Andrew Winchester

Electrochemical characterization of liquid phase exfoliated two-dimensional layers of molybdenum disulfide.

We report on the electrochemical charge storage behavior of few-layered flakes of molybdenum disulfide (MoS2) obtained by liquid phase exfoliation of bulk MoS2 powder in 1-dodecyl-2-pyrrolidinone. The specific capacitances of the exfoliated flakes obtained using a 6 M KOH aqueous solution as an electrolyte were found to be an order of magnitude higher than those of bulk MoS2 (∼0.5 and ∼2 mF cm(-2) for bulk and exfoliated MoS2 electrodes, respectively). The exfoliated MoS2 flakes also showed significant charge storage in different electrolytes, such as organic solvents [1 M tetraethylammonium tetrafluoroborate in propylene carbonate (Et4NBF4 in PC)] and ionic liquids [1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6)]. The values of specific capacitances obtained using Et4NBF4 in PC and BMIM-PF6 were ∼2.25 and ∼2.4 mF cm(-2), respectively. An analysis of electrochemical impedance spectroscopy using an equivalent circuit modeling was performed to understand the charge storage mechanism of these exfoliated MoS2 flakes using different electrolytes. Our findings indicate that liquid phase exfoliation methods can be used to produce large quantities of electrochemically active, two-dimensional layers of MoS2 and can act as an ideal material in several applications related to electrochemistry.

Pinhole-free hole transport layers significantly improve the stability of MAPbI3-based perovskite solar cells under operating conditions

Pinhole-free 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-MeOTAD) hole transport layers (HTLs) were deposited on perovskite films. MAPbI3-based perovskite solar cells employing the pinhole-free HTL showed a prolonged lifetime under one sun and was operated at the maximum power point. The solar cell architecture (planar versus mesoporous-layers) was also observed to strongly influence the cells stability.

Chemical Vapor Deposition Synthesized Atomically Thin Molybdenum Disulfide with Optoelectronic-Grade Crystalline Quality

The ability to synthesize high-quality samples over large areas and at low cost is one of the biggest challenges during the developmental stage of any novel material. While chemical vapor deposition (CVD) methods provide a promising low-cost route for CMOS compatible, large-scale growth of materials, it often falls short of the high-quality demands in nanoelectronics and optoelectronics. We present large-scale CVD synthesis of single- and few-layered MoS2 using direct vapor-phase sulfurization of MoO2, which enables us to obtain extremely high-quality single-crystal monolayer MoS2 samples with field-effect mobility exceeding 30 cm(2)/(V s) in monolayers. These samples can be readily synthesized on a variety of substrates, and demonstrate a high-degree of optoelectronic uniformity in Raman and photoluminescence mapping over entire crystals with areas exceeding hundreds of square micrometers. Because of their high crystalline quality, Raman spectroscopy on these samples reveal a range of multiphonon processes through peaks with equal or better clarity compared to past reports on mechanically exfoliated samples. This enables us to investigate the layer thickness and substrate dependence of the extremely weak phonon processes at 285 and 487 cm(-1) in 2D-MoS2. The ultrahigh, optoelectronic-grade crystalline quality of these samples could be further established through photocurrent spectroscopy, which clearly reveal excitonic states at room temperature, a feat that has been previously demonstrated only on samples which were fabricated by micro-mechanical exfoliation and then artificially suspended across trenches. Our method reflects a big step in the development of atomically thin, 2D-MoS2 for scalable, high-quality optoelectronics.

Imaging the motion of electrons across semiconductor heterojunctions

Technological progress since the late twentieth century has centred on semiconductor devices, such as transistors, diodes and solar cells. At the heart of these devices is the internal motion of electrons through semiconductor materials due to applied electric fields or by the excitation of photocarriers. Imaging the motion of these electrons would provide unprecedented insight into this important phenomenon, but requires high spatial and temporal resolution. Current studies of electron dynamics in semiconductors are generally limited by the spatial resolution of optical probes, or by the temporal resolution of electronic probes. Here, by combining femtosecond pump-probe techniques with spectroscopic photoemission electron microscopy, we imaged the motion of photoexcited electrons from high-energy to low-energy states in a type-II 2D InSe/GaAs heterostructure. At the instant of photoexcitation, energy-resolved photoelectron images revealed a highly non-equilibrium distribution of photocarriers in space and energy. Thereafter, in response to the out-of-equilibrium photocarriers, we observed the spatial redistribution of charges, thus forming internal electric fields, bending the semiconductor bands, and finally impeding further charge transfer. By assembling images taken at different time-delays, we produced a movie lasting a few trillionths of a second of the electron-transfer process in the photoexcited type-II heterostructure-a fundamental phenomenon in semiconductor devices such as solar cells. Quantitative analysis and theoretical modelling of spatial variations in the movie provide insight into future solar cells, 2D materials and other semiconductor devices.

Protecting the properties of monolayer MoS2 on silicon based substrates with an atomically thin buffer

Semiconducting 2D materials, like transition metal dichalcogenides (TMDs), have gained much attention for their potential in opto-electronic devices, valleytronic schemes, and semi-conducting to metallic phase engineering. However, like graphene and other atomically thin materials, they lose key properties when placed on a substrate like silicon, including quenching of photoluminescence, distorted crystalline structure, and rough surface morphology. The ability to protect these properties of monolayer TMDs, such as molybdenum disulfide (MoS2), on standard Si-based substrates, will enable their use in opto-electronic devices and scientific investigations. Here we show that an atomically thin buffer layer of hexagonal-boron nitride (hBN) protects the range of key opto-electronic, structural, and morphological properties of monolayer MoS2 on Si-based substrates. The hBN buffer restores sharp diffraction patterns, improves monolayer flatness by nearly two-orders of magnitude, and causes over an order of magnitude enhancement in photoluminescence, compared to bare Si and SiO2 substrates. Our demonstration provides a way of integrating MoS2 and other 2D monolayers onto standard Si-substrates, thus furthering their technological applications and scientific investigations.

Ultrafast Intrinsic Photoresponse and Direct Evidence of Sub-gap States in Liquid Phase Exfoliated MoS2Thin Films

2-Dimensional structures with swift optical response have several technological advantages, for example they could be used as components of ultrafast light modulators, photo-detectors, and optical switches. Here we report on the fast photo switching behavior of thin films of liquid phase exfoliated MoS2, when excited with a continuous laser of λ = 658 nm (E = 1.88 eV), over a broad range of laser power. Transient photo-conductivity measurements, using an optical pump and THz probe (OPTP), reveal that photo carrier decay follows a bi-exponential time dependence, with decay times of the order of picoseconds, indicating that the photo carrier recombination occurs via trap states. The nature of variation of photocurrent with temperature confirms that the trap states are continuously distributed within the mobility gap in these thin film of MoS2, and play a vital role in influencing the overall photo response. Our findings provide a fundamental understanding of the photo-physics associated with optically active 2D materials and are crucial for developing advanced optoelectronic devices.

Adsorption energy of oxygen molecules on graphene and two-dimensional tungsten disulfide

Adsorption of gas molecules on the surface of atomically layered two-dimensional (2D) materials, including graphene and transition metal dichalcogenides, can significantly affect their electrical and optical properties. Therefore, a microscopic and quantitative understanding of the mechanism and dynamics of molecular adsorption and desorption has to be achieved in order to advance device applications based on these materials. However, recent theoretical calculations have yielded contradictory results, particularly on the magnitude of the adsorption energy. Here, we have experimentally determined the adsorption energy of oxygen molecules on graphene and 2D tungsten disulfide using temperature-programmed terahertz (THz) emission microscopy (TPTEM). The temperature dependence of THz emission from InP surfaces covered with 2D materials reflects the change in oxygen concentration due to thermal desorption, which we used to estimate the adsorption energy of oxygen molecules on graphene (~0.15 eV) and tungsten disulphide (~0.24 eV). Furthermore, we used TPTEM to visualize relative changes in the spatial distribution of oxygen molecules on monolayer graphene during adsorption and desorption. Our results provide much insight into the mechanism of molecular adsorption on the surface of 2D materials, while introducing TPTEM as a novel and powerful tool for molecular surface science.

Pulling apart photoexcited electrons by photoinducing an in-plane surface electric field

We make a movie, on the nanometer and femtosecond scale, as we pull apart electrons with a photoinduced surface field. The study and control of spatiotemporal dynamics of photocarriers at the interfaces of materials have led to transformative modern technologies, such as light-harvesting devices and photodetectors. At the heart of these technologies is the ability to separate oppositely charged electrons and holes. Going further, the ability to separate like charges and manipulate their distribution could provide a powerful new paradigm in opto-electronic control, more so when done on ultrafast time scales. However, this requires one to selectively address subpopulations of the photoexcited electrons within the distribution—a challenging task, particularly on ultrafast time scales. By exploiting the spatial intensity variations in an ultrafast light pulse, we generate local surface fields within the optical spot of a doped semiconductor and thereby pull apart the electrons into two separate distributions. Using time-resolved photoemission microscopy, we directly record a movie of this redistribution process lasting a few hundred picoseconds, which we control via the spatial profile and intensity of the photoexciting pulse. Our quantitative model explains the underlying charge transport phenomena, thus providing a roadmap to the more generalized ability to manipulate photocarrier distributions with high spatiotemporal resolution.

Visualization of Electron Transport in 2D Semiconductor Heterojunctions

Transfer of electrons through semiconductor heterojunctions is the key process in modern electronics. We have visualized dynamics of electrons in InSe/GaAs through time and spectrally-resolved photoemission electron microscopy, which provides insight to this fundamental operating phenomenon.