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Figure 1. Potential energy surface of the ground and excited states of OV(0).
The different configura-tions of the defect are de-picted in (a-e). (f) depicts the one-electron orbitals participating in this excita-tion process. An electron is promoted from the 1a' level to 3a'. (g) shows the ground state geometry in a different orientation. If the defect is excited at the 1.72 eV resonance, the sys-tem non-radiatively relaxes back through an intersyssys-tem crossing to its ground state without an emission of a photon. While the ground state of OV(0) is very similar to that of the well-known NV(-) system, the excitation process above is very different from that of NV(-) according to our results.
Figure 2. Photoemission spec-trum (PES) of adamantane:
theory and experiment. Our theoretical method includes the electron-electron correla-tion with the so-called GW calculation. In the ionization process, we also include elec-tron-phonon interaction in order to properly describe the dynamic Jahn-Teller or polaronic nature of an ionized adamantane molecule. This effect is depicted on the left, if an electron is removed from the system, the atoms start vibrating. We could reproduce the ionization threshold at 9 eV, as well as the overall lineshape is fully ab initio - no empirical factors from experiments are used. The right panel depicts the structure of the adamantane molecule for which the PES was calculated.
Significant results have been achieved in the research of solid-state quantum bits, which are the building blocks of a future implementation of the quantum computer. A prominent candidate is the so-called divacancy defect in silicon carbide which has a high-spin ground state. This electron spin may interact with the nearby nuclear spins in the lattice that can naturally occur in SiC. We developed a detailed theory on the optical dynamic spin polarization of the nuclear spins driven by the coherent control of the electron spins of the point defect. Our simulations unraveled that certain nuclear spins can be optically spin-polarized at a given direction depending on the magnitude of a small external magnetic field, thus a bidirectional spin-polarization can be achieved without the need of radiofrequency excitation of the nuclear spins. The proof-of-principle measurement was carried out for proximate nuclear spins by Awschalom group at Chicago University for which theory predicted 25% spin inversion probability at a certain magnitude of the external
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magnetic field (see Figure 3). These results suggest the incorporation of optical dynamic spin polarization techniques into future quantum information processing and quantum sensing protocols. We contributed to the characterization of nitrogen-vacancy defect in hexagonal SiC, that might be a near-infrared (NIR) counterpart of the famous nitrogen-vacancy center in diamond that operates rather in the visible region.
Figure 3. Experimental and theoretical 29Si nuclear spin polarization and ODMR spectrum of the mS = 0 to mS = +1 spin transition of PL6 qubits in 4H-SiC at the GSLAC region. (a) The measured (points) and calculated (thick line) magnetic field dependence of the nuclear spin polarization of a 29Si nucleus at the SiIIb site. DNP is highly efficient up until the LAC-c, at which point it exhibits a sharp drop and reversal. Measurements are carried out at room temperature. (b) The experimental low-microwave-power ODMR spectrum. The measurements are carried out on an ensemble of PL6 divacancy-related qubits in 4H-SiC at room temperature. f0 = f0(B) describes the zero-field-splitting and Zeeman shift of the mS = +1 spin state. (c) Theoretical simulation of the ODMR spectrum which takes into account the DNP of the 29Si nucleus at the SiIIb site and the microwave transition strength in the mS =
|0,+1> manifold. The green ellipsoids on (b) and (c) highlight the signs of the nuclear spin polarization reversal.
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. Such systems have been developed so far, but most of them are either unstable or toxic, thus they are not suitable for therapy. Our Momentum Semiconductor Nanostructures Research Group is, however, seeking such solutions that can be applied in vivo. Molecular-sized colloid SiC nanoparticles (NP) are very promising candidates to realize bioinert
non-B [Gauss]
P
-0.2 0 0.2 0.4 0.6 0.8 1 1.2
465 470 475 480 485 490 495
B [Gauss] B [Gauss]
f – f0 [MHz] 5
4 3 2 1 0
f – f0 [MHz]
(b) (c)
ΔPL/PL [a.u.]
(a)
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perturbative fluorescent nanoparticles for in vivo bioimaging. Fluorescent water-soluble silicon carbide (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. Understanding the fluorescence of this complex system 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. We identified 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 (see Figure 4). 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.
Figure 4. Surface- and environment-dependent luminescence of SiC NPs.
The combination of advanced time-resolved spectroscopy and ab initio simulations, aided by surface engineering, is able to identify the luminescence centers of complex systems. Using such a method, SiOx -defect-related color centers (pink regions on NPs) at the surface of SiC NPs have been identified. From the experimental data, it is possible to build a framework for the surface-related luminescence which can describe the connection between luminescence and surface chemistry.
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)
Visegrad Group (V4) + Japan Joint Research Project on Advanced Materials: Nanophotonics with metal - group IV-semiconductor nanocomposites: From single nanoobjects to functional ensembles (NaMSeN), 2016-2018
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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), Prof. W. J. Choyke experimental group, SiC (nano)particles University of Linköping (Sweden), Prof. Erik Janzén experimental group, point defects in SiC Harvard University (USA), Prof. Michael Lukin experimental group, defects for quantum computing
University of Chicago (USA), Prof. David D. Awschalom experimental group, SiC defects for quantum computing
University of Stuttgart (Germany), Prof. Jörg Wrachtrup experimental group, defects for quantum computing
University of Ulm (Germany), Prof. Fedor Jelezko experimental group, defects for quantum computing
Hasselt University (Belgium), Prof. Milos Nesladek experimental group, defects in diamond Kaunas University of Technology (Lithuania), Dr. Audrius Alkauskas theoretician group, defects in diamond and SiC
University of Erlangen-Nürnberg (Germany), Dr. Michel Bockstedte theoretician group, defects in diamond and SiC
University of Kobe (Japan), Prof. Minoru Fujii experimental group, Si nanoparticles Charles University (Czechia), Prof. Jan Valenta experimental group, Si nanoparticles
Slovakian Academy of Sciences (Slovakia), Prof. Ivan Stich theoretician group, quantum Monte Carlo methods in Si nanoparticles
Warsaw University of Technology (Poland), Prof. Romuald B. Beck experimental group, Si layers and devices
University of Mainz (Germany), Prof. Dmitrii Budker experimental group, diamond defects University of Saarland (Germany), Prof. Christoph Becher experimental group, diamond defects
Publications
Articles
1. Beke D, Jánosi TZ, Somogyi B, Major DÁ, Szekrényes Zs, Erostyák J, Kamarás K, Gali A:
Identification of luminescence centers in molecular-sized silicon carbide nanocrystals.
J PHYS CHEM C 120:(1) 685-691 (2016)
2. Csóré A, Gällström A, Janzén E, Gali A: Investigation of Mo defects in 4H-SiC by means
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of density functional theory. MATER SCI FORUM 858: 261-264 (2016)
3. Gali A, Demján T, Vörös M, Thiering G, Cannuccia E, Marini A: Electron-vibration coupling induced renormalization in the photoemission spectrum of diamondoids.
NAT COMMUN 7: 11327/1-9 (2016)
4. Gruznev DV, Bondarenko LV, Matetskiy AV, Mihalyuk AN, Tupchaya AY, Utas OA, Eremeev SV, Hsing C-R, Chou J-P, Wei C-M, Zotov AV, Saranin AA: Synthesis of two-dimensional Tlx Bi1-x compounds and Archimedean encoding of their atomic structure.
SCI REP-UK 6: 19446/1-9 (2016)
5. Ivády V, Szász K, Falk AL, Klimov PV, Janzén E, Abrikosov IA, Awschalom DD, Gali A:
First principles identification of divacancy related photoluminescence lines in 4H and 6H-SiC. MATER SCI FORUM 858: 322-325 (2016)
6. Ivády V, Klimov PV, Miao KC, Falk AL, Christle DJ, Szász K, Abrikosov IA, Awschalom DD, Gali A: High-fidelity bidirectional nuclear qubit initialization in SiC. PHYS REV LETT 117:(22) 220503/1-6 (2016)
7. Ivády V, Szász K, Falk AL, Klimov PV, Christle DJ, Koehl WF, Janzén E, Abrikosov IA, Awschalom DD, Gali A: Optical nuclear spin polarization of divacancies in SiC. MATER SCI FORUM 858: 287-290 (2016)
8. Jarmola A, Bodrog Z, Kehayias P, Markham M, Hall J, Twitchen DJ, Acosta VM, Gali A, Budker D: Optically detected magnetic resonances of nitrogen-vacancy ensembles in
13C-enriched diamond. PHYS REV B 94:(9) 094108/1-5 (2016)
9. Lohrmann A, Johnson BC, Almutairi AFM, Lau DWM, Negri M, Bosi M, Gibson BC, McCallum JC, Gali A, Ohshima T, Castelletto S: Engineering single defects in silicon carbide bulk, nanostructures and devices. MATER SCI FORUM 858: 312-317 (2016) Proc. Conf. Silicon Carbide and Related Materials 2015.
10. Norambuena A, Reyes SA, Mejía-Lopéz J, Gali A, Maze JR: Microscopic modeling of the effect of phonons on the optical properties of solid-state emitters. PHYS REV B 94:(13) 134305/1-8 (2016)
11. Thiering G, Gali A: Characterization of oxygen defects in diamond by means of density functional theory calculations. PHYS REVB 94:(12) 125202/1-15 (2016)
12. von Bardeleben H J, Cantin J L, Csóré A, Gali A, Rauls E, Gerstmann U: NV centers in 3C,4H, and 6H silicon carbide: A variable platform for solid-state qubits and nanosensors. PHYS REV B 94:(12) 121202/1-6 (2016)
See also: S-Q.1
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