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polarization derives from hyperfine-mediated level anticrossings. These results lay a foundation for SiC-based quantum memories, nuclear gyroscopes, and hyperpolarized probes for magnetic resonance imaging. The study is featured in Physics as Viewpoint and highlighted as Editor's suggestion in Physical Review Letters. We further developed a theory about the dynamic nuclear spin polarization of defects in solids that we published in Physical Review B.
In addition, our ab initio calculations predicted that the so-called carbon antisite-vacancy defect could have a high-spin ground state when it is neutral in the 4H polytype of silicon carbide. Furthermore, we proposed what conditions are needed to observe this defect and realize a new solid state qubit. The results were published in Physical Review B as Rapid Communication. We also worked together with scientists at Melbourne University: they fabricated a diode from SiC where single-photon emitters are engineered into this junction.
These single-photon emitters can be driven electrically to realize a quantum light-emitting diode operating at room temperature. We developed a model by simulations for the origin of these emitters that is a combination of stacking faults with a point defect (silicon antisite).
These results were published in Nature Communications.
Figure 2. (a) Calculated spin density on a divacancy defect in 4H-SiC Ground state at so-called hh configuration. (b) The scheme of level anti-crossing is shown for the ground and excited states. The dynamic spin polarization of the nuclear spin represented by an arrow is shown for (c) excited-state branches and (d) ground-state branches, respectively.
Biologists urgently need biomarker systems which trace, e.g., cancer cells in the blood stream or provide fluorescent signals depending on the activity of neurons in brain. Although such systems have already been developed, but most of them are either unstable or toxic, thus they are not suitable for therapy. Our “Lendület” Semiconductor Nanostructures Research Group is, however, seeking such solutions that can be applied in vivo. Molecular-sized colloidal SiC nanoparticles are very promising candidates to realize bioinert non-perturbative fluorescent nanoparticles for in vivo bioimaging. Furthermore, SiC nanoparticles with engineered vacancy-related emission centres may realize magneto-optical probes operating at nanoscale resolution. Understanding the nature of molecular-sized SiC nanoparticle emission is essential for further applications. Our group members at Wigner ADMIL laboratory have succeeded to develop an efficient and simple method to produce a relatively narrow size distribution of water-soluble molecular-sized SiC nanoparticles. The tight control of their size distribution makes it possible to demonstrate a switching mechanism in the luminescence
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correlated with particle size. We show that molecular-sized SiC nanoparticles of 1–3 nm show a relatively strong and broad surface-related luminescence whilst the larger ones exhibit a relatively weak band edge and structural defect luminescence with no evidence of quantum confinement effect (see Figure 3). These results were published in Nanoscale.
Figure 3. (a) PL spectra of the sample (filtrate) at different excitation wavelengths after SiC NCs were filtered through a 30 kDa centrifuge filter. After filtration, the red shoulder does not occur in the PL spectra. (b) AFM image and size distribution of the sample. The average size is about 1.5 nm and most of the particles are smaller than 4 nm.
Understanding the fluorescence of complex systems such as small nanocrystals with various surface terminations in solution is still a scientific challenge. We showed that the combination of advanced time-resolved spectroscopy and ab initio simulations, aided by surface engineering, is able to identify the luminescence centers of such complex systems.
Fluorescent water-soluble SiC nanocrystals have been previously identified as complex molecular systems of silicon, carbon, oxygen, and hydrogen, held together by covalent bonds that made the identification of their luminescence centers unambiguous. The aqueous solutions of molecular-sized SiC nanocrystals are exceedingly promising candidates to realize bioinert nonperturbative fluorescent nanoparticles for in vivo bioimaging, and thus the identification of their luminescent centers is of immediate interest. We presented identification of two emission centers of this complex system: surface groups involving carbon−oxygen bonds and a defect consisting of silicon−oxygen bonds that becomes the dominant pathway for radiative decay after total reduction of the surface. The identification of these luminescent centers reconciles previous experimental results on the surface- and pH-dependent emission of SiC nanocrystals and helps design optimized fluorophores and nanosensors for in vivo bioimaging (see Figure 4). The results were published in The Journal of Physical Chemistry C.
Finally, we mention that the Wigner ADMIL infrastructure could be significantly developed by the installation of a new induction generator and heat chamber for alloying new materials and annealing. The investment was financed by the Infrastructure program of the Hungarian Academy of Sciences.
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Figure 4. Schematic diagram showing the surface- and environment-dependent luminescence of SiC nanocrystals
Grants
OTKA No. K101819: Design, fabrication and analysis of luminescent silicon carbide nanocrystals for in vivo biomarker applications (Adam Gali, 2012-2016)
OTKA No. K106114: Development of novel silicon carbide nanomarkers and more effective glutamate and GABA uncaging materials for measurement of neuronal network activity and dendritic integration with three-dimensional real-time two-photon microscopy (Adam Gali, 2012-2016)
EU FP7 No. 611143: DIADEMS-Diamond devices enabled metrology and sensing (Adam Gali, 2013-2017)
International cooperation
Pontificia Universidad Católica de Chile (Santiago de Chile, Chile), Biophysics with color centers in diamond and related materials (J. R. Maze)
RMIT (Melbourne, Australia), Color centers in SiC nanoparticles for bioimaging (S. Castalletto) University of Melbourne (Melbourne, Australia), Single photon emitters in SiC devices (B.C.
Johnson)
University of Pittsburgh (USA), SiC (nano)particles (Prof. W. J. Choyke) University of Linköping (Sweden), point defects in SiC (Prof. Erik Janzén)
Harvard University (USA), defects for quantum computing (Prof. Michael Lukin)
University of Chicago (USA), SiC defects for quantum computing (Prof. David D. Awschalom) University of Stuttgart (Germany), defects for quantum computing (Prof. Jörg Wrachtrup) University of Ulm (Germany), defects for quantum computing (Prof. Fedor Jelezko) Hasselt University (Belgium), defects in diamond (Prof. Milos Nesladek)
Kaunas University of Technology (Lithuania), defects in diamond and SiC (Dr. Audrius Alkauskas)
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University of Erlangen-Nürnberg (Germany), defects in diamond and SiC (Dr. Michel Bockstedte)
University of Kobe (Japan), Si nanoparticles (Prof. Minoru Fujii)
Charles University (Czech Republic), Si nanoparticles (Prof. Jan Valenta)
Slovakian Academy of Sciences (Slovakia), quantum Monte Carlo methods in Si nanoparticles (Prof. Ivan Stich)
Warsaw University of Technology (Poland), Si layers and devices (Prof. Romuald B. Beck) University of Mainz (Germany), diamond defects (Prof. Dmitrii Budker)
University of Saarland (Germany), diamond defects (Prof. Christoph Becher)
Publications
Articles
1. Beke D, Szekrényes Zs, Czigány Zs, Kamarás K, Gali Á: Dominant luminescence is not due to quantum confinement in molecular-sized silicon carbide nanocrystals. NANOSCALE 7:(25) pp. 10982-10988. (2015)
2. Falk AL, Klimov PV, Ivády V, Szász K, Christle DJ, Koehl WF, Gali Á, Awschalom DD: Optical Polarization of Nuclear Spins in Silicon Carbide. PHYS REV LETT 114:(24) Paper 247603.
6 p. (2015)
3. Fashandi H, Ivády V, Eklund P, Spetz AL, Katsnelson MI, Abrikosov IA: Dirac points with giant spin-orbit splitting in the electronic structure of two-dimensional transition-metal carbides. PHYS REV B 92:(15) Paper 155142. 9 p. (2015)
4. Gällström A, Magnusson B, Leone S, Kordina O, Son NT, Ivády V, Gali Á, Abrikosov IA, Janzén E, Ivanov IG: Optical properties and Zeeman spectroscopy of niobium in silicon carbide. PHYS REV B 92:(7) Paper 075207. 14 p. (2015)
5. Ivády V, Szász K, Falk AL, Klimov PV, Christle DJ, Janzen E, Abrikosov IA, Awschalom DD, Gali Á: Theoretical model of dynamic spin polarization of nuclei coupled to paramagnetic point defects in diamond and silicon carbide. PHYS REV B 92:(11) Paper 115206. 18 p. (2015)
6. Kürti J, Koltai J, Gyimesi B, Zólyomi V: Hydrocarbon chains and rings: bond length alternation in finite molecules. THEOR CHEM ACC 134:(10) Paper 114. 8 p. (2015) 7. Lohrmann A, Iwamoto N, Bodrog Z, Castelletto S, Ohshima T, Karle TJ, Gali Á, Prawer S,
McCallum JC, Johnson BC: Single-photon emitting diode in silicon carbide. NAT COMMUN 6: Paper 7783. 10 p. (2015)
8. Stacey A, O'Donnell KM, Chou J-P, Schenk A, Tadich A, Dontschuk N, Cervenka J, Pakes Ch, Gali Á, Hoffman A, Prawer S: Nitrogen Terminated Diamond. ADV MATER INTERFACES 2:(10) Paper 1500079. 6 p. (2015)
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9. Szász K, Ivády V, Abrikosov IA, Janzén E, Bockstedte M, Gali Á: Spin and photophysics of carbon-antisite vacancy defect in 4H silicon carbide: A potential quantum bit. PHYS REV B 91:(12) Paper 121201. 5 p. (2015)
10. Thiering G, Gali Á: Complexes of silicon, vacancy, and hydrogen in diamond: A density functional study. PHYS REV B 92:(16) Paper 165203. 15 p. (2015)
11. Widmann M, Lee S-Y, Rendler T, Son NT, Fedder H, Paik SY, Yang L-P, Zhao N, Yang S, Booker I, Denisenko A, Jamali M, Momenzadeh SA, Gerhardt I, Ohshima T, Gali Á, Janzén E, Wrachtrup J: Coherent control of single spins in silicon carbide at room temperature.
NAT MATER 14: pp. 164-168. (2015)
12. Zólyomi V, Kurti J: Towards improved exact exchange functionals relying on GW quasiparticle methods for parametrization. PHYS REV B 92:(3) Paper 035150. 8 p. (2015)
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