InSe

/Tag: InSe

Experimental identification of critical condition for drastically enhancing thermoelectric power factor of two-dimensional layered materials. (arXiv:1811.11592v1 [cond-mat.mes-hall])

2018-11-29T04:30:19+00:00November 29th, 2018|Categories: Publications|Tags: , |

Nano-structuring is an extremely promising path to high performance thermoelectrics. Favorable improvements in thermal conductivity are attainable in many material systems, and theoretical work points to large improvements in electronic properties. However, realization of the electronic benefits in practical materials has been elusive experimentally. A key challenge is that experimental identification of the quantum confinement length, below which the thermoelectric power factor is significantly enhanced, remains elusive due to lack of simultaneous control of size and carrier density. Here we investigate gate tunable and temperature-dependent thermoelectric transport in $gamma$ phase indium selenide ($gamma$ InSe, n type semiconductor) samples with thickness varying from 7 to 29 nm. This allows us to properly map out dimension and doping space. Combining theoretical and experimental studies, we reveal that the sharper pre-edge of the conduction-band density of states arising from quantum confinement gives rise to an enhancement of the Seebeck coefficient and the power factor in the thinner InSe samples. Most importantly, we experimentally identify the role of the competition between quantum confinement length and thermal de Broglie wavelength in the enhancement of power factor. Our results provide an important and general experimental guideline for optimizing the power factor and improving the thermoelectric performance of two-dimensional layered semiconductors.

Published : "arXiv Mesoscale and Nanoscale Physics".

Photoquantum Hall Effect and Light‐Induced Charge Transfer at the Interface of Graphene/InSe Heterostructures

2018-11-28T22:32:38+00:00November 28th, 2018|Categories: Publications|Tags: , , |

InSe‐capped graphene field effect transistors combine the photosensitivity of InSe with the unique electrical properties of graphene. The light‐induced charge transfer at the InSe/graphene interface can induce an increase or decrease of the carrier density in graphene and a light‐induced transition from a hole‐ to an electron‐carrier current, with potential for quantum metrology and miniaturized optical sensors and switches. Abstract The transfer of electronic charge across the interface of two van der Waals crystals can underpin the operation of a new class of functional devices. Among van der Waals semiconductors, an exciting and rapidly growing development involves the “post‐transition” metal chalcogenide InSe. Here, field effect phototransistors are reported where single layer graphene is capped with n‐type InSe. These device structures combine the photosensitivity of InSe with the unique electrical properties of graphene. It is shown that the light‐induced transfer of charge between InSe and graphene offers an effective method to increase or decrease the carrier density in graphene, causing a change in its resistance that is gate‐controllable and only weakly dependent on temperature. The charge transfer at the InSe/graphene interface is probed by Hall effect and photoconductivity measurmentes and it is demonstrated that light can induce a sign reversal of the quantum Hall voltage and photovoltaic effects in the graphene layer. These findings demonstrate the potential of light‐induced charge transfer in gate‐tunable InSe/graphene phototransistors for optoelectronics and quantum metrology.

Published in: "Advanced Functional Materials".

Photoquantum Hall Effect and Light‐Induced Charge Transfer at the Interface of Graphene/InSe Heterostructures

2018-11-27T14:32:32+00:00November 27th, 2018|Categories: Publications|Tags: , , |

InSe‐capped graphene field effect transistors combine the photosensitivity of InSe with the unique electrical properties of graphene. The light‐induced charge transfer at the InSe/graphene interface can induce an increase or decrease of the carrier density in graphene and a light‐induced transition from a hole‐ to an electron‐carrier current, with potential for quantum metrology and miniaturized optical sensors and switches. Abstract The transfer of electronic charge across the interface of two van der Waals crystals can underpin the operation of a new class of functional devices. Among van der Waals semiconductors, an exciting and rapidly growing development involves the “post‐transition” metal chalcogenide InSe. Here, field effect phototransistors are reported where single layer graphene is capped with n‐type InSe. These device structures combine the photosensitivity of InSe with the unique electrical properties of graphene. It is shown that the light‐induced transfer of charge between InSe and graphene offers an effective method to increase or decrease the carrier density in graphene, causing a change in its resistance that is gate‐controllable and only weakly dependent on temperature. The charge transfer at the InSe/graphene interface is probed by Hall effect and photoconductivity measurmentes and it is demonstrated that light can induce a sign reversal of the quantum Hall voltage and photovoltaic effects in the graphene layer. These findings demonstrate the potential of light‐induced charge transfer in gate‐tunable InSe/graphene phototransistors for optoelectronics and quantum metrology.

Published in: "Advanced Functional Materials".

Electronic and optical properties of SnX2(X=S, Se)-InSe van der Waals heterostructures from first- principle calculations. (arXiv:1811.09031v1 [cond-mat.mtrl-sci])

2018-11-26T02:29:28+00:00November 26th, 2018|Categories: Publications|Tags: , |

In this work from first-principles simulations we investigate bilayer van der Waals heterostructures (vdWh) of emerging 2-dimensional (2D) optical materials SnS 2 and SnSe 2 with monolayer InSe. With density functional theory (DFT) calculations, we study the structural, electronic, optical and carrier transport properties of the SnX 2 (X=S,Se)-InSe vdWh. Calculations show SnX 2 -InSe in its most stable stacking form (named AB-1) to be a material with a small (0.6- 0.7eV) indirect band-gap. The bilayer vdWh shows broad spectrum optical response, with number of peaks in the infra-red to visible region. In terms of carrier transport properties, asymmetry in conductance was observed with respect to the transport direction and electron and hole transmission. The findings are promising from the viewpoint of nanoelectronics and photonics.

Published in: "arXiv Material Science".

High Mobilities in Layered InSe Transistors with Indium‐Encapsulation‐Induced Surface Charge Doping

2018-11-10T22:34:25+00:00November 10th, 2018|Categories: Publications|Tags: , |

A robust layered indium selenide (InSe) field‐effect transistor (FET) with superior high mobility (3700 cm2 V−1 s−1 at room temperature) is demonstrated by depositing an indium doping layer. With tunable carrier transport, the surface‐doped InSe FETs present flexible operations to realize various logic circuits, such as inverters and not‐or and not‐and gates. Abstract Tunability and stability in the electrical properties of 2D semiconductors pave the way for their practical applications in logic devices. A robust layered indium selenide (InSe) field‐effect transistor (FET) with superior controlled stability is demonstrated by depositing an indium (In) doping layer. The optimized InSe FETs deliver an unprecedented high electron mobility up to 3700 cm2 V−1 s−1 at room temperature, which can be retained with 60% after 1 month. Further insight into the evolution of the position of the Fermi level and the microscopic device structure with different In thicknesses demonstrates an enhanced electron‐doping behavior at the In/InSe interface. Furthermore, the contact resistance is also improved through the In insertion between InSe and Au electrodes, which coincides with the analysis of the low‐frequency noise. The carrier fluctuation is attributed to the dominance of the phonon scattering events, which agrees with the observation of the temperature‐dependent mobility. Finally, the flexible functionalities of the logic‐circuit applications, for instance, inverter and not‐and (NAND)/not‐or (NOR) gates, are determined with these surface‐doping InSe FETs, which establish a paradigm for 2D‐based materials to overcome the bottleneck in the development of electronic devices.

Published in: "Advanced Materials".

High Mobilities in Layered InSe Transistors with Indium‐Encapsulation‐Induced Surface Charge Doping

2018-11-07T10:34:02+00:00November 7th, 2018|Categories: Publications|Tags: , |

A robust layered indium selenide (InSe) field‐effect transistor (FET) with superior high mobility (3700 cm2 V−1 s−1 at room temperature) is demonstrated by depositing an indium doping layer. With tunable carrier transport, the surface‐doped InSe FETs present flexible operations to realize various logic circuits, such as inverters and not‐or and not‐and gates. Abstract Tunability and stability in the electrical properties of 2D semiconductors pave the way for their practical applications in logic devices. A robust layered indium selenide (InSe) field‐effect transistor (FET) with superior controlled stability is demonstrated by depositing an indium (In) doping layer. The optimized InSe FETs deliver an unprecedented high electron mobility up to 3700 cm2 V−1 s−1 at room temperature, which can be retained with 60% after 1 month. Further insight into the evolution of the position of the Fermi level and the microscopic device structure with different In thicknesses demonstrates an enhanced electron‐doping behavior at the In/InSe interface. Furthermore, the contact resistance is also improved through the In insertion between InSe and Au electrodes, which coincides with the analysis of the low‐frequency noise. The carrier fluctuation is attributed to the dominance of the phonon scattering events, which agrees with the observation of the temperature‐dependent mobility. Finally, the flexible functionalities of the logic‐circuit applications, for instance, inverter and not‐and (NAND)/not‐or (NOR) gates, are determined with these surface‐doping InSe FETs, which establish a paradigm for 2D‐based materials to overcome the bottleneck in the development of electronic devices.

Published in: "Advanced Materials".

High Mobilities in Layered InSe Transistors with Indium‐Encapsulation‐Induced Surface Charge Doping

2018-11-07T00:35:18+00:00November 6th, 2018|Categories: Publications|Tags: , |

A robust layered indium selenide (InSe) field‐effect transistor (FET) with superior high mobility (3700 cm2 V−1 s−1 at room temperature) is demonstrated by depositing an indium doping layer. With tunable carrier transport, the surface‐doped InSe FETs present flexible operations to realize various logic circuits, such as inverters and not‐or and not‐and gates. Abstract Tunability and stability in the electrical properties of 2D semiconductors pave the way for their practical applications in logic devices. A robust layered indium selenide (InSe) field‐effect transistor (FET) with superior controlled stability is demonstrated by depositing an indium (In) doping layer. The optimized InSe FETs deliver an unprecedented high electron mobility up to 3700 cm2 V−1 s−1 at room temperature, which can be retained with 60% after 1 month. Further insight into the evolution of the position of the Fermi level and the microscopic device structure with different In thicknesses demonstrates an enhanced electron‐doping behavior at the In/InSe interface. Furthermore, the contact resistance is also improved through the In insertion between InSe and Au electrodes, which coincides with the analysis of the low‐frequency noise. The carrier fluctuation is attributed to the dominance of the phonon scattering events, which agrees with the observation of the temperature‐dependent mobility. Finally, the flexible functionalities of the logic‐circuit applications, for instance, inverter and not‐and (NAND)/not‐or (NOR) gates, are determined with these surface‐doping InSe FETs, which establish a paradigm for 2D‐based materials to overcome the bottleneck in the development of electronic devices.

Published in: "Advanced Materials".

Infrared-to-violet tunable optical activity in atomic films of GaSe, InSe, and their heterostructures. (arXiv:1810.01838v1 [cond-mat.mes-hall])

2018-10-04T04:30:23+00:00October 4th, 2018|Categories: Publications|Tags: , , , |

Two-dimensional semiconductors – atomic layers of materials with covalent intra-layer bonding and weak (van der Waals or quadrupole) coupling between the layers – are a new class of materials with great potential for optoelectronic applications. Among those, a special position is now being taken by post-transition metal chalcogenides (PTMC), InSe and GaSe. It has recently been found that the band gap in 2D crystals of InSe more than doubles in the monolayer compared to thick multilayer crystals, while the high mobility of conduction band electrons is promoted by their light in-plane mass. Here, we use Raman and PL measurements of encapsulated few layer samples, coupled with accurate atomic force and transmission electron microscope structural characterisation to reveal new optical properties of atomically thin GaSe preserved by hBN encapsulation. The band gaps we observe complement the spectral range provided by InSe films, so that optical activity of these two almost lattice-matched PTMC films and their heterostructures densely cover the spectrum of photons from violet to infrared. We demonstrate the realisation of the latter by the first observation of interlayer excitonic photoluminescence in few-layer InSe-GaSe heterostructures. The spatially indirect transition is direct in k-space and therefore is bright, while its energy can be tuned in a broad range by the number of layers.

Published : "arXiv Mesoscale and Nanoscale Physics".

Transformation of 2D group-III selenides to ultra-thin nitrides: enabling epitaxy on amorphous substrates

2018-09-28T10:33:37+00:00September 28th, 2018|Categories: Publications|Tags: , , |

The experimental realization of two-dimensional (2D) gallium nitride (GaN) has enabled the exploration of 2D nitride materials beyond boron nitride. Here we demonstrate one possible pathway to realizing ultra-thin nitride layers through a two-step process involving the synthesis of naturally layered, group-III chalcogenides (GIIIC) and subsequent annealing in ammonia (ammonolysis) that leads to an atomic-exchange of the chalcogen and nitrogen species in the 2D-GIIICs. The effect of nitridation differs for gallium and indium selenide, where gallium selenide undergoes structural changes and eventual formation of ultra-thin GaN, while indium selenide layers are primarily etched rather than transformed by nitridation. Further investigation of the resulting GaN films indicates that ultra-thin GaN layers grown on silicon dioxide act as effective ‘seed layers’ for the growth of 3D GaN on amorphous substrates.

Published in: "Nanotechnology".

Infrared-to-violet tunable optical activity in atomic films of GaSe, InSe, and their heterostructures

2018-09-25T12:33:45+00:00September 25th, 2018|Categories: Publications|Tags: , , |

Two-dimensional (2D) semiconductors—atomic layers of materials with covalent intra-layer bonding and weak (van der Waals or quadrupole) coupling between the layers—are a new class of materials with great potential for optoelectronic applications. Among those, a special position is now being taken by post-transition metal chalcogenides (PTMC), InSe and GaSe. It has recently been found (Bandurin et al 2017 Nat. Nanotechnol . 12 223–7) that the band gap in 2D crystals of InSe more than doubles in the monolayer compared to thick multilayer crystals, while the high mobility of conduction band electrons is promoted by their light in-plane mass. Here, we use Raman and PL measurements of encapsulated few layer samples, coupled with accurate atomic force and transmission electron microscope structural characterisation to reveal new optical properties of atomically thin GaSe preserved by hBN encapsulation. The band gaps we observe complement the spectral range provided…

Published in: "2DMaterials".

Gate-tunable weak antilocalization in a few-layer InSe

2018-09-18T16:33:13+00:00September 18th, 2018|Categories: Publications|Tags: |

Author(s): Junwen Zeng, Shi-Jun Liang, Anyuan Gao, Yu Wang, Chen Pan, Chenchen Wu, Erfu Liu, Lili Zhang, Tianjun Cao, Xiaowei Liu, Yajun Fu, Yiping Wang, Kenji Watanabe, Takashi Taniguchi, Haizhou Lu, and Feng MiaoIndium selenide (InSe) has attracted tremendous research interest due to its high mobility and potential applications in next-generation electronics. However, the underlying transport mechanism of carriers in thin InSe at low temperatures remains unknown. Here we report the gate voltage and temperat…[Phys. Rev. B 98, 125414] Published Tue Sep 18, 2018

Published in: "Physical Review B".

Charged impurity scattering in two-dimensional materials with ring-shaped valence bands: GaS, GaSe, InS, and InSe. (arXiv:1808.10853v1 [cond-mat.mes-hall])

2018-09-03T04:30:17+00:00September 3rd, 2018|Categories: Publications|Tags: |

The singular density of states and the two Fermi wavevectors resulting from a ring-shaped or “Mexican hat” valence band give rise to unique trends in the charged impurity scattering rates and charged impurity limited mobilities. Ring shaped valence bands are common features of many monolayer and few-layer two-dimensional materials including the III-VI materials GaS, GaSe, InS, and InSe. The wavevector dependence of the screening, calculated within the random phase approximation, is so strong that it is the dominant factor determining the overall trends of the scattering rates and mobilities with respect to temperature and hole density. Charged impurities placed on the substrate and in the 2D channel are considered. The different wavevector dependencies of the bare Coulomb potentials alter the temperature dependence of the mobilities. Moving the charged impurities 5 $AA$ from the center of the channel to the substrate increases the mobility by an order of magnitude.

Published : "arXiv Mesoscale and Nanoscale Physics".

Optical second harmonic generation in encapsulated single-layer InSe. (arXiv:1808.05874v1 [physics.optics])

2018-08-20T04:30:23+00:00August 20th, 2018|Categories: Publications|Tags: , , |

We report the observation of optical second harmonic generation (SHG) in single-layer indium selenide (InSe). We measure a second harmonic signal of $>10^3$ $textrm{cts/s}$ under nonresonant excitation using a home-built confocal microscope and a standard pulsed pico-second laser. We demonstrate that polarization-resolved SHG serves as a fast, non-invasive tool to determine the crystal axes in single-layer InSe and to relate the sharp edges of the flake to the armchair and zigzag edges of the crystal structure. Our experiment determines these angles to an accuracy better than $pm$ $0.2^{circ}$. Treating the two-dimensional material as a nonlinear polarizable sheet, we determine a second-order nonlinear sheet polarizability $| chi_{textrm{sheet}}^{(2)}|=(17.9 pm 11.0)times 10^{-20}$ $textrm{m}^2 textrm{V}^{-1}$ for single-layer InSe, corresponding to an effective nonlinear susceptibility value of $| chi_textrm{eff}^{(2)}| approx (223 pm 138) times 10^{-12}$ $textrm{m} textrm{V}^{-1}$ accounting for the sheet thickness ($textrm{d} approx 0.8$ $textrm{nm}$). We demonstrate that the SHG technique can also be applied to encapsulated samples to probe their crystal orientations. The method is therefore suitable for creating high quality van der Waals heterostructures with control over the crystal directions.

Published : "arXiv Mesoscale and Nanoscale Physics".

Enhanced Light Emission from the Ridge of Two-dimensional InSe Flakes. (arXiv:1807.08862v1 [cond-mat.mes-hall])

2018-07-25T04:30:22+00:00July 25th, 2018|Categories: Publications|Tags: |

InSe, a newly rediscovered two-dimensional (2D) semiconductor, possesses superior electrical and optical properties as a direct bandgap semiconductor with high mobility from bulk to atomically thin layers, drastically different from transition metal dichalcogenides (TMDCs) in which the direct bandgap only exists at the single layer limit. However, absorption in InSe is mostly dominated by an out-of-plane dipole contribution which results in the limited absorption of normally incident light which can only excite the in-plane dipole at resonance. To address this challenge, we have explored a unique geometric ridge state of the 2D flake without compromising the sample quality. We observed the enhanced absorption at the ridge over a broad range of excitation frequencies from photocurrent and photoluminescence (PL) measurements. In addition, we have discovered new PL peaks at low temperature due to defect states on the ridge, which can be as much as ~ 60 times stronger than the intrinsic PL peak of InSe. Interestingly, the PL of the defects is highly tunable through an external electrical field, which can be attributed to the Stark effect of the localized defects. InSe ridges thus provide new avenues for manipulating light-matter interaction and defect-engineering which are vitally crucial for novel optoelectronic devices based on 2D semiconductors.

Published : "arXiv Mesoscale and Nanoscale Physics".

Magnetotransport and lateral confinement in an InSe van der Waals Heterostructure

2018-06-21T10:34:16+00:00June 21st, 2018|Categories: Publications|Tags: , , , |

In the last six years, indium selenide (InSe) has appeared as a new van der Waals heterostructure platform which has been extensively studied due to its unique electronic and optical properties. Such as transition metal dichalcogenides (TMDCs), the considerable bandgap and high electron mobility can provide a potential optoelectronic application. Here we present low-temperature transport measurements on a few-layer InSe van der Waals heterostructure with graphene-gated contacts. For high magnetic fields, we observe magnetoresistance minima at even filling factors related to two-fold spin degeneracy. By electrostatic gating with negatively biased split gates, a one-dimensional channel is realized. Close to pinch-off, transport through the constriction is dominated by localized states with charging energies ranging from 2 to 5 meV. This work opens new possibility to explore the low-dimensional physics including quantum point contact and quantum dot.

Published in: "2DMaterials".

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