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Zhu, Heyuan

Publications and source records attributed to Zhu, Heyuan.

6 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

High thermoelectric efficiency in monolayer PbI 2 from 300 K to 900 K

By using a first-principles approach, monolayer PbI 2 is found to have great potential in thermoelectric applications. The linear Boltzmann transport equation is applied to obtain the perturbation to the electron distribution by different scattering mechanisms. The mobility is mainly limited by the deformation-potential interaction with long-wavelength acoustic vibrations at low carrier concentrations. At high concentrations, ionized impurity scattering becomes stronger. The electrical conductivity and Seebeck coefficient are calculated accurately over various ranges of temperature and carrier concentration. The lattice thermal conductivity of PbI 2 , 0.065 W/mK at 300 K, is the lowest among other 2D thermoelectric materials. Such ultralow thermal conductivity is attributed to large atomic mass, weak interatomic bonding, strong anharmonicity, and localized vibrations in which the vast majority of heat is trapped. These electrical and phonon transport properties enable high thermoelectric figure of merit over 1 for both p-type and n-type doping from 300 K to 900 K. A maximum zT of 4.9 is achieved at 900 K with an electron concentration of 1.9X10 12 cm -2 . Our work shows exceptionally good thermoelectric energy conversion efficiency in monolayer PbI 2 , which can be integrated to the existing photovoltaic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2018

The role of Anderson’s rule in determining electronic, optical and transport properties of transition metal dichalcogenide heterostructures

Two-dimensional (2D) transition metal dichalcogenides (TMDs) MX 2 (M = Mo, W; X = S, Se, Te) possess unique properties and novel applications in optoelectronics, valleytronics and quantum computation. In this work, we performed first-principles calculations to investigate the electronic, optical and transport properties of the van der Waals (vdW) stacked MX 2 heterostructures formed by two individual MX 2 monolayers. Here, we found that the so-called Anderson's rule can effectively classify the band structures of heterostructures into three types: straddling, staggered and broken gap. The broken gap is gapless, while the other two types possess direct (straddling, staggered) or indirect (staggered) band gaps. The indirect band gaps are formed by the relatively higher energy level of Te-d orbitals or the interlayer couplings of M or X atoms. For a large part of the formed MX2 heterostructures, the conduction band maximum (CBM) and valence band minimum (VBM) reside in two separate monolayers, thus the electron–hole pairs are spatially separated, which may lead to bound excitons with extended lifetimes. The carrier mobilities, which depend on three competitive factors, i.e. elastic modulus, effective mass and deformation potential constant, show larger values for electrons of MX 2 heterostructures compared to their constituent monolayers. Finally, the calculated optical properties reveal strong absorption in the ultraviolet region.

36 MATERIALS SCIENCE↗
DOE OSTI2018

1D SbSeI, SbSI, and SbSBr With High Stability and Novel Properties for Microelectronic, Optoelectronic, and Thermoelectric Applications

Mechanical exfoliation of 2D materials has triggered an explosive interest in lowdimensional material research. We extend this idea to 1D van derWaals materials. Three 1D semiconductors (SbSeI, SbSI and SbSBr) with high stability and novel electronic properties are discovered using first principles calculations. Both the dynamical and the thermal stability of these 1D materials are examined. We demonstrate that their nanowire thinner than 7 Å can be easily obtained by mechanical exfoliation, hydrothermal method or sonochemical method. The bulk-to-1D transition results in dramatic changes in band gap, effective mass and static dielectric constant due to quantum confinement, making 1D SbSeI a highly promising channel material for transistors with gate length shorter than 1 nm. Under small uniaxial strain, these materials are transformed from indirect into direct band gap semiconductors, paving the way for optoelectronic devices and mechanical sensors. Moreover, the thermoelectric performance of these materials is significantly improved over their bulk counterparts. These highly desirable properties render SbSeI, SbSI and SbSBr promising 1D materials for applications in future microelectronics, optoelectronics, mechanical sensors, and thermoelectrics.

36 MATERIALS SCIENCE↗
DOE OSTI2017

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

The 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, revealing a direct band gap of 1.61 eV for monolayer α-GeSe and an indirect band gap of 2.47 eV for monolayer β-GeSe. For monolayer β-GeSe, the electronic/hole transport is anisotropic, with an extremely high electron mobility of 2.93×104cm2/Vs along the armchair direction, comparable to that of black phosphorene. However, for β-GeSe, robust band gaps nearly independent of the applied tensile strain along the armchair direction are observed. Both monolayer α- and β-GeSe exhibit anisotropic optical absorption in the visible spectrum.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Chemical intuition for high thermoelectric performance in monolayer black phosphorus, α-arsenene and aW-antimonene

Identifying materials with intrinsically high thermoelectric performance remains a challenge even with the aid of a high-throughput search. Here, using a chemically intuitive approach based on the bond-orbital theory, three anisotropic 2D group-V materials (monolayer black phosphorus, α-arsenene, and aW-antimonene) are identified as candidates for high thermoelectric energy conversion efficiency. Concepts, such as bond length, bond angle, and bond strength, are used to explain the trends in their electronic properties, such as the band gap and the effective mass. Our first principles calculations confirm that high carrier mobilities and large Seebeck coefficients can be obtained at the same time in these materials, due to complex Fermi surfaces originating from the anisotropic structures. An intuitive understanding of how the bonding character affects phonon transport is also provided with emphasis on the importance of bonding strength and bond anharmonicity. High thermoelectric performance is observed in these materials. In conclusion, our approach provides a powerful tool to identify new thermoelectric materials and evaluate their transport properties.

30 DIRECT ENERGY CONVERSION↗