In the search for nano-scale, highly customizable materials for next-generation electronic devices, two-dimensional (2D) materials have generated much interest. 2D materials have complex, layer-dependent optical and electronic properties of which many aspects remain yet to be explored and fully understood. The aim of this thesis was to investigate and explain optoelectronic properties of several 2D materials systems towards device design. This was accomplished using predictive physical modelling at the density functional theory level (DFT) as well as many-body theory (GW+BSE). The optical transitions of bulk ReS2 and ReSe2 were studied using DFT in comparison with experiment. We found that the orbital composition of the band edges determined the sign of the pressure coefficient of the optical gap. Our results provide a step towards understanding the perceived layer-independence of the optical properties of ReS2 and ReSe2. The exciton landscape of MoS2 monolayer was explored in detail using many-body theory (GW+BSE). We found dark excitons very close to bright excitons and even lower in energy. Our results help reverse the common assumption that the lowest-energy exciton in MoS2 is bright. The ideal band offset between recently predicted monolayers of the CaP3 family was predicted using GW theory. We observed chemical trends in the band offsets and explained their origin. Our results serve as indicators for heterojunction design with these novel materials. The effective mass of a test set of eighteen semiconductors including several 2D materials was calculated using DFT with semi-local and non-local hybrid exchange-functionals and compared for accuracy with respect to experimental data. Our analysis details the effect of the nonlocal exchange potential on the accuracy of the effective mass. Our results give guidelines for high-throughput calculations of the effective mass for different material classes, including 2D materials. / Thesis / Doctor of Philosophy (PhD)

京都大学 / 0048 / 新制・課程博士 / 博士(エネルギー科学) / 甲第21315号 / エネ博第371号 / 新制||エネ||73(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー応用科学専攻 / (主査)教授 松田 一成, 教授 佐川 尚, 教授 大垣 英明 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

<div>Recently, atomically thin two-dimensional (2D) layered materials such as graphene and transition metal dichalcogenides (TMDCs) have emerged as a new class of materials due to their unique electronic structures and optical properties at the nanoscale limit. 2D materials also hold great promises as building blocks for creating new heterostructures for optoelectronic applications such as atomically thin photovoltaics, light emitting diodes, and photodetectors. Understanding the fundamental photo-physics process in 2D semiconductors and heterostructures is critical for above-mentioned applications. </div><div>In Chapter 1, we briefly describe photo-generated charge carriers in two-dimensional (2D) transition metal dichalcogenides (TMDCs) semiconductors and heterostructures. Due to the reduced dielectric screening in the single-layer or few-layer of TMDCs semiconductors, Columbo interaction between electron and hole in the exciton is greatly enhanced that leads to extraordinary large exciton binding energy compared with bulk semiconductors. The environmental robust 2D excitons provide an ideal platform to study exciton properties in TMDCs semiconductors. Since layers in 2D materials are holding by weak van de Waals interaction, different 2D layers could be assembled together to make 2D heterostructures. The successful preparation of 2D heterostructures paves a new path to explore intriguing optoelectronic properties.</div><div>In Chapter 2, we introduce various optical microscopy techniques used in our work for the optical characterization of 2D semiconductors and heterostructures. These optical imaging tools with high spatial and temporal resolution allow us to directly track charge and energy flow at 2D interfaces.</div><div>Exciton recombination is a critical factor in determining the efficiency for optoelectronic applications such as semiconductor lasers and light-emitting diodes. Although exciton dynamics have been investigated in different 2D semiconductor, large variations in sample qualities due to different preparation methods have prevented obtaining intrinsic exciton lifetimes from being conclusively established. In Chapter 3, we study exciton dynamics in 2D TMDCs semiconductors using ultrafast PL and transient absorption microscopy. Here we employ 2D WS2 semiconductor as a model system to study exciton dynamics due to the low defect density and high quantum yield of WS2. We mainly focus on how the exciton population affects exciton dynamics. At low exciton density regime, we demonstrate how the interlayer between the bright and dark exciton populations influence exciton recombination. At high exciton density regime, we exhibit significant exciton-exciton annihilation in monolayer WS2. When comparing with the bilayer and trilayer WS2, the exciton-exciton annihilation rate in monolayer WS2 increases by two orders of magnitude due to enhanced many-body interactions at single layer limit. </div><div>Long-range transport of 2D excitons is desirable for optoelectronic applications based on TMDCs semiconductors. However, there still lacks a comprehensive understanding of the intrinsic limit for exciton transport in the TMDCs materials currently. In Chapter 4, we employ ultrafast transient absorption microscopy that is capable of imaging excitons transport with ~ 200 fs temporal resolution and ~ 50 nm spatial precision to track exciton motion in 2D WS2 with different thickness. Our results demonstrate that exciton mobility in single layer WS2 is largely limited by extrinsic factors such as charge impurities and surface phonons of the substrate. The intrinsic phonon-limited exciton transport is achieved in WS2 layers with a thickness greater than 20 layers.</div><div>Efficient photocarrier generation and separation at 2D interfaces remain a central challenge for many optoelectronic applications based on 2D heterostructures. The structural tunability of 2D nanostructures along with atomically thin and sharp 2D interfaces provides new opportunities for controlling charge transfer (CT) interactions at 2D interfaces. A largely unexplored question is how interlayer CT interactions contribute to interfacial photo-carrier generation and separation in 2D heterostructures. In Chapter 5, we present a joint experimental and theoretical study to address carrier generation from interlayer CT transitions in WS2-graphene heterostructures. We use spatially resolved ultrafast transient absorption microscopy to elucidate the role of interlayer coupling on charge transfer and photo-carrier generation in WS2-graphene heterostructures. These results demonstrate efficient broadband photo-carrier generation in WS2-graphene heterostructures which is highly desirable for atomically thin photovoltaic and photodetector applications based on graphene and 2D semiconductors.</div><div>CT exciton transport at heterointerfaces plays a critical role in light to electricity conversion using 2D heterostructures. One of the challenges is that direct measurements of CT exciton transport require quantitative information in both spatial and temporal domains. In order to address this challenge, we employ transient absorption microscopy (TAM) with high temporal and spatial resolution to image both bright and dark CT excitons in WS2-tetrance and CVD WS2-WSe2 heterostructure. In Chapter 6, we study the formation and transport of interlayer CT excitons in 2D WS2-Tetracene vdW heterostructures. TAM measurements of CT exciton transport at these 2D interfaces reveal coexistence of delocalized and localized CT excitons. The highly mobile delocalized CT excitons could be the key factor to overcome large CT exciton binding energy in achieving efficient charge separation. In Chapter 7, we study stacking orientational dependent interlayer exciton recombination and transport in CVD WS2-WSe2 heterostructures. Temperature-dependent interlayer exciton dynamics measurements suggest the existence of moiré potential that localizes interlayer excitons. TAM measurements of interlayer excitons transport reveal that CT excitons at WS2-WSe2 heterointerface are much more mobile than intralayer excitons of WS2. We attributed this to the dipole-dipole repulsion from bipolar interlayer excitons that efficiently screen the moiré potential fluctuations and facilitate interlayer exciton transport. Our results provide fundamental insights in understanding the influence of moiré potential on interlayer exciton dynamics and transport in CVD WS2-WSe2 heterostructures which has important implications in optoelectronic applications such as atomically thin photovoltaics and light harvesting devices. </div><div><br></div>

Semiconducting two-dimensional (2D) materials have gained considerable attention in recent years owing to their potential in future electronics. On the one hand, the conventional 2D semiconductors, such as transition metal dichalcogenides (TMDCs (MoS2, WS2, etc.) are being exhaustively studied, on the other hand, search for novel 2D materials is at a rapid pace. In this thesis, we explore 2D materials beyond graphene and TMDCs in terms of their intrinsic electronic properties and underlying charge transport mechanisms. We introduce 2D semiconducting materials of indium selenide (InSe) and gallium selenide (GaSe), and a novel π-d conjugated Fe3(THT)2(NH4)3 metal-organic framework (MOF) as potential candidates for their use as active elements in (opto)electronic applications. Owing to the air-sensitivity of InSe and GaSe, their integration into active devices has been severely constrained. Here, we report a hexagonal boron nitride (hBN) based encapsulation, where 2D layers of InSe and GaSe are sandwiched between two layers of hBN; top hBN passivating the 2D layer from the environment and bottom hBN acting as a spacer and suppressing charge transfer to the 2D layer from the SiO2 substrate. To fabricate the devices from fully encapsulated InSe and GaSe layers, we employ the technique of lithography-free via-contacts, which are metal contacts embedded within hBN flakes. Based on our results, we find that full hBN encapsulation preserves InSe in its pristine form and suppresses its degradation with time. Consequently, the electronic properties of encapsulated InSe devices are significantly improved, leading to a mobility of 30–120 cm2 V−1 s−1 as compared to a mere ∼1 cm2 V−1 s−1 obtained for unencapsulated devices. In addition, encapsulated InSe devices are stable for a prolonged period of time, overcoming their limitation to be air-sensitive. On employing full hBN encapsulation to GaSe, a high photoresponsivity of 84.2 A W−1 at 405 nm is obtained. The full hBN encapsulation technique passivates the air-sensitive layers from various degrading factors and preserves their unaltered properties. In the future, this technique can be applied to other 2D materials that have been restricted so far in their fundamental study and applications due to their environmental sensitivity. MOFs are another emerging class of semiconducting 2D materials investigated in this thesis. They are hybrid materials that consist of metal ions connected with organic ligands via coordination bonds. In recent years, advances in synthetic approaches have led to the development of electrically conductive MOFs as a new generation of electronic materials. However, to date, poor mobilities and hopping-type charge transport dominant in these materials have prevented them from being considered for electronic applications. In this work, we investigate a newly developed π-d conjugated Fe3(THT)2(NH4)3 (THT: 2,3,6,7,10,11-hexathioltriphenylene) MOF. The MOF films are characterized with a direct bandgap lying in the infrared (IR) region. By employing Hall-effect measurements, we demonstrate band-like transport and a record-high mobility of 230 cm2 V−1 s−1 in Fe3(THT)2(NH4)3 MOF films. The temperature-dependent conductivity confirms a thermally activated charge carrier population in the samples induced by the small bandgap of the analyzed MOFs. Following these results, we demonstrate the feasibility of using this high-mobility semiconducting MOF as an active material in thin-film optoelectronic devices. The MOF photodetectors fabricated in this work are capable of detecting wavelengths from UV to NIR (400–1575 nm). The narrow IR bandgap of the active layer constrains the performance of the photodetector at room temperature by band-to-band thermal excitation of the charge carriers. At 77 K, the device performance is significantly improved; two orders of magnitude higher voltage responsivity, lower noise equivalent power, and higher specific detectivity of 7 × 10^8 cm Hz1/2 W−1 are achieved at 785 nm excitation, which is a direct consequence of suppressing the thermal generation of charge carriers across the bandgap. These figures of merit are retained over the analyzed spectral region (400–1575 nm) and are comparable to those obtained with the first demonstrations of graphene and black phosphorus based photodetectors, thus, revealing a promising application of MOFs in optoelectronics. / Zweidimensionale (2D) Halbleiter haben dank ihres Potenzials für elektronische Anwendungen in den letzten Jahren große Aufmerksamkeit erregt. Dabei werden einerseits konventionelle 2D-Materialien, wie die Übergangsmetall-Chalkogenide (TMDCs) (MoS2, WS2, usw.) intensiv erforscht. Andererseits schreitet auch die Suche nach neuen 2D-Materialien rasch voran. Diese Dissertation stellt Forschungsergebnisse zu elektrischen Eigenschaften und den zugrundeliegenden Ladungstransportmechanismen von 2D-Materialien jenseits von Graphen und TMDCs vor. Untersucht wurden die 2D-Halbleiter Indiumselenid (InSe) und Galliumselenid (GaSe), sowie eine neuartige π-d konjugierte Metallorganische Gerüstverbindung (Metal-Organic Framework, MOF) Fe3(THT)2(NH4)3. Diese Materialien sind vielversprechende Kandidaten für elektronische und optoelektronische Anwendungen. InSe und GaSe sind besonders luftempfindliche Materialien. Aus diesem Grund ist ihre Verwendung für aktive Bauteile trotz ihrer hervorragenden elektrischen Eigenschaften bis heute sehr begrenzt. In dieser Arbeit wird ein effektives Verkapselungsverfahren vorstellt, bei dem InSe- oder GaSe-2D-Schichten zwischen zwei Schichten aus hexagonalem Bornitrid (hBN) eingebettet werden. Die untere Schicht hBN isoliert das Material vom Substrat Siliziumdioxid (SiO2), während die obere Schicht das 2D-Material luftdicht isoliert. Um Bauteile aus komplett eingekapseltem InSe oder GaSe herzustellen, wurden lithographiefreie, sogenannte via-Kontakte hergestellt. Dies sind Metallkontakte, die bereits vor der Verkapselung in die hBN-Schichten integriert werden. Die hBN-Verkapselung erhält InSe in seiner ursprünglichen Form. Die hier vorgestellten Ergebnisse zeigen, dass sich die elektronischen Eigenschaften von InSe durch Verkapselung signifikant verbessern, was zu elektrischen Mobilitäten von 30–120 cm2 V−1 s−1 gegenüber nur rund ∼1 cm2 V−1 s−1 in unverkapselten Bauteilen führt. Darüber hinaus bleiben die Eigenschaften der verkapselten InSe-Bauteile über einen langen Zeitraum erhalten und degradieren nicht mehr bei Kontakt mit Luft. Die Verkapselung von GaSe ermöglicht den Einsatz in Fotodetektoren, bei einer Wellenlänge von 405 nm wird eine Fotoempfindlichkeit von 84.2 A W−1 gemessen; auch hier bewahrt die Verkapselung die empfindlichen Schichten vor schädlichen Einflüssen und konserviert so ihre unveränderten Eigenschaften. In der Zukunft kann diese Technik auch für andere 2D-Materialien eingesetzt werden, insbesondere für solche, deren Erforschung und Anwendung durch die große Empfindlichkeit bis heute eingeschränkt ist. Darüber hinaus untersucht diese Dissertation mit Metallorganischen Gerüstverbindungen (MOFs) eine zweite Klasse halbleitender 2D-Materialien. MOFs sind hybride Materialien aus Metallionen, die mit organischen Molekülen als Verbindungselementen eine meist kristalline Struktur bilden. In den letzten Jahren haben Fortschritte in der synthetischen Herstellung zur Entwicklung von elektronisch leitfähigen MOFs geführt. Die niedrige Mobilität und der sogenannte hopping-Ladungstransport der gängigsten MOFs haben jedoch verhindert, dass diese für Anwendungen betrachtet wurden. In dieser Arbeit wird eine kürzlich neu entwickelte, π-d-konjugierte Fe3(THT)2(NH4)3 (THT: 2,3,6,7,10,11-hexathioltriphenylene) MOF vorgestellt. Der MOF Film hat eine direkte Bandlücke im Infrarot(IR)-Bereich liegend. Mithilfe von Hall-Effekt-Messungen wurde gezeigt, dass der Transport in den Fe3(THT)2(NH4)3 MOF Filmen mit dem Drude-Modell konsistent ist. Darüber hinaus wird eine bis jetzt nicht übertroffene Mobilität von 230 cm2 V−1 s−1 gemessen. Die Temperaturabhängigkeit der Leitfähigkeit bestätigt, dass die kleine Bandlücke zu thermisch aktivierten Ladungstragerdichten in den Proben führt. Auf Grundlage dieser Ergebnisse wird die Machbarkeit von hochmobilen halbleitenden Fe3(THT)2(NH4)3 MOFs als aktives Material in dünnen optoelektronischen Bauteilen gezeigt. Die hier vorgestellten MOF Fotodetektoren reagieren auf Wellenlängen im UV bis Nahinfrarotspektrum (400–1575 nm). Die schmale Bandlücke schränkt die Leistung des Fotodetektors bei Raumtemperatur durch thermische Band-zu-Band-Anregung der Ladungsträger ein. Bei einer Temperatur von 77 K verbessert sich die Leistung des Detektors signifikant: Bei 785 nm wird eine um zwei Größenordnungen erhöhte Spannungsempfindlichkeit, eine niedrigere äquivalente Rauschleistung sowie eine höhere spezifische Empfindlichkeit von 7 × 10^8 cm Hz1/2 W−1 erhalten. Dies ist eine direkte Konsequenz der Unterdrückung thermischer Anregung von Ladungsträgern über die Bandlücke. Diese Leistungszahlen sind über das analysierte Spektrum (400–1575 nm) gültig und vergleichbar mit den ersten Fotodetektoren auf Grundlage von Graphen und Schwarzem Phosphor. Die Ergebnisse zeigen deutlich das Potenzial von MOFs für optoelektronische Anwendungen.

<p dir="ltr">In the realm of two-dimensional (2D) materials, monolayer (ML) transition metal dichalcogenides (TMDCs) have gained significant interest due to their direct bandgap transition, high carrier mobility, strong light-matter interaction, and robust spin and valley degrees of freedom, starkly contrasting their bulk counterparts. Owing to their large surface-to-volume ratio, the integration of ML TMDCs with other various 2D semiconductors and microcavities offers opportunities to study fundamental photo-physics processes at the heterointerfaces, paving the way for implementation of next-generation devices.</p><p dir="ltr">Chapter 1 provides a concise introduction to 2D materials, particularly TMDCs, and their fascinating optical and electronic properties. It examines the role of excitons in 2D materials, and the impact of energy transfer (ET) and charge transfer (CT) on exciton’s properties in TMDC through the construction of 2D van der Waals (vdW) heterostructures and coupling with optical microcavities. This chapter also delves into the potential enhancement of TMDCs’ optical properties by integrating 2D hybrid lead halide perovskites and ultra-thin three-dimensional (3D) halide perovskites with TMDCs. Furthermore, it sets the general context for light-matter interaction, another form of ET, considering both weak and strong coupling regimes.</p><p dir="ltr">Chapter 2 outlines the optical techniques employed to gather data for this work. A focus is placed on ultrafast optical techniques like transient absorption spectroscopy, which allow for direct probing and analysis of ET and CT dynamics at the heterointerface.</p><p dir="ltr">Photoinduced interfacial CT plays a critical role in the field of energy conversion involving vdW heterostructures constructed by inorganic nanostructures and organic materials. However, the control of atomic-scale stacking configurations to modulate charge separation at interfaces remains challenging. Chapter 3 aims to illustrate tunability of interfacial charge separation in a Type-II heterojunction between ML-WS₂ and an organic semiconducting molecule by rational design of relative stacking configurations using 2D perovskites as scaffoldings. This chapter investigates how different molecular stacking, face-to-face versus face-to-edge, affects CT at the heterointerface. Our findings reveal that the CT process heavily depends on the relative stacking configurations at the organic-TMDCs heterointerface, with charge separation being notably slowed down for face-to-edge configuration compared to face-to-face configuration. These investigations open new opportunities for designing efficient charge separation processes in energy conversion applications by judiciously engineering interfaces between organic and inorganic semiconductors, using 2D perovskites as scaffolds.</p><p dir="ltr">Though TMDCs’ large surface-to-volume ratios make them excellent platforms for studying interfacial properties, the presence of bulky ligands on the surface of 2D perovskite poses a challenge, impeding direct interfacial coupling in their heterostructures. Chapter 4 details the fabrication of ML-WS₂ and ultra-thin CH₃NH₃PbX₃ (MAPbX₃, X=Br, I) heterostructures with tunable energy levels, to study the dynamics of CT and ET at these hybrid interfaces. Notably, heterojunctions of WS₂ with pure MAPbBr₃ and MAPbI₃ were elucidated as Type-I and Type-II respectively, using photoluminescence (PL) and time-resolved photoluminescence (TR-PL) measurements. Transit absorption (TA) spectroscopy investigations unambiguously revealed a rapid ET facilitated by CT in the WS₂/MAPbBr₃ heterostructure, with a time constant of ~20 ps, and a predominantly CT in the WS₂/MAPbI₃ heterostructure with a time constant of ~50 femtosecond (fs). The successful interfacing of low-dimensional perovskites with an extensive array of traditional 2D materials such as TMDCs opens up possibilities for novel optoelectronic properties and applications within the field of 2D material systems. Furthermore, the ultrafast and efficient ET and CT processes hold promise for the creation of advanced energy conversion devices.</p><p dir="ltr">In the last chapter, we successfully fabricated a ML-WS₂ in conjunction with a silver (Ag) nanoparticle (NP) array. Our findings affirmed a weak light-matter coupling between ML-WS₂ and the Ag NP array, as evidenced by angle-resolved photoluminescence spectroscopy. Furthermore, an enhancement in the bright exciton emission from ML-WS₂ was observed at reduced temperatures. The analysis of PL enhancement factor at varying temperatures suggested that an upper bound of the enhancement factor for the bright exciton could reach ~51 or even higher at 7 K, given the imperfect uniformity of the electric filed generated around the NPs. This discovery carries significant implications for the manipulation of excitons in TMDCs and expands their potential applications in the field of optoelectronics.</p>