UV Raman Spectra Research Articles

Optical, Structural, and Synchrotron X-ray Absorption Studies for GaN Thin Films Grown on Si by Molecular Beam Epitaxy.

GaN on Si plays an important role in the integration and promotion of GaN-based wide-gap materials with Si-based integrated circuits (IC) technology. A series of GaN film materials were grown on Si (111) substrate using a unique plasma assistant molecular beam epitaxy (PA-MBE) technology and investigated using multiple characterization techniques of Nomarski microscopy (NM), high-resolution X-ray diffraction (HR-XRD), variable angular spectroscopic ellipsometry (VASE), Raman scattering, photoluminescence (PL), and synchrotron radiation (SR) near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. NM confirmed crack-free wurtzite (w-) GaN thin films in a large range of 180-1500 nm. XRD identified the w- single crystalline structure for these GaN films with the orientation along the c-axis in the normal growth direction. An optimized 700 °C growth temperature, plus other corresponding parameters, was obtained for the PA-MBE growth of GaN on Si, exhibiting strong PL emission, narrow/strong Raman phonon modes, XRD w-GaN peaks, and high crystalline perfection. VASE studies identified this set of MBE-grown GaN/Si as having very low Urbach energy of about 18 meV. UV (325 nm)-excited Raman spectra of GaN/Si samples exhibited the GaN E2(low) and E2(high) phonon modes clearly without Raman features from the Si substrate, overcoming the difficulties from visible (532 nm) Raman measurements with strong Si Raman features overwhelming the GaN signals. The combined UV excitation Raman-PL spectra revealed multiple LO phonons spread over the GaN fundamental band edge emission PL band due to the outgoing resonance effect. Calculation of the UV Raman spectra determined the carrier concentrations with excellent values. Angular-dependent NEXAFS on Ga K-edge revealed the significant anisotropy of the conduction band of w-GaN and identified the NEXAFS resonances corresponding to different final states in the hexagonal GaN films on Si. Comparative GaN material properties are investigated in depth.

Pre-resonance effects in deep UV Raman spectra of normal and deuterated water.

We have investigated the shape of the OH/OD stretching Raman band of water as a function of the excitation wavelength in the deep UV region (200-266 nm). By analyzing the spectral profiles, we highlighted selective pre-resonance effects in the high wavenumber component of the OH/OD stretching band, associated to distorted H-bonded water configurations. A van't Hoff treatment of the temperature-dependent Raman spectra provides an estimate of the thermal energy associated to the change from ordered (ice-like) to disordered configurations that agrees with values obtained by related methods based on a two-state model of water. These results open the possibility of exploiting the observed pre-resonance deep-UV signal enhancement to investigate H-bonding properties in aqueous media.

Deep-UV Raman spectroscopy: A novel heuristic method to characterize volcanologically relevant glasses on Mars

Deep ultraviolet Raman spectroscopy is an essential component of the Perseverance rover operating on Mars. Here we propose a proof of concept of deep-UV Raman structural characterization of volcanologically relevant silicate glasses to provide a suitable analytical method of UV Raman spectra collected on Mars. The results show few but substantial spectral differences concerning those obtained by conventional Raman scattering using visible light laser sources. The evolution of the UV Raman spectra between 825 and 1300 cm−1 is confirmed to be more sensitive to the silica network's short-range structure than that below 700 cm−1 when compared to their visible counterpart. We adopted a Gaussian fitting model to parametrize the number of bridging oxygens resulting from the tetrahedral‑oxygens stretching vibrations along a sub-alkaline join of volcanic glasses. We used these parameters to empirically provide a relation with the glass silica content. The model was externally validated with glasses having different and known compositions (in particular different amounts of iron, alkali and alumina) extending the validation set also to other magmatic series (i.e. alkaline and shoshonitic). Our approach enables a fast screening of deep-UV Raman spectra collected on Mars to disentangle the glass structure and retrieve its silica content within a given magmatic series. This method is relevant for planetary exploration and applies both to microanalysis of dry or hydrate volcanologically relevant glasses and general technical glasses since deep-UV Raman spectroscopy is particularly effective in reducing the unwished photoluminescence and increasing the signal-to-noise ratio.

Study on all solid-state 213 nm laser Raman spectrometer

Due to the adverse effects formed by fluorescence interference difficult to remove, intense interest has been directed towards the development of techniques and instrumentation to inhibit fluorescence. When the wavelength of Raman excitation light is less than 250 nm, we can completely separate the fluorescence noise from the Raman features in the UV spectrum. Based on this, we describe a new deep UV Raman spectroscopy system composed of a compact acousto-optically Q-switched diode-pumped solid-state (DPSS) neodymium-doped yttrium vanadate laser and a UV Raman spectrometer. The laser is capable of producing ∼140 mW of 213 nm power quasi-continuous wave as 9.6 ns pulses at a 20 kHz repetition rate. The UV Raman spectrometer with the incident slit of 50 μm width is based on Littrow structure, of which the spectral resolution is better than 0.05 nm in the band of 211.5 nm ∼ 235.5 nm meeting the requirements of 10 cm−1 Raman spectral resolution. We effectively reduce the volume of the test system and improve the robustness of the test system. We measure the deep UV Raman spectra of solution methanol, which verified the feasibility and rationality of the system.

TS-1 Synthesis via Subcrystal Aggregation: Construction of Highly Active Hydrogen-Bonded Titanium Species for Alkene Epoxidation.

The construction and determination of highly active Ti sites comprise one of the most significant challenges in the rational design and synthesis of Ti-containing porous catalysts. The pathway to efficiently build highly catalytically active titanium species remains to be proposed in spite of deliberate post treatments or ambiguous batch composition adjustments. In this study, we developed a bottom-up strategy to construct a TS-1 catalyst with highly active hydrogen-bonded Ti species via subcrystal aggregation crystallization. The microstructure of the hydrogen-bonded Ti species was verified by vacuum FT-IR and 1H MAS SSNMR spectroscopies. Noteworthy features of the hydrogen-bonded Ti species were also revealed, including a pentahedral coordination state and Brønsted acidity, as identified by the UV-Raman, XPS, XAFS, and FT-IR spectra of adsorbed pyridine. Significantly, the hydrogen-bonded Ti species exhibits extraordinary activity in allyl chloride epoxidation (nearly 70% higher than that of traditional Ti species). This study provides a new approach to building highly active Ti sites, which may provide new insights into the design and synthesis of high-performance titanosilicate catalysts.

Structural, electronic, and spectral properties of AlnCuk (n = 1–16; k= 0, ±1) alloy clusters

The structural, electronic, and spectral properties of AlnCuk (n = 1–16; k = 0, ±1) clusters have been examined via ab initio calculations combined with the CALYPSO method. It is found that the Cu atom in the ground-state AlnCuk cluster is situated on the surface for n = 1–11 and prefers a central site for n = 12–16 (except for in Al15Cu, Al12Cu+, and Al16Cu+). The stability, chemical activity, ionization potential, and electron affinity of the AlnCuk clusters are closely linked to the cluster size, the number of valence electrons, the point group, and the spin state. Simultaneously, we simulated UV spectra, photoelectron spectra, and vibrational and Raman spectra of the AlnCuk clusters. The global minimum structure of the AlnCu− cluster is recognized by our comparing the simulated and measured photoelectron spectra.

Synthesis and Room Temperature Structural, Optical, Electrical Properties of Holmium Doped GdMn2O5 Multiferroics

Multiferroic materials with the compositional formulae Gd1-xHoxMn2O5 (x = 0, 0.1, 0.2, 0.3, and 0.4) were prepared by the conventional solid-state reaction method. The structural analysis has been carried out by X-ray diffraction technique which confirms the orthorhombic structure with the Pbam space group. The optical properties have been studied by UV Visible, FTIR, and Raman spectra. The energy gap that was calculated using UV visible absorption spectra shows that the material is semiconducting in nature. The observed 14 Raman and 17 IR active modes are attributed to the relative motions of corresponding atoms and by increasing the doping concentration of Ho, the emergence of phonon shifting was observed which suggests the possible multiferroic coupling behaviour. The electrical properties including dielectric constant, dielectric loss, and ac conductivity at room temperature indicate the spin-lattice coupling.

Co-Crystalline ZSM-5/ZSM-11 Nanostructures for Alkylation of Benzene with Ethanol

The interzeolite transformation approach has been used extensively for the quick synthesis of zeolites at a lower cost. Herein, an interzeolite transformation method for the quick synthesis of co-crystalline ZSM-5/ZSM-11 (CDM-5) nanoaggregates has been developed by precisely controlling the synthesis parameters and using 1,6-hexamethylenediamine as an organic structure-directing agent. In contrast, the irregular CDM-5 zeolite block obtained from an amorphous aluminosilicate gel takes a longer crystallization time using the conventional method. Combined with the related results of characterization, UV-Raman spectra show that double 6-membered rings in the parent USY zeolite framework are broken, and an ordered aluminosilicate aggregate species (OAAS) containing 4-membered rings (4Rs) are formed during the interzeolite transformation. HRTEM and SEM images demonstrate that the block OAAS is dissolved into amorphous nanoparticles, followed by converting into CDM-5 nanoaggregates. These results reveal that an OAAS is essential for the synthesis of CDM-5 nanoaggregates via the interzeolite transformation method. In addition, the CDM-5 nanoaggregate catalyst exhibits superior stability and resistance to carbon deposition in the alkylation reaction of benzene with ethanol compared with the irregular block CDM-5 catalyst.

New insights into the structure and formation of coals, terrestrial and extraterrestrial kerogens from resonant UV Raman spectroscopy

Resonant UV (244 nm-excitation) Raman spectroscopy was used to characterize a series of coals, type II kerogens, Insoluble Organic Matter (IOM) extracted from primitive chondrites and two stratospheric Interplanetary Dust Particles. UV-Raman spectra of immature terrestrial kerogens and coals are distinct from those of extraterrestrial kerogens extracted from type 1 and 2 primitive chondrites, allowing unambiguous identification. The degree of disorder in the refractory carbonaceous matter in the considered IDPs is found to be higher than in primitive chondrites, confirming former interpretation of visible (514 nm-excitation) Raman spectra of IDPs. Spectral and structural features of laboratory analogues of chondritic organic matter formed in cold plasma reactors at 500–700 °C differ from those of natural samples, challenging the hypothesis that chondritic organic matter within the protosolar disk may form through plasma reactions. Finally, UV Raman data shed new light in the structural evolution of coals during maturation and support the presence of an sp2 structural transition at vitrinite reflectance Ro ∼ 0.4–0.5%. Comparison with former visible Raman and infrared spectroscopic data confirms the existence of this structural transition, which is related to a dramatic drop of oxygenated species (carboxyl, hydroxyl, ketone and ether groups). Oxygen thus plays an indirect but significant role in the control of the Raman spectra of polyaromatic materials. A potential application of this technique is the determination of the biotic versus abiotic origin of kerogens in ancient rocks on Earth, and of soils and rocks on Mars that is otherwise difficult with classical visible-excitation Raman spectroscopy.

Low temperature, pressureless sp2 to sp3 transformation of ultrathin, crystalline carbon films

Nanosized and crystalline sp3-bonded carbon materials were prepared over large surface areas up to ∼33 × 51 μm2 from the exposure of few-layer graphene (FLG) to H radicals produced by the hot-filament process at low temperature (below 325 °C) and pressure (50 Torr). Hybrid materials were also obtained from the partial conversion of FLG. sp3-C related peaks from diamond and/or lonsdaleite and/or hybrids of both were detected in UV and visible Raman spectra. CH bonding was directly detected by Fourier Transform Infrared (FTIR) microscopy over an area of ∼150 μm2 and one single component attributed to sp3-CH mode was detected in the CH stretching band showing that carbon is bonded to one single hydrogen and strongly suggesting that the sp3-C materials obtained are ultrathin films with basal planes hydrogenated. The experimental results are compared to computational predictions and comprehensively discussed. Those materials constitute new synthetic carbon nanoforms after fullerenes, nanodiamonds, carbon nanotubes and graphene. This opens the door to new research in multiple areas for the development of new potential applications and may have wide scientific impact, including for the understanding of extraterrestrial diamond-related structures and polytype formation mechanism(s).

UV-Raman scattering of thin film Si with ultrathin silicon oxide tunnel contact for high efficiency crystal silicon solar cells

A precise characterization of thin film silicon in devices is a great challenge for device optimization because the material properties depend on the substrate and change with film thickness. A specific situation is thin poly-Si layer in tunnel oxide passivated contact (TOPCon) structure in high-efficiency solar cells. In this paper, we present a systematic study using a wide range UV-Raman spectroscopy to measure the crystalline structure and chemical bonding configurations in the TOPCon structure. Because of the high absorption coefficient of 325-nm laser in silicon films, the Raman probes only the top surface region and minimizes the contribution from c-Si substrate. However, the comparison with the Raman spectra collected with a green laser of 532 nm, the UV-Raman spectrum of a-Si:H shows a very different spectral shape with the disappearance of the LA mode of Si-Si vibration and an appearance of additional signal at 325 cm−1. In addition, the UV-Raman spectrum of the pre-crystallized P doped a-Si:H layer shows most of the vibration modes of Si–Si, Si–H, Si–O–Si and Si–O–H bonds, where the Si–O–H bonds are in the top surface oxide layer. After the crystallization, the Si-Si TO mode shows a high degree of crystallization, the Si-H vibration modes disappear, and an additional Si-O-Si mode is observed which is believed from the ultrathin SiOx passivation layer. In addition, we provide a set of UV-Raman measurements on the samples made with the same layer structure but at different annealing temperatures to show the correlation between the UV-Raman spectra and the passivation quality of the TOPCon structure on the n-type c-Si wafers. These observations prove that UV-Raman spectroscopy is a useful characterization tool for high efficiency c-Si solar cells with TOPCon structure. In addition, the impacts of the material properties on the passivation quality are discussed.

Mechanism of zeolite X crystallization from diatomite

Here we systemically investigated the crystallization process of zeolite X using diatomite, a natural cost-effective silica precursor by the hydrothermal method. To decipher the mechanism, solids were separated from the mixtures at various stages of crystallization and were characterized by XRD, SEM, solid state NMR, FT-IR, UV-Raman and XRF. The XRD patterns revealed that the crystallization started between 3 and 3.5 h and ended by 5 h. The SEM images showed that the intermediate species of zeolite X appear on the surface of particles. The FT-IR and UV-Raman spectra suggested the presence of double six-membered rings (D6R) in the framework of zeolite X and they played a critical role in the crystallization process. Furthermore, the UV-Raman spectra indicated that the secondary building units of β cages formed the intermediate species and they were interconnected via double six-membered rings to form the framework of zeolite X.

The Anion Effect on Zeolite Linde Type A to Sodalite Phase Transformation

This study reports on the effect of three different salts (Na2CO3, NaCl, and Na2SO4) on desilication product (DSP) precipitation in synthetic Bayer liquor. Na2SO4 had a distinct effect compared with both Na2CO3 and NaCl as only nosean-sodalite formed. With the addition of Na2SO4, both the templating effect of SO42– and subunit condensation processes appear to be involved. Na2CO3 promoted the transformation of zeolite Linde Type A (LTA) to sodalite, while NaCl did not have a significant effect on phase transformation compared with the control test. The unit cell size of zeolite LTA did not change as a function of time and anions present. The unit cell size of sodalite did significantly change as a function of salts added as follows: NaCl < control < Na2CO3 < Na2SO4. UV-Raman spectra exhibited signature peaks for sodalite with Na2CO3 and Na2SO4 which is useful since the differences in the XRD patterns for the same materials are subtle.

Stand-off deep-UV Raman spectroscopy

UV Raman spectra were measured using a novel experimental configuration. This configuration allows many of the difficulties associated with UV excitation and high-power pulsed laser sources to be mitigated. Large sample areas are imaged into the detection system allowing high power excitation sources to be used while simultaneously avoiding sample degradation and multi-photon absorption effects. Such large detection areas allow large numbers of molecular scatters to be probed even with minimal penetration depth. Alignment issues between sample and collection optics are also simplified. Several common solvents were studied using 213 nm light and their spectra reported.

Structural Dynamics of Dispersed Titania During Dehydration and Oxidative Dehydrogenation Studied by In Situ UV Raman Spectroscopy

The structural dynamics of dispersed titania, i.e., silica supported titania, is investigated during dehydration and oxidative dehydrogenation (ODH) of ethanol using optical spectroscopy. UV Raman spectroscopy enabling resonance enhancements proves to be a valuable tool to identify Ti–OH, Ti–O–Si, and Ti–O–Ti groups. Upon dehydration, a transformation of Ti–OH into Ti–O–Si and Ti–O–Ti groups is observed. Two types of Ti–OH vibrations (isolated, geminal) are identified at around 700 and 800 cm− 1 in agreement with theoretical models. Dispersed titania is catalytically active in ethanol ODH with a performance comparable to dispersed vanadia. In situ UV Raman spectra reveal a consumption of Ti–O–Ti, Ti–O–Si, and Ti–OH groups during ethanol adsorption to the titania surface. The presented results are consistent with an ODH reaction mechanism involving a structural transformation of oligomerized or closely neighbored monomeric TiOX structures. The relevance of the proposed mechanism is discussed in the context of other supported transition metal oxide catalysts.

Contributions of tailored oxygen vacancies in ZnO/Al2O3 composites to the enhanced ability for H2S removal at room temperature

A series of Ni-doped ZnO/Al2O3 adsorbents with various compositions were fabricated through the sol–gel method. They were then tested dynamically in H2S removal from a gas stream containing H2S in moist N2. The adsorbents obtained were characterized using Scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2 adsorption and desorption, Powder X-ray diffraction (XRD), CO2 temperature-programmed desorption (CO2-TPD), UV-Raman spectroscopy, photoluminescence (PL) spectroscopy and X-ray photoelectron spectroscopy (XPS) techniques. The dynamic tests for H2S removal showed that the sulfur capacity tended to gradually increase at first and then decrease with the increase in Ni2+ dopant content. XPS, PL and UV-Raman spectra results indicated that the concentrations of oxygen vacancies increased after doping Ni species. The increased breakthrough capacity was strongly related to the concentrations of oxygen vacancies. Oxygen vacancies played a dual role in removing H2S in the dynamic tests. They could efficiently promote H2S dissociation by providing high OH concentrations on the ZnO surface by acid–base interaction. Furthermore, oxygen vacancies could efficiently promote the counterdiffusion of HS− or S2− and O2− in the bulk of ZnO. This mechanism of anion diffusion in the ZnO lattice was supposed in this study.

Damage and annealing recovery of boron-implanted ultra-shallow junction: The correlation between beam current and surface configuration

Abstract The condition of the beam current in the implantation process is a key issue in the damage rate and structural evolution in the sequent annealing process, especially for ultra-shallow layers. In this work, we develop a compensative optical method combined with UV Raman, X-ray photoelectron spectroscopy (XPS), and X-ray absorption near edge spectroscopy (XANES) to inspect the influence of the beam current in the implantation process. The optima condition of the beam current in the implantation process is determined by higher effective Si-B bond portion in UV Raman spectra and less the peak of B–B bond in XPS spectra which is caused by B cluster defects. Results of XANES indicate that the B oxide layer is formed on the surface of the ultra-shallow junction. The defects in the ultra-shallow junction after annealing are analyzed by novel optical analyses, which cannot be inspected by a traditional thermal wave and resistance measurement. This work exhibits the structural variation of the ultra-shallow junction via a variant beam current and provides a valuable metrology in examining the chemical states and the effective activation in the implantation technology.

The chemical states and atomic structure evolution of ultralow-energy high-dose Boron implanted Si(110) via laser annealing

Further scale down the dimension of silicon-based integrated circuit is a crucial trend in semiconductor fabrication. One of the most critical issues in the nano-device fabrication is to confirm the atomic structure evolution of the ultrathin shallow junction. In this report, UV Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge structure (XANES) and reflective second harmonic generation (RSHG) are utilized to monitor the pulse laser induced atomic structure evolution of ultralow-energy high-dose Boron implanted Si(110) at room and cold substrate temperature. A peak feature around 480 cm−1 resolved in UV Raman spectra indicates the formation of Si-B bond after the laser irradiation. The red shift of binding energy of Si element (~99 eV) in XPS and the evolution of absorption peak (~196.2 eV) in XANES reveal that the changes in the chemical states of ultra shallow junction strongly correlate to the activation process of Boron implantation, which is confirmed by RSHG measurement. The substrate temperature effect in the recrystallization of Boron implanted region is also realized by cross-section high-resolution TEM (HRTEM). The phenomena of Si-B bond formation and ultra-shallow junction recrystallization can be traced and applied to improve the reliability of Si ultra shallow junction in the future.

Identifying the Elusive Framework Niobium in NbS-1 Zeolite by UV Resonance Raman Spectroscopy.

It was found that bands at 739, 963, and 1107 cm-1 in the resonant Raman spectra are characteristics of framework penta-coordinated NbV -OH species, and that a band at 1336 cm-1 in the UV Raman spectrum with excitation line at 320 nm is a sensitive detector for identifying extra-framework niobium species. The change of framework penta-coordinated NbV -OH species into Nb+ and NbO- species due to dehydration was definitively confirmed based on UV resonance Raman and UV/Vis results.

Deep UV Raman spectroscopy for planetary exploration: The search for in situ organics

Raman spectroscopy has emerged as a powerful, non-contact, non-destructive technique for detection and characterization of in situ organic compounds. Excitation using deep UV wavelengths (< 250nm), in particular, offers the benefits of spectra obtained in a largely fluorescence-free region while taking advantage of signal enhancing resonance Raman effects for key classes of organic compounds, such as the aromatics. In order to demonstrate the utility of this technique for planetary exploration and astrobiological applications, we interrogated three sets of samples using a custom built Raman instrument equipped with a deep UV (248.6nm) excitation source. The sample sets included: (1) the Mojave Mars Simulant, a well characterized basaltic sample used as an analog for Martian regolith, in which we detected ∼0.04 wt% of condensed carbon; (2) a suite of organic (aromatic hydrocarbons, carboxylic acids, and amino acids) and astrobiologically relevant inorganic (sulfates, carbonates, phosphates, nitrates and perchlorate) standards, many of which have not had deep UV Raman spectra in the solid phase previously reported in the literature; and (3) Mojave Mars Simulant spiked with a representative selection of these standards, at a concentration of 1 wt%, in order to investigate natural 'real world' matrix effects. We were able to resolve all of the standards tested at this concentration. Some compounds, such as the aromatic hydrocarbons, have especially strong signals due to resonance effects even when present in trace amounts. Phenanthrene, one of the aromatic hydrocarbons, was also examined at a concentration of 0.1 wt% and even at this level was found to have a strong signal-to-noise ratio. It should be noted that the instrument utilized in this study was designed to approximate the operation of a 'fieldable' spectrometer in order to test astrobiological applications both here on Earth as well as for current and future planetary missions. It is the foundation of SHERLOC, an arm mounted instrument recently selected by NASA to fly on the next rover mission to Mars in 2020.