Abstract
Recently, sulfur (S)-vacancies created on the basal plane of 2
H
-molybdenum disulfide (MoS
2
) using argon plasma exposure exhibited higher intrinsic activity for the electrochemical ...hydrogen evolution reaction than the edge sites and metallic 1
T
-phase of MoS
2
catalysts. However, a more industrially viable alternative to the argon plasma desulfurization process is needed. In this work, we introduce a scalable route towards generating S-vacancies on the MoS
2
basal plane using electrochemical desulfurization. Even though sulfur atoms on the basal plane are known to be stable and inert, we find that they can be electrochemically reduced under accessible applied potentials. This can be done on various 2
H
-MoS
2
nanostructures. By changing the applied desulfurization potential, the extent of desulfurization and the resulting activity can be varied. The resulting active sites are stable under extended desulfurization durations and show consistent HER activity.
Band offsets between different monolayer transition metal dichalcogenides are expected to efficiently separate charge carriers or rectify charge flow, offering a mechanism for designing atomically ...thin devices and probing exotic two-dimensional physics. However, developing such large-area heterostructures has been hampered by challenges in synthesis of monolayers and effectively coupling neighboring layers. Here, we demonstrate large-area (>tens of micrometers) heterostructures of CVD-grown WS2 and MoS2 monolayers, where the interlayer interaction is externally tuned from noncoupling to strong coupling. Following this trend, the luminescence spectrum of the heterostructures evolves from an additive line profile where each layer contributes independently to a new profile that is dictated by charge transfer and band normalization between the WS2 and MoS2 layers. These results and findings open up venues to creating new material systems with rich functionalities and novel physical effects.
Layered transition metal dichalcogenides (TMDs) draw much attention as the key semiconducting material for two-dimensional electrical, optoelectronic, and spintronic devices. For most of these ...applications, both n- and p-type materials are needed to form junctions and support bipolar carrier conduction. However, typically only one type of doping is stable for a particular TMD. For example, molybdenum disulfide (MoS2) is natively an n-type presumably due to omnipresent electron-donating sulfur vacancies, and stable/controllable p-type doping has not been achieved. The lack of p-type doping hampers the development of charge-splitting p–n junctions of MoS2, as well as limits carrier conduction to spin-degenerate conduction bands instead of the more interesting, spin-polarized valence bands. Traditionally, extrinsic p-type doping in TMDs has been approached with surface adsorption or intercalation of electron-accepting molecules. However, practically stable doping requires substitution of host atoms with dopants where the doping is secured by covalent bonding. In this work, we demonstrate stable p-type conduction in MoS2 by substitutional niobium (Nb) doping, leading to a degenerate hole density of ∼3 × 1019 cm–3. Structural and X-ray techniques reveal that the Nb atoms are indeed substitutionally incorporated into MoS2 by replacing the Mo cations in the host lattice. van der Waals p–n homojunctions based on vertically stacked MoS2 layers are fabricated, which enable gate-tunable current rectification. A wide range of microelectronic, optoelectronic, and spintronic devices can be envisioned from the demonstrated substitutional bipolar doping of MoS2. From the miscibility of dopants with the host, it is also expected that the synthesis technique demonstrated here can be generally extended to other TMDs for doping against their native unipolar propensity.
Using in situ electrical biasing transmission electron microscopy, structural and chemical modification to n–i–p‐type MAPbI3 solar cells are examined with a TiO2 electron‐transporting layer caused by ...bias in the absence of other stimuli known to affect the physical integrity of MAPbI3 such as moisture, oxygen, light, and thermal stress. Electron energy loss spectroscopy (EELS) measurements reveal that oxygen ions are released from the TiO2 and migrate into the MAPbI3 under a forward bias. The injection of oxygen is accompanied by significant structural transformation; a single‐crystalline MAPbI3 grain becomes amorphous with the appearance of PbI2. Withdrawal of oxygen back to the TiO2, and some restoration of the crystallinity of the MAPbI3, is observed after the storage in dark under no bias. A subsequent application of a reverse bias further removes more oxygen ions from the MAPbI3. Light current–voltage measurements of perovskite solar cells exhibit poorer performance after elongated forward biasing; recovery of the performance, though not complete, is achieved by subsequently applying a negative bias. The results indicate negative impacts on the device performance caused by the oxygen migration to the MAPbI3 under a forward bias. This study identifies a new degradation mechanism intrinsic to n–i–p MAPbI3 devices with TiO2.
Using an in situ biasing TEM experiment, a new intrinsic degradation mechanism of methylammonium lead triiodide (MAPbI3) solar cells with a titanium dioxide (TiO2) electron‐transporting layer is identified: oxygen migration from the TiO2 layer to the MAPbI3 under forward biasing, which leads to severe structural modification of the MAPbI3 and the process is pseudo‐reversible.
The potential impact of encapsulated molecules on the thermal properties of individual carbon nanotubes (CNTs) has been an important open question since the first reports of the strong modulation of ...electrical properties in 2002. However, thermal property modulation has not been demonstrated experimentally because of the difficulty of realizing CNT-encapsulated molecules as part of thermal transport microstructures. Here we develop a nanofabrication strategy that enables measurement of the impact of encapsulation on the thermal conductivity (κ) and thermopower (S) of single CNT bundles that encapsulate C
, Gd@C
and Er
@C
. Encapsulation causes 35-55% suppression in κ and approximately 40% enhancement in S compared with the properties of hollow CNTs at room temperature. Measurements of temperature dependence from 40 to 320 K demonstrate a shift of the peak in the κ to lower temperature. The data are consistent with simulations accounting for the interaction between CNTs and encapsulated fullerenes.
Black phosphorus attracts enormous attention as a promising layered material for electronic, optoelectronic and thermoelectric applications. Here we report large anisotropy in in-plane thermal ...conductivity of single-crystal black phosphorus nanoribbons along the zigzag and armchair lattice directions at variable temperatures. Thermal conductivity measurements were carried out under the condition of steady-state longitudinal heat flow using suspended-pad micro-devices. We discovered increasing thermal conductivity anisotropy, up to a factor of two, with temperatures above 100 K. A size effect in thermal conductivity was also observed in which thinner nanoribbons show lower thermal conductivity. Analysed with the relaxation time approximation model using phonon dispersions obtained based on density function perturbation theory, the high anisotropy is attributed mainly to direction-dependent phonon dispersion and partially to phonon-phonon scattering. Our results revealing the intrinsic, orientation-dependent thermal conductivity of black phosphorus are useful for designing devices, as well as understanding fundamental physical properties of layered materials.
Soft and conformable wearable electronics require stretchable semiconductors, but existing ones typically sacrifice charge transport mobility to achieve stretchability. We explore a concept based on ...the nanoconfinement of polymers to substantially improve the stretchability of polymer semiconductors, without affecting charge transport mobility. The increased polymer chain dynamics under nanoconfinement significantly reduces the modulus of the conjugated polymer and largely delays the onset of crack formation under strain. As a result, our fabricated semiconducting film can be stretched up to 100% strain without affecting mobility, retaining values comparable to that of amorphous silicon. The fully stretchable transistors exhibit high biaxial stretchability with minimal change in on current even when poked with a sharp object. We demonstrate a skinlike finger-wearable driver for a light-emitting diode.
Doping of traditional semiconductors has enabled technological applications in modern electronics by tailoring their chemical, optical and electronic properties. However, substitutional doping in ...two-dimensional semiconductors is at a comparatively early stage, and the resultant effects are less explored. In this work, we report unusual effects of degenerate doping with Nb on structural, electronic and optical characteristics of MoS2 crystals. The doping readily induces a structural transformation from naturally occurring 2H stacking to 3R stacking. Electronically, a strong interaction of the Nb impurity states with the host valence bands drastically and nonlinearly modifies the electronic band structure with the valence band maximum of multilayer MoS2 at the Γ point pushed upward by hybridization with the Nb states. When thinned down to monolayers, in stark contrast, such significant nonlinear effect vanishes, instead resulting in strong and broadband photoluminescence via the formation of exciton complexes tightly bound to neutral acceptors.
Recent debates about the conventional traditional threshold used in the fields of neuroscience and psychology, namely
< 0.05, have spurred researchers to consider alternative ways to analyze fMRI ...data. A group of methodologists and statisticians have considered Bayesian inference as a candidate methodology. However, few previous studies have attempted to provide end users of fMRI analysis tools, such as SPM 12, with practical guidelines about how to conduct Bayesian inference. In the present study, we aim to demonstrate how to utilize Bayesian inference, Bayesian second-level inference in particular, implemented in SPM 12 by analyzing fMRI data available to public via NeuroVault. In addition, to help end users understand how Bayesian inference actually works in SPM 12, we examine outcomes from Bayesian second-level inference implemented in SPM 12 by comparing them with those from classical second-level inference. Finally, we provide practical guidelines about how to set the parameters for Bayesian inference and how to interpret the results, such as Bayes factors, from the inference. We also discuss the practical and philosophical benefits of Bayesian inference and directions for future research.
Developing materials, interfaces, and devices with improved stability remains one of the key challenges in the field of photoelectrochemical water splitting. As a barrier to corrosion, molybdenum ...disulfide is a particularly attractive protection layer for photocathodes due to its inherent stability in acid, the low permeability of its basal planes, and the excellent hydrogen evolution reaction (HER) activity the MoS2 edge. Here, we demonstrate a stable silicon photocathode containing a protecting layer consisting of molybdenum disulfide, molybdenum silicide, and silicon oxide which operates continuously for two months. We make comparisons between this system and another molybdenum sulfide–silicon photocathode embodiment, taking both systems to catastrophic failure during photoelectrochemical stability measurements and exploring mechanisms of degradation. X-ray photoelectron spectroscopy and transmission electron microscopy provide key insights into the origins of stability.
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