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U N I V E R S I TAT I S O U L U E N S I SACTAC

TECHNICA

U N I V E R S I TAT I S O U L U E N S I SACTAC

TECHNICA

OULU 2018

C 652

Georgies Alene Asres

SYNTHESIS, CHARACTERIZATION AND APPLICATION OF WS2 NANOWIRE-NANOFLAKE HYBRID NANOSTRUCTURES

UNIVERSITY OF OULU GRADUATE SCHOOL;UNIVERSITY OF OULU,FACULTY OF INFORMATION TECHNOLOGY AND ELECTRICAL ENGINEERING

C 652

AC

TAG

eorgies Alene A

sres

C652eukansi.kesken.fm Page 1 Tuesday, March 27, 2018 2:44 PM

ACTA UNIVERS ITAT I S OULUENS I SC Te c h n i c a 6 5 2

GEORGIES ALENE ASRES

SYNTHESIS, CHARACTERIZATION AND APPLICATION OF WS2 NANOWIRE-NANOFLAKE HYBRID NANOSTRUCTURES

Academic dissertation to be presented with the assent ofthe Doctoral Training Committee of InformationTechnology and Electrical Engineering of the University ofOulu for public defence in Martti Ahtisaari -sali (L2),Linnanmaa, on 27 April 2018, at 12 noon

UNIVERSITY OF OULU, OULU 2018

Copyright © 2018Acta Univ. Oul. C 652, 2018

Supervised byProfessor Krisztian KordasProfessor Anita Lloyd SpetzDoctor Gabriela Lorite

Reviewed byDoctor Ana Laura Elias ArriagaProfessor Harri Lipsanen

ISBN 978-952-62-1888-5 (Paperback)ISBN 978-952-62-1889-2 (PDF)

ISSN 0355-3213 (Printed)ISSN 1796-2226 (Online)

Cover DesignRaimo Ahonen

JUVENES PRINTTAMPERE 2018

Asres, Georgies Alene, Synthesis, characterization and application of WS2nanowire-nanoflake hybrid nanostructures. University of Oulu Graduate School; University of Oulu, Faculty of Information Technologyand Electrical EngineeringActa Univ. Oul. C 652, 2018University of Oulu, P.O. Box 8000, FI-90014 University of Oulu, Finland

Abstract

Transition metal dichalcogenide (TMD) materials crystalize in a layered structure with astoichiometry MX2 where M is a transition metal (Mo, W, Tc, Re, V, Nb, Ta, Ti, Zr, Hf) and X isa chalcogen (S, Se, Te). While there is a strong covalent bond between the chalcogen and the metalatoms in each 2-dimensional (2D) sheet, the bulk 3-dimensional crystals are held together by weakvan der Waals forces acting on the adjacent 2D sheets allowing for micromechanical and liquidphase exfoliation into nanostructures composed of either a single layer or a few layers. Since theelectronic band structure depends not only on the chemistry but also on the number of layers, awhole new range of metal, semimetal and semiconductor materials may be achieved. Theseproperties, among many other advantages (e.g. tunable band structure, high mobility of carriers,easy intercalation with ions), make TMDs appealing and timely for applications in solar cells andphotodetectors, heterogeneous catalysis, electrocatalytic electrodes, energy storage and in(electro) chemical sensing. Motivated by the anticipated fascinating properties of TMDs, thisresearch work focuses on the synthesis, characterization and application of a novel hybrid WS2nanomaterial. While the original goal of the research work was to develop a simple method tosynthesize WS2 nanowires, it became clear that instead of nanowires a hybrid nanowire-nanoflake(NW-NF) structure could be synthesized by a simple thermal sulfurization of hydrothermallygrown WO3 nanowires. The structure, morphology and composition of the new materials wereanalyzed by X-ray diffraction, Raman spectroscopy, electron microscopy and X-ray photoelectronspectroscopy. Temperature dependent electrical measurements carried out on random networks ofthe nanostructures showed nonlinear characteristics and a negative temperature coefficient ofresistance indicating that the hybrids were semiconducting. Resistive gas sensors were preparedand exposed to H2S, CO, NH3, H2 and NO and to which the devices displayed ultra-highsensitivity (0.043 ppm-1) towards H2S with a detection limit of 20 ppb. The results suggest furtherexploration of gas sensing with TMDs as potential competitive alternatives to the classical metaloxide based devices. Moreover, photodetector devices with excellent visible light response werealso demonstrated using an individual WS2 NW-NF hybrid as well as its random networks havingphotoresponsivity of up to 400 mAW-1. This was two orders of magnitude higher than thatmeasured for other 2D materials based devices. Overall, the WS2 nanowire-nanoflake hybrid is atruly multipurpose and multifunctional semiconductor making it a promising material foradvanced micro, nano and optoelectronics devices.

Keywords: gas sensor, nanoflake, nanohybrids, nanowire, photodetector, WO3, WS2

Asres, Georgies Alene, WS2 -nanolanka-nanohiutale -hybridirakenteen synteesi,karakterisointi ja sovellukset. Oulun yliopiston tutkijakoulu; Oulun yliopisto, Tieto- ja sähkötekniikan tiedekuntaActa Univ. Oul. C 652, 2018Oulun yliopisto, PL 8000, 90014 Oulun yliopisto

Tiivistelmä

Siirtymämetallidikalkogenidistä (transition metal dichalcogenide, TMD) olevat materiaalitkiteytyvät kerroksittaisiksi rakenteiksi, joiden stoikiometria on MX2, missä M on siirtymämetalli(Mo, W, Tc, Re, V, Nb, Ta, Ti, Zr, Hf) ja X on kalkogeeni (S, Se, Te). 2-ulotteisessa (2D) tasossakalkogeenin ja metallin välillä on voimakas kovalenttinen sidos, mutta suuremmassa kolmiulot-teisessa kiteessä viereisiä tasoja sitoo toisiinsa vain heikot van der Waals-voimat, jolloin tasot onmahdollista erottaa mikromekaanisesti ja nestefaasikuorinnalla yksittäisiksi tai muutamasta ker-roksesta koostuvaksi nanorakenteeksi. Koska elektronivyörakenne ei riipu ainoastaan kemialli-sesta koostumuksesta vaan myös kerrosten lukumäärästä, voidaan muodostaa täysin uusia metal-lisia, puolimetallisia tai puolijohdemateriaaleja. Nämä ominaisuudet monien muiden lisäksi(esim. räätälöity vyörakenne, korkeanliikkuvuuden varauksen kuljettajat, helppo ionien interke-laatio) tekevät TMD-materiaaleista kiinnostavia ja ajankohtaisia aurinkokennoihin, valokennoi-hin, heterogeeniseen katalyysiin, sähkökatalyyttisiin elektrodeihin, energiavarastoihin ja sähkö-kemiallisiin antureihin. TDM-materiaalien oletettavasti kiehtovien ominaisuuksien motivoima-na tämä tutkimus keskittyy uusien hybridi-WS2 –nanomateriaalien synteesiin, karakterisoimi-seen ja sovellutuksiin. Tutkimuksen alkuperäinen tavoite oli kehittää yksinkertainen menetelmäWS2-nanolankojen syntetisoimiseksi, mutta kävi ilmi että nanolankojen sijaan syntyi nanolanka-nanohiutale –hybridirakenne (nanowire-nanoflake, NW-NF), kun hydrotermisesti kasvatettujaWO3-nanolankoja rikitettiin termisesti. Näiden uusien materiaalien rakenne, morfologia ja koos-tumus on analysoitu röntgendiffraktiolla, Raman-spekstrokopialla, elektronimikroskoopilla jaröntgenfotoelektronispektroskopialla. Valikoimattomista nanorakenteista koostuvien verkosto-jen lämpötilasta riippuvien sähköisten ominaisuuksien mittaukset osoittavat epälineaarisia piir-teitä ja negatiivinen resistanssin lämpötilakerroin viittaa hybridien puolijohtavuuteen. Materiaa-lista valmistettiin resistiivisiä kaasuantureita, jotka altistettiin H2S:lle, CO:lle, NH3:lle, H2:lle jaNO:lle, näistä anturi osoitti erittäin suurta herkkyyttä H2S:lle (0.043 ppm) havaintorajan ollessa20 ppb. Tulokset kannustavat TMD-materiaalien kaasuanturisovellutusten jatkotutkimukseentarjoten potentiaalisesti kilpailukykyisen vaihtoehdon perinteisille metallioksidi-pohjaisille lait-teille. Lisäksi, yksittäisillä WS2–nanolanka-nanohiutalepartikkeleilla sekä valikoimattomillananolanka-nanohiutalehybridiverkostoilla demonstroitiin valokenno, jonka vaste näkyväänvaloon oli jopa 400 mAW-1 ollen kaksi kertaluokkaa korkeampi kuin muilla 2D-materiaaleihinperustuvilla kennoilla. Kaiken kaikkiaan, WS2 nanolanka-nanohiutalehybridi on todella moni-käyttöinen ja monipuolinen puolijohde ollen lupaava materiaali kehittyneille mikro- nano- jaoptoelektronisille laitteille.

Asiasanat: kaasuanturi, nanohiutale, nanohybridit, nanolanka, valokenno, WO3, WS2

7

Acknowledgements

This work has been conducted in the Microelectronics Research Unit of the

University of Oulu under a grant provided by the Graduate School Infotech Oulu.

First, I would like to thank my advisor Prof. Krisztian Kordas for his

continuous support of my PhD study and related research work, for his patience,

motivation, encouragement and knowledge. Without his kind guidance it would not

have been possible to accomplish it. My sincere thanks also goes to my co-

supervisors Prof. Anita Lloyd Spetz and Dr. Gabriela Lorite for their guidance and

help.

Besides my advisors, I would like to thank my follow-up group members:

Docent Juha Hagberg, Dr. Jani Perantie and Dr. Merja Teirikangas for their

constructive comments and for checking that the research was proceeding as planned.

I am grateful to my current and former research group members: Dr. Melinda

Mohl, Dr. Geza Toth, Dr. Jhih-Fong Lin, Topias Järvinen, Olli Pitkänen, Aron

Dombovari, Teemu Sipola and Rashad Hajimammadov for their unfailing support

and assistance during my research work. Also to everyone in the Microelectronics

Research Unit and Microscopy and Nanotechnology Centre of the University of

Oulu; it was great sharing the laboratory with all of you during last four years. I

also very much appreciate the extensive support of collaborators from the Umeå

University, University of Szeged, Max Planck Institute, Universidad del País

Vasco, Rice University, Southern Illinois University, Linköping University and

VTT Technical Research Centre of Finland.

Last but not least, I would like to thank my family: my parents and my brothers

and sisters for supporting me spiritually throughout this research work and my life

in general.

Oulu, February 2018 Georgies Alene

8

9

List of original publications

This thesis is based on the following three original publications:

I Asres, G. A., Dombovari, A., Sipola, T., Puskás, R., Kukovecz, A., Kónya, Z.,Popov, A., Lin, J.F. Lorite, G.S. Mohl, M., Toth, G., Spetz, A.L., Kordas, K. (2016). A novel WS2 nanowire-nanoflake hybrid material synthesized from WO3 nanowires in sulfur vapor. Scientific Reports 6, 25610.

II Asres, G.A., Baldovi, J.J., Dombovari, A., Järvinen, T., Lorite, G.S., Mohl, M., Shchukarev, A., Paz, A.P., Xian, L., Mikkola, J-P., Spetz, A.L., Jantunen, H., Rubio, A., Kordas, K.(in press) Ultra-sensitive H2S gas sensors based on p-type WS2 hybrid materials. Nano Research.

III Asres, G.A., Järvinen, T., Lorite, G.S., Mohl, M., Pitkänen, O., Dombovari, A., Toth, G., Spetz, A.L., Vajtai, R., Ajayan, P.M., Lei, S., Talapatra, S., Kordas, K. (2018). Excellent photoresponse of individual WS2 nanowire-nanoflake hybrid materials. Manuscript submitted for publication.

The synthesis methods used to obtain WS2 nanowire-nanoflake hybrid materials

and their characterization are reported in Paper I. Specifically, a gas-phase

sulfurization method is developed to synthesize the WS2 nanohybrid materials from

hydrothermally grown WO3 nanowires. To assess the chemical composition, micro

and crystal structure as well as the electrical and optical properties of the

synthesized new nanomaterials, a broad range of analytics including X- ray

photoelectron spectroscopy (XPS), scanning and transmission electron microscopy

(SEM and TEM), X-ray diffraction (XRD), Raman spectroscopy are applied, as

well as temperature dependent 2- and 4-point probe current-voltage (I-V)

measurements.

In Paper II, resistive gas sensors based on drop-cast WS2 nanowire-nanoflake

networks are demonstrated on lithographically defined Si chips. The sensor

sensitivity for five different analyte gases (CO, H2, NH3, H2S and NO) are measured

and evaluated at 30°C and 200°C. The results reveal the sensors have excellent

sensitivity and selectivity towards H2S gas. To shed light on the interaction of H2S

with the WS2 sensing element ab initio calculations are carried out. The Badar

analysis indicates that the local charge transfer between the analytes and pristine

WS2 is very small, and thus suggests that the excellent sensitivity towards H2S

observed in the experimental study might be due to a different phenomenon. XPS is

implemented to study further the possible origin of the high sensitivity of the sensor

towards H2S. The XPS study shows that O2 in the environment during the gas

sensing measurements induces a partial (and reversible) substitution of S with O at

the anionic sites of the lattice. This is then reset in the presence of S in the ambient

10

giving a reasonable qualitative explanation for the chemoresistive properties of the

material.

Paper III reports on photosensor devices constructed from individual WS2

nanowire-nanoflake hybrid materials electrically connected to sputtered Pt probe

pads using FIB deposited Pt wires. The sensors are illuminated by three different

lasers (401 nm, 552 nm and 661 nm) tuned between 1 and 100 mW power to

evaluate the spectral and power dependence of the photoresponse. The measured

highest responsivity ~400 mA·W-1 is significantly better than most TMDs or early

reported graphene based photodetector devices.

The experiments such as materials synthesis, XRD, Raman, EDS and SEM

analysis and gas sensing measurements were carried out by the Author. Optical

measurement for the optical band gap calculation was performed by Dr. Alexey

Popov. Electrical and HR-TEM analyses were jointly conducted by Aron

Dombovari, Sami Saukko, Teemu Sipola and Dr. Jhih-Fong Lin. Photoconductivity

measurements were performed by the Author with the kind assistance of Topias

Järvinen. FIB processing was carried out by Esa Heinonen. XPS measurements

were performed by Dr. Andrey Shchukarev (Umeå University). First-principles

calculations were conducted by Dr. José J. Baldoví, Dr. Lede Xian, Dr. Alejandro

Pérez Paz and Prof. Angel Rubio (Universidad del País Vasco and Max Planck

Institute for the Structure and Dynamics of Matter). Conductive AFM analysis was

carried out by Dr. Gabriela S. Lorite. Finite-element simulations were done by Dr.

Geza Toth (VTT Technical Research Centre of Finland). The experiments were

planned by Prof. Krisztian Kordas. The manuscripts were written by the Author

with the kind help of all co-authors.

11

Contents

Abstract

Tiivistelmä

Acknowledgements 7 List of original publications 9 Contents 11 1 Motivation, objectives and outline of the thesis 13 2 Introduction 15

2.1 Inorganic layered transition metal dichalcogenide nanostructures .......... 15 2.2 Synthesis of TMDs .................................................................................. 17 2.3 TMD based field effect transistors .......................................................... 19 2.4 Photodetectors and other optoelectronic devices .................................... 19 2.5 Gas sensors .............................................................................................. 20

3 Materials and methods 23 3.1 Materials synthesis .................................................................................. 23 3.2 Structural and chemical characterization ................................................ 23 3.3 Optical characterization .......................................................................... 24 3.4 Electrical measurements.......................................................................... 24 3.5 Gas sensor device fabrication and measurements ................................... 25 3.6 Photosensor device fabrication and characterization .............................. 25

4 Results and discussion 27 4.1 The structure of the synthesized materials (Paper I) ............................... 27 4.2 Electrical properties and light sensing behavior of WS2

nanohybrid networks (Paper I) ................................................................ 29 4.3 Resistive gas sensing with WS2 nanohybrid networks (Paper II) ........... 32 4.4 Photo sensing using individual WS2 nanohybrid particles (Paper

III) ........................................................................................................... 36 5 Summary and conclusions 41 List of references 43 Original publications 51

12

13

1 Motivation, objectives and outline of the thesis

Among the different transition metal dichalcogenide materials, in this work,

tungsten disulfide (WS2) nanowire-nanoflake hybrid was synthesized, investigated

and applied in a number of different applications. Although the chemical and

atomic structure of WS2 are similar to those of molybdenum disulfide (MoS2), WS2

has received less attention than MoS2. However, similarly to MoS2, WS2 also has

interesting properties such as layer number dependent electrical and optical

properties as well as catalytic, electrocatalytic and photocatalytic activity making

it a promising material for diverse applications [1].

The new nanowire-nanoflake hybrid material developed in this work, in which

nanoflakes protrude from the surface and are thus an integral part of the nanowires,

may combine the unique structural and electronic properties of one dimensional

nanowires and the outstanding properties of two dimensional nanoflakes. The

above mentioned reasons are the major motivations behind the synthesis and

characterization of WS2 nanostructure in this research with the hope of finding at

least a few functionalities having practical importance or which can benefit the

development of other similar materials and devices.

New types of semiconducting materials are needed for future electronics devices

that may be based on the new materials alone or in combination with existing

ceramic, polymer or Si platforms. Novel types of transistor materials, which enable

a high channel current on/off ratio and fast switching demand high carrier mobility,

scalability to nanoscopic dimensions and compatibility with C-MOS technologies

[2]. Two dimensional materials, mainly TMDs, fulfill several of these requirements

and may complement or even compete with existing classical 3D electronic

materials. They are very thin, which supports efficient control over switching and

can help to decrease short-channel effects and power dissipation, the main factors

limiting transistor miniaturization [2]. This suggests that WS2, one of the common

TMDs, is a promising material for field effect transistors (FETs) and thus gives

encouragement to fabricate and study the new WS2 nanohybrid as a channel

material.

On the other hand, being nanostructured, the semiconducting WS2 nanohybrid

is expected to have a large specific surface area and is known to exhibit some

catalytic behavior, prompting its analysis for gas sensing applications. To address

this hypothesis, simple Taguchi-type resistive gas sensor devices have been

produced, their properties measured, and the results assessed in the light of

14

previously reported [3] and revisited theoretical data obtained by first principles

calculations.

Further, again because of the material’s semiconducting nature with a low band

gap, it is anticipated to have some functionalities that may be exploited for

optoelectronics using visible light. For this purpose simple photosensor devices

based on individual particles of the nanohybrid were also prepared and studied.

Accordingly, the objectives of this thesis were to develop a simple method to

synthesize WS2 nanostructures, and after exploring their chemical and physical

properties, to find some relevant applications in micro and nanoelectronics.

The corresponding work is summarized in this thesis using the scheme as

follows: After the introductory section describing the properties, synthesis methods

and applications of TMDs (Chapter 1), the details of experiments are given in

Chapter 2, which is followed by the major findings and their discussion in Chapter

3. Finally, the thesis is summarized and the implications of the work are concluded

in Chapter 4.

15

2 Introduction

2.1 Inorganic layered transition metal dichalcogenide

nanostructures

The recent rapid developments in electronics, medicine, health care, energy

production, environmental remediation and transportation have been fostered with

the help of new nanomaterials and associated technologies. Among the new

materials, owing to their layered structure and promising physical and chemical

properties [4] inorganic layered transition metal dichalcogenides (TMDs) represent

an important and fascinating family of nanomatrials.

Although all group 16 chemical elements (i.e. the oxygen group) of the

periodic table are defined as chalcogens, the term chalcogenides is commonly

reserved for sulfides, selenides and tellurides. In brief, TMDs are a group of

materials with a formula MX2; where M is a transition metal (Mo, W, Tc, Re, V,

Nb, Ta, Ti, Zr, Hf) and X is a chalcogen (S, Se, Te). TMDs of groups 4-7 metals

tend to crystallize in layered structures, while those in groups 8-10 are commonly

found in non-layered structures [5]. In each layer, the metal and the chalcogen are

covalently bonded on both sides (see Figure 1a). These covalent bonds provide the

structural integrity of a single layer. To form a bulk crystal, several layers are held

together by weak van der Waals forces acting between adjacent chalcogen slabs.

Due to this weak inter-plane interaction it is easy to separate this material by

micromechanical and liquid phase exfoliation, even allowing the isolation of single

layers [6]. TMDs have a hexagonal or rhombohedral symmetry, and the

coordination of the metal atoms are octahedral or trigonal prismatic. [2]

Fig. 1. Schematic representation of a typical MX2 structure, with the chalcogen atoms

(X) in yellow and the metal atoms (M) in black [2]. The monolayer comprises a metal

layer sandwiched between two chalcogen layers in a trigonal prismatic arrangement,

forming an X-M-X structure.

16

TMDs exhibit remarkably diverse electronic properties that can display a metallic,

semiconducting or even a superconducting nature, i.e. all the kinds of electronic

materials we use in daily life (See detail in Table 1). Nanoscale TMDs are rapidly

emerging with new discoveries of their unique optical, electronic, catalytic and

mechanical properties [7]. Interestingly, their electron structure and optical

properties are layer number dependent. For instance, WS2 changes from indirect

gap (1.4 eV) [2] to direct gap (2.1 eV) when the bulk sample is reduced in size to a

monolayer. The possibility of creating single or several stable atomic-thickness

layers of these materials was realized following the success of graphene, although

practical applications of graphene as a zero band gap semiconductor are limited

without additional manipulation [2]. Different approaches have already been

implemented to engineer the band gap in graphene, such as using nanostructuring

[8,9], chemical functionalization [10], and the application of extreme external fields

[11]. However, these methods have the drawback of adding complexity and

diminishing carrier mobility [2]. This is where semiconducting TMDs with a finite

band gap can complement or even compete with graphene. Furthermore, as it is

possible to tune their electronic structure by simply varying the chemical

composition and particle geometry, TMDs hold great promise for nano and

optoelectronics [12].

Although there have been very many studies reporting the electronic and other

relevant properties of graphene and its derivatives, layered TMDs are just

emerging, and materials other than MoS2 have not been given deep coverage.

Furthermore, suitable methods for the large scale synthesis of layered

semiconductors are yet to be developed. This research work attempts to bridge these

gaps by focusing on synthesis, characterization and some potential applications of

WS2 nanostructures.

Table 1.Electrical properties of TMDs.

Group Transition Metal Chalcogen Properties Reference

4 Ti, Hf, Zr S Diamagnetic semiconductors

(Eg: 0.2-2.0 eV)

4

Se Diamagnetic semiconductors (Eg=0.2-2

eV)

Te Diamagnetic semiconductors (Eg=0.2-2

eV)

17

Group Transition Metal Chalcogen Properties Reference

5 V, Nb, Ta S VS2 (Ferromagnetic

Semiconductor),NbS2 (nonmagnetic

metal), TaS2 (semiconductor).(ρ ~10-4

Ωcm)

13-17

Se VSe2 (ferromagnetic, narrow band

metals), NbSe2 (metallic), TaSe2

(Semimetal)

Te VTe2 (Ferromagnetic), NbTe2

(superconducting)

6 Mo, W S Semiconducting Diamagnetic (Eg ≈ 1

eV).

18

Se Semiconducting Diamagnetic. (Eg ≈ 1

eV).

Te Semimetallic (ρ~10-3 Ωcm).

7 Tc, Re S Small gap semiconductors. Diamagnetic 18

Se Small gap semiconductors. Diamagnetic

Te Small gap semiconductors. Diamagnetic

10 Pd, Pt S Sulfides (PdS2, PtS2) semiconducting

and Eg≈0.4 eV.

18-20

Se PdSe2 (semiconducting), PtSe2

(semimetal)

Te PtTe2 (metallic) and PdTe2

( superconducting)

Symbol ρ denotes in-plane electrical resistivity

2.2 Synthesis of TMDs

Chemical vapor deposition (CVD) and doping are arguably among the best

methods to prepare nanostructured materials and thus these techniques have been

under intense development in the last two decades. Being compatible with existing

semiconductor technologies, scalable and economically competitive, these

methods are key to the fabrication of many industrial products [21]. In a typical

CVD process, inside a reactor, source gases containing the precursors are

transported towards a substrate. With energy supplied by either heating or plasma

discharge, the precursors decompose and/or react with each other and a thin film of

product forms on the substrate. The spent gases are removed by gas flow through

an outlet. Doping is similar in terms of apparatus but the reaction takes place

between a solid phase and the vapor of doping atoms or their precursors, thus

18

facilitating doping and/or chemical reaction at elevated temperatures in which the

chemical composition (and sometimes also the microstructure) of the solid changes.

When the doping process is excessive, practically all of the atoms in the anionic (or

cationic) sites can be replaced.

One of the first studies into the formation of polyhedral and cylindrical WS2

nanowires (or one dimensional WS2) was reported in 1992 [22]. These

nanostructures were achieved by heating thin tungsten films in an atmosphere of

hydrogen sulfide. Since then, several recipes have been shown to produce sulfide

nanostructures using a strategy that involves heating (or even evaporation) of metals

or their oxides in sulfur vapor or H2S gas. Recently, WS2 nanotubes decorated on

their outer surface with fullerene-like WS2 nanoparticles have been produced by

sulfurizing W5O14 (WO2.8) nanowhiskers [23]. Reproducible mass production (a

few 100 g/batch) of a pure WS2 nanotube phase using a large fluidized bed reactor

was also demonstrated by annealing mixtures of different WOx (2.83 < x ≤ 3)

phases and morphologies in H2S and H2/N2 at 800-900°C [24]. Synthesis of WS2

and WSe2 nanowires on stainless steel using metallic tungsten and chalcogen

powder as precursors under autogenic pressure at 750°C has also been

demonstrated [25]. WO3 nanorods annealed in H2S at 840°C resulted in the

formation of multi-walled WS2 [26]. MoS2 and WS2 onion-like nanoparticles and

three dimensional nanoflowers were produced by atmospheric pressure CVD with

the reaction of chlorides (MoCl5/WCl6) and sulfur [27]. Under optimized

conditions of deposition temperature, sample position in the reactor and flux of the

carrier gas, control over the synthesized product morphologies was demonstrated.

In other work [28], WS2 flakes on Si wafers were synthesized using a low-pressure

chemical vapor deposition technique. It was shown that with a proper positioning

of S and WO3 precursors and the growth substrate in three different zones of the

furnace, and adjustment of other growth parameters (e.g. the time of S precursor

introduced, the temperature of WO3 precursor and deposition temperature) the

morphology of the WS2 product could be tuned.

In this thesis, a simple CVD method is developed to synthesize one dimensional

WS2 nanowires covered with two dimensional nanoflakes using hydrothermally

grown WO3 nanowires and sulfur as precursors. Since the shape and thickness of

the WS2 flakes can significantly influence their electronic, photoelectric and

mechanical properties, [28] fascinating properties of the new WS2 nanohybrids may

be expected.

19

2.3 TMD based field effect transistors

Typically, smaller and faster electronic devices are currently realized by

miniaturization of silicon based transistors [29]. However, some fundamental limits

such as high leakage current, short channel effect, pronounced carrier and phonon

scattering and contact resistance restrict any significant further improvement in the

future. Moreover, the brittle nature of silicon limits its straightforward application

in flexible electronics [30, 31]. Numerous efforts to overcome the above mentioned

limitations have already been attempted using novel device architectures and

alternative materials [32,33]. Semiconducting 2D TMDs are among the

investigated and promising alternative materials. Properties such as structural

stability, small band gap, lack of dangling bonds and thus high mobility compared

to that of silicon make semiconducting 2D TMDs attractive for FET applications

[34]. In 2004 Podzorov et al. reported [35] WSe2 crystals with p-type conductivity

and carrier mobility as high as 500 cm2V-1s-1 at room temperature. The above result

was soon followed by bottom gated FETs using a thin layer of MoS2 as a channel

material having mobility values in the range of 0.1-10 cm2V-1s-1 [2, 36] and a high

~104 on/off ratio measured at 60 K.

2.4 Photodetectors and other optoelectronic devices

Photodetectors are ubiquitous devices with the main function of collecting and

converting information, associated with light, into electrical signals that can be

processed by standard electronics. Among different materials, silicon based

photodetectors are preferred because of their compatibility with complementary

metal–oxide semiconductors (CMOS) technology that enables device

miniaturization and scalability at affordable cost. However, in order to make color

sensitive silicon devices, it is necessary to use filters. In addition, silicon’s indirect

band gap reduces the photoefficiency. Therefore, the industry and scientific

communities are expending considerable efforts to find novel nanoscale materials

that may complement or even compete with traditional Si, Ge, GaAs and other

compound semiconductors [37].

Semiconducting layered TMDs, with their layer number and composition

dependent electronic structure, make it possible to select any predefined band gap

thus enabling detection of light in different spectral regions [38]. Furthermore,

owing to the direct band gap of several compositions and structures, fast and

efficient detectors may be achieved. Accordingly, in recent years, intense research

20

into the understanding and engineering of the optoelectronic properties of TMDs

and related devices has been started [37]. With respect to this, there are some

properties of WS2 that makes it even more interesting than other semiconducting

TMDs. Firstly, WS2 displays ambipolar field modulation behavior[38,39] and its

suitably reduced (hole and electron) effective mass, as found by theoretical study,

enables a high carrier mobility[40]. For instance, recently a field effect mobility of

234 cm2 V−1 s−1 has been measured for multilayer WS2 at room temperature using

direct Au contacts [41]. Secondly, compared to MoS2 crystals, monolayer WS2 has

approximately a twenty-fold greater photoluminescence emission intensity [42]

which promises optoelectronic devices with a relatively high quantum efficiency.

Similar to other applications of TMDs, most of the reports refer to MoS2 and

only very recent studies have shown that tungsten disulfide is also a promising

material for photodetectors, such as CVD grown WS2 nanoparticle thin films [43],

a CVD grown monolayer WS2 photodetector with graphene electrodes [44] and a

monolayer WS2 phototransistor [45,46].

2.5 Gas sensors

The environment around us is composed of different gases with various properties,

origins and concentrations. Some are toxic while others are vital for life or

indicators of health. Accordingly, gas sensors are needed in different applications

such as environmental protection, industrial process monitoring and safety,

amenity, energy saving, health, foods, and so on [47]

Studies concerning semiconductor gas sensors undertook a considerable

expansion in the late 1980s, and became one of the most active research areas

within the sensor community [47]. Among them, metal oxide semiconductors stand

out as the ones most commonly used. The recent rapid progress in nanotechnology

and materials science resulted in new nanomaterials and devices that extended the

field of semiconducting gas sensors [48-51]. Metal oxide semiconductor based gas

sensors have high sensitivity, particularly at high temperatures, but one of their

drawbacks is their poisoning and deactivation by even the smallest traces of sulfur

or its compounds. Further, to achieve selectivity for a particular analyte, it is

necessary to sensitize them e.g. by decorating the oxide surface with various metals

or other metal oxides [52]. Due to these reasons and to improve further the existing

sensing performance, scientists are trying different approaches to substantiate new

frontiers of materials for gas sensing including carbon nanotubes [53], graphene

21

[54], conducting polymers [55] and lately, transition metal dichalcogenide

nanostructures [56].

Research results suggest that charge transfer based conductance modulation [3,

56, 57, 58] is the working mechanism of these sensors. Since the electronic density

of states is dependent on the dimensions of the crystal lattice, it is reasonable to

speculate that gas sensing properties are also influenced by the dimensions and

microstructure of the sensing material. This gives the main motivation to investigate

the gas sensing properties of one dimensional WS2 covered with two dimensional

nanoflakes in this study.

22

23

3 Materials and methods

3.1 Materials synthesis

The starting material WO3 for synthesizing WS2 was achieved by a hydrothermal

process. 18.74 g of Na2WO4·2H2O (Sigma-Aldrich, ACS reagent 99%) and 22.49

g of Na2SO4 (Sigma-Aldrich, ACS reagent ≥99.0%) were dissolved in 600 mL

distilled water and the pH = 1.5 was adjusted using 3 M HCl solution. After 10 min

stirring, the solution was transferred to an autoclave (Parr Series 4520 Bench Top

Reactor with PTFE lining) and stirred (35 rpm) for 48 h at 180°C under autogenic

pressure. The product suspension was collected, centrifuged (Hettich, Universal

320, 20 min, 3000 rpm), washed with distilled water and ethanol and then dried at

60°C for 24 h.

WS2 nanomaterials were obtained by sulfurization of the WO3. Sulfur powder

(1.0 g, reagent grade, purified by sublimation) and WO3 nanowires (0.1 g) were

placed in two different alumina boats at the center of a tubular reactor. Before

sulfurization, the reactor was evacuated to a base pressure of ~1 torr and filled with

N2 at 1 bar. The evacuation and flushing steps were repeated three times to

minimize O2 in the reactor. Sulfurization was performed for a period of 10 min at

500, 600, 700 or 800°C in a flow of 400 sccm N2. The product synthesized at 800°C

was used for the investigations.

3.2 Structural and chemical characterization

The synthesized products were analyzed by the means of X-ray diffraction (Bruker

D8 Discovery, Cu Kα), micro-Raman spectroscopy (Horiba Jobin-Yvon Labram

HR800 at λ = 488 nm), scanning electron microscopy (FESEM, Zeiss Ultra Plus)

and transmission electron microscopy (FEI Tecnai G2 20 X-Twin at 200 kV

acceleration) to investigate the crystal structure and microstructure of the materials.

X-ray photoelectron spectroscopy (monochromatic Al Kα source operated at 150 W)

equipped with a delay line detector and a charge neutralizer was conducted to examine

in detail the elemental compositions and oxidation states.

24

3.3 Optical characterization

The optical properties of the powder samples were studied using a

spectrophotometer system (Optronic Laboratories, USA) equipped with an

integrating sphere. The powder was mounted on a microscope slide with double-

sided scotch tape. The total reflectance from the scattering powder was measured

within the 400-1100 nm spectral range. Subsequently the optical absorption was

calculated, from which the optical band gap of the products prepared under different

conditions was derived.

3.4 Electrical measurements

For the I-V measurement (two-point probe and four-point probe), Pt electrodes were

defined by photolithography on top of an SiO2/Si wafer. The WS2 nanowire-

nanoflake hybrid was dispersed in acetone and drop cast to bridge the interdigital

Pt electrodes. Then the electrodes were probed under a probe station (Wentworth

labs station) connected to a computer-controlled source meter (Keithley 2636A)

and the I-V curve was measured. The measurement was conducted at room

temperature in ambient air.

Field effect transistors (FETs) were fabricated using conventional

photolithography techniques in a clean room using the WS2 NW-NF hybrid as a

channel material. The procedure was as follows. A SiO2/Si wafer (consisting of a

thermally grown 300 nm oxide film on Si) was purchased. Then the interdigital

Au/Ti layer was defined by photolithography on top of the insulating SiO2 layer.

On the reverse side an Al/Ti/Au multilayer was deposited consecutively by

sputtering (as a back gate contact). The WS2 NW-NF hybrid material dispersed in

acetone was then drop cast on the front side between the gold electrodes as a

channel. Then the back gated FET device was ready for test. The gold contacts on

the front sides and the gold on the reverse were probed in the probe station and

electrical and optical measurement performed using a computer-controlled

Keithley 2636A source meter.

The output and transfer characteristics curves of the FETs were measured using

a probe station. A LabVIEW script was developed for this purpose. In addition, the

photosensing properties of the FET were investigated by illuminating the sample

with a light emitting diode (LED). A standard collimated RGB light-emitting diode

was used (emission centered at 623 nm, 517.5 nm and 466 nm with corresponding

25

luminosity values of 800, 4000 and 900 mcd, respectively; chip-to-diode distance

of 5 cm at an illumination angle of ~45°).

3.5 Gas sensor device fabrication and measurements

Resistive type gas sensors were fabricated as follows. First, platinum electrodes

were defined by photolithography on the front side of the SiO2/Si wafer in a similar

manner to that discussed above in section 2.4. Then, the WS2 sensing material was

dispersed in acetone and drop cast between the Pt finger electrodes. The platinum

electrode probed by the contact needles of the probe station enabled the subsequent

electrical and gas sensing analyses. All the prepared devices were dried overnight

in ambient air before the gas sensing measurements to desorb moisture and stabilize

the base resistance of the sensor. The gas sensing measurements were conducted

using a Linkam heating and freezing stage. The sensors were investigated upon

exposure to five different analyte gases (H2S, CO, NH3, H2 and NO) with nominal

concentrations between 1 and 600 ppm, adjusted using MKS type 247 mass-flow

controllers. For the H2S gas sensing measurements, lower gas concentrations were

also applied between 10 and 1000 ppb. All measurements were performed in air

buffer at 30°C and 200°C. A LabVIEW script was used to control the temperature

and gas concentration by computer during the gas sensing measurements. The

resistance of the sensors was monitored by an Agilent 3458 multimeter using a

constant bias of 1V.

3.6 Photosensor device fabrication and characterization

Photosensor devices were prepared using individual particles of WS2 NW-NF

hybrids. A Si/SiO2 wafer was cut with a laser into 7×7 mm squares. A TEM grid

was taped on the top of the chip and 200 nm thick Pt was deposited by sputtering

(using the grid as a shadow mask) to define the probe pads. After that WS2

dispersed in acetone was drop cast on the surface and Pt nanowires were deposited

on the surface by the means of focused-ion beam-assisted CVD to connect the

individual WS2 structures to the sputtered Pt pads. The Pt contact pads were probed

in the (photo) electrical measurements using a probe station. The photo sensing

data were averaged over 16384 measurement sets. For the photo sensing

measurements a Coherent® High-Performance OBIS™ Laser System (TEM00

mode, λ=661, 552 and 401 nm with a corresponding 1/e2 beam diameter of 9, 7 and

8 mm) was used to illuminate the samples. The laser beams were modulated with a

26

10 kHz square wave signal from a signal generator (TGA1244, Thurlby Thandar

Instruments Ltd.). The device was connected in series with a 19.6 MΩ load

resistance and probed with an oscilloscope (Agilent InfiniiVision DSO-X 3024A,

Agilent Technologies, Inc.). The 10 MΩ oscilloscope probe (Agilent N2863B,

Agilent Technologies, Inc.) formed a parallel resistor circuit with the load resistor

resulting in a net impedance of 6.62 MΩ. A synchronization signal was used for

triggering the measurement with the laser. The photocurrent and responsivity of the

devices were evaluated as a function of laser power (1, 2, 5, 10 and 20 mW at all

wavelengths plus 50 mW at 401 nm as well as 50 and 100 mW at 661 nm).

27

4 Results and discussion

4.1 The structure of the synthesized materials (Paper I)

In Paper I, hydrothermally grown WO3 nanowires and sulfur powder were used as

precursors to synthesize WS2 nanostructures at four different temperatures (500°C,

600°C, 700°C and 800°C). The morphology of the synthesized WO3 nanowires and

the products after sulfurization are shown in Figure 2 (a-e). The diameter of the

WO3 nanowires was found to be between 270 and 390 nm and their length varied

between 1-3 µm. The effect of sulfurization on the morphology of the product was

insignificant at 500 and 600°C, whereas tiny flakes appeared on the surface at

higher temperatures. Thus, the products synthesized at 700°C and above had

features of both one and two dimensional materials combined in a single

nanostructured particle.

Fig. 2. Structural change of WO3 during sulfurization. (a) Scanning electron micrograph

of WO3 nanowires. Panels (b–e) display the corresponding products after sulfurization

at 500, 600, 700 and 800°C, respectively. (f) X-ray diffraction patterns of the

corresponding products. Note: the reflection located around 24° corresponds to

elemental sulfur on the surface of the WS2. (Paper I).

The crystal structure of the original material was hexagonal according to XRD [data

sheet PDF 01-075-2187]. The sulfurized products showed a different diffraction

pattern at higher temperature suggesting the conversion of the original hexagonal

WO3 to hexagonal WS2 (see Figure 2(f)). From the relative intensity of the most

28

intense peaks (e.g. at 28.2° and 36.6° corresponding to the (200) and (201) planes

of WO3, and the (002) and (103) of WS2 at 14.4° and 39.6°, respectively), it was

found that the WO3 phase dominated up to 500°C but the samples produced above

this temperature were dominantly hexagonal WS2 [data sheet PDF#841398], with

an apparently complete WO3 to WS2 conversion at 800°C. Since this work was

concentrated on metal dichalcogenides, all subsequent discussion is focused on the

WS2 product produced at 800°C.

Transmission electron microscopy (TEM) was employed to investigate the

micro and nanostructure of the WS2 products. In Figure 3(a), flakes surrounding

the nanowire are visible, thus the name of the nanostructured particle hybrid nanowire-nanoflake. Analysis of the distance between the fringes of the thin flakes

(Figure 3(b)) gave 0.62 nm, which is consistent with the (002) d-spacing for the

layered WS2 structure. Figure 3(d) shows the selected area electron diffraction that

indicated that the product was indeed hexagonal WS2, agreeing well with the XRD

result. In addition, TEM analysis revealed that the thickness of the 2D structures

was below 10 nm, implying that the number of layers in the flakes was 15 or less.

The EDS result supports the argument concluded by XRD regarding the conversion

of WS2 at 800°C.

29

Fig. 3. Structure of WS2 hybrid materials. (a) Low magnification transmission electron

micrograph of WS2 nanowire-nanoflake hybrid synthesized at 800°C. (b) High-resolution

micrograph of a flake with layered crystal structure. Inset shows the d-spacing of the

layers. (c) SAED pattern of the WS2 nanowire-nanoflake hybrid material. (d) Raman

spectrum of the corresponding WS2 nanowire/nanoflake hybrid material.(e) Raman

spectroscopy of pristine WO3 nanowire. (Paper I).

The Raman spectrum of the WS2 nanowire-nanoflake hybrid prepared at 800°C

(Figure 3 (d)) showed two significant peaks that matched the in-plane E2g mode at

(~352 cm−1) and the out-of-plane A1g mode at (~420 cm−1) of crystalline WS2. For

comparison, the Raman spectrum of WO3 is also included (Figure 3 (e)) displaying

characteristic peaks centered at 242 cm−1, 325 cm−1 (bending modes of O-W-O)

and at 668 cm−1, 810 cm−1 (stretching modes of O-W-O).[59] [60][61]

4.2 Electrical properties and light sensing behavior of WS2

nanohybrid networks (Paper I)

The electrical properties of WS2 nanowire-nanoflake hybrid networks exhibited a

nonlinear current-voltage characteristic as shown in Figure 4(a) in both 2- and 4-

point measurement arrangements. In the 4-point probe setup, the effect of any

possible barrier caused by the WS2-Au interface was excluded but the I-V curve

was still nonlinear, and might be attributed to the contacts between the nanohybrids

in the random networks. The resistance of nanohybrids measured in air showed a

decrease with increasing temperature (Figure 4(b)) that implied the product was

30

semiconducting. The data could be well fitted by both thermal activation and

variable-range hopping (VRH) models. Evaluating the fitting parameters however,

it was found that thermal activation was probably the best mechanism to describe the

conduction process with an apparent activation energy of 0.29 eV (see inset in

Figure 4(b)), as the fitting parameters for the VRH model are non-physical.

Fig. 4. (a) Current-voltage characteristics (2- and 4-point probe) and temperature

dependent resistance (2-point probe) of WS2 hybrid films. Inset in panel (b) displays the

corresponding Arrhenius plot for the thermally activated conduction. (Paper I).

Back-gated field effect transistor (FET) measurements of the networks of products

synthesized at different temperatures showed no field effect at all for the materials

obtained at 500°C or 600°C. On the other hand, a FET prepared using the sample

synthesized at 800°C displayed a small but measurable field effect as shown in

Figure 5 (a) with a corresponding field effect mobility of 0.02 cm2V-1s-1. The

obtained value is reasonable and comparable to MoS2 and WS2 based FET (0.004-

0.04 cm2 V-1S-1) [62, 63] and CVD grown MoS2 [64] or CVD grown WS2 [65]. It

was assumed that the obtained low field effect mobility was caused by the large

numbers of contacts between the nanowires in the nanowire network. If so, there is

a possibility to improve the field effect mobility by using very few or individual

WS2 nanoparticles in the channel. This argument is supported by experimental

results published for single crystal WS2 with ~20 μm crystal size (100 cm2V−1s−1)

[66] and for WS2 films (9–15 cm2V−1s−1) [67]. Taking this into consideration, a low

density nanowire-nanoflake based FET was prepared and its output characteristics

and transfer curves were measured. Consistent with the above assumption, the field

effect mobility showed one order of magnitude improvement (0.18 cm2V-1s-1), as

displayed in Figure 5 (b). While the high contact resistance between the nanowires

in the network was expected to be the major reason for the poor gate effect, the

high thickness of the SiO2 (300 nm) used in the experiments could also be

31

responsible for such poor behavior. An additional investigation reducing the oxide

layer thickness (< 100nm) is required before any further conclusions can be drawn.

Fig. 5. Output characteristics of the WS2 nanowire nanoflake hybrid based FET devices.

Current-voltage curves for (a) high and (b) low density random networks of the hybrid

nanowires between the source and drain electrodes. Inset in panel (a) displays a

magnified plot for the outlined regime between 4.4 and 4.5 V for better visibility of the

current values at different gate voltages. Inset in panel (b) shows a dark field optical

micrograph of a device. (Paper I).

To obtain information about the optical properties of the product, the total

reflectance of films made of the nanostructures was measured, and the band gaps

calculated from the corresponding Tauc plots (Figure 6(a)). The results, clearly

indicating that the band gap of WO3 nanowires dropped from 2.6 eV to 1.4 eV as a

result of sulfurization, are in good agreement with the literature data [68].

Fig. 6. (a) Tauc plot for direct band-to-band transition derived from total reflectance

measurements on the original and sulfurized powders. Dashed lines represent fitting of

the linear sections of each curve. Intersections of the dashed lines with the horizontal

axis define the band gap. (b) Current-voltage characteristics of a high density FET under

different LED illumination conditions (red, green and blue centered at 623 nm, 517.5 nm

and 466 nm, respectively). (Paper I).

32

In addition to the electrical characterization, the FET was illuminated by light

emitting diodes (LEDs) to assess the photoresponse of the material. As depicted in

Figure 6 (b), the channel current was modulated by the LED illumination. Among

the tested three different colors, the effect was somewhat more significant for blue

light, probably due to the better optical absorption and more efficient

photogeneration of carriers in the semiconductor as compared to the longer

wavelengths.

4.3 Resistive gas sensing with WS2 nanohybrid networks (Paper II)

Paper II focuses on using the synthesized WS2 nanowire-nanoflake hybrid material

as a gas sensing element in resistive (Taguchi-type) devices. The sensors were

exposed to five different analytes (H2S, CO, NH3, H2 and NO) at two different

temperatures (30°C and 200°C). The three key performance indicators to assess gas

sensor performance, response, sensitivity and recovery, were evaluated.

The sensors showed a positive/negative resistance response for

reducing/oxidizing gases (Figure 7) indicating that WS2 nanowire-nanoflake

hybrids are p-type semiconductors and the possible working mechanism of the

sensor is charge transfer conduction modulation, similar to other reported results

[3,57, 69].

33

Fig. 7. Sensor response curves for (a) H2S, (b) CO, (c) NH3, (d) H2 and (e) NO in air buffer

at ppm gas concentration levels measured at 30°C and 200°C. (Paper II).

34

The typical response times obtained were 1 and 2 min at 200°C and longer (3-6 min)

at 30°C. Since reaction adsorption/desorption and reaction rates increase with

temperature, the observed quicker response at 200°C was reasonable.

The sensitivity to H2S was particularly high (0.023 ppm-1) and prompted further

testing of the WS2-H2S system in the ppb concentration regime (Figure 8). The

detection limit of the sensor was found to be 20 ppb at 200°C with a corresponding

sensitivity of 0.043 ppm-1 i.e. the WS2 nanohybrid was competitive with other

known materials such as Fe2O3 nanochains and nanoparticles [70,71], CuO-SnO2

[72], CuO nanoparticles [73] and nanosheets [74], mesoporous WO3 [75], CeO2

nanowires [76] and PbS quantum dots [77].

Table 2. Resistive and field-effect transistor H2S sensors and their properties as

reported in the literature (Paper II).

Method Sensing material Sensitivitya Operating

temperature

Lowest experimental

H2S concentrationb

Ref.

Resistive WS2 nanowire-

nanoflake hybrid

0.043 ppm-1 200 ºC 0.02 ppm This

work

Resistive α-Fe2O3 nanoparticles ~5.2 ppm-1 300 ºC 0.05 ppm [71]

Resistive CuO nanosheets ~3.3 ppm-1 240 ºC 0.03 ppm [74]

Resistive Mesoporous WO3 ~50 ppm-1 250 ºC 0.25 ppm [75]

Resistive CeO2 nanowire ~2 ppm-1 Room temp. 0.05 ppm [76]

Resistive PbS quantum dots ~140 ppm-1 135 ºC 10 ppm [77]

Resistive ZnO nanostructures ~0.14 ppm-1 300 ºC 20 ppm [78]

Resistive Pt-loaded WO3 thin

films

~1200 ppm-1 100 ºC 1 ppm [79]

Resistive Nanocrystalline In2O3-

SnO2

~20 ppm-1 40 ºC 2 ppm [80]

Resistive SnO2 thin film 0.2 ppm-1 300 ºC 5 ppm [81]

Resistive SnO2 nanocolumns 2.8 ppm-1 300 ºC 5 ppm [81]

Resistive SnO2 nanocolumns

decorated with Au

22 ppm-1 300 ºC 5 ppm [81]

Resistive SnO2 nanocolumns

decorated with Ag

12.8 ppm-1 300 ºC 5 ppm [81]

FET Ultrathin Ph5T2

microplates

240 ppm-1 Room temp. 0.5 ppm [82]

FET CuPc 0.004 ppm-1 Room temp. 100 ppm [83]

FET In2O3 nanowires 0.41 ppm-1 Room temp. 1 ppm [84]

35

Method Sensing material Sensitivitya Operating

temperature

Lowest experimental

H2S concentrationb

Ref.

a The highest sensitivity values were calculated from the published references as the relative change of

electrical resistance normalized to the corresponding H2S concentration i.e. ΔR/(R0·CH2S). When the

electrical current values were reported, the sensitivity values were calculated from the relative change of

the current at the corresponding gas concentration, i.e. ΔI/(I0·CH2S).

Fig. 8. H2S sensing performance at ppb level. (a) Sensor resistance at 200°C. (b)

Sensitivity of the five different sensors displaying high selectivity towards H2S (0.023

ppm-1 at 1 ppm according to data in Fig. 7). Data point labeled with an asterisk denotes

sensitivity (0.043 ppm-1) measured at 20 ppb H2S. (Paper II).

To further elaborate the interaction of H2S with WS2 ab initio calculations were

conducted using a simplified model that considered single and bilayer pristine

undoped WS2 [3]. The adsorption of each analyte on the surface was found to be

thermodynamically stable (i.e. having negative adsorption energy). On the other

hand, the adsorbed gases had little or insignificant contribution to the band structure

in the band gap. Furthermore, Bader analysis suggested that a very small amount of

charge was transferred for NH3, NO and H2S, and no sizable charge transfer at all

for H2 and CO. Accordingly, the modeling data indicated that the cause of the

excellent sensitivity of the hybrids towards the H2S molecule stemmed from a

source other than direct charge transfer.

To reveal the changes of the surface chemistry in a typical H2S sensing

experiment, additional XPS analysis was conducted on three samples. Sample #1

represented an original WS2 hybrid, Sample #2 represented materials that had

undergone exposure to air at 200°C and Sample #3 was treated as Sample #2 but

was subsequently also exposed to H2S (at 200°C for 1 h in 1 ppm in air) . The

results shown in Table 3 reveal that when the WS2 hybrid was heated in air, oxygen

36

partially substituted the surface sulfur. This could be reversed by inserting even 1

ppm H2S into the gas. Therefore, the H2S sensing process may be interpreted as

reversible adsorption/desorption and red-ox reaction sequences, in which oxygen

and sulfur compete for the anionic sites in the WS2 lattice, resulting in a particularly

high sensitivity of the material (and its electrical properties) for this analyte.

Table 3. Surface composition of the WS2 hybrid in various stages of the H2S sensing

process (Paper II).

Line Sample #1

(Original WS2 hybrid)

Sample #2

(WS2 hybrid heated in air)

Sample #3

(sample heated in air and

subsequently in 1 ppm

H2S)

BE (eV) FWHM

(eV)

AC

(at.%)

BE (eV) FWHM

(eV)

AC

(at.%)

BE (eV) FWHM

(eV)

AC

(at.%)

Assignment

C 1s 284.2 1 8.53 284.3 1.05 9.99 284.3 0.85 3.52 W-C (?)

285.3 1.25 3.34 285.3 1.1 3.38 285.3 1.45 3.93 C-(C, H)

O 1s 530.6 1.3 9.31 530.6 1.35 9.7 530.8 1.35 9.83 WO3

532.1 1.6 7.01 532.2 1.45 7.95 532.3 1.55 8.53 NaOH

533.8 1.25 0.51 533.9 2 1.17 534.3 1.8 1.11 H2Oads (?)

Na 1s 1071.7 1.7 6.28 1071.7 1.65 7.78 1071.9 1.6 7.44 NaOH

W 4f7/2 32.7 0.65 20.41 32.7 0.65 18.49 32.7 0.6 20.38 WS2

35.7 1.15 3.17 35.7 1.1 2.85 35.9 1.15 3.29 WO3

S 2p3/2 162.3 0.65 41.44 162.3 0.65 37.28 162.4 0.6 41.94 WS2

168.9 1 1.42 SO42-

The aging effect is one of the concerns of the sulfide based nanomaterials. A recent

study [85] revealed that WS2 flakes undergo oxidation along their grain boundaries

even at room temperature. However, there is no conclusive result regarding the long

term aging effects.[86]

4.4 Photo sensing using individual WS2 nanohybrid particles

(Paper III)

Photodetector devices based on individual nanohybrid particles were prepared

using FIB deposited Pt contacts and sputtered Pt probe pads (Figure 9(a)). Current-

voltage curves measured for five different devices (Figure 9(b) showed linear

behavior that indicated that the contacts were ohmic and there was good agreement

with the Fermi level as well as the valence and conduction band positions of Pt and

WS2, respectively (inset in Figure 9(b)). [87-91].

37

Fig. 9. (a) SEM image of the individual WS2 nanohybrid between platinum contacts, and

(b) I-V curves of five devices with the corresponding band diagram of Pt-WS2 contact.

The photosensitivity of individual nanohybrids was assessed at three different

wavelengths (401 nm, 552 nm and 661 nm) between 1 and 100 mW total power

(Figure 10). The photocurrent showed significant light intensity dependence, and a

rapid response of ~1 µs for each wavelength. The larger time constant in the fitting

parameters was probably associated with the thermal momentum of the system

according to simulations on time dependent surface heating. It is worth noting here,

that the simulated increase of surface temperature expected in the experiments was

very low and thus the increased current was truly due to the photogeneration.

The power function fit on the photocurrent vs. laser power data gave very

similar exponents of ~0.21 for each wavelength. Such a low exponent value implies

a complex photogeneration and relaxation mechanism through various trap states

in the WS2 nanohybrids. The low exponent is similar to the values obtained for

other materials such as graphene (~0.25) [92], ZnO (~0.28) [93], single layer WS2

(0.4) [94] and InSb nanowires (0.2) [95]. The maximum obtained

photoresponsivity of ~0.4 A·W-1 is excellent considering that it is close to those of

38

bulk Si based detectors [96] and composites of carbon nanotube and perovskite

films [97], and is significantly higher than the earlier reported values for graphene

[98, 99] or for most TMDs [100-102]. However it does not compete with CdSe and

CdSe/CdS core–shell nanocrystals [103], arrays of ZnO [104] or Si nanowires [105].

Fig. 10. (a) Photocurrent modulation by red laser pulses (λ=661 nm, w0=9 mm 1/e2 beam

waist, TEM00, 10 kHz square wave with duration of 50 µs, power of 1, 2, 5, 10, 20, 50 and

100 mW) measured on a serially connected load resistor. (b) Photoresponse to laser

pulses (λ=661, 552 and 401 nm denoted as red, green and blue plots) of 20 mW total

power corresponding to 29, 52 and 40 nW power on the WS2 nanohybrid considering

the 1/e2 beam waists of 9, 7 and 8 mm, respectively (nanohybrid length and diameter of

5 µm and 330 nm, respectively). The time constants correspond to the fitting parameters

of bi-exponential growth and decay fitting parameters. (c) Log-log plot of photocurrent

vs. total laser power and wavelength, and corresponding power function fittings. (d)

Photoresponsivity vs. light power on the nanohybrid.

39

Table 4. Photoresponsivity Performance parameters [113].

Description Responsivity Detector type Reference

WS2nanowire-nanoflake 0.4 AW-1 Photodetector This work

Bulk Si 0.1-0.7 AW-1 photodiode 96

Composites of carbon nanotube

and perovskite films

13.8 AW-1 Photodetector 97

Single layer MoS2 7.5 mAW-1 Phototransistor 100

Few-Layered WS2 Films 21.2 μA W−1 Photodetector 102

ZnO nanowire 4.7 ×104 AW-1 Phototransistor 104

Silicon nanowire 105 A/W Phototransistor 105

Graphene-metal junction 6.1mA W-1 Photocurrent(PV/PTE) 106

Graphene p-n junction 10mAW-1 Photocurrent (PTE) 107

Graphene coupled to waveguide 0.13AW-1 Photocurrent(PV/PTE) 108

Hybrid graphene-QD 108 AW-1 Phototransistor 109

Graphene double layer

hetrostructure

> 1AW-1 Phototransistor 110

Biased MoS2 880 AW-1 Photoconductor 111

WSe2 p-n junction 16 mAW-1 p-n photodiode 112

40

41

5 Summary and conclusions

In this thesis a novel WS2 nanowire-nanoflake hybrid nanostructure has been

synthesized, characterized and then demonstrated to be a versatile nanomaterial.

Treatment of hydrothermally grown WO3 nanowires in the vapor of S at 800°C

resulted in the formation of WS2 nanowire-nanoflake hybrids instead of the

anticipated WS2 nanowires. The obtained nanostructured material showed p-type

semiconducting behavior with a band gap and field effect mobility of 1.4 eV and

0.18 cm2V-1s-1, respectively. In resistive gas sensors, the WS2 nanohybrid displayed

excellent selectivity to H2S (among the tested analytes such as H2S, CO, NH3, H2

and NO) with a high sensitivity of 0.043 ppm-1 and a detection limit of 20 ppb.

Further, this new nanomaterial also showed high photoresponsivity (0.4 A·W-1) in

the entire visible spectrum.

Based on these results, it may be concluded that the WS2 nanowire-nanoflake

hybrid has a number of fascinating properties making it a versatile nanomaterial

similar to other transition metal disulfides and prompting applications in nano and

optoelectronics as well as in gas sensing. In a broader perspective, this thesis also

sheds light on a potential large-scale synthesis method that may be applied to obtain

a large variety of other highly complex nanomaterials by simply doping metal oxide

nanostructures in the vapor of S or other chalcogens.

42

43

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Original publications

I Asres, G. A., Dombovari, A., Sipola, T., Puskás, R., Kukovecz, A., Kónya, Z.,Popov, A., Lin, J.F. Lorite, G.S. Mohl, M., Toth, G., Spetz, A.L., Kordas, K.( 2016) A novel WS2 nanowire-nanoflake hybrid material synthesized from WO3 nanowires in sulfur vapor. Scientific Reports, 6, 25610.

II Asres, G.A., Baldovi, J.J., Dombovari, A., Järvinen, T., Lorite, G.S., Mohl, M., Shchukarev, A., Paz, A.P., Xian, L., Mikkola, J-P., Spetz, A.L., Jantunen, H., Rubio, A., Kordas, K.(in press) Ultra-sensitive H2S gas sensors based on p-type WS2 hybrid materials. Nano Research,

III Asres, G.A., Järvinen, T., Lorite, G.S., Mohl, M., Pitkänen, O., Dombovari, A., Toth, G., Spetz, A.L., Vajtai, R., Ajayan, P.M., Lei, S., Talapatra, S., Kordas, K.(2018) Excellent photoresponse of individual WS2 nanowire-nanoflake hybrid materials. Manuscript submitted for publication.

Reprinted with permission from Springer Nature (paper I and paper II)

Original publications are not included in the electronic version of the dissertation.

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645. Leppänen, Teemu (2018) Resource-oriented mobile agent and softwareframework for the Internet of Things

646. Klets, Olesya (2018) Subject-specific finite element modeling of the knee joint tostudy osteoarthritis development and progression

647. Shams Shafigh, Alireza (2018) New networking paradigms for future wirelessnetworks

648. Mikhaylov, Konstantin (2018) Plug and play reconfigurable solutions forheterogeneous IoT

649. Verrollot, Jordan (2018) Mature supply management as an enabler for rapidproduct development and product portfolio renewal

650. Kemppainen, Anssi (2018) Adaptive methods for autonomous environmentalmodelling

651. Nyländen, Teemu (2018) Application specific programmable processors forreconfigurable self-powered devices

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UNIVERSITY OF OULU P .O. Box 8000 F I -90014 UNIVERSITY OF OULU FINLAND

A C T A U N I V E R S I T A T I S O U L U E N S I S

University Lecturer Tuomo Glumoff

University Lecturer Santeri Palviainen

Postdoctoral research fellow Sanna Taskila

Professor Olli Vuolteenaho

University Lecturer Veli-Matti Ulvinen

Planning Director Pertti Tikkanen

Professor Jari Juga

University Lecturer Anu Soikkeli

Professor Olli Vuolteenaho

Publications Editor Kirsti Nurkkala

ISBN 978-952-62-1888-5 (Paperback)ISBN 978-952-62-1889-2 (PDF)ISSN 0355-3213 (Print)ISSN 1796-2226 (Online)

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TECHNICA

U N I V E R S I TAT I S O U L U E N S I SACTAC

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OULU 2018

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SYNTHESIS, CHARACTERIZATION AND APPLICATION OF WS2 NANOWIRE-NANOFLAKE HYBRID NANOSTRUCTURES

UNIVERSITY OF OULU GRADUATE SCHOOL;UNIVERSITY OF OULU,FACULTY OF INFORMATION TECHNOLOGY AND ELECTRICAL ENGINEERING

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