing the early time relaxation
dynamics in the investigated Si nanocrystals are summarized
in Table I. Comparison of the trapping times of the undoped
Si NCs with the sample possessing randomly dispersed Er3ϩ
centers, indicates that the trapping probability for the inves-
tigated system is greatly increased with the addition of Er3ϩ
.
The incorporation of Er3ϩ ions into the core of the nano-
crystals appears to open up additional nonradiative relaxation
pathways, which greatly shortens the fastest trapping time
component of the conduction band electrons from 134 ps to
Upon photoexcitation of the randomly dispersed erbium-
doped Si nanocrystal sample, the conduction band electron is
very quickly localized within 220 fs to a shallow trap. A
subsequent transport to a deeper trap with a lifetime of 100
ps is then observed, indicative of a possible trap-to-trap elec-
tron hopping mechanism. This type of mechanism has been
proposed for semiconductor systems with a substantial num-
ber of defect sites,31,32 which is likely the case for the inves-
tigated erbium-doped nanocrystal samples. The photoexcited
electron will remain in the traps until recombination with a
hole will occur. In the case of an indirect bandgap material
such as Si, the recombination process is phonon assisted.
However, with the addition of erbium centers the number of
deep traps increases, resulting in additional nonradiative re-
laxation pathways which shorten the lifetime of the photoex-
cited electron. As a consequence, the erbium-induced defect
sites help promote the electron–hole recombination by pro-
viding a fast nonradiative route for the electron to hop from
trap to trap site until it undergoes radiative or nonradiative
FIG. 7. Femtosecond transient absorption of Er3ϩ surface-enriched Si NCs
with BQ adsorbed on the surface after excitation with 390 nm pulses. The
inset shows the kinetics observed for the transient absorption at 520 nm and
the transient bleach at 650 nm.
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J. Chem. Phys., Vol. 120, No. 18, 8 May 2004
Effect of erbium on relaxation dynamics in Si nanocrystals
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TABLE I. Early time relaxation dynamics in silicon nanocrystals.
Trap state lifetime͑s͒
BQ charge transfer
Nanocrystal type
Undoped Si NCs
Risetime ␶rise/Lifetime͑s͒ ␶
Risetime/Lifetime͑s͒ ␶
dec
dec
р130 fsa
р130 fsa
р130 fsa
134Ϯ7 ps
150Ϯ30 fs
220Ϯ12 fs
214Ϯ10 fs
36Ϯ2 ps ͑58%͒
130Ϯ9 ps ͑42%͒
Avg.: 75 ps
40Ϯ1 ps ͑66%͒
165Ϯ6 ps ͑34%͒
Avg.: 82.5 ps
67Ϯ2 ps ͑85%͒
109Ϯ5 ps ͑15%͒
Avg.: 73.3 ps
Randomly dispersed
Er-doped Si NCs
360Ϯ30 fs
100Ϯ9 ps
Surface Er-doped Si NCs
2.4Ϯ0.1ps
182Ϯ8 ps
aRise of the signal within the excitation laser pulse.
recombination with the hole. Thereby, energy transfer to the
Er3ϩ center can occur.
cesses in the internally doped nanocrystal sample seems to
be more favorable and lead to faster trapping times ͑100 ps
compared to 182 ps͒.
The presence of low-lying trap sites in the doped Si
sample is further confirmed with the addition of BQ of the
surface of the nanocrystals. The back electron transfer seems
not to recover completely to the ground state in the doped Si
NCs. Instead of the electron–hole recombination, the elec-
trons appear to be captured at deep trap states hindering the
complete recovery of the NC ground state and its spectrum.
For these studies BQ was the molecule chosen because it
is well known as an electron acceptor molecule. In addition,
from previous work we have discovered via FT-infrared
spectroscopy that BQ binds directly to the semiconductor
NC surface via its oxygen electrons ͑lone pairs͒.29 This direct
surface adsorption is responsible for the observed ultrafast
electron transfer process. Furthermore, the redox potential
The data also suggest that the charge transfer complex
formed between the nanocrystal and the electron acceptor
BQ possesses a longer rise time than when the Er3ϩ centers
are randomly dispersed throughout the nanocrystal than
when clustered at the surface, therby indicating that there are
deeper trap states initially occupied in the internally doped
nanocrystals compared to the undoped and surface-doped Si
NCs. Furthermore, the back electron transfer lifetimes are
longer in the core-doped nanocrystals, which confirms the
presence of low-lying trap sites that favor the interception of
the returning electron. These observations are also consistent
with the near infrared photoluminescence mechanism of each
type of Si nanocrystal ͑Fig. 9͒. Previous measurements have
shown that the internally Er-doped material emits at 1540
nm. This is attributed to an excitation pathway involving
energy transfer from the recombination process of the pho-
toexcited carriers at an interfacial site. Efficient energy trans-
fer requires resonance between the electron–hole recombina-
tion process and the Er3ϩ excited state. This is consistent
with the presence of low-lying trap states in the internally
doped Si nanocrystal sample. On the other hand, the lack of
emission in the surface-enriched sample can simply mean
that high local concentrations of Er3ϩ ions can result in a
co-operative upconversion, in which an Er3ϩ ion de-excites
by transferring its energy to a neighboring excited ion, pro-
moting it into higher energy levels.
33
for BQ is well documented ͑Ϫ0.45 V versus SCE͒ and
allows a relative estimation of the energy levels of the initial
trapping sites in the doped and undoped NCs ͑Fig. 8͒.
B. Influence of Er3¿ architecture
on the electron dynamics
A close comparison of the trap state lifetimes of the er-
bium surface-enriched with that of the internally doped Si
nanocrystals shows considerably shorter lifetimes for the lat-
ter. This observation leads to the conclusion that Er3ϩ in the
surface-enriched Si NCs has a different effect on the conduc-
tion band electron dynamics than when the dopant is ran-
domly dispersed throughout the Si lattice. The trapping pro-
FIG. 8. Schematic diagram of the electron transfer process to benzonquinone ͑BQ͒ across the surface of Si nanocrystals.
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8722
J. Chem. Phys., Vol. 120, No. 18, 8 May 2004
Samia et al.
FIG. 9. Schematic diagram of the different relaxation processes in Si nanocrystals.
In addition, the forward electron transfer rate to BQ
Addition of the electron acceptor, BQ, on the surface of
the Si nanocrystal samples provides a method for assessing
the relative depths of the trapping sites in the investigated
system. Analysis of the charge transfer rise time and the
subsequent back electron transfer to the NCs supports the
presence of low-lying trap states in the erbium-doped Si
nanocrystals.
gives interesting insights into the effect of Er3ϩ doping on
the energy level structure of the NC. Here, we observe that
the change in forward electron transfer rates ͑from 150 to
220 fs͒ indicate that there could be an ultrafast electron trap-
ping process occurring in the Er3ϩ doped NCs, consistent
with the overall slower forward electron transfer of the
doped systems. Moreover, it was shown that the internal dop-
ing has a greater effect than the surface doping in the Si NCs.
This could imply a stronger lattice distortion and deeper de-
fect formation for internal doping compared to surface dop-
ing ͑see Table I͒. In a similar way one could explain the
different CT lifetimes. The randomly dispersed erbium-
doped NCs show longer back electron transfer times, possi-
bly because low-lying trapping sites could intercept the back
electron transfer and cause the electron/hole recombination
rate to slow down. The presented results may explain the
observed emission in internally doped NCs and the lack
thereof in surface-doped ones. In internally doped Si NCs,
the excited electrons get trapped faster and efficiently relax
into lower lying trapping sites, which then allow the ob-
served resonance energy transfer to the Er3ϩ dopant.
ACKNOWLEDGMENTS
C.B. would like to acknowledge financial support by
NSF ͑No. CHE-0239688͒, ACS-PRF and the Ohio Board of
Regents. J.C. gratefully acknowledges the National Science
Foundation and the Robert A. Welch Foundation for financial
support of this research.
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