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单晶LaB6是一种理想的热发射和场发射阴极材料，其不同晶面表现出不同的发射性能.采用基于密度泛函理论的第一性原理计算分析了LaB6单晶的（100），（110），（111），（210），（211）和（310）典型晶面的差分电子密度、能带结构和态密度，并对光学区熔法制备的高质量单晶LaB6的上述典型晶面的热发射性能进行了测试.理论计算结果表明LaB6各晶面结构的不同和电子结构的差异导致LaB6发射性能具有各向异性，晶面内La原子的密度越大、费米能级进入导带越深、费米能级附近态密度越大及其在导带区域的分布宽度越宽、导带在费米能级附近分布越多，晶面的逸出功越低，发射性能越好.热发射测试结果表明，当阴极测试温度为1773 K，测试电压为1 kV时，（100），（110），（111），（210），（211）和（310）晶面的最大发射电流密度分别为42.4，36.4，18.4，32.5，30.5和32.2 A/cm2，其中（100）晶面具有最佳的发射性能.The electron emission properties of lanthanum hexaboride (LaB6) have received much attention because its low work function, low volatility, high brightness, thermal stability and high mechanical strength. However, single crystal LaB6 is an ideal thermionic emission and field emission cathode material, its different crystal surfaces exhibit different emission properties. So far the physical factors of the emission properties of different crystal surfaces of LaB6 single crystal have been rarely reported. In this paper, the density function theory based first-principles calculations are used to analyze the electron density differences, band structures and densities of states of the typical LaB6 (100), (110), (111), (210), (211) and (310) surfaces, and the thermionic emission properties of the high-quality single crystal LaB6 typical surfaces are tested. The theoretical calculation results show that single crystal LaB6 has metal properties, electron emission characteristics and anisotropy of emission performance which are mainly caused by different crystal structures and electronic structures of LaB6 typical surfaces. The densities of La atoms in different surfaces of LaB6 single crystal are different, and a high density of La atoms in a surface is beneficial to its emission performance. The difference between relative positions for the Fermi level of different surfaces has different effect on their emission performance, and a surface with high position of Fermi level against the bottom of conduction band could have small work function and good emission performance. In addition, a surface structure of single crystal LaB6 has a large density of states and a high number of distributions of conduction band near the Fermi level, which are in favor of its electron emission. The (100) surface of single crystal LaB6 with the highest density of La atoms and electronic structure in favor of electron emission could have optimal electron emission performance compared with the remaining crystal surfaces. Thermionic emission test results show that maximum emission current densities of the (100), (110), (111), (210), (211) and (310) surfaces are 42.4, 36.4, 18.4, 32.5, 30.5 and 32.2 A/cm2 at the cathode temperature 1773 K and the voltage 1 kV. The (100) surface of LaB6 single crystal has a maximum emission current density under the same test condition, meaning that this surface has a smallest work function and best emission property compared with the other crystal surface. The thermionic emission test results show that the actual performances are basically accordant with the calculated results, demonstrating that the first principle calculation could provide a good theoretical guidance for studying the electron emission properties of rare earth hexaborides (REB6) and other cathode materials.
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[1] Duan J, Zhou T, Zhang L, Du J G, Jiang G, Wang H B 2015 Chin. Phys. B 24 096201
[2] Zhang H, Tang J, Yuan J, Yamauchi Y, Suzuki T T, Shinya N, Nakajima K, Qin L C 2016 Nat. Nanotechnol. 11 273
[3] Bao L H, Zhang J X, Zhou S L, Zhang N 2011 Acta Phys. Sin. 60 106501 (in Chinese)[包黎红, 张久兴, 周身林, 张宁 2011 60 106501]
[4] Bao L H, Zhang J X, Zhou S L, Zhang N, Xu H 2011 Chin. Phys. Lett. 28 088101
[5] Zhang H, Tang J, Yuan J S, Ma J, Shinya N, Nakajima K, Murakami H, Ohkubo T, Qin L C 2010 Nano Lett. 10 3539
[6] Bao L H, Narengerile, Tegus O, Zhang X, Zhang J X 2013 Acta Phys. Sin. 62 196105 (in Chinese)[包黎红, 那仁格日乐, 特古斯, 张忻, 张久兴 2013 62 196105]
[7] Zhou S, Zhang J, Liu D, Lin Z, Huang Q, Bao L, Ma R, Wei Y 2010 Acta Mater. 58 4978
[8] Nishitani R, Aono M, Tanaka T, Oshima C, Kawai S, Iwasaki H, Nakamura S 1980 Surf. Sci. 93 535
[9] Oshima C, Bannai E, Tanaka T, Kawai S 1977 J. Appl. Phys. 48 3925
[10] Uijttewaal M A, de Wijs G A, de Groot R A 2006 J. Phys. Chem. B 110 18459
[11] Oshima C, Aono M, Tanaka T, Nishitani R, Kawai S 1980 J. Appl. Phys. 51 997
[12] Gesley M, Swanson L W 1984 Surf. Sci. 146 583
[13] Swanson L W, Gesley M A, Davis P R 1981 Surf. Sci. 107 263
[14] Liu H, Zhang X, Ning S, Xiao Y, Zhang J 2017 Vacuum 143 245
[15] Payne M C, Teter M P 1992 Rev. Mod. Phys. 64 1045
[16] Segall M D, Lindan P J D, Probert M J, Pickard C J, Hasnip P J, Clark S J, Payne M C 2002 J. Phys.-Condens. Mater. 14 2717
[17] Perdew J P, Burke K, Ernzerhof M 1996 Phys. Rev. Lett. 77 3865
[18] Yamamoto S 2006 Rep. Prog. Phys. 69 181
[19] Yamauchi H, Takagi K, Yuito I, Kawabe U 1976 Appl. Phys. Lett. 29 638
[20] Mogren S, Reifenberger R 1991 Surf. Sci. 254 169
[21] Waldhauser W, Mitterer C, Laimer J, Stori H 1998 Surf. Coat. Technol. 98 1315
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