Room-temperature Cathodoluminescence Spectra Research Articles

Branched MgO Nanowires Synthesized by Thermal Evaporation Method in Air at Atmospheric Pressure

MgO nanowires with a branched structure were fabricated using a thermal evaporation method in air at atmospheric pressure. The branched MgO nanowire was made up of two parts: a primary central nanowire trunk and lots of secondary nanowire branches. The branched MgO nanowires had a 4-fold symmetrical structure. The secondary nanowire branches grew perpendicular on the four side facets of the central nanowire trunks with square cross-sections. The nanowire branches also grew in a single row and were vertically well aligned in the same direction with each other. The scanning electron microscopy images of the branched nanowires grown at 1000<sup>o</sup>C showed that the diameter of branches gradually decreased along the growth direction and no catalyst particle was found at the tips of the branches, indicating that the branches were grown by a vapor-solid process. For the branched nanowires grown at 1150<sup>o</sup>C, spherical particles which were shown to be catalysts were observed at the tips of the branches. The chemical analysis by energy dispersive spectroscopy showed that the spherical particles were composed of Mg and O elements. These results suggest that the branches’ growth resulted from a self-catalyzed vapor-liquid-solid process. The structural characterization by X-ray diffraction confirmed that the branched MgO nanowires had a cubic lattice structure. The room temperature cathodoluminescence spectra of the branched MgO nanowires exhibited a very strong visible emission which was associated with oxygen vacancies.

Catalyst-Free Thermal Evaporation Synthesis of TiO<sub>2</sub> Nanostructures in Atmospheric Air

TiO<sub>2</sub> nanostructures were synthesized using a thermal evaporation method without a catalyst. TiO powders mixed with graphite powders were used as the source materials. The synthesis process was performed in air atmosphere at 1000 °C. When the mass ratio of TiO/graphite in the source material was 2:1, TiO<sub>2</sub> nanowires and nanobelts started to form. As the mass ratio of graphite to TiO increased to 1:1, TiO<sub>2</sub> nanowires and nanobelts were formed in large quantity. The nanowires had an average diameter of 80 nm and lengths in the range of 3 ~ 11 µm. The average width and length of the nanobelts were 500 nm and 3.4 µm, respectively. However, with further increase in the mass ratio of TiO/graphite to 1:2, no nanostructures were observed. The mass ratio of graphite to TiO in the source material had an important effect on the formation of the TiO<sub>2</sub> nanowires and nanobelts. The X-ray diffraction data confirmed that the TiO<sub>2</sub> nanostructures had a rutile crystal structure. Two emission bands centered at 410 nm and 510 nm were observed in the room temperature cathodoluminescence spectrum of the TiO<sub>2</sub> nanostructures. The emission at 410 nm is attributed to the electron transition from the conduction band to the valence band in rutile TiO<sub>2</sub> crystal, which is indicative of the high crystallinity of the TiO<sub>2</sub> nanostructures.

Optoelectronic attenuation behavior of Al2O3/ZnO nanolaminates grown by Atomic Layer Deposition

Zinc oxide (ZnO) and aluminum oxide (Al2O3) nanolaminates deposited in a homemade Atomic Layer Deposition (ALD) system are presented. In this work, three samples with a multi-films structure formed by four Al2O3/ZnO bi-layers of 20 nm were synthetized by thermal ALD. To analyze the optoelectronic response, the thickness of the Al2O3 was varied at 1%, 3% and 5% of the total bi-layer width for each sample. A curve of saturation was performed by means of spectroscopic ellipsometry to determine the dose time for the precursors. As a result, the synthesis of ZnO films was obtained using 50 ms for Diethylzinc dosification. To estimate the thickness of Al2O3 layers of the nanolaminates, X-ray photoelectron spectroscopy was used. To characterize the optical response of the samples, room temperature cathodoluminescence spectra were obtained from different multilayer structures. The attenuation of the luminescence was related to the integration of the aluminum oxide on the nanolaminates. As a result, a correlation between the reduction of the luminescence intensity of the multilayer and the increment of the Al2O3 layer thickness was demonstrated. The accurate control on the deposition of thin films makes this process a suitable method to be used in the optoelectronic industry as a precise coating technique with attenuation applications.

Room-Temperature 2.5 eV Pulsed Cathodoluminescence Band of High-Purity Silicon Dioxide

We study the room-temperature (RT) pulsed cathodoluminescence (PCL) spectra of a set of pure synthetic (both crystalline and amorphous) silicon dioxide samples. We show that the PCL spectra of all samples (both amorphous and crystalline) possess bands with intensity maxima in the region of 487 – 500 nm (2.54 – 2.48 eV). These bands are the most intense in the PCL spectra of disordered materials. We investigate the annealing behavior of RT PCL spectra of the crystalline and amorphous samples. Annealing has no significant effect on this emission. We demonstrate that the surface area of the material plays no role in the emission of PCL bands at 415 and 490 nm in the spectra of α-quartz single crystal and crystalline powder with grain sizes of 10 – 100 μm. Our results show that the bands in the region of 2.5 eV are the universal property of all synthetic pure SiO2 samples. The nature of the SiO2 emission band in the region of 2.5 eV is not clear; we discuss two possible explanations. The first one is based on considering the intrinsic emission due to self-trapped exciton (STE) decay with the transient O-O (oxygen–oxygen) bond formation. The second one is based on the role of Li ions in the emission process.

Structure of the Room-Temperature 2.5 eV α-Quartz Single-Crystal Pulsed Cathodoluminescence Band

We study the room-temperature (RT) pulsed cathodoluminescence (PCL) spectrum of a high-purity synthetic α-quartz single crystal. The spectrum consists of two wide bands with intensity maxima at 415 and 490 nm (2.99 and 2.53 eV). The band at 490 nm (2.5 eV) is polarized in the XY crystal plane (perpendicular to the third-order symmetry axis) and possesses a structure with three peaks at 480±2, 487±2, and 493±2 nm (2.58±0.01, 2.55±0.01, and 2.52±0.01 eV). The intensities of the peaks at 480±2 and 493±2 nm increased with increase in the irradiation dose up to 45 kGy. Peaks are equidistant at the energetic scale. The energy separation between the peaks Δ = 0.03 ± 0.01 eV is equal in order of magnitude to energies of Li x O y molecular vibrations and to the energy of the optical phonon in α-quartz. We propose an explanation of the experimental data obtained. According to this explanation, the structure observed may be attributed to the amplitude modulation of the quartz 2.5 eV emission band by the crystalline electric fields on frequencies of optical phonons. The nonequilibrium phonons may arise during the electron-beam irradiation.

Novel, low-cost solid-liquid-solid process for the synthesis of α-Si3N4 nanowires at lower temperatures and their luminescence properties.

Ultra-long, single crystal, α-Si3N4 nanowires sheathed with amorphous silicon oxide were synthesised by an improved, simplified solid-liquid-solid (SLS) method at 1150 °C without using flowing gases (N2, CH4, Ar, NH3, etc.). Phases, chemical composition, and structural characterisation using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM/HRTEM), Fourier-transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS) showed that the nanowires had Si3N4@SiOx core-shell structures. The growth of the nanowires was governed by the solid-liquid-solid (SLS) mechanism. The room temperature photoluminescence (PL) and cathodoluminescence (CL) spectra showed that the optical properties of the α-Si3N4 nanowires can be changed along with the excitation wavelength or the excitation light source. This work can be useful, not only for simplifying the design and synthesis of Si-related nanostructures, but also for developing new generation nanodevices with changeable photoelectronic properties.

Zn-Mn 혼합물의 열 증발에 의한 ZnO 결정의 성장에 미치는 Mn의 영향

ZnO crystals with different morphologies were synthesized through a thermal evaporation of Zn-Mn mixtures in air. The morphology was dependant on the Mn content in Zn-Mn mixture. The morphology was changed from rod to tetrapod shape with decreasing Mn content in Zn-Mn mixture. The result indicates that the concentration of Mn might be responsible for the different morphologies of ZnO crystals. XRD spectra showed that the ZnO crystals had a hexagonal wurtzite crystal strutures. For all the samples, room temperature cathodoluminescence spectra showed a ultra-violet emission at 380 nm and a green emission at around 500 nm. However, the intensity ratio of ultra-violet emission to green emission was significantly different with the Mn content in the source material.

Synthesis of CuO/ZnO Composite Nanostructures, Their Optical Characterization and Valence Band Offset Determination by X-Ray Photoelectron Spectroscopy

The study of the optical properties of zinc oxide (ZnO) nanorods and its composite nanostructures is an appealing issue in order to have highly efficient optoelectronic devices in the future. Zinc oxide, copper oxide (CuO) and their composite nanostructures were grown by the hydrothermal growth technique. The structural properties of the grown ZnO, CuO and their composite NSs were investigated by scanning electron microscopy and X-ray diffraction techniques. UV-visible spectroscopy, Cathodoluminescence and photoluminescence techniques were used for the study of optical properties of the as synthesized nanomaterials. X-ray photoelectron spectroscopy was used to measure the valence band offset of the CuO/ZnO composite nanostructures. Cathodoluminescence study of pure ZnO nanorods showed more insight for lateral luminescence compared to the top surface of the nanorods which opened a novel investigation in the area of optical properties of ZnO nanomaterial. While the room temperature cathodoluminescence spectra of CuO/ZnO composite nanostructures have demonstrated excellent luminescence in the UV region compared to the cathodoluminescence spectra observed at 4 K. The measured values for valence band offset and conduction band offset are found to be 2.83 eV and 0.73 eV, respectively for the prepared CuO/ZnO composite nanostructures. It was observed that CuO/ZnO composite nanostructures have type-II band alignment. The conclusion from both the cathodoluminescence and the photoluminescence studies showed that the luminescence in the visible region is only originated from the ZnO nanomaterial and that the CuO absorbs this visible emission as it covers the ZnO. This provides evidence suggesting that the use of CuO/ZnO in the fabrication of LEDs in the visible range is not appropriate.

공기 중에서 열증발법에 의하여 제작된 튜브 형상의 ZnO 결정

Tubular-shaped ZnO crystals were synthesized by thermal evaporation technique under air atmosphere. Mixture of Zn and Mg powder was used as the source material. The thermal evaporation and oxidation of Zn/Mg mixture were carried out for 1 hr at and under in air under atmospheric pressure. When only Zn powder was used as a source material, tetrapod-shaped ZnO crystals were synthesized. This provides that Mg played a key role in the formation of the tubular-shaped crystals. SEM images showed that the tubular-shaped ZnO crystals grew along [0001] direction. XRD spectrum revealed that the ZnO tubes had hexagonal wurtzite structure. Two emission peaks at 380 nm and 510 nm were observed in the room temperature cathodoluminescence spectrum.

Rapid synthesis of SnO2 nanowires employing solar thermal energy

SnO2 nanowires were synthesized by an unique thermal evaporation technique employing solar energy at atmospheric pressure in air. Sunlight was used to create heat, which is referred herein to as solar thermal energy, a clean form of energy. The sunlight was focused onto a small area containing the tin (Sn) powder by using a magnifying lens. The concentrated sunlight created sufficient heat to melt and vaporize the Sn powder. Furthermore, the Sn vapor was made to react with oxygen in air to form tin oxide (SnO2) product. The product was confirmed to consist of high purity SnO2 nanowires. The SnO2 nanowires was obtained within just several seconds by the solar thermal evaporation technique. The SnO2 nanowires were about several tens of nanometers in diameter and a few tens of micrometers in length. The ultraviolet (UV) emission at 355nm as well as a visible emission at around 580nm was observed in the room temperature cathodoluminescence (CL) spectrum, which confirms the high quality of the SnO2 nanowires. These results establish a novel rapid synthesis process for the formation of SnO2 nanowires.

PREPARATION, CHARACTERIZATION AND LUMINESCENCE PROPERTIES OF Eu DOPED CaAl2O4 NANOCONES

Europium doped CaAl 2 O 4 nanocones have been grown first time by thermal evaporation method. Scanning electron microscope (SEM) and transmission electron microscope (TEM) were used to analyze the morphology, size and crystal structure of the nanocones. The body of the nanocones are about 2–20 μm in length and their diameters are 200 nm to 1 μm at one end and tapers off to a ~ 40–200 nm at the tip end. The as-synthesized nanocones are single crystalline in monoclinic structure and grow along the [010] direction and the normal direction of (100) and (001). The room temperature photoluminescence (PL) and cathodoluminescence (CL) spectrum measurement reveals that CaAl 2 O 4: Eu 2+ nanocones emit light at about 440 nm.

A facile method for the preparation of Eu2+-doped nanocrystalline BaFCl

A facile method for the preparation of Eu2+-doped BaFCl is reported. The method is based on the co-precipitation of aqueous solutions of BaCl2 and NH4F to yield BaFCl. The doping by europium in the 2+ oxidation state is realized by the reduction of Eu3+ to Eu2+ employing granular zinc in the BaCl2 solution under nitrogen. Powder X-ray diffraction and electron microscopy have been used to confirm the BaFCl phase and photoluminescence, in the temperature range of 2.5–290K, and room-temperature cathodoluminescence spectra have been measured to characterize the Eu2+ ions in the sample.

Hybrid organic zinc oxide white-light-emitting diodes on disposable paper substrate

ZnO–organic hybrid white-light-emitting diodes (WLEDs) were demonstrated on a paper substrate. The configuration used for ZnO–organic hybrid WLEDs consists a layer of poly (9,9)-(dioctylfluorene) (PFO) on poly (3,4-ethylenedioxythiophene) poly (styrenesulfonate) (PEDOT: PSS) on n-type ZnO nanorods grown by a low-temperature chemical aqueous method on paper substrate. Room temperature photoluminescence, electroluminescence, and cathodoluminescence (CL) spectra reveal a broad visible region covering the range from 420 to 800 nm. By using room temperature-CL, we got luminescence information, especially to verify the origin of specific emissions, the internal absorption of the ultraviolet and the spatial distribution of radiative defects. It was observed that the visible wavelength range depends on the penetration depth of the excitation. This suggests that the concentration of deep levels responsible for the visible luminescence is at the sample surface to a depth of 1–2 µm when using an accelerating voltage up to 20–30 kV. The results indicate that demonstration of WLEDs on paper substrate with reasonable electrical performance greatly influences the reduction of substrate cost, furthermore, this may open way to fabricate optoelectronics devices on disposable substrates for large-area applications.

Low Temperature Thermal Evaporation Process for the Synthesis of ZnO Nanowires

ZnO nanowires were synthesized by thermal evaporation of ZnBr2 powder at relatively low temperatures of 600­700°C under air atmosphere. Any catalysts and substrates were not used in the synthesis of ZnO nanowires. The ZnO nanowires had a typical diameter of 100nm and lengths up to several tens micrometers. The X-ray diffraction (XRD) pattern indicated that the ZnO had hexagonal wurtzite structure. The scanning electron microscope (SEM) image showed clearly that catalyst particles existed at the tips of nanowires. The energy dispersive X-ray (EDX) analysis revealed that the catalyst particles were composed of Zn and O. Based on the SEM and EDX results, it was suggested that the nanowires were grown via self-catalytic growth mechanism. A strong green emission was observed in the room temperature cathodoluminescence spectrum. [doi:10.2320/matertrans.M2013156]

Selective β-SiC/SiO<sub>2</sub> Core-Shell NW Growth on Patterned Silicon Substrate

β-SiC/SiO2 core-shell NWs have been synthesized on patterned silicon wafers in a CVD system, using carbon oxide as single precursor and nickel nitrate as catalytic element, in nitrogen or argon atmosphere at 1100°C. The coaxial structure and the crystallinity of the NW core are examined by (scanning) transmission electron microscopy. The patterning of the substrate allows to grow NWs in selected areas only, as imaged by SEM. Cathodoluminescence (CL) panchromatic images of the same areas point out that the light emitted under electron excitation is localized only in the area covered with NWs. The room-temperature CL spectrum has three different components peaked at 2.45 eV, related to the 3C-SiC near-band-edge emission, and at 2.75 and 3.75 eV, that are induced by the triplet and singlet states of oxygen-deficiency centers ODC(II) in silicon dioxide shell.

Crystal growth and characterization of gallium oxynitride nanowires grown on seed crystals

Nanowires of gallium oxynitride with wurtzite type structure were grown using seed crystals obtained by ammonia nitridation of an amorphous gallium oxide precursor containing 3 at% nickel additive. The seed crystals on a silica substrate were annealed with the amorphous gallium oxide precursor under ammonia flow to grow gallium oxynitride nanowires. The nanowires grew to lengths of about 150 μm along the seed crystals parallel to the hexagonal c-plane at 750 and 800 °C but they did not grow in the lateral direction. When the growth temperature was increased above 900 °C, the growth direction gradually changed to become parallel with the c-axis with a copresenting of gallium-nitride-like crystals. Room-temperature cathodoluminescence spectra of nanowires grown at 800 °C exhibited strong blue emission at 2.69 eV along with weak band-edge emission at 3.39 eV, similar to those of GaN. The latter emission was intense for nanowires grown at 1000 °C, which had an improved crystallinity and a higher nitride/oxide ion ratio. Nanowires grown at 750 and 900 °C exhibited persistent photoconductivity under UV irradiation at 393 nm.

Luminescent properties of ZnO thin films treated by pulse-modulated high-power inductively coupled plasma

The effect of hydrogen plasma irradiation on luminescence properties of ZnO thin films was studied by using a pulse-modulated inductively coupled plasma technique. H-plasma exposure distance was changed to investigate the effect of hydrogen plasma irradiation on luminescent properties. Room temperature cathodoluminescence (CL) spectrum shows that hydrogen plasma irradiation can increase the efficiency of UV emission at 3.27 eV, and the improvement is strongly dependent on H-plasma exposure distance. For low temperature CL spectra, the intensity of donor–acceptor pair (DAP) transition at 3.315 eV has been increased more rapidly after hydrogen plasma irradiation, leading DAP to be the dominant transition.

Growth, Characterization, and Polarity Identification of (0001) Zn1 − xMgxO Epitaxial Films on Lattice-Matched β-LiGaO2 (001) Substrates

Small lattice-mismatched (001) substrates have been used for the epitaxial growth of (0001) films using the metallorganic chemical vapor deposition method. Structure characterizations by employing X-ray diffraction and transmission electron microscope demonstrate that no MgO or Mg cluster exists throughout the films. The epitaxial (0001) films are successfully grown on the (001) substrates with the epitaxial relationship of and . Polarity of the films is determined by the convergent beam electron diffraction technique as well as wet chemical etching. Both results indicate the Zn-polarity of films. The room-temperature cathodoluminescence spectra of the epitaxial (0001) films show an obvious blueshift of the near-bandedge emission with increasing Mg content, suggesting a successful bandgap engineering in the epitaxial (0001) films on the (001) substrates.

Study of Au/ZnO nanorods Schottky light-emitting diodes grown by low-temperature aqueous chemical method

High quality vertically aligned ZnO nanorods (NRs) were grown by low-temperature aqueous chemical technique on 4H-n-SiC substrates. Schottky light-emitting diodes (LEDs) were fabricated. The current-voltage (I–V) characteristics of Schottky diodes reveal good rectifying behavior. Optical properties of the ZnO nanorods (NRs) were probed by cathodoluminescence (CL) measurements at room temperature complemented with electroluminescence (EL). The room-temperature CL spectra of the ZnO NRs exhibit near band edge (NBE) emission as well as strong deep level emission (DLE) centered at 690 nm. At room temperature the CL spectra intensity of the DLE was enhanced with the increase of the electron beam penetration depth due to the increase of defect concentration at the interface and due to the conversion of self-absorbed UV emission. We observed a variation in the DLE along the nanorod depth. This indicates a relatively lower structural quality near the interface between ZnO NRs and n-SiC substrate. The room-temperature CL spectra of SiC show very weak emission, which confirms that most of the DLE is originating from the ZnO NRs, and SiC has a minute contribution to the emission.

Self-standing device-quality single-crystalline diamond films on vicinal (001) surfaces fabricated using microwave-plasma chemical-vapor-deposition method

Device-quality self-standing diamond films have been fabricated by growing sufficiently thick homoepitaxial films on high-pressure/high-temperature-synthesized (HPHT) Ib-type (001) and vicinal (001) diamond substrates with the microwave-plasma chemical-vapor-deposition (CVD) method. The HPHT substrates used were separated from the homoepitaxial layers with a laser cutting technique. Higher-quality thick films were more successfully obtained in the case of the vicinal substrates, compared with the case of the substrate without substantial off-angle, indicating advantages of vicinal (001) Ib substrates. In room-temperature cathodoluminescence (CL) spectra taken on both sides of the self-standing diamond films thus fabricated, strong free-exciton emissions were observed even at room temperature whereas no nitrogen-related CL peaks were detectable, which are often observed even from rather-thick homoepitaxial CVD diamond films grown on HPHT Ib substrates.