Thermal reaction and luminescence of long-lived N 2D in N2 ice.

Photochemistry of an N2 ice and thermal reaction of the irradiated sample were studied with vacuum-ultraviolet (VUV) light from a synchrotron. Concurrent detection of infrared absorption and visible emission spectra provide evidence for the generation of energetic products N (2D) and N (2P) atoms, N2 (A) molecule and linear-N3 (l-N3) radical after excitation of icy N2 at 121.6 nm. Irradiation at 190 nm is shown to be an effective way to eliminate the l-N3 radical. After the photolysis and photoelimination of the l-N3, we initiate synthesis of l-N3 via the thermal ramping of the sample in temperature range 3.5 to 20 K. In addition, the emission from the N (2D) atom was observed during the thermal ramping process. These behaviors indicate that a long-lived N (2Dlong) atom is generated in the VUV-photolyzed N2 ice. A comparison of the variations of the visible emission of N (2D) and the infrared absorption of l-N3 with time indicates that the long-lived N (2Dlong) dominated the thermal synthesis of l-N3 The results have enhanced suggestion and understanding of the conversion for nitrogen species in cold astrophysical environments with VUV irradiation.

Photoluminescence of optical windows excited with extreme ultraviolet radiation.

Upon excitation with extreme ultraviolet (EUV) radiation, optical windows CaF2 and sapphire emit strong photoluminescence (PL) in the ultraviolet region 200-400 nm. The spectral profiles of the windows observed in the PL spectra appear strongly dependent on their temperature. We suggest the use of PL spectra of CaF2 and sapphire excited with EUV light to indicate the temperature for EUV applications such as nano-photolithography technology in manufacturing semiconductor devices; potentially, the method is applicable to a wide range of radiation including the vacuum-ultraviolet (VUV) and EUV regions and in all fields.

Photoluminescence of diamond containing nitrogen vacancy defects as a sensor of temperature upon exposure to vacuum- and extreme-ultraviolet radiation.

Upon excitation with vacuum-ultraviolet (VUV) and extreme-ultraviolet (EUV) radiation, diamond with nitrogen vacancies (DNV) emits strong photoluminescence (PL) in the wavelength region of 550-800 nm. The spectral profiles of the DNV in the PL spectra appear to be strongly dependent on the temperature of the diamond. Moreover, all PL spectra intersect at one isosbestic point, 570 nm; this result is evidence that the NV0 and NV- defects in diamond interconvert with each other upon VUV and EUV radiation. We suggest the use of PL spectra of DNV excited with VUV or EUV light to indicate the temperature for applications such as in nano-photolithography technology for the manufacture of semiconductor devices.

Electroluminescence from h-BN by using Al2O3/h-BN multiple heterostructure.

Two-dimensional (2-D) hexagonal boron nitride (h-BN) has attracted considerable attention for deep ultraviolet optoelectronics and visible single photon sources, however, realization of an electrically-driven light emitter remains challenging due to its wide bandgap nature. Here, we report electrically-driven visible light emission with a red-shift under increasing electric field from a few layer h-BN by employing a five-period Al2O3/h-BN multiple heterostructure and a graphene top electrode. Investigation of electrical properties reveals that the Al2O3 layers act as potential barriers confining injected carriers within the h-BN wells, while suppressing the electrostatic breakdown by trap-assisted tunneling, to increase the probability of radiative recombination. The result highlights a promising potential of such multiple heterostructure as a practical and efficient platform for electrically-driven light emitters based on wide bandgap two-dimensional materials.

Photolysis of O2 dispersed in solid neon with far-ultraviolet radiation.

Irradiation at 173 or 143 nm of samples of 16O2 or 18O2 in solid Ne near 4 K produced many new spectral lines in absorption and emission from the mid-infrared to the near-ultraviolet regions. The major product was ozone, O3, that was identified with its mid-infrared and near-ultraviolet absorption lines. Oxygen atoms were formed on photolysis of O2 and stored in solid neon until the temperature of a sample was increased to 9 K, which enabled their migration and combination to form O3 and likely also O2. O2 in five excited states and O in two excited states detected through the emission spectra indicate that complicated processes occurred in solid Ne after far-ultraviolet excitation. For the transition 1D2 → 3P1,2 of O, the lifetime was determined to be 5.87 ± 0.10 s; the lifetime of the upper state of an unidentified transition associated with an emission feature at 701.7 nm was determined to be 2.34 ± 0.07 s.

Thresholds of photolysis of O2 and of formation of O3 from O2 dispersed in solid neon.

Irradiation of O2 dispersed in solid Ne with ultraviolet light produced infrared absorption lines of O3 and emission lines from atomic O (1D2 → 3P1,2), molecular O2 (A' 3Δu → X 3Σg) and radical OH (A 2Σ+ → X 2ΠI) in the visible and near-ultraviolet regions. The threshold wavelength for the formation of O3 was determined to be 200 ± 4 nm, corresponding to energy 6.20 ± 0.12 eV, which is hence the threshold for dissociation of O2. The thresholds of emission from excited O (1D2), O2 (A' 3Δu) and OH (A 2Σ+) were all observed to be 200 ± 4 nm, the same as for the formation of O3 in this photochemical system. The results indicate that, once O3 was generated, it was readily photolyzed to produce the long-lived atom O (1D2). Further reactions of O (1D2) with O3 produced excited O2 (A' 3Δu); reaction with water yielded radical OH (A 2Σ+). These results enhance our understanding of the evolution of the transformation of oxygen and open a window for the understanding of complicated processes in the solid phase.

Ultraviolet and Infrared Spectra of Diboron in Solid Neon at 4 K.

Apart from products H, B, BH, BH2 and BH3 identified from their emission spectra in the UV/Vis region, photolysis of diborane(6) dispersed in solid neon at 4 K with far-ultraviolet light from a synchrotron led to observation of absorption line (0,0) of the electronic transition A 3 Σu- ←X 3 Σg- of B2 at 326.39 nm. Absorption lines (1,0) of 11 B2 , 11 B10 B and 10 B2 were recorded at 316.63, 316.40 and 316.15 nm, respectively. ΔG1/2 of state A 3 Σu- for 11 B2 , 11 B10 B and 10 B2 in solid neon are accordingly derived to be 945, 968 and 993 cm-1 , respectively. Weak lines (0,1) of 11 B2 at 29586 cm-1 and of 11 B10 B at 29560 cm-1 , corresponding to 1042±30 and 1068±30 cm-1 for vibrational modes in the electronic ground state, were recorded in emission. An absorption line recorded at 1066.5±0.5 cm-1 in infrared spectra after photolysis of either B2 H6 in Ne or B2 D6 with D2 in Ne is thus attributed to 11 B10 B.

Far-UV-Excited Luminescence of Nitrogen-Vacancy Centers: Evidence for Diamonds in Space.

The nitrogen-vacancy (NV) centers in diamond are among the most thoroughly investigated defects in solid-state matter; however, our understanding of their properties upon far-UV excitation of the host matrix is limited. This knowledge is crucial for the identification of NV as the carrier of extended red emission (ERE) bands detected in a wide range of astrophysical environments. Herein, we report a study on the photoluminescence spectra of NV-containing nanodiamonds excited with synchrotron radiation over the wavelength range of 125-350 nm. We observed, for the first time, an emission at 520-850 nm with a quantum yield greater than 20 %. Our results share multiple similarities with the ERE phenomena, suggesting that nanodiamonds are a common component of dust in space.

Infrared and Ultraviolet Spectra of Diborane(6): B2H6 and B2D6.

We recorded absorption spectra of diborane(6), B2H6 and B2D6, dispersed in solid neon near 4 K in both mid-infrared and ultraviolet regions. For gaseous B2H6 from 105 to 300 nm, we report quantitative absolute cross sections; for solid B2H6 and for B2H6 dispersed in solid neon, we measured ultraviolet absorbance with relative intensities over a wide range. To assign the mid-infrared spectra to specific isotopic variants, we applied the abundance of (11)B and (10)B in natural proportions; we undertook quantum-chemical calculations of wavenumbers associated with anharmonic vibrational modes and the intensities of the harmonic vibrational modes. To aid an interpretation of the ultraviolet spectra, we calculated the energies of electronically excited singlet and triplet states and oscillator strengths for electronic transitions from the electronic ground state.

Analysis of Nickel Defect in Diamond with Photoluminescence upon Excitation near 200 nm.

Photoluminescent (PL) spectra of synthetic diamond powders at temperatures between 10 and 300 K were excited with synchrotron radiation in the wavelength range 125-375 nm. Prominent spectral PL features were detected at 484.6 and 489.0 nm (2.559 and 2.535 eV), associated with nickel defect. During our measurement of PL excitation (PLE) spectra of Ni defect in diamond, we observed a distinct PLE line at 215 nm for the first time. We thereby suggest the use of UV-PL spectra excited in the region 200-220 nm to analyze and to identify nickel defect in diamonds.

Quantitative analysis of nitrogen defect N4 in diamond with photoluminescence excited in the 170-240 nm region.

Upon excitation at 170-240 nm, diamonds emit strong luminescence in wavelength range of 300-700 nm. The spectral features observed in the photoluminescence excitation (PLE) spectra show two vibrational progressions, A and B, related to nitrogen defects N2 and N4, respectively. We used PLE spectra excited in region 170-240 nm to identify the type of diamond and demonstrate quantitative analysis of the B center as a N4 nitrogen defect in diamonds; the least detectable concentration of the N4 nitrogen defect is about 13 ppb, and the sensitivity of PLE is about 30 times than that practicable with infrared absorption spectra.

Vacuum-ultraviolet photolysis of methane at 3 K: synthesis of carbon clusters up to C20.

Samples of pure methane and of methane dispersed in solid neon at 3 K subjected to irradiation at wavelengths less than 165 nm with light from a synchrotron yielded varied products that were identified through their infrared absorption spectra, including CH3, C2H2, C2H3, C2H4, C2H6, C4H2, C4H4, C5H2, C8H2, CnH with n = 1-5, and carbon chains Cn with n = 3-20. The efficiency of photolysis of methane and the nature of the photoproducts depended on the concentration of methane and the wavelength selected for irradiation; an addition of H2 into solid neon enhanced the formation of long carbon chains.

Production of N3 upon photolysis of solid nitrogen at 3 K with synchrotron radiation.

The photodissociation of gaseous molecular nitrogen has been investigated intensively, but the corresponding knowledge in a solid phase is lacking. Irradiation of pure solid nitrogen at 3 K with vacuum-ultraviolet light from a synchrotron produced infrared absorption lines of product l-N3 at 1657.8 and 1652.6 cm(-1). The threshold wavelength to generate l-N3 was determined to be (143.7±1.8) nm, corresponding to an energy of (8.63±0.11) eV. Quantum-chemical calculations support the formation of l-N3 from the reaction N2 +N2, possibly through an activated complex l-N4 upon photoexcitation with energy above 8.63 eV. The results provide a possible application to an understanding of the nitrogen cycle in astronomical environments.

Identification of nitrogen defects in diamond with photoluminescence excited in the 160-240 nm region.

Photoluminescence (PL) spectra of natural diamond powders of type IaAB at 300 and 13 K were excited with synchrotron radiation in the wavelength range of 150-260 nm. The spectral features observed in the excitation spectra at 13 K show four vibrational progressions related to nitrogen defects in diamond: A, B, B', and N3. Progression A has a spacing of 1258 ± 40 cm(-1), associated with the N2 (or A) center; progression B has a spacing of 1181 ± 40 cm(-1) and progression B' has a spacing of 744 ± 40 cm(-1) related to the N4 (or B) center; and progression N3 has a spacing of 1417 ± 40 cm(-1) associated with the N3 center. The PL of these defects comprise continuous emission with two broad lines with maxima of ∼420 and 469 nm at 300 K. Upon excitation with light at wavelengths of <200 nm, the distinct zero-phonon lines of N3 and N4 centers in diamond at a temperature of 13 K become prominent at 416.0 and 491.2 nm, respectively. The vibrational progressions in the photoluminescence excitation (PLE) spectra of N2, N3, and N4 centers in diamond of type IaAB at 13 K are identified for the first time. We suggest the use of PL spectra excited in the region of 160-240 nm to analyze and identify the type of diamond.

Nitrogen-Vacancy Centers in Diamond for High-Performance Detection of Vacuum Ultraviolet, Extreme Ultraviolet, and X-rays.

Fluorescent nanodiamonds (FNDs) containing nitrogen-vacancy (NV) centers as built-in fluorophores exhibit a nearly constant emission profile over 550-750 nm upon excitation by vacuum-ultraviolet (VUV), extreme ultraviolet (EUV), and X-radiations from a synchrotron source over the energy (wavelength) range of 6.2-1450 eV (0.86-200 nm). The photoluminescence (PL) quantum yield of FNDs increases steadily with the increasing excitation energy, attaining a value as great as 1700% at 700 eV (1.77 nm). Notably, the yield curve is continuous, having no gap in the VUV to X-ray region. In addition, no significant PL intensity decreases were observed for hours. Applying the FND sensor to measure the absorption cross-sections of gaseous O2 over 110-200 nm and comparing the measurements with the sodium-salicylate scintillator, we obtained results in agreement with each other within 5%. The superb photostability and broad applicability of FNDs offer a promising solution for the long-standing problem of lacking a robust and reliable detector for VUV, EUV, and X-radiations.

Mid-infrared spectra of silane dispersed in solid neon.

We report mid-infrared spectra of silane dispersed in solid neon at relative concentrations 1:1000 and 1:5000, recorded with spectral resolution 0.15 cm-1. Apart from major lines associated with internal vibrational motions of 28SiH4, 29SiH4 and 30SiH4 in fundamental modes ν3 and ν4, several weak accompanying lines in each region become discernible at the resolution of our experimental measurements, and are tentatively associated with librational motions of silane molecules in the solid neon lattice. The wavenumbers associated with a few overtone and combination modes are also presented.

Electronic and Vibrational Absorption Spectra of NH2 in Solid Ne.

Irradiation of ammonia dispersed in solid neon near 4 K with tunable far-ultraviolet light from a synchrotron yielded amidogen, NH2, and imidogen, NH, radicals as products. The electronic absorption spectra of amidogen radicals in isotopic variants NH2, NHD, and ND2 were recorded in the visible and near-ultraviolet regions after photolysis of NH3 and ND3. The infrared absorption lines of NH2 associated with vibration-rotational levels of vibrational modes ν1 at 3234.3 (00,0-10,1), 3244.9 (00,0-11,1), and 3249.3 cm-1 (00,0-11,0), and ν2 at 1498.7 (10,1-11,1), 1509.5 (11,0-10,1), 1516.5 (00,0-10,1), 1528.6 (00,0-11,1), and 1533.7 cm-1 (00,0-11,0) were unambiguously identified according to the results of experiments with deuterium isotopes. The 00,0-00,0 lines of ν1 and ν2 for NH2 were derived to be at 3213.5 and 1494.6 cm-1 in solid neon.

Improvements in structural and optical properties of wafer-scale hexagonal boron nitride film by post-growth annealing.

Remarkable improvements in both structural and optical properties of wafer-scale hexagonal boron nitride (h-BN) films grown by metal-organic chemical vapor deposition (MOCVD) enabled by high-temperature post-growth annealing is presented. The enhanced crystallinity and homogeneity of the MOCVD-grown h-BN films grown at 1050 °C is attributed to the solid-state atomic rearrangement during the thermal annealing at 1600 °C. In addition, the appearance of the photoluminescence by excitonic transitions as well as enlarged optical band gap were observed for the post-annealed h-BN films as direct consequences of the microstructural improvement. The post-growth annealing is a very promising strategy to overcome limited crystallinity of h-BN films grown by typical MOCVD systems while maintaining their advantage of multiple wafer scalability for practical applications towards two-dimensional electronics and optoelectronics.

Identification of cyc-B3H3 with Three Bridging B-H-B Bonds in a Six-Membered Ring.

Irradiation of samples of diborane(6), B2H6 and B2D6, separately and together, dispersed in solid neon near 4 K with tunable far-ultraviolet light from a synchrotron yielded new infrared absorption lines that are assigned to several carriers. Besides H, B, BH, BH2, BH3, B2, B2H2, and B2H4, previously identified, a further species is assigned on the basis of quantum-chemical calculations of vibrational wavenumbers and intensities to be cyc-B3H3 (D 3h , singlet state) in several isotopic variants, which feature three bridging B-H-B bonds in a six-membered ring.