Addressing and tailoring the electronic properties of semiconductor nanostructures: nanowires and transition metal dichalcogenides

Semiconductor materials played and still play a pivotal role in the technological development of modern life. From personal computer to data storage (e.g. solid-state disk-drives), from solar cells and cell phones to LEDs and biological sensors, there has always been a new system to study, a novel application to develop, and a solution to an otherwise unsolvable problem in which semiconductors play an essential role. All those technological goals have been achieved thanks to the synergic works carried out by basic researches in materials science, in particular in the semiconductor field. In the last decades, tremendous efforts have been made to miniaturize semiconductor devices at a nanometer scale, aiming at obtaining more compact devices with optimized speed and reduced power consumption. Unfortunately, or fortunately, the physical properties of any material dramatically change when the material dimensions are reduced to nanometer-lengths. Therefore, many efforts are required to understand the properties of any desired nanostructure, if they have to play a central role in technological applications. In recent years, a great interest has grown in the investigation and applications of nanowires (NWs). NWs are several micron-long filamentary-crystals whose diameters range from few to hundreds of nanometers. Their dimensions make NWs suitable to bridge the gap between the microscopic and the nanoscopic world in both research and technology fields. Although several types of materials can be grown in a NW form, e.g. metals, insulators, and semiconductors, the latter are the most interesting and promising materials. As a matter of fact, owing to their peculiar shape and dimensions, semiconductor NWs are valuable candidates for novel nanoscale devices, in which they act as both functionalized components and interconnects. Moreover, semiconductor NWs represent nanostructured systems for which some key parameters in device engineering, e.g. chemical composition, size, and crystal phase, are well controlled nowadays. This is mainly due to the technique used to grow NWs. NWs are usually fabricated via the vapor-liquid-solid (VLS) technique, in which metal nanoparticles are used as catalyst seeds to induce a one-dimensional crystal growth. This well-controlled process allows for the synthesis of a wide range of semiconductor systems in the NW form, ranging from IV-IV to II-VI, with a high degree of manageability of both the chemical composition and morphology. In addition, under suitable VLS conditions, non-nitride III-V NWs can crystallize in the hexagonal wurtzite (WZ) structure in materials that, instead, are notoriously stable in the cubic zinc-blende (ZB) structure. The opportunity to controllably grow NWs in different crystal phases, namely, the polytypism, adds a new degree of freedom in device engineering. The presence of a WZ crystal phase in many III-V NWs offers also the opportunity to address the electronic band structure of this poorly known structure, vii viii whose presence itself is a subject of fundamental interest in materials science and chemistry. As an example, there is no experimental information concerning the variation of the spin and transport properties, i.e. gyromagnetic factors and carrier effective-masses, respectively, when the phase transition from ZB to WZ occurs. Even the fundamental band-gap value of some WZ semiconductor materials have not been determined, yet. Therefore, a comprehensive study aimed at the investigation of the correlation between the NW electronic properties and NW crystal structure is mandatory nowadays. A great interest has grown also in the field of layered materials. Since the discovery of graphene in 2004, it has been understood the great potential of layered systems for advanced-technological applications. As a matter of fact, layered materials thinned to their physical limits -and usually referred to as two-dimensional (2D) materials- exhibit properties quite different from those of their bulk counterparts. A very wide spectrum of 2D materials has been then investigated. The most studied material is graphene because of its exceptional electronic and mechanical properties. Group VI transition metal dichalcogenides (TMDs) have also attracted the attention of researchers involved in the semiconductor field. TMDs have a crystal structure similar to that of graphite. Their layered structure, X-M-X, where M is the transition metal and X is the chalcogen atom, is characterized by weak interlayer van der Waals bonds and strong intralayer covalent bonds. That structure allows for an easy mechanical exfoliation, as in the graphene case, which is a major advantage of 2D materials, together with their synthesis techniques, cheap and easy as compared to the molecular-beam-epitaxy or metal-organic chemical-vapor-deposition techniques used for the fabrication of other nanostructured systems. The most surprising feature observed in 2D TMDs is the transition from an indirect band-gap in the infrared region to a direct band-gap in the visible region when they are thinned to the mono- layer limit. That feature, coupled with the TMD extremely high flexibility, elasticity, and resistance, makes TMDs suitable in the field of low-dimensional optoelectronic devices. In addition, the TMD high surface-to-volume ratio is valuable in biological fields, as they can be used as highly reactive sensors. Besides, the TMD unique properties in the single-layer limit of valley-valley coupling and valley-spin coupling render TMDs the suitable candidates for novel technologies based on valleytronic and spintronic. However, almost all these aforementioned properties are at the early stage of investigation and systematic studies are necessary before TMDs could be exploited in future applications. In this thesis, the electronic properties of InP NWs and MX2 TMDs, with M=Mo or W and X=S or Se, are thoroughly investigated mainly by means of optical spectroscopy, in particular photoluminescence (PL) in combination with external perturbations, e.g. high magnetic fields. The response of semiconductor TMDs to hydrogen irradiation is studied, too. The thesis is therefore structured in two parts, the first one, from chap. 1 to chap. 3, is devoted to InP NWs, the second one, from chap. 4 to chap. 6, is devoted to 2D TMDs. • In the first chapter, the high degree of freedom achieved in NW fabrication is presented and accounted for by the VLS technique, which is also discussed in details together with its recent development: the selective-area-epitaxy technique. Then, the differences between the structural, electronic, and optical ix properties of WZ and ZB crystal phases are discussed. The striking variation induced in the band structure by the crystal phase-transition is highlighted, too. Moreover, the different optical anisotropies of the two crystal phases are summarized. The chapter is concluded by a review of the technological applications of semiconductor NWs in the fields of optoelectronic, energy conversion, biosensoring, and as probes of elusive quantum effects. • The second chapter comprehends a systematic investigation of InP NWs in both the ZB and WZ crystal-phases. The morphological characteristics of the investigated samples as accessed through scanning-electron-microscopy, transmission-electron-microscopy, and selective-area-diffraction patterning are also presented. The basic optical properties of InP in both crystal phases are assessed by either PL or μ-PL experiments as a function of lattice temperature and power excitation. Polarization-resolved measurements are shown, too. The three lowest-energy critical points of the WZ band-structure are investigated by PL excitation (PLE) as a function of lattice temperature. A quantitative reproduction of those spectra allows for establishing the temperature depen- dence of the A, B, and C inter-band transitions. A comparison with ZB results is made, too. Finally, the hot-carrier effect in NWs is found and its dependence on NW morphology is investigated. • In the third chapter, the transport and spin properties of WZ InP are assessed by PL spectroscopy under high magnetic fields (up to 28 T ). A brief review of the effects that a magnetic field has on the energy and symmetry of exciton recombinations and of free-electron-to-acceptor and donor-to-acceptor transi- tions in WZ crystal is presented. Both diamagnetic shift and Zeeman splitting depend on the magnetic-field direction with respect to the NW symmetry-axis, namely the WZ cˆ-axis. That dependence has been investigated by applying the magnetic field either parallel or orthogonal to the NW axis. The obtained results are compared with the literature of both theoretical models of WZ InP and experimental results in other WZ compounds, such as GaN, InN, and ZnO. Finally, the non-linearity observed in the Zeeman splitting for magnetic fields above 10T and parallel to the NW axis is compared to a theoretical prediction. • In the fourth chapter, the lattice, electronic, and vibrational properties of 2D TMDs are described. In particular, the lattice structures of several polytypes are shown, with special emphasis on the 2H polytype, whose electronic and vibrational properties are investigated and its different properties in the bulk and single-layer regimes highlighted. Then, several methods aimed at reaching the mono-layer limit are presented and top-down exfoliations from bulk mate- rials are singled out from bottom-up syntheses. The chapter ends with a brief review of the technological applications of semiconductor 2D TMDs in the fields of optoelectronic, energy conversion and storage, and molecular sensing. • The fifth chapter comprehends a systematic investigation of the effects of hydrogen irradiation on the emission properties of single- and bi-layer TMDs, such as MoSe2 and WSe2. Firstly, a wide variety of experimental results con- x cerning MX2 optical band-gaps and vibrational mode-energies are summarized. A brief description of the investigated samples is presented, too. The optical properties of pristine samples are assessed by means of either μ-Raman or μ-PL experiments whose room- and low-temperature results agree well with the existing literature. Then, the pristine flakes are irradiated with progressively increasing doses of hydrogen and the results thus obtained are reported. In the single-layer regime, a worsening of the material optical quality is observed together with the appearances of very sharp peaks below the band-gap energy. Conversely, a small improvement in the PL efficiency is obtained in the bi-layer regime. Finally, a solution to the worsening of the optical quality observed in hydrogenated single-layer flakes is provided. • In the sixth chapter, the effects of hydrogen irradiation on the morphological and optical properties of multi-layer TMDs are discussed. Surprisingly, hy- drogenation favors unique conditions for the production and accumulation of molecular hydrogen just one or few layers beneath the crystal surface of all the multi-layer MX2 compounds investigated. That turns into the creation of atomically-thin domes filled with hydrogen molecules. The results of an atomic-force-microscopy and optical investigation of these new fascinating nanostructures are discussed. Finally, the possibility to tailor the dome posi- tion, size, and density is demonstrated, which provides a tool to manage the mechanical and electronic structure of 2D materials. • The main results obtained in this work are summarized in the conclusive remarks. • In the appendix, the theoretical basis of the optical-spectroscopy techniques here used, such as PL, PLE, magneto-PL, and Raman spectroscopy, are provided. PL and PLE are complementary techniques that enable a complete characterization of the electronic states of any optically-efficient material. Indeed, PL is an extremely sensitive probe of low-density electronic states, such as impurities or defects, while PLE can address the full density of states, i.e, it mimics absorption measurements, at least under certain approximations. On the other hand, PL spectroscopy under magnetic field allows for the determination of carrier effective-masses and g-factors, while Raman allows for getting information about the lattice properties of solids. A description of all the used experimental setups is also given. Finally, a description of the experimental apparatus used for hydrogen irradiation and atomic-force- microscopy measurements is provided. • Finally, a list of the publications to which the author of this thesis has contributed is provided, along with a list of poster/oral contributions to international conferences given by the author of this thesis during his PhD studies.