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Dive into the research topics where Dequan Er is active.

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Featured researches published by Dequan Er.


ACS Applied Materials & Interfaces | 2014

Ti3C2 MXene as a High Capacity Electrode Material for Metal (Li, Na, K, Ca) Ion Batteries

Dequan Er; Junwen Li; Michael Naguib; Yury Gogotsi; Vivek B. Shenoy

Two-dimensional (2-D) materials are capable of handling high rates of charge in batteries since metal ions do not need to diffuse in a 3-D lattice structure. However, graphene, which is the most well-studied 2-D material, is known to have no Li capacity. Here, adsorption of Li, as well as Na, K, and Ca, on Ti3C2, one representative MXene, is predicted by first-principles density functional calculations. In our study, we observed that these alkali atoms exhibit different adsorption energies depending on the coverage. The adsorption energies of Na, K, and Ca decrease as coverage increases, while Li shows little sensitivity to variance in coverage. This observed relationship between adsorption energies and coverage of alkali ions on Ti3C2 can be explained by their effective ionic radii. A larger effective ionic radius increases interaction between alkali atoms, thus lower coverage is obtained. Our calculated capacities for Li, Na, K, and Ca on Ti3C2 are 447.8, 351.8, 191.8, and 319.8 mAh/g, respectively. Compared to materials currently used in high-rate Li and Na ion battery anodes, MXene shows promise in increasing overall battery performance.


Nature Materials | 2016

The role of electronic coupling between substrate and 2D MoS2 nanosheets in electrocatalytic production of hydrogen

Damien Voiry; Raymond Fullon; Jieun Yang; Cecilia de Carvalho Castro e Silva; Rajesh Kappera; Ibrahim Bozkurt; Daniel Kaplan; Maureen J. Lagos; Philip E. Batson; Gautam Gupta; Aditya D. Mohite; Liang Dong; Dequan Er; Vivek B. Shenoy; Tewodros Asefa; Manish Chhowalla

The excellent catalytic activity of metallic MoS2 edges for the hydrogen evolution reaction (HER) has led to substantial efforts towards increasing the edge concentration. The 2H basal plane is less active for the HER because it is less conducting and therefore possesses less efficient charge transfer kinetics. Here we show that the activity of the 2H basal planes of monolayer MoS2 nanosheets can be made comparable to state-of-the-art catalytic properties of metallic edges and the 1T phase by improving the electrical coupling between the substrate and the catalyst so that electron injection from the electrode and transport to the catalyst active site is facilitated. Phase-engineered low-resistance contacts on monolayer 2H-phase MoS2 basal plane lead to higher efficiency of charge injection in the nanosheets so that its intrinsic activity towards the HER can be measured. We demonstrate that onset potentials and Tafel slopes of ∼-0.1 V and ∼50 mV per decade can be achieved from 2H-phase catalysts where only the basal plane is exposed. We show that efficient charge injection and the presence of naturally occurring sulfur vacancies are responsible for the observed increase in catalytic activity of the 2H basal plane. Our results provide new insights into the role of contact resistance and charge transport on the performance of two-dimensional MoS2 nanosheet catalysts for the HER.


Nano Letters | 2014

Tailoring the Physical Properties of Molybdenum Disulfide Monolayers by Control of Interfacial Chemistry

Sina Najmaei; Xiaolong Zou; Dequan Er; Junwen Li; Zehua Jin; Weilu Gao; Qi Zhang; Sooyoun Park; Liehui Ge; Sidong Lei; Junichiro Kono; Vivek B. Shenoy; Boris I. Yakobson; Antony George; Pulickel M. Ajayan; Jun Lou

We demonstrate how substrate interfacial chemistry can be utilized to tailor the physical properties of single-crystalline molybdenum disulfide (MoS2) atomic-layers. Semiconducting, two-dimensional MoS2 possesses unique properties that are promising for future optical and electrical applications for which the ability to tune its physical properties is essential. We use self-assembled monolayers with a variety of end termination chemistries to functionalize substrates and systematically study their influence on the physical properties of MoS2. Using electrical transport measurements, temperature-dependent photoluminescence spectroscopy, and empirical and first-principles calculations, we explore the possible mechanisms involved. Our data shows that combined interface-related effects of charge transfer, built-in molecular polarities, varied densities of defects, and remote interfacial phonons strongly modify the electrical and optical properties of MoS2. These findings can be used to effectively enhance or modulate the conductivity, field-effect mobility, and photoluminescence in MoS2 monolayers, illustrating an approach for local and universal property modulations in two-dimensional atomic-layers.


ACS Nano | 2017

Janus Monolayer Transition-Metal Dichalcogenides

Jing Zhang; Shuai Jia; Iskandar Kholmanov; Liang Dong; Dequan Er; Weibing Chen; Hua Guo; Zehua Jin; Vivek B. Shenoy; Li Shi; Jun Lou

The crystal configuration of sandwiched S-Mo-Se structure (Janus SMoSe) at the monolayer limit has been synthesized and carefully characterized in this work. By controlled sulfurization of monolayer MoSe2, the top layer of selenium atoms is substituted by sulfur atoms, while the bottom selenium layer remains intact. The structure of this material is systematically investigated by Raman, photoluminescence, transmission electron microscopy, and X-ray photoelectron spectroscopy and confirmed by time-of-flight secondary ion mass spectrometry. Density functional theory (DFT) calculations are performed to better understand the Raman vibration modes and electronic structures of the Janus SMoSe monolayer, which are found to correlate well with corresponding experimental results. Finally, high basal plane hydrogen evolution reaction activity is discovered for the Janus monolayer, and DFT calculation implies that the activity originates from the synergistic effect of the intrinsic defects and structural strain inherent in the Janus structure.


Scientific Reports | 2015

Elastic Deformations in 2D van der waals Heterostructures and their Impact on Optoelectronic Properties: Predictions from a Multiscale Computational Approach

Hemant Kumar; Dequan Er; Liang Dong; Junwen Li; Vivek B. Shenoy

Recent technological advances in the isolation and transfer of different 2-dimensional (2D) materials have led to renewed interest in stacked Van der Waals (vdW) heterostructures. Interlayer interactions and lattice mismatch between two different monolayers cause elastic strains, which significantly affects their electronic properties. Using a multiscale computational method, we demonstrate that significant in-plane strains and the out-of-plane displacements are introduced in three different bilayer structures, namely graphene-hBN, MoS2-WS2 and MoSe2-WSe2, due to interlayer interactions which can cause bandgap change of up to ~300 meV. Furthermore, the magnitude of the elastic deformations can be controlled by changing the relative rotation angle between two layers. Magnitude of the out-of-plane displacements in graphene agrees well with those observed in experiments and can explain the experimentally observed bandgap opening in graphene. Upon increasing the relative rotation angle between the two lattices from 0° to 10°, the magnitude of the out-of-plane displacements decrease while in-plane strains peaks when the angle is ~6°. For large misorientation angles (>10°), the out-of-plane displacements become negligible. We further predict the deformation fields for MoS2-WS2 and MoSe2-WSe2 heterostructures that have been recently synthesized experimentally and estimate the effect of these deformation fields on near-gap states.


ACS Nano | 2017

Toward a Mechanistic Understanding of Vertical Growth of van der Waals Stacked 2D Materials: A Multiscale Model and Experiments

Han Ye; Jiadong Zhou; Dequan Er; Christopher C. Price; Zhongyuan Yu; Yumin Liu; John Lowengrub; Jun Lou; Zheng Liu; Vivek B. Shenoy

Vertical stacking of monolayers via van der Waals (vdW) interaction opens promising routes toward engineering physical properties of two-dimensional (2D) materials and designing atomically thin devices. However, due to the lack of mechanistic understanding, challenges remain in the controlled fabrication of these structures via scalable methods such as chemical vapor deposition (CVD) onto substrates. In this paper, we develop a general multiscale model to describe the size evolution of 2D layers and predict the necessary growth conditions for vertical (initial + subsequent layers) versus in-plane lateral (monolayer) growth. An analytic thermodynamic criterion is established for subsequent layer growth that depends on the sizes of both layers, the vdW interaction energies, and the edge energy of 2D layers. Considering the time-dependent growth process, we find that temperature and adatom flux from vapor are the primary criteria affecting the self-assembled growth. The proposed model clearly demonstrates the distinct roles of thermodynamic and kinetic mechanisms governing the final structure. Our model agrees with experimental observations of various monolayer and bilayer transition metal dichalcogenides grown by CVD and provides a predictive framework to guide the fabrication of vertically stacked 2D materials.


Nano Letters | 2018

Prediction of Enhanced Catalytic Activity for Hydrogen Evolution Reaction in Janus Transition Metal Dichalcogenides

Dequan Er; Han Ye; Nathan C. Frey; Hemant Kumar; Jun Lou; Vivek B. Shenoy

Significant efforts have been made in improving the hydrogen evolution reaction (HER) catalytic activity in transition metal dichalcogenides (TMDs), which are promising nonprecious catalysts. However, previous attempts have exploited possible solutions to activate the inert basal plane, with little improvement. Among them, the most successful modification requires a careful manipulation of vacancy concentration and strain simultaneously. To fully realize the promise of TMD catalysts for HER in an easier and more effective way, a new means in tuning the HER catalytic activity is needed. Herein, we propose exploiting the inherent structural asymmetry in the recently synthesized family of Janus TMDs as a new means to stimulate HER activity. We report a density functional theory (DFT) study of various Janus TMD monolayers as HER catalysts, and identify the WSSe system as a promising candidate, where the basal plane can be activated without large applied tensile strains and in the absence of significant density of vacancies. We predict that it is possible to realize a strain-free Janus TMD-based catalyst that can readily provide promising intrinsic HER catalytic performance. The calculated density of states and electronic structures reveal that the introduction of in-gap states and a shift in the Fermi level in hydrogen adsorbed systems due to Janus asymmetry is the origin of enhanced HER activity. Our results should pave the way to design high-performance and easy-accessible TMD-based HER catalysts.


Physical Chemistry Chemical Physics | 2018

Prediction of optimal structural water concentration for maximized performance in tunnel manganese oxide electrodes

Nathan C. Frey; Bryan W. Byles; Hemant Kumar; Dequan Er; Ekaterina Pomerantseva; Vivek B. Shenoy

Crystal water has been shown to stabilize next-generation energy storage electrodes with structural tunnels to accommodate cation intercalation, but the stabilization mechanism is poorly understood. In this study, we present a simple physical model to explain the energetics of interactions between the electrode crystal lattice, structural water, and electrochemically cycled ions. Our model is applied to understand the effects of crystal water on sodium ion intercalation in a tunnel manganese oxide structure, and we predict that precisely controlling the crystal water concentration can optimize the ion intercalation voltage and capacity and promote stable cycling. The analysis yields a critical structural water concentration by accounting for the interplay between elastic and electrostatic contributions to the free energy. Our predictions are validated with first-principles calculations and electrochemical measurements. The theoretical framework used here can be extended to predict critical concentrations of stabilizing molecules to optimize performance in next-generation battery materials.


Nanoscale | 2016

Synthesis of two-dimensional titanium nitride Ti4N3 (MXene).

Patrick Urbankowski; Babak Anasori; Taron Makaryan; Dequan Er; Sankalp Kota; Patrick L. Walsh; Meng-Qiang Zhao; Vivek B. Shenoy; Michel W. Barsoum; Yury Gogotsi


Physical Review Letters | 2016

Two-Dimensional π-Conjugated Covalent-Organic Frameworks as Quantum Anomalous Hall Topological Insulators.

Liang Dong; Youngkuk Kim; Dequan Er; Andrew M. Rappe; Vivek B. Shenoy

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Vivek B. Shenoy

University of Pennsylvania

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Junwen Li

University of Pennsylvania

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Hemant Kumar

University of Pennsylvania

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Liang Dong

University of Pennsylvania

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Nathan C. Frey

University of Pennsylvania

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Han Ye

University of Pennsylvania

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