Microporous poly- and monocrystalline diamond films produced from chemical vapor deposited diamond-germanium composites

We report on a novel method for porous diamond fabrication, which is based on the synthesis of diamond-germanium composite films followed by etching of Ge component. The composites were grown by microwave plasma assisted CVD in CH4-H2-GeH4 mixtures on (100) silicon, microcrystalline- and single-crystal diamond substrates. The structure and the phase composition of the films before and after the etching were analyzed with scanning electron microscopy and Raman spectroscopy. The films revealed a bright emission of GeV color centers due to diamond doping with Ge, as evidenced by photoluminescence spectroscopy. The possible applications of the porous diamond films include thermal management, surfaces with superhydrophobic properties, chromatography, supercapacitors etc

Luminescent diamond composites

Diamond is valuable material with extraordinary high thermal conductivity and transparency in a wide spectral range from UV to IR and longer wavelengths. Defects and impurities in the diamond lattice can absorb and emit light at wavelengths specific for each of such “color centers.” Particularly, the vacancy-related defects in diamond, such as nitrogen-vacancy (NV) or silicon-vacancy (SiV) centers, are actively investigated due to their potential for biomedicine, quantum optics, local thermometry and magnetometry. Although a great variety of different color centers in diamond are discovered, only a limited number of those point defects can be reliably reproduced in synthetic diamond, obtained either by chemical vapor deposition (CVD) or high-pressure high-temperature (HPHT). An alternative approach to producing luminescent diamond-based materials is to integrate stable non-diamond sources of luminescence in the form of nano- or microparticles of foreign materials into the pristine diamond. The produced diamond composites possess excellent properties of diamond combined with optical emission characteristics, which cannot be provided with intrinsic defects in diamond. The good candidates for the materials of such impurities are well-investigated fluorides and oxides doped by rare-earth elements (RE) or other luminescent chalcogenides such as sulfides, selenides and tellurides. Here we briefly review recent achievements in fabrication and properties of these new luminescent diamond-RE composites, compare them with luminescent properties of doped diamond, and outline prospects for applications of the luminescent diamond composites for photonics, markers, monitors of high-power synchrotron, X-ray beams and X-ray lasers

Charge-transfer complexes of conjugated polymers as intermediates in charge photogeneration for organic photovoltaics

Ultrafast visible-pump/IR-probe spectroscopy is applied to study the wavelength dependence of charge photogeneration in materials based on donor–acceptor charge-transfer complexes (CTCs) of the conjugated polymer MEH-PPV. In binary polymer–acceptor blends, photoexcitation in the absorption band of either CTC or polymer results in similar dynamics of the charge-associated transient absorption. Likewise, in polymer/CTC–acceptor/fullerene ternary blends, where charge separation occurs via a two-step pathway, the photophysics is also independent of excitation wavelength. These similarities in charge dynamics indicate that CTC excited states serve as an intermediate for charge photogeneration. The conclusions of the ultrafast study are supported by photocurrent spectroscopy.

Ultrafast Charge Photogeneration Dynamics in Ground-State Charge-Transfer Complexes Based on Conjugated Polymers

The charge photogeneration and early recombination in MEH-PPV-based charge-transfer complexes (CTCs) and in MEH-PPV/PCBM blend as a reference are studied by ultrafast visible-pump-IR-probe spectroscopy. After excitation of the CTC band, an immediate (<100 fs) electron transfer is observed from the polymer chain to the acceptor with the same yield as in the MEH-PPV/PCBM blend. The forward charge transfer in the CTCs is followed by an efficient (~95%) and fast (<30 ps) geminate recombination. For comparison, the recombination efficiency obtained in the MEH-PPV/PCBM blend does not exceed a mere 50%. Polarization-sensitive experiments demonstrate high (~0.3) values of transient anisotropy for the CTCs polaron band. In contrast, in the MEH-PPV/PCBM blend the dipole moment orientation of the charge-induced transition is less correlated with the polarization of the excitation photon. According to these data, photogeneration and recombination of charges in the CTCs take place locally (i.e., within a single pair of a polymer conjugation segment and an acceptor) while in the MEH-PPV/PCBM blend exciton migration precedes the separation of charges. Results of the ultrafast experiments are supported by photocurrent measurements on the corresponding MEH-PPV/acceptor photodiodes.

Effect of diamond seeds size on the adhesion of CVD diamond coatings on WC-Co instrument

In this study, we investigated the effect of the size of diamond seeds on the adhesion of multilayered polycrystalline diamond (PCD) films, grown by microwave plasma-assisted chemical vapor deposition (MPCVD). For that, identical WC-Co substrates were separately seeded by a set of diamond powders with various average particle sizes from water-based suspensions using similar seeding procedures. This investigation included powders with a difference in particle sizes of nearly 3 orders of magnitude: from 5 nm up to 2-4 μm. Seeded substrates were used to grow 8-10 μm thick multilayered PCD films using MPCVD with time-limited cycling injections of N2 gas. The Raman spectra and scanning electron microscopy (SEM) studies showed the similarity of microstructure and phase composition of all grown films, which confirmed that all films were grown in similar conditions. The performed scratch tests revealed sufficient differences in the adhesion of the films seeded with different diamond particles. The PCD film grown on 250-500 nm particles delaminated even before any mechanical investigations. The substrates seeded with 50 nm particles allowed the formation of the stable PCD film, but it started flaking under a load as small as 15 N. The 2-4 μm powder allowed the formation of PCD film with decent adhesion, which had local flaking under scratch test, which can be explained by the inhomogeneity of seeds distribution. Detonation nanodiamond (DND) powders allowed the formation of continuous diamond films with decent adhesion, however, powders with positive zeta potential were superior due to a much lower agglomeration of separate particles

Synthesis of Y3Al5O12:Ce Powders for X-ray Luminescent Diamond Composites

A concentration series of Y3Al5O12:Ce solid solutions were prepared, and the composition demonstrating the highest X-ray luminescence intensity of cerium was identified. Based on the best composition, a series of luminescent diamond&ndash;Y3Al5O12:Ce composite films were synthesized using microwave plasma-assisted chemical vapor deposition (CVD) in methane&ndash;hydrogen gas mixtures. Variations in the amounts of the embedded Y3Al5O12:Ce powders allowed for the fine-tuning of the luminescence intensity of the composite films

Synthesis of Y₃Al₅O₁₂:Ce Powders for X-ray Luminescent Diamond Composites

A concentration series of Y3Al5O12:Ce solid solutions were prepared, and the composition demonstrating the highest X-ray luminescence intensity of cerium was identified. Based on the best composition, a series of luminescent diamond–Y3Al5O12:Ce composite films were synthesized using microwave plasma-assisted chemical vapor deposition (CVD) in methane–hydrogen gas mixtures. Variations in the amounts of the embedded Y3Al5O12:Ce powders allowed for the fine-tuning of the luminescence intensity of the composite films

CVD synthesis of multi-layered polycrystalline diamond films with reduced roughness using time-limited injections of N2 gas

Multi-layered polycrystalline diamond (PCD) films were synthesized using microwave plasma-assisted chemical vapor deposition (CVD) with periodical addition (injections) of N2 gas to the standard CH4-H2 gas mixture. The aim of such an approach was to reduce the roughness of the films while preserving the overall high quality and phase purity of the PCD material. The thicknesses of the films were in the range of 5 to 51 μm, while the number of layers was from 1 to 15. The introduction of even the smallest amount of nitrogen leads to a significant (more than 2-fold) increase in the growth rate of PCD films. Optimized injection regimes allowed the reduction of the relative roughness (Sq/thickness) of the PCD films by more than 3 times in comparison with standard microcrystalline diamond film grown under similar conditions without N2 addition. The proposed method of periodic injection of N2 during growth restricted the formation of continuous NCD layers, which improved the overall sp3/sp2 ratio in comparison with standard multi-layered MCD/NCD materials. The obtained multi-layered PCD materials with reduced roughness may be used for the formation of protective and hard covers, optical coatings, electrochemical and thermal management applications. Prime novelty statement The multi-layered PCD films were synthesized in a microwave plasma CVD in regimes with short-term periodic N2 injections in CH4-H2 process gas; the average growth rate is doubled owing to the pulsed nitrogen injection, while the surface roughness is reduced by more than 3 times in comparison with a standard (no N2 injection) microcrystalline film