WSe2

/Tag: WSe2

Two dimensional crystals in three dimensions: electronic decoupling of single-layered platelets in colloidal nanoparticles. (arXiv:1811.05238v1 [cond-mat.mtrl-sci])

2018-11-14T05:29:24+00:00November 14th, 2018|Categories: Publications|Tags: , |

Two-dimensional crystals, single sheets of layered materials, often show distinct properties desired for optoelectronic applications, such as larger and direct band gaps, valley- and spinorbit effects. Being atomically thin, the low amount of material is a bottleneck in photophysical and photochemical applications. Here, we propose the formation of stacks of two-dimensional crystals intercalated with small surfactant molecules. We show, using first principles calculations, that already the very short surfactant methyl amine electronically decouples the layers. We demonstrate the indirect-direct band gap transition characteristic for Group 6 transition metal dichalcogenides experimentally by observing the emergence of a strong photoluminescence signal for ethoxide-intercalated WSe2 and MoSe2 multilayered nanoparticles with lateral size of about 10 nm and beyond. The proposed hybrid materials offer the highest possible density of the two-dimensional crystals with electronic properties typical for monolayers. Variation of the surfactant’s chemical potential allows fine-tuning of electronic properties and potentially elimination of trap states caused by defects.

Published in: "arXiv Material Science".

Enhanced Raman Scattering of CuPc Films on Imperfect WSe2 Monolayer Correlated to Exciton and Charge‐Transfer Resonances

2018-11-11T04:32:58+00:00November 11th, 2018|Categories: Publications|Tags: |

Here, it is demonstrated that the surface‐enhanced Raman scattering performance of the WSe2 monolayer can be enhanced via tailoring the atomic ratio of WSe2, which is correlated to the exciton and charge‐transfer resonances. The amplitude of the exciton and charge‐transfer resonances is estimated by the femtosecond optical pump‐probe measurement and a bipolar junction transistor consisting of the probe molecules and WSe2. Abstract Recently, 2D transition‐metal dichalcogenides (2D TMDCs) are identified as ideal substrates for surface‐enhanced Raman scattering (SERS). However, the effect of enhancement factor (EF) on TMDCs is lower than metal‐based SERS substrates. Here, it is demonstrated that the SERS performance of WSe2 monolayer can be enhanced via tailoring the atomic ratio of WSe2; this correlates to the exciton and charge‐transfer resonances. CuPc molecules are adsorbed onto WSe2 monolayers as the probe molecules, and the atomic ratio (Se:W) of WSe2 is tailored from 2 to 1.92. For an atomic ratio of 1.96, the maximum EF on irradiated WSe2 is more than 120; this is enhanced by more than 40 times compared with pristine WSe2. The amplitude of exciton and charge‐transfer resonances is estimated by femtosecond optical pump‐probe measurement and a bipolar junction transistor (BJT) consisting of CuPc film and 2D materials. It is found that the intensity of resonances in the CuPc–WSe2 system is tailored by the atomic ratio of WSe2. This is closely correlated to the SERS performance of WSe2. This study shows that the SERS performance of WSe2 is enhanced by tuning the atomic ratio of WSe2

Published in: "Advanced Functional Materials".

Super-ideal diodes at the Schottky-Mott limit in gated graphene-WSe$_2$ heterojunctions. (arXiv:1811.02660v1 [cond-mat.mes-hall])

2018-11-08T04:30:19+00:00November 8th, 2018|Categories: Publications|Tags: , , , , |

Metal-semiconductor interfaces, known as Schottky junctions, have long been hindered by defects and impurities. Such imperfections dominate the electrical characteristics of the junction by pinning the metal Fermi energy. We report measurements on a boron nitride encapsulated graphene-tungsten diselenide (WSe$_2$) Schottky junction which exhibits ideal diode characteristics and a complete lack of Fermi-level pinning. The Schottky barrier height of the device is rigidly tuned by electrostatic gating of the WSe$_2$, enabling experimental verification of the Schottky-Mott limit in a single device. Utilizing this exceptional gate control, we realize a super-ideal gated-Schottky diode which surpasses the ideal diode limit. Our results provide a pathway for defect-free electrical contact to two-dimensional semiconductors and open up possibilities for circuits with efficient switching characteristics and higher efficiency optoelectronic devices.

Published : "arXiv Mesoscale and Nanoscale Physics".

2D Crystals in Three Dimensions: Electronic Decoupling of Single‐Layered Platelets in Colloidal Nanoparticles

2018-11-07T08:35:19+00:00November 7th, 2018|Categories: Publications|Tags: , |

Optoelectronic properties of 2D crystals are distinct from their bulk counterparts, and advantageous for applications. Band gaps are larger, and sometimes direct, well‐suited for optoelectronic and photocatalytic applications. However, being atomically thin, low density is a bottleneck for quantitative performance. The proposed intercalation of layered materials with short surfactants yields electronically decoupled layers with similar properties as monolayers, but higher cross‐sections. Abstract 2D crystals, single sheets of layered materials, often show distinct properties desired for optoelectronic applications, such as larger and direct band gaps, valley‐ and spin‐orbit effects. Being atomically thin, the low amount of material is a bottleneck in photophysical and photochemical applications. Here, the formation of stacks of 2D crystals intercalated with small surfactant molecules is proposed. It is shown, using first principles calculations, that the very short surfactant methyl amine electronically decouples the layers. The indirect–direct band gap transition characteristic for Group 6 transition metal dichalcogenides is demonstrated experimentally by observing the emergence of a strong photoluminescence signal for ethoxide‐intercalated WSe2 and MoSe2 multilayered nanoparticles with lateral size of about 10 nm and beyond. The proposed hybrid materials offer the highest possible density of the 2D crystals with electronic properties typical of monolayers. Variation of the surfactant’s chemical potential allows fine‐tuning of electronic properties and potentially elimination of trap states caused by defects.

Published in: "Small".

Enhanced Raman Scattering of CuPc Films on Imperfect WSe2 Monolayer Correlated to Exciton and Charge‐Transfer Resonances

2018-11-07T00:32:48+00:00November 6th, 2018|Categories: Publications|Tags: |

Here, it is demonstrated that the surface‐enhanced Raman scattering performance of the WSe2 monolayer can be enhanced via tailoring the atomic ratio of WSe2, which is correlated to the exciton and charge‐transfer resonances. The amplitude of the exciton and charge‐transfer resonances is estimated by the femtosecond optical pump‐probe measurement and a bipolar junction transistor consisting of the probe molecules and WSe2. Abstract Recently, 2D transition‐metal dichalcogenides (2D TMDCs) are identified as ideal substrates for surface‐enhanced Raman scattering (SERS). However, the effect of enhancement factor (EF) on TMDCs is lower than metal‐based SERS substrates. Here, it is demonstrated that the SERS performance of WSe2 monolayer can be enhanced via tailoring the atomic ratio of WSe2; this correlates to the exciton and charge‐transfer resonances. CuPc molecules are adsorbed onto WSe2 monolayers as the probe molecules, and the atomic ratio (Se:W) of WSe2 is tailored from 2 to 1.92. For an atomic ratio of 1.96, the maximum EF on irradiated WSe2 is more than 120; this is enhanced by more than 40 times compared with pristine WSe2. The amplitude of exciton and charge‐transfer resonances is estimated by femtosecond optical pump‐probe measurement and a bipolar junction transistor (BJT) consisting of CuPc film and 2D materials. It is found that the intensity of resonances in the CuPc–WSe2 system is tailored by the atomic ratio of WSe2. This is closely correlated to the SERS performance of WSe2. This study shows that the SERS performance of WSe2 is enhanced by tuning the atomic ratio of WSe2

Published in: "Advanced Functional Materials".

The origin of single photon emission in 2D WSe2. (arXiv:1811.00221v1 [cond-mat.mtrl-sci])

2018-11-02T02:29:18+00:00November 2nd, 2018|Categories: Publications|Tags: , |

Several experimental groups have shown that defect structures in 2D WSe2 result in single photon emission (SPE). However, the origin of SPE is still unknown. We present a first principles study of the nature and optical properties of point defects in 2D WSe2, together with scanning tunneling microscopy (STM) and scanning transmission electron microscopy images. We predict that O2 can dissociate easily at Se vacancies, resulting in O-passivated Se vacancies (O-Se) and O interstitials (O-ins), which give STM images in good agreement with experiment. Our GW-Bethe-Salpeter-equation calculations show that O-ins defects give exciton peaks ~50-100 meV below the free exciton peak, in good agreement with the localized excitons observed in independent SPE experiments. No other point defect (O-Se, Se vacancies, W vacancies, and Se-W antisite defects) gives excitons in the same energy range. We conclude that the O-ins defect is a source for the SPE previously observed in 2D WSe2.

Published in: "arXiv Material Science".

Band Structure Engineering of Interfacial Semiconductors Based on Atomically Thin Lead Iodide Crystals. (arXiv:1810.08927v1 [cond-mat.mes-hall])

2018-10-23T04:30:27+00:00October 23rd, 2018|Categories: Publications|Tags: , , , |

To explore new constituents in two-dimensional materials and to combine their best in van der Waals heterostructures, are always in great demand as being unique platform to discover new physical phenomena and to design novel functionalities in modern electronics. Herein, we introduce PbI2 into two-dimensional system by synthesizing PbI2 crystals down to atomic scale, and further assemble them with transition metal dichalcogenide monolayers, as a good demonstration of flexibly designing different types of band alignment and interlayer interactions in PbI2-based interfacial semiconductors. The photoluminescence of MoS2 is strongly enhanced in MoS2/PbI2 stacks, while a dramatic quenching of WS2 or WSe2 is revealed in WS2/PbI2 and WSe2/PbI2 stacks. This is attributed to the effective heterojunction formation between PbI2 and these two-dimensional materials, but type I band alignment in MoS2/PbI2 stacks where fast-transferred charge carriers accumulate in the semiconductor with a smaller band gap and type II in WS2/PbI2 and WSe2/PbI2 stacks with separated electrons and holes. Our results demonstrate that MoS2, WS2, WSe2 monolayers with very similar electronic structures themselves, show distinct optical properties when interfacing with atomically thin PbI2 crystals, providing unprecedent capabilities to engineer and optimize the device performance based on two-dimensional heterostructures.

Published : "arXiv Mesoscale and Nanoscale Physics".

High-Quality Reconfigurable Black Phosphorus p-n Junctions

2018-10-23T00:32:37+00:00October 23rd, 2018|Categories: Publications|Tags: , , , , |

The p-n junction is the fundamental building block for electronics and photonics applications. In a conventional p-n junction, the performance parameters cannot be tuned due to the fixed doping level after ion implantation. In this paper, reconfigurable p-n or n-p junctions were built based on the ambipolar black phosphorus (BP) and other 2-D materials, namely, graphene and hexagonal boron nitride (h-BN). High-quality graphene/h-BN/BP/h-BN sandwich device was created by the dry transfer method in an inert environment. Graphene serves as the local back gate, which can tune BP partially into either n-type or p-type material. The rest of the BP channel can be controlled by the back gate. The BP encapsulated device can enable the high performance of the ideality factor down to 1.08 (p-n junction) and 1.18 (n-p junction). The device developed in this paper has a more balanced ideality factor than previously reported reconfigurable WSe2 and BP devices. By tuning the back gate and graphene gate, the BP device can be tuned into four operational quadrants, namely, n-n, n-p, p-n, and p-p junctions. Moreover, the hole mobility of BP is up to 404 cm2/$text {V}cdot text {s}$ at room temperature. Our work shows that the BP heterostructure is very promising for high-speed reconfigurable logic functions and circuits.

Published in: "IEEE Transactions on Electron Devices".

Tuning Bandgap and Energy Stability of Organic-Inorganic Halide Perovskites through Surface Engineering. (arXiv:1810.07297v1 [physics.app-ph])

2018-10-18T02:29:28+00:00October 18th, 2018|Categories: Publications|Tags: , , , , , , |

Organohalide perovskite with a variety of surface structures and morphologies have shown promising potential owing to the choice of the type of heterostructure dependent stability. We systematically investigate and discuss the impact of 2-dimensional molybdenum-disulphide (MoS2), molybdenum-diselenide (MoSe2), tungsten-disulphide (WS2), tungsten-diselenide (WSe2), boron- nitiride (BN) and graphene monolayers on band-gap and energy stability of organic-inorganic halide perovskites. We found that MAPbI3ML deposited on BN-ML shows room temperature stability (-25 meV~300K) with an optimal bandgap of ~1.6 eV. The calculated absorption coefficient also lies in the visible-light range with a maximum of 4.9 x 104 cm-1 achieved at 2.8 eV photon energy. On the basis of our calculations, we suggest that the encapsulation of an organic-inorganic halide perovskite monolayers by semiconducting monolayers potentially provides greater flexibility for tuning the energy stability and the bandgap.

Published in: "arXiv Material Science".

Growth and Thermo-driven Crystalline Phase Transition of Metastable Monolayer 1T’-WSe2 Thin Film. (arXiv:1810.05981v1 [cond-mat.mtrl-sci])

2018-10-16T02:29:37+00:00October 16th, 2018|Categories: Publications|Tags: |

Two-dimensional (2D) transition metal dichalcogenides MX2 (M = Mo, W, X = S, Se, Te) attracts enormous research interests in recent years. Its 2H phase possesses an indirect to direct bandgap transition in 2D limit, and thus shows great application potentials in optoelectronic devices. The 1T’ phase transition can drive the monolayer MX2 to be a 2D topological insulator. Here we realized the molecular beam epitaxial (MBE) growth of both the 1T’ and 2H phase monolayer WSe2. The crystalline structure of these two phases was characterized using scanning tunneling microscopy. The monolayer 1T’-WSe2 was found to be metastable, and can transform into 2H phase under post-annealing procedure. This thermo-driven crystalline phase transition makes the monolayer WSe2 to be an ideal platform for the controlling of topological phase transitions.

Published in: "arXiv Material Science".

Thickness-Dependent Differential Reflectance Spectra of Monolayer and Few-Layer MoS2, MoSe2, WS2 and WSe2. (arXiv:1810.04745v1 [cond-mat.mtrl-sci])

2018-10-12T02:29:42+00:00October 12th, 2018|Categories: Publications|Tags: , , , , |

The research field of two dimensional (2D) materials strongly relies on optical microscopy characterization tools to identify atomically thin materials and to determine their number of layers. Moreover, optical microscopy-based techniques opened the door to study the optical properties of these nanomaterials. We presented a comprehensive study of the differential reflectance spectra of 2D semiconducting transition metal dichalcogenides (TMDCs), MoS2, MoSe2, WS2, and WSe2, with thickness ranging from one layer up to six layers. We analyzed the thickness-dependent energy of the different excitonic features, indicating the change in the band structure of the different TMDC materials with the number of layers. Our work provided a route to employ differential reflectance spectroscopy for determining the number of layers of MoS2, MoSe2, WS2, and WSe2.

Published in: "arXiv Material Science".

Energy-resolved Photoconductivity Mapping in a Monolayer-bilayer WSe2 Lateral Heterostructure. (arXiv:1810.04253v1 [cond-mat.mtrl-sci])

2018-10-11T02:29:15+00:00October 11th, 2018|Categories: Publications|Tags: , , |

Vertical and lateral heterostructures of van der Waals materials provide tremendous flexibility for band structure engineering. Since electronic bands are sensitively affected by defects, strain, and interlayer coupling, the edge and heterojunction of these two-dimensional (2D) systems may exhibit novel physical properties, which can be fully revealed only by spatially resolved probes. Here, we report the spatial mapping of photoconductivity in a monolayer-bilayer WSe2 lateral heterostructure under multiple excitation lasers. As the photon energy increases, the light-induced conductivity detected by microwave impedance microscopy first appears along the hetero-interface and bilayer edge, then along the monolayer edge, inside the bilayer area, and finally in the interior of the monolayer region. The sequential emergence of mobile carriers in different sections of the sample is consistent with the theoretical calculation of local energy gaps. Quantitative analysis of the microscopy and transport data also reveals the linear dependence of photoconductivity on the laser intensity and the influence of interlayer coupling on carrier recombination. Combining theoretical modeling, atomic scale imaging, mesoscale impedance microscopy, and device-level characterization, our work suggests an exciting perspective to control the intrinsic band-gap variation in 2D heterostructures down to the few-nanometer regime.

Published in: "arXiv Material Science".

Nondegenerate chiral phonons in the Brillouin-zone center of $sqrt{3}×sqrt{3}$ honeycomb superlattices

2018-10-08T16:33:43+00:00October 8th, 2018|Categories: Publications|Tags: |

Author(s): Xifang Xu, Hao Chen, and Lifa ZhangThe theoretical finding on chiral phonons at Brillouin-zone corners (valleys) of two-dimensional honeycomb lattices and its experimental verification in monolayer tungsten diselenide [H. Zhu, J. Yi, M. Li, J. Xiao, L. Zhang, C. Yang, R. A. Kaindl, L. Li, Y. Wang, and X. Zhang, Science 359, 579 (2018…[Phys. Rev. B 98, 134304] Published Mon Oct 08, 2018

Published in: "Physical Review B".

Dependence of the hBN Layer Thickness on the Band Structure and Exciton Properties of Encapsulated WSe2 Monolayers. (arXiv:1810.02130v1 [cond-mat.mtrl-sci])

2018-10-05T02:29:19+00:00October 5th, 2018|Categories: Publications|Tags: , , , |

The optical properties of two-dimensional transition metal dichalcogenide monolayers such as MoS$_2$ or WSe$_2$ are dominated by excitons, Coulomb bound electron-hole pairs. Screening effects due the presence of hexagonal-BN surrounding layers have been investigated by solving the Bethe Salpeter Equation on top of GW wave functions in density functional theory calculations. We have calculated the dependence of both the quasi-particle gap and the binding energy of the neutral exciton ground state E$_b$ as a function of the hBN layer thickness. This study demonstrates that the effects of screening at this level of theory are more short-ranged that it is widely believed. The encapsulation of a WSe$_2$ monolayer by three sheets of hBN (around 1 nm) already yields a 20 % decrease of E$_b$ whereas the maximal reduction is 27% for thick hBN. We have performed similar calculations in the case of a WSe$_2$ monolayer deposited on stacked hBN layers. These results are compared to the recently proposed Quantum Electrostatic Heterostructure approach.

Published in: "arXiv Material Science".

Piezoelectric conversion in TMDCs Heterostructure Atomic Layers. (arXiv:1810.00052v1 [cond-mat.mes-hall])

2018-10-02T04:30:39+00:00October 2nd, 2018|Categories: Publications|Tags: , , , |

Two-dimensional heterostructure of WSe2/MoS2 atomic layers has unique piezoelectric characteristics which depends on the number of atomic layer, stacking type, and interlayer interaction size. The van der Waals heterostructure of p- and n-type TMDC atomic layers with different work functions forms a staggered gap alignment. The large band offset of conduction band minimum and valence band maximum between p-type WSe2 and n-type MoS2 atomic layers leads to the large electric polarization and piezoelectricity. The output voltages for a MoS2/WSe2 partial vertical heterostructure with a size of 3.0 nm and 1.5 nm were 0.137 V and 0.183 V for 4% and 8% tensile strains, respectively. The output voltage of AB-stacking MoS2/WSe2 heterostructure was larger than that of AA-stacking heterostructure for 4% tensile strain due to the contribution of intrinsic piezoelectricity and the symmetric condition in out-of-plane. The AB-stacking has a lower formation energy and better structural stability compared to AA-stacking. The large output voltage of nanoscale partial or full vertical heterostructure of 2D WSe2/MoS2 atomic layers in addition to the increased output voltage through series connection of multiple nanoscale piezoelectric devices will enable the realization of nano electromechanical systems (NEMS) with TMDC heterostructure atomic layers.

Published : "arXiv Mesoscale and Nanoscale Physics".

Low temperature photoconductivity of few layer p -type tungsten diselenide (WSe 2 ) field-effect transistors (FETs)

2018-10-01T12:33:45+00:00October 1st, 2018|Categories: Publications|Tags: |

We report on the low-temperature photoconductive properties of few layer p -type tungsten diselenide (WSe 2 ) field-effect transistors (FETs) synthesized using the chemical vapor transport method. Photoconductivity measurements show that these FETs display room temperature photo-responsivities of ∼7 mAW −1 when illuminated with a laser of wavelength λ = 658 nm with a power of 38 nW. The photo-responsivities of these FETs showed orders of magnitude improvement (up to ∼1.1 AW −1 with external quantum efficiencies reaching as high as ∼188%) upon application of a gate voltage ( V G  = −60 V). A temperature dependent (100K < T < 300 K) photoconductivity study reveals a weak temperature dependence of responsivity for these WSe 2 phototransistors. We demonstrate that it is possible to obtain stable photo-responsivities of ∼0.76 ± 0.2 AW −1 (with applied V G = −60 V), at low tem…

Published in: "Nanotechnology".

Some say, that 2D Research is the best website in the world.