The Growth Methods and Field Emission Studies of Low-Dimensional Boron-Based Nanostructures

Based on the morphology characteristics, low-dimensional (LD) nanostructures with high aspect ratio can be usually divided into nanowire, nanocone, nanotube, nanorod, nanoribbon, nanobelt and so on. Among numerous LD nanostructures, boron-based nanostructures attracted much interest in recent years because they have high melting-point, large electric and thermal conductivity, and low work function. Compared to traditional thermal emission, field emission (FE) has notable advantages, such as lower power dissipation, longer working life, room-temperature operation, higher brightness and faster switching speed. Most studies reveal they have lower turn-on and threshold fields as well as high current density, which are believed as ideal cold cathode nanomaterials. In this review, we will firstly introduce the growth methods of LD boron-based nanostructures (boron monoelement and rare-earth metal hexaboride). Then, we will discuss their FE properties and applications. At last, the conclusions and outlook will be summarized based on the above studies.


Introduction
Field emission (FE) is a typical quantum tunneling effect, in which electrons can tunnel through the barrier and enter the vacuum with the help of the applied electric field instead of going across the barrier [1].R. H. Fowler and L. Nordheim [2] first proposed the classical FE theory (FN theory) for the metallic flat plane in 1928.With the developments of FE theories [3][4][5][6][7][8], different field emitters have been explored, such as microtip or nanotip arrays [9][10][11], thin film [8,12], and low-dimensional (LD) nanostructures [13][14][15].During the FE studies of metal microtips, it is found that the existence of a field enhancement factor can remarkably decrease the turn-on or threshold field of field emission in comparison with flat plane [16], which can be approximately calculated by the ratio of length to the curvature radius of the tip [1].Under this circumstance, LD nanostructures (metal and metal oxides [17], carbon-based [13,18,19], silicon-based [20], and III-V group-based [21,22]) nanostructures inevitably attract considerable attention in cold cathode applications because their larger aspect ratio suggests they should have ultrahigh field enhancement factors.There are usually several crucial requirements for ideal nanostructures with excellent FE performances, which are respectively high field enhance factor, low work function, large electrical and thermal conductivity, high melting-point, and strong endurance to harsh conditions [1].

CVD Method
CVD method is a commonly-used way for deposition of thin film.Nowadays, it has been a typical method to grow LD nanostructures, which can be almost suitable for all the reported LD boron-based nanostructures.As shown in Figure 1, vapors, liquids or solids can be chosen as the reaction precursors in CVD method based on the products.In CVD reaction, the transition change from solids or liquids into vapors usually occurs for the precursors at appropriate temperature, and the vapors can be transported to the upsides of substrate and form LD nanostructures via the vapor-liquid-solid (VLS) or vapor-solid (VS) process.In the VLS process, the gas precursors continuously dissolve into the catalyst droplets, and subsequently the solid solutes precipitate from oversaturated droplets and formed the LD nanoamterials with the proceeding of the reaction.Different from the VLS process, the gaseous precursors can directly change into solid-phase nanomaterials without undergoing the intermediate liquid phase in the VS process.In addition, some precursors can be pre-deposited on the substrate and participate in the reaction [39].
Appl.Sci.2019, 9, x FOR PEER REVIEW 2 of 16 current.For example, boron monoelement nanotube (NT) structures [24] have been proven to have metallic conductivity whether their chirality belongs to armchair or zigzag, which are very beneficial for actual FE applications.In addition, rare-earth metal hexaboride (REB6) nanostructures should be very promising nanomaterials for FE application because they possess metallic conductivity, low work function of 2.5~4.4 eV [32,39,40], large current density and stable electron emission ability.Therefore, in this review, we will mainly focus on the recent progress of the growth methods and FE properties of LD boron-based nanostructures (boron monoelement and rare-earth metal hexaborides).

CVD Method
CVD method is a commonly-used way for deposition of thin film.Nowadays, it has been a typical method to grow LD nanostructures, which can be almost suitable for all the reported LD boron-based nanostructures.As shown in Figure 1, vapors, liquids or solids can be chosen as the reaction precursors in CVD method based on the products.In CVD reaction, the transition change from solids or liquids into vapors usually occurs for the precursors at appropriate temperature, and the vapors can be transported to the upsides of substrate and form LD nanostructures via the vaporliquid-solid (VLS) or vapor-solid (VS) process.In the VLS process, the gas precursors continuously dissolve into the catalyst droplets, and subsequently the solid solutes precipitate from oversaturated droplets and formed the LD nanoamterials with the proceeding of the reaction.Different from the VLS process, the gaseous precursors can directly change into solid-phase nanomaterials without undergoing the intermediate liquid phase in the VS process.In addition, some precursors can be predeposited on the substrate and participate in the reaction [39].Typical morphologies of LD boron-based nanostructures are demonstrated in Figure 2. As seen in Table 1, the boron precursors in these CVD methods can be divided into two categories, which are respectively gaseous and solid phases.As for the use of the gaseous precursors, there are lots of advantages, for example, the morphology control of nanostructures is facilely realized by adjusting the flux or ratio of precursors and the reaction period is relatively shorter.However, the disadvantage for this way is very obvious because these gaseous precursors are usually toxic, flammable or explosive, which inevitably produce potential threat for our daily lives.The other strategy of CVD method is to use B and B2O3 powders as source materials instead of gaseous precursors for preparation of LD boron nanostructures, as described in Refs.[23,24,29,48,65].Very recently, our group [28,39,66] further developed this method to synthesize the LaB6 and SmB6 nanowires (NWs).In comparison with the CVD method using gaseous precursors, this solid-precursor CVD way often needs relatively higher growth temperature, as seen in Table 1.Therefore, low temperature and environment-friendly growth method remains a challenge for the LD boron-based nanostructures.Typical morphologies of LD boron-based nanostructures are demonstrated in Figure 2. As seen in Table 1, the boron precursors in these CVD methods can be divided into two categories, which are respectively gaseous and solid phases.As for the use of the gaseous precursors, there are lots of advantages, for example, the morphology control of nanostructures is facilely realized by adjusting the flux or ratio of precursors and the reaction period is relatively shorter.However, the disadvantage for this way is very obvious because these gaseous precursors are usually toxic, flammable or explosive, which inevitably produce potential threat for our daily lives.The other strategy of CVD method is to use B and B 2 O 3 powders as source materials instead of gaseous precursors for preparation of LD boron nanostructures, as described in Refs.[23,24,29,48,65].Very recently, our group [28,39,66] further developed this method to synthesize the LaB 6 and SmB 6 nanowires (NWs).In comparison with the CVD method using gaseous precursors, this solid-precursor CVD way often needs relatively higher growth temperature, as seen in Table 1.Therefore, low temperature and environment-friendly growth method remains a challenge for the LD boron-based nanostructures.

Laser Ablation Way
Laser ablation is also an effective method to fabricate LD boron nanostructures [53][54][55][56][57][58][59].In laser ablation method, high-power continuous or pulsed laser sources are needed to irradiate the solid precursors and turn them into vapors or plasmas, which are subsequently transferred to the substrate and produce nanostructures.By this way, boron nanobelts (NBs) [55][56][57][58]81] and NWs [53,59] can be fabricated by using a pure boron or a B/NiCo as target, as shown in Figure 3a.Both the boron NBs and NWs were indexed as tetragonal structures.As seen in Table 1, the NBs are observed to have a width-to-thickness ratio of about 5, and their length ranges from several micrometers to millimeters [55].And the boron NWs have a diameter less than 100 nm and a averaged length of hundreds nanometers [59].Although laser ablation way has special advantage for growth of high-melting LD boron nanostructures, the experimental equipment is too expensive to be widely used for many researchers compared with CVD methods.

Magnetron Sputtering Way
Magnetron sputtering way is another synthesis method for boron NWs [60][61][62][63], in which direct-current (DC) or radio frequency (RF) sputtering techniques are used to produce the plasma by bombarding the targets for the formation of nanostructures on substrate [60].As shown in Figure 3b,c, well-aligned and large-scale boron NW arrays can be prepared on different substrates by this way [60].The mean length and diameter of the as-grown NWs are tens of micrometers and 45 nm, respectively.However, it is hard to fabricate crystalline boron nanostructures by magnetron sputtering method [62,63].

Other Methods
Besides the above-mentioned methods, there exist other methods to synthesize LD boron-based nanostructures.By a solid-state reaction way, Y. M. Zhao et al. [64] synthesized single crystalline SmB 6 NWs with cubic structure at low temperature of 220~240 • C, and they are 50~120 nm in diameter and 8~12 µm in length.As presented in Figure 3d.X.J. Wang et al. [33,34] respectively prepared the LaB 6 FE arrays and microtriodes by etching the LaB 6 single crystal or micro-fabrication technology.Single crystalline boron NTs with β-rhombohedral structure can be fabricated in a Cu alloy at 1573 K and separated by 50% HNO 3 solution [82].As described above, there are many choices for preparation of LD boron-based nanostructures, and the researchers should adopt the suitable method by comprehensive consideration of the intrinsic characteristics of the products and the precursors.At the viewpoint of actual FE applications, the CVD method should take great advantages over other methods because the products by CVD way usually have higher crystallinity, larger aspect ratio and better morphology controllability, which corresponds to better FE performances.

Field Emission Properties of LD Boron-Based Nanostructures
LD boron-based nanostructures were found to have outstanding FE properties in recent studies.Based on the research goal in experiments, FE measurement techniques can be simply divided into two kinds, which are respectively individual nanostructure and nanostructured film measurement methods.In this section, we will discuss the research advances in FE properties and applications of LD boron-based nanostructures.

FE Studies of a Single Boron-Based Nanostructure
FE measurement on individual nanostructures are very necessary because of which efficiently avoid the field shielding effect [83][84][85], and thus it can reveal the intrinsic FE properties and mechanisms of LD boron-based nanostructures.There are usually two methods for investigating the FE properties of a single nanostructure.One is W-tip assisted anode probe measurement technique, which can be depicted as follows.In the first step, the researchers are required to peel a suitable single nanowire from the substrate.Subsequently, the researchers need to stick a nanostructure on the W tip by laser welding way.Finally, the emission behaviors of individual nanostructure attaching on the W tip can be studied in the commonly-used FE measurement system, in which a fluorescent screen or metal stage is used as anode probe, and the distance between cathode and anode keeps constant in experiments.The other is in-situ microprobe measurement technique [23][24][25][26][27][28].By this technique, one can freely manipulate the distance between the W microprobe anode and individual nanostructures on substrate in in-situ microprobe measurement system, in which stepping nanomotor is used to control the movement of microprobe to approach a single nanostructure at nanometer precision in the modified SEM or TEM system.In this system, the electric conductivity and FE behaviors of a single LD boron-based nanostructures as cathode can be easily studied, as indicated in Figure 4a.Compared with W-tip assisted anode probe technique, in-situ microprobe measurement technique has significant merits, such as higher accuracy, more handily manipulation and in-situ observation of the FE behaviors of nanostructures in experiments.As a result, in-situ

Field Emission Properties of LD Boron-Based Nanostructures
LD boron-based nanostructures were found to have outstanding FE properties in recent studies.Based on the research goal in experiments, FE measurement techniques can be simply divided into two kinds, which are respectively individual nanostructure and nanostructured film measurement methods.In this section, we will discuss the research advances in FE properties and applications of LD boron-based nanostructures.

FE Properties of LD Boron-Based Nanostructures
3.1.1.FE Studies of a Single Boron-Based Nanostructure FE measurement on individual nanostructures are very necessary because of which efficiently avoid the field shielding effect [83][84][85], and thus it can reveal the intrinsic FE properties and mechanisms of LD boron-based nanostructures.There are usually two methods for investigating the FE properties of a single nanostructure.One is W-tip assisted anode probe measurement technique, which can be depicted as follows.In the first step, the researchers are required to peel a suitable single nanowire from the substrate.Subsequently, the researchers need to stick a nanostructure on the W tip by laser welding way.Finally, the emission behaviors of individual nanostructure attaching on the W tip can be studied in the commonly-used FE measurement system, in which a fluorescent screen or metal stage is used as anode probe, and the distance between cathode and anode keeps constant in experiments.The other is in-situ microprobe measurement technique [23][24][25][26][27][28].By this technique, one can freely manipulate the distance between the W microprobe anode and individual nanostructures on substrate in in-situ microprobe measurement system, in which stepping nanomotor is used to control the movement of microprobe to approach a single nanostructure at nanometer precision in the modified SEM or TEM system.In this system, the electric conductivity and FE behaviors of a single LD boron-based nanostructures as cathode can be easily studied, as indicated in Figure 4a.Compared with W-tip assisted anode probe technique, in-situ microprobe measurement technique has significant merits, such as higher accuracy, more handily manipulation and in-situ observation of the FE behaviors of    4b,d, the maximum FE currents of a single boron or LaB6 nanostructures are far higher than 1 μA.Especially for individual boron NT and LaB6 NW, their maximum emission current respectively reach as high as 88.9 μA [24] and 96.0 μA [28], which are much better than many other nice cold cathode nanostructures (boron NW [86], GdB6 NW [25] and Mo nanoscrew [87]).Moreover, their FE performances can be comparable to individual carbon NT [88,89] and graphene carbon NT tree with excellent emission performances [90].However, the FE current stability of boron-based nanostructures needs to be further improved due to their relatively larger current fluctuation.4b,d, the maximum FE currents of a single boron or LaB 6 nanostructures are far higher than 1 µA.Especially for individual boron NT and LaB 6 NW, their maximum emission current respectively reach as high as 88.9 µA [24] and 96.0 µA [28], which are much better than many other nice cold cathode nanostructures (boron NW [86], GdB 6 NW [25] and Mo nanoscrew [87]).Moreover, their FE performances can be comparable to individual carbon NT [88,89] and graphene carbon NT tree with excellent emission performances [90].However, the FE current stability of boron-based nanostructures needs to be further improved due to their relatively larger current fluctuation.In addition, J. Tang et al. [40] used field ion microscopy (FIM) and field emission microscopy (FEM) to investigate the emission site distribution of a single LaB 6 NW.The FIM patterns of LaB 6 NWs with <001> and <012> orientations are respectively demonstrated in Figure 5a,b.It is found that LaB 6 NW with <001> orientation has the highest FE symmetry while LaB 6 NW with <012> orientation has the lowest apical work function.Also, H. J. Gao et al. [41] researched on the diameter-dependent FE properties of a single boron NW by in-situ microprobe technique.They found that the FE mechanism of individual boron NWs deviates from the classical FN theory when their diameter gradually decreases.Further effort is still needed for mastering the intrinsic FE mechanisms of LD boron-based nanostructures.
Appl.Sci.2019, 9, x FOR PEER REVIEW 8 of 16 LaB6 NW with <001> orientation has the highest FE symmetry while LaB6 NW with <012> orientation has the lowest apical work function.Also, H. J. Gao et al. [41] researched on the diameter-dependent FE properties of a single boron NW by in-situ microprobe technique.They found that the FE mechanism of individual boron NWs deviates from the classical FN theory when their diameter gradually decreases.Further effort is still needed for mastering the intrinsic FE mechanisms of LD boron-based nanostructures.

FE Behaviors of Boron-Based Nanostructured Film
The FE properties of boron-based nanostructured film are very significant for evaluating their actual applications, in which the turn-on field and threshold field, current stability and maximum emission current density are the crucial parameters.To investigate the FE properties of boron-based nanostructure film, a column-like probe with smaller diameter of about several mm, a fluorescent screen or a metal platform is often used as an anode.FE current of the nanostructured film sample can be auto-recorded by the programmed amperemeter when the electric voltage is applied between cathode film and anode.Sometimes, a gate voltage may be introduced to assist the anode to extract the electrons from the cathode.Usually, the turn-on and threshold fields are defined as the applied electrical field, corresponding to the FE current density reaches 10 μA/cm 2 and 1 mA/cm 2 .And the current stability directly reflects the current fluctuation at fixed field.There is a very simply and classical calculation expression of current fluctuation Ifluct, i.e., = ( − )/( + ), where Imax and Imin are respectively the maximum and minimum currents at given field during a measurement period over 2 h.It is known to us all that lower turn-on and threshold field as well smaller current fluctuation of nanostructured film suggest that they have excellent FE properties.The FE properties of LD boron-based nanostructures are listed in Table 3. H. J. Gao's group [29][30][31] and N. S. Xu's group [23] separately carried out the FE studies of boron nanostructured film.As shown in Figure 6a, LD boron nanostructures with higher work function and smaller electric conductivity have a relatively higher turn-on field of 2.8~5.1 V/μm and threshold field of 4~10.5 V/μm than LaB6 nanowires (about 2 and 5 V/μm).Meanwhile, the emission current density of boron nanostructured film can reach up to 45 mA/cm 2 and their emission current stability is less than 3%.In addition, the patterned boron nanocones (NCs) have better FE properties (such as lower turn-on and threshold fields) compared to the continuous boron NC films [30,31].Also, the FE properties of REB6 nanostructured films were investigated.As demonstrated in Figure 6b, CeB6 NW film exhibits a turn-on field of 7.6 V/μm and their maximum FE current is about 287 μA.Our group has compared the FE properties of SmB6 NW and nanopencil (NP) films, as seen in Figure 6c.It is found that the SmB6 NWs have a lower turn-on field (6.5 V/μm) than SmB6 NPs (6.9 V/μm), and the maximum emission current density of both nanostructures can reach several hundred μA/cm 2 .As seen in Figure 6(d), X.J. Wang et al. [33,34] found that the emission current density of LaB6 FE microtriode arrays can arrive as high as 558 mA/cm 2 at the gate voltage of 145 V.By using a flat metal stage as anode, we [28] investigated the FE properties of LaB6 NWs film with an area of 0.34 cm 2 .It is seen in Figure 6c,d, the FE current density of LaB6 NW film arrives at 16.7 mA/cm 2 and their current fluctuation is only about 1.7% for 2 hour continuous measurements.In addition, J. Q.Xu et al. [23] studied the effect

FE Behaviors of Boron-Based Nanostructured Film
The FE properties of boron-based nanostructured film are very significant for evaluating their actual applications, in which the turn-on field and threshold field, current stability and maximum emission current density are the crucial parameters.To investigate the FE properties of boron-based nanostructure film, a column-like probe with smaller diameter of about several mm, a fluorescent screen or a metal platform is often used as an anode.FE current of the nanostructured film sample can be auto-recorded by the programmed amperemeter when the electric voltage is applied between cathode film and anode.Sometimes, a gate voltage may be introduced to assist the anode to extract the electrons from the cathode.Usually, the turn-on and threshold fields are defined as the applied electrical field, corresponding to the FE current density reaches 10 µA/cm 2 and 1 mA/cm 2 .And the current stability directly reflects the current fluctuation at fixed field.There is a very simply and classical calculation expression of current fluctuation I fluct , i.e., I f luct = (I max − I min )/(I max + I min ), where I max and I min are respectively the maximum and minimum currents at given field during a measurement period over 2 h.It is known to us all that lower turn-on and threshold field as well smaller current fluctuation of nanostructured film suggest that they have excellent FE properties.
The FE properties of LD boron-based nanostructures are listed in Table 3. H. J. Gao's group [29][30][31] and N. S. Xu's group [23] separately carried out the FE studies of boron nanostructured film.As shown in Figure 6a, LD boron nanostructures with higher work function and smaller electric conductivity have a relatively higher turn-on field of 2.8~5.1 V/µm and threshold field of 4~10.5 V/µm than LaB 6 nanowires (about 2 and 5 V/µm).Meanwhile, the emission current density of boron nanostructured film can reach up to 45 mA/cm 2 and their emission current stability is less than 3%.In addition, the patterned boron nanocones (NCs) have better FE properties (such as lower turn-on and threshold fields) compared to the continuous boron NC films [30,31].Also, the FE properties of REB 6 nanostructured films were investigated.As demonstrated in Figure 6b, CeB 6 NW film exhibits a turn-on field of 7.6 V/µm and their maximum FE current is about 287 µA.Our group has compared the FE properties of SmB 6 NW and nanopencil (NP) films, as seen in Figure 6c.It is found that the SmB 6 NWs have a lower turn-on field (6.5 V/µm) than SmB 6 NPs (6.9 V/µm), and the maximum emission current density of both nanostructures can reach several hundred µA/cm 2 .As seen in Figure 6d, X.J. Wang et al. [33,34] found that the emission current density of LaB 6 FE microtriode arrays can arrive as high as 558 mA/cm 2 at the gate voltage of 145 V.By using a flat metal stage as anode, we [28] investigated the FE properties of LaB 6 NWs film with an area of 0.34 cm 2 .It is seen in Figure 6c,d, the FE current density of LaB 6 NW film arrives at 16.7 mA/cm 2 and their current fluctuation is only about 1.7% for 2 hour continuous measurements.In addition, J. Q.Xu et al. [23] studied the effect of the morphologies (sparse nanoneedles (NNs), dense NNs, dense nanorods (NRs), and NW arrays) on the FE properties of LaB 6 nanostructures, and their turn-on fields were found to be respectively 4.12, 2.16, 2.80 and 1.80 V/µm.In this case, larger aspect ratio is observed to be very essential to lower their turn-on field of LaB 6 nanostructures because it represents larger field emission enhancement factors.Also, H. J. Gao et al. [30] investigated the FE properties of different growth density (low, medium, and high) of boron NCs.They found that the boron NCs with medium growth density have the most excellent FE properties (the lowest turn-on and threshold fields).It suggests their FE properties can be improved by adjusting the distance among neighbored boron NCs to close to the length of NCs, which can be attributed to the weakening of field screening effect at suitable nanostructure density.In conclusion, better crystallinity, larger aspect ratio and suitable growth density are highly demanded for the FE applications of LD boron-based nanomaterials.The temperature-dependent FE properties of LD boron-based nanostructure film also attract researchers' interest because of which care about their practical applications.Y. M. Zhao et al. [35,37,49] compared the temperature-dependent FE properties among several REB 6 NW films.As shown in Figure 7a,b, the research results show that the FE current density of rare-earth hexaboride (LaB 6 , SmB 6 , and PrB 6 ) NW film at 573 K increases to be about 10 times larger than that of it at room temperature.J. Q.Xu and our group [28,32] respectively investigated the temperature-dependent FE behaviors of LaB 6 nanowire film, as shown in Figure 7c-f.The FE current densities at 723 K and 773 K were observed to respectively increase about 45 and 2.3 times larger than the room-temperature current density.Both of these two research groups attributed the FE improvement mechanism of LaB 6 nanowires to the decrease of the effective work function resulting from desorption of the adsorbent gases during temperature-rising process.Moreover, F. Liu et al. [28] found out that the FE current density of LaB 6 NWs unvaried with the drop of temperature and almost recovered to the original excellent FE performances after detachment of the surface oxygen molecules in O 2 injecting experiments.As seen in Table 3, LD boron-based nanomaterials have comparable FE properties with many excellent cathode nanostructures [87,91,93,94].Combined the FE performances of individual LD boron-based nanomaterials and nanostructured film, the LD boron-based nanostructures should have promising future in cold cathode electron sources.Of course, the FE current density and stability of LD boron-based nanostructures are worse than thermal emission in present [95][96][97].Therefore, it is still a long way to fabricate high-quality nanostructures and design a much better FE device structure to enhance their emission performances for actual applications.The temperature-dependent FE properties of LD boron-based nanostructure film also attract researchers' interest because of which care about their practical applications.Y. M. Zhao et al. [35,37,49] compared the temperature-dependent FE properties among several REB6 NW films.As shown in Figure 7a,b, the research results show that the FE current density of rare-earth hexaboride (LaB6, SmB6, and PrB6) NW film at 573 K increases to be about 10 times larger than that of it at room temperature.J. Q.Xu and our group [28,32] respectively investigated the temperature-dependent FE behaviors of LaB6 nanowire film, as shown in Figure 7c-f.The FE current densities at 723 K and 773 K were observed to respectively increase about 45 and 2.3 times larger than the room-temperature current density.Both of these two research groups attributed the FE improvement mechanism of LaB6 nanowires to the decrease of the effective work function resulting from desorption of the adsorbent gases during temperature-rising process.Moreover, F. Liu et al. [28] found out that the FE current density of LaB6 NWs unvaried with the drop of temperature and almost recovered to the original excellent FE performances after detachment of the surface oxygen molecules in O2 injecting experiments.As seen in Table 3, LD boron-based nanomaterials have comparable FE properties with many excellent cathode nanostructures [87,91,93,94].Combined the FE performances of individual LD boron-based nanomaterials and nanostructured film, the LD boron-based nanostructures should have promising future in cold cathode electron sources.Of course, the FE current density and stability of LD boron-based nanostructures are worse than thermal emission in present [95][96][97].Therefore, it is still a long way to fabricate high-quality nanostructures and design a much better FE device structure to enhance their emission performances for actual applications.

FE Applications of Boron-Based Nanostructures
In recent studies, low-power nanostructure-based microwave electron tubes were observed to exhibit nice performances, in which 27.5 W output power and 19.5 dB gain have been achieved at a frequency of 10.5 GHz, a helix voltage of 3.35 kV and a cathode current of 58.6 mA [98,99].In addition,

FE Applications of Boron-Based Nanostructures
In recent studies, low-power nanostructure-based microwave electron tubes were observed to exhibit nice performances, in which 27.5 W output power and 19.5 dB gain have been achieved at a frequency of 10.5 GHz, a helix voltage of 3.35 kV and a cathode current of 58.6 mA [98,99].In addition, carbon NTs [100][101][102][103][104] and ZnO NWs [105] were used as X-ray sources with a spatial resolution of less than 25 µm.In the meantime, a single LaB 6 NW with growing along <001> direction was fabricated into the electron gun of the SEM system by H. Zhang et al. [97] because of its outstanding FE behaviors.As shown in Figure 8, individual LaB 6 NW was fixed on the end of W needle as the cathode filament by in-situ e-beam-induced deposition (EBID) technique in the modified SEM system.It was found that the current density gain of LaB 6 NW is about 1000 times greater than that of the W(310) tip and no emission decay was observed during tens of hours of operation.Moreover, the SEM equipped with a single LaB 6 NW electron source can capture an enough low resolution (up to 2.6 nm) image and have a rapid mapping ability of chemical compositions at room N. S. Xu et al. [24] have demonstrated the prototype luminescent tube using boron NT cathode.The luminance of boron NT-based luminescent tube was observed to increase nearly linearly with the cathode current, and the maximum luminance can arrive at 1412 cd/m 2 at 3 mA/cm 2 current density.Moreover, the boron NT-based luminescent tube exhibited very stable operation and uniform brightness distribution in 2 h continuous measurements, in which the fluctuation was less than 7% and the emission uniformity was over 95 %.Therefore, it suggests that LD boron-based nanostructures should be excellent candidates for future cathode electron source of high-power microwave electron tubes, X-ray tubes or cold cathode electron guns in SEM/TEM system.

Outlook and Conclusions
In summary, the researches on the synthesis ways and FE properties of LD boron-based nanostructures have made great progress in the past two decades, but there still exist two big challenges for their future developments.One is the growth of pure boron single crystalline NTs with controlled layers and aspect ratio because the boron NTs with metallic properties usually coexisted with the p-type NWs as well as their length and diameter are hard to control using the known ways.In this situation, the electron transport and FE properties of boron NTs can't be effectively modulated and improved.The other is to fabricate high-performance LD boron-based nanostructure cathodes to meet the critical requirements for cold cathode electron sources, which can bear enough larger current density with a long lifetime and higher stability.In present studies, the emission current of nearly all individual boron-based nanostructures can arrive at 1 μA current and their maximum current can be even as high as 96.0 μA in measurements [23,24,28,86].But when their nanostructured film are used,

Outlook and Conclusions
In summary, the researches on the synthesis ways and FE properties of LD boron-based nanostructures have made great progress in the past two decades, but there still exist two big challenges for their future developments.One is the growth of pure boron single crystalline NTs with controlled layers and aspect ratio because the boron NTs with metallic properties usually coexisted with the p-type NWs as well as their length and diameter are hard to control using the known ways.In this situation, the electron transport and FE properties of boron NTs can't be effectively modulated and improved.
The other is to fabricate high-performance LD boron-based nanostructure cathodes to meet the critical requirements for cold cathode electron sources, which can bear enough larger current density with a long lifetime and higher stability.In present studies, the emission current of nearly all individual boron-based nanostructures can arrive at 1 µA current and their maximum current can be even as high as 96.0 µA in measurements [23,24,28,86].But when their nanostructured film are used, the averaged emission current of a single nanostructure on film is found to be only about 0.2 µA [33], which is far lower than that of individual nanostructure.As a result, the FE performances of LD boron-based nanostructures are not good enough for their cold cathode electron source applications nowadays.
The aim to these two main problems, there are the following possible solutions by our analysis.As for the synthesis of pure boron NTs, the researchers can realize this goal by adjusting the growth parameters, such as heating or cooling rate, the flow rate ratio of carrier gas, the mass ratio of source materials and the collection area of products.If we hope to efficiently improve the FE performances of LD boron-based nanostructure films, two strategies can be considered.One is to control the nanostructure density on substrate, which can effectively decrease the field shielding effect and ensure more nanostructures involve in emission.The other is to enhance the heat dissipation capacity by designing a more effective device structure [106], fabricating composite structure [90], increasing the electron conductivity of boron-based nanostructures by element doping and lowering the contact resistance between nanostructures and substrate, which may remarkably improve their endurance to large emission current.
Considering their excellent emission properties, REB 6 nanostructures should have great potential applications as the cold cathode electron sources in SEM/TEM system, high-power microwave electron tubes or X-ray sources.Moreover, boron monoelement nanowires or nanotubes with large Young's modulus [42] may have a more promising future in portable, flexible and low-power cathode electron sources.

Figure 1 .
Figure 1.Schematic diagram of a typical CVD system.

Figure 1 .
Figure 1.Schematic diagram of a typical CVD system.
nanostructures in experiments.As a result, in-situ microprobe measurement technique should a better choice for the studies of individual boron-based or other nanostructures.Appl.Sci.2019, 9, x FOR PEER REVIEW 7 of 16 microprobe measurement technique should a better choice for the studies of individual boron-based or other nanostructures.

Figure 4 .
Figure 4. (a) Typical SEM image during in-situ microprobe measurements of individual nanostructure [28].Copyright 2017 The Royal Society of Chemistry.(b) I-E curves of a single boron NW [23].Copyright 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.(c) I-E curves of a single boron NT [24].Copyright 2010 The Royal Society of Chemistry.(d) FE properties of individual LaB6 NW [28].Copyright 2017 The Royal Society of Chemistry.

Figure 4 .
Figure 4. (a) Typical SEM image during in-situ microprobe measurements of individual nanostructure [28].Copyright 2017 The Royal Society of Chemistry.(b) I-E curves of a single boron NW [23].Copyright 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.(c) I-E curves of a single boron NT [24].Copyright 2010 The Royal Society of Chemistry.(d) FE properties of individual LaB 6 NW [28].Copyright 2017 The Royal Society of Chemistry.
Appl.Sci.2019, 9, x FOR PEER REVIEW 11 of 16 FE behaviors.As shown in Figure8, individual LaB6 NW was fixed on the end of W needle as the cathode filament by in-situ e-beam-induced deposition (EBID) technique in the modified SEM system.It was found that the current density gain of LaB6 NW is about 1000 times greater than that of the W(310) tip and no emission decay was observed during tens of hours of operation.Moreover, the SEM equipped with a single LaB6 NW electron source can capture an enough low noise, high resolution (up to 2.6 nm) image and have a rapid mapping ability of chemical compositions at room temperature.N. S. Xu et al.[24] have demonstrated the prototype luminescent tube using boron NT cathode.The luminance of boron NT-based luminescent tube was observed to increase nearly linearly with the cathode current, and the maximum luminance can arrive at 1412 cd/m 2 at 3 mA/cm 2 current density.Moreover, the boron NT-based luminescent tube exhibited very stable operation and uniform brightness distribution in 2 h continuous measurements, in which the fluctuation was less than 7% and the emission uniformity was over 95 %.Therefore, it suggests that LD boron-based nanostructures should be excellent candidates for future cathode electron source of high-power microwave electron tubes, X-ray tubes or cold cathode electron guns in SEM/TEM system.

Figure 8 .
Figure 8. Individual LaB6 NW as a FE electron gun in a modified SEM system [97].Copyright 2015 Springer Nature.(a-c) Schematic diagram, SEM picture and FE images of a single LaB6 NW fixed on the top of W needle.(d) Typical 100,000 times magnification image captured by SEM with individual LaB6 NW cathode at 5.0 kV extraction voltage.(e) EDS mapping result.The inset is the spectrum of the mapping area.(f) The dependence of normalized angular current density on extraction voltage.The corresponding FN plots are shown in the inset.(g) Electron energy distribution from a LaB6 NW and a W(310) tip.

Figure 8 .
Figure 8. Individual LaB 6 NW as a FE electron gun in a modified SEM system [97].Copyright 2015 Springer Nature.(a-c) Schematic diagram, SEM picture and FE images of a single LaB 6 NW fixed on the top of W needle.(d) Typical 100,000 times magnification image captured by SEM with individual LaB 6 NW cathode at 5.0 kV extraction voltage.(e) EDS mapping result.The inset is the spectrum of the mapping area.(f) The dependence of normalized angular current density on extraction voltage.The corresponding FN plots are shown in the inset.(g) Electron energy distribution from a LaB 6 NW and a W(310) tip.

Table 1 .
Typical CVD growth parameters of boron-based nanostructure.

Table 2
lists the recent FE results of individual boron-based nanostructures.As seen in Figure

Table 2 .
[40]ary of FE properties of individual boron-based nanostructures and other nanostructures.In addition, J. Tang et al.[40]used field ion microscopy (FIM) and field emission microscopy (FEM) to investigate the emission site distribution of a single LaB6 NW.The FIM patterns of LaB6 NWs with <001> and <012> orientations are respectively demonstrated in Figure5a,b.It is found that

Table 2
lists the recent FE results of individual boron-based nanostructures.As seen in Figure

Table 2 .
Summary of FE properties of individual boron-based nanostructures and other nanostructures.

Table 3 .
FE characteristics of typical boron-based nanostructures.