Room-temperature photoluminescence mediated by sulfur vacancies in 2D molybdenum disulfide

Atomic defects in monolayer transition metal dichalcogenides (TMDs) such as chalcogen vacancies significantly affect their properties. In this work, we provide a reproducible and facile strategy to rationally induce chalcogen vacancies in monolayer MoS2 by annealing at 600 °C in an argon/hydrogen (95%/5%) atmosphere. Synchrotron X-ray photoelectron spectroscopy shows that a Mo 3d5/2 core peak at 230.1 eV emerges in the annealed MoS2 associated with nonstoichiometric MoSx (0 < x < 2), and Raman spectroscopy shows an enhancement of the ∼380 cm–1 peak that is attributed to sulfur vacancies. At sulfur vacancy densities of ∼1.8 × 1014 cm–2, we observe a defect peak at ∼1.72 eV (referred to as LXD) at room temperature in the photoluminescence (PL) spectrum. The LXD peak is attributed to excitons trapped at defect-induced in-gap states and is typically observed only at low temperatures (≤77 K). Time-resolved PL measurements reveal that the lifetime of defect-mediated LXD emission is longer than that of band edge excitons, both at room and low temperatures (∼2.44 ns at 8 K). The LXD peak can be suppressed by annealing the defective MoS2 in sulfur vapor, which indicates that it is possible to passivate the vacancies. Our results provide insights into how excitonic and defect-mediated PL emissions in MoS2 are influenced by sulfur vacancies at room and low temperatures

Production of Magnetic Arsenic–Phosphorus Alloy Nanoribbons with Small Band Gaps and High Hole Conductivities

Quasi-1D nanoribbons provide a unique route to diversifying the properties of their parent 2D nanomaterial, introducing lateral quantum confinement and an abundance of edge sites. Here, a new family of nanomaterials is opened with the creation of arsenic–phosphorus alloy nanoribbons (AsPNRs). By ionically etching the layered crystal black arsenic–phosphorus using lithium electride followed by dissolution in amidic solvents, solutions of AsPNRs are formed. The ribbons are typically few-layered, several micrometers long with widths tens of nanometers across, and both highly flexible and crystalline. The AsPNRs are highly electrically conducting above 130 K due to their small band gap (ca. 0.035 eV), paramagnetic in nature, and have high hole mobilities, as measured with the first generation of AsP devices, directly highlighting their properties and utility in electronic devices such as near-infrared detectors, quantum computing, and charge carrier layers in solar cells

Air-stable bismuth sulfobromide (BiSBr) visible-light absorbers : optoelectronic properties and potential for energy harvesting

ns2 compounds have recently attracted considerable interest due to their potential to replicate the defect tolerance of lead-halide perovskites and overcome their toxicity and stability limitations. However, only a handful of compounds beyond the perovskite family have been explored thus far. Herein, we investigate bismuth sulfobromide (BiSBr), which is a quasi-one-dimensional semiconductor, but very little is known about its optoelectronic properties or how it can be processed as thin films. We develop a solution processing route to achieve phase-pure, stoichiometric BiSBr films (ca. 240 nm thick), which we show to be stable in ambient air for over two weeks without encapsulation. The bandgap (1.91 ± 0.06 eV) is ideal for harvesting visible light from common indoor light sources, and we calculate the optical limit in efficiency (i.e., spectroscopic limited maximum efficiency, SLME) to be 43.6% under 1000 lux white light emitting diode illumination. The photoluminescence lifetime is also found to exceed the 1 ns threshold for photovoltaic absorber materials worth further development. Through X-ray photoemission spectroscopy and Kelvin probe measurements, we find the BiSBr films grown to be n-type, with an electron affinity of 4.1 ± 0.1 eV and ionization potential of 6.0 ± 0.1 eV, which are compatible with a wide range of established charge transport layer materials. This work shows BiSBr to hold promise for indoor photovoltaics, as well as other visible-light harvesting applications, such as photoelectrochemical cells, or top-cells for tandem photovoltaics

Phase Engineering and Optical Property Tuning of Transition Metal Dichalcogenides

This thesis investigates the cation-assisted crystallographic phase transitions and optical property modifications of two-dimensional transition metal dichalcogenides (2D TMDs) using optical techniques. The thesis begins with an introduction to the context of the research (Chapter 1), followed by an overview to the key materials and theoretical concepts in Chapter 2. In Chapter 3 we introduce in the key experimental methods used in the work. Chapter 4 examines the mechanism of semiconducting hexagonal (1H, 2H) to metallic tetragonal (1T, distorted 1T) phase transition reactions in 2D TMDs during chemical intercalation of lithium cations, employing real-time optical visualization. We directly quantify the dynamics of the phase transition in micrometer-sized TMD flakes with diffraction limited resolution. In addition, we complement the results with *ex-situ* Raman and photoluminescence measurement. We show this reaction to be a charge-limited surface driven intercalation reaction. After establishing our ability to probe reaction dynamics *in-situ* via optical microscopy, in Chapter 5, we explore the effect of optical excitation on this phase transition reaction. We demonstrate that illuminating the material with photons having energy above the band gap accelerates the transition of 2H to 1T phases by more than two orders of magnitudes. This finding also enables a novel and rapid spatial photo-redox phase patterning within mono- and few layered 2D-TMDs. We then demonstrate the improved performance of a phase-engineered photodetector based on mono-layer 1H-MoS2 using this method. We compare chemical lithiation to electrochemical lithiation to develop a detailed mechanistic picture of this process. Based on our findings, we propose a universal route for chemical cation intercalation and phase engineering of TMDs based on redox-potential matching. This is supported by our demonstration of the same phase transition using a newly synthesized solvent of polycyclic aromatic hydrocarbon with lithium and sodium, which significantly shortens the reaction time from several days to just a few minutes, and replaces the highly pyrophoric chemical n-butyllithium, which has been used in this process for the past five decades. This advanced phase engineering method can be applied to a various type of TMDs, such as powder, crystal, and thin flakes, and offers a promising pathway for scalable production of phase-engineered TMDs. Finally, in Chapter 6 we conduct chemical treatments using the cation-(bis(trifluoromethane) sulfonimide) system on MoS2 grown by metal-organic chemical vapour deposition (MOCVD). By altering the surrounding chemical nature of the cation, we were able to maintain the phase of MoS2 in its semiconducting hexagonal, and only enhance the radiative recombination intensity. This effect is particularly enhanced by adding additional prior treatment using sulfides, which passivate sulfur defects. In conclusion, through the utilization of various optical characterization methods, we have explored the versatile role of cations in phase engineering and luminescence properties for TMDs

Direct Imaging of Carrier Funneling in a Dielectric Engineered 2D Semiconductor

In atomically thin transition-metal dichalcogenides (TMDCs), the environmental sensitivity of the strong Coulomb interaction offers promising approaches to create spatially varying potential landscapes in the same continuous material by tuning its dielectric environment. Thus, allowing for control of transport. However, a scalable and CMOS-compatible method for achieving this is required to harness these effects in practical applications. In addition, because of their ultrashort lifetime, observing the spatiotemporal dynamics of carriers in monolayer TMDCs, on the relevant time scale, is challenging. Here, we pattern and deposit a thin film of hafnium oxide (HfO2) via atomic layer deposition (ALD) on top of a monolayer of WSe2. This allows for the engineering of the dielectric environment of the monolayer and design of heterostructures with nanoscale spatial resolution via a highly scalable postsynthesis methodology. We then directly image the transport of photoexcitations in the monolayer with 50 fs time resolution and few-nanometer spatial precision, using a pump probe microscopy technique. We observe the unidirectional funneling of charge carriers, from the unpatterned to the patterned areas, over more than 50 nm in the first 20 ps with velocities of over 2 × 103 m/s at room temperature. These results demonstrate the possibilities offered by dielectric engineering via ALD patterning, allowing for arbitrary spatial patterns that define the potential landscape and allow for control of the transport of excitations in atomically thin materials. This work also shows the power of the transient absorption methodology to image the motion of photoexcited states in complex potential landscapes on ultrafast time scales.TRU

Direct Imaging of Carrier Funneling in a Dielectric Engineered 2D Semiconductor.

Publication status: PublishedIn atomically thin transition-metal dichalcogenides (TMDCs), the environmental sensitivity of the strong Coulomb interaction offers promising approaches to create spatially varying potential landscapes in the same continuous material by tuning its dielectric environment. Thus, allowing for control of transport. However, a scalable and CMOS-compatible method for achieving this is required to harness these effects in practical applications. In addition, because of their ultrashort lifetime, observing the spatiotemporal dynamics of carriers in monolayer TMDCs, on the relevant time scale, is challenging. Here, we pattern and deposit a thin film of hafnium oxide (HfO2) via atomic layer deposition (ALD) on top of a monolayer of WSe2. This allows for the engineering of the dielectric environment of the monolayer and design of heterostructures with nanoscale spatial resolution via a highly scalable postsynthesis methodology. We then directly image the transport of photoexcitations in the monolayer with 50 fs time resolution and few-nanometer spatial precision, using a pump probe microscopy technique. We observe the unidirectional funneling of charge carriers, from the unpatterned to the patterned areas, over more than 50 nm in the first 20 ps with velocities of over 2 × 103 m/s at room temperature. These results demonstrate the possibilities offered by dielectric engineering via ALD patterning, allowing for arbitrary spatial patterns that define the potential landscape and allow for control of the transport of excitations in atomically thin materials. This work also shows the power of the transient absorption methodology to image the motion of photoexcited states in complex potential landscapes on ultrafast time scales

Photoredox phase engineering of transition metal dichalcogenides

Acknowledgements: We thank P. Knight and the members of the workshop at the Department of Material Science and Metallurgy at the University of Cambridge for their technical support for experimental design and cell fabrication. This project received funding from the European Research Council under the Horizon 2020 research and innovation programme of the European Union (grant agreement no. 758826 (SOLARX) to A.R.). This work was supported by the Engineering and Physical Sciences Research Council (grant EP/W017091/1). E.S. received funding from the UKRI postdoctoral individual fellowship (grant reference no. EP/Y026659/1). A.R., C.S. and J.L. acknowledge support from the Faraday Institution Degradation Project. J.-I.L., Y.W. and M.C. acknowledge support from the Faraday Institution LiSTAR programme and characterization project (EP/S003053/1, FIRG014 and FIRG012), the NEXGENNA programme (FIRG018), the Royal Society Wolfson Merit Award (WRM\FT\180009), and the European Research Council (ERC) Advanced Grant under the Horizon 2020 research and innovation programme of the European Union (grant agreement no. GA 101019828-2D-LOTTO). This work was funded by the UKRI. For open access, the author has applied a Creative Commons Attribution (CC BY) licence to any Author Accepted Manuscript version arising.Crystallographic phase engineering plays an important part in the precise control of the physical and electronic properties of materials. In two-dimensional transition metal dichalcogenides (2D TMDs), phase engineering using chemical lithiation with the organometallization agent n-butyllithium (n-BuLi), to convert the semiconducting 2H (trigonal) to the metallic 1T (octahedral) phase, has been widely explored for applications in areas such as transistors, catalysis and batteries1–15. Although this chemical phase engineering can be performed at ambient temperatures and pressures, the underlying mechanisms are poorly understood, and the use of n-BuLi raises notable safety concerns. Here we optically visualize the archetypical phase transition from the 2H to the 1T phase in mono- and bilayer 2D TMDs and discover that this reaction can be accelerated by up to six orders of magnitude using low-power illumination at 455 nm. We identify that the above-gap illumination improves the rate-limiting charge-transfer kinetics through a photoredox process. We use this method to achieve rapid and high-quality phase engineering of TMDs and demonstrate that this methodology can be harnessed to inscribe arbitrary phase patterns with diffraction-limited edge resolution into few-layer TMDs. Finally, we replace pyrophoric n-BuLi with safer polycyclic aromatic organolithiation agents and show that their performance exceeds that of n-BuLi as a phase transition agent. Our work opens opportunities for exploring the in situ characterization of electrochemical processes and paves the way for sustainably scaling up materials and devices by photoredox phase engineering

Photo-Redox Phase Engineering of Transition Metal Dichalcogenides

Crystallographic phase engineering plays a pivotal role in the precise control of physical and electronic properties of materials. In two-dimensional transition metal dichalcogenides (2D TMDs), phase engineering using chemical lithiation with the organometalization agent n-butyllithium (n-BuLi), to convert the semiconducting 2H (trigonal) to the metallic 1T (octahedral) phase, has been widely explored for applications in areas such as transistors, catalysis and batteries1,2,3,4,5,6,7,8,9,10,11,12,13,14,15. While this chemical phase engineering can be performed at ambient temperatures and pressures, the underlying mechanisms are poorly understood and the use of n-BuLi raises significant safety concerns. Here, we optically visualize the archetypical phase transition from the 2H to the 1T phase in mono- and bi-layer 2D TMDs, and discover that this reaction can be accelerated by up to six orders of magnitude using low-power illumination at 455 nm. We identify that above-gap illumination improves the rate-limiting charge-transfer kinetics through a photo-redox process. We employ this method to achieve rapid and high-quality phase engineering of TMDs and demonstrate that this methodology can be harnessed to inscribe arbitrary phase patterns with diffraction limited edge-resolution into few-layer TMDs. Finally, we replace pyrophoric n-BuLi with safer polycyclic-aromatic organolithiation agents, and show that their performance exceeds that of n-BuLi as a phase transition agent. Our work opens new avenues for the in- situ characterization of electro-chemical processes and paves the way for sustainably scaling up materials and devices via photo-redox phase engineering.ER

Photoredox phase engineering of transition metal dichalcogenides

Acknowledgements: We thank P. Knight and the members of the workshop at the Department of Material Science and Metallurgy at the University of Cambridge for their technical support for experimental design and cell fabrication. This project received funding from the European Research Council under the Horizon 2020 research and innovation programme of the European Union (grant agreement no. 758826 (SOLARX) to A.R.). This work was supported by the Engineering and Physical Sciences Research Council (grant EP/W017091/1). E.S. received funding from the UKRI postdoctoral individual fellowship (grant reference no. EP/Y026659/1). A.R., C.S. and J.L. acknowledge support from the Faraday Institution Degradation Project. J.-I.L., Y.W. and M.C. acknowledge support from the Faraday Institution LiSTAR programme and characterization project (EP/S003053/1, FIRG014 and FIRG012), the NEXGENNA programme (FIRG018), the Royal Society Wolfson Merit Award (WRM\FT\180009), and the European Research Council (ERC) Advanced Grant under the Horizon 2020 research and innovation programme of the European Union (grant agreement no. GA 101019828-2D-LOTTO). This work was funded by the UKRI. For open access, the author has applied a Creative Commons Attribution (CC BY) licence to any Author Accepted Manuscript version arising.Crystallographic phase engineering plays an important part in the precise control of the physical and electronic properties of materials. In two-dimensional transition metal dichalcogenides (2D TMDs), phase engineering using chemical lithiation with the organometallization agent n-butyllithium (n-BuLi), to convert the semiconducting 2H (trigonal) to the metallic 1T (octahedral) phase, has been widely explored for applications in areas such as transistors, catalysis and batteries1–15. Although this chemical phase engineering can be performed at ambient temperatures and pressures, the underlying mechanisms are poorly understood, and the use of n-BuLi raises notable safety concerns. Here we optically visualize the archetypical phase transition from the 2H to the 1T phase in mono- and bilayer 2D TMDs and discover that this reaction can be accelerated by up to six orders of magnitude using low-power illumination at 455 nm. We identify that the above-gap illumination improves the rate-limiting charge-transfer kinetics through a photoredox process. We use this method to achieve rapid and high-quality phase engineering of TMDs and demonstrate that this methodology can be harnessed to inscribe arbitrary phase patterns with diffraction-limited edge resolution into few-layer TMDs. Finally, we replace pyrophoric n-BuLi with safer polycyclic aromatic organolithiation agents and show that their performance exceeds that of n-BuLi as a phase transition agent. Our work opens opportunities for exploring the in situ characterization of electrochemical processes and paves the way for sustainably scaling up materials and devices by photoredox phase engineering