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Formation and Photoluminescence of “Cauliflower” Silicon Nanoparticles Wingjohn Tang, Joren Jaromir Eilers, Marijn A. van Huis, Da Wang, Ruud E. I. Schropp, and Marcel Di Vece J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 18 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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The Journal of Physical Chemistry

Formation and Photoluminescence of “Cauliflower” Silicon Nanoparticles

Wingjohn Tang1, Joren Eilers2, Marijn A. van Huis1 3, Da Wang1, Ruud E.I. Schropp4, Marcel Di Vece1 5

1

Soft Condensed Matter, Debye Institute for Nanomaterials Science, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands

2

Condensed Matter and Interfaces, Debye Institute for Nanomaterials Science, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands 3

Kavli Institute of Nanoscience, Delft University of Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands

4

Energy research Center of the Netherlands (ECN), Solar Energy, High Tech Campus Building 21, 5656 AE Eindhoven; and Eindhoven University of Technology (TU/e),

Department of Applied Physics, Plasma & Materials Processing, P.O. Box 513, 5600 MB Eindhoven 5

Nanophotonics—Physics of Devices, Debye Institute for Nanomaterials Science, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands

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Abstract

The technological advantages of silicon make silicon nanoparticles, which can be used as quantum dots in a tandem configuration, highly relevant for photovoltaics. However, producing a silicon quantum dot solar cell structure remains a challenge. Here we use a gas aggregation cluster source to produce silicon nanoparticles. At a particular experimental condition, “cauliflower” silicon particles are formed as observed by high resolution transmission electron microscopy. They are likely formed by aggregation of smaller silicon nanoparticles and are in an intermediate state in the evolution towards large crystalline particles. These “cauliflower” silicon particles consist of nanodomains of crystalline and amorphous silicon (a-Si) of which the latter exhibits photoluminescence. The luminescence decay times of the silicon “cauliflower” particles are increased a thousand times by isolating the particles. Here were present a detailed study of such “cauliflower” silicon nanoparticles which is part of the quest for using silicon quantum dots in solar cells.


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Introduction

The photovoltaics (PV) market is currently dominated by silicon based solar cell technology because of its relative ease of production and its consistence of abundant and non-toxic materials only. Although new concepts and materials are intensively investigated to further increase efficiencies, silicon remains a very attractive PV material. Combining silicon with one of such new concepts, the quantum dot, may lead to enhanced device performance1. The advantages of silicon quantum dots are their tuneable band gap2, which can be employed in a tandem cell, and the direct band gap which may appear for the smallest sizes3. In principle a direct band gap increases absorption as compared to the indirect band gap of bulk crystalline silicon4. However, small quantum dots have a low density of states which in turn reduces their optical absorption. Moreover, for small atomic clusters the concept of bands and bandgaps is not applicable. Various methods to fabricate silicon particles for solar cells have been studied with proof of principle devices. From silicon rich silicon oxide or silicon carbide, nanoparticles are formed upon annealing whereby the silicon atoms aggregate to form particles. A similar process is possible by Si ion implantation5. However, these silicon nanoparticles have a large size distribution and are often separated by large distances, which 3 ACS Paragon Plus Environment


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hinders electrical conductance. The large size distribution will prevent accurate tuning of the band gap and forms exciton trapping centres. The fabrication of mono disperse silicon quantum dots with controllable separation distance is a prerequisite for successful application in a solar cell. Measuring photo luminescence of silicon nanoparticles is a method to study its optical properties, which yielded a clear size dependence of the luminescence emission energy due to quantum confinement

6 7 8 9 10

. Photoluminescence on silicon particles which were

fabricated by laser ablation, showed radiative decay lifetimes of micro seconds at room temperature which also depends on size and the available phonon contribution11. Multiple exciton generation in silicon nanoparticles was observed by step-like enhancement of the luminescence quantum yield12. In recent work by Tuan Trinh et al.13 the direct generation of multiple excitons was even possible in adjacent silicon nanoparticles. In this work we produced silicon nanoparticles with a gas aggregation cluster source by magnetron sputtering. This production method has the advantage of being simple, fast14

15

with high production volume and resembles the laser vaporisation technique16. The silicon particles were deposited on a glass substrate and investigated with a suit of techniques such as transmission electron microscopy (TEM), Raman spectroscopy and (time resolved) photoluminescence spectroscopy. Various stages of silicon nanoparticle formation were 4 ACS Paragon Plus Environment

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identified, depending on the cluster source operational parameters.

“Cauliflower”-like

particles of about 50-100 nm exhibited the most interesting optical features. In this study we explored the optical and structural properties of such “cauliflower” silicon nanoparticles, which forms a diving board to using cluster source produced silicon particles for PV.

Experimental

The silicon nanoparticles were produced by a gas aggregation magnetron sputtering cluster source (NC200U-B, Oxford Applied Research Ltd.) in RF mode with a power of 120 W and Ar/He flow of 15/10 sccm17

18 19

. The silicon target had a purity of 99.99 %. The

presence of helium was to stimulate the aggregation to smaller particles. It also contained a 10% volume fraction of hydrogen to enhance sputtering and to passivate the silicon surface of the particles. The particles were deposited on glass substrates which were cleaned with iso-propanol and sonication. In another set of samples the silicon particles were implanted in vacuum grease, containing silicones and SiO2 (Dow Corning, high vacuum grease), smeared on a glass substrate. The reference sample was produced by sputtering an a-Si layer from a Si target on an identical glass substrate. Photoluminescence was performed by recording emission and excitation spectra on an Edinburgh FLS920 dedicated fluorescence spectrometer with a single monochromator, a 5 ACS Paragon Plus Environment


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Hamamatsu R928 detector and a Xe900 continuous Xenon Arc lamp as excitation source. Emission spectra measurements were carried out at an excitation wavelength of 375 nm. Photoluminescence lifetime measurements were performed on a Edinburgh FLS920 equipped with a Hamamatsu H4722-02 photo sensor module, double monochromator and an Edinburgh 375 EPLED nm picosecond pulsed diode laser as excitation source. Raman spectra were recorded with a Renishaw InVia Raman microscope using an Ar-ion Spectra-Physics laser as the excitation source at 514 nm. UV-Vis measurements were performed on a Vertex 70 FT-IR spectrometer (Bruker). A FEI Tecnai 10 operated at 100 kV was used with a standard TWIN objective lens , as well as a FEI Tecnai 12 operating at 120 kV. A cubed FEI Titan 80-300kV operating at 300 kV was used for high resolution (HRTEM) imaging. For scanning transmission electron microscopy (STEM) a FEI Tecnai 20F with a Field Emission Gun (FEG) was used, operating at 200 kV in secondary electron mode (SE-STEM) and in high-angle annular dark-field mode (HAADF-STEM).

Results and Discussion

Initial characterization of the silicon particles by TEM as shown in Fig 1 A-D showed that the shape and size depended on the aggregation distance, the distance between magnetron and exit aperture.

Since the aggregation of particles is a process which


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Fig 1. TEM Micrograph of silicon particle deposited on a holy carbon film TEM grid made with an aggregation distance of (a) 40 mm, (b) 60 mm, (c) 20 mm and close-up image, (d) close-up image of “cauliflower” particle of sample with 60mm AL. Scale bar included. All micrographs were taken with the Tecnai 12 (120kV).

depends on time, gas pressure and sputtered atom density, the aggregation length, i.e. the distance between magnetron head and exit aperture influences those parameters. In Fig 1 the silicon particles are imaged for different aggregation distances. In Fig 1 A, the silicon



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particles have a size in the order of 80 nm and are faceted, indicating a crystalline structure. For the aggregation distance of 60 mm the majority of particles is structured and appear to consist of smaller particles. Both, the crystalline and structured particles have a large size distribution. A high resolution image of the structured particle shows a “cauliflower” shape and confirms the presence of smaller particles which are the building blocks for the particle. Two stages in the formation of nanoparticles can be distinguished: 1) aggregation of atoms into small particles, 2) aggregation of small particles in bigger particles20

21

. A third and final

stage encompasses the crystallization of the small particle aggregate into a single (crystalline) particle. In the “cauliflower” structure space between the branches is likely created by the charge on the small silicon particles. Since neutral particles would not aggregate into branches but instead would form a supra-sphere, we can conclude that only the presence of a repulsive force such as created by electric charge can cause the gaps between branches. The charge repels some of the particles by Coulomb force and hence creates the gaps between branches. The “cauliflower” particles are produced in the largest quantity with the 60 mm aggregation length. This is caused by a relatively lower Ar pressure since the total space is larger. This lower Ar pressure enhances the diffusion of smaller clusters, which are then able to form aggregates. In the high resolution TEM images of the “cauliflower” particles in Fig 2 A and B, nanodomains wherein atomic lattice fringes are clearly observable, indicate the presence of 8 ACS Paragon Plus Environment

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a

c

b

d

Fig 2. Bright-field TEM images of (a) the 60mm AL sample and (b) HR-TEM close-up of a “cauliflower” particle showing many crystalline nanodomains indicated with red circles. The inset shows the electron diffraction rings, indicating presence of a-Si. The images were recorded with a cubed, aberration-corrected Titan microscope operating at 300 kV. c) HAADF-STEM image and (d) SE-STEM image of the sample with 60 mm aggregation distance.

nanocrystalline domains. 9 ACS Paragon Plus Environment


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Additional electron diffraction (ED) patterns (Fig 2b)) show not also diffraction spots but also the presence of rings, which can be attributed to the presence of amorphous Si (a-Si) domains. Therefore, the “cauliflower” particle consists of crystalline and amorphous nanodomains which was confirmed by Raman analysis (not shown), which revealed an a-Si fraction of 39%, 19% and 86% for the 20, 40 and 60 mm aggregation length respectively. Since the deposition with 60 mm aggregation distance has a very high a-Si content and yielded the highest number of “cauliflower“ particles, they are therefore mainly amorphous. The granular structure of the “cauliflower” nanoparticles becomes particularly pronounced when using high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and secondary electron STEM (SE-STEM) imaging (Figs. 2 C and D). Whether the c-Si domains are surrounded by a-Si layer or vise verse cannot be determined. The exact position of crystalline or amorphous domains in the “cauliflower” particles could not be resolved. . In Fig 3 the Tauc plot of the optical absorption versus photon energy is shown for a layer of particles deposited with 20 and 60 mm aggregation distance. In contrast to the Tauc plot of the particles deposited with 20 (Fig 3 A) and 40 mm (not shown) aggregation distance, this Tauc plot is likely the result of a combination of different phases, leading to a less linear relation. However, fitting of the approximately linear slope of this plot provides the band gap 10 ACS Paragon Plus Environment

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A

B

0,10

0,08

0,08

(αhv)0,5 (eV/cm)0.5

(αhv)0.5 (eV/cm)0.5

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0,06

0,06

0,04

0,04 0,02

0,02

0,00 1,4

1,6

1,8 2,0 hv (eV)

2,2

2,4

0,00

1,4

1,6

1,8 2,0 hv (eV)

2,2

Fig. 3 Tauc plot of silicon nanoparticles on glass produced with a cluster source aggregation distance of A) 20 mm B) 60 mm.

of the semiconductor. With a layer thickness of 90 +/- 20 nm the band gaps for Si particles deposited with a 20, 40 and 60 mm aggregation distance are 1.40±0.06, 1.57±0.03, 1.50±0.22 eV respectively. This agrees well with the band gap of a-Si (1.5-1.6 eV) although a small effect of c-Si (1.1 eV22) could be responsible for the slightly lower value. The presence of quantum confined c-Si is likely causing the band gap to shift to higher energies, which is particularly prominent for the silicon particles produced with a 40 mm aggregation distance and only 19% a-Si fraction. In Fig 4 A the photo luminescence results of Si particles deposited with an aggregation distance of 60 mm are shown. The photo luminescence of the silicon particles produced with



10,0 PL Intensity (a.u.)

A

7,5 5,0 2,5 0,0 400

500

600 700 Wavelength (nm)

800

5,0

B

PL Intensity (a.u.)

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2,5

0,0 400

500

600 700 Wavelength (nm)

800

Fig. 4 Emission spectrum of A) silicon particles produced with 60 mm aggregation length, B) Glass reference (gray) and a-Si on glass reference. All samples were excited at 365nm. The a-Si signal is indicated with the arrows.


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the other aggregation lengths are similar and therefore not shown. By comparing this spectrum with the glass and a-Si layer reference photo luminescence spectra in Fig 4 B, the peak positions which correspond with a-Si are at 513 and 630 nm (arrows). This photoluminescence identification follows from the photo luminescence emission at this wavelength of the a-Si reference sample, which does not contain crystalline silicon. Although in general the photoluminescence of a-Si is weak, this is in agreement with exciton recombination in the direct bad gap of a-Si23. The photoluminescence of defects at the Si-SiO2 interface24 are unlikely to be positioned at those wavelengths due to the relatively small surface to volume ratio for the reference a-Si film. No significant peak position shifts were observed with respect to the reference a-Si sample or as a function of aggregation length. This suggests that quantum confinement25

26

in these silicon nanoparticles is absent. Since the

luminescence of c-Si particles is expected in the red to infra-red part of the spectrum27, which is not observed in this experiment, the contribution of the small amount of c-Si may not be detectable in these particles. The photo luminescence spectrum of silicon particles in grease was similar to the spectra without grease and therefore not shown. The time resolved luminescence decay plots in Fig 5 A and B could be fitted with a bi-exponential decay function from which the decay times are extracted and shown in Table 1. The fast decay of about half a nanosecond at 515 nm corresponds to the minimum detector 13 ACS Paragon Plus Environment


A

emission at 630nm

PL Intensity (a.u.)

160 120 80 40 0 10

20 30 Lifetims (ns)

40

50

12

B

emission at 627nm

PL intensity (a.u.)

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8

4

0 0

2

4 6 Lifetime (µs)

8

10

Fig. 5 Time resolved photo luminescence recordings of A) silicon nanoparticles assembled on glass and B) silicon nanoparticles implanted in vacuum grease. For both graphs an aggregation distance of 60 mm of the cluster source was used. The excitation wavelength was 375 nm. The red solid line represents the bi-exponential fit. The lower amounts of counts in B) are caused by a smaller repetition rate due to the longer time scale. 14 ACS Paragon Plus Environment

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response as seen from the glass reference sample. A decay time at 515 nm of several nanoseconds for silicon particles deposited on glass was observed, which is in agreement with previous reports on a-Si nanoparticles which are in close proximity28. The lifetime at the 630 nm emission is somewhat larger as compared to the 515 nm emission. This is in agreement with the different transition probabilities for the two levels. These short lifetimes of several nanoseconds are in accordance with the lifetime of a-Si nanoparticles29. The excited state lifetime of a-Si particles is much shorter than for c-Si nanoparticles, which can be about 10 µs at room temperature. By depositing the silicon particles directly in vacuum grease smeared on a glass substrate the luminescence life time was extended markedly by about a 1000 times (Table 1). The first decay has a lifetime of about 200 ns while the second decay has a lifetime of about a microsecond. Due to the vacuum grease the silicon nanoparticles are separated from each other in contrast to deposition on glass, where they from a film of assembled particles. The vacuum grease itself gave no detectable photoluminescence signal. The separation of nanoparticles in vacuum grease reduces energy transfer between the particles and therefore confines the excitons to a single particle. By reducing the exciton path length, the probability of encountering recombination centers is reduced considerably, thereby extending its lifetime. Therefore the micro second life time corresponds to that of a single isolated particle containing mainly a-Si. It is an indirect proof that agglomeration of the 15 ACS Paragon Plus Environment


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Aggregation

τ1 (515nm)

τ 2 (515nm)

τ1 (630nm)

τ 2 (630nm)

τ1 (627nm) τ 2 (627nm)

length

(ns)

(ns)

(ns)

(ns)

(ns)

(ns)

40mm

0.51 ± 0.01

2.8 ± 0.1

0.47 ± 0.03

17.3 ± 3.1

191 ± 54

1256 ± 175

60mm

0.75 ± 0.01

3.6 ± 0.1

0.76 ± 0.03

5.3 ± 0.7

166 ± 16

1280 ± 84

20mm

0.36 ± 0.01

2.3 ± 0.1

-

-

292 ± 86

1483 ± 151

Glass ref.

1.04 ± 0.03

5.4 ± 0.4

-

-

Table 1 Photo luminescence decay times as obtained from a bi-exponential fit of the time resolved luminescence measurements on silicon particles deposited with different aggregation lengths. The first 4 columns are lifetimes for particles deposited on glass while the last two columns are silicon particles deposited in vacuum grease. The excitation wavelength was 375 nm.

silicon particles in grease is much less. Particularly the non-radiative recombination at surface defects are responsible for the reduction in radiative life time30. Recent work demonstrated the existence

of

dark

silicon

particles

in

which

excitons

preferentially

recombine

non-radiatively 31 . By localizing an exciton, it will not be trapped in such dark silicon nanoparticles and therefore also has a longer lifetime. It is unlikely that the vacuum grease matrix itself affects the

luminescent decay time as it consists only of stable silicones and 16 ACS Paragon Plus Environment

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silicon oxide, which will not interact strongly with the particles.

Although the main

bottleneck in quantum dot photovoltaics is charge carrier extraction, the extended lifetime of electron-hole pairs in silicon particles is interesting since a longer lifetime creates more opportunity for the charge carriers to become converted to electrical energy. Concluding, silicon nanoparticles are produced with a gas aggregation cluster source, which often adopt a “cauliflower” morphology. TEM analysis showed that the “cauliflower” consists of small silicon particles which are aggregated but did not merge before landing. Raman spectroscopy, photo absorption and photoluminescence measurements show the presence of mainly a-Si nanodomains. By preventing aggregation of the nanoparticles, which is achieved by immersing the silicon nanoparticles in vacuum grease, the photoluminescence lifetime is increased by 3 orders of magnitude due to the localization of the exciton. By preventing energy transfer and at the same time providing electric contact to harvest charge carriers a more efficient silicon quantum dot solar cells can be fabricated.

Acknowledgement

Discussions with Andries Meijerink and Alfons van Blaaderen are very much appreciated. The authors thank Maarten Dorenkamper from ECN for Raman assistance. 17 ACS Paragon Plus Environment


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