Zero-phonon Line Research Articles

Local defects in two-dimensional gallium sulfide as potential single-photon emitters: First-principles evaluation

The single-photon emitters (SPEs) are essential components of emerging quantum communication technology. It has been shown that the local defects in the wide-bandgap semiconductors can be sources of sharp zero-phonon lines (ZPLs) of emission, which is favorable for applications. Most of the reported SPEs emit photons in the visible light region, while fiber-based quantum communication applications require sources of near-infrared (NIR) emission. Therefore, the purpose of this paper is to search for defects which are promising candidates for SPEs in the NIR region by applying computational methods. To this end, based on an educated guess, 12 local doping substitutional defects in two-dimensional (2D) gallium sulfide were selected for consideration. The selected defects are ${Y}_{X}$, where $X=\mathrm{Ga}$ or S, and $Y=\mathrm{C}$, Si, Ge, N, P, As. Here, $X$ stands for a substituted element and $Y$ for a dopant. To evaluate the stability of the defects, their formation energy, binding energy of the dopants, and phonon spectra were calculated within the density functional theory (DFT) approximation. Three defects, ${\mathrm{Ge}}_{\mathrm{Ga}}, {\mathrm{N}}_{\mathrm{Ga}}$, and ${\mathrm{P}}_{\mathrm{Ga}}$, are found to have imaginary-frequency vibration modes, indicating their dynamical instability. They were removed from further consideration. A preliminary analysis of the DFT densities of electronic states (DOS) revealed six remaining defects, ${\mathrm{As}}_{\mathrm{Ga}}, {\mathrm{Ge}}_{\mathrm{S}}, {\mathrm{Si}}_{\mathrm{S}}, {\mathrm{C}}_{\mathrm{S}}, {\mathrm{N}}_{\mathrm{S}}$, and ${\mathrm{P}}_{\mathrm{S}}$, with DOS favorable for formation of local optical excitations in the NIR region. Next, for these six defects, the state-of-the-art linear response GW and the Bethe-Salpeter equation (BSE) methods were used to obtain their electronic structures, in terms of the independent quasiparticle (IQP) states, and optical excitation spectra, in terms of the frequency-dependent dielectric function and the dipole optical transition oscillator strength. The calculation results indicate that the ${\mathrm{C}}_{\mathrm{S}}, {\mathrm{N}}_{\mathrm{S}}$, and ${\mathrm{P}}_{\mathrm{S}}$ defects have intense sharp excitation peaks in the NIR region. The analysis of the contributions of the IQP states to the corresponding BSE eigenstates brings us to the conclusion that these excitations can result in emission with dominating narrow ZPL peaks. I thus propose here the ${\mathrm{C}}_{\mathrm{S}}, {\mathrm{N}}_{\mathrm{S}}$, and ${\mathrm{P}}_{\mathrm{S}}$ defects in 2D GaS as promising candidates for SPEs in the NIR emission region.

A Comparative Study of the Band-Edge Exciton Fine Structure in Zinc Blende and Wurtzite CdSe Nanocrystals

In this paper, we studied the role of the crystal structure in spheroidal CdSe nanocrystals on the band-edge exciton fine structure. Ensembles of zinc blende and wurtzite CdSe nanocrystals are investigated experimentally by two optical techniques: fluorescence line narrowing (FLN) and time-resolved photoluminescence. We argue that the zero-phonon line evaluated by the FLN technique gives the ensemble-averaged energy splitting between the lowest bright and dark exciton states, while the activation energy from the temperature-dependent photoluminescence decay is smaller and corresponds to the energy of an acoustic phonon. The energy splittings between the bright and dark exciton states determined using the FLN technique are found to be the same for zinc blende and wurtzite CdSe nanocrystals. Within the effective mass approximation, we develop a theoretical model considering the following factors: (i) influence of the nanocrystal shape on the bright-dark exciton splitting and the oscillator strength of the bright exciton, and (ii) shape dispersion in the ensemble of the nanocrystals. We show that these two factors result in similar calculated zero-phonon lines in zinc blende and wurtzite CdSe nanocrystals. The account of the nanocrystals shape dispersion allows us to evaluate the linewidth of the zero-phonon line.

Evaluating the defects in CVD diamonds: A statistical approach to spectroscopy

Laboratory-grown diamonds produced by chemical vapor deposition (CVD) can have similar sizes and colors to natural diamonds, and their color can be enhanced by post-growth treatment. Despite considerable published research on CVD diamond growth, post-growth treatment remains difficult to detect in some CVD diamonds. To improve the detection of post-growth treatment, we apply multivariate statistics to photoluminescence (PL) emission spectra and surface fluorescence images from 11,606 gem-quality CVD diamonds within the D-Z color range. Unsupervised statistics indicate as-grown CVD diamonds commonly have 3H defects, with zero-phonon line (ZPL) at 503.5 nm, and relatively intense PL peaks at 467.6 nm. These features are absent or minor in diamonds treated under high-pressure, high-temperature (HPHT) conditions. HPHT-treated CVD diamonds tend to exhibit the H3 system (with ZPL at 503.2 nm) as well as larger 491.9 nm and 566.6 nm peak areas. A subset of CVD diamonds showing the H3 system could have experienced treatment at low-pressure high-temperature (LPHT) conditions (including inside the reactor during or after CVD growth).

Direct synthesis of group IV-vacancy center-containing nanodiamonds via detonation process using aromatic compound as group IV element source

Nanodiamonds (NDs) containing group IV-vacancy (G4V) centers—silicon-vacancy (SiV), germanium-vacancy (GeV) and tin-vacancy (SnV) centers—have shown promising potential as fluorescent markers for bioimaging and -sensing. However, the scale of fabrication has been limited to the laboratory scale. In this study, a detonation process was applied that enables practical scale fabrication of NDs for the direct synthesis of these G4V center-containing NDs (G4V-NDs). This detonation process for the direct synthesis of G4V-NDs employed explosives with the addition of dopant molecules with group IV atoms centered on tetraphenyl compounds. The successful synthesis of negatively charged SiV and GeV center-containing NDs (SiV- and GeV-NDs) was evidenced by photoluminescence (PL) spectra with zero-phonon lines (ZPLs) attributed to such color centers. However, as a result of the same strategy, NDs containing the SnV centers were not obtained in detectable concentrations in PL measurements. When the generated concentrations of SiV- and GeV-NDs synthesized under identical conditions were evaluated based on the number of data points that clear ZPLs were observed on the PL mappings, the SiV-NDs were found to be produced more predominantly than the GeV-NDs. The physics behind such results is explained by the difference in the reaction thermodynamics for each group IV atom. • Detonation process (DP) enables practical scale production of nanodiamonds (NDs). • Direct synthesis of NDs containing SiV or GeV centers was achieved by DP. • As Si or Ge sources in DP, atoms centered on tetraphenyl compounds were used. • SnV centers were not detected in NDs obtained via same scheme. • This is due to the difference in the reaction thermodynamics for each heteroatom.

Luminescent properties of a double-perovskite Rb2KScF6:Mn4+ red-emitting phosphor for white light-emitting diodes

Searching for efficient Mn4+ red phosphors for white lighting and display backlighting has drawn great attention. In this work, an efficient Mn4+-doped fluoroscandate, Rb2KScF6:Mn4+, with short fluorescence lifetime is prepared via a facile co-precipitation method. It was found that the nonequivalent occupation of Mn4+ for Sc3+ leads to the identifications of sharp red emissions, enhanced zero-phonon line, high quantum efficiency and short fluorescence lifetime upon blue light excitation. Thanks to the luminescent properties, good optical parameters of high color rendering index, low correlated color temperature, and acceptable color gamut were achieved by coating the red Rb2KScF6:Mn4+ phosphor with yellow Y3Al5O12:Ce3+ or green β-SiAlON:Eu2+ phosphor in various white light-emitting diodes, demonstrating its potential role for white light illumination and backlight display applications.

Cation-exchange reaction to prepare Na2WO2F4:Mn4+ with intense zero phonon line at 619 nm avoiding disproportionation reaction

We report that in the case of synthesizing the Na2WO2F4:Mn4+ red luminescent phosphor, the cation-exchange reaction is superior to a conventional co-precipitation reaction for a brighter luminescence, because it avoided the disproportionation reaction of K2MnF6. As evidenced by the UV–Vis transmittance spectra, during the co-precipitation reaction, the WO42− contained in Na2WO4 induced a catalytic effect on the disproportionation reaction of MnF62−, which led to valence change (from Mn4+ to Mn7+, Mn3+, etc.) and fluorescence quenching of Mn4+. Fluorination of Na2WO4 to get Na2WO2F4 for cation-exchange reaction with K2MnF6 under presence of HF solution was found to be effective in suppressing this disproportionation reaction. At optimized conditions, the phosphor prepared via the cation-exchange reaction shows a luminescence intensity 19-fold higher than that prepared via the co-precipitation method. The Na2WO2F4:Mn4+ obtained via cation-exchange reaction exhibits red luminescence peaking at 619 nm that is ascribed to the zero phonon line (ZPL) emission, and the ZPL to Stokes ν6 phonon sideband intensity ratio reached as high as 117.7%–138.2%.

Synthesis, crystal structure, electronic structure, and intensive zero-phonon-line emission of Mn4+ in K5Nb3OF18 lattice

Manganese ions-activated fluorides and oxyfluorides have found applications in solid state lighting and display, but the coordination environments of Mn4+ activator and their effects on the photoluminescence (PL) properties in some host lattices have remained unclear. In this study, microcrystal powders of oxyfluoride K5Nb3OF18 with and without Mn4+-doped were synthesized by using a coprecipitation method. The morphology, crystal structure, electronic structure of K5Nb3OF18 and the coordination environments and PL properties of Mn4+ activator in K5Nb3OF18:Mn4+ were investigated. It is revealed that K5Nb3OF18 crystallizes in the tetragonal structure with the space group I4cm. There are two crystallographic sites for Nb5+ ions in the lattice, both of which have C4v symmetry. Mn4+ ions-activated K5Nb3OF18 shows characteristic emission due to the transition from 2E to 4A2, while with intensive zero-phonon-line (ZPL). Structure analysis indicates that the local environments of Nb5+ ions in K5Nb3OF18 are more contracted and distorted than those in Rb5Nb3OF18. These features along with the non-centrosymmetric nature of the Nb5+ sites are considered to response for the intensive ZPL emission.

Up/Down-Scaling Photoluminescent Micromarks Written in Diamond by Ultrashort Laser Pulses: Optical Photoluminescent and Structural Raman Imaging.

Elongated photoluminescent micromarks were inscribed inside a IaAB-type natural diamond in laser filamentation regime by multiple 515 nm, 0.3 ps laser pulses tightly focused by a 0.25 NA micro-objective. The micromark length, diameter and photoluminescence contrast scaled as a function of laser pulse energy and exposure, coming to a saturation. Our Raman/photoluminescence confocal microscopy studies indicate no structural diamond damage in the micromarks, shown as the absent Raman intensity variation versus laser energy and exposition along the distance from the surface to the deep mark edge. In contrast, sTable 3NV (N3)-centers demonstrate the pronounced increase (up to 40%) in their 415 nm zero-phonon line photoluminescence yield within the micromarks, and an even higher-ten-fold-increase in NV0-center photoluminescence yield. Photogeneration of carbon Frenkel "interstitial-vacancy" (I-V) pairs and partial photolytic dissociation of the predominating 2N (A)-centers were suggested to explain the enhanced appearance of 3NV- and NV-centers, apparently via vacancy aggregation with the resulting N (C)-centers or, consequently, with 2N- and N-centers.

Computational Insights into Electronic Excitations, Spin-Orbit Coupling Effects, and Spin Decoherence in Cr(IV)-Based Molecular Qubits.

The great success of point defects and dopants in semiconductors for quantum information processing has invigorated a search for molecules with analogous properties. Flexibility and tunability of desired properties in a large chemical space have great advantages over solid-state systems. The properties analogous to point defects were demonstrated in the Cr(IV)-based molecular family, Cr(IV)(aryl)4, where the electronic spin states were optically initialized, read out, and controlled. Despite this kick-start, there is still a large room for enhancing properties crucial for molecular qubits. Here, we provide computational insights into key properties of the Cr(IV)-based molecules aimed at assisting the chemical design of efficient molecular qubits. Using the multireference ab initio methods, we investigate the electronic states of Cr(IV)(aryl)4 molecules with slightly different ligands, showing that the zero-phonon line energies agree with the experiment and that the excited spin-triplet and spin-singlet states are highly sensitive to small chemical perturbations. By adding spin-orbit interaction, we find that the sign of the uniaxial zero-field splitting (ZFS) parameter is negative for all considered molecules and discuss optically induced spin initialization via non-radiative intersystem crossing. We quantify (super)hyperfine coupling to the 53Cr nuclear spin and to the 13C and 1H nuclear spins, and we discuss electron spin decoherence. We show that the splitting or broadening of the electronic spin sub-levels due to superhyperfine interaction with 1H nuclear spins decreases by an order of magnitude when the molecules have a substantial transverse ZFS parameter.

Antisite Defect SiC as a Source of the DI Center in 4H‐SiC

DI center, as a widely existed defect in 4H‐SiC, has attracted much attention in recent years, while the origin of it is still unclear. Herein, by comparing first‐principles calculated potential point defects‐related zero‐phonon lines (ZPLs) and nonradiative capture cross‐sections with the DI center‐related values in the experiment, it is proposed that the transition from the bound exciton states about 57 meV below the CBM to the “+1/+2” level of antisite defect SiC is responsible for the DI center in 4H‐SiC. This work describes the temperature and transition energy level dependence of the hole capture cross‐section of antisite defect in 4H‐SiC, which provides an idea for the optimal design of point defects in semiconductor materials.

Tailored light emission from color centers in nanodiamond using self-assembled photonic crystals

The defect centers in solid-state materials especially the nitrogen-vacancy (NV) centers in diamond have shown a tremendous potential for their utilization in quantum technology applications. However, they exhibit certain drawbacks such as the feeble zero phonon line with huge phonon contribution and the higher lifetime values. Here, we present a novel approach to control the spontaneous emission from NV centers in nanodiamond using engineered self-assembled photonic crystals. Using two complimentary emission measuring geometries at room temperature, we show a 63% suppression and 17% enhancement of NV center emission intensity using photonic stopgap, supported with simulations. The emission rates are modified in a broad spectral range of NV center emission and are consistent with the Barnett–Loudon sum rule. The results are crucial for emerging quantum technologies using NV centers in diamond.

Bistable carbon-vacancy defects in h-BN

Single-photon emitters in hexagonal boron nitride have been extensively studied recently. Although unambiguous identification of the emitters is still under intense research, carbon-related defects are believed to play a vital role for the emitter producing zero-phonon lines in the range of 1.6–2.2 eV. In this study, we systematically investigate two configurations of carbon-vacancy defects, VNCB and CNVB, by means of density functional theory calculations. We calculated the reaction barrier energies from one defect to the other to determine relative stability. We find that the barrier energies are charge dependent, and CNVB could easily transform to VNCB in neutral- and positive-charge states while it is stable when negatively charged. Formation energy calculations show that the VNCB is the dominant defect over CNVB. However, neither VNCB nor CNVB has suitable fluorescence spectra that could reproduce the observed ones. Our results indicate that the origin of the 1.6-to-2.2-eV emitters should be other carbon-related configurations.

Tin disulfide with bright near-IR luminescence centers obtained at high pressures

In this work, the high-pressure and high-temperature (HPHT) method was applied to obtain crystalline tin disulfide (${\mathrm{SnS}}_{2}$). The synthesis was performed in a sulfur-tin-carbon growth system by placing the reaction mixture of sulfur and tin in a graphite capsule. The synthesized ${\mathrm{SnS}}_{2}$ demonstrated to be a highly crystalline $2H$-polytypic form suitable for mechanical exfoliation of atomically thin films. The crystals reveal strong near-IR photoluminescence attributed to a new luminescence center in ${\mathrm{SnS}}_{2}$. At 5 K, this center is characterized by a narrow zero-phonon line at 885 nm and phonon sideband whose structure is governed by ${\mathrm{SnS}}_{2}$ phonon density of states. Based on $\mathit{ab}$ initio calculations, the center is attributed to carbon in the tin sublattice. The side product of the HPHT synthesis is the diamond phase containing optically active tin-vacancy centers (${\mathrm{SnV}}^{\ensuremath{-}}$).

Stacking-dependent deep level emission in boron nitride

We report hyperspectral microscopy in ${sp}^{2}$-bonded boron nitride polytypes with a simultaneous inspection of the photoluminescence signal in the UV-C and UV-B spectral ranges. By comparing two different polytypes extracted from the same polycrystalline sample, we reveal that the well-known ``4 eV defect'' does not emit at the same energy for polytypes in the $AB$ and $A{A}^{\ensuremath{'}}$ stackings. The zero-phonon line is either at 4.14 or 4.16 eV for the noncentrosymmetric $AB$ stacking, instead of the usual 4.09 eV energy in the $A{A}^{\ensuremath{'}}$ stacking. Our results open the way for different characterization methods of the stacking order in ${sp}^{2}$-bonded boron nitride, and bring alternative inputs for the elucidation of the atomistic configuration of points defects by advanced ab initio calculations.

Exhaustive characterization of modified Si vacancies in 4H-SiC

Abstract The negatively charged silicon vacancy V S i − $\left({\mathrm{V}}_{\mathrm{S}\mathrm{i}}^{-}\right)$ in silicon carbide is a well-studied point defect for quantum applications. At the same time, a closer inspection of ensemble photoluminescence and electron paramagnetic resonance measurements reveals an abundance of related but so far unidentified signals. In this study, we search for defects in 4H-SiC that explain the above magneto-optical signals in a defect database generated by automatic defect analysis and qualification (ADAQ) workflows. This search reveals only one class of atomic structures that exhibit silicon-vacancy-like properties in the data: a carbon antisite (CSi) within sub-nanometer distances from the silicon vacancy only slightly alters the latter without affecting the charge or spin state. Such a perturbation is energetically bound. We consider the formation of V S i − + C S i ${\mathrm{V}}_{\mathrm{S}\mathrm{i}}^{-}+{\mathrm{C}}_{\mathrm{S}\mathrm{i}}$ up to 2 nm distance and report their zero phonon lines and zero field splitting values. In addition, we perform high-resolution photoluminescence experiments in the silicon vacancy region and find an abundance of lines. Comparing our computational and experimental results, several configurations show great agreement. Our work demonstrates the effectiveness of a database with high-throughput results in the search for defects in quantum applications.

Structural stability and optical properties of Ni complex defects in diamond

Structural stability of defect complexes composed of several Ni atoms and C vacancies in diamond are studied by first-principles calculations. Results show that Ni atoms prefer accumulating with C vacancies, forming Ni-Ni complex defects, rather than to be isolated as separated point defects. Focusing on complex defects consisting of one or two Ni, the electronic structures associated with the defects near the band gap are analyzed, and their optical properties including zero-phonon line energies are discussed by comparing with experimental results.

All‐Optical Thermometry with NV and SiV Color Centers in Biocompatible Diamond Microneedles (Advanced Optical Materials 15/2022)

Biocompatible single-crystal diamond needles enriched with nitrogen and silicon vacancy (NV and SiV) color centers in a controllable manner via modified chemical vapor deposition synthesis are employed by Lena Golubewa, Polina Kuzhir and co-workers (article number 2200631) for improved all-optical thermometry based on the temperature dependence of SiV zero-phonon-line (ZPL) peak position, FWHM, photoluminescence lifetime, and ratiometrically determined ZPL intensity. Simultaneous spectrally separated bioimaging with both color centers is performed.

All-optical thermometry using a single multimode fiber endoscope and diamond nanoparticles containing nitrogen vacancy centers.

Photoluminescence (PL) spectra from diamond nanoparticles containing negative nitrogen vacancy centers were measured by using a single multimode fiber endoscope combined with a high-sensitivity spectroscopy system. A laser light spot was produced at the distal end of the endoscope and the PL spectra from a temperature-controlled ensemble of diamond nanoparticles were measured. After calibrating the sensitivity and wavelength of the spectroscopy system, the temperature dependence of the zero-phonon line peak wavelength similar to those previously reported was obtained.

Site-splitting inhibition and near-infrared luminescence properties of Cr3+ activated magnetoplumbite SrAl12O19 modified by La and Mg

Near-infrared phosphor-converted light-emitting diodes (pc-LEDs) are attractive in application like in vivo imaging and nondestructive examination in food industry. Plenty of aluminium oxides with high melting point and rigidity have been researched towards high photoluminescence quantum yield and thermal stability. In this work, a series of Cr3+ activated La, Mg modified SrAl12O19 phosphors, i.e. Sr0.7La0.3Mg0.3Al11.7O19, are prepared via a solid-state reaction route. The site splitting and structural evolution towards relative thermodynamic stability in the modified host are discussed, Cr3+ ions with multisite occupation in the host show intense zero-phonon-line (ZPL) emission and tunable broadband emission. The optimized phosphor exhibits excellent luminescence thermal stability, with 98% residual integrity intensity of that at room temperature at 473 K. The results provide a reference for developing novel NIR-emitting magnetoplumbite phosphor with high efficiency and excellent thermal quenching resistance.

Detection of Single W-Centers in Silicon

Controlling the quantum properties of individual fluorescent defects in silicon is a key challenge towards advanced quantum photonic devices prone to scalability. Research efforts have so far focused on extrinsic defects based on impurities incorporated inside the silicon lattice. Here we demonstrate the detection of single intrinsic defects in silicon, which are linked to a tri-interstitial complex called W-center, with a zero-phonon line at 1.218${\mu}$m. Investigating their single-photon emission properties reveals new information about this common radiation damage center, such as its dipolar orientation and its photophysics. We also identify its microscopic structure and show that although this defect does not feature electronic states in the bandgap, Coulomb interactions lead to excitonic radiative recombination below the silicon bandgap. These results could set the stage for numerous quantum perspectives based on intrinsic luminescent defects in silicon, such as quantum integrated photonics, quantum communications and quantum sensing.