Silyl effusion from plasma-produced silicon nanocrystals

Rebecca Anthony and Uwe Kortshagen

Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota

USA

Abstract: In this study, we examine the surfaces of silicon nanoparticles produced in a non-

thermal plasma reactor. We see enhanced photoluminescence (PL) upon heating of the

nanoparticles, corresponding to changes in the Si-Hx bonds at the particle surface. Residual

gas analysis that shows that particle heating causes desorption of silyl groups from the particle

surface. The relationship between these silyl radicals and nanoparticle PL is explored.

Keywords: silicon, nanocrystals, photoluminescence, surface

Introduction The efficient and tunable optical emission characteristics of

nanoscale silicon have attracted much interest in recent years.

We have previously shown that the non-thermal plasma flow-

through reactor is an effective tool for the synthesis of high-

quality, monodisperse, and efficiently-emitting nanocrystals

[1]. The surfaces of silicon nanoparticles can be

functionalized with alkene ligands to eliminate some surface

defects and improve photoluminescence (PL) characteristics,

but features of the native hydrogenated surface as well as

non-functionalizing surface treatments can influence the

optical behavior of these nanoparticles. In fact, hydrogen

passivation of surface defects and dangling bonds in

nanoscale silicon is generally shown to increase silicon

photoluminescence intensity [2]. Here we present surface

studies on silicon nanocrystals produced in our plasma

reactor, with the goal of understanding how to engineer the

nanoparticles and their environment for better device

functionality. We see that the hydride surface is altered

through plasma input power as well as post-synthesis thermal

treatment, and that these changes result in enhanced optical

behavior of the nanoparticles. Additionally, residual gas

analysis (RGA) measurements from the nanoparticles

indicate that heating the nanoparticles results in a removal of

silicon-hydride radicals from the samples. The increase in

photoluminescence of the nanoparticles upon heating may be

due to the removal of these radical groups, which can act as

non-radiative recombination sites for excitons in the

nanoparticles.

Background

The presence of incompletely-bound silicon atoms at the

nanocrystal surface (dangling bonds) and other defects

influence the optical behavior. In fact, full hydrogen

passivation of surface defects and dangling bonds in

nanoscale silicon is generally shown to increase silicon

photoluminescence intensity [3]. Early studies on porous

silicon noted that heating of the silicon can change hydrogen

surface species and correspondingly change the

photoluminescence intensity [4]. In this case, hydrogen is

removed by annealing, and full hydride passivation is

restored by reacting the silicon with hydrofluoric acid.

Seraphin et al. [5] demonstrated that oxide passivation,

hydride passivation, and halide passivation all increase the

PL intensity from silicon nanocrystals compared to

unpassivated particles. They speculate that even one

dangling bond per nanocrystal can trap charge carriers,

leading to a decrease in radiative rate. The dangling bonds

and free electrons in nanoparticles can act as defects and

charge traps, reducing the radiative recombination rate in the

nanocrystals by increasing non-radiative recombination. In

many nanocrystal synthesis schemes, the particle surface is

incompletely hydrogen-passivated and contains some of these

defects. Heat treatment and surface passivation, as

mentioned above, decrease the dangling bond density,

yielding an increase in photoluminescence intensity [6, 7].

In this work we will examine the effects of heat treatment on

silicon nanoparticle surface and photoluminescence.

Experimental details The flow-through nonthermal plasma reactor used in this

work operates by dissociating reactant gases via an applied rf

power, leading to silicon cluster formation and growth. The

reactor chamber itself consists of 9.5 mm O.D. quartz tube

connected to a mechanical pump. Radiofrequency power at

13.56 MHz is applied to the gases in the tube via a dual-ring

electrode encircling the tube. The reactant gases used are

argon, silane (5% in helium), and hydrogen at total flowrates

between 140-200 sccm. Tuning the inert gas flowrate in the

plasma allows control over nanoparticle size, and thus

emission wavelength. The input power for nanocrystals is

near 85 W; lower powers result in a lower crystalline fraction

among the particles produced. Heating experiments on the

nanoparticles were carried out under nitrogen atmosphere, in

the case of PL experiment, and under vacuum, in the case of

RGA measurements. FTIR data were taken using a Nicolet

Series II Magna-IR System 750 FTIR. The residual gas

analysis information was taken using a Stanford Research

Systems RGA100 instrument.

Results and Discussion

As-produced nanocrystals produced in our reactor do no

photoluminesce efficiently. Trihydride presence at the

nanocrystal surface or other sources of dangling bonds and

defects are likely the root of the low PL

improve the particles’ solubility and also enhance the PL

intensity, we carry out surface-modification

hydrosilylation. This is typically accomplished by refluxing

the nanorystals in a 5:1 by volume mixture of mesitylene and

an alkene such as 1-dodecene. In hydrosilylation of the

nanocrystals, the surface silicon-hydrogen bonds are cleaved

and alkene ligands attach to the nanoparticles, y

alkane chain coverage. In this process, the surface is not

fully hydrosilylated; there are some residual silicon

monohydride groups seen in FTIR.

hydrosilylation, the nanocrystals typically photolu

with quantum yields of ~ 50% or higher [8].

However, the particle surface and PL intensity can be

changed without this reaction. We have seen

improvements in PL intensity resulting from post

treatment in the absence of passivating ligands, such as under

nitrogen atmosphere. This is accomplished in by heating in

inert gas atmosphere or by boiling in a non-p

as mesitylene (see Figure 1a). Simultaneously, the density of

different hydrogen bonds at the nanoparticle surface is

changed by heating. The nanocrystals emerge from the

plasma reactor with a surface hydride layer comprised of

silicon mono-, di-, and tri-hydrides. Following heating, the

density of Si-H3 bonds has reduced, leaving predominantly

Si-H2 at the surface (see Figure 1b).

Marra et al. showed that silicon trihydrides on silicon

nanoparticles can be the result of adsorption of SiH

dangling bond sites, and that these trihydrides can be

dissociated or removed at temperatures below 250°C[

source of these dangling bonds may be silicon trihydride

species, residually left at the nanoparticle surface due to the

reaction kinetics. The group of Kessels [10

radicals present near the substrate during thin

nanocrystalline silicon growth in a

silane/argon/hydrogen plasma, and showed that the silicon

trihydride radical is the most prevalent. If the same is true for

our nonthermal plasma reactor, the predominance of SiH

could lead to adsorbed radicals on the nanoparticles’ surfaces,

creating dangling bonds and limiting the PL intensity from

as-produced nanoparticles.

By using temperature-programmed desorption with a

residual gas analyzer (RGA), we studied the hydrogen

evolution of the surfaces of silicon nanocrystals.

experiment, the silicon nanoparticle sample was heated under

vacuum to 450°C in 10° increments, and desorbed gases were

measured as a function of temperature. The RGA

instrument’s mass spectrometer allows desorbed molecules to

be identified by their mass.

nanocrystals produced in our reactor do not

Trihydride presence at the

nanocrystal surface or other sources of dangling bonds and

of the low PL intensity. To

improve the particles’ solubility and also enhance the PL

modification by

y accomplished by refluxing

the nanorystals in a 5:1 by volume mixture of mesitylene and

In hydrosilylation of the

hydrogen bonds are cleaved

and alkene ligands attach to the nanoparticles, yielding an

alkane chain coverage. In this process, the surface is not

fully hydrosilylated; there are some residual silicon

monohydride groups seen in FTIR. Following

hydrosilylation, the nanocrystals typically photoluminesce

However, the particle surface and PL intensity can be

have seen slight

from post-synthesis

treatment in the absence of passivating ligands, such as under

his is accomplished in by heating in

polar solvent such

). Simultaneously, the density of

different hydrogen bonds at the nanoparticle surface is

The nanocrystals emerge from the

plasma reactor with a surface hydride layer comprised of

Following heating, the

bonds has reduced, leaving predominantly

showed that silicon trihydrides on silicon

nanoparticles can be the result of adsorption of SiH3 on

dangling bond sites, and that these trihydrides can be

ed at temperatures below 250°C[9]. One

g bonds may be silicon trihydride

species, residually left at the nanoparticle surface due to the

netics. The group of Kessels [10] examined the

radicals present near the substrate during thin-film

nanocrystalline silicon growth in a low-pressure

silane/argon/hydrogen plasma, and showed that the silicon

trihydride radical is the most prevalent. If the same is true for

lasma reactor, the predominance of SiH3

could lead to adsorbed radicals on the nanoparticles’ surfaces,

limiting the PL intensity from

programmed desorption with a

residual gas analyzer (RGA), we studied the hydrogen

evolution of the surfaces of silicon nanocrystals. In this

he silicon nanoparticle sample was heated under

vacuum to 450°C in 10° increments, and desorbed gases were

measured as a function of temperature. The RGA

instrument’s mass spectrometer allows desorbed molecules to

Rather than hydrogen and silyl radicals simply

reorganizing on the nanoparticle surface, we discovered that

these silyl radicals are in fact desorbing from the nanocrystals

entirely, leaving behind primarily a di

Fig. 2a,b). The desorption of surface silyls from the

nanocrystals occurs at a temperature corresponding to the

release of physisorbed silyl groups from silicon crystals [1

This indicates that during particle synthesis in the plasma,

some dissociated silane

Fig. 1: (a) Photoluminescence quantum yield as a function of

heating temperature and (b) FTIR absorbance for as

(orange), dry-heated (red), and mesitylene

nanocrystals (green). PL quantum yield error bars reflect

standard deviation.

fragments do not experience the temperature and time

necessary to chemically bond to the

nanocrystals, and simply reside physisorbed on the

than hydrogen and silyl radicals simply

reorganizing on the nanoparticle surface, we discovered that

these silyl radicals are in fact desorbing from the nanocrystals

entirely, leaving behind primarily a di-hydride coverage (see

of surface silyls from the

nanocrystals occurs at a temperature corresponding to the

groups from silicon crystals [11].

This indicates that during particle synthesis in the plasma,

luminescence quantum yield as a function of

heating temperature and (b) FTIR absorbance for as-produced

heated (red), and mesitylene-boiled

PL quantum yield error bars reflect

do not experience the temperature and time

necessary to chemically bond to the growing silicon

nanocrystals, and simply reside physisorbed on the

nanoparticle surface. To verify this, we heated the

nanoparticles to 400° C and measured the residual gas

spectrum after 1 minute, 5 minutes, and 10 minutes (see Fig.

3). The removal of the silyl groups is confirmed by the

reduction in signal from the silyl region of the mass spectrum

as time passed.

Fig. 2: (a) Residual gas analysis of silicon nanoparticles

heated under vacuum. Circled regions correspond to silyl

groups desorbing from the sample. (b) FTIR absorbance

spectra of the nanocrystal sample before (orange) and after

(red) residual gas analysis.

To verify this, we heated the

nanoparticles to 400° C and measured the residual gas

ctrum after 1 minute, 5 minutes, and 10 minutes (see Fig.

3). The removal of the silyl groups is confirmed by the

reduction in signal from the silyl region of the mass spectrum

: (a) Residual gas analysis of silicon nanoparticles

heated under vacuum. Circled regions correspond to silyl

(b) FTIR absorbance

spectra of the nanocrystal sample before (orange) and after

As heating the nanoparticles results in an increased PL

intensity as well as a shift in hydride coverage of the

particles, as seen in FTIR, it could be that the heating reaction

results in the removal of the silyl radicals on the particles’

surfaces. This removal of silyls would reduce the dangling

bond density on the nanoparticles, thus improving PL.

Fig. 3: Residual gas analysis spectrum for silicon

nanocrystals heated to 400°C, taken at 3 time intervals. The

signals from silyl groups, near 30 and 60 amu,

passes, indicating removal of these groups from the sample.

. In summary, we have studied the effects of heating on the

hydrogen bound to the surfaces of silicon nanocrystals

produced in a nonthermal plasma reactor. Heating the

nanocrystals to 150°C leads to an increase in

photoluminescence quantum yield, and a decrease in Si

infrared absorption signal as evidenced in FTIR. When

examined using residual gas analysis, it is clear that heating

the nanocrystals leads to the removal of silyl radicals from

the particles. These residual silyls may act as dangling bonds

on the particles, reducing PL. Therefore, removing the silyl

radicals by heating would contribute

photoluminescence of the nanoparticles seen upon heating.

Acknowledgements:

This work was supported primarily by the MRSEC Program

of the National Science Foundation under Award Number

DMR-0819885 and partially by the University of Minnesota

Center for Nanostructure Applications.

References:

1] L. Mangolini, E. Thimsen, and U. Kortshagen. High yield

plasma synthesis of luminescent silicon

Letters, 5(4):655–659, 2005.

As heating the nanoparticles results in an increased PL

intensity as well as a shift in hydride coverage of the

particles, as seen in FTIR, it could be that the heating reaction

results in the removal of the silyl radicals on the particles’

ces. This removal of silyls would reduce the dangling

bond density on the nanoparticles, thus improving PL.

Fig. 3: Residual gas analysis spectrum for silicon

nanocrystals heated to 400°C, taken at 3 time intervals. The

from silyl groups, near 30 and 60 amu, shrink as time

passes, indicating removal of these groups from the sample.

In summary, we have studied the effects of heating on the

hydrogen bound to the surfaces of silicon nanocrystals

asma reactor. Heating the

leads to an increase in

photoluminescence quantum yield, and a decrease in Si-H3

infrared absorption signal as evidenced in FTIR. When

examined using residual gas analysis, it is clear that heating

nanocrystals leads to the removal of silyl radicals from

. These residual silyls may act as dangling bonds

on the particles, reducing PL. Therefore, removing the silyl

would contribute to the improvement in

photoluminescence of the nanoparticles seen upon heating.

his work was supported primarily by the MRSEC Program

of the National Science Foundation under Award Number

the University of Minnesota

Center for Nanostructure Applications.

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