BMe Research Grant
BMe Research Grant - 2015
Silicon Carbide1 (SiC) is an excellent material known as semiconductor2 that may substitute silicon in high power electronics  and as ceramic3 with the hardness of diamond . SiC shows new properties in nano size where quantum confinement4 takes place. The surface of SiC nanocrystal acts like an organic compound while the core crystal becomes an excellent luminescence source opening the door for new generations of luminescent dyes5. I synthesize these nanocrystals and study their chemical and physical properties. My aim is to better understand the physics behind these nanoclusters and produce the best possible dyes for in vivo bioimaging by changing the physical and chemical properties of the material.
My PhD work has been carried out as a member of Prof. Adam Gali's group at Wigner Research Centre6. Wigner RCP was founded by uniting two research institutes of the Hungarian Academy of Science, namely the Research Institute for Particle and Nuclear Physics and the Research Institute for Solid State Physics and Optics. Our group’s7 research focuses on the theoretical and experimental characterization of silicon carbide based nanoclusters.
Visual analysis of biological systems is an integral avenue of basic and applied biological research. Fluorescence microscopy techniques are often used for studying intracellular mechanisms . The importance of this method is clearly demonstrated by the 2014 Nobel Prize8 in chemistry that was awarded for ultra high resolution microscopy9 , but fluorescence dyes have been used to label cancer cells10 during surgery, too [5a,b].
There are groups that already synthesized SiC nanocrystals [6a,b] and studied their physiological properties , however, the initial methods were not suitable to prepare SiC nanoclusters of molecular size and provide the narrow size distribution required for biological applications and to understand their optical properties.
My research goal is to synthesize SiC nanocrystals that are suitable for in-vivo11 and in-vitro bioimaging. To this end, nanocrystals should be biocompatible, sufficiently small for clearance  after the treatment and applicable in aqueous environment. While most dyes are toxic , the biocompatibility of SiC has been known for a long time. The first task was to find proper synthesis method capable of producing SiC nanocrystals in large amount with narrow size distribution. The origin of luminescence has to be clarified, too. A further aim is to make the emission independent of size by the inclusion of luminescence centers12. Certain luminescence centres in SiC are single photon sources which renders our nanocrystals proper candidates for quantum information processing as well.
Figure 1.: vacancies in SiC
In my research work several experimental methods were used. For producing SiC nanocrystals top-down methods and bulk SiC have to be used. Development of bulk SiC synthesis is crucial for luminescence centres too. The applied method, volume synthesis13 provides a fast reaction between silicon and carbon .
To produce 1–4 nm SiC a different chemical reaction is required. Although SiC is a very resistant material, wet chemical etching method [6b], applying with a mixture of hydrofluoric acid and nitric acid at 100 °C is applicable for SiC..
Figure 2: Synthesis of SiC nanocrystals
For further measurements, SiC nanocrystals are dispersed in a suitable solvent. Optical properties of the colloid sol14 are determined by fluorescence spectrometer15 and infrared spectroscopy16 was used for analysing their surface chemistry.
Figure 3: Infrared spectrum of SiC nanocrystals
I joined Gali’s group in the initial phase of experimental research. My first task was the elaboration of synthesis method for SiC nanocrystals at our Institute. As a start, a synthesis route was developed based on literature by applying closed reaction chamber, similar to what is used at hydrothermal synthesis [B5] These development make the production safer, ensured higher yield and better quality [B5]. The size of our nanocrystal is about 1–3 nm. This narrow size distribution is very rare in top-down methods17.
Synthesis of bulk SiC was also developed by applying changes to the existing furnace system and to the reaction route. These developments enable us to synthesize SiC nanocrystals with better quality [B3] and synthesize SiC nanocrystals of about 100 nm in size containing red single photon centres [B2] in high density18.
In the case of the smallest, 1–3 nm nanocrystals the luminescence is complex mechanism where quantum confinement and surface chemistry jointly determine the emission. The energy of emitted photon is highly influenced by the oxygen containing surface groups as was proven both experimentally and theoretically [B1]. The effect of different surface groups (carboxyl, carbonyl, hydroxyl, etc) was also studied by making different surface terminated SiC nanocrystals. Using time dependent fluorescence spectroscopy19 an increase in the emission wavelength with increasing reduction degree of the surface was identified, too. Another emission centre was also found that was due to the Si-O bonds on the surface.
Figure 4.: Emission vs. surface at SiC nanocrystals.
A small step toward to application was taken by measuring the toxicity of the nanocrystals and using SiC as fluorescence probe for two-photon imaging [B4].
Figure 5.: SiC nanocrystal labeled neuron cell under two-photon microscope.
Although we have already proven the applicability of SiC nanocrystals in biological systems, the real break-through would be the creation of SiC nanoclusters with red-infrared emission. To this end, we have to synthesize red emission centre-containing SiC nanocrystals, but we also have to optimize the surface of the nanoparticles. Currently, we can create SiC nanocrystals with 620 nm emission. The next step is to analyse this emission centre and continue development for more optimal synthesis. Further investigation of the connection between surface chemistry and luminescence and the identification of the new emission centre would realize a new, SiC-based dye and nano-sensor family which, thanks to the low toxicity and clearance of SiC, may open new perspectives in biology and quantum computing.
[B1]. Beke D., Szekrényes Zs., Czigány Zs., Kamarás K., Gali A., Dominant Luminescence is not Due to Quantum Confinement in Molecular Sized Silicon Carbide Nanocrystals, Nanoscale, 2015, DOI: 10.1039/C5NR01204J
[B2] Castelletto S., Johnson B, Zachreson C., Beke D., Balogh I., Ohshima T. Aharonovich I., Gali A., Room Temperature Quantum Emission from Cubic Silicon Carbide Nanoparticles, ACS Nano, 8(8), 7938, (2014)
[B3] Szekrényes Zs., Somogyi B., Beke D., Károlyházi Gy., Balogh I., Kamarás K., Gali A., Chemical Transformation of Carboxyl Groups on the Surface of Silicon Carbide Quantum Dots, Journal of Physical Chemistry C – Nanomaterials and Interfaces, 118(34), 19995, (2014)
[B4] Beke D., Szekrényes Zs., Pálfi D., Róna G., Balogh I., Maák P.A., Katona G., Czigány Zs., Kamarás K., Rózsa B., Buday L., Vértessy B., Gali A., Silicon carbide quantum dots for bioimaging, Journal of Materials Research, 10(28), 205 (2013)
[B5] Beke D, Szekrényes Z, Balogh I, Czigány Z, Kamarás K, Gali A., Preparation of small silicon carbide quantum dots by wet chemical etching, Journal of Materials Research, 28(1), 44 (2013).
[B6] Beke D, Szekrenyes Zs, Balogh I, Veres M, Fazakas E, Varga L.K, Kamaras K, Czigany Zs, Gali A., Characterization of luminescent silicon carbide nanocrystals prepared by reactive bonding and subsequent wet chemical etching, Applied Physics Letters, 99(21) 213108. (2011)
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