Thin Films of TiO2 and Related Oxides by ALD/MLD: Tailoring of Transport Properties

Janne-Petteri Niemelä

Thin Films of TiO2 and Related Oxides by ALD/MLD: Tailoring of Transport Properties

Julkaisun otsikon käännös: Titaanidioksidin ja vastaavien oksidiohutkalvojen ALD/MLD-kasvatus: Kuljetusominaisuuksien muokkaus

Janne-Petteri Niemelä

Tutkimustuotos: Doctoral Thesis › Collection of Articles

Abstrakti

Uusien energiateknologioiden kehittäminen edellyttää usein uusien materiaalien valmistusta, mikä vaatii yhä useammin materiaalien muokkaamista nanomittakaavassa. Erinomaisia menetelmiä tähän tarkoitukseen ovat atomikerroskasvatus- (ALD) ja molekyylikerroskasvatustekniikka (MLD), jotka mahdollistavat kerrosrakenteisten, substituoitujen ja epäorgaanis-orgaanisten ohutkalvojen kasvatuksen moninaisten nanorakenteiden pinnoille. Titaanidioksidi on monipuolinen materiaali, joka tunnetaan parhaiten fotokatalyyttinä. Viime aikoina erityisesti Nb-substituoitu TiO2 on herättänyt kiinnostusta myös läpinäkyvänä sähkönjohteena sekä lämpösähköisenä materiaalina. Tässä väitöskirjassa taustoitetaan aluksi näiden sovellusten asettamia vaatimuksia materiaalien ominaisuuksille. Seuraavaksi esitellään tärkeimmät työssä hyödynnetyt kokeelliset menetelmät. Lopuksi vedetään yhteen väitöskirjatyön aikana syntyneet, kemian ja fysiikan alan tieteellisissä julkaisuissa esitellyt tulokset. Kokeellisessa osassa kehitettiin aluksi ALD-prosessi Nb-substituoitujen TiO2-ohutkalvojen valmistukseen käyttäen TiCl4-, Nb(OEt)5- ja H2O-lähtöaineita. Niobiumin todettiin liukenevan pelkistävän lämpökäsittelyn myötä viidenarvoisena TiO2-kalvojen anataasi-rakenteeseen, minkä seurauksena materiaalit käyttäytyivät degeneroituneille puolijohteille ominaisella tavalla. Lämpökäsittelyä edeltävä kiteisyys materiaaleissa vaikutti huomattavasti niiden lopullisiin kuljetusominaisuuksiin; amorfisina kasvaneissa kalvoissa elektronien kuljetusta rajoittivat rakeiden sisäiset ominaisuudet, kun taas kiteisinä kasvaneissa kalvoissa raerajasironta alensi elektronien mobiliteettia huomattavasti. Valmistetuista materiaaleista erityisen lupaavasti läpinäkyvinä johteina toimivat 160–175 ⁰C lämpötilassa kasvatetut Ti0.93Nb0.07O2-ohutkalvot. Lisäksi tässä väitöskirjatyössä valmistettiin epäorgaanis-orgaanisia TiO2:HQ-superhilaohutkalvoja yhdistämällä MLD-prekursori hydrokinoni (HQ) titaanidioksidin TiCl4/H2O-pohjaiseen ALD-prosessiin. Titaanidioksidikerrosten väliin kasvatettujen yksittäisten molekyylikerrosten havaittiin herkistävän TiO2 näkyvälle valolle, mikä on erittäin kiinnostavaa fotokatalyytti- ja aurinkokennosovellusten kannalta. Lämpökäsittelyn avulla molekyylikerrokset kyettiin muuntamaan ohuiksi grafiittisiksi kerroksiksi, ja näin saatiin aikaan uudentyyppisiä TiO2:C-superhilarakenteita. On huomattavaa, että sekä TiO2:HQ- että TiO2:C-superhilojen orgaaniset rajapinnat sirottivat fononeja tehokkaasti madaltaen materiaalien lämmönjohtavuuden erittäin alhaiseksi – tämä on lupaavaa lämpöeriste- sekä lämpösähkösovellusten kannalta.

Julkaisun otsikon käännös	Titaanidioksidin ja vastaavien oksidiohutkalvojen ALD/MLD-kasvatus: Kuljetusominaisuuksien muokkaus
Alkuperäiskieli	Englanti
Pätevyys	Tohtorintutkinto
Myöntävä instituutio	Aalto-yliopisto
Ohjaaja	Karppinen, Maarit, Vastuuprofessori
Kustantaja	Aalto University
Painoksen ISBN	978-952-60-6605-9
Sähköinen ISBN	978-952-60-6606-6
Tila	Julkaistu - 2015
OKM-julkaisutyyppi	G5 Artikkeliväitöskirja

Tutkimusalat

atomi/molekyylikerroskasvatus
ohutkalvo
kuljetusominaisuudet

Pääsy asiakirjaan

http://urn.fi/URN:ISBN:978-952-60-6606-6Lisenssi: Määrittelemätön

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Karppinen, M. (Manager)
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@phdthesis{4ff8101aac8140f19eacee7201ebc41f,

title = "Thin Films of TiO2 and Related Oxides by ALD/MLD: Tailoring of Transport Properties",

abstract = "The key for improving the performance of energy conversion devices is to develop new functional materials – a challenging task that often requires engineering of the material structures in nanoscale. This challenge can be met by employing atomic and molecular layer deposition (ALD and MLD) techniques that allow for fabrication of layered structures, doped and hybrid inorganic-organic materials in a conformal manner on nanostructured substrate surfaces. One interesting multifunctional material is TiO2 that has recently, in addition to its traditional use as a photocatalyst, arisen interest as a transparent conductor and a thermoelectric material for conversion of waste heat into electricity, particularly when doped with Nb. In this dissertation, first, an introductory background is presented regarding the properties of and requirements for the materials for energy applications relevant to this work. Second, a brief introduction to the experimental methods used and their application in this work is given. Third, the results of the experimental work communicated via the chemistry and physics journals of the research field are summarized. In the experimental part of this dissertation an ALD route employing TiCl4, Nb(OEt)5 and H2O as precursors for fabrication of Nb-doped TiO2 thin films was developed. Niobium was found to readily incorporate as pentavalent in anatase-structured films upon a reductive post-deposition annealing treatment such that the materials could be identified as degenerate semiconductors with metal-like transport properties. Initial crystallinity in the as-deposited films heavily affected the final transport properties of the films; in the initially amorphous films the intra-grain properties were found to govern the electron transport, while any crystallinity present prior to the annealing resulted in films where grain boundaries substantially suppressed electron mobility. The Ti0.93Nb0.07O2 films deposited at 160-175 ⁰C showed particularly promising transparent conducting oxide properties. In particular, the ALD precursors TiCl4 and H2O for TiO2 were combined with the MLD precursor hydroquinone in order to fabricate inorganic-organic superlattices. First, in the as-deposited films the organic component was found to sensitize TiO2 to visible light – a fact that could potentially lead to applications in the fields of photocatalysis and solar cells. Second, post-deposition annealing enabled conversion of the as-deposited TiO2:HQ superlattice films into TiO2:C films, new type of inorganic-organic thin film structures where graphitic carbon layers were periodically confined between TiO2 layers. Remarkably, incoherent phonon-boundary scattering enabled ultra-low thermal conductivities in both TiO2:HQ and TiO2:C superlattices, interesting for thermal barrier and thermoelectric applications.",

keywords = "atomic/molecular layer deposition, thin film, transport properties, atomi/molekyylikerroskasvatus, ohutkalvo, kuljetusominaisuudet, atomic/molecular layer deposition, thin film, transport properties",

author = "Janne-Petteri Niemel{\"a}",

year = "2015",

language = "English",

isbn = "978-952-60-6605-9",

series = "Aalto University publication series DOCTORAL DISSERTATIONS",

publisher = "Aalto University",

number = "136",

address = "Finland",

school = "Aalto University",

}

TY - BOOK

T1 - Thin Films of TiO2 and Related Oxides by ALD/MLD: Tailoring of Transport Properties

AU - Niemelä, Janne-Petteri

PY - 2015

Y1 - 2015

N2 - The key for improving the performance of energy conversion devices is to develop new functional materials – a challenging task that often requires engineering of the material structures in nanoscale. This challenge can be met by employing atomic and molecular layer deposition (ALD and MLD) techniques that allow for fabrication of layered structures, doped and hybrid inorganic-organic materials in a conformal manner on nanostructured substrate surfaces. One interesting multifunctional material is TiO2 that has recently, in addition to its traditional use as a photocatalyst, arisen interest as a transparent conductor and a thermoelectric material for conversion of waste heat into electricity, particularly when doped with Nb. In this dissertation, first, an introductory background is presented regarding the properties of and requirements for the materials for energy applications relevant to this work. Second, a brief introduction to the experimental methods used and their application in this work is given. Third, the results of the experimental work communicated via the chemistry and physics journals of the research field are summarized. In the experimental part of this dissertation an ALD route employing TiCl4, Nb(OEt)5 and H2O as precursors for fabrication of Nb-doped TiO2 thin films was developed. Niobium was found to readily incorporate as pentavalent in anatase-structured films upon a reductive post-deposition annealing treatment such that the materials could be identified as degenerate semiconductors with metal-like transport properties. Initial crystallinity in the as-deposited films heavily affected the final transport properties of the films; in the initially amorphous films the intra-grain properties were found to govern the electron transport, while any crystallinity present prior to the annealing resulted in films where grain boundaries substantially suppressed electron mobility. The Ti0.93Nb0.07O2 films deposited at 160-175 ⁰C showed particularly promising transparent conducting oxide properties. In particular, the ALD precursors TiCl4 and H2O for TiO2 were combined with the MLD precursor hydroquinone in order to fabricate inorganic-organic superlattices. First, in the as-deposited films the organic component was found to sensitize TiO2 to visible light – a fact that could potentially lead to applications in the fields of photocatalysis and solar cells. Second, post-deposition annealing enabled conversion of the as-deposited TiO2:HQ superlattice films into TiO2:C films, new type of inorganic-organic thin film structures where graphitic carbon layers were periodically confined between TiO2 layers. Remarkably, incoherent phonon-boundary scattering enabled ultra-low thermal conductivities in both TiO2:HQ and TiO2:C superlattices, interesting for thermal barrier and thermoelectric applications.

AB - The key for improving the performance of energy conversion devices is to develop new functional materials – a challenging task that often requires engineering of the material structures in nanoscale. This challenge can be met by employing atomic and molecular layer deposition (ALD and MLD) techniques that allow for fabrication of layered structures, doped and hybrid inorganic-organic materials in a conformal manner on nanostructured substrate surfaces. One interesting multifunctional material is TiO2 that has recently, in addition to its traditional use as a photocatalyst, arisen interest as a transparent conductor and a thermoelectric material for conversion of waste heat into electricity, particularly when doped with Nb. In this dissertation, first, an introductory background is presented regarding the properties of and requirements for the materials for energy applications relevant to this work. Second, a brief introduction to the experimental methods used and their application in this work is given. Third, the results of the experimental work communicated via the chemistry and physics journals of the research field are summarized. In the experimental part of this dissertation an ALD route employing TiCl4, Nb(OEt)5 and H2O as precursors for fabrication of Nb-doped TiO2 thin films was developed. Niobium was found to readily incorporate as pentavalent in anatase-structured films upon a reductive post-deposition annealing treatment such that the materials could be identified as degenerate semiconductors with metal-like transport properties. Initial crystallinity in the as-deposited films heavily affected the final transport properties of the films; in the initially amorphous films the intra-grain properties were found to govern the electron transport, while any crystallinity present prior to the annealing resulted in films where grain boundaries substantially suppressed electron mobility. The Ti0.93Nb0.07O2 films deposited at 160-175 ⁰C showed particularly promising transparent conducting oxide properties. In particular, the ALD precursors TiCl4 and H2O for TiO2 were combined with the MLD precursor hydroquinone in order to fabricate inorganic-organic superlattices. First, in the as-deposited films the organic component was found to sensitize TiO2 to visible light – a fact that could potentially lead to applications in the fields of photocatalysis and solar cells. Second, post-deposition annealing enabled conversion of the as-deposited TiO2:HQ superlattice films into TiO2:C films, new type of inorganic-organic thin film structures where graphitic carbon layers were periodically confined between TiO2 layers. Remarkably, incoherent phonon-boundary scattering enabled ultra-low thermal conductivities in both TiO2:HQ and TiO2:C superlattices, interesting for thermal barrier and thermoelectric applications.

KW - atomic/molecular layer deposition

KW - thin film

KW - transport properties

KW - atomi/molekyylikerroskasvatus

KW - ohutkalvo

KW - kuljetusominaisuudet

KW - atomic/molecular layer deposition

KW - thin film

KW - transport properties

M3 - Doctoral Thesis

SN - 978-952-60-6605-9

T3 - Aalto University publication series DOCTORAL DISSERTATIONS

PB - Aalto University

ER -

Thin Films of TiO2 and Related Oxides by ALD/MLD: Tailoring of Transport Properties

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