Electrical transport properties driven by unique bonding configuration in gamma-GeSe. (arXiv:2304.06954v1 [cond-mat.mtrl-sci])

2023-04-17T02:29:33+00:00April 17th, 2023|Categories: Publications|Tags: |

Group-IV monochalcogenides have recently shown great potential for their thermoelectric, ferroelectric, and other intriguing properties. The electrical properties of group-IV monochalcogenides exhibit a strong dependence on the chalcogen type. For example, GeTe exhibits high doping concentration, whereas S/Se-based chalcogenides are semiconductors with sizable bandgaps. Here, we investigate the electrical and thermoelectric properties of gamma-GeSe, a recently identified polymorph of GeSe. gamma-GeSe exhibits high electrical conductivity (~106 S/m) and a relatively low Seebeck coefficient (9.4 uV/K at room temperature) owing to its high p-doping level (5×1021 cm-3), which is in stark contrast to other known GeSe polymorphs. Elemental analysis and first-principles calculations confirm that the abundant formation of Ge vacancies leads to the high p-doping concentration. The magnetoresistance measurements also reveal weak-antilocalization because of spin-orbit coupling in the crystal. Our results demonstrate that gamma-GeSe is a unique polymorph in which the modified local bonding configuration leads to substantially different physical properties.

Published in: "arXiv Material Science".

Versatile van der Waals Heterostructures of Gamma-GeSe with h-BN/Graphene/MoS2. (arXiv:2210.09991v1 [cond-mat.mtrl-sci])

2022-10-19T02:29:37+00:00October 19th, 2022|Categories: Publications|Tags: , , , |

Recent discovery of a novel hexagonal phase of GeSe (Gamma-GeSe) has triggered great interests in nanoelectronics applications owing to its electrical conductivity of bulk phase even higher than graphite while its monolayer is a semiconductor. For potential applications, construction of functional two-dimensional (2D) contacts is indispensable. Herein, via first-principles calculations, we propose the design of van der Waals heterostructures (vdWHs) of Gamma-GeSe contacting respectively with graphene, 2D h-BN and MoS2, as representatives of metallic, insulator, and semiconductor partners. Our work shows that the h-BN or graphene layer donates electrons to the Gamma-GeSe layer, resulting in n doping in Gamma-GeSe, while the MoS2 layer accepts electrons from the Gamma-GeSe layer leading to p doping of the latter. The Gamma-GeSe/BN heterostructure has a type-I band alignment with large band offsets, indicating that BN can be used as an effective passivating layer to protect Gamma-GeSe from its environmental disturbance while maintaining its major electronic and optical characteristics. For Gamma-GeSe/graphene heterostructure, it is prone to have a very low-Schottky barrier down to tens of meV, easily overcome by thermal excitation, which can be tunable by strain and external electric field. The Gamma-GeSe/MoS2 vdWH forms a Z-scheme interface, which is beneficial for carriers splitting and photon utilization. Our work indicates that Gamma-GeSe can be well passivated by BN, and form intimate contact with graphene for high charge injection efficiency and with MoS2 for efficient carriers splitting for redox reactions.

Published in: "arXiv Material Science".

Highly Modulated Dual Semimetal and Semiconducting Gamma-GeSe with Strain Engineering. (arXiv:2210.08704v1 [cond-mat.mtrl-sci])

2022-10-18T02:29:45+00:00October 18th, 2022|Categories: Publications|Tags: , , , |

Layered hexagonal Gamma–GeSe, a new polymorph of GeSe synthesized recently, shows strikingly high electronic conductivity in its bulk form (even higher than graphite) while semiconducting in the case of monolayer (1L). In this work, by using first-principles calculations, we demonstrate that, different from its orthorhombic phases of GeSe, the Gamma–GeSe shows a small spatial anisotropic dependence and a strikingly thickness-dependent behavior with transition from semimetal (bulk, 0.04 eV) to semiconductor (1L, 0.99 eV), and this dual conducting characteristic realized simply with thickness control in Gamma-GeSe has not been found in other 2D materials before. The lacking of d-orbital allows charge carrier with small effective mass (0.16 m0 for electron and 0.23 m0 for hole) which is comparable to phosphorene. Meanwhile, 1L Gamma–GeSe shows a superior flexibility with Young’s modulus of 86.59 N/m, only one-quarter of that of graphene and three-quarters of that of MoS2, and Poisson’s ratio of 0.26, suggesting a highly flexible lattice. Interestingly, 1L Gamma-GeSe shows an in-plane isotropic elastic modulus inherent with hexagonal symmetry while an anisotropic in-plane effective mass owing to shifted valleys around the band edges. We demonstrate the feasibility of strain engineering in inducing indirect-direct and semiconductor-metal transitions resulting from competing bands at the band edges. Our work shows that the free 1L Gamma-GeSe shows a strong light absorption (~106 cm-1) and an indirect bandgap with rich valleys at band edges, enabling high carrier concentration and a low rate of direct electron-hole recombination which would be promising for nanoelectronics and solar cell applications.

Published in: "arXiv Material Science".

Fast Intercalation of Lithium in Semi-Metallic {gamma}-GeSe Nanosheet: A New Group-IV Monochalcogenide for Lithium-Ion Battery Application. (arXiv:2206.04939v1 [cond-mat.mtrl-sci])

2022-06-13T02:29:42+00:00June 13th, 2022|Categories: Publications|Tags: , |

Existence of van der Waals gaps renders two-dimensional (2D) materials ideal passages of lithium for being used as anode materials. However, the requirement of good conductivity significantly limits the choice of 2D candidates. So far only graphite is satisfying due to its relatively high conductivity. Recently, a new polymorph of layered germanium selenide (Gamma-GeSe) was proven to be semimetal in its bulk phase with a higher conductivity than graphite while its monolayer behaves semiconducting. In this work, by using first-principles calculations, we examined the possibility of using this new group-IV monochalcogenide, Gamma-GeSe, as anode in the Li-ion battery (LIBs). Our studies revealed that Li atom would form an ionic adsorption with adjacent selenium atoms at the hollow site and exist in cationic state (lost 0.89 e to Gamma-GeSe). Results of climbing image-nudged elastic band show the diffusion barrier of Li is 0.21 eV in the monolayer limit, which can activate a relatively fast diffusion even at room temperature on the Gamma-GeSe surface. The calculated theoretical average voltages range from 0.071 to 0.015 V at different stoichiometry of LixGeSe with minor volume variation, suggesting its potential application as anode of LIBs. The predicted moderate binding energy, a low open circuit voltage (comparable to graphite) and a fast motion of Li suggests that Gamma-GeSe nanosheet can be chemically exfoliated via Li intercalation and a promising candidate as the anode material for LIBs.

Published in: "arXiv Material Science".

Analytical Theory of Near-Field Electrostatic Effects in Two-Dimensional Materials and van der Waals Heterojunctions. (arXiv:2205.04606v1 [cond-mat.mtrl-sci])

2022-05-11T02:29:24+00:00May 11th, 2022|Categories: Publications|Tags: |

We derive and validate a quantitative analytical model of the near-field electrostatic effects in the vicinity (>=3AA) of two-dimensional (2D) materials. In solving the Poisson equation of a near-planar point charge ansatz for the electronic density of a 2D material, our formula quantitatively captures the out-of-plane decay and the in-plane modulation of density functional theory (DFT)-calculated potentials. We provide a method for quickly constructing the electronic density ansatz, and apply it to the case of hexagonal monolayers (BN, AlN, GaN) and monochalcogenides (GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbS, PbSe, PbTe) and their flexural and polar distortions. We demonstrate how our model can be straightforwardly applied to predict material-/angle-specific moir’e potentials arising in twisted superlattices with periodicities beyond the reach of DFT calculations.

Published in: "arXiv Material Science".

Polarization switching induced by domain wall sliding in two-dimensional ferroelectric monochalcogenides. (arXiv:2204.12209v1 [cond-mat.mtrl-sci])

2022-04-27T04:30:24+00:00April 27th, 2022|Categories: Publications|Tags: |

Monolayer ferroelectric materials have emerged as a promising avenue for miniaturization of ferroelectric devices, and as potential candidate materials for future photovoltaics or thermoelectrics. The properties of ferroelectrics are strongly influenced by the presence of domain walls, but a detailed theoretical understanding of their role in polarization switching and influence on optical properties is still lacking in two-dimensional materials. In this work we study structural, dynamic, electronic and optical properties of 90 and 180 degree DWs in monolayer GeS, GeSe, SnS and SnSe, by means of DFT calculations. We find that, in all cases, 180 degree walls impact the properties stronger than 90 degree walls and exhibit higher formation energies. In particular, calculations of the optical absorption show that 180 degree walls may induce a red shift of the optical absorption peak of the materials, significantly enhancing their solar light absorption at high DW densities. We then calculate the formation energies and migration barriers of DWs and find that these are strongly dependent on lattice anisotropy. The barriers allow us to extract the electric field required to induce DW migration and we find much lower values of the coercive fields compared to coherent bulk polarization switching. Our results are in agreement with experimental coercive field measured for SnS and thus elucidate the crucial role of domain walls in polarization switching.

Published : "arXiv Mesoscale and Nanoscale Physics".

Polarization switching induced by domain wall sliding in two-dimensional ferroelectric monochalcogenides. (arXiv:2204.12209v1 [cond-mat.mtrl-sci])

2022-04-27T02:29:28+00:00April 27th, 2022|Categories: Publications|Tags: |

Monolayer ferroelectric materials have emerged as a promising avenue for miniaturization of ferroelectric devices, and as potential candidate materials for future photovoltaics or thermoelectrics. The properties of ferroelectrics are strongly influenced by the presence of domain walls, but a detailed theoretical understanding of their role in polarization switching and influence on optical properties is still lacking in two-dimensional materials. In this work we study structural, dynamic, electronic and optical properties of 90 and 180 degree DWs in monolayer GeS, GeSe, SnS and SnSe, by means of DFT calculations. We find that, in all cases, 180 degree walls impact the properties stronger than 90 degree walls and exhibit higher formation energies. In particular, calculations of the optical absorption show that 180 degree walls may induce a red shift of the optical absorption peak of the materials, significantly enhancing their solar light absorption at high DW densities. We then calculate the formation energies and migration barriers of DWs and find that these are strongly dependent on lattice anisotropy. The barriers allow us to extract the electric field required to induce DW migration and we find much lower values of the coercive fields compared to coherent bulk polarization switching. Our results are in agreement with experimental coercive field measured for SnS and thus elucidate the crucial role of domain walls in polarization switching.

Published in: "arXiv Material Science".

Outstanding thermoelectric performance predicted for out-of-plane p-doped GeSe. (arXiv:2202.05986v1 [cond-mat.mtrl-sci])

2022-02-15T05:29:25+00:00February 15th, 2022|Categories: Publications|Tags: |

The record-breaking thermoelectric performance of tin selenide (SnSe) has motivated the investigation of analogue compounds with the same structure. A promising candidate that emerged recently is germanium selenide (GeSe). Here, using extensive first-principles calculations of the hole-phonon and hole-impurity scattering, we investigate the thermoelectric transport properties of the orthorhombic phase of p-doped GeSe. We predict outstanding thermoelectric performance for GeSe over a broad range of temperatures due to its high Seebeck coefficients, extremely low Lorenz numbers, ultralow total thermal conductivity, and relatively large band gap. In particular, the out-of-plane direction in GeSe presents equivalent or even higher performance than SnSe for temperatures above 500 K. By extending the analysis to 900 K, we obtained an ultrahigh value for the thermoelectric figure of merit (zT = 3.2) at the optimal hole density of 4×10^19 cm^-3. Our work provides strong motivation for continued experimental work focusing on improving the GeSe doping efficiency in order to achieve this optimal hole density.

Published in: "arXiv Material Science".

Activation of Phosphorene-like Two-dimensional GeSe for Efficient Electrocatalytic Nitrogen Reduction via States Filtering of Ru. (arXiv:2201.11892v1 [cond-mat.mtrl-sci])

2022-01-31T02:29:24+00:00January 31st, 2022|Categories: Publications|Tags: , , |

Nitrogen reduction reaction (NRR) which converts nitrogen (N2) to ammonia (NH3) normally requires harsh conditions to break the bound nitrogen bond. Herein, via first-principles calculation we reveal that a superior NRR catalytic activity could be obtained through anchoring atomic catalyst above a phosphorene-like puckering surface of germanium selenide (GeSe). Through examining the single- and double- atoms (B, Fe, W, Mo and Ru) decorated on GeSe, we find that its rippled structure allows an intimate contact between the deposited species and the GeSe which significantly promotes the states hybridization. Amongst the various atomic catalyst, we predict that the Ru dimer decorated GeSe monolayer (Ru2@GeSe) has superior catalytic activity for the N2 fixation and reduction. Through examining the three NRR pathways (distal, alternating and enzymatic), the distal and enzymatic pathway is both the thermodynamically favorable with the maximum Gibbs free energy change ({Delta}GMAX) of 0.25 and 0.26 eV, respectively. Such a superior activity could be attributed to the filtered states of GeSe by Ru dimer which leads to the effective activation of the adsorbed N2 bond. As an efficient near-infrared absorber of GeSe, the Ru mediated hybridization of GeSe-Ru-N2 complex enables an in-gap state which further broadens the absorbing window, rendering for a broadband solar absorption and possible photocatalysis.

Published in: "arXiv Material Science".

Substitutional Doped GeSe: Tunable Oxidative States with Strain Engineering. (arXiv:2201.11890v1 [cond-mat.mtrl-sci])

2022-01-31T02:29:21+00:00January 31st, 2022|Categories: Publications|Tags: , |

Layered chalcogenide materials have a wealth of nanoelectronics applications like resistive switching and energy-harvesting such as photocatalyst owing to rich electronic, orbital, and lattice excitations. In this work, we explore monochalcogenide germanium selenide GeSe with respect to substitutional doping with 13 metallic cations by using first-principles calculations. Typical dopants including s-shell (alkali elements Li and Na), p-shell (Al, Pb and Bi), 3d (Fe, Cu, Co and Ni), 4d (Pd and Ag) and 5d (Au and Pt) elements are systematically examined. Amongst all the cationic dopants, Al with the highest oxidation states, implying a high mobility driven by electric field, and Al-doped GeSe may be a promising candidate for novel resistive switching devices. We show that there exist many localized induced states in the band gap of GeSe upon doping Fe, Co, or Ni, while for Cu, Ag, and Au cases there is no such states in the gap. The Ag and Cu are + 0.27 and + 0.35 charged respectively and the positive charges are beneficial for field-driven motion in GeSe. In contrast, Au is slightly negatively charged renders Au-doped GeSe a promising photocatalyst and enhanced surface plasmon. Moreover, we explore the coexistence of dopant and strain in GeSe and find dynamical adjustments of localized states in GeSe with levels successive shifting upward/downward with strain. This induces dynamic oxidative states of the dopants under strain which should be quite popular in composites where motion of metal adatoms causes significant deformation.

Published in: "arXiv Material Science".

Strain-tunable in-plane ferroelectricity and lateral tunnel junction in monolayer group-IV monochalcogenides. (arXiv:2201.08715v1 [cond-mat.mtrl-sci])

2022-01-24T02:29:34+00:00January 24th, 2022|Categories: Publications|Tags: |

2D Ferroelectric materials are promising for designing low-dimensional memory devices. Here, we explore strain tunable ferroelectric properties of group-IV monochalcogenides MX (M=Ge, Sn; X=S, Se) and their potential application in lateral field tunnel junction devices. We find that these monolayers have in-plane ferroelectricity, with their ferroelectric parameters being on par with other known 2D ferroelectric materials. Amongst SnSe, SnS, GeSe, and GeS, we find that GeS has the best ferroelectric parameters for device applications, which can be improved further by applying uniaxial tensile strain. We use the calculated ferroelectric properties of these materials to study the tunneling electroresistance (TER) of a 4 nm device based on lateral ferroelectric tunnel junction. We find a substantial TER ratio $10^3-10^5$ in the devices based on these materials, which can be further improved up to a factor of 40 on the application of tensile strain.

Published in: "arXiv Material Science".

Metastable piezoelectric group IV monochalcogenide monolayers with a buckled honeycomb structure. (arXiv:2101.05736v1 [cond-mat.mtrl-sci])

2021-01-15T02:29:19+00:00January 15th, 2021|Categories: Publications|Tags: , , |

Twelve two-dimensional group-IV monochalcogenide monolayers (SiS, SiSe, SiTe, GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbS, PbSe, and PbTe) with a buckled honeycomb atomistic structure–belonging to symmetry group P3m1–and an out-of-plane intrinsic electric polarization are shown to be metastable by three independendent methods. First, we uncover a coordination-preserving structural transformation from the low-buckled honeycomb structure onto the lower-energy Pnm2$_1$ (or Pmmn for PbS, PbSe, and PbTe) phase to estimate {em energy barriers} $E_B$ that must be overcome during such structural transformation. Using the curvature of the local minima and $E_B$ as inputs to Kramers escape formula, large escape times are found, implying the structural metastability of the buckled honeycomb phase (nevertheless, and with the exception of PbS and PbSe, these phases display escape times ranging from 700 years to multiple times the age of the universe, and can be considered “stable” for practical purposes only in that relative sense). The second demonstration is provided by phonon dispersion relations that include the effect of long-range Coulomb forces and display no negative vibrational modes. The third and final demonstration of structural metastability is furnished by room-temperature {em ab initio} molecular dynamics for selected compounds. The magnitude of the electronic band gap evolves with chemical composition. Different from other binary two-dimensional compounds such as transition metal dichalcogenide monolayers and hexagonal boron nitride monolayers which only develop an in-plane piezoelectric response, the twelve group-IV monochalcogenide monolayers with a buckled honeycomb structure also display out-of-plane piezoelectric properties.

Published in: "arXiv Material Science".

Localized Excitons in Defective Monolayer Germanium Selenide. (arXiv:2005.10102v1 [cond-mat.mtrl-sci])

2020-05-21T02:30:05+00:00May 21st, 2020|Categories: Publications|Tags: , |

Germanium Selenide (GeSe) is a van der Waals-bonded layered material with promising optoelectronic properties, which has been experimentally synthesized for 2D semiconductor applications. In the monolayer, due to reduced dimensionality and, thus, screening environment, perturbations such as the presence of defects have a significant impact on its properties. We apply density functional theory and many-body perturbation theory to understand the electronic and optical properties of GeSe containing a single selenium vacancy in the $-2$ charge state. We predict that the vacancy results in mid-gap “trap states” that strongly localize the electron and hole density and lead to sharp, low-energy optical absorption peaks below the predicted pristine optical gap. Analysis of the exciton wavefunction reveals that the 2D Wannier-Mott exciton of the pristine monolayer is highly localized around the defect, reducing its Bohr radius by a factor of four and producing a dipole moment along the out-of-plane axis due to the defect-induced symmetry breaking. Overall, these results suggest that the vacancy is a strong perturbation to the system, demonstrating the importance of considering defects in the context of material design.

Published in: "arXiv Material Science".

Ab initio analysis of some Ge-based 2D nanomaterials. (arXiv:2001.07358v1 [cond-mat.mtrl-sci])

2020-01-22T02:29:29+00:00January 22nd, 2020|Categories: Publications|Tags: |

The structural, electronic and dynamical properties of a group of 2D germanium-based compounds, including GeC, GeN, GeO, GeSi, GeS, GeSe, and germanene, are investigated by employing first-principles calculations. The most stable structure of each of these systems is identified after considering the most probable configurations and performing accurate phonon calculations. We introduce a new phase of germanene, which we name the tile germanene, which is significantly more stable than the known hexagonal germanene. We apply the modern modified Becke-Johnson (mBJ) and DFT1/2 schemes to obtain an accurate band structure for our selected 2D materials. It is seen that GeO and GeC exhibit the highest band gaps of more than 3 eV in this group of materials. Moreover, we argue that, in contrast to the semi-metallic nature of hexagonal germanene, the tile germanene is a very good conductor. The band edges of our semiconducting 2D materials are accurately aligned to the vacuum level to address the potential photocatalytic application of this system for water splitting and carbon dioxide reduction. The optical properties, including dielectric functions, refractive index, reflectivity, and Loss function of the samples are investigated in the framework of the Bethe-Salpeter approach.

Published in: "arXiv Material Science".

Edge phonons in layered orthorhombic GeS and GeSe monochalcogenides

2019-09-03T16:39:28+00:00September 3rd, 2019|Categories: Publications|Tags: , |

Author(s): H. B. Ribeiro, S. L. L. M. Ramos, L. Seixas, C. J. S. de Matos, and M. A. PimentaGermanium sulfide (GeS) and germanium selenide (GeSe) are layered orthorhombic crystals whose structure bears a strong resemblance with that of black phosphorus and, additionally, are expected to exhibit high piezoelectricity in the few layer domain. In this work, we investigate the Raman properties…[Phys. Rev. B 100, 094301] Published Tue Sep 03, 2019

Published in: "Physical Review B".

Direction and strain controlled anisotropic transport behaviors of 2D GeSe-phosphorene vdW heterojunctions

2019-08-20T10:38:59+00:00August 20th, 2019|Categories: Publications|Tags: , , |

Vertical van der Waals (vdW) heterostructures made up of two or more 2D monolayer materials provide new opportunities for 2D devices. Herein, we study the electronic transport properties of vertical integration of 2D GeSe-phosphorene(GeSe–BP) heterostructure, using the nonequilibrium Green’s function formalism combined with the density-functional theory. The results reveal that the directional dependency and strain tunable transport anisotropic behavior appears in GeSe/BP-stacking vdW heterostructures. The current–voltage ( I – V ) characteristics indicate that the electric current propagates more easily through the perpendicular buckled direction ( Y ) than the linear atomic chain direction ( X ) in the low bias regime regardless of the GeSe–BP stacking, which is supported by the underlying electronic structures along Γ– Y and Γ– X lines. The anisotropic transmission spectra indicate an over 10 5 on/off ratio between the I …

Published in: "Nanotechnology".

Recent progress in 2D group IV–IV monochalcogenides: synthesis, properties and applications

2019-04-08T06:34:46+00:00April 8th, 2019|Categories: Publications|Tags: , |

Coordination-related, 2D structural phase transitions are a fascinating facet of 2D materials with structural degeneracy. Phosphorene and its new phases, exhibiting unique electronic properties, have received considerable attention. The 2D group IV–IV monochalcogenides (i.e. GeS, GeSe, SnS and SnSe) like black phosphorous possess puckered layered orthorhombic structure. The 2D group IV–IV monochalcogenides with advantages of earth-abundance, less toxicity, environmental compatibility and chemical stability, can be widely used in optoelectronics, piezoelectrics, photodetectors, sensors, Li-batteries and thermoelectrics. In this review, we summarized recent research progress in theory and experiment, which studies the fundamental properties, applications and fabrication of 2D group IV–IV monochalcogenides and their new phases, and brings new perspectives and challenges for the future of this emerging field.

Published in: "Nanotechnology".

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

2018-11-28T22:32:23+00:00November 28th, 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".

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

2018-11-17T22:32:26+00:00November 17th, 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".

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