Recent Developments on Rare-Earth Hexaboride Nanowires

Abstract

With the rise of topological insulator samarium hexaboride (SmB₆), rare-earth hexaboride (RB₆) nanowires are the focus of the second wave of a research boom. Recent research has focused on new preparation methods, novel electronic properties, and extensive applications. Here, we review the recent developments in RB₆ nanowires in the past five years. Two main synthesis methods (chemical vapor deposition and high-pressure solid-state) of RB₆ nanowires are introduced and compared. Moreover, their electronic transport, magnetic properties, and superconducting properties are revealed. Furthermore, the applications of RB₆ nanowires are presented, including as field emitters, photodetectors, and in energy storage. Finally, we detail further research directions for RB₆ nanowires.

1. Introduction

Rare-earth hexaborides (RB₆) 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 RB₆ [1,2,3]. For example, yttrium hexaboride (YB₆) is a superconductor with a Tc of 7.2 K, which is the second highest transition temperature among all borides [4]. Moreover, lanthanum hexaboride (LaB₆), possessing low work function of 2.7 eV, is a famous thermionic electron emission material with high current density and stability [5]. Cerium hexaboride (CeB₆) 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 (EuB₆) recently exhibited a colossal magnetoresistance effect [7]. In recent years, the emergent topological insulator has increased interest in samarium hexaboride (SmB₆), which possesses both insulating bulk state and metallic surface state due to the inversion of the d and f bands. Experimental evidence proves that SmB₆ 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, RB₆ experienced the first wave of a research boom from 2005 to 2015, and many RB₆ nanowires were prepared by chemical vapor deposition (CVD) [9,10,11,12,13,14,15,16,17,18,19,20]. These RB₆ nanowires achieved excellent field emission properties and mechanical properties [21,22,23,24,25,26,27,28,29]. From 2016, the second wave of research boom of RB₆ began as SmB₆ proved to be a topological insulator, and researchers began to explore the difference in topological properties between nanowires and bulk single crystals [8].

In this review, we summarize the recent developments in RB₆ nanowires. Two main synthesis methods of RB₆ nanowires are summarized. Furthermore, their electronic transport and magnetic properties are summarized. Finally, the applications of RB₆ nanowires are presented, including as field emitters, photodetectors, and in energy storage.

2. Growth of RB₆ Nanowires

The structural models of rare-earth hexaborides are shown in Figure 1a. RB₆ crystals are CsCl-type structures with a space group of Pm-3m. Among 17 rare-earth elements, only 13 can form hexaborides with boron, which are YB₆, LaB₆, CeB₆, PrB₆, NdB₆, SmB₆, EuB₆, GdB₆, TbB₆, DyB₆, HoB₆, ErB₆, and YbB₆. On the left side of Figure 1a, one B₆ octahedron is surrounded by eight R atoms, and RB₆ crystals generally have suitable conductivity. On the right side of Figure 1a, one R atom is encircled by eight B₆ octahedra, and B₆ octahedra are connected by covalent bonds, which give RB₆ high melting points, high hardness, and high chemical stability. From the study of electronic structure and bonding characteristics of LaB₆, lanthanum and adjacent boron atoms are not sufficiently bonded, indicating that lanthanum atoms can migrate efficiently [30]. During the thermal field emission, the lanthanum atoms can freely migrate in the boron frame to replace the lanthanum atoms evaporated on the surface, thus showing excellent field emission performance [31]. Rare-earth hexaborides share common properties, but the special electronic structure of each material determines their characteristic properties. For instance, YbB₆ was once proposed to be a topological insulator, but new evidence for the electronic structure suggests that its electronic states originate from the hybridization of the Yb d- and B p-orbits. This indicates that YbB₆ has a non-topological insulator electronic structure [32]. Although most of the RB₆ crystals are metals, SmB₆ can open the band gap at low temperature due to the hybridization of the 4f bands and 5d bands, and meanwhile, metallic surface states are topologically protected [33]. To study and utilize the properties of RB₆, high-quality crystals, especially low-dimensional nanowires, need to be prepared. Concerning materials preparation, only two methods are reported to obtain RB₆ nanowires, chemical vapor deposition (CVD) and high-pressure solid-state (HPSS), as depicted in Figure 1b. CVD is a tradition method to grow RB₆ nanowires using vapor–liquid–solid (VLS) or vapor–solid (VS) mechanisms at a high temperature. HPSS using autoclave is a new method to grow RB₆ nanowires at a low temperature.

2.1. CVD Growth

In the past 5 years, a series of RB₆ nanowires were prepared by the CVD method, namely, LaB₆, CeB₆, NdB₆, SmB₆, and ternary La_xPr_1−xB₆ nanowires, as shown in Figure 2. Different methods use different source materials and substrates, as summarized below.

RCl₃ + B + B₂O₃ + H₂ → RB₆ (R = La, Sm)

(1)

From 2017 to 2019, Gan et al. used a Ni-catalyzed low-pressure CVD method to prepare high-quality LaB₆ and SmB₆ nanowires with a length of tens of microns, as depicted in Figure 2a [34,38]. The source materials of this method are LaCl₃ (SmCl₃), H₂, B, and B₂O₃, and they are non-toxic. Halides are common rare-earth sources, easy to decompose and reactive. The innovation of this method lies in the use of B and B₂O₃ as the boron source, because boron powder alone is extremely difficult to change to a gaseous state and has low reactivity. At a high temperature of 1000 °C, the mixture of B and B₂O₃ can produce active B₂O₂ vapor, and then B₂O₂ reacts with LaCl₃ (SmCl₃) and H₂ to grow LaB₆ and SmB₆ nanowires on Ni-coated Si substrates. The LaB₆ nanowires exhibit excellent field emission properties and stability, both at room temperature and at high temperatures [34]. Compared with bulk single crystals, the transport properties prove that SmB₆ nanowires have less residual resistance due to their large surface area [38].

CeCl₃·7H₂O + B₂H₆ → CeB₆

(2)

In another method, Fu et al. applied a low-pressure CVD route to grow CeB₆ nanowires on Au-coated flexible carbon cloths using CeCl₃·7H₂O and B₂H₆ as source materials, as depicted in Figure 2b [35]. In this method, the CeCl₃·7H₂O is safe, but the B₂H₆ gas is deleterious to humans. The field emission properties of flexible CeB₆ nanowire arrays are outstanding, showing a low turn-on field and a high field emission enhancement factor. Meanwhile, the field current density can remain stable under bending conditions.

SmCl₃ + BCl₃ +H₂ → SmB₆

(3)

Besides B₂H₆ gas, BCl₃ gas is also a common source of gaseous boron. In 2016, Zhou et al. used a CVD route to grow SmB₆ nanowires on Au-coated Si substrates [39]. The electron transport testing on four-probe single-nanowire devices showed that the SmB₆ nanowire has a saturated resistance under 10 K due to the presence of both insulating state in bulk and conductive state on the surface.

R + BCl₃ + H₂ → RB₆ (R = Nd, La_xPr_1−x)

(4)

In addition to the catalytic growth using metal particles (Au, Ni), there is also self-catalytic growth using rare-earth metals themselves as catalysts. In 2016, Han et al. reported the self-catalytic growth of NdB₆ and ternary La_xPr_1−xB₆ nanowires by an ordinary-pressure CVD method, as shown in Figure 2c,d [36,37]. Besides the NdB₆ nanowires, they also acquired NdB₆ nanoawls and nanotubes. The growth of ternary La_xPr_1−xB₆ nanowires reveals that this self-catalytic method is suitable for doping and preparation of RB₆ alloys.

2.2. HPSS Growth

Along with the CVD route, the solid-state method is also a route to prepare RB₆ crystals, including the high-pressure solid-state method [40,41,42,43,44,45,46,47], solution combustion method [48,49], and molten salt method [50,51]. However, because the diffusion rate of atoms in solid-state materials is extremely slow, it is difficult to obtain the nanowire morphology. At the same time, the low reactivity of solid source materials is also a problem restricting the development of 1D RB₆ nanomaterials. To solve such problems, from 2016, Zhao group utilized a rare-earth metal, self-catalytic, high-pressure solid-state method (HPSS) route to prepare various RB₆ nanowires, as shown in Figure 3a–f [52,53,54,55,56,57]. It is noteworthy that, until now, this is the only report on the synthesis of YbB₆ nanowires [56]. The general chemical reaction of the HPSS method is given below.

R + 6 H₃BO₃ + 10 Mg + I₂ → RB₆ + 9 MgO + MgI₂ + 9 H₂O

In this equation, Mg is used for the reduction of H₃BO₃, and I₂ acts as the catalyst to boost the reaction of R and B atoms. From the literature, the Gibbs free energy (Δ_rG) and heat function (Δ_rH) of this equation are about −1900 kJ mol⁻¹ and −2000 kJ mol⁻¹, respectively, demonstrating that the reaction is spontaneous and exothermic. Moreover, the high pressure in the autoclave is generated by iodine (higher than 45 atm), which is also a key to obtaining RB₆ nanowires. Due to high exothermic and high pressure, the trigger temperature of this HPSS method (200–260 °C) is generally much lower than that of the CVD method (950–1100 °C). From the ex situ time-dependent morphology study (5 min, 30 min, 360 min), we speculate that the growth of nanowires has three steps: (i) diffusion and reaction of R and B atoms; (ii) nucleation of RB₆ crystals; (iii) growth of RB₆ nanowires [53]. This HPSS route is a general method for the synthesis of rare-earth hexaborides, which we believe can be extended to the synthesis of other metal boride nanowires.

3. Properties and Applications of RB₆ Nanowires

3.1. Electronic Transportation

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 4a–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 4c, 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 4d–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 5) [55]. As shown in Figure 5b, 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 5c, 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 6). 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 7) [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.

3.2. Optoelectronic Properties

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 8). 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.

3.3. Electrochemical Performances

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 9a). 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 9b). 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.

4. Conclusions and Outlook

In conclusion, we review in this paper the recent developments in RB₆ nanowires in the past five years. Two main synthesis methods (CVD and HPSS) of RB₆ nanowires are outlined and compared. Moreover, their electronic transport, magnetic properties, and superconducting properties are summarized. Finally, the applications of RB₆ nanowires are revealed, including as field electron emitters, photodetectors, and in energy storage.

With the rise of two-dimensional (2D) materials, RB₆ nanowires should absorb some of the advantages of 2D material, such as atomically thin and large area lateral size. If RB₆ nanowires become thinner and wider, also called RB₆ nanobelts, they may reveal novel properties (Figure 10). In a recent study, Lee et al. reported the perfect Andreev reflection in a topological superconducting state based on SmB₆/YB₆ heterostructures [71]. We believe the heterostructures based on combinations of RB₆ nanowires or films may find new physical phenomena and represent future trends. In terms of the synthesis methods, CVD, solid-state, MBE, and PLD methods are all applicable, and only few improvements are needed. For instance, when using the CVD method to grow RB₆ nanobelts, mica substrates may be the best. Furthermore, adding some salts can improve the growth efficiency [72]. Meanwhile, 2D rare-earth materials have shown novel properties and applications, and thus, new discoveries and properties will also arise regarding the atomically thin 2D RB₆ nanobelts.