Efficient luminescence from silicon nanocrystals:
Role of the plasma afterglow
Uwe Kortshagen, Rebecca Anthony, and David Rowe
Department of Mechanical Engineering, University of Minnesota, Minneapolis MN 55455
Abstract:
For nanocrystals produced in plasmas, the plasma synthesis
parameters can be important players in determining the attributes of the
nanocrystals. Here we demonstrate the effects of gases injected into the plasma
afterglow on the photoluminescence quantum yields from silicon nanocrystals
produced in a nonthermal plasma reactor. We show that hydrogen injection into
the plasma afterglow leads to silicon nanocrystals with photoluminescence
quantum yields exceeding 50%. We demonstrate that hydrogen injection leads to
a more fully passivated SiNC surface due to cooling effects and chemical binding
with the nanocrystal surfaces. Exchange of hydrogen with injection of other inert
gases leads to diminished luminescence efficiency.
Keywords: silicon, nanocrystal, luminescence, surface, plasma
1. Introduction
The pursuit of efficient and sustainable lighting
technologies has led to great interest in the optical
characteristics of nanoscale silicon. Silicon is
abundant and largely nontoxic, and silicon
nanocrystals (SiNCs) exhibit efficient and tunable
light emission that can be used in the fabrication of
visible and IR light-emitting devices (LEDs) [1-3],
as well as in other applications [4-6] . In order to
maximize the performance of SiNCs in LED
structures, it is important to study and understand the
parameters of SiNC synthesis and processing that
lead to optimal light-emission.
Plasma technology is one of the more
effective approaches to synthesis of SiNCs. Plasmas
offer a simple synthesis route through a gas-phase
decomposition of silane or dislane and subsequent
formation of nanocrystals [7-11]. However, reports
of optical emission from SiNCs have been
inconsistent with regards to photoluminescence (PL)
quantum yields (QYs) [7-12]. In this work, we
demonstrate the important role played by hydrogen
injection into the afterglow of a nonthermal plasma
reactor for the synthesis of SiNCs. We propose that
hydrogen injection is essential in synthesis of SiNCs
with PL QYs greater than 50%, nearly 4x greater
than in other gas-injection scenarios.
The nonthermal plasma reactor is a
streamlined and flexible tool for the synthesis of
high-quality SiNCs [12,13]. Following a surface
functionalization step, these SiNCs can demonstrate
PL QYs of >60% [11]. All samples in previous work
have included the injection of hydrogen into the
Ar + SiH4
(5% in He)
9.5mm O.D.
5.5mm I.D.
electrodes
6.4mm O.D.
3.9mm I.D.
injected
gas
25.6mm O.D.
20.4mm I.D.
primary
plasma
region
plasma
afterglow
25mm
15mm
to collection
mesh
Figure 1: Schematic of the nonthermal plasma reactor and gas
injection region.
plasma afterglow. However, the exact role of
hydrogen injection has not been studied. Due to the
exceptionally high quantum yields measured from
SiNCs produced in the nonthermal plasma reactor
under the injection scenario, examination of the
effect of hydrogen injection is a worthy endeavor. It
has been shown that hydrogen treatments of SiNCs
following synthesis can change or improve PL QYs
[9,14]. Here we investigate hydrogen treatment of
SiNCs in the immediate post-synthesis environment,
while the SiNCs are freshly emerged from the
plasma.
as-produced nanocrystals, while the SiNCs for PL
were surface-functionalized with 1-dodecene in a
thermal hydrosilylation reaction.
a)
2. Experimental Details
For the purposes of these gas injection
experiments, the gas flowrates of silane (5% in
helium) and argon (13sccm and 35sccm
respectively) through the synthesis region of the
reactor were the same for each sample. The applied
rf power at 13.56 MHz was 70-80 W, and the
pressure was held constant at 1.4 Torr through use of
a feedback-monitored throttle valve. The only
parameter changed was the injection of gas through
a downstream sidearm of the reactor into the plasma
afterglow. A schematic of the reactor is shown in
Figure 1.
We hypothesized that the two possible effects of
hydrogen in the plasma afterglow are quenching of
particle temperature and chemical passivation of
surface sites. In order to examine these effects, we
chose four injection schemes. To test the effect of
cooling, we exchanged hydrogen injection for argon
and helium. Both gases are inert, but the thermal
conductivity of helium is near that of hydrogen,
while argon’s thermal conductivity is much lower.
As for examining the propensity of injected
hydrogen to bind with SiNC surfaces, we chose
deuterium injection. Deuterium and hydrogen are
nearly chemically identical, but have differentiable
absorbance peaks in Fourier transform infrared
(FTIR) spectra—so any chemical binding which
occurs in the plasma afterglow would be evidenced
by Si-Dx peaks in infrared absorbance spectroscopy.
Following synthesis, the SiNC samples were
removed from the reactor into a nitrogen-purged
glove bag to prevent oxidation. The SiNC samples
were examined using a Bruker Alpha FTIR in
diffuse reflectance mode, to examine changes in
surface bonding as a result of this gas injection. PL
quantum yield (QY) measurements were taken using
an Ocean Optics, Inc. USB2000 spectrometer
equipped with an integrating sphere and 395-400nm
LED excitation source. FTIR spectra were taken on
b)
Figure 2: a) normalized PL spectra for SiNC samples and b)
quantum yields based on injection gases.
3. Results and Discussion
The normalized PL spectra from these
samples are plotted in Figure 2a, and show a change
in PL intensity based on injection gas. The PL QYs
are plotted in Fig. 2b, showing a span in QYs
between 12% (argon injection) and >50% (hydrogen
injection and deuterium injection), a dramatic
change. It is reasonable to expect that these changes
in PL must be due to interactions with the SiNCs in
the afterglow region of the plasma rather than some
innate change in the synthesis region of the plasma.
A quick calculation of binary diffusion of hydrogen
through argon shows that the time for gas molecules
to travel from the bottom of the synthesis region to
the afterglow injection region is 4.4 ms, whereas the
time needed for diffusion of hydrogen back to the
bottom of the synthesis region is roughly two times
as long. Thus, any interactions of the injection gas
with the rest of the plasma species should be
confined to the region downstream of where the
SiNCs are synthesized and grown.
To better understand what happens to the
SiNCs in the afterglow region, we performed FTIR
analysis of the samples to look at the nanocrystal
surfaces. As hydrogen is a well-known passivator of
SiNC surface sites, and because the remediation of
surface defects in nanocrystals has been shown to
improve PL [15], one possibility is that injection of
hydrogen leads to a better-passivated SiNC surface.
To start, we will discuss the FTIR spectra for the
Ar/He/H2 samples. The SiNC surfaces are capped
with hydrogen, as shown in Figure 3. The three main
features shown here are from Si-H (2086cm-1), Si-H2
(2112cm-1), and Si-H3 (2136cm-1) stretching
vibrations [16]. The most notable difference in these
three spectra is an increase in the Si-H3 absorbance
peak, and a relative decrease in the Si-H absorbance
peak, as the samples move from argon injection up
to hydrogen injection. This corresponds with an
increase in PL QY, as well. The prevalence of
higher-order hydrides on the H2-injection samples
indeed indicates a better-passivated surface.
One method to confirm the surface
passivation of dangling bonds is to measure the
dangling bonds on the SiNCs using electron
paramagnetic spin resonance (EPR). This work was
done by packing SiNCs made with argon and with
hydrogen injection into Suprasil quartz tubes, and
measuring dangling bonds in a Bruker CW EleXsys
E500 spectrometer. EPR spectra normalized to
sample mass are shown in Figure 3. It is clear that
the dangling bond density for the argon-injection
sample is greater than that for the hydrogen-injection
sample, and so based on this data and on FTIR
analysis, it is reasonable to assert that injection of
hydrogen leads to improvement in PL due to a
reduction of surface defects by more complete SiNC
surface passivation.
Figure 3: FTIR absorbance spectra for as-synthesized SiNs
made with different injection gases.
After synthesis, when a SiNC passes
through the plasma afterglow, interactions with
energetic gas species could cause desorption of H
from the SiNC surface. One possibility is that
sidearm gas has a quenching effect on this H
desorption.
H2 and He have similar thermal
conductivities, while Ar has a lower thermal
conductivity and also easily absorbs energy through
production of excited state. This may explain why
argon sidearm gas leads to the poorest PL
performance.
However, hydrogen has the special
capability to further passivate the SiNC surface in
the plasma effluent. The deuterium injection
experiment shows that the injected gas plays an
active role in structuring the surface of SiNCs. As
shown in Fig. 3, the native hydride surface of the
SiNCs before the sidearm plasma region is almost
fully exchanged for a deuterated surface. Deuterium
injection does not lead in a decrease in PL QY over
hydrogen injection.
Figure 4: EPR spectra for argon injection and hydrogen
injection samples. The amplitude of the signal indicates the
dangling bond density.
4. Conclusions
In conclusion, we have shown that the
injection of gas into the afterglow of the plasma
leads to measureable changes in PL performance of
the nanocrystals. We hypothesize that hydrogen
injection yields SiNCs with the most efficient
photoluminescence by a combination of quenching
of surface hydrogen desorption and additional
hydrogen passivation of the SiNC surfaces.
Acknowledgements: This work was primarily
supported by NSF through the MRSEC grant DMR0819885. The authors would like to thank Ryan
Mello and Nic Kramer for their help with EPR
measurements.
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