六硼化铕亚微米管的电子场发射性能

许军旗王艳蕊赵彦明张勤远余本海孙海斌

(1信阳师范学院，信阳 464000)

(2华南理工大学，广州 510640)

六硼化铕亚微米管的电子场发射性能

许军旗＊,1王艳蕊1赵彦明＊,2张勤远2余本海1孙海斌1

(1信阳师范学院，信阳 464000)

(2华南理工大学，广州 510640)

以铕(Eu)粉末和三氯化硼(BCl3)作为前驱体，用简单的化学气相沉积方法制备了六硼化铕(EuB6)亚微米管。用XRD，SEM拉曼光谱，和TEM来表征EuB6微米管的形貌和晶体结构。场发射测试结果表明：根据F-N公式，场发射电流和电场之间在不同的阶段分别满足直线关系。

EuB6；化学气相沉积；纳米管；场发射；稀土

0 Introduction

Rare-earth(RE)hexaborides have been widely used in modern technology as an excellent electron source,both as hot and cold cathodes,as they offers high brightness and longer life.The advantage of using RE hexaborides originated from the crystal structure,which is composed of RE metal atoms embedded inside a stable boron octahedron network.This arrangement imparts a unique combination of all the desired properties for an excellent cathode material such as low work function,low volatility at high temperature,long service life time,high conductivity,high chemical resistance,and high mechanical strength,while these properties seldom coexist in any other materials[1-2].The synthesis,structure and field-emission(FE)properties of one-dimensional(1D)nanostructural RE hexaborides have attracted considerable interests from both scientific and technological areas of research over the past several years[2-10].Recently,Zhang et al.[2]hasdemonstrated field emission from a single LaB6nanowire emitter with a current density as high as 5×105A·cm-2working at room temperature with a working voltage of 800 V,which is one order of magnitude higher than that of the state-of-the-art W/ZrO thermal field emitter working at 1 800℃with a 3 000 V extraction voltage.The LaB6nanowire has also an emission current density of the same order of magnitude as that of an individual carbon nanotube,which is 10～50 times smaller in diameter.Moreover,a single GdB6nanowire emitter could produce more than 50 nA emission current at 750 V,which is more than 5 times larger than the single LaB6nanowire emitter working under the same condition[3].Europium hexaboride(EuB6)submicrotubes,one of the rare earth hexaborides,have attracted the attention of many scientists due to their interesting properties,such as the very high neutron absorption for the fast neutrons,fluctuating-valence semiconductor and Kondo insulators effect[11-12].Knowledge of the electrical properties of EuB61D submicrotubes is the first step to realizing their application in electronic/photonic nanodevices.However,to the best of our knowledge there has been no report on the electrical properties of 1D EuB6submicrotubes.In a recent paper,we reported a successful synthesis of[100]oriented EuB6nanowires with diameter of 100 ～300 nm[13].Here,we report the synthesis and structural characterization of polycrystalline EuB6submicrotubes with diameter of 0.5 ～1.5 μm and length more than a few tens of micrometers.FE from the 1D EuB6submicrotubes has also been investigated for the purpose of better understanding the FE mechanism of RE hexaborides.

1 Experimental

EuB6submicrotubes were prepared by a simple,one-step chemical vapor deposition(CVD)method on silicon substrates by using Eu powders and BCl3gas as precursors without Pt/Pd catalyst.The reaction was conducted in a tube furnace operated at 950℃where Eu powders(99.99%)direct reacted with the introduced BCl3gas under an atmosphere of hydrogen and argon(50%H2+50%Ar with gas flow of about 120 mL·min-1).The products were characterized by means of the XRD,SEM,TEM and HRTEM,respectively.The phase purity of the as-synthesized products were examined by XRD using a DRIC Y-2000 X-ray powder diffractometer(30 kV,20 mA)equipped with monochromatizedCu Kα radiation(λ=0.154 06 nm).A step scan mode was adopted with a scanning step of 0.02°and a sampling time of 3 s,and the patterns were recorded in the 2θ range of 20～90 ℃.The morphology of the prepared products was observed on a scanning electron microscope(LEO 1530 VP,30 kV).The TEM(HRTEM)was carried out on a JEOL 2010 transmission electron microscope(operated at 200 kV).

To assess the performance of the 1D EuB6submicrotubes as a field emitter,the FE properties were measured with a high vacuum level around 1.33×10-4Pa or lower.The measurement was conducted on a standard parallel-plate-electrode configuration where the indium tin oxide glass substrate(ITO)was used as the anode shown in Fig.1.The anode was separated from the EuB6surface by a 120 μm gap using a teflon film as spacer.Both the anode and the cathode were connected to a computer-controlled Keithley 248 source meter.The diameter of the current emission area was 5 mm.

2 Results and discussion

The XRD pattern of the products is shown in Fig.2(a).All of the diffraction peaks can be indexed according to the cubic phase of EuB6with a lattice constant a of 0.416 1 nm,which is consistent with the value in thestandard card(PDF No.65-5933).In addition,intense Raman bands are found at 761,1 098,and 1 240 cm-1,respectively,in the range of 600 to 1 500 cm-1(Fig.2(b)),which is assigned to the Raman active vibrational modes of T2g,Egand A1g,respectively[14-15].These three prominent peaks are additional confirmation for the formation of hexaborides since the peaks obey the selection rule for the hexaborides cubic symmetry.

Figs.3(a)to(c)show typical SEM images of the assynthesized EuB6products.From Fig.3(a),one can see that the EuB6product contains numerous tube-like submicrotubes with diameters from 0.5 to 1.5 μm and lengths up to several micrometers.It is worth noting that nanostructural EuB6(lateral dimension within 100 nm)is also found on the same substrates,although the amount is less.Fig.3(d)is a low-magnification TEM micrograph,showing a EuB6nanotube.Fig.3(e)is the high-resolution TEM image of the section as marked by a small square in Fig.3(d).The HR-TEM stripes are not parallel,which confirms that the tube is polycrystalline in nature,and the interplanar d-spacing is 0.298 nm,corresponding to the lattice of(110).

The growth of EuB61D submicrotubes is unlikely to obey the vapor-liquid-solid(VLS)growth mechanism.In the VLS mechanism,a liquid nanoparticle is present in the growth frontier of each 1D submicrotube to receive the gas phase precursor and guide submicrotubes growth[16].In our case,all of the obtained EuB6submicrotubes are absent of siliconcontaining nanoparticles from condensed melts in the tip part as shown in Fig.3,excluding the VLS mechanism possiblely due to the self-catalysis[9,17].Therefore,we suggest that the growth mechanism of ourEuB6submicrotubes is the vapor-solid(VS)mechanism[18].

Theoretically,The FE characteristics can be evaluated by the simplified Flower-Nordheim(FN)equation,

J=(Aβ2E2/ø)exp[-Bø3/2/(βE)]

where J is the current density,E is the applied field strength,β is the field enhancement factor.φ is the work function of the electron emitter,A and B are constant with the value of 1.56×10-10A·V-2·eV and 6.83×103V·eV-3/2,respectively[19-20].Fig.4(a)shows the dependence of the field emission current density J on the applied electric field strength E of the 1D EuB6submicrotubes,from which we can see that the J rises rapidly with increase in E.In FN equation,β is introduced to reflect the true value of the electric field at the tip compared to its average macroscopic value.The value of β can be calculated from the slope of the plot of ln(J/E2)-1/E.For example,the plot of ln(J/E2)-1/E(Fig.4(b))presents two distinct stages with the estimated β value of 505 at lower applied fields(E＜15 V·μm-1)and 1 319 at high higher fields(E ＞15 V·μm-1),respectively,assuming a fixed EuB6work function of 4.11 eV.Two-stage phenomena in FE from EuB6submicrotubes may be the origin to space charge effect,localized state,and adsorbate-enhanced tunneling state,which arefound in carbon nanotubesand ZnO submicrotubes[21-22].The field-emission performance is related to the morphology and the size in addition to intrinsic properties(such as electronic affinity),for example,the conical tips and the small sizes of the nanostructures are helpful for electron emission[23-24].However,in ourfield-emission experiments,the disorder distributing submicrotubes,the polycrystalline of the submicrotubes and the poor conductivity of the Si substrate may reduce the β value.Therefore,the intrinsic field-emission performance of EuB6submicrotubes is probably higher than the values estimated from our experimental data,and thus,the EuB6submicrotubes have the potential application in field-emission microelectronic device.

3 Conclusions

In summary,w e have developed a simple CVD approach to produce 1D functional EuB6submicrotubes using Eu powders and BCl3gas as precursors.XRD,SEM and TEM characterizationsshow thatthe submicrotubes have a diameter of 0.5～1.5 μm and are polycrystalline in nature.The field enhancement factor shows a larger value at higher fields and smaller value at lower voltage.It is attributed to space charge effect,localized state,and adsorbate-enhanced tunneling state in shallow levels,which are excited at low electric field.

Acknowledgement:This work was supported by the National Nature Science Foundation of China(NSFC,No:50472055),Science and Technology Program of Henan Province(No.082300410050, 092300410209, 092300410207) and Foundation of Henan Educational Committee(2010A140015).

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Filed-Emission of Electrons from EuB6Submicrotubes

XU Jun-Qi＊,1WANG Yan-Rui1ZHAO Yan-Ming＊,2ZHANG Qin-Yuan2YU Ben-Hai1SUN Hai-Bin1
(1Xinyang Normal University,Xinyang,Henan 464000)
(2South China University of Technology,Guangzhou 510640)

Functional polycrystalline EuB6submicrotubes were synthesized via a simple,one-step chemical vapor deposition(CVD)method using Eu powders and BCl3gas as precursors without any catalysts.The morphology and crystal structure of the EuB6submicrotubes were characterized by XRD,Raman spectroscopy,SEM and TEM.The dependence of the field emission current density J on the applied electric field E follows a two-stage slope behavior in ln(J/E2)-1/E plot according to Fowler-Nordheim equation.

EuB6;chemical vapor deposition;nanotubes;field emission;rare-earth

O641.3

A

1001-4861(2010)06-1056-05

2009-11-25。收修改稿日期：2010-03-13。

国家自然科学基金(No.50972046)；河南省科技厅科技计划项目(No.082300410050，092300410209，09230041020T)；河南省教育厅项目(No.2010A140015)资助。

＊通讯联系人。E-mail：xujunqi@yahoo.cn；zhaoym@scut.edu.cn

许军旗，男，30岁，博士；研究方向：材料微结构和物性。