Manipulating the anisotropy and field emission of lanthanum hexaboridenanorods

Menaka,a Rajkumar Patra,b Santanu Ghoshb and Ashok Kumar Ganguli*a

Received 5th March 2012, Accepted 17th June 2012

DOI: 10.1039/c2ra20399e

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 (y500 uC lower than the conventional solid state route)

and allows for variation of morphology of nanostructured films. Vertically aligned nanorods (30 nm

6 200–400 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.

1. Introduction

Lanthanum hexaboride is widely used for high electron density

cathodes because of its high melting point of 2500 uC,1 high

electrical conductivity,2 low work function (y2.6 eV)3 and low

vapour pressure at high temperatures.4 Recent reviews on the

field emission properties of various metal oxides, nitrides and

borides5–7 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

(y1800 uC),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 (y500 uC 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

aDepartment of Chemistry, Indian Institute of Technology, Hauz Khas,New Delhi, 110016, India. E-mail: ashok@chemistry.iitd.ernet.in;http://www.chemistry.iitd.ac.in/faculty/ganguli.html;Fax: 91-11-26854715; Tel: No: 91-11-26591511bDepartment of Physics, Indian Institute of Technology, Hauz Khas, NewDelhi, 110016, India

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temperature of 1300 uC (normally lanthanum hexaboride is

synthesized at 1800 uC). 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 mm (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 uC), 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 crystal-

lographic 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 mm in diameter are formed. Investigation of the time-

dependent hydrothermal reaction shows27 that the growth of the

lanthanum hydroxide nanorods at 150 uC 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 6 200 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).

The reaction of the lanthanum hydroxide precursor (obtained

using Tergitol as a capping agent) with boron at 1300 uC leads to

the formation of pure lanthanum hexaboride (cubic, Pm3m, a =

4.159 A, (#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 uC, 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 uC and 1500 uC, 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 uC; the morphology and size of the lanthanum

hexaboride nanostructures depends on the initial precursor

yielding nanorods (30 nm 6 200–400 nm) and other nanos-

tructures. Transmission electron micrographs of LaB6 show

the presence of lanthanum hexaboride nanorods (30 nm 6 200–

400 nm) (Fig. 4a), nanoparticles (25 nm) (Fig. 4b) and much

larger particles of 0.2–0.25 mm (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 nanoparticlesFig. 1 PXRD studies of the lanthanum hydroxide precursor.

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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.

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 A

(#JCPDS: 340427) (Fig. 7a–c). Atomic force microscopy study

of the film fabricated using polycrystalline LaB6 nanorods

(30 nm 6 200–400 nm), LaB6 nanoparticles (y25 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 (y96%

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.

The field emission characteristics of the LaB6 films obtained

using the hydrothermal route are shown in Fig. 8. The current

Fig. 2 Transmission electron microscopic study of the lanthanum hydroxide precursor obtained at 120 uC 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.

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density (J) is given by the Fowler–Nordheim equation29

J~1:54|10{6 b2E2

Wexp

{6:83|107W32

bE

�����

�����

where E is the applied field strength (in V mm21), W is the work

function of the emitters, b is the field enhancement factor and

can be expressed as b = 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 emitters30–32 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 6 200–400 nm) show much higher (nearly 8

fold) current density of 51.66 mA cm22 compared to the 25 nm

nanoparticles (6.63 mA cm22). The films with larger particles (200–

250 nm) show an even lower current density of 2.10 mA cm22. The

comparison of the current density was carried out at a field

strength of 12 V mm21. The LaB6 film consisting of nanorods

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

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.

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shows low turn-on fields of 3.06 V mm21 (measured at 1 mA cm22),

while the LaB6 film with smaller nanoparticles (25 nm) shows a

turn-on field of 3.15 V mm21 compared to the larger particles of

200–250 nm (turn on field 8.28 V mm21). 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

(b), which is equal to bW3/2/slope, where b is a constant with

a value of 6.83 6 103 V eV23/2 mm21 and W 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 mm V21) for the nanorods (30 nm 6 200–400 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 mm V21), the corresponding field

enhancement factors are 4191 (nanorods, 30 nm 6 200–400 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 mm V21).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.

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.,

Fig. 4 Transmission electron microscopic study of lanthanum hexaboride obtained at 1300 uC using a lanthanum hydroxide precursor synthesized

using (a) Tergitol/initial pH = 8, (b) Tergitol/initial pH = 10 and (c) Tergitol/initial pH = 13.

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Fig. 5 HRTEM of lanthanum hexaboride obtained at 1300 uC 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 uC starting from a lanthanum hydroxide precursor synthesized

using (a) Tergitol/initial pH = 8, (b) Tergitol/initial pH = 10 and (c)

Tergitol/initial pH = 13.

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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 uC 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 uC for 6 h and

subsequently at 1300 uC 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 ml of the resulting suspension was subjected to spin coating

on the Si substrate (1 inch 6 1 inch) at 3000 rpm for 1 min. The

procedure was repeated twice. After spin coating, the samples

were subjected to drying at 200 uC via a slow drying process (with

slow heating rate and cooling rate of 20 uC h21) 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-Ka radiation.

A step size of 0.02 deg. with a step time of 1 s was used for the

2-theta range 10u–70u. The raw data was subjected to back-

ground correction, and Ka2-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

0.002u 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-Ka radiation. The diffractograms were obtained for 2h

ranges between 10u and 70u at a fixed glancing angle of 0.5 uC.

Fig. 7 GAXRD studies of films fabricated using lanthanum hexaboride (a) nanorods (30 nm 6 200–400 nm), (b) nanoparticles (25 nm) and (c)

particles (200–250 nm).

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Fig. 8 AFM studies of lanthanum hexaboride film fabricated using lanthanum hexaboride (a) nanorods (30 nm 6 200–400 nm), (b) nanoparticles

(25 nm) and (c) particles (200–250 nm).

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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 y (psi) and D (delta) values were found for angles of

incidence at 55u, 65u and 75u, 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) mea-

surements were performed in an indigenously developed UHV

FE set-up using cathode anode planar geometry, with a base

Fig. 9 Field emission study of films fabricated using polycrystalline LaB6 (a) nanorods (30 nm 6 200–400 nm), (b) nanoparticles (25 nm), (c) particles

(200–250 nm) and (d) bare Si.

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

Reaction conditionsLa3+ (0.1 M) + OH2 (x molar)+ 10% surfactant hydrothermalprocess: 120 uC/2 days

Shape and size ofthe lanthanumprecursor

Size and morphology oflanthanum hexaboride

Surface investigation oflanthanum hexaboridefilm using atomic forcemicroscopic 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 M3. Surfactant: non- ionic, Tergitol Dia. 8–10 nm and

length 120 nmNanorods (dia.30 nm, length200–400 nm,aspect ratio 6.6)

Vertically alignednanorods

Turn on field (at 1 mA cm22)= 3.06 V mm21, maximumcurrent density at 12 V mm21

= 51.66 mA cm22

x = 0.1 M. [Fig. 2b] [Fig. 4a] [Fig. 8a] Field enhancement factor, b1

= 1289, b2 = 4191 [Fig. 9,10a]4. Surfactant: non- ionic, Tergitol Dia. 8–10 nm and

length 240 nmSphericalparticles 25 nm

Flat sphericalparticles 25 nm

Turn on field (at 1 mA cm22)= 3.15 V mm21

Maximum current density at12 V mm21 = 6.63 mA cm22

x = 0.2 M [Fig. 2c] [Fig. 4b] [Fig. 8b] Field enhancement factor b1

= 1240, b2 = 4009 [Fig. 9,10b]5. Surfactant: non- ionic, Tergitol Dia. 8–10 nm and

length 300–500 nmNanoparticlesy200–250 nm

Flat sphericalparticles 0.2–0.25 mm

Turn on field (at 1 mA cm22)= 8.28 V mm21

Maximum current density at12 V mm21 = 2.10 mA cm22

Field enhancement factor,b1 = 1216, b2 = 3652

x = 0.3 M [Fig. 2d] [Fig. 4c] [Fig. 8c] [Fig. 9,10c]

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pressure of y2.5 6 1026 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) char-

acteristics. The applied electric field was estimated by dividing

the applied voltage by the inter-electrode gap (typically, 250 mm).

The turn-on field (Ft) is taken as the field strength where the

emission current density (J) reaches 1 mA cm22. 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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