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ARIZONA STATE UNIVERSITY. May 2017 ... XRD, AFM, and Raman spectroscopy were used to determine the ... 6.1 Summary of Results .
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Angle Resolved Polarization and Vibrational Studies of Transition Metal Trichalcogenides and Related Alloys by Wilson Kong
A Thesis Presented in Partial Fulfillment of the Requirements for the Degree Master of Science
Graduate Supervisory Committee:Approved April 2017 by the Sefaattin Tongay, Chair Liping Wang Matthew Green
May 2017
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A new class of layered materials called the transition metal trichalcogenides (TMTCs) exhibit strong anisotropic properties due to their quasi-1D nature. These 2D materials are composed of chain-like structures which are weakly bound to form planar sheets with highly directional properties. The vibrational properties of three materials from the TMTC family, specifically TiS 3 , ZrS 3 , and HfS 3 , are relatively unknown and studies performed in this work elucidates the origin of their Raman characteristics. The crystals were synthesized through chemical vapor transport prior to mechanical exfoliation onto Si/SiO 2 substrates. XRD, AFM, and Raman spectroscopy were used to determine the crystallinity, thickness, and chemical signature of the exfoliated crystals. Vibrational modes and anisotropic polarization are investigated through density functional theory calculations and angle-resolved Raman spectroscopy. Particular Raman modes are explored in order to correlate select peaks to the b-axis crystalline direction. Mode III vibrations for TiS 3 , ZrS 3 , and HfS 3 are shared between each material and serves as a unique identifier of the crystalline orientation in MX 3 materials. Similar angle-resolved Raman studies were conducted on the novel Nb0.5Ti0.5S 3 alloy material grown through chemical vapor transport. Results show that the anisotropy direction is more difficult to determine due to the randomization of quasi-1D chains caused by defects that are common in 2D alloys. This work provides a fundamental understanding of the vibrational properties of various TMTC materials which is needed to realize applications in direction dependent polarization and linear dichroism.
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I wish to express my sincere appreciation to my advisor, Dr. Sefaattin Tongay, for accepting me into his group, providing support in both my academics and personal life, and shaping me into a more capable researcher. His vast knowledge of 2D materials and accommodating attitude has eased the difficulty of my research projects. I cannot think of a better advisor to supervise and mentor me during my time at Arizona State University. I would like to thank each of my many lab group members, both past and present, for all of your guidance and assistance on my various research projects. Your work ethic and knowledge served as a strong model for me to follow as a young graduate student. Not only was the work in this group productive, it was very enjoyable. You were a fantastic group of people to spend the last two years with. I would also like to thank the various collaborators from ASU and other universities for your contributions to my research and for providing me with excellent results. Special thanks to Dr. Hasan Sahin, Mr. David Wright, and Mr. Cihan Bacaksiz for your tremendous contributions towards making my first journal publication possible. Lastly, I want to thank my family for all the love and support they have shown me throughout my entire life. I am the person I am today because of you.
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LIST OF FIGURES .......................................................................................................... vii
CHAPTER
1.1 Two-Dimensional Materials ................................................................................. 1 1.1.1 Graphene ....................................................................................................... 3 1.1.2 Transition Metal Dichalcogenides ................................................................ 4 1.1.3 Transition Metal Monochalcogenides........................................................... 5 1.2 Anisotropic Two-Dimensional Materials ............................................................. 6 1.2.1 Black Phosphorus and Phosphorene ............................................................. 6 1.2.2 Rhenium Disulfide and Gallium Telleride .................................................... 7 1.2.3 Transition Metal Trichalcogenides ............................................................... 8
2.1 Chemical Vapor Transport ................................................................................. 12 2.1.1 Methodology ............................................................................................... 12 2.1.2 Growth of MX 3 Materials ........................................................................... 13 2.1.3 Titanium Trisulfide Growth ........................................................................ 14 2.1.4 Zirconium and Hafnium Trisulfide Growth ................................................ 15
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1.1 Two-Dimensional Materials
In recent years, the field of nanomaterials has gained much attention due to the study of an emerging class of layered materials known as two-dimensional (2D) materials [1]. Layered materials are comprised of atomically thin sheets with a thickness of one unit cell bound together through weak Van der Waals interactions [2]. This enables the facile separation of individual atomic sheets from a bulk crystal either through mechanical or liquid exfoliation techniques [3–6]. Pristine surfaces of 2D materials are minimally rough, contain few dangling bonds, and generally free of defect sites [7]. Such chemically passivated surfaces enables the formation of vertical heterostructures between several types of layered materials [8]. Advances in bottom-up synthesis methods such as chemical vapor deposition (CVD) [9], physical vapor deposition (PVD) [10], and molecular beam epitaxy (MBE) [11] improve the scalability of these materials for additional research purposes or to one day achieve commercial production. Unique mechanical, electronic, and optical properties derived from strong quantum confinement effects are seen in 2D materials [12]. Much of this research began when physicists Andre Geim and Konstantin Novoselov first discovered the widely known and popular 2D material, graphene [13]. This material and its superior properties enable its many applications in optoelectronics [14], electrochemical energy storage [15], and
coatings for aircraft [16]. Despite the merits of graphene, this 2D material still faces many limitations that prevent it from achieving greater potential applications. Many other discovered 2D materials beyond graphene possess electronic properties that range from insulating and semiconducting to metallic or even superconducting [17]. Hexagonal boron nitride (h-BN) is an analogue to graphene, possessing a hexagonal crystal structure and a wide band gap ranging from 3.6 to 7.1 eV [18]. Graphene is known to be synthesized on h-BN sheets due to their similar lattice constants and surface structure [19,20]. Environmentally unstable 2D materials use h-BN as a passivation layer to protect it from oxidation or other destructive processes. Other commonly studied 2D materials include the semiconducting transition metal dichalcogenides (TMDCs) with layer dependent optical and electronic behavior [21]. Lamellar TMDCs span a wide range of band gap values which make them attractive candidates for device fabrication. Vertical and lateral heterojunction growth of layered materials leads nano-scale field-effect transistors with strong interlayer excitonic transitions and enhanced photoluminescence (PL) [22,23]. Even beyond TMDCs, elemental layered materials, (phosphorene, silicene, etc.) [24], transition metal monochalcogenides (GaSe, InSe, etc.) [25-29], anisotropic materials (ReS 2 , ReSe 2 , etc.) [30–34], and 2D covalent organic frameworks [35] add to the increasingly large number of 2D materials available. Such a wide variety of low dimensional materials, potential applications, and novel synthesis methods encourage the development of many new fields of materials research. In the following sections, many notable 2D materials are discussed in greater detail to illuminate their importance to materials science research.
1.1.2 Transition Metal Dichalcogenides
Analogous to graphene, transition metal dichalcogenides (TMDCs) are layered materials that potentially serve as attractive alternatives for device applications. TMDCs circumvent graphene’s limitations by offering a similar flexible structure and a tunable range of band gap energies [45]. A typical TMDC material is characterized by the chemical formula, MX 2 where M is a transition metal (i.e. Mo, W), and X is an element from the chalcogen family (i.e. S, Se, Te). Metal atoms are arranged in a trigonal or octahedral prismatic coordination. Overall, TMDC crystals generally have hexagonal or rhombohedral crystal structures with behaviors that range from semiconducting to metallic. First principles calculations show the TMDC band structure for MoX 2 and WX 2 undergoing an indirect to direct band transition as the thickness decreases from bulk to a monolayer. The change in electron hybridization due to quantum confinement effectively changes the band structure of these materials as the number of layers change. For WS 2 , the band gap energy ranges from 1.1 to 1.9 eV as the bulk crystal is thinned down to a monolayer [12]. These electronic and optical properties can be further tuned through strain engineering [46], defect engineering [47], doping [7], or alloying [48]. Few-layer TMDCs have been explored in the fabrication of FETs [8,22,23,49], as a potential replacement for platinum as a catalytic material [50,51], in atomically thin photovoltaics [52], and in label-free biosensors [53]. Due to the unique band structure of TMDCs, the confinement of carriers at the conduction and valance band extrema leads to potential applications in valleytronics [54]. One of the primary challenges to utilizing these properties is being able to synthesize TMDCs in large quantities. While exfoliation
produces limited area, yet pristine single crystals of few-layer sheets, the advancement of vapor deposition techniques will ultimately dictate the scalability of these materials.
1.1.3 Transition Metal Monochalcogenides
Materials similar to TMDCs, transition metal monochalcogenides (TMMCs) are layered materials that are known to exhibit nonlinear optical behavior. TMMCs follow the standard chemical formula MX (M = Ga, In, Sn and X = S, Se, Te). The layered structures are composed of atoms bonded in the following X-M-M-X orientation either with hexagonal or rhombohedral crystal structure. In contrast to TMDCs, TMMCs are direct gap semiconductors in bulk form that transition to an indirect gap for flakes fewer than 7 layers [55]. Gallium selenide (GaSe) is a layered semiconductor with a direct band gap energy of 2 eV [28,56]. Large band renormalization for GaSe can be seen on monolayers grown onto silicon (111) through Van der Waals epitaxy. Precise control over kinetic factors such as cooling rates can alter the growth morphologies of few-layer GaSe [57]. Most notably, the second harmonic generation (SHG), a nonlinear optical process, seen in GaSe is strongest of all known 2D materials, being 1 to 2 orders of magnitude larger than MoS 2 monolayers [25]. Indium selenide (InSe) is another layered semiconductor exhibiting a direct to indirect gap transition as the thickness of the material decreases. Recent reports have shown few layer InSe possessing light electron mass and high mobility values of up to 2000 cm^2 V-1^ s-1^ at low temperature values [58]. InSe is additionally explored for its thickness
to exhibit linearly polarized two-lobed plots for PL which indicate a direction dependent polarization response to light [61]. Thickness dependent band gap energies spanning from 0.3 eV for bulk BP and 2.0 eV for phosphorene effectively connects the band gap range between graphene and the TMDCs [62]. Anisotropic carrier mobility values can be seen for few layered BP measured from 10K to 300K [63]. The x-direction shows light effective- mass values which make this an attractive materials for high performance transistors. However, monolayer phosphorene is not environmentally stable with hinders its use in many device applications unless encapsulated by an inert barrier material.
1.2.2 Rhenium Disulfide and Gallium Telleride
Rhenium disulfide (ReS 2 ) is another member of the TMDC family which distinguishes itself from other TMDCs like MoS 2 through its anisotropic crystal structure and direction dependent properties. These layered materials stack via Van der Waals forces with strong covalent bonding within each planar sheet. Structural in-plane anisotropy originates from the strong dimerization and interaction of adjacent Re atoms. This feature enables ReS 2 to be used in linear dichroic applications. ReS 2 is known to have optical band gaps ranging from 1.4 to 1.6 eV as the material is thinned from bulk to monolayer [34]. Recently, it has been shown that CVD grown truncated triangular and hexagonal ReS 2 flakes contain oriented sub-domains with Re-Re chains arranged in particular directions with respect to the nucleation center [64]. High resolution scanning transmission electron microscopy (HRSTEM) studies provide strong insight into the atomic restructuring of
chains at grain boundaries due to vacancy defects. Re-chains are known to change direction near grain boundaries which can reduce the anisotropic polarization response of ReS 2 nanosheets. Monoclinic gallium telluride (GaTe) is another layered material belonging to the transition metal monochalcogenide family. Unlike GaSe, a structurally isotropic material, GaTe sheets have reduced in-plane symmetry. Each primitive unit cell contains 6 Ga and 6 Te atoms with a C2/m symmetry. This material has a direct band gap of 1.65 eV with additional emission lines shown at 1.29, 1.39, and 1.5 eV [65]. Its strong in-plane anisotropic polarization has been demonstrated through angle-resolved photoluminescence (ARPL) [66]. The incident laser light polarized along the GaTe flake achieves its maximum when the polarization direction is parallel to <010> chain direction which indicates maximum optical absorbance along this direction. GaTe also exhibits bright PL emission in the forbidden energy band which has not been previously reported [65]. Researchers have recently synthesized this material through a physical vapor transport (PVT) method on various substrates which demonstrates the versatility of vapor phase grown anisotropic materials [65].
1.2.3 Transition Metal Trichalcogenides
Transition metal trichalcogenides (TMTCs) are a new class of layered materials with the chemical formula MX 3 where M is a transition metal from group IVB (i.e. Ti, Zr, Hf) or group VB (i.e. Nb, Ta) and X is an element from the chalcogen family (i.e. S, Se,
a visibly long edge parallel to the b-axis direction of the flake. Typically, TMTC crystals grow along this b-axis direction which leads to whisker or hair-like growth.
Figure 2. a) Top down view of MX 3 chain direction along the b-axis shown with black arrows. b) Bonding between adjacent chains in the a-c plane. The chain-like structures impart a quasi-1D behavior, similar to nanowires or nanotubes, to TMTC materials which subsequently show anisotropic properties for a 2D material. TiS 3 mobility is known to be strongest along the b-axis at 80 cm^2 V-1^ s-1^ while showing a weaker response along the a-axis at 40 cm^2 V-1^ s-1^ [69]. ZrS 3 is known to show strong linear dichroism along its b-axis direction when laser polarization is parallel to the chains [70]. Under strained conditions, HfS 3 transitions from an indirect to direct band gap semiconductor [71]. Much of this quasi-1D behavior affords TMTCs a variety of applications in FETs, photodetectors, and other photonic devices. TMTCs represent a system of 2D materials with greater advantages over traditional 2D materials. However, a fundamental understanding of their vibrational properties is still being pioneered. Knowledge of TMTC characteristic vibrations would provide greater insight into their unique materials properties. The following questions are addressed in this
work which provide a greater understanding of TMTCs: What is the origin of the Raman peaks for TMTCs? Do TMTCs have similar vibrational characteristics? Can these vibrational characteristics be used to correlate particular Raman peaks to the b-axis chain direction? Is the anisotropy response thickness dependent? Does the anisotropy of TMTCs change when alloyed? This study employs density functional theory (DFT) calculations and angle-resolved Raman spectroscopy in order to shed light onto the vibrational characteristics and polarization of TMTCs which will establish a foundation of knowledge for researchers to build upon.
