Manipulating the anisotropy and field emission of lanthanum hexaboride nanorods

Abstract

The present study describes the controlled synthesis of lanthanum hexaboride nanostructures with efficient field emission properties. The synthesis is mediated by a nanostructured lanthanum hydroxide precursor, which is controlled by varying the capping agent and pH using a hydrothermal route. The effect of charge on the capping agent (surfactant) strongly affects the shape and size of the precursor (neutral surfactants lead to the formation of nanorods while a cationic surfactant results in the formation of particles). This precursor mediated route leads to lanthanum hexaboride nanostructures at much lower temperatures (∼500 °C lower than the conventional solid state route) and allows for variation of morphology of nanostructured films. Vertically aligned nanorods (30 nm × 200–400 nm), nanoparticles (25 nm) and sub-micron particles (0.2–0.25 microns) could be precisely obtained. Field emission studies of these vertically aligned nanorods show a very high field enhancement factor (4191), which is required for an efficient field emitter.

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1. Introduction

Lanthanum hexaboride is widely used for high electron density cathodes because of its high melting point of 2500 °C,¹ high electrical conductivity,² low work function (∼2.6 eV)³ and low vapour pressure at high temperatures.⁴ Recent reviews on the field emission properties of various metal oxides, nitrides and borides^5–7 suggest that the metal hexaborides show superior performance with a high field enhancement factor, as well as good current stability. Among the known hexaboride based field emitters, lanthanum hexaboride shows the most promising performance compared to other metal hexaborides and hence it has been used in most of the applications as an electron source. Lanthanum hexaboride has been synthesized via the solid state route,^8–13 hydrothermal route,^14,15 gas phase reaction (CVD and PVD) and magnetic sputtering.^16–23 The synthesis of lanthanum hexaboride via the solid state route tends to give micron-sized particles due to the formation of LaB₆ at high temperature (∼1800 °C),^1,2,6 while the gas phase reactions like CVD are quite expensive and are not used for commercial purposes. In addition, the fabrication of LaB₆ films has earlier been reported via chemical vapor deposition and magnetic sputtering,^16–23 which require expensive equipment and chemicals. These techniques do not enable fine control of the shape and size of micron-sized particles of lanthanum hexaboride. Chemical processes like spin coating are cost effective and simple, and can be used routinely for deposition of nanostructured films of oxide, selenide and sulphides.^24–26 However, there is no report for fabrication of metal boride films via low temperature chemical routes. Therefore, the motivation of the present study is to obtain vertically aligned nanorods of lanthanum hexaboride, which would be efficient field emitters, since anisotropic nanostructures with sharp tips enhance the field emission properties. We have optimized the synthetic process mediated by nanostructured lanthanum hydroxide precursors (particles and nanorods with various aspect ratios), which efficiently react with boron at much lower temperatures (∼500 °C or less) than earlier reports to yield nanostructured lanthanum hexaboride. Furthermore, films of lanthanum hexaboride were fabricated by the spin coating route using optimal conditions. We have investigated the field emission of the LaB₆ nanostructures where the vertically aligned nanorods offer excellent properties with a high field enhancement factor of 4191 compared to the maximum reported field enhancement factor of 3585.^18,19

2. Results and discussion

The present study is based on investigation of the critical parameters involved in controlling the shape and size of lanthanum hydroxide using the hydrothermal method and further heating this precursor with boron to yield lanthanum hexaboride. In the first stage, we optimize conditions for control of the shape and size of lanthanum hydroxide precursors via the hydrothermal route using various surfactants (cationic (CTAB), anionic (AOT) and non-ionic (Tergitol)) and also the pH of the solution. In the second stage, the as-obtained lanthanum hydroxide precursors (with various sizes and morphologies) were reacted with boron to synthesize pure LaB₆ at a low temperature of 1300 °C (normally lanthanum hexaboride is synthesized at 1800 °C). In the third stage lanthanum hexaboride films on silicon substrate are fabricated by the spin coating route. We then investigated the field emission properties of these nanostructured lanthanum hexaboride films.

The PXRD studies of the lanthanum hydroxide precursor, obtained via the hydrothermal route using Tergitol as a non-ionic surfactant and sodium hydroxide at varying concentrations (0.1 M, 0.2 M and 0.3 M), show that the products are poorly crystalline and all the reflections could be indexed on the basis of a hexagonal cell of lanthanum hydroxide with space group P6₃/m (Fig. 1a–c, JCPDS No. 060585). A similar hydrothermal reaction in the presence of a cationic surfactant (CTAB) and sodium hydroxide (0.1 M) also resulted in the formation of lanthanum hydroxide (Fig. 1d). Transmission electron microscopy studies of these precursors show a drastic change in morphology. TEM studies of lanthanum hydroxide precursors obtained using CTAB–NaOH (0.1 M, pH 8), Tergitol–NaOH (0.1 M, pH 8), Tergitol–NaOH (0.2 M, pH 10) and Tergitol–NaOH (0.3 M, pH 13) show the formation of particles with an average diameter of 0.4 μm (Fig. 2a), nanorods with d = 8–10 nm, l = 120 nm and AR = 12 (Fig. 2b), nanorods with d = 8–10 nm, l = 240 nm and AR = 24 (Fig. 2c) and nanorods with d = 8–10 nm, l = 300–500 nm, AR = 30–50 (Fig. 2d), respectively. It may be noted that the above mentioned pH is measured before initiating the hydrothermal reaction. After completion of the reaction (2 days at 120 °C), the observed pH was found to be lower (pH = 7), which is due to the consumption of hydroxyl ions to form lanthanum hydroxide. The change in morphology of lanthanum hydroxide (from nanorods to nanoparticles) can be attributed to two factors, the surfactant charge and the initial pH of the reaction. Tergitol is a non-ionic surfactant, containing a surface hydroxyl group, which is adsorbed to the surface of lanthanum hydroxide via hydrogen bonding and facilitates anisotropic growth in the favored crystallographic directions. Earlier reports on lanthanum hydroxide nanorods via the hydrothermal route in the absence of a surfactant also show²⁷ the formation of anisotropic nanostructures of La(OH)₃, which is the thermodynamically favored crystal morphology (it has a hexagonal crystal system). On the contrary, in the presence of the cationic surfactant (CTAB), spheres of 0.4 μm in diameter are formed. Investigation of the time-dependent hydrothermal reaction shows²⁷ that the growth of the lanthanum hydroxide nanorods at 150 °C involves four different steps: nucleation, aggregation, dissolution–recrystallization and the growth process. Lanthanum hydroxide nanoparticles form during the nucleation–dissolution process, while during the growth–recrystallization process, the anisotropic nanostructures crystallize. During the recrystallization and growth process, the stabilization of the isotropic morphology may be explained due to the strong binding of CTAB, being a cationic surfactant, to the negatively charged La(OH)₃ surface by electrostatic interaction, which is much stronger than the hydrogen bonding interaction. This uniformly capped moiety does not favour the orientational growth and hence leads to the formation of spherical particles instead of nanorods (Scheme 1). Earlier reports on lanthanum hydroxide²⁷ describe much thicker nanorods (20 nm × 200 nm) with a small aspect ratio AR = 10–12. The thickness of the nanorods shown in our method is 8–10 nm and it also provides excellent control over the aspect ratio (AR = 12, 24 and 30–50). Varying the initial pH by increasing the concentration of hydroxyl ions in the reaction led to an increase in the aspect ratio of the nanorods from 12 to 30 (Fig. 2b–d). Thus, under hydrothermal conditions where the surfactant acts as a capping agent, cationic surfactants facilitate the growth of lanthanum hydroxide particles, while non-ionic surfactants help in the growth of anisotropic nanostructures of lanthanum hydroxide, and the aspect ratio can be controlled by a change in initial pH (Fig. 2e).

Fig. 1 PXRD studies of the lanthanum hydroxide precursor.

Fig. 2 Transmission electron microscopic study of the lanthanum hydroxide precursor obtained at 120 °C using (a) CTAB, (b) Tergitol/initial pH = 8, (c) Tergitol/initial pH = 10, (d) Tergitol/initial pH = 13 and (e) variation of the aspect ratio of the lanthanum hydroxide precursor with pH in the presence of the Tergitol surfactant.

Scheme 1 Schematics representing the growth mechanism of lanthanum hydroxide in the presence of surfactants and at various pH values.

The reaction of the lanthanum hydroxide precursor (obtained using Tergitol as a capping agent) with boron at 1300 °C leads to the formation of pure lanthanum hexaboride (cubic, Pm3̄m, a = 4.159 Å, (#JCPDS: 340427)) (Fig. 3a–c) (in some batches 5–10% LaBO₃ impurity was observed and was removed by acid wash). Earlier reports on the synthesis of lanthanum hexaboride via the solid state route suggest that under ambient pressure, the formation of LaB₆ is only possible at 1800 °C, in which La₂O₃ and boron have been used as starting materials.⁹ Other solid state routes via the carbothermal and the aluminium flux method (not the cleanest method) suggest that the stabilization of LaB₆ is possible at 1700 °C and 1500 °C, respectively.^10–12 In the borothermal process, as in the present study, high purity LaB₆ can be obtained by the reduction of the nanostructured lanthanum hydroxide precursor at the much lower temperature of 1300 °C; the morphology and size of the lanthanum hexaboride nanostructures depends on the initial precursor yielding nanorods (30 nm × 200–400 nm) and other nanostructures. Transmission electron micrographs of LaB₆ show the presence of lanthanum hexaboride nanorods (30 nm × 200–400 nm) (Fig. 4a), nanoparticles (25 nm) (Fig. 4b) and much larger particles of 0.2–0.25 μm (Fig. 4c). The nanostructured lanthanum hexaborides are single crystalline, as shown by the lattice fringes corresponding to the (110) planes (Fig. 5a–c) and sharp spots in the electron diffraction pattern (Fig. 6a–c). Fig. 6a shows the electron diffraction pattern with a row of well-defined spots running along the a^*- axis. Also, there are weak diffraction spots with streaks running nearly normal to the nanorod length axis. These kinds of diffraction pattern in the electron diffraction studies have been commonly observed for nanorods of oxides.²⁸ However, the electron diffraction patterns of LaB₆ nanoparticles 25 nm and 250 nm show the presence of sharp spots forming rings due to their nanocrystalline nature. So, from the electron diffraction study, we can clearly decipher the presence of highly uniform nanorods (Fig. 4a), and their growth direction (Fig. 6a) and nanoparticles (Fig. 4b–c, 6b–c) of lanthanum hexaborides. Earlier, the formation of LaB₆ nanowires of 100 nm in diameter and micron sized length has been reported by Zhang et al.^17,19 and there is no report so far that suggests the formation of thin nanorods (30 nm) of LaB₆ as obtained here.

Fig. 3 PXRD studies of lanthanum hexaboride obtained using a lanthanum hydroxide precursor using (a) Tergitol/initial pH = 8, (b) Tergitol/initial pH = 10 and (c) Tergitol/initial pH = 13.

Fig. 4 Transmission electron microscopic study of lanthanum hexaboride obtained at 1300 °C using a lanthanum hydroxide precursor synthesized using (a) Tergitol/initial pH = 8, (b) Tergitol/initial pH = 10 and (c) Tergitol/initial pH = 13.

Fig. 5 HRTEM of lanthanum hexaboride obtained at 1300 °C using a lanthanum hydroxide precursor synthesized using (a) Tergitol/initial pH = 8, (b) Tergitol/initial pH = 10 and (c) Tergitol/initial pH = 13.

Fig. 6 Electron diffraction studies of lanthanum hexaboride obtained at 1300 °C starting from a lanthanum hydroxide precursor synthesized using (a) Tergitol/initial pH = 8, (b) Tergitol/initial pH = 10 and (c) Tergitol/initial pH = 13.

Further, LaB₆ films were fabricated by spin coating using a dispersion of LaB₆ powder (obtained by the hydrothermal route). Glancing angle X-ray diffraction of LaB₆ films was successfully indexed on the basis of a cubic cell with a = 4.141 Å (#JCPDS: 340427) (Fig. 7a–c). Atomic force microscopy study of the film fabricated using polycrystalline LaB₆ nanorods (30 nm × 200–400 nm), LaB₆ nanoparticles (∼25 nm) and LaB₆ particles (200–250 nm) shows that the nanorods are vertically aligned (Fig. 8a) (the inset of Fig. 8a shows that the surface consists of well aligned nanorods) while the nanoparticles are homogenously distributed over the Si substrate. The vertical assembly of nanorods is of significance for applications and very limited methods are known to obtain such alignment without using a template. The solvent evaporation technique has been shown to control the orientational order of nanorods (∼96% in the case of CdS).²⁴ We could optimize the spin coating methodology using a suspension of LaB₆ nanorods to fabricate films made up of vertically oriented LaB₆ nanorods. The various spin coating parameters such as spin rate and spinning time, as well as the suspension density, play a crucial role in obtaining such an orientation of the LaB₆ nanorods.

Fig. 7 GAXRD studies of films fabricated using lanthanum hexaboride (a) nanorods (30 nm × 200–400 nm), (b) nanoparticles (25 nm) and (c) particles (200–250 nm).

Fig. 8 AFM studies of lanthanum hexaboride film fabricated using lanthanum hexaboride (a) nanorods (30 nm × 200–400 nm), (b) nanoparticles (25 nm) and (c) particles (200–250 nm).

The field emission characteristics of the LaB₆ films obtained using the hydrothermal route are shown in Fig. 8. The current density (J) is given by the Fowler–Nordheim equation²⁹

where E is the applied field strength (in V μm⁻¹), Φ is the work function of the emitters, β is the field enhancement factor and can be expressed as β = h/r,³⁰ (h is the height and r is the radius of curvature of an emitting center). For good field emitters, apart from the low work function, the shape and size of the materials are also important and it is known that the anisotropic nanostructures are better field emitters^30–32 compared to other morphologies. The current density vs. applied field behaviour of nanorods and nanoparticles has been shown in Fig. 9 and Table 1. LaB₆ films and bare Si show an increase in emission current density with increase in applied field as expected. The nanorods (30 nm × 200–400 nm) show much higher (nearly 8 fold) current density of 51.66 μA cm⁻² compared to the 25 nm nanoparticles (6.63 μA cm⁻²). The films with larger particles (200–250 nm) show an even lower current density of 2.10 μA cm⁻². The comparison of the current density was carried out at a field strength of 12 V μm⁻¹. The LaB₆ film consisting of nanorods shows low turn-on fields of 3.06 V μm⁻¹ (measured at 1 μA cm⁻²), while the LaB₆ film with smaller nanoparticles (25 nm) shows a turn-on field of 3.15 V μm⁻¹ compared to the larger particles of 200–250 nm (turn on field 8.28 V μm⁻¹). The nanorod-based film offers high brightness and current density compared to the nanoparticulated film. The Fowler–Nordheim (F–N) plots (Fig. 10 a–c) show linear behavior, which indicates that emission from the LaB₆ film follows the tunneling mechanism. The nanorods were homogeneously distributed in the films as confirmed by transmission electron and atomic force micrographs earlier (Fig. 4a–c and Fig. 8a–c). As shown in Fig. 10 a–c, the F–N plot is linear, from which we obtain the field enhancement factor (β), which is equal to bΦ^3/2/slope, where b is a constant with a value of 6.83 × 10³ V eV^−3/2 μm⁻¹ and Φ is the work function of the emitter (2.6 eV). The field enhancement factors obtained from the ln(J/E²) vs. 1/E plot in the high field region (1/E = 0.08–0.20 μm V⁻¹) for the nanorods (30 nm × 200–400 nm), nanoparticles (25 nm) and submicron particles (200–250 nm) are 1289, 1240 and 1216 respectively (Fig. 10 a–c). However, in the low field region (1/E = 0.25–0.70 μm V⁻¹), the corresponding field enhancement factors are 4191 (nanorods, 30 nm × 200–400 nm), 4009 (nanoparticles, 25 nm) and 3652 (particles, 200–250 nm). Note that these values are much higher than the maximum reported field enhancement factor of 3585 (in low field range of 1/E = 0.2–0.55 μm V⁻¹).^15,16 The mechanism of the two-slope field enhancement behaviour of LaB₆ is not yet clear. However, for zinc oxide and metal borides,^33,34 the two-slope behavior has been reported earlier, which is due to space charge,³⁵ surface adsorbate³⁶ and damage of emission site³⁷ at high voltage. The high field enhancement factor obtained in our study is mainly attributed to three factors (a) small size of nanorod, (b) uniform size with sharp tip and (c) most importantly the vertical orientation of nanorods.

Fig. 9 Field emission study of films fabricated using polycrystalline LaB₆ (a) nanorods (30 nm × 200–400 nm), (b) nanoparticles (25 nm), (c) particles (200–250 nm) and (d) bare Si.

Fig. 10 The F–N plot for the lanthanum hexaboride film fabricated using polycrystalline LaB₆ (a) nanorods (30 nm × 200–400 nm), (b) nanoparticles (25 nm) and (c) particles (200–250 nm).

Table 1 Size and morphology of nanocrystalline lanthanum hexaboride and fabrication of LaB₆ film

Reaction conditions La^3+ (0.1 M) + OH^− (x molar) + 10% surfactant hydrothermal process: 120 °C/2 days	Shape and size of the lanthanum precursor	Size and morphology of lanthanum hexaboride	Surface investigation of lanthanum hexaboride film using atomic force microscopic study	Field emission study
1. Surfactant: cationic, CTAB	Particles	—	—	—
x = 0.1 M	[Fig. 2a]
2. Surfactant: anionic, AOT	Stable dispersion was formed	—	—	—
x = 0.1 M
3. Surfactant: non- ionic, Tergitol	Dia. 8–10 nm and length 120 nm	Nanorods (dia. 30 nm, length 200–400 nm, aspect ratio 6.6)	Vertically aligned nanorods	Turn on field (at 1 μA cm^−2) = 3.06 V μm^−1, maximum current density at 12 V μm^−1 = 51.66 μA cm^−2
x = 0.1 M.	[Fig. 2b]	[Fig. 4a]	[Fig. 8a]	Field enhancement factor, β1 = 1289, β2 = 4191 [Fig. 9,10a]
4. Surfactant: non- ionic, Tergitol	Dia. 8–10 nm and length 240 nm	Spherical particles 25 nm	Flat spherical particles 25 nm	Turn on field (at 1 μA cm^−2) = 3.15 V μm^−1
				Maximum current density at 12 V μm^−1 = 6.63 μA cm^−2
x = 0.2 M	[Fig. 2c]	[Fig. 4b]	[Fig. 8b]	Field enhancement factor β1 = 1240, β2 = 4009 [Fig. 9,10b]
5. Surfactant: non- ionic, Tergitol	Dia. 8–10 nm and length 300–500 nm	Nanoparticles ∼200–250 nm	Flat spherical particles 0.2–0.25 μm	Turn on field (at 1 μA cm^−2) = 8.28 V μm^−1
				Maximum current density at 12 V μm^−1 = 2.10 μA cm^−2
				Field enhancement factor, β1 = 1216, β2 = 3652
x = 0.3 M	[Fig. 2d]	[Fig. 4c]	[Fig. 8c]	[Fig. 9,10c]

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3. Experimental

Lanthanum nitrate hexahydrate (Spectrochem, 99.98%, India), Tergitol (Sigma Aldrich, 99.99%), NaOH (99%, Fischer Scientific, India), CTAB (cetyltrimethyl ammonium bromide) (99.99% Spectrochem, India) and boron (99.98%, Loba Chem., India) were used as starting materials. Refluxing in acid medium was carried out to further purify the boron powder.

3.1 Synthesis of lanthanum hydroxide precursor

To an aqueous solution of lanthanum nitrate (0.1 M, 250 ml) containing 10% surfactant, 250 ml of NaOH (0.1 M) containing 10% surfactant (CTAB or Tergitol) was added and subsequently stirred for 30 min. The resulting solutions were further loaded into a one litre Teflon lined autoclave and heated at 120 °C for two days. Three different surfactants (CTAB or Tergitol or AOT) with different charges were used to investigate the effect of surfactant charge on the morphology of the lanthanum hydroxide precursor. The resulting white products were separated by centrifugation, followed by washing with acetone, and dried in air. It may be noted that the use of AOT as a surfactant leads to the formation of stable dispersion and hence the product could not be isolated. The effect of pH in the reaction has been studied by varying the concentration of sodium hydroxide (0.2 M and 0.3 M) in the presence of non ionic surfactant Tergitol.

3.2 Synthesis of polycrystalline lanthanum hexaboride

The as-obtained lanthanum hydroxide precursors, obtained using 0.1 M, 0.2 M and 0.3 M NaOH, were mixed with boron in 1 : 9, 1 : 19 and 1 : 23 weight ratios, respectively, and annealed under an argon atmosphere at 800 °C for 6 h and subsequently at 1300 °C for 6 h. The as-obtained product was further purified by washing with concentrated acid and then with water and acetone and dried in a vacuum desiccator.

3.3 Fabrication of lanthanum hexaboride film

50 mg of as-obtained lanthanum hexaborides (obtained using Tergitol surfactant at various pH) were dispersed in 2 ml of a mixture of ethanol and ethylene glycol (1 : 1 volume ratio). 200 μl of the resulting suspension was subjected to spin coating on the Si substrate (1 inch × 1 inch) at 3000 rpm for 1 min. The procedure was repeated twice. After spin coating, the samples were subjected to drying at 200 °C via a slow drying process (with slow heating rate and cooling rate of 20 °C h⁻¹) under an argon atmosphere.

3.4 Characterization

X-ray diffraction studies were carried out on a Bruker D8 advance X-ray diffractometer using Ni-filtered Cu-Kα radiation. A step size of 0.02 deg. with a step time of 1 s was used for the 2-theta range 10°–70°. The raw data was subjected to background correction, and Kα2-reflections were removed by a stripping procedure to obtain accurate lattice constants. The grain size was evaluated from slow scans with a step size of 0.002° in 2-theta and a step time of 2 s. TEM studies were carried out with a Technai G²20 electron microscope operated at 200 kV. A drop of ethanol with the dispersed powder was taken onto a porous carbon film supported on a copper grid, and then dried under vacuum. Characterization of the CeB₆ film on the silicon substrate was carried out by glancing-angle X-ray diffraction (GAXRD) studies using a Phillips X'Pert, PRO-PW 3040 diffractometer operating in Bragg–Brentano geometry with Cu-Kα radiation. The diffractograms were obtained for 2θ ranges between 10° and 70° at a fixed glancing angle of 0.5 °C. The ellipsometry studies were performed to measure the refractive index in the range 200–1000 nm using a M-2000F ellipsometer (J. A. Woollam Co., Inc.). The experimentally determined ψ (psi) and Δ (delta) values were found for angles of incidence at 55°, 65° and 75°, and the data were fitted using optical models based on WVASE32 software. The atomic force microscopy investigation was carried out using Digital Instruments/Nanoscope-IIIa AFM. Field emission (FE) measurements were performed in an indigenously developed UHV FE set-up using cathode anode planar geometry, with a base pressure of ∼2.5 × 10⁻⁶ Pa at room temperature. An APLAB high voltage DC power supply (0.5–5 kV; 20 mA) and a Keithley multimeter were used to measure current–voltage (I–V) characteristics. The applied electric field was estimated by dividing the applied voltage by the inter-electrode gap (typically, 250 μm). The turn-on field (F_t) is taken as the field strength where the emission current density (J) reaches 1 μA cm⁻². High voltage was stepped up and down three times to check the repeatability of the field emission (FE) results.

4. Conclusions

A novel route to control the anisotropy of lanthanum hexaboride nanostructures mediated by a hydroxide precursor has been developed. The precursor mediated route leads to boride nanostructures at much lower temperature and has the potential for application to a wide variety of metal borides. The hydrothermally grown hydroxide precursor plays a significant role in obtaining the desired morphology, which, combined with the spin coating, was effectively used to obtain highly uniform vertically aligned nanorods that show significantly superior field emission properties. The next step would be to scale up the synthesis so as to obtain large scale production of the precursor (lanthanum hydroxide) with an appropriate aspect ratio, and also to further lower the temperature of the borothermal reaction along with large scale synthesis of the lanthanum hexaboride powder.

Acknowledgements

AKG thanks the CSIR and DST, Govt. of India for the financial support. Menaka thanks UGC, Govt. of India for a fellowship.

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