Optical and structural properties of Si nanocrystals in SiO₂ film

Silicon nanocrystals (Si-nc) embedded in a SiO₂ matrix is a promising system for silicon-based photonics. We studied optical and structural properties of Si-rich silicon oxide SiOₓ (x < 2) films annealed in a furnace at temperatures up to 1200 °C and containing Si-nc. The measured optical properties of SiOₓ films are compared with the values estimated by using the effective medium approximation and X-ray photoelectron spectroscopy (XPS) results. A good agreement is found between the measured and calculated refractive index. The results for absorption suggest high transparency of nanoscale suboxide. The extinction coefficient for elemental Si is found to be between the values for crystalline and amorphous Si. Thermal annealing increases the degree of Si crystallization; however, the Si–SiO₂ phase separation is not complete after annealing at 1200 °C. The 1.5-eV photoluminescence probably originates from small (~1 nm) oxidized Si grains or oxygen-related defects, but not from Si-nc with sizes of about 4 nm. The SiOx films prepared by molecular beam deposition and ion implantation are structurally and optically very different after preparation but become similar after annealing at ~1100 °C. The laser-induced thermal effects found for SiOₓ films on silica substrates illuminated by focused laser light should be taken into account in optical measurements. Continuous-wave laser irradiation can produce very high temperatures in free-standing SiOₓ and Si/SiO₂ superlattice films, which changes their structure and optical properties. The center of a laser-annealed area is very transparent and consists of amorphous SiO₂. Large Si-nc (up to 300 nm) are observed in the ring around the central region. These Si-nc produce high absorption and they are typically under compressive stress, which is connected with the crystallization from the melt phase. Some of the large Si-nc exhibit surface features, which is interpreted in terms of eruption of pressurized Si from the film. A part of large Si-nc is removed from the film forming holes of similar sizes. The presence of oxygen in the laser-annealing atmosphere decreases the amount of removed Si-nc. The structure of laser-annealed areas is explained by thermodiffusion, which leads to the macroscopic Si–SiO₂ phase separation. Comparison of the structure of central regions for laser annealing in oxygen, air, and inert atmospheres excludes the dominating effect of Si oxidation in the formation of laser-annealed area. By using a strongly focused laser beam, the structural changes in the free-standing films can be obtained in submicron areas, which suggests a concept of nonvolatile optical memory with high information density and superior thermal stability.Fotoniikan yhdistämisellä elektroniikkaan voidaan ratkaista monia modernien elektronisten laitteiden rajoittuneisuuksia. Piihin perustuva fotoniikka on erityisen lupaavaa, sillä pii on elektroniikan perusmateriaali. Yhdistämisessä haastavin tehtävä liittyy piihin perustuvan tehokkaan valolähteen konstruointiin. Piidioksidiin SiO₂ vangitut pii-nanokiteet (Si-nc) ovat osoittautuneet lupaaviksi, sillä valon emissiotehokkuus nanorakenteisessa piissä on merkittävästi makroskooppisen piin emissiotehokkuutta suurempi. Tämän väitöskirjan ensimmäisessä osassa tarkastellaan piillä rikastettujen ja aina 1200 Celsiusasteessa päästettyjen piidioksidikalvojen SiOₓ (x < 2) optisia ja rakenteellisia ominaisuuksia. Kalvot on valmistettu joko pii-ioni-istutuksella tai molekyylisuihkukasvatuksella. Näin muodostetuissa kalvoissa piidioksidiin vangitut pii-nanokiteet ovat halkaisijaltaan noin 4 nm. Tutkimuksessa on sovellettu fotoluminesenssia, Ramansirontaa, läpäisyspektroskopioita ja Röntgen fotoelektronispektroskopiaa (XPS). SiOₓ kalvojen mitatut taitekertoimet ja absorptiokertoimet on onnistuneesti arvioitu soveltaen effektiivisen väliaineen approksimaatiota ja XPS:llä mitattua kemiallista koostumusta. Työssä esitetään myös 1.5 eV:n fotoluminesenssin muodostumisen mahdollisia mekanismeja. Työn toisessa osassa tutkitaan laserilla tapahtuvan kuumennuksen vaikutuksia vapaisiin SiOₓ kalvoihin ja Si/SiO₂ superhiloihin. Yhteistyö useiden tutkimusryhmien kesken mahdollisti laajan mikroskooppisten menetelmien käytön (mikro –Raman, läpäisyspektroskopia, fotoluminesenssi, infrapunaspektroskopia, läpäisy –elektronimikroskopia, elektronin energiahäviöspektroskopia, pyyhkäisyelektronimikroskopia, atomivoimamikroskopia ja XPS). Fokusoidulla laservalolla tapahtuva kuumennus aiheuttaa merkittäviä rakenteellisia ja optisia muutoksia kalvoissa. Laservalolla käsitellyillä alueilla havaitaan amorfisessa SiO₂ matriisissa jopa 300 nm:n suuruisia pii-nanokiteitä ja samalla Ramansignaali kasvaa voimakkaasti. Kuumennuksen yhteydessä käytetyn kaasuympäristön (ilma, Ar tai O₂) ja säteilytysajan (1 ms, 1 s, 100 s) vaikutuksia pii-nanokiteisiin tutkittiin. Rakenteelliset muutokset voidaan havaita alle mikrometrin suuruisilla alueilla valokuvaamalla, Ramansironnalla ja läpäisyspektroskopialla. Tulosten perusteella voidaan toteuttaa optinen muisti, jonka tietotiheys on suuri, se on termisesti erittäin kestävä ja muistitieto voidaan pyyhkiä pois ja kirjoittaa uudelleen optisin menetelmin

Optical and Structural Properties of Si Nanocrystals in SiO2 Films

Optical and structural properties of Si nanocrystals (Si-nc) in silica films are described. For the SiOx (x <2) films annealed above 1000 degrees C, the Raman signal of Si-nc and the absorption coefficient are proportional to the amount of elemental Si detected by X-ray photoelectron spectroscopy. A good agreement is found between the measured refractive index and the value estimated by using the effective-medium approximation. The extinction coefficient of elemental Si is found to be between the values of crystalline and amorphous Si. Thermal annealing increases the degree of Si crystallization; however, the crystallization and the Si-SiO2 phase separation are not complete after annealing at 1200 degrees C. The 1.5-eV PL quantum yield increases as the amount of elemental Si decreases; thus, this PL is probably not directly from Si-nc responsible for absorption and detected by Raman spectroscopy. Continuous-wave laser light can produce very high temperatures in the free-standing films, which changes their structural and optical properties. For relatively large laser spots, the center of the laser-annealed area is very transparent and consists of amorphous SiO2. Large Si-nc (up to ~300 nm in diameter) are observed in the ring around the central region. These Si-nc lead to high absorption and they are typically under compressive stress, which is connected with their formation from the liquid phase. By using strongly focused laser beams, the structural changes in the free-standing films can be made in submicron areas.Peer reviewe

Ion irradiation of multi-walled boron nitride nanotubes

Non Peer reviewe

Characterization of ion-irradiation-induced defects in multi-walled carbon nanotubes

Peer reviewe

Broadband optical properties of monolayer and bulk MoS2

Layered semiconductors such as transition metal dichalcogenides (TMDs) offer endless possibilities for designing modern photonic and optoelectronic components. However, their optical engineering is still a challenging task owing to multiple obstacles, including the absence of a rapid, contactless, and the reliable method to obtain their dielectric function as well as to evaluate in situ the changes in optical constants and exciton binding energies. Here, we present an advanced approach based on ellipsometry measurements for retrieval of dielectric functions and the excitonic properties of both monolayer and bulk TMDs. Using this method, we conduct a detailed study of monolayer MoS2 and its bulk crystal in the broad spectral range (290–3300 nm). In the near- and mid-infrared ranges, both configurations appear to have no optical absorption and possess an extremely high dielectric permittivity making them favorable for lossless subwavelength photonics. In addition, the proposed approach opens a possibility to observe a previously unreported peak in the dielectric function of monolayer MoS2 induced by the use of perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS) seeding promoters for MoS2 synthesis and thus enables its applications in chemical and biological sensing. Therefore, this technique as a whole offers a state-of-the-art metrological tool for next-generation TMD-based devices

The genetic history of the Southern Arc: a bridge between West Asia and Europe

By sequencing 727 ancient individuals from the Southern Arc (Anatolia and its neighbors in Southeastern Europe and West Asia) over 10,000 years, we contextualize its Chalcolithic period and Bronze Age (about 5000 to 1000 BCE), when extensive gene flow entangled it with the Eurasian steppe. Two streams of migration transmitted Caucasus and Anatolian/Levantine ancestry northward, and the Yamnaya pastoralists, formed on the steppe, then spread southward into the Balkans and across the Caucasus into Armenia, where they left numerous patrilineal descendants. Anatolia was transformed by intra–West Asian gene flow, with negligible impact of the later Yamnaya migrations. This contrasts with all other regions where Indo-European languages were spoken, suggesting that the homeland of the Indo-Anatolian language family was in West Asia, with only secondary dispersals of non-Anatolian Indo-Europeans from the steppe

Matrix isolation study of methyl propiolate in argon and nitrogen matrices

Methyl propiolate (HCtriple bondCCOOCH3, MP) isolated in argon and nitrogen matrices was experimentally studied by infrared spectroscopy and by quantum chemical calculations. The calculations, carried out at the DFT(B3LYP) and MP2 levels of theory, predict the existence of two conformers with a planar skeleton. In situ broadband UV light (λ > 200 nm) irradiation of matrix-isolated MP led to the production of new bands ascribed to conformer II testifying the occurrence of the I →II photoisomerization. No other photochemical processes were observed. Assignment of the IR spectra of both conformers is presented, considerably extending the vibrational information available for this molecule hitherto

Structural, spectroscopic, and photochemical study of ethyl propiolate isolated in cryogenic argon and nitrogen matrices

Ethyl propiolate (HC ≡ CCOOCH2CH3, EP) was studied experimentally by infrared spectroscopy in argon and nitrogen cryomatrices (15 K) and by quantum chemical calculations (at the DFT(B3LYP) and MP2 levels of theory). Calculations predict the existence of four conformers: two low-energy conformers (I and II) possessing the carboxylic moiety in the cis configuration (O=C-O-C dihedral equal to ~0°) and two higher-energy trans forms (O=C-O-C dihedral equal to ~180°; III and IV). The conformation of the ethyl ester group within each pair of conformers is either anti (C-O-C-C equal to 180°; in conformers I and III) or gauche (C-O-C-C equal to ±86.6° in II, and ± 92.5° in IV). The two low-energy cis conformers (I and II) were predicted to differ in energy by less than 2.5 kJ mol-1 and were shown to be present in the studied cryogenic matrices. Characteristic bands for each one of these conformers were identified in the infrared spectra of the matrix-isolated compound and assigned taking into account the results of normal coordinate analysis, which used the geometries and harmonic force constants obtained in the DFT calculations. The two trans conformers (III and IV) were estimated to be 17.5 kJ mol-1 higher in energy than the conformational ground state (form I) and were not observed experimentally. The unimolecular photochemistry of matrix-isolated EP (in N2 matrix) was also investigated. In situ irradiation with UV light (λ > 235 nm) leads mainly to decarbonylation of the compound, with generation of ethoxyethyne, which in a subsequent photoreaction generates ketene (plus ethene)

Giant Raman gain in silicon nanocrystals

Nanostructured silicon has generated a lot of interest in the past decades as a key material for silicon-based photonics. The low absorption coefficient makes silicon nanocrystals attractive as an active medium in waveguide structures, and their third-order nonlinear optical properties are crucial for the development of next generation nonlinear photonic devices. Here we report the first observation of stimulated Raman scattering in silicon nanocrystals embedded in a silica matrix under non-resonant excitation at infrared wavelengths (~1.5 μm). Raman gain is directly measured as a function of the silicon content. A giant Raman gain from the silicon nanocrystals is obtained that is up to four orders of magnitude greater than in crystalline silicon. These results demonstrate the first Raman amplifier based on silicon nanocrystals in a silica matrix, thus opening new perspectives for the realization of more efficient Raman lasers with ultra-small sizes, which would increase the synergy between electronic and photonic devices.Peer reviewe