As an emerging topological insulator, many experiments and theoretical studies have been conducted on bulk SmB₆ single crystals [8]. From 2016, researchers began to investigate the novel electronic transport and magneto-transport properties of SmB₆ nanowires [37,38,53,58,59,60,61,62]. In 2017, Kong et al. reported the spin-polarized surface state transport of single SmB₆ nanowires (Figure 1a–c) [58]. 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 SmB₆ nanowire has a bulk gap ~3.2 meV, which is opened by the hybridization of the 4f bands and 5d bands in SmB₆ nanowires. As shown in Figure 1c, the magnetoresistance (MR) of SmB₆ 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 SmB₆ nanowires is spin-polarized. In another interesting work, Zhou et al. reported the positive planar Hall effect (PHE) of SmB₆ nanowires (Figure 1d–f) [59]. 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 SmB₆. In other studies, the researchers found the anomalous magnetoresistance and the hysteresis of magnetoresistance in SmB₆ nanowires [60,61,62].

In the RB₆ family, like SmB₆, YbB₆ is proposed to be a mixed-valent (Yb²⁺/Yb³⁺) topological insulator and demonstrates new quantum phenomena [63,64,65]. In 2018, Han et al. reported the semiconductor–insulator transition behavior in a YbB₆ nanowire (Figure 2) [55]. As shown in Figure 2b, as the temperature decreases from 300 to 2 K, the resistivity of the YbB₆ 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 YbB₆ nanowires. Furthermore, the magnetoresistance (MR) of the YbB₆ 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 B² at 2 K and 10 K, which follows Kohler’s law. Because a semiconductor–insulator transition occurred at 2 K for YbB₆ nanowires, the hole-dominant transport is credible at 2 K and the transport at 10 K is electron-dominant.

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 T_c = 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.

LaB₆ bulk single crystals have been applied in commercial scanning electron microscopy and transmission electron microscopy. For RB₆ nanowires, the most attractive application is also the field emitter of an electronic gun of an electron microscope (Figure 4) [66,67,68]. Published in Nature Nanotechnology, Zhang et al. reported the first application of a single LaB₆ nanowire to scanning electron microscopy, revealing excellent performance [66]. Their LaB₆ 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 LaB₆ 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 LaB₆ nanowire into an aberration-corrected transmission electron microscope [67]. The LaB₆ 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 LaB₆ nanowire electron source in SEM and TEM reveals that the RB₆ nanowires have notable application prospects and commercial value both in electron microscopy and other electron-emitting devices.

Most of the RB₆ 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, SmB₆ shows a small gap (3 meV), evidenced by electrical transport measurements, and may have potential in fabricating devices. Recently, Zhou et al. [69] first reported the self-powered SmB₆ 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 SmB₆ 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 × 10⁷ Jones, respectively. The demonstration of a SmB₆ nanowire photodetector reveals its application potential in mid-infrared photodetectors.

RB₆ crystals show excellent metal-like conductivity (>10³ S m⁻¹) and they are suitable for active electrochemical electrode materials for energy storage. Recently, Wang et al. [52] reported the application of CeB₆ nanowires as lithium-ion battery anode materials, and they obtained a capacity of ~225 mA h g⁻¹ after 60 cycles (Figure 6a). The kinetic analysis shows that the Li⁺ storage mechanism mainly comes from the surface capacitive behavior. Xue et al. [70] reported the LaB₆ nanowires on carbon fiber as electrode materials for supercapacitors (Figure 6b). The LaB₆ electrode materials showed a high areal capacitance of 17.34 mF cm⁻² and revealed suitable cycling stability after 10,000 cycles. The successful application of RB₆ nanowires in batteries and capacitors demonstrates their potential in the field of electrochemical energy storage.