Title Ispitivanje niskoenergetskih Ramanovih modova u dvodimenzionalnim materijalima Title (english) Observation of Low-Energy Raman Modes inTwo-Dimensional Materials Author Ana Senkić Mentor Nataša Vujičić (mentor)Mentor Mihael Srđan Grbić (mentor)Committee member Mihael Srđan Grbić (predsjednik povjerenstva)Committee member Nataša Vujičić (član povjerenstva)Committee member Ivo Batistić (član povjerenstva)Committee member Mario Basletić (član povjerenstva)Committee member Danko Radić (član povjerenstva)Granter University of Zagreb Defense date and country 2019-07-15, Croatia Scientific / art field, NATURAL SCIENCES Abstract Još od izoliranja jednoslojnog (1L) grafena 2004. godine [1], dvodimenzionalni (2-D) slojeviti materijali su jedna od najistraživanijih klasa materijala. Grafen je materijal sačinjen od jedne ravnine ugljikovih atoma koji su međusobno povezani kovalentnom vezom, a unutar višesloja ravnine grafena su povezane slabim van der Waalsovim silama. Iako su potencijalne primjene grafena raznovrsne, nepostojanje energijskog procijepa ograničava njegovu primjenu u elektronici, primjerice u proizvodnji tranzistora koji bi zamijenili današnje silicijeve. Kao potencijalni kandidati nameću se materijali koji imaju poluvodička svojstva poput dihalkogenida prijelaznih metala. Njihova općenita formula je MX2 te se jedan sloj takvog materijala sastoji od tri ravnine X-M-X gdje X predstavlja halkogeni atom (S, Se ili Te) dok je M atom prijelaznog metala (Mo ili W). Osim tzv. homostruktura, struktura koje su sačinjene od jedne vrste materijala, primjerice MoS2 ili WS2, moguće je sintetizirati i van der Waalsove heterostrukture - strukture nastale kombinacijom različitih 2-D materijala. Takvi hibridni materijali imaju nova, kombinirana i sinergijska svojstva svojih sastavnih dijelova te je stoga potrebna univerzalna neinvazivna tehnika određivanja vrste materijala, njegove debljine, relativne orijentacije kristalnih osi susjednih slojeva itd. Tehnika koja odgovara takvim zahtjevima je Ramanova spektroskopija kojom se proučavaju vibracije kristalne rešetke materijala. Posebno je zanimljiv niskoenergetski dio spektra koji je direktna posljedica načina titranja jednog sloja kao cjeline u odnosu na susjedne slojeve. Niskoenergetski Ramanovi modovi obuhvaćaju dvije vrste titranja: smicanje slojeva - slojevi titraju u protufazi duž ravnine prostiranja, te disanje slojeva - slojevi titraju duž z-osi koja je okomita na ravninu prostiranja materijala i na taj način se ili približavaju ili odmiču jedan od drugog. Pokazalo se da su upravo ti niskoenergetski Ramanovi modovi puno učinkovitiji u određivanju broja slojeva, načina njihovog slaganja te interakcije. Niskoenergetski Ramanovi modovi imaju znantno manji intenzitet u odnosu na visokoenergetske, a blizina jake Rayleigheve linije čini njihovo opažanje još izazovnijim i zahtjevnijim. Razvojem spektroskopskih metoda, danas smo u mogućnosti eksperimentalno opaziti i ove niskoenergetske Ramanove modove. U ovom radu je nadograden postojeći eksperimentalni postav odgovarajućim filterima koji su omogućili snimanje niskoenergetskih Ramanovih modova raznih materijala: mehanički eksfoliranih uzoraka grafita i WSe2 te uzoraka dobivenih CVD metodom MoS2 i WS2, a nakon toga je napravljena i analiza dobivenih spektara. Uočeno je da u slučaju grafitnih i WS2 uzoraka niskoenergetski dio Ramanovih spektara nema uočljivih razlika za različite debljine slojeva. Uzrok tomu je preveliko pobuđivanje nositelja naboja u Si/SiO2 podlozi u slučaju grafitnih uzoraka, dok je kod WS2 riječ o rezonantnom Ramanovom raspršenju: energija lasera pobudne valne duljine 532 nm je slična energiji apsorpcijskog B ekscitona koji zasjenjuje slabe modove smicanja i disanja slojeva. U druga dva materijala, MoS2 te WSe2 su uočeni teorijski predviđeni rezultati: frekvencija moda smicanja se povećava s porastom debljine materijala dok se frekvencija moda disanja slojeva smanjuje s porastom debljine materijala. Kao komplementarna metoda određivanja broja slojeva materijala korišten je i mikroskop atomskih sila - AFM koji je za CVD naraˇstane uzorke (MoS2 i WS2) dao rezultate približne teorijskim, dok su za eksfolirane uzorke debljine materijala bile veće zbog prisutnosti nečistoća na podlozi te zbog stvaranja mjehurića vodene pare ili zraka prilikom mehaničkog eksfoliranja materijala.
Abstract (english) Ever since the isolation of single-layer (1L) graphene in 2004 [1], two-dimensional (2-D) layered materials have been among the most extensively studied classes of materials. Graphene consists of one plane of carbon atoms that are interconnected by covalent bonds and within the multilayer material adjacent graphene layers are interacting with weak van der Waals forces. Although potential applications of graphene are diverse, the absence of energy band-gap limits its use in electronics, for example in the production of transistors that would replace traditional ones made up of silicon. Potential replacement candidates are materials with semiconducting properties such as transition metal dichalgogenides (TMDs). Their general formula is MX2 and one layer of such material consists of three planes: X-M-X where X represents chalcogen atom (S, Se or Te) and M is an atom of transition metal (Mo or W). In addition to so called homostructures - structures composed of only one type of material (MoS2, WS2 etc.) it is possible to synthesize van der Waals heterostructures - structures created by combination of different 2-D materials. Such hybrid materials have new, combined and synergic properties of their constituents and therefore a universal technique of determining the type of material, its thickness, the relative orientation of crystalline axes etc. is required. Technique which meets these requirements is Raman spectroscopy which investigates vibration of the crystal lattice in materials. Particularly interesting is the low-energy Raman spectra which is a direct consequence of the rigid oscillations of adjacent layers. One layer, which can contain three planes of atoms, is considered to be one particle which is connected to adjacent layer (particle) by a spring. Low-energy Raman modes include two different types of oscillation: shear modes - two adjacent layers oscillate in anti-phase along the x-y plane and layer-breathing modes - two adjacent layers oscillate along z-axis, which is perpendicular to the x-y plane and thus are either approaching or moving away from each other. It has been demonstrated that these low-energy Raman modes are more effective at determining layer numbers, stacking configurations and provide a unique opportunity to study interlayer coupling. Low-energy Raman modes have weaker intensity compared to the high-energy Raman modes and due to their proximity to the strong Rayleigh line background, it makes their detection more challenging and demanding. Recent progress in spectroscopic methods makes it possible to experimentally observe these low-energy Raman modes. In this work, existing experimental setup was upgraded with appropriate filters that enabled investigation of low-energy Raman modes of various materials: mechanically exfoliated graphite samples and WSe2 and samples synthesized by CVD method MoS2 and WS2, followed by an analysis of the obtained Raman spectra. It was observed that in the case of graphite and WS2 samples the low-energy Raman spectra have no noticeable differences for different layer thicknesses. The reason for this behaviour is large excitation of carriers in doped Si/SiO2 substrate in case of graphite samples, while in WS2 there is a resonant Raman scattering: laser with 532 nm wavelength has similar energy as the absorption B exciton which then diminishes shear and layer-breathing modes. The other two materials: MoS2 and WSe2 reproduced theoretically predicted results: the frequency of shear mode increases with the increase of the number of layers while the frequency of layer-breathing mode decreases. As a complementary technique in determining the number of layers in materials, the atomic force microscope - AFM was used. It reproduced theoretical results for CVD grown samples (MoS2 and WS2) while for the mechanically exfoliated materials measured thicknesses were much larger than expected. The discrepancy is caused by the presence of impurities on the substrate or confined bubbles of water vapour or air which formed during mechanical exfoliation of the material.
Keywords
2-D materijali
niskoenergetski Ramanovi modovi
mod smicanja slojeva
mod disanja slojeva
Keywords (english)
2-D materials
low-energy Raman spectroscopy
shear mode
layer-breathing mode
Language croatian URN:NBN urn:nbn:hr:217:963090 Study programme Title: Physics; specializations in: Research Type of resource Text File origin Born digital Access conditions Open access Terms of use Created on 2019-09-04 12:02:27
