Hundredfold Enhancement of Light Emission via Defect Control in Monolayer Transition-Metal Dichalcogenides

Two dimensional (2D) transition-metal dichalcogenide (TMD) based semiconductors have generated intense recent interest due to their novel optical and electronic properties, and potential for applications. In this work, we characterize the atomic and electronic nature of intrinsic point defects found in single crystals of these materials synthesized by two different methods - chemical vapor transport and self-flux growth. Using a combination of scanning tunneling microscopy (STM) and scanning transmission electron microscopy (STEM), we show that the two major intrinsic defects in these materials are metal vacancies and chalcogen antisites. We show that by control of the synthetic conditions, we can reduce the defect concentration from above $10^{13} /cm^2$ to below $10^{11} /cm^2$. Because these point defects act as centers for non-radiative recombination of excitons, this improvement in material quality leads to a hundred-fold increase in the radiative recombination efficiency

Luminescent Harnessing of 2D Transition Metal Dichalcogenide Excitons

Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) are regarded as viable candidates for future high-performance optoelectronic and electronic devices due to their chemical stability, low dimensionality, direct bandgap and favourable electronic mobilities. Their direct bandgap facilitates strong light coupling, yielding photoluminescence (PL). Their quantum confined nature produces tightly bound excitons that exhibit intriguing many-body phenomena. 2D excitons may be transferred to other emissive materials in a heterostructure system. This has applications in e.g., photon harvesting with luminescent solar concentrators (LSCs). Newly prepared monolayers are however susceptible to chalcogen atom vacancies, which quench bright excitons and trap mobile charges, amounting to material with poor PL yields and low mobilities, which is of little practical use. Post-fabrication defect passivation schemes offer a means to recover and enhance optical and electronic properties of newly fabricated monolayers. This thesis presents a novel surface treatment based on oleic acid (OA) ligands, which unlike previously reported schemes, is applicable to both sulphide and selenide TMDs. As separate studies, we investigate the effects of OA on monolayer tungsten disulphide (WS2) and molybdenum diselenide (MoSe2). Steady state and time resolved PL (TRPL) microscopy uncover the photophysics of PL enhancement by OA treatment, and provides insights into the surface passivation mechanism. Electronic measurements of 2D TMD field effect transistors support the conclusions drawn from optical measurements. The following study reports exciton transfer from a 2D TMD absorber to a quantum dot (QD) emitter in a 2D-QD heterostructure. WS2 is harnessed as an optical antenna, from which excitons are funnelled to near infrared (NIR) lead sulphide-cadmium sulphide QDs. This describes the opposite process to what has been reported for similar hybrid systems, where 2D TMDs quench excitons. Steady state PL techniques confirm excitation energy transfer (ET), and the ET mechanism. TRPL studies reveal ET dynamics and confirm ET efficiency. Combining steady state PL and TRPL elucidates the ET pathway and competing loss channels. Finally, the concept of an LSC based on 2D-QD heterostructure luminophores is developed with the aid of Monte Carlo light transport simulations. Using an idealised luminophore model, Heterostructure LSC performance is compared to other LSCs based on typical luminophore materials namely, Lumogen Red 305 dye and NIR QDs.ERC (758826 & 756962); EPSRC (EP/P027741/1, EP/M006360/1, EP/R023980/1, EP/L015978/1, EP/L016087/1, EP/P027741/1, & EP/P005152/1); Winton program for physics of sustainabilit

Carbon incorporation in MOCVD of MoS2 thin films grown from an organosulfide precursor

Altres ajuts: CERCA Programme/Generalitat de CatalunyaWith the rise of two-dimensional (2D) transition-metal dichalcogenide (TMD) semiconductors and their prospective use in commercial (opto)electronic applications, it has become key to develop scalable and reliable TMD synthesis methods with well-monitored and controlled levels of impurities. While metal-organic chemical vapor deposition (MOCVD) has emerged as the method of choice for large-scale TMD fabrication, carbon (C) incorporation arising during MOCVD growth of TMDs has been a persistent concern-especially in instances where organic chalcogen precursors are desired as a less hazardous alternative to more toxic chalcogen hydrides. However, the underlying mechanisms of such unintentional C incorporation and the effects on film growth and properties are still elusive. Here, we report on the role of C-containing side products of organosulfur precursor pyrolysis in MoS2 thin films grown from molybdenum hexacarbonyl Mo(CO)6 and diethyl sulfide (CH3CH2)2S (DES). By combining in situ gas-phase monitoring with ex situ microscopy and spectroscopy analyses, we systematically investigate the effect of temperature and Mo(CO)6/DES/H2 gas mixture ratios on film morphology, chemical composition, and stoichiometry. Aiming at high-quality TMD growth that typically requires elevated growth temperatures and high DES/Mo(CO)6 precursor ratios, we observed that temperatures above DES pyrolysis onset (â 600 °C) and excessive DES flow result in the formation of nanographitic carbon, competing with MoS2 growth. We found that by introducing H2 gas to the process, DES pyrolysis is significantly hindered, which reduces carbon incorporation. The C content in the MoS2 films is shown to quench the MoS2 photoluminescence and influence the trion-To-exciton ratio via charge transfer. This finding is fundamental for understanding process-induced C impurity doping in MOCVD-grown 2D semiconductors and might have important implications for the functionality and performance of (opto)electronic devices

Janus monolayers of transition metal dichalcogenides.

Structural symmetry-breaking plays a crucial role in determining the electronic band structures of two-dimensional materials. Tremendous efforts have been devoted to breaking the in-plane symmetry of graphene with electric fields on AB-stacked bilayers or stacked van der Waals heterostructures. In contrast, transition metal dichalcogenide monolayers are semiconductors with intrinsic in-plane asymmetry, leading to direct electronic bandgaps, distinctive optical properties and great potential in optoelectronics. Apart from their in-plane inversion asymmetry, an additional degree of freedom allowing spin manipulation can be induced by breaking the out-of-plane mirror symmetry with external electric fields or, as theoretically proposed, with an asymmetric out-of-plane structural configuration. Here, we report a synthetic strategy to grow Janus monolayers of transition metal dichalcogenides breaking the out-of-plane structural symmetry. In particular, based on a MoS2 monolayer, we fully replace the top-layer S with Se atoms. We confirm the Janus structure of MoSSe directly by means of scanning transmission electron microscopy and energy-dependent X-ray photoelectron spectroscopy, and prove the existence of vertical dipoles by second harmonic generation and piezoresponse force microscopy measurements

Systems of Transition Metal Dichalcogenides : Controlling Applied Strain and Defect Density With Direct Impact on Material Properties

Transition metal dichalcogenides (TMDs) are crystalline layered materials that have significantly impacted the field of condensed matter physics. These materials were the first exfoliatable semiconductors to be discovered after the advent of graphene. The focus of this dissertation is utilizing multiple imaging and characterization techniques to improve and understand the impact of strain and lattice defects in these materials. These inclusions to the lattice, alter the semiconducting performance in controllable ways. A comprehensive study using scanning tunneling spectroscopy (STM), spectroscopy (STS), scanning transmission electron microscopy (STEM), and photoluminescence (PL) in this work will provide a breadth of ways to pinpoint and cross-examine the impact of these factors on these materials. In the first half of this work we focus on the control of lattice defects through two growth processes: chemical vapor transport (CVT) and self-flux. By fine tuning the growth procedure we are both able to determine the intrinsic defects of the material, their electronics, and consistently diminish their density. The second half uses an in-situ strain device to reversibly control and examine the effects of applied strain on transition metal dichalcogenide layers. Utilizing the scanning tunneling microscope to image the lattice, we characterize the change of lattice parameters and observe the formation of strain solitons within the lattice. Measuring these solitons directly we look at the dynamics of a special class of line defects, folds within the top layer of the material, that occur naturally as strain is relieved within the monolayer. With the available imaging techniques and theoretical models we uncover a host of properties of these materials that are only accessible within the high strain regim