/Tag: GeSe

Saddle‐Point Excitons and Their Extraordinary Light Absorption in 2D β‐Phase Group‐IV Monochalcogenides

2018-11-13T10:32:45+00:00November 13th, 2018|Categories: Publications|Tags: |

Monolayer β‐phase group‐IV monochalcogenides possess saddle‐points in the joint density of states, which leads to a remarkable absorption peak within the fundamental gap. Accordingly, the power conversion efficiencies for monolayer β‐GeSe and β‐SnSe are significantly higher than reported high‐performance ultrathin solar cells using transition metal dichalcogenides. Abstract In 2D materials, saddle‐points in the electronic structure give rise to diverging density of states, which leads to intriguing physical phenomena useful for applications, including magnetism, superconductivity, charge density wave, as well as enhanced optical absorption. Using first‐principles calculations, monolayer β‐phase group‐IV monochalcogenides (MX, M = Ge or Sn; X = S or Se) are shown to be a new class of 2D materials that possess saddle‐points in both the lowest conduction band and the highest valence band as well as in the joint density of states. Due to the existence of saddle‐points, a remarkable absorption peak within the fundamental gap is found in these materials when the light polarization is along the armchair (y) direction. The properties of saddle‐point excitons can be effectively tuned by both the strain and thickness of these materials. Importantly, the strong optical absorbance induced by saddle‐point exciton absorptions and the appropriate bandgap give ideal power conversion efficiencies as large as 1.11% for monolayer β‐SnSe, significantly higher than reported high‐performance ultrathin solar cells using transition metal dichalcogenides. These results not only open new avenues for exploring novel many‐body physics, but also suggest β‐phase MXs could be promising candidates for future optoelectronic devices.

Published in: "Advanced Functional Materials".

First-principles study on the electronic, optical, and transport properties of monolayer $α$- and $β$-GeSe

2017-12-22T16:30:49+00:00December 22nd, 2017|Categories: Publications|Tags: , |

Author(s): Yuanfeng Xu, Hao Zhang, Hezhu Shao, Gang Ni, Jing Li, Hongliang Lu, Rongjun Zhang, Bo Peng, Yongyuan Zhu, Heyuan Zhu, and Costas M. SoukoulisThe extraordinary properties and the novel applications of black phosphorene induce the research interest in the monolayer group-IV monochalcogenides. Here using first-principles calculations, we systematically investigate the electronic, transport, and optical properties of monolayer α- and β-GeSe,…[Phys. Rev. B 96, 245421] Published Fri Dec 22, 2017

Published in: "Physical Review B".

Tuning the ferro- to para-electric transition temperature and dipole orientation of group-IV monochalcogenide monolayers. (arXiv:1709.04581v1 [cond-mat.mes-hall])

2017-09-15T19:58:51+00:00September 15th, 2017|Categories: Publications|Tags: , |

Coordination-related, two-dimensional (2D) structural phase transitions are a fascinating and novel facet of two-dimensional materials with structural degeneracies. Nevertheless, a unified theoretical account of these transitions remains absent, and the following points are established through {em ab-initio} molecular dynamics and 2D discrete clock models here: Group-IV monochalcogenide (GeSe, SnSe, SnTe, …) monolayers have four degenerate structural ground states, and a 2D phase transition from a three-fold coordinated onto a five-fold coordinated structure takes place at finite temperature. On unstrained samples, the 2D phase transition requires lattice parameters to freely evolve. A fundamental energy scale permits understanding this transition. The transition temperature $T_c$ and the orientation of the in-plane intrinsic electric dipole can be controlled by moderate uniaxial tensile strain, and a modified discrete clock model describes the transition on strained samples. These results establish a general underlying theoretical background to understand structural phase transitions in 2D materials and their effects on material properties.

Published : "arXiv Mesoscale and Nanoscale Physics".

Layered material GeSe and vertical GeSe/MoS2 p-n heterojunctions. (arXiv:1708.06421v1 [cond-mat.mes-hall])

2017-08-23T19:58:45+00:00August 23rd, 2017|Categories: Publications|Tags: , , , , |

Group-IV monochalcogenides are emerging as a new class of layered materials beyond graphene, transition metal dichalcogenides (TMDCs), and black phosphorus (BP). In this paper, we report experimental and theoretical investigations of the band structure and transport properties of GeSe and its heterostructures. We find that GeSe exhibits a markedly anisotropic electronic transport, with maximum conductance along the armchair direction. Density functional theory calculations reveal that the effective mass is 2.7 times larger along the zigzag direction than the armchair direction; this mass anisotropy explains the observed anisotropic conductance. The crystallographic orientation of GeSe is confirmed by angleresolved polarized Raman measurements, which are further supported by calculated Raman tensors for the orthorhombic structure. Novel GeSe/MoS2 p-n heterojunctions are fabricated, combining the natural p-type doping in GeSe and n-type doping in MoS2. The temperature dependence of the measured junction current reveals that GeSe and MoS2 have a type-II band alignment with a conduction band offset of ~0.234 eV. The anisotropic conductance of GeSe may enable the development of new electronic and optoelectronic devices, such as high-efficiency thermoelectric devices and plasmonic devices with resonance frequency continuously tunable through light polarization direction. The unique GeSe/MoS2 p-n junctions with type-II alignment may become essential building blocks of vertical tunneling field-effect transistors for low-power applications. The novel p-type layered material GeSe can also be combined with n-type TMDCs to form heterogeneous complementary metal oxide semiconductor (CMOS) circuits.

Published : "arXiv Mesoscale and Nanoscale Physics".

Two-Dimensional Multiferroics: Ferroelasticity, Ferroelectricity, Domain Wall, and Potential Mechano-Opto-Electronic Applications

2016-10-15T11:03:51+00:00September 1st, 2016|Categories: Publications|Tags: |

Low-dimensional multiferroic materials hold great promises in miniaturized device applications such as nanoscale transducers, actuators, sensors, photovoltaics, and nonvolatile memories. Here, using first-principles theory we predict that two-dimensional (2D) monolayer Group IV monochalcogenides including GeS, GeSe, SnS, and SnSe are a class of 2D semiconducting multiferroics with strongly coupled giant in-plane spontaneous ferroelectric polarization and spontaneous ferroelastic lattice strain that are thermodynamically stable at room temperature and beyond, and can be effectively modulated by elastic strain engineering. Their optical absorption spectra exhibit strong in-plane anisotropy with visible-spectrum excitonic gaps and sizable exciton binding energies, rendering the unique characteristics of low-dimensional semiconductors. More importantly, the predicted low domain wall energy and small migration barrier together with the coupled multiferroic order and anisotropic electronic structures suggest their great potentials for tunable multiferroic functional devices by manipulating external electrical, mechanical, and optical field to control the internal responses, and enable the development of four device concepts including 2D ferroelectric memory, 2D ferroelastic memory, and 2D ferroelastoelectric nonvolatile photonic memory as well as 2D ferroelectric excitonic photovoltaics.

Published in: “arXiv Materials Science

Phosphorene analogues: Isoelectronic two-dimensional group-IV monochalcogenides with orthorhombic structure

2015-08-08T20:48:09+00:00August 8th, 2015|Categories: Publications|Tags: , |

Author(s): Lídia C. Gomes and A. Carvalho

The group-IV monochalcogenides SnS, SnSe, GeS, and GeSe form a family within the wider group of semiconductor “phosphorene analogues.” Here, we used first-principles calculations to investigate systematically their structural, electronic, and optical properties, analyzing the changes associated with…

[Phys. Rev. B 92, 085406] Published Thu Aug 06, 2015

Published in: “Physical Review B”

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