Showing papers by "Jingbo Li published in 2018"

Universality of electronic characteristics and photocatalyst applications in the two-dimensional Janus transition metal dichalcogenides

Abstract: Due to mirror symmetry breaking, two-dimensional Janus transition metal dichalcogenides $MXY$ ($M=\text{Mo,W}$; $X,Y=\text{S,Se,Te}$) present charming electronic properties. However, there have not been many related studies as of yet, and the intrinsic physical pictures are unclear. Here, we use first-principles calculations to explore the universality of electronic characteristics and photocatalyst applications of Janus $MXY$, finding that the induced dipole moment, vibrational frequency, Rashba parameters, and direct-indirect band transition of monolayer $MXY$ are deeply associated with the atomic radius and electronegativity differences of chalcogen $X$ and $Y$ elements. The internal electric field renders Janus $MXY$ the ideal photocatalysts. Moreover, the stacking-dependent on/off switch of the dipole moment further confirms that asymmetric Janus $MXY$ serves as a promising candidate for highly efficient photocatalysts within a broad range from infrared, visible, to ultraviolet light.

Perpendicular Optical Reversal of the Linear Dichroism and Polarized Photodetection in 2D GeAs.

Abstract: The ability to detect linearly polarized light is central to practical applications in polarized optical and optoelectronic fields and has been successfully demonstrated with polarized photodetection of in-plane anisotropic two-dimensional (2D) materials. Here, we report the anisotropic optical characterization of a group IV-V compound-2D germanium arsenic (GeAs) with anisotropic monoclinic structures. High-quality 2D GeAs crystals show the representative angle-resolved Raman property. The in-plane anisotropic optical nature of the GeAs crystal is further investigated by polarization-resolved absorption spectra (400-2000 nm) and polarization-sensitive photodetectors. From the visible to the near-infrared range, 2D GeAs nanoflakes demonstrate the distinct perpendicular optical reversal with a 75-80° angle on both the linear dichroism and polarization-sensitive photodetection. Obvious anisotropic features and the high dichroic ratio of Ipmax /Ipmin ∼ 1.49 at 520 nm and Ipmax /Ipmin ∼ 4.4 at 830 nm are achieved by the polarization-sensitive photodetection. The polarization-dependent photocurrent mapping implied that the polarized photocurrent mainly occurred at the Schottky photodiodes between electrode/GeAs interface. These experimental results are consistent with the theoretical calculation of band structure and band realignment. Besides the excellent polarization-sensitive photoresponse properties, GeAs-based photodetectors also exhibit rapid on/off response. These results demonstrate that the 2D GeAs crystals have promising potential for polarization optical applications.

Two-dimensional n -InSe/ p -GeSe(SnS) van der Waals heterojunctions: High carrier mobility and broadband performance

Abstract: Recently, constructing van der Waals (vdW) heterojunctions by stacking different two-dimensional (2D) materials has been considered to be effective strategy to obtain the desired properties. Here, through first-principles calculations, we find theoretically that the 2D $n$-InSe/$p$-GeSe(SnS) vdW heterojunctions are the direct-band-gap semiconductor with typical type-II band alignment, facilitating the effective separation of photogenerated electron and hole pairs. Moreover, they possess the high optical absorption strength ($\ensuremath{\sim}{10}^{5}$), broad spectrum width, and excellent carrier mobility ($\ensuremath{\sim}{10}^{3}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$). Interestingly, under the influences of the interlayer coupling and external electric field, the characteristics of type-II band alignment is robust, while the band-gap values and band offset are tunable. These results indicate that 2D $n$-InSe/$p$-GeSe(SnS) heterojunctions possess excellent optoelectronic and transport properties, and thus can become good candidates for next-generation optoelectronic nanodevices.

Tunable electronic and optical properties of InSe/InTe van der Waals heterostructures toward optoelectronic applications

Abstract: Forming novel van der Waals (vdW) heterostructures by combining different two-dimensional (2D) materials is significant to achieve more desirable properties. Using first-principles calculations, we demonstrate the electronic and optical properties of the InSe/InTe van der Waals heterostructure. Our results suggest that this heterostructure has an intrinsic type-II band alignment with a direct band gap. The electrons and holes are respectively localized in the InSe and InTe layers. The spatial separation of the lowest energy electron–hole pairs can occur, implying that the InSe/InTe heterostructure is a good candidate for a high-efficiency solar cell. In addition, the optical absorption in heterostructures can be enhanced compared with both of the monolayers. Moreover, tuning of the values with a direct band gap can be induced by applying normal strain, and the band gap exhibits linear variation. Meanwhile, an intrinsic type-II band alignment can be tuned to become type-I, which is suitable for light emission applications. These results indicate that the flexible InSe/InTe vdW heterostructure can provide new ways to utilize two-dimensional materials in future optoelectronic devices.

Highly polarization sensitive photodetectors based on quasi-1D titanium trisulfide (TiS3)

Abstract: Photodetectors with high polarization sensitivity are in great demand in advanced optical communication. Here, we demonstrate that photodetectors based on titanium trisulfide (TiS3) are extremely sensitive to polarized light (from visible to the infrared), due to its reduced in-plane structural symmetry. By density functional theory calculation, TiS3 has a direct bandgap of 1.13 eV. The highest photoresponsivity reaches 2500 A W−1. What is more, in-plane optical selection caused by strong anisotropy leads to the photoresponsivity ratio for different directions of polarization that can reach 4:1. The angle-dependent photocurrents of TiS3 clearly display strong linear dichroism. Moreover, the Raman peak at 370 cm−1 is also very sensitive to the polarization direction. The theoretical optical absorption of TiS3 is calculated by using the HSE06 hybrid functional method, in qualitative agreement with the observed experimental photoresponsivity.

Chemical vapor deposition growth of two-dimensional heterojunctions

Abstract: The properties of two-dimensional (2D) layered materials with atom-smooth surface and special interlayer van der Waals coupling are different from those of traditional materials. Due to the absence of dangling bonds from the clean surface of 2D layered materials, the lattice mismatch influences slightly on the growth of 2D heterojunctions, thus providing a flexible design strategy. 2D heterojunctions have attracted extensive attention because of their excellent performance in optoelectronics, spintronics, and valleytronics. The transfer method was utilized for the fabrication of 2D heterojunctions during the early stage of fundamental research on these materials. This method, however, has limited practical applications. Therefore, chemical vapor deposition (CVD) method was recently developed and applied for the preparation of 2D heterojunctions. The CVD method is a naturally down-top growth strategy that yields 2D heterojunctions with sharp interfaces. Moreover, this method effectively reduces the introduction of contaminants to the fabricated heterojunctions. Nevertheless, the CVD-growth method is sensitive to variations in growth conditions. In this review article, we attempt to provide a comprehensive overview of the influence of growth conditions on the fabrication of 2D heterojunctions through the direct CVD method. We believe that elucidating the effects of growth conditions on the CVD method is necessary to help control and improve the efficiency of the large-scale fabrication of 2D heterojunctions for future applications in integrated circuits.

Type-II InSe/MoSe2(WSe2) van der Waals heterostructures: vertical strain and electric field effects

Abstract: Two-dimensional (2D) InSe, a new graphene-like semiconducting material, is gaining significant attention due to its particular optoelectronic characteristics. However, the studies of InSe-based van der Waals heterostructures (vdWHs) are in its initial stages, which limits its applications in future nano-devices. In this work, we utilize first-principles calculations to investigate 2D InSe/MoSe2(WSe2) vdWHs considering stacking configurations, vertical strain and electric field effects. The results show that the most stable 2D InSe/MoSe2(WSe2) vdWHs possess an intrinsic type-II band alignment. Additionally, the vertical strain not only tunes the band gap but also induces the band alignment transition from type-II to type-I in the vdWHs. Besides, application of external electric field can also transfer the intrinsic type-II band alignment to type-I or type-III. This work predicates the feasibility of 2D InSe/MoSe2(WSe2) vdWHs and extends the applications of 2D InSe materials in the field of optoelectronics.

Effects of Electric Field on the Electronic Structures of Broken-gap Phosphorene/Sn X 2 (X = S , Se ) van der Waals Heterojunctions

Abstract: Custom-made stacks of two-dimensional materials can be constructed to achieve specific optoelectronic properties in devices. This work uses density functional theory to investigate the electronic structures and transport properties of phosphorene--tin-dichalcogenide van der Waals heterostructures in an external electric field. The results show that these systems exhibit rare type-III (broken-gap) band alignment, which will facilitate the development of tunnel field-effect transistors. Moreover, the band alignment can be tuned effectively by the applied electric field.

Tin dioxide quantum dots coupled with graphene for high-performance bulk-silicon Schottky photodetector

Abstract: Commercial photodetectors have been dominated by bulk silicon (B-Si) due to the maturity of Si technology. However, its relatively poor mobility has impeded B-Si from high-performance applications. Herein, we demonstrate that tin dioxide quantum dots (SnO2-QDs) coupled with graphene produce a Schottky junction with B-Si to drastically promote the performance of the SnO2-QDs/graphene/B-Si Schottky photodetector. This hybrid device is sensitive to broadband illumination covering the UV-vis-NIR region and shows high responsivity of 967.6 A W−1 (nearly 4 orders higher than that of commercial B-Si Schottky photodetectors), with corresponding external quantum efficiency of 2.3 × 105% and detectivity of 1.8 × 1013 Jones. In addition, the hybrid device manifests fast rise and decay times of 0.1 and 0.23 ms, respectively. These figures-of-merit are among the best values of the recently reported B-Si Schottky photodetectors. We also established that the superior performances are attributed to the strong light absorption of the hybrid structure and increased built-in potential of the graphene/B-Si Schottky junction, which allows efficient separation of photoexcited electron–hole pairs. These findings pave the way toward the rational design of optoelectronic devices through the synergetic effects of 2D materials with 0D and 3D semiconductors.

Hydrothermal One-Step Synthesis of Highly Dispersed M-Phase VO2 Nanocrystals and Application to Flexible Thermochromic Film.

Abstract: Preparation of ultrafine highly dispersed VO2(M) nanoparticles that are essential materials to fabricate thermochromic flexible films remains a challenge, preventing effective use of their promising properties. Here, we report an original hydrothermal approach by controlling oxidizing atmosphere of reaction with hydrogen peroxide to prepare ultrafine VO2(M) nanoparticles free from annealing. Hydrogen peroxide is separated from precursor solution in a reactor, which creates a moderate oxygenation environment, enabling the formation of stoichiometric VO2(M) nanoparticles. The obtained VO2(M) nanoparticles are well-dispersed, highly uniform, and single-phase, with an average particle size ∼30 nm. The flexible thermochromic films fabricated with the VO2(M) nanoparticles exhibit excellent thermochromic performance with a solar modulation efficiency of 12.34% and luminous transmittance of 54.26%. While the films prepared with annealed nanoparticles show reduced transmittance due to light scattering of the large size particles resulting from agglomeration and growth during annealing. This work demonstrates a promising technique to realize moderate oxidizing atmosphere by hydrothermal process for preparing well-dispersed stoichiometric nano-oxides.

Highly anisotropic solar-blind UV photodetector based on large-size two-dimensional α-MoO 3 atomic crystals

Abstract: Orthorhombic MoO3 (α-MoO3) is a typical layered n-type semiconductor with optical band gap over 2.7 eV, which have been widely studied in catalysis, gas sensing, lithium-ion batteries, field-emission, photoelectrical, photochromic and electrochromic devices, supercapacitors and organic solar cells. However, the bottleneck of generation large size atomic thin two-dimensional (2D) α-MoO3 crystals remain challenging this field (normally several micrometers size). Herein, we developed a facile vapor–solid (VS) process for controllable growth of large-size 2D α-MoO3 single crystals with a few nanometers thick and over 300 μm in lateral size. High-performance solar-blind photodetectors were fabricated based on individual 2D α-MoO3 single crystal. The detectors demonstrate outstanding optoelectronic properties under solar-blind UV light (254 nm), with a photoresponsivity of 67.9 A W−1, external quantum efficiency of 3.3 × 104%. More important, the devices showed strong in-plane anisotropy in optoelectronic response and transport properties, e.g. the photocurrent along b-axis was found to be 5 times higher than the values along c-axis under 254 nm UV light, and current ON/OFF ratio and mobility anisotropy is about 2 times high. Our work suggests an optimized synthesis routine for 2D crystals, and the great potential of 2D oxides in functional optoelectronics.

Neat Design for the Structure of Electrode To Optimize the Lithium-Ion Battery Performance

Abstract: The appearance of mechanical cracks originated from anisotropic expansion and shrinkage of electrode particles during Li+ de/intercalation is a major cause of the capacity fading in Li-ion batterie...

Large tunneling magnetoresistance in magnetic tunneling junctions based on two-dimensional CrX3 (X = Br, I) monolayers.

Abstract: Magnetic tunneling junctions (MTJs) have atomic thickness due to the use of two-dimensional (2D) materials. Combining density functional theory with non-equilibrium Green's function formalism, we systematically investigate the structural and magnetic properties of CrX3/h-BN/CrX3 (X = Br, I) MTJs, as well as their spin-dependent transport characteristics. Through calculation of the transmission spectrum, the large tunneling magnetoresistance (TMR) effect was observed in these MTJs. Moreover, their conductance based on two-dimensional materials was greatly improved over that of traditional MTJs. The transmission mechanism was analyzed using the symmetry of the orbit and the eigenstates of the transmitted electrons. We also discuss the problem of Schottky contact between different metal electrodes and devices. Our results suggest that MTJs based on two-dimensional ferromagnets are feasible.

Type-I Transition Metal Dichalcogenides Lateral Homojunctions: Layer Thickness and External Electric Field Effects.

Abstract: Transition metal dichalcogenide (TMD) heterostructures have been widely explored due to the formation of type-II band alignment and interlayer exciton. However, the studies of type-I TMD heterostructures are still lacking, which limit their applications in luminescence devices. Here, the 1L/nL MX2 (n = 2, 3, 4; M = Mo, W; X = S, Se) lateral homojunction based on the layer-dependent band gaps of TMD nanosheets is theoretically simulated. The studies show that the TMD homojunction presents with high thermal stability and type-I band alignment. The band offset and quantum confinement of carriers can be easily tuned by controlling the thickness of the multilayer region. Moreover, the electric field can decrease the band gaps of 1L/3L and 1L/4L homojunctions linearly. Interestingly, for the 1L/2L MX2 homojunction, the gap value is robust to the weak electric field, while it drops sharply under a strong electric field. This study sheds light on the physical pictures in the TMD lateral homojunction, and provides a practicable and general approach to engineer a type-I homojunction based 2D semiconductor materials.

Narrow-gap physical vapour deposition synthesis of ultrathin SnS1−xSex (0 ≤ x ≤ 1) two-dimensional alloys with unique polarized Raman spectra and high (opto)electronic properties

Abstract: Here we report ultrathin SnS1−xSex alloyed nanosheets synthesized via a narrow-gap physical vapour deposition approach. The SnS1−xSex alloy presents a uniform quadrangle shape with a lateral size of 5–80 μm and a thickness of several nanometers. Clear orthorhombic symmetries and unique in-plane anisotropic properties of the 2D alloyed nanosheets were found with the help of X-ray diffraction, high resolution transmission electron microscopy and polarized Raman spectroscopy. Moreover, 2D alloyed field-effect transistors were fabricated, exhibiting a unipolar p-type semiconductor behavior. This study also provided a lesson that the thickness of the alloyed channels played the major role in the current on/off ratio, and the high ratio of 2.10 × 102 measured from a large ultrathin SnS1−xSex device was two orders of magnitude larger than that of previously reported SnS, SnSe nanosheet based transistors because of the capacitance shielding effect. Obviously enhanced Raman peaks were also found in the thinner nanosheets. Furthermore, the ultrathin SnS0.5Se0.5 based photodetector showed a highest responsivity of 1.69 A W−1 and a short response time of 40 ms under illumination of a 532 nm laser from 405 to 808 nm. Simultaneously, the corresponding highest external quantum efficiency of 392% and detectivity of 3.96 × 104 Jones were also achieved. Hopefully, the narrow-gap synthesis technique provides us with an improved strategy to obtain large ultrathin 2D nanosheets which may tend to grow into thicker ones for stronger interlayer van der Waals forces, and the enhanced physical and (opto)electrical performances in the obtained ultrathin SnS1−xSex alloyed nanosheets prove their great potential in the future applications for versatile devices.

Out of plane stacking of InSe-based heterostructures towards high performance electronic and optoelectronic devices using a graphene electrode

Abstract: Due to their tailored energy band alignments, the integration of two-dimensional materials with out of plane stacking structures provides unprecedented opportunities to fabricate novel electronic and optoelectronic devices. Here, we report the vertical integration of Au–InSe–graphene and graphene–InSe/WSe2–graphene heterostructures to achieve superior properties. The InSe/graphene heterostructure shows a large current density up to 1646 A cm−2, which makes it a potential candidate for high current flexible devices to enable three dimensional integration. Meanwhile, the graphene–InSe/WSe2–graphene heterostructure exhibits a high rectification ratio of 103, a decent responsivity of 83 A W−1, and a superior detectivity of 1.55 × 1012 Jones, simultaneously. Theoretical calculation indicates that the superior device performance can be attributed to the type II band alignment of InSe and WSe2, which enables efficient separation of photo-generated electron–hole pairs. This study paves the way for the facile fabrication of various functional heterostructures for next-generation electronic and optoelectronic applications.

Fabrication of a high performance ZnIn2S4/Si heterostructure photodetector array for weak signal detection

Abstract: Owing to their exciting electronic and optical attributes, layered materials have attracted great interest in the field of next-generation photodetectors. However, owing to their large dark current, low detectivity and small signal-to-noise ratio, the performance of photodetectors based merely on layered materials is unsatisfactory for use in weak signal detection. Integrating layered materials with mature silicon (Si) technology offers a feasible scenario to overcome these drawbacks. Herein, we report the facile synthesis of layered ZnIn2S4 nanosheets and the construction of a ZnIn2S4/Si heterostructure photodetector array for weak signal detection. Owing to the interfacial charge transfer, which significantly suppresses the dark current and accelerates the separation of the photoexcited electron–hole pairs, the fabricated ZnIn2S4/Si photodetector array presents an ultralow dark current (18 pA), superior signal-to-noise ratio (11 572), stable photoswitching and high detectivity (2 × 1012 Jones). Notably, the heterostructure device exhibits an outstanding weak signal detection capability, which has been successfully demonstrated using the detection of weak light sources including a cell phone screen, flashlight and lighter. These results demonstrate that the synergetic effect of layered materials and mature semiconductor technology shows great potential for application in next-generation optoelectronics.

PtSe2/graphene hetero-multilayer: gate-tunable Schottky barrier height and contact type.

Abstract: Graphene-based two-dimensional hybrid materials are attracting significant attention because they can preserve novel characteristics of Dirac cone. Here, based on first-principles methods, we focus on the electronic characteristics of PtSe2/graphene hetero-multilayer. The negative binding energies indicate that the hybrid materials can be fabricated easily in practice. Also, the n-type Schottky contact is formed and its barrier height is robust to the number of graphene layer. Moreover, the gate-voltage can effectively induce the Schottky barrier transformation from n-type to p-type and contact type transformation from Schottky to Ohmic in the PtSe2/graphene hetero-multilayer. Thus, the work demonstrates that the graphene stacking configuration and gate-voltage will tune the electronic characteristics of PtSe2/graphene-based nanodevices.

Porous layer assembled hierarchical Co3O4 as anode materials for lithium-ion batteries

Abstract: The flower-like Co3O4 particles with three-dimensional structure have been achieved by inheriting the flower-like framework of β-Co(OH)2 particles fabricated by a facile solvothermal method without any surfactant. The obtained Co3O4 microflower, which was composed of large amounts of self-assembled porous ultrathin nanosheets, exhibited excellent electrochemical performances in terms of high specific capacity and good cycle stability when being evaluated as anode materials for lithium-ion battery. Specifically, a high reversible capacity of above 1100 mA h g−1 was achieved after 50 cycles at the current density of 296 mA g−1. Hierarchical flower-like structure with mesoporous was considered as providing more active sites for Li+ insertion and paths for transport of Li+, which led to faster lithium-ion diffusion. Co3O4 porous flower-like nanostructures possessed significant potential application in energy storage systems.

Tunable electric properties of bilayer InSe with different interlayer distances and external electric field

Abstract: Using density functional theory we explore the band structure of bilayer Indium selenide (InSe), and we find that the van der Waals interaction has significant effects on the electric and optical properties. We then explore the tuning electronic properties by different interlayer distances and by an external vertical electric field. Our results demonstrate that the band gaps of bilayer InSe can be continuously tuned by different interlayer coupling. With decreasing interlayer distances, the tunable band gaps of bilayer decrease linearly, owing to the enhancement of the interlayer interaction. Additionally, the band structure of bilayer InSe under external vertical fields is discussed. The presence of a small external electric field can make a new spatial distribution of electron–hole pairs. A well separation based on the electrons and holes, localized in different layers can be obtained using this easy method. These properties of bilayer InSe indicates potential applications in designing new optoelectronic devices.

The synthesis of FeCoS2 and an insight into its physicochemical performance

Abstract: In this work, FeCoS2 was synthesised via a modulated hydrothermal method. With the hydrothermal temperature being increased, the microstructure of the as-obtained FeCoS2 evolved from the initial ultrathin nanosheets to the final microspheres, which were also assembled from these nanosheets. Meanwhile, X-ray diffraction results illustrated that the degree of the [101] preferred orientation was gradually reduced with increasing hydrothermal temperature. When being evaluated as a photocatalyst toward the photodegradation of methylene blue, FeCoS2 presented itself as a promising candidate for the photocatalytic degradation. When the as-obtained FeCoS2 was tested as an anode material for Na-ion batteries, it could deliver a first discharge capacity of 806 mA h g−1 at 50 mA g−1. In addition, the magnetic properties of the temperature-dependent FeCoS2 products were also investigated, demonstrating that FeCoS2 nanosheets obtained at 170 °C exhibited the most evident ferromagnetic characteristics. The current research illustrates FeCoS2's potential for diverse applications in modern materials science and devices.

Gate-tunable and high optoelectronic performance in multilayer WSe2 P–N diode

Abstract: Two-dimensional (2D) material-based P–N junctions are unique building blocks for next-generation optoelectronics with the advantages of atomic thinness, flexibility and low cost. However, their device performances are limited due to either weak photo-absorption or low-quality junctions. Herein, we demonstrate a high-quality homogeneous and seamless P–N junction based on multilayer WSe2 instead of the reported monolayer structure or van der Waals junction and present its optoelectronic applications as a diode, photovoltaic cell and photodetector with high performances. Our WSe2 P–N junction exhibits ideal diode behaviour with an ideal factor close to 1 and rectifying ratio of 105. It also shows significant photovoltaic properties with VOC of ∼0.8 V and PCE of up to 6.2% under 635 nm light illumination. As a photodiode, it produces the maximum responsivity of 486 mA W−1, external quantum efficiency of ∼90%, fast speed below 4 ms and broadband spectral coverage up to the near-infrared region. This study demonstrates a high-performance 2D-based P–N diode as multifunctional optoelectronics in a single device, which holds promise to revolutionize electronic and optoelectronic technology.