ARTICLE pubs.acs.org/JPCC
Formation of Single-Crystal Spherical Particle Architectures by Plasma-Induced Low-Temperature Coalescence of Silicon Nanocrystals Synthesized by Laser Ablation in Water Vladimir Svrcek,*,† Davide Mariotti,‡,§ Keerti Kalia,§ Calum Dickinson,^ and Michio Kondo† †
Research Center for Photovoltaics, National Institute of Advanced Industrial Science and Technology (AIST), Central 2 1-1-1 Umezono, Tsukuba, Ibaraki, 305 6568, Japan ‡ Nanotechnology & Advanced Materials Research Institute, University of Ulster, Shore Road, Newtownabbey, BT37 0QB, United Kingdom § Department of Microelectronic Engineering, Rochester Institute of Technology, 82 Lomb Memorial Drive, Rochester, New York 14623, United States ^ Materials & Surface Science Institute, University of Limerick, Limerick, Ireland ABSTRACT: We report that the synthesis of silicon nanocrystals (SiNCs) by laser ablation in water produces unique surface characteristics and, in particular, hydroxyl-terminated surfaces, which can induce coalescence and formation of micrometer-sized single-crystal Si spherical particles under a low-temperature (∼550 C) plasma process. We demonstrate that the spherical particles can be self-organized into aggregates that extend with varying gas concentrations. At the same time, SiNCs that were sufficiently apart not to coalesce have shown peculiar photoluminescence properties, which suggest an increased tunneling probability from self-trapped excitonic surface states.
1. INTRODUCTION Si-based technologies have greatly contributed to the past progress of our society, and today, we continue to borrow and improve silicon technological know-how to overcome the new global challenges (e.g., efficient solar energy harvesting). More recently, silicon has also demonstrated its potential in future nanotechnology applications with a range of nanostructures that possess advantageous physical and chemical characteristics.19 The unique properties that originate from quantum confinement effects and from surface characteristics suggest the possibility of utilizing SiNCs as functional building blocks for optoelectronic and photovoltaic applications in many diverse ways.1016 Another aspect that could play an interesting role in novel nanodevice design is the geometry, whereby spherical structures might offer new approaches to advanced nanoscale devices. For example, photovoltaic devices might gain from a spherical geometry where light could be absorbed at any angle and where multiple reflection paths could enhance the overall efficiency.17,18 Therefore, solar cell architectures that exploit SiNCs’ advantageous properties are very appealing and currently under intensive research. Furthermore, SiNCs may also have a significant impact in the fabrication processes of solar cell technologies that are closer to the marketplace. This is because SiNCs can be used as seeds or nucleation sites for rapid and low-temperature crystallization of amorphous silicon (a-Si) to form the active layer of microcrystalline solar cells. The biphasic a-Si/SiNCs system tends to crystallize when annealed, r 2011 American Chemical Society
and thanks to the presence of nanosized Si clusters, no energy is required to overcome the nucleation barrier. This SiNC-seeded approach to microcrystalline solar cells, which is based on the solid-phase crystallization (SPC) mechanism, is, therefore, very advantageous and can reduce fabrication costs while increasing throughput. However, one of the crucial items in SiNC research is represented by the surface characteristics that are strongly affecting our ability to accurately measure and assess SiNCs' properties. For example, stable and intense photoluminescence in the green-blue region has been elusive for sometime due to surface and defect states.19 Surface characteristics that are fueling a vibrant debate on SiNCs' optical properties20 may also strongly affect the interactions of SiNCs with various processing environments. This latter aspect is less frequently considered in relation to nanoparticle surface composition; however, it is very important to allow integration of SiNCs in application devices. Some very interesting experimental and computational work has been carried out, for instance, to elucidate sintering, coalescence, and oxidation mechanisms of hydrogen-terminated SiNCs at different temperatures where it was found that surface hydrogen determines coalescence temperatures21 and can prevent oxygen passivation.22 It is clear that different termination types as well as Received: November 30, 2010 Revised: March 10, 2011 Published: March 23, 2011 6235
dx.doi.org/10.1021/jp111387q | J. Phys. Chem. C 2011, 115, 6235–6242
The Journal of Physical Chemistry C diverse processing conditions (e.g., plasmas) will need to be further investigated. Here, we present our observations on plasma and heat treatment of a particular type of SiNC produced by laser ablation in water. The characteristic laser processing in liquid induces unique surface passivation with OH terminations that may provide SiNCs with different characteristics compared to the more common H-terminated SiNCs. Furthermore, we have exposed the laser-produced SiNCs to a low-pressure plasma treatment (30 mTorr, C2H2/H2) that promotes low-temperature SiNC coalescence and the formation of large micrometersized single-crystal spherical particles arranged in aggregated patterns. However, due to the distribution of SiNCs on the substrates, it is observed that some SiNCs do not coalesce and instead undergo modification of the surface characteristics with optical properties that may reveal to be beneficial for potential PV applications. Both SiNCs' surface characteristics and plasma conditions seem to play a crucial role either in activating the coalescence process or in determining the SiNCs' photoluminescence (PL) characteristics.
2. EXPERIMENTAL SECTION The synthesis of SiNCs is achieved by laser ablation in water of a Si wafer using a nanosecond pulsed excimer laser (KrF, 245 nm, 20 Hz).23 The silicon wafer (p-type boron doped, Æ100æ) is placed at the bottom of a glass container and immersed in 10 mL of water. The laser beam is focused onto a 1.5 mm diameter spot on the wafer surface by a lens. The ablation process is continued for 2 h at room temperature and ambient pressure. The type of surface terminations of the SiNCs was analyzed by Fourier transform infrared (FTIR) spectroscopy. The SiNCs were drop-cast on silicon samples and left to dry. The measurements were taken in ratio mode with a PerkinElmer Spectrum 2000 using as reference a section from the same silicon wafer where the SiNCs were deposited. The range of 3707800 cm1 was scanned with a resolution of 1 cm1 and 0.2 cm1 steps. Measurements were repeated several times. Following laser ablation, SiNCs were stored in water. After about 4 weeks, a drop of the SiNCwater dispersion was left to dry on a silicon substrate in ambient conditions and exposed to ambient air. The sample was then treated with a commercial parallel-plates rf-driven plasma (Drytek 482 Quad Etcher). The plasma process included a 7 min pretreatment in hydrogen gas (30 mTorr) without plasma and at a substrate temperature of 550 C. Following, acetylene was introduced in the chamber and the plasma process was initiated at the same pressure and with the same substrate temperature. The plasma power was set at 200 W, and the process was continued for 15 min. Different samples were treated with a different gas mixture; specifically, the gas concentration of C2H2/H2 was varied (1:9, 2:8, and 3:7). The system was then cooled in an Ar atmosphere at 50 mTorr for 10 min. Transmission electron diffraction and micro-Raman were employed to perform structural analysis. Raman spectra were measured on a Renishaw inVia Raman microscope using the 633 nm line of a HeNe laser as the excitation source. The focusing spot size was about 1 μm, and back scattering light was collected. The samples were kept in air during the Raman spectral measurements. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were taken with a Hitachi S-4300 microscope at 20 kV and a JEOL
ARTICLE
JEM-22011 with a LaB6 filament at 200 kV acceleration voltages, respectively. Energy-dispersive X-ray spectroscopy (EDS) was employed to determine local elemental concentrations. For the temperature dependence of the PL, a He:Cd laser has been used. The excitation laser intensity was approximately ∼70 mW/cm2. The samples were placed in the cryostat with a varying temperature from 4 to 300 K.
3. RESULTS AND DISCUSSION Silicon generally presents hydrophobic surfaces. Most SiNC synthesis techniques are also producing SiNCs with a hydrophobic character, and their dispersion in aqueous solutions is rather difficult.24 The synthesis of SiNCs by laser ablation in water induces surface passivation and reduces the surface zeta potential, allowing easy dispersion of the SiNCs in water.23 Therefore, compared to other techniques, laser ablation in water offers the possibility of synthesizing SiNCs with unique surface characteristics. Furthermore, SiNCs produced with this technique are sufficiently small (
