Microscopic investigation of single-crystal diamond following ultrafast laser irradiation

We present a structural investigation of single-crystal diamond following ultrafast laser irradiation of the surface and the bulk material. Optical microscopy, atomic force microscopy, scanning electron microscopy, and focused-ion beam and transmission electron microscopy techniques were utilized to selectively examine the final state of the samples. Laser induced periodic surface structures (LIPSS) with high- and low-spatial frequencies were obtained with multiple-pulse surface irradiation under both stationary and translated target conditions. High magnification transmission electron microscopy analysis of cross sections of the LIPSS revealed modified layers of a few tens of nanometers in thickness capping the crystalline diamond matrix. Sub-surface irradiation of diamond at high laser fluences led to damaged regions and cracks in the bulk material. When translational bulk irradiation of the diamond was performed, substantially sub-wavelength periodic structures were observed at the unpolished side facet of the diamond plate where the laser focus was translated out of the bulk. Spatial periods were 140 nm and the structures largely consisted of single-crystal diamond with a 10 nm modified layer. Finally, preliminary studies of single-shot laser ablation craters at high laser fluences exhibited suppression of material removal for peak values above 45 J/cm².     

Metastable carbon allotropes in picosecond-laser-modified diamond

In this paper, we report on the bulk modifications of type IIa single-crystal diamond with visible 10-ps pulses (at λ = 532 nm) and microstructural changes characterized by the appearance of several ‘unidentifiable’ vibrational modes in the frequency range of 1000–1400 cm^?1 in the Raman spectra of laser-modified diamond. It is found that the new Raman modes are strongly pronounced in the spectra of high-stress regions in immediate proximity to the bulk microstructures in the absence of the G mode at ~1580 cm^?1 characteristic of the sp² phase. The high internal stresses are determined from the splitting of the triply degenerate diamond Raman line. The revealed structure transformation is localized within a narrow bulk layer near the bulk microstructures formed, and the stress relaxation is found to result in disappearance of the detected vibrational modes in the spectra. It is suggested that the formation of bulk regions with a sp³ carbon structure consisting of Z-carbon and hexagonal diamond is responsible for the appearance of new Raman modes in the spectra of laser-modified diamond. These findings evidence that the stress-assisted formation of novel metastable carbon phases or defect structures occur in the course of bulk modification of diamond with ps-laser pulses. In addition, we report the results of simulations of internal stresses in the system ‘graphitized cylinder-in-diamond’ to show (1) the effect of the mechanical properties of laser-modified diamond on the resulting stresses and (2) formation of bulk microscopic regions with high stresses of >10 GPa, i.e., the conditions at which various sp³ carbon allotropes and defect structures become more stable than graphite.     

Photoluminescence of SiV centers in CVD diamond particles with specific crystallographic planes

We prepared the isolated micrometer-sized diamond particles without seeding on the substrate in hot filament chemical vapor deposition. The diamond particles with specific crystallographic planes and strong silicon-vacancy(SiV) photoluminescence(PL) have been prepared by adjusting the growth pressure. As the growth pressure increases from 2.5 to 3.5 kPa,the diamond particles transit from composite planes of {100} and {111} to only smooth {111} planes. The {111}-faceted diamond particles present better crystal quality and stronger normalized intensity of SiV PL with a narrower bandwidth of 5 nm. Raman depth profiles show that the SiV centers are more likely to be formed on the near-surface areas of the diamond particles, which have poorer crystal quality and greater lattice stress than the inner areas. Complex lattice stress environment in the near-surface areas broadens the bandwidth of SiV PL peak. These results provide a feasible method to prepare diamond particles with specific crystallographic planes and stronger SiV PL.     

Ion-beam studies of synthetic single-crystal diamonds

A surface layer of polished single-crystal diamond plates with the (100) orientation, which can be applied to planar high-frequency high-power microstructures, is investigated by means of ion-beam and X-ray methods. It is demonstrated that the diamond plates are characterized by a high degree of structural perfection and a low level of surface roughness.     

Solvothermally synthesized europium-doped CdS nanorods: applications as phosphors

To exploit the photoluminescent behavior of CdS at nanoscale with different doping concentration of europium—a rare earth element, we report the synthesis of Eu-doped CdS nanorods by using low temperature solvothermal process by using ethylenediamine. The outcomes can have future applications as phosphors, photovoltaic cells, lasers, light emitting diodes, bio-imaging, and sensors. The doping was confirmed by electron dispersive spectroscopy supported by X-ray diffraction. From scanning electron microscopy and transmission electron microscopy analysis it was observed that the average diameter of the Cd_1−x Eu _x S nanorods is about 10–12 nm having lengths in the range of 50–100 nm. UV–Visible spectroscopy study was carried out to determine the band gap of the nanorods and the absorbance peaks showed blue shift with respect to the bulk CdS. The blue shift was also observed as the doping concentration of Eu increases. From photoluminescence (PL) studies at λ_ex = 450 nm, peaks at 528 and 540 nm were observed due to CdS, peak at 570 nm is due to defects related transitions, while the peak at 613 nm is due to Eu. As the doping concentration of Eu is increased the intensity of the luminescent peak at 613 nm is increased. Thermogravimetric analysis showed the nanorods are thermally stable up to 300 °C. The traces of impurities adsorbed on the nanorods were confirmed by Fourier transform infrared spectroscopy.     

Diode-pumped laser with Yb:YAG single-crystal fiber grown by the micro-pulling down technique

Laser emission obtained from an Yb:YAG single-crystal fiber directly grown by the micro-pulling down technique is demonstrated for the first time. We achieved 11.2 W of continuous wave (CW) output power at 1031 nm for 55 W of incident pump power at 940 nm. In the Q-switched regime, we obtained pulses as short as 17 ns, for an average power of 2.3 W at 2 kHz corresponding to an energy of 1.15  mJ. In both cases, the M ² factor was 2.5. This single-crystal fiber showed performance similar to a standard rod elaborated by the Czochralski method. The potential of Yb³⁺-doped single-crystal fibers is presented for scalable high-average and high-peak-power laser systems.     

Aging effect on blue luminescent silicon nanocrystals prepared by pulsed laser ablation of silicon wafer in de-ionized water

Blue luminescent colloidal silicon nanocrystals (Si-ncs) were synthesized at room temperature by nanosecond pulsed laser ablation of a single-crystal silicon target in de-ionized water. Irregular Si-nc fragments obtained by laser ablation are stabilized into regularly shaped, spherical, and well-separated aggregates during the aging process in water. Aging in de-ionized water for several weeks improved the photoluminescence (PL) intensity. At least two weeks of aging are necessary for observation of broad blue room temperature PL with a maximum centered at 420 nm. Detailed structural analysis revealed that agglomerates after aging for several months contain Si-ncs with irregular shape smaller than the quantum confinement limit (<5 nm). These blue luminescent Si-ncs dispersed in de-ionized water exhibited a PL decay time of 6 ns, which is much faster than that of Si-ncs prepared in traditional ways (usually on the order of microseconds). The oxidized Si-ncs with quantum confinement effects are responsible for a PL band around 400 nm visible to the naked eye at room temperature.     

Evidence of light guiding in ion-implanted diamond

We demonstrate the feasibility of fabricating light-waveguiding microstructures in bulk single-crystal diamond by means of direct ion implantation with a scanning microbeam, resulting in the modulation of the refractive index of the ion-beam damaged crystal. Direct evidence of waveguiding through such buried microchannels is obtained with a phase-shift micro-interferometric method allowing the study of the multimodal structure of the propagating electromagnetic field. The possibility of defining optical and photonic structures by direct ion writing opens a range of new possibilities in the design of quantum-optical devices in bulk single-crystal diamond.     

Two-photon-excited luminescence spectra in diamond nanocrystals

Two-photon-excited luminescence (TEL) spectra have been recorded in the blue (400–500 nm) and near-ultraviolet (300–400 nm) ranges for diamond particles with 4 nm average size, which were obtained by detonation synthesis from explosives. The observed TEL bands are attributed, by comparing the obtained spectra with the impurity luminescence spectra in large diamond crystals, to N2 and N3 defects associated with the presence of nitrogen impurities in diamond. The TEL spectra presented are found to have certain distinguishing features: short-wavelength shift of the maximum and changes in the shape and width of the spectral bands for ultradispersed diamond compared with the spectrum in bulk crystals. Fiz. Tverd. Tela (St. Petersburg) 41, 1110–1112 (June 1999)     

Dosimeter properties of MgO transparent ceramic doped with C

We have investigated the photoluminescence (PL), scintillation and thermally-stimulated luminescence (TSL) dosimeter properties of MgO ceramic doped with C ions (0.001, 0.01 and 0.1%). The samples were synthesized by a Spark Plasma Sintering (SPS) technique. The PL emission peaks appeared around 400 and 750 nm in all the samples. The PL decay time constants at 400 nm were ∼10 and ∼100 ns which were on the typical order of F+ center in the undoped MgO. The scintillation emission peaks were detected at 330, 400 and 750 nm under X-ray irradiation. The TSL glow curves showed the ∼250 °C peak in 0.1% C-doped sample. The TSL response was confirmed to be linear to the irradiation dose over the dose range from 0.1 to 1000 mGy. As a result, the sensitivity of MgO was improved by C-doping.     

Formation of Germanium–Vacancy Color Centers in CVD Diamond

We study Ge-doped polycrystalline diamond films synthesized, using microwave plasma chemical vapor deposition (CVD) in CH₄-H₂ base mixtures. We compare two sources of the dopant – gaseous monogermane (GeH₄) and solid Ge plates. We investigate the structure and phase composition of the obtained films, using scanning electron microscopy, photoluminescence (PL), and Raman spectroscopy. We vary the precursor gas composition to maximize the intensity of the Germanium–vacancy (Ge-V) PL signal at 602 nm and discover that, using [C]-rich gas mixtures ([CH₄]=20%), we are able to increase the intensity of Ge-V signal by two orders of magnitude in comparison with Ge-doped high-quality microcrystalline films of the same thickness but grown at [CH₄]=4%. The attained results may be used for the fabrication of polycrystalline diamond films and plates with high concentrations of Ge-V centers, which may serve as source material for the fabrication of submicrometer-sized luminescent diamond particles for local optical thermometry.

     

Synthesis and photoluminescence properties of aqueous CdWO4 quantum dots with WO6 luminescence center

Aqueous CdWO₄ QDs were synthesized by the reaction of CdCl₂ and Na₂WO₄ in the presence of mercaptoacetic acid (TGA) as capping reagent. The crystal morphology, particle size and its distribution of as-prepared products were characterized by transmission electron microscopy (TEM, SAED) atomic force microscopy (AFM), high-resolution transmission electron microscopy (HRTEM), and photon correlation spectroscopy (PCS), respectively. Qualitative assays for functional groups on the QDs’ surface were measured by fourier transform infrared spectroscopy (FTIR). Photoluminescence properties of QDs were studied by photoluminescence spectroscopy (PL). The results showed that the single QD with diameter of about 8 ± 2 nm was single-crystal. The particle size distribution of QDs was normal. Infrared absorption bands of carboxylic group on the surface of CdWO₄ QDs were observed around 1610-1550 cm⁻¹ (nonsymmetrical vibration of -COO⁻) and 1400 cm⁻¹ (symmetric vibration of C-O). With reaction-time going, PL peak position shifted from 498 to 549 nm and intensity of PL increased first and then decreased. PL peak position of QDs was blue-shift compared with 570 nm WO₆⁶⁻ luminescence center of bulk CdWO₄.     

Catalyst-free synthesis and luminescence of aligned ZnO nanorods

Quasi-aligned undoped ZnO nanorods with diameter in the range 100–300 nm and length of several micrometers have been grown catalyst-free on Si(1 0 0) wafer in a one-step process by direct heating of Zn powders. All nanowires are single crystals and are aligned vertically to the substrate surface with c-axis preferred orientation. XRD, HRTEM and Raman studies revealed that the ZnO nanorods have wurtzite phase, are highly crystalline and well aligned with the lattice parameters a=0.32 nm and c=0.52 nm. The PL spectra measured at different temperatures are dominated by excitonic emission at 380 nm and less intense below band gap emission band centered at 520 nm.     

Photoluminescence characterization of beryllium-implanted 6H–silicon carbide

Beryllium has been implanted into both n- and p-type 6H–SiC with post-implantation annealing at 1600 °C. Photoluminescence (PL) measurements have been performed, and PL lines at 420.5, 431 nm, and a broad band at around 505 nm have been observed. The line at 420.5 nm is attributed to an intrinsic defect D_II-center induced by beryllium implantation. The effects of excitation intensity and temperature during the PL experiments are investigated. Based on the excitation laser dependence PL result, the new doublet lines at around 431 nm are thought to be associated with beryllium related bound excitons. The broad band corresponding to the green luminescence at room temperature has been attributed to the recombination of free carriers to beryllium bound levels.     

Computer simulation of the photoluminescence of nanostructured aluminum oxide excited with pulsed synchrotron radiation

An algorithm and a program are developed to calculate the photoluminescence (PL) parameters for bulk single-crystal and nanoscale dielectrics excited with pulsed synchrotron radiation. The luminescence spectra of F and F ⁺ centers and the PL decay kinetics in single-crystal and nanoscale aluminum-oxide samples containing oxygen anion vacancies are calculated for various nanoparticle sizes. It is shown that a noticeable broadening of the bands and a decrease in the afterglow time is observed for nanoparticle sizes that are less than 20 nm.     

Photoluminescence properties of thenardite activated with Eu

Na₂SO₄:Eu phosphors were prepared by heating pure natural thenardite with EuF₃ at 900 °C for 20 min in air. The photoluminescence (PL) and excitation spectra of as-prepared and γ-ray-irradiated phosphors were observed at 300 K. The PL spectrum under 394 nm excitation consisted of strong narrow bands with peaks at 579, 592, 616, 652, 697 and 741 nm, assigned to the ⁵D₀→⁷F_J (J=0, 1, 2, …, 5) transitions, respectively, within Eu³⁺. The PL spectrum under 340 nm excitation consisted of a broad Eu²⁺ band with a peak at 435 nm. The excitation spectrum obtained by monitoring the violet luminescence consisted of a weak band with a peak at approximately 261 nm and a broad Eu²⁺ band with a peak at approximately 338 nm. The relative efficiency of the violet luminescence of the γ-ray-irradiated phosphor at the exposure of 46 kGy increased up to 3.0 times that of the unirradiated phosphor. The enhancement of violet luminescence by γ-ray irradiation was ascribed to the conversion of Eu³⁺ to Eu²⁺ in Na₂SO₄.     

Aqueous phase-synthesized small CdSe quantum dots: adsorption layer structure and strong band-edge and surface trap emission

We synthesized, in aqueous solution at room temperature, small water-soluble CdSe quantum dots (QDs) with strong photoluminescence (PL) and then correlated the PL with their adsorption layer structure. For synthesizing the QDs, their initial synthesis condition was controlled to form small Cd-containing species capable of passivating dangling bonds on the CdSe core surface. Each CdSe QD (d ~ 2.5 nm) consisted of a CdSe core (d ~ 2.1 nm), a cysteine (cys)-ligand shell, and an adsorption layer composed of Cd–cys complexes (mainly CdL(-H)⁻, cys ≡ H₂L), cys (as L²⁻), Cd(OH)₂, and CdO _x (x ≥ 1). Our CdSe QDs showed strong blue band-edge PL as well as strong green surface trap PL. Their PL quantum yield (QY) of ~18% was unexpectedly high, considering their extremely small core size and their absence of any wide-bandgap inorganic shell. We attributed the QY to their adsorption layer species. The small weakly charged Cd–cys complex and the small neutral cadmium oxides in the adsorption layer could relatively readily diffuse into the unprotected surface sites on the core. These wide-bandgap species coalesced selectively on the unprotected surface sites with minimal spatial disturbance to the preexisting surface Cd-ligand coordination, and passivated them effectively. These decreased nonradiative recombination of the excitons significantly and thus led to the unexpectedly high QYs.     

Synthesis and characterization of ZnO columns grown on MgO-coated silicon substrates by carbon-thermal evaporation

Zinc oxide columns have been grown on an MgO-coated silicon (111) substrate by the carbon-thermal evaporation method at 1050 °C. The MgO layer obtained from the substrate pre-dripped in Mg(NO₃)₂ solution by the use of a dropper can solve the troublesome lattice mismatch problem in the heteroepitaxy and promote the growth of ZnO columns effectively. The as-prepared ZnO structures were characterized by using X-ray diffraction (XRD), field-emission transmission electron microscope (FETEM), selection area electron diffraction (SAED), and photoluminescence (PL) spectrum. The results show that the columns are highly crystalline with the wurtzite hexagonal structure, and grow along the [0001] in the c-axis direction. Photoluminescence (PL) spectra of the as-synthesized microstructures exhibit broad green emission peaks at ∼514 nm. In addition, the growth mechanism of the two ZnO structures is discussed based on the analysis briefly based on the time-dependent experiment.     

Novel NdCo<Subscript>5</Subscript> nanoflakes and nanoparticles produced by surfactant-assisted high-energy ball milling

High-energy ball milling has been shown to be a promising method for the fabrication of rare earth—transition metal nanopowders. In this work, NdCo₅ nanoflakes and nanoparticles have been produced by a two-stage high-energy ball milling (HEBM), by first using wet HEBM to prepare precursor nanocrystalline powders followed by surfactant-assisted HEBM. NdCo₅ flakes have a thickness below 150 nm and an aspect ratio as high as 10²–10³; the nanoparticles have an average size of 7 nm. Both the nanoparticles and nano-flakes exhibited high coercivities at low temperatures, with values at 50 K of 3 and 3.7 kOe, respectively. The high values of coercivity can be attributed to the large surface anisotropy of nanoparticles that leads to an effective uniaxial-type of behavior in contrast to the planar anisotropy of the bulk samples. Angle-dependent magnetization measurements at different temperatures were used to determine the spin reorientation transitions in the nanopowders and nanoparticles. The nanoparticles showed spin reorientation temperatures, T _SR1 = 276 and T _SR2 = 237 K which are lower when compared with the values of 290 and 245 K, respectively for bulk.     

1.3 μm range effectively cylindrical In0.53Ga0.47As/In0.52Al0.48As quantum wires grown on (2 2 1)A InP substrates by molecular beam epitaxy

Vertically coupled quantum wires (QWRs) have been made by alternately stacking nominally 3.6 nm thick In_0.53Ga_0.47As self-organized QWR layers and 1 nm thick In_0.52Al_0.48As barrier layers on (2 2 1)A-oriented InP substrates by molecular beam epitaxy. The surface of In_0.53Ga_0.47As QWR layers was corrugated at an amplitude of 1.1 nm and period of 27 nm, and lateral confinement potential is induced by their thickness modulation. The wavelength of photoluminescence (PL) from the stacked QWRs at 15 K becomes longer from 1220 to 1327 nm with increasing total number of stacked QWR layers, N_SL, from 1 to 9, while PL full-width at half-maximum is reduced from 22 to 8.6 meV. The PL intensity with the polarization parallel to the wire direction, I, is 1.30 times larger than that with the normal polarization, I, when N_SL=1. The PL intensity ratio, I/I, reaches as large as 4 when N_SL=9, indicating successful control of relative strength between vertical confinement and lateral confinement of carriers. The value of I/I obtained for the stacked QWRs with N_SL=9 is the same value as cylindrical QWRs have. The results indicate that effectively cylindrical QWRs with the best uniformity and 1.3 μm range emission were realized by stacking of self-organized QWR layers.