One-step synthesis of photoluminescent nanofluids by direct loading of reactively sputtered cubic ZrN nanoparticles into organic liquids

ZrN nanofluids may exhibit unique optoelectronic properties because of the matching of the solar spectrum with interband transitions and localized surface plasmon resonance (LSPR). Nevertheless, these nanofluids have scarcely been investigated, mainly because of the complexity of the current synthetic routes that involve aggressive chemicals and high temperatures. This work aims to validate reactive dc magnetron sputtering of zirconium in Ar/N2 as an environmentally benign, annealing-free method to produce 22 nm-sized, highly crystalline, stoichiometric, electrically conductive, and plasmonic ZrN nanoparticles (NPs) of cubic shape and to load them into vacuum-compatible liquids of different chemical compositions (polyethylene glycol (PEG), paraffin, and pentaphenyl trimethyl trisiloxane (PTT)) in one step. The nanofluids demonstrate LSPR in the red/near-IR range that gives them a bluish color in transmittance. The nanofluids also demonstrate complex photoluminescence behavior such that ZrN NPs enhance the photoluminescence (PL) intensity of paraffin and PEG, whereas the PL of PTT remains almost invariable. Based on DFT calculations, different energetic barriers to charge transfer between ZrN and the organic molecules are suggested as the main factors that influence the observed optoelectronic response. Overall, our study provides a novel approach to the synthesis of transition metal nitride nanofluids in an environmentally friendly manner, deepens the understanding of the interactions between ZrN and organic molecules, and unveils new optoelectronic phenomena in such systems.

Light emission dynamics of silicon vacancy centers in a polycrystalline diamond thin film

Diamond thin films can be, at a relatively low-cost, prepared with a high-density of light-emitting negatively charged silicon vacancy (SiV) centers, which opens up the possibility of their application in photonics or sensing. The films are composed of diamond grains with both the SiV centers and sp²-carbon phase, the ratio of these two components being dependent on the preparation conditions. The grain surface and the sp²-related defects might act as traps for the carriers excited within the SiV centers, consequently decreasing their internal photoluminescence (PL) quantum efficiency. Here, we show that in a 300 nm thick polycrystalline diamond film on a quartz substrate, the SiV centers in the diamond grains possess similar temperature-dependent (13-300 K) PL decay dynamics as the SiV centers in monocrystalline diamond, which suggests that most of the SiV centers are not directly interconnected with the defects of the diamond thin films, i.e. that the carriers excited within the centers do not leak into the defects of the film. The activation energy ΔE = 54 meV and the attempt frequency α = 2.6 were extracted from the measured data. These values corresponded very well with the published values for SiV centers in monocrystalline diamond. We support this claim by measuring the transient absorption via a pump and probe technique, where we separated the nanosecond recombination dynamics of carriers in SiV centers from the picosecond decay dynamics of polycrystalline diamond defects. Our results show that PL emission properties of SiV centers in polycrystalline diamond thin films prepared via chemical vapor deposition are very similar to those in monocrystalline diamond thereby opening the door for their application in diamond photonics and sensing.

Photonic crystal cavity-enhanced emission from silicon vacancy centers in polycrystalline diamond achieved without postfabrication fine-tuning

Diamond optical centers have recently emerged as promising single-photon sources for quantum photonics. Particularly, negatively charged silicon vacancy (SiV-) centers show great promise due to their narrow zero-phonon emission line present also at room temperature. However, due to fabrication tolerances it is challenging to prepare directly photonic structures with optical modes spectrally matching the emission of SiV- centers. To reach the spectral overlap, photonic structures must typically undergo complicated post-processing treatment. In this work, suspended photonic crystal cavities made of polycrystalline diamond are engineered and more than 2.5-fold enhancement of the SiV- center zero-phonon line intensity via coupling to the cavity photonic mode is demonstrated. The intrinsic non-homogeneous thickness of the diamond thin layer within the sample is taken as an advantage that enables reaching the spectral overlap between the emission from SiV- centers and the cavity modes without any post-processing. Even with lower optical quality compared to monocrystalline diamond, the fabricated photonic structures show comparable efficiency for intensity enhancement. Therefore, the results of this work may open up a promising route for the application of polycrystalline diamond in photonics.

Random temporal laser speckles for the robust measurement of sub-microsecond photoluminescence decay

Time-resolved photoluminescence (PL) is commonly used to track dynamics in a broad range of materials. Thus, the search for simplification of the acquisition of PL kinetics attracts continuous attention. This paper presents a new robust and straightforward approach to the measurement of PL decay, which is based on randomly fluctuating excitation intensity. The random excitation waveform is attained by using laser speckles generated on a rotating diffuser. Owing to this, the presented technique is able to utilize any coherent excitation source without the necessity to generate short pulses or to controllably modulate the light. PL decay can be computationally reconstructed from the Fourier image of the PL trace. The paper demonstrates the performance of the method, which is able to acquire sub-microsecond dynamics as the impulse response function reaches 300 ns. The reconstructed PL decays were compared to streak camera measurements to verify the method. Finally, potential limitations and applications of the technique are discussed.

Charge Carrier Injection Electroluminescence with CO-Functionalized Tips on Single Molecular Emitters

We investigate electroluminescence of single molecular emitters on NaCl on Ag(111) and Au(111) with submolecular resolution in a low-temperature scanning probe microscope with tunneling current, atomic force, and light detection capabilities. The role of the tip state is studied in the photon maps of a prototypical emitter, zinc phthalocyanine (ZnPc), using metal and CO-metal tips. CO-functionalization is found to have an impact on the resolution and contrast of the photon maps due to the localized overlap of the p-orbitals on the tip with the molecular orbitals of the emitter. The possibility of using the same CO-functionalized tip for tip-enhanced photon detection and high resolution atomic force is demonstrated. We study the electroluminescence of ZnPc, induced by charge carrier injection at sufficiently high bias voltages. We propose that the distinct level alignment of the ZnPc frontier orbitals with the Au(111) and Ag(111) Fermi levels governs the primary excitation mechanisms as the injection of electrons and holes from the tip into the molecule, respectively. These findings put forward the importance of the tip status in the photon maps and contribute to a better understanding of the photophysics of organic molecules on surfaces.

Two-dimensional photonic crystals increasing vertical light emission from Si nanocrystal-rich thin layers

We have fabricated two-dimensional photonic crystals (PhCs) on the surface of Si nanocrystal-rich SiO₂ layers with the goal to maximize the photoluminescence extraction efficiency in the normal direction. The fabricated periodic structures consist of columns ordered into square and hexagonal pattern with lattice constants computed such that the red photoluminescence of Si nanocrystals (SiNCs) could couple to leaky modes of the PhCs and could be efficiently extracted to surrounding air. Samples having different lattice constants and heights of columns were investigated in order to find the configuration with the best performance. Spectral overlap of the leaky modes with the luminescence spectrum of SiNCs was verified experimentally by measuring photonic band diagrams of the leaky modes employing angle-resolved spectroscopy and also theoretically by computing the reflectance spectra. The extraction enhancement within different spatial angles was evaluated by means of micro-photoluminescence spectroscopy. More than 18-fold extraction enhancement was achieved for light propagating in the normal direction and up to 22% increase in overall intensity was obtained at the spatial collection angle of 14°.

Enhanced Extraction of Silicon-Vacancy Centers Light Emission Using Bottom-Up Engineered Polycrystalline Diamond Photonic Crystal Slabs

Silicon vacancy (SiV) centers are optically active defects in diamond. The SiV centers, in contrast to nitrogen vacancy (NV) centers, possess narrow and efficient luminescence spectrum (centered at ≈738 nm) even at room temperature, which can be utilized for quantum photonics and sensing applications. However, most of light generated in diamond is trapped in the material due to the phenomenon of total internal reflection. In order to overcome this issue, we have prepared two-dimensional photonic crystal slabs from polycrystalline diamond thin layers with high density of SiV centers employing bottom-up growth on quartz templates. We have shown that the spectral overlap between the narrow light emission of the SiV centers and the leaky modes extracting the emission into almost vertical direction (where it can be easily detected) can be obtained by controlling the deposition time. More than 14-fold extraction enhancement of the SiV centers photoluminescence was achieved compared to an uncorrugated sample. Computer simulation confirmed that the extraction enhancement originates from the efficient light-matter interaction between light emitted from the SiV centers and the photonic crystal slab.

A complex study of the fast blue luminescence of oxidized silicon nanocrystals: the role of the core

Silicon nanocrystals (SiNCs) smaller than 5 nm are a material with strong visible photoluminescence (PL). However, the physical origin of the PL, which, in the case of oxide-passivated SiNCs, is typically composed of a slow-decaying red-orange band (S-band) and of a fast-decaying blue-green band (F-band), is still not fully understood. Here we present a physical interpretation of the F-band origin based on the results of an experimental study, in which we combine temperature (4-296 K), temporally (picosecond resolution) and spectrally resolved luminescence spectroscopy of free-standing oxide-passivated SiNCs. Our complex study shows that the F-band red-shifts only by 35 meV with increasing temperature, which is almost 6 times less than the red-shift of the S-band in a similar temperature range. In addition, the F-band characteristic decay time obtained from a stretched-exponential fit decreases only slightly with increasing temperature. These data strongly suggest that the F-band arises from the core-related quasi-direct radiative recombination governed by slowly thermalizing photoholes.

Efficient light amplification in low gain materials due to a photonic band edge effect

One of the possibilities of increasing optical gain of a light emitting source is by embedding it into a photonic crystal (PhC). If the properties of the PhC are tuned so that the emission wavelength of the light source with gain falls close to the photonic band edge of the PhC, then due to low group velocity of the light modes near the band edge caused by many multiple reflections of light on the photonic structure, the stimulated emission can be significantly enhanced. Here, we perform simulation of the photonic band edge effect on the light intensity of spectrally broad source interacting with a diamond PhC with low optical gain. We show that even for the case of low gain, up to 10-fold increase of light intensity output can be obtained for the two-dimensional PhC consisting of only 19 periodic layers of infinitely high diamond rods ordered into a square lattice. Moreover, considering the experimentally feasible structure composed of diamond rods of finite height - PhC slab - we show that the gain enhancement, even if reduced compared to the ideal case of infinite rods, still remains relatively high. For this particular structure, we show that up to 3.5-fold enhancement of light intensity can be achieved.

Femtosecond luminescence spectroscopy of core states in silicon nanocrystals

We present a study of ultrafast carrier transfer from highly luminescent states inside the core of silicon nanocrystal (due to quasidirect transitions) to states on the nanocrystal-matrix interface. This transfer leads to a sub-picosecond luminescence decay, which is followed by a slower decay component induced by carrier relaxation to lower interface states. We investigate the luminescence dynamics for two different surface passivation types and we propose a general model describing spectral dependence of ultrafast carrier dynamics. Our results stress the crucial role of the energy distribution of the interface states on surface-related quenching of quasidirect luminescence in silicon nanocrystals. We discuss how to avoid this quenching in order to bring the attractive properties of the quasidirect recombination closer to exploitation.