Rare-earth hexaborides (RB
6) have received substantial attention thanks to their high electrical conductivity, high melting points, and high chemical stability. Meanwhile, the strong correlation effect of 4f–5d electrons of rare-earth elements also brings some newfangled physical properties of RB
6 [1,2,3][1][2][3]. For example, yttrium hexaboride (YB
6) is a superconductor with a Tc of 7.2 K, which is the second highest transition temperature among all borides
[4]. Moreover, lanthanum hexaboride (LaB
6), possessing low work function of 2.7 eV, is a famous thermionic electron emission material with high current density and stability
[5]. Cerium hexaboride (CeB
6) is an antiferromagnetic heavy-fermion metal, but recently, it was found to demonstrate low-energy ferromagnetic fluctuation
[6]. Furthermore, as a ferromagnetic semimetal, europium hexaboride (EuB
6) recently exhibited a colossal magnetoresistance effect
[7]. In recent years, the emergent topological insulator has increased interest in samarium hexaboride (SmB
6), which possesses both insulating bulk state and metallic surface state due to the inversion of the d and f bands. Experimental evidence proves that SmB
6 is the first strongly correlated 3D topological Kondo insulator
[8].
Due to the small size effect and quantum confinement effect, one-dimensional (1D) nanomaterials have new properties compared with bulk crystals. With the rise of 1D nanomaterials, RB
6 experienced the first wave of a research boom from 2005 to 2015, and many RB
6 nanowires were prepared by chemical vapor deposition (CVD)
[9,10,11,12,13,14,15,16,17,18,19,20][9][10][11][12][13][14][15][16][17][18][19][20]. These RB
6 nanowires achieved excellent field emission properties and mechanical properties
[21,22,23,24,25,26,27,28,29][21][22][23][24][25][26][27][28][29]. From 2016, the second wave of research boom of RB
6 began as SmB
6 proved to be a topological insulator, and researchers began to explore the difference in topological properties between nanowires and bulk single crystals
[8].
2.1. Electronic Transportation
As an emerging topological insulator, many experiments and theoretical studies have been conducted on bulk SmB
6 single crystals
[8]. From 2016, researchers began to investigate the novel electronic transport and magneto-transport properties of SmB
6 nanowires
[37,38,53,58,59,60,61,62][30][31][32][33][34][35][36][37]. In 2017, Kong et al. reported the spin-polarized surface state transport of single SmB
6 nanowires (
Figure 1a–c)
[58][33]. Under 5 K, the resistance appears saturated and flat, indicating that the surface states control the transport behavior. The appearance of topological surface states is caused by the reversal of
d and
f electrons. The fitting of a temperature-dependent resistance curve reveals that SmB
6 nanowire has a bulk gap ~3.2 meV, which is opened by the hybridization of the 4
f bands and 5
d bands in SmB
6 nanowires. As shown in
Figure 1c, the magnetoresistance (MR) of SmB
6 nanowires is negative and the MR shows no sign of saturation at high magnetic field up to 14 T. The negative MR indicates that this transport behavior is spin-dependent. Furthermore, the nonlocal tests reveal that the surface state transport of SmB
6 nanowires is spin-polarized. In another interesting work, Zhou et al. reported the positive planar Hall effect (PHE) of SmB
6 nanowires (
Figure 1d–f)
[59][34]. They found that as the temperature decreases, the amplitude increases sharply, but saturates at 5 K. This positive PHE is due to the surface states of SmB
6. In other studies, the researchers found the anomalous magnetoresistance and the hysteresis of magnetoresistance in SmB
6 nanowires
[60,61,62][35][36][37].
Figure 1. (
a) SEM image of a SmB
6 nanowire device, the scalebar is 2 μm. (
b) Temperature-dependent resistance of the SmB
6 nanowire. (
c) Magnetoresistance curves under a parallel magnetic field at various temperatures
[58][33]. Copyright 2017, American Physical Society. (
d) Planar Hall resistivity with various angles at 1.6 K. (
e) PHE amplitude and resistivity. Inset is the definition of tilting angle θ. (
f) Planar Hall resistivity with various angles at 80 K
[59][34]. Copyright 2019, American Physical Society.
In the RB
6 family, like SmB
6, YbB
6 is proposed to be a mixed-valent (Yb
2+/Yb
3+) topological insulator and demonstrates new quantum phenomena
[63,64,65][38][39][40]. In 2018, Han et al. reported the semiconductor–insulator transition behavior in a YbB
6 nanowire (
Figure 2)
[55][41]. As shown in
Figure 2b, as the temperature decreases from 300 to 2 K, the resistivity of the YbB
6 nanowire device undergoes a dramatic 49-fold increase (ρ
2 K/ρ
300 K = 49). They propose that the semiconductor–insulator transition is due to a small band gap opening at a low temperature induced by the slightly boron-rich or boron-deficient segments in YbB
6 nanowires. Furthermore, the magnetoresistance (MR) of the YbB
6 nanowire was tested with perpendicular magnetic field B = 0–7 T at various temperatures. As displayed in
Figure 2c, the MR shows no sign of saturation at high magnetic field up to 14 T and has a linear dependence with B
2 at 2 K and 10 K, which follows Kohler’s law. Because a semiconductor–insulator transition occurred at 2 K for YbB
6 nanowires, the hole-dominant transport is credible at 2 K and the transport at 10 K is electron-dominant.
Figure 2. (
a) SEM image of the YbB
6 nanowire device. (
b) Resistivity as a function of temperature from 2 to 300 K. (
c) Magnetoresistance (MR) as a function of B
2 at various temperatures
[55][41]. Copyright 2018, Elsevier Science B.V.
Of all the metal borides, YB6 bulk crystals have the second highest superconducting transition temperature of 7.2 K after MgB2. More superconducting properties have been studied in bulk YB6 single crystals, but the superconducting properties of YB6 nanowires have not been reported. Recently, Wang et al. reported the synthesis of 1D YB6 nanowires by a high-pressure solid-state method and studied their magnetic properties (Figure 3). The temperature-dependent magnetization under zero-field cooling and field cooling revealed that the YB6 nanowires have a superconducting transition with Tc = 7.8 K. Meanwhile, they found that the YB6 nanowires exhibited a peak effect in the superconducting state observed from the magnetic hysteresis loops obtained at 2 K and 10 K, indicating that YB6 nanowires pertain to a type-II superconductor.
Figure 3. (
a) The temperature-dependent magnetization under zero-field cooling and field cooling modes of superconducting YB
6 nanostructure. (
b) The magnetic hysteresis loops obtained at 2 K and 10 K
[57][42]. Copyright 2021, Elsevier Science B.V.
LaB
6 bulk single crystals have been applied in commercial scanning electron microscopy and transmission electron microscopy. For RB
6 nanowires, the most attractive application is also the field emitter of an electronic gun of an electron microscope (
Figure 4)
[66,67,68][43][44][45]. Published in Nature Nanotechnology, Zhang et al. reported the first application of a single LaB
6 nanowire to scanning electron microscopy, revealing excellent performance
[66][43]. Their LaB
6 nanowire electron source shows low work function, is chemically inert, and has high monochromaticity. When assembled into a field-emission gun of SEM, it demonstrates ultra-low emission decay, and its current density gain is three orders of magnitude higher than traditional W tips. By this LaB
6 nanowire-based SEM, they obtained low-noise and high-resolution images, better than W-tip-based SEM. Recently, published in Nature Nanotechnology in 2021, Zhang et al. reported the installation of a single LaB
6 nanowire into an aberration-corrected transmission electron microscope
[67][44]. The LaB
6 NW-based TEM achieved atomic resolution and probe-forming modes at 60 kV energy. Compared with the state-of-the-art W (310) electron source, the nanostructured electron source provides higher temporal coherence at a spatial frequency of 105 pm, showing a higher contrast transfer amplitude of 84% and a spectral energy resolution of 35%. The first demonstration of the LaB
6 nanowire electron source in SEM and TEM reveals that the RB
6 nanowires have notable application prospects and commercial value both in electron microscopy and other electron-emitting devices.
Figure 4.
Illustrations of the LaB
6
bulk crystal and nanowire electron-emission sources in electron microscopy.
2.2. Optoelectronic Properties
Most of the RB
6 crystals are metals with zero band gap, and thus, they are not suitable for semiconductor devices, such as field effect transistors and photodetectors. However, as a topological Kondo insulator, SmB
6 shows a small gap (3 meV), evidenced by electrical transport measurements, and may have potential in fabricating devices. Recently, Zhou et al.
[69][46] first reported the self-powered SmB
6 nanowire photodetectors with broadband wavelengths covering from 488 nm to 10.6 μm (
Figure 5). They claimed that the photocurrent stemmed from the interface of SmB
6 nanowire and Au electrodes owing to the built-in potential, proved by the spatially resolved photocurrent mapping. The current on/off ratio, responsibility, and specific detectivity are 100, 1.99 mA/W, and 2.5 × 10
7 Jones, respectively. The demonstration of a SmB
6 nanowire photodetector reveals its application potential in mid-infrared photodetectors.
Figure 5. (
a) Current–time measurement of SmB
6 nanowire photodetector under illuminating of 10.6 μm light source. (
b) Current–time curves of SmB
6 nanowire photodetector under illuminating with different light wavelengths
[69][46]. Copyright 2018, AIP Publishing.