Hui-Xiong Deng

First-principles study of magnetic properties in Mo-doped graphene

The geometric structure, electronic structure and magnetic properties of substitutionally Mo-doped graphene are studied based on first-principles calculations. Mo introduces a magnetic moment of 2 μB in graphene. The magnetic properties and band structure can be well understood using a hybridization model. Magnetic coupling between two Mo impurities is also discussed. Depending on the relative position of the two Mo impurities, the ground state of the system can be ferromagnetic, antiferromagnetic or paramagnetic. A Ruderman-Kittel-Kasuya-Yosida (RKKY)-like behavior is observed when the distance between Mo atoms is relatively large. However, when the distance between Mo atoms is rather small, the RKKY model is not suitable to describe the magnetic ordering due to their non-neglectable direct interactions.

Flexible photodetectors based on phase dependent PbI2 single crystals

As a precursor of perovskites, lead iodide (PbI2) is a typical layered material with a direct bandgap. Perovskites are widely utilized in highly efficient photovoltaics, but the low-dimensional PbI2 nanostructures and their (opto)electronic properties are rarely reported. Herein, single-crystalline PbI2 nanosheets (phase I) and nanowires (phase II) are controllably synthesized via a facile physical vapor deposition method. Their different crystal morphology and crystallographic symmetry show obvious phase dependence. The corresponding photodetectors on both SiO2/Si and flexible polyethylene terephthalate (PET) substrates are investigated systematically. Compared with PbI2 nanowire based photodetectors, PbI2 nanosheet based photodetectors exhibit a relatively high sensitivity (with a high photoresponsivity of 147.6 A W−1 and fast response time) to the 450 nm laser. Both the PbI2 nanosheet and nanowire devices with flexible PET substrates exhibit comparable performance to their photodetectors fabricated on SiO2/Si, and also show excellent mechanical stability and durability. At the same time, the photoelectric properties vary greatly with different bending angles for such flexible PbI2 photodetectors. By modeling the band structures under different compressive strains, the theoretical simulations fit very well with experimental results. These findings provide a scientific basis for exploiting high-performance flexible photodetectors based on low-dimensional PbI2 single crystals.

Origin of Reduced Efficiency in Cu(In,Ga)Se

It is well known that adding Ga to CuInSe₂ forming CuIn_1-xGa_xSe₂ (CIGS) alloy can significantly improve the solar cell efficiency, but adding too much Ga will lead to a decline of the solar cell efficiency. The exact origin of this puzzling phenomenon is currently still under debate. It is especially unclear whether it is caused by either structural or electronic issues. In this paper, we conclude that the defect issue, especially antisite defects M_Cu (M = In, Ga), rather than the alloy solubility is the key problem for the reduced efficiency in CIGS. The deep levels that are induced by M_Cu defects can pin the open-circuit voltage (V_oc) of CIGS. Self-compensation in CIGS, which forms 2V_Cu + M Cu defect complexes, is found to be beneficial to quenching the deep-trap levels induced by M_Cu in CIGS. Unfortunately, the density of isolated M_Cu is quite high and cannot be largely converted into 2V_Cu + M_Cu complexes under thermal equilibrium condition. Thus, nonequilibrium growth conditions or low growth temperature that can suppress the formation of the deep-trap centers M_Cu will be necessary to improve the efficiency of CIGS solar cells, especially with high Ga concentrations.

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We have investigated the origin of antiferromagnetism of CoO in the rocksalt structure using spin-polarized density functional theory calculations. We find that in the rocksalt structure, the superexchange interaction between the occupied and unoccupied eg states plays the dominant role, which leads to an antiferromagnetic ground state, but the system also has a strong direct exchange interaction between the partially occupied minority spin t2g states that leads to the unusual situation that the ferromagnetic phase is more stable than most antiferromagnetic configurations.

Solar Cells With High Ga Concentration: Alloy Solubility Versus Intrinsic Defects

2D layered materials have attracted increasing interest, owing to their unique properties and large potential for versatile applications. As one of the 2D layered semiconductors, tin disulfide (SnS2) is rarely reported compared with other 2D materials like molybdenum disulfide (MoS2). Herein, high quality SnS2 flakes were grown by a facile and low-cost path, and photodetectors based on thin SnS2 flakes were fabricated and characterized. These flakes are of high quality according to the results of XRD, Raman and TEM measurements, and present hexagonal and half-hexagonal forms with an average diameter of 100 μm. The devices based on these SnS2 flakes showed wavelength dependent photo-responsive characteristics as the illuminating wavelength varied in the UV-Vis range (from 100 to 800 nm). They also showed excellent photo-responsive characteristics under monochromic illumination using three different wavelengths (533, 405 and 255 nm) with high photo-responsivity and high external quantum efficiency (EQE). The experimental results agree well with the first-principles calculated band structure and optical absorption coefficient curve.

Origin of antiferromagnetism in CoO: A density functional theory study

Efficient bandgap engineering is a significant strategy for the utilization of widely concerned two-dimensional (2D) layered materials in versatile devices such as nanoelectronics, optoelectronics, and photonics. Alloying transition-metal dichalcogenides (TMDs) with different components has been proved as a very effective way to get 2D nanostructured semiconductors with artificially designed tunable bandgaps. Here we report a systematically study of chemical vapor transport (CVT) grown SnSe2(1−x)S2x alloys with continuously bandgaps ranging from 1.37 eV (SnSe2) to 2.27 eV (SnS2). The carrier mobility of 2D SnSe2(1−x)S2x nanosheets can be tuned from 2.34 cm2 V−1 s−1 (SnS2) to 71.30 cm2 V−1 s−1 (SnSe2) by controlling the S composition in the alloy. Furthermore, the carrier mobility of SnSeS increase from 10.34 to 12.16 cm2 V−1 s−1 under illumination, showing excellent optoelectronic properties. Our study suggests that SnSe2(1−x)S2x nanosheets is a highly qualified 2D materials for next-generation nanoelectronics and optoelectronics application.

Wavelength dependent UV-Vis photodetectors from SnS2 flakes

Deep level defects are usually harmful to solar cells. Here we show that incorporation of selected deep level defects in the carrier-collecting region, however, can be utilized to improve the efficiency of optoelectronic devices. The designed defects can help the transport of the majority carriers by creating defect levels that are resonant with the band edge state, and/or reduce the concentration of minority carriers through Coulomb repulsion, thus suppressing the recombination at the carrier-collecting region. The selection process is demonstrated by using Si solar cell as an example. Our work enriches the understanding and utilization of the semiconductor defects.

Composition-tunable 2D SnSe2(1−x)S2x alloys towards efficient bandgap engineering and high performance (opto)electronics

Current thermoelectric (TE) materials often have low performance or contain less abundant and/or toxic elements, thus limiting their large-scale applications. Therefore, new TE materials with high efficiency and low cost are strongly desirable. Here we demonstrate that SiS and SiSe monolayers made from nontoxic and earth-abundant elements intrinsically have low thermal conductivities arising from their low-frequency optical phonon branches with large overlaps with acoustic phonon modes, which is similar to the state-of-the-art experimentally demonstrated material SnSe with a layered structure. Together with high thermal power factors due to their two-dimensional nature, they show promising TE performances with large figure of merit (ZT) values exceeding 1 or 2 over a wide range of temperatures. We establish some basic understanding of identifying layered materials with low thermal conductivities, which can guide and stimulate the search and study of other layered materials for TE applications.

Suppress carrier recombination by introducing defects: The case of Si solar cell

We have studied the magnetic properties of Co (2–12 MLs)/L10-Mn1.5Ga (15 nm) bilayers without and with annealing at 300 °C by a combination of superconducting quantum interference device (SQUID) magnetometry and x-ray magnetic circular dichroism (XMCD). We find that the Co layer can remain perpendicularly magnetized when its thickness is less than six monolayers due to the coupling between Co and L10-Mn1.5Ga layers, which is doubly confirmed by both SQUID and XMCD measurements. Such an exchange coupling between L10-Mn1.5Ga and Co layers changes from ferromagnetic coupling to antiferromagnetic coupling after the annealing process. Furthermore, the magnetic coupling can also be tailored from ferromagnetic to antiferromagnetic by changing the L10-Mn1.5Ga surface from Mn-rich to Ga-rich. The first-principles calculations show that the interfacial coupling type is ferromagnetic for a Mn-terminated L10-Mn1.5Ga bilayer, while antiferromagnetic for a Ga-terminated bilayer. The spin and orbital moments of Co in the Co/L10-Mn1.5Ga bilayers are calculated according to the sum rules and the ratio of the orbital to spin magnetic moments for Co is not enhanced like other perpendicularly magnetized Co-based multilayers such as Co/Pd and Co/Pt.

Earth-Abundant and Non-Toxic SiX (X = S, Se) Monolayers as Highly Efficient Thermoelectric Materials

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.