ODMR Spectra Research Articles

NV-center distribution and ODMR spectra in two types of natural diamonds

Two types of natural diamonds were investigated by means of PL scanning and ODMR. One was irradiated by neutrons to create NV-centers and the second wasn’t specifically treated. The pattern of NV-center distribution was found such that NV-centers in untreated diamond are localized in a special direction, supposedly sliding planes. Furthermore, the spin Hamiltonian parameters E and D of NV-center ground state, calculated by ODMR spectra in untreated diamond, depend on the surface point.

High growth rate synthesis of diamond film containing perfectly aligned nitrogen-vacancy centers by high-power density plasma CVD

Diamond film sensors with large volumes that are synthesized by microwave plasma chemical vapor deposition (MPCVD) and contain perfectly aligned nitrogen-vacancy (NV) ensembles are promising material for achieving highly sensitive quantum magnetometers. The step-flow growth mode of MPCVD on diamond (111) surface is required to realize aligned NV center ensembles. However, it is difficult to control the growth mode on diamond (111) surface due to the presence of twins and the conventional growth rate of aligned NV center ensembles is lower than 0.5 μm/h. In this study, we achieved a high growth rate (6.6 μm/h) of diamond (111) film containing perfectly aligned NV ensembles with high contrast (30%) by applying high power density plasma (103 W/cm3) and precisely controlling the gas flow rate ratios of CH4/H2 and N2/CH4. The growth mode was controlled to realize step flow growth by inducing the hydrogen etching of the nucleation with decreasing the CH4/H2. It was considered that increasing the microwave power density produces a large amount of atomic hydrogen and carbon precursor, leading to a high growth rate of perfectly aligned NV center diamond film. By controlling the N2/CH4 ratio, the nitrogen density in the diamond film could be precisely controlled to obtain a high contrast. Moreover, we measured the quality of the obtained diamond sensor films using the ODMR spectra and Ramsey sequence. We confirmed a high Rabi contrast (approximately 30%) and no significant decrease in T2∗ due to the deterioration of the crystallinity of the diamond film was observed. This result is promising for building material technology for highly sensitive quantum sensors with large sensor volumes.

Characterization of strong NV− gradient in the e-beam irradiated diamond sample

We report on studies of a diamond sample with a strong gradient of NV− concentration (between 4 and 36 ppm over 1.5 mm) created by inhomogeneous irradiation with an electron beam. The sample was characterized by spatially resolved luminescence and ODMR spectra and spatial distributions of NV− concentration, strain splitting, and relaxation times T1, T2. Such inhomogeneous sample may find applications in designing optimum conditions for numerous applications in spectroscopy and sensorics.

Imaging of stray magnetic field vectors from a magnetic particle with an ensemble of nitrogen-vacancy centers in diamond

We report the imaging of stray magnetic field vectors from a magnetic micro particle structure in which the magnetization direction is obscured by harnessing an ensemble of spins of nitrogen-vacancy (NV) centers in diamond. By virtue of spin-dependent fluorescence (optically detected magnetic resonance, ODMR) and the Zeeman effect, a variety of ODMR spectra of NV spins at local sites around a Nd–Fe–B magnetic micro particle are obtained. With four different spectra arising from the symmetry in the tetrahedral diamond structure and deconvoluted from the ODMR spectrum and subtractive fluorescence image, the stray magnetic field vectors and the magnetization direction of the magnetic structure are recovered uniquely in three dimensions from many possible candidates. A method for determining the polarity of magnetization of the magnetic particle by applying different polarities of an external magnetic field along the and diamond lattice directions is introduced.

Multi-frequency ODMR of Nitrogen-Vacancy Color Centers in Diamond Crystals in zero magnetic fields

Techniques of optical detection of magnetic resonance, as applied tothe nitrogen-vacancy center in diamond, provide a tool for controlling both electron and nuclear spins. These techniques generally use a combination of microwave (MW) drive field for electronic spin transitions and radiofrequency (RF) drive field for nuclear spin transitions. They are most effective in strong magnetic fields where levels anticrossing (LAC) occurs; therefore NV spin system dynamics is mostly studied under these conditions. Nevertheless, zero-field LAC can probably also be used for controlling electron and nuclear spin states as well for exciting narrow resonances for metrological application, and these perspectives are not yet properly investigated. Here we present two-quantum resonances appearing in ODMR spectra of NV centers in bulk diamond under multi-frequency (MW+RF) resonance excitation in zero and ultra-low magnetic fields, and discuss their specifics.

Peculiarities of optical and ODMR Spectra of Nitrogen–Vacancy Color Centers in Diamond Crystals

The optical and ODMR spectra of nitrogen vacancy (NV) centers in diamond were investigated as applied to the development of micro-scale diamond magnetometer sensor. It is shown that the NV0 center (unlike the NV− center) has an absorption line in the UV range. The inversion of the zero phonon line of the NV− center was observed and explained. ODMR signals in high magnetic fields (as compared with inner fields in the crystal) as well as in weak fields also were investigated. We propose the solution to the problem of the magnetometer sensitivity reduction at low magnetic field, that is of interest for quantum magnetometry applications.

Spin-dependent recombination of defects in bulk ZnO crystals and ZnO nanocrystals as studied by optically detected magnetic resonance

Abstract Long-lasting (several hours) afterglow at helium temperatures was observed in ZnO nanocrystals, 36 nm in size, after switching off the UV excitation. A quenching of the afterglow intensity in the magnetic field was found, which appears because of the preferential orientation of the recombining spins along the field and the spin-dependence of the recombination. ODMR was detected as resonance increase in the afterglow intensity at saturation of the EPR transitions of defects, which take part in the recombination. Shallow donors and different types of acceptors responsible for the afterglow in Al doped ZnO nanocrystals were identified. ODMR of donors and Li acceptors was recorded via the afterglow of bulk ZnO:Li, which could last for about an hour. In the photoluminescence-detected ODMR of bulk ZnO:Li crystals exchange-coupled donor–acceptor pairs were observed in addition to the Li acceptor and donor signals. Exchange-coupled pairs dominated the ODMR spectra recorded on the photoluminescence of ZnO nanocrystals.

Photophysics of Sulfur-Containing Centers on AgBr Surfaces

Silver halide microcrystals, the basic element in photographic products, are commonly reacted with labile sulfur-containing compounds to increase their photographic efficiency (speed). Such AgBr microcrystals have been examined with computational and photophysical techniques. Density functional and lattice-relaxation computations have given estimates of the optical transition energies and the electron- and hole-trapping abilities of a number of small sulfur-containing clusters that may be deposited on, or buried in, an AgBr surface. These data indicate that single sulfides, whatever their structure, are not deep traps for electrons or holes. Disulfides, in general, can trap holes and certain structures; for example, a disulfide at a double-kink site can trap electrons. Computations indicate that single sulfides substituted for a halogen in the (001) surface and charge compensated with an interstitial silver ion in the surface or in the bulk do not trap electrons; neither do disulfides in the bulk. A single sulfide substituted for a bromide at a positive kink and compensated with an interstitial silver ion appears to be capable of initiating the formation of small, stable silver clusters (Ag n , n 4) that become latent image, the centers that eventually lead to a visible photographic image. Photographic data from sulfur-sensitized AgBr microcrystals indicate that the optimum formal sulfur level is about 10 000 S atoms/μm 2 . Radio-frequency photoconductivity measurements show that deposited sulfur centers trap photoelectrons. The optimum and higher levels of sulfur deposited on the surface of AgBr microcrystals produce an absorption band (∼525 nm) and associated low-temperature luminescence (∼725 nm). The luminescence lifetime behavior and ODMR data clearly indicate that this sulfur-related absorption and emission is due to a donor-acceptor processes. ODMR spectra obtained by monitoring the intrinsic AgBr luminescence (590 nm) have a resonance that is assigned to a Ag 2 + center. This Ag 2 + species is thought to arise from a silver dimer associated with a sulfur species at the surface.

Electron (hole) paramagnetic resonance of spherical CdSe nanocrystals

A new mechanism of electron paramagnetic resonance in spherical zinc-blende semiconductor nanocrystals, based on the extended orbital motion of electrons in the entire nanocrystal, is presented. Quantum confinement plays a crucial role in making the resonance signal observable. The mechanism remains operative in nanocrystals with uniaxially distorted shape. A theoretical model based on the proposed mechanism is in good quantitative agreement with unusual ODMR spectra observed in nearly spherical CdSe nanocrystals.

A 95 GHz ODMR study of AgCl nanocrystals embedded in crystalline KCl matrix

The W-band (95 GHz) ODMR spectra of AgCl nanocrystals (NCs) embedded in a KCl single crystal lattice have been measured for various orientations of the magnetic field in a {100} crystal plane. Only self-trapped exciton (STE) signals are observed while no ODMR is found from distant electron-hole pairs. This confirms the conclusion of a previous ODMR study in the Q-band (at 35 GHz) concerning the spatial confinement effect and impurity exclusion on exciton recombination in NCs of AgCl. Through increased resolution, more accurate g- and zero-field splitting values of the triplet state were obtained in good agreement with those of the STE in bulk AgCl.

Resonant nonradiative energy transfer to erbium ions in amorphous hydrogenated silicon

The simultaneous study of absorption, luminescence, and ODMR spectra of erbium doped a-Si:H and a SiCx:H alloy reveals that Er3+ ions are pumped by a resonant but nonradiative energy transfer from electron–hole pairs excited in the host. Direct optical pumping into absorption lines of Er3+ is not observed. The emission of the Er3+ ions is strong and decreases only moderately from 77 K to room temperature. We propose an energy transfer by a Förster mechanism, based on resonant dipole coupling, which quenches efficiently the luminescence of the host in the case of large erbium concentration. Resonance of electron–hole pairs to the excited state of the rare earth ion is achieved as electrons thermalize in tail states.

Optically detected magnetic resonance studies of CdS nanoparticles grown in phosphate glass

CdS nanoparticles, with an average diameter between 2.3 and 5.5 nm, were grown in phosphate glasses. The photoluminescence of the aforementioned materials showed a broad band, Stokes shifted from the corresponding absorption edge by about 1.2 eV and blue shifted upon reduction of the particle size. The ODMR spectra of the discussed materials are composed of three resonance signals. Theoretical simulation of the latter spectra, utilizing phenomenological spin Hamiltonian, revealed that the luminescence band is associated with a recombination between trapped electrons and holes, at asymmetric sites, more likely located at the surface of the nanoparticles. Moreover, this simulation showed a distribution of electron–hole distances around the circumference of the nanoparticles.

Effects of film order on the optical properties of α-sexithiophene

The optical properties of ordered and disordered α-sexithiophene (α-6T) thin films are investigated by a variety of spectroscopies. The dominant low-lying even and odd parity excitons are identified by absorption and electroabsorption spectroscopies. Intermolecular interactions in ordered α-6T films results in Davydov splitting of the absorption band of the lowest odd parity exciton. Charged excitations in α-6T films were studied by photoinduced absorption and optically detected magnetic resonance. Both polarons and bipolarons are observed in disordered samples. As a consequence of higher carrier mobilities, only bipolarons were observed in ordered samples. Spin pairing of excitations broadens the observed ODMR spectra.

Electron paramagnetic resonance of deep boron in silicon carbide

In this article we report the first EPR observation of deep boron centres in silicon carbide. A direct identification of the boron atom involved in the defect centre, considered as deep boron, has been established by the presence of a hyperfine interaction with and nuclei in isotope-enriched 6H-SiC:B crystals. Deep boron centres were shown from EPR spectra to have axial symmetry along the hexagonal axis. A correspondence between the EPR spectra and the luminescence, ODMR and DLTS spectra of deep boron centres has been indicated. The structural model for a deep boron centre as a boron - vacancy pair is presented and the evidence for bistable behaviour of deep boron centres is discussed.

Optical Absorption, Luminescence, and UV-Excited Optically Detected Magnetic Resonance (UV-ODMR) Study of Poly(P-Phenyleneethynyleneaniline) (PPEA) Derivatives

Abstract The absorption, photoluminesence (PL), and X-band (9.35 GHz) ODMR spectra of several PPEA derivative films is described and discussed. The absorption of the solutions and films peak at 390–415 and 415–422 nm, resp. The intense PL of the solutions and films peak at 425–450 and 450–490 nm, resp. Interestingly, the vibronic structure of the PL of the solutions is clearer than that of the films. The ODMR excited at 254 ≤ Λex ≤ 400 nm includes the following familiar features: (i) a narrow polaron resonance at g ≊ 2.002 and (ii) full- and half-field triplet exciton powder patterns. However, the widths of the polaron and full-field triplet powder pattern resonances are ∼30 and ∼1500 G, resp., as compared to 10–15 and 600–900 G, resp., of other π-conjugated polymers. The results are discussed in relation to structural disorder and defects in these polymers.

A method of altering the modulation type and frequency of commercial ESR spectrometers

A strategy of altering the modulation type and frequency of commercial continuous wave ESR spectrometers without changes in the main console is discussed. A simple converter unit, permitting the use of various external receivers, widening the possibilities for the basic commercial set-up is suggested. Block diagrams for altering the modulation frequency and type are presented and the use of boxcar averaging systems for the time-resolved experiments is discussed in detail. As an example of a successful application of this method, some results obtained with two different RYDMR accessories, based on the commercial CW ESR spectrometers, are discussed. X-band OD ESR studies of hydrocarbon solutions are illustrated by the spectra of p-xylene and trans-phenyl-propene radical cations, and methyl acrylate and trans-azabenzene radical ions, detected in situ under X-irradiation. Semiconductor studies are illustrated by CW and time-resolved X-band ODMR spectra of PGa-P3Y point defects as well as its OD ENDOR and ENDOR-induced ODMR spectra.

Fine Structure of the Triplet Relaxed Excited State of Ga+ Centres in Alkali Chlorides Induced by the Off‐Centre Position of the Excited Ga+ Ion

AbstractThe luminescence of a RbCl:Ga crystal is investigated by the methods of time‐resolved polarization spectroscopy in the temperature range 0.09 to 400 K. Manifestations of an off‐centre displacement of an excited Ga+ ion and its tunnelling motion are found in the decay kinetics of the triplet AT emission of the Ga+ centre. No such manifestations are observed for the Ax emission. A comparison of luminescence characteristics and peculiarities of the triplet relaxed excited state (RES) structure of the Ga+ centre in RbCl:Ga crystal with those of NaCl:Ga and KCl:Ga crystals is performed. The fine structure of the tetragonal Jahn‐Teller minima of the triplet RES of Ga+ centres in RbCl:Ga is found to be analogous to that of KCl:Ga: the metastable state is split due to tunnelling transitions between various equivalent off‐centre positions of the excited Ga+ ion, but in the emitting state the spin splitting caused by the off‐centre displacement of Ga+ ion considerably dominates over the tunnel splitting of this state. Basing on the analysis of the ODMR spectra as well as of the circular polarization of Ga+ triplet luminescence and its decay kinetics, the assumption is made that the tunnel splitting energy of the triplet metastable state of Ga+ centres is less than 10−5 eV.

Optical, zero-field ODMR and EPR studies of the triplet states from singlet fission in biphenyl-TCNQ and biphenyl-tetrafluoro-TCNQ charge-transfer crystals

Biphenyl-TCNQ and biphenyl-tetrafluoro-TCNQ crystals, when excited with visible light, give rise to triplet excitons which were investigated by optical, EPR and zero-field (ZF) ODMR spectroscopies in a wide temperature range. The analysis of spin polarization carried by EPR and ZF ODMR spectra at room temperature enabled us to reveal the singlet depletion path S→ (T, T) which is thermally activated. The activation energies are Δ E = 1940 cm −1 for B-TCNQ and Δ E = 1720 cm −1 for B-TCNQF 4. From the relation 2 E T= E S +Δ E the energies of the triplet state in the CT complexes were derived: E T=9770 ± 100 cm −1 for B-TCNQ and E T=8695 ± 100 cm −1 for B-TCNQF 4. It was also established that the triplet species detected by ODMR at 1.8 K are populated by ISC.

Time-Resolved Polarization Spectroscopy and Off-Centre Position of Excited Ga+ Ions in NaCl:Ga Crystal

In the temperature region from 0.35 to 4.2 K the decay kinetics of the triplet AT and Ax emission of NaCl:Ga crystals as well as the spectral and polarization charcteristics of the single decay components of the emission are investigated. An off-centre displacement of the Ga+ ion in the tetragonal (T) minima of the relaxed excited state (RES) is found. No manifestations of the off-centre displacement of the Ga+ ion in the trigonal (X) minima are observed. It is shown that fast tunnelling transitions between various equivalent off-centre positions of an excited Ga+ ion in the plane normal to the tetragonal Jahn-Teller distortion axis lead to the tunnel splitting of the metastable as well as of the radiative T minima of the triplet RES of the Ga+ centre. It is evident from the existence of two slow and two fast components in the low-temperature decay kinetics of the AT emission. In case of NaCl:Ga the energy (δ) of tunnel splitting of the radiative minimum is assumed to be larger than the energy (2E) of the spin–orbit splitting of this minimum which is caused by the lowering of the RES symmetry due to the off-centre displacements of the Ga+ ion. As a result, off-centre effects are not observable in the ODMR spectra of Ga+ centres in NaCl:Ga crystals, unlike Ga+ centres in other alkali halides, where the energy δ is probably smaller than 2E. [Russian text ignoured]

Multiple localized exciton species in sulfied-doped AgBr detected by luminescence and ODMR

Abstract The low temperature photoluminescence of sulfide-doped AgBr has been examined with both steady state and pulsed excitation. Emission bands peaking at about 495, 566, 596, and 725 nm are observed, depending upon excitation conditions. Subband-gap excitation resonances at 464, 496, 530, and 558 nm are observed, depending upon monitoring wavelength. Excitation at 496 nm preferentially enhances 596 nm emission, which gives rise to an ODMR spectrum. ODMR data indicate that at least three species are contributing to the 596 nm emission. These species are interpreted as triplet excitons. A highly antisotropic species observed in the ODMR spectra is interpreted as a triplet exciton bound to a substitutional divalent sulfur ion/interstitial silver ion complex with approximate [111] symmetry.