液相法制备取向ZnO纳米线阵列的场发射特性

September 物理化学学报(Wuli Huaxue Xuebao) Acta Phys．一Chim．Sin．，2010，26(9)：2563—2568 2563 【Article】 www．whxb．pku．edu ．an 液相法制备取向ZnO纳米线阵列的场发射特性 张 欢 李梦轲 ” 张 竞 于丽媛 刘玲玲 杨 志 z ( 辽宁师范大学物理与电子技术学院，辽宁 大连 1 16029； 上海交通大学微纳科学技术研究院， 微米／纳米加工技术国家级重点实验室，薄膜与微细技术教育部重点实验室，上海 200240) 摘要： 采用水热合成工艺，在不同条件下制备了不同的一维取向 ZnO纳米线阵列样品．用 x射线衍射仪 (XRD)、扫描电镜(SEM)J~透射电镜(TEM)对样品的晶体结构和形貌等进行了 表 关于同志近三年现实表现材料材料类招标技术评分表图表与交易pdf视力表打印pdf用图表说话 pdf 征，对样品的场发射特性进行了 分析和比较，并用 Fowler-Nordheim方程对影响ZnO纳米线场发射的因素进行了研究．结果表明，具有较低生长 密度分布、较高的长径比和较尖锐生长端的 ZnO纳米线阵列样品具有较好的场发射特性． 关键词： ZnO； 纳米线阵列；场发射； 液相法 中图分类号： 0649 Field Emission Properties of Aligned ZnO Nanowire Arrays Prepared by Simple Solution-Phase M ethod ZHANG Huan LI Meng—Ke · ZHANG Jing YU Li—Yuan LIU Ling—Ling YANG Zhi。 School ofPh sics and Electronic Technology．Liaoning Normal University,Dalian 116029,Liaoning Province．P．R．China； 2NationalKeyLaboratory ofNano／Micro Fabrication Technology,KeyLaboratoryfor Thin FizmandMicrofabrication of theMinistryofEducation，ResearchInstituteofMicro／NanoScienceandTechnology, Shanghai Jiao Tong University,Shanghai 200240,P．R．China) Abstract： One—dimensional(1D)aligned ZnO nanowire arrays wit}l different morphologies were synthesized by a solution．phase method．The morphology and microstructure of the products were characterized by X—ray diffraction (XRD)．scanning electron microscopy(SEM)．and transmission electron microscopy(TEM)．The field emission property ofdifferent ZnO nan owire array samples was compared．The factors that influence the field emission prope rty ofthe ID ZnO nanowire arrays were analyzed using the Fowler-Nordheim equation．The results showed that the ZnO nanowire samples wim the 1ower areal density．higher aspect ratio．and thin tips showed much better field emission characteristics． Key W ords： ZnO； Nanowire array； Field emission； Solution—phase method High—quality field emitters are very desirable for applications in a wide range of field．emission—based devices such as flat—pan． el displays and other electronic devices．1D ZnO semiconductor nanostructures with its inherent properties of larger length--to--di·- ameter，higher surface—area—to．volume ratio，thermal stability， oxidation resistance，and high chemical stability should be a good candidate for field emission applications．Recently，the 丘eld emission property of different 1D ZnO nanostructures．such as nanowires，nanoneedles，nanopins，and nanotubes has been studiedt1--5]．The previous research on 1D ZnO nanostructure field emitters showed that the shape，aspect ratio，screening effect， and contact behavior(both rnechanical and electrica1)are the Received：May 20，2010；Revised：M．dy 25，2010；Published on W eb：July 15，2010． ’Corresponding au~or．Emall：lmknwnu@sina．corn；Tel：+86—41 1-82159023． The project was supported by the Innovation Team Foundation of Educational Department of Liaoning Province，China(2007T088)，Natural Science Foundation ofLiaoningProvince，China(20072155)，ConstructionCapitalforKeyLaboratoryofLiaoningProvince，Doctoral ScientificResearch Starting Foundation of Liaoning Province，China(20081081)，and National Natural Science Foundation of China(10804040)． 辽宁省教育厅创新团队(2007T088)，辽宁省自然科学基金(20072155)，辽宁省重点实验室建设基金，辽宁省博士后科研启动资金(20081081)及国 家自然科学基金(10804040)资助项 目 ⑥Editorial office of Acta Phyrsico—Chimica Sinica 2564 Acta P s．一Chim．Sin．．2010 VO1．26 primary influence factors on the field emission properties[ 卅． However，the field emission properties of 1 D ZnO nanowires with different geometrical structures still have many obscure problems to deal with．For example，the field emission proper— ties of ZnO nanowire arrays with regular high aspect ratio and the effect of emitter density on the field—screening were rarely stud—ied due to the difficulty in the preparation of those emitters wim different densities．Therefore．it is essential to synthesize well·-aligned 1 D ZnO nanostructures and pursue the physical ori— gins of the dependence of the field emission of 1 D ZnO nano· wire emitters on the geometrical factors for improving their field emission properties． There are various methods for synthesizing 1 D ZnO nanos— tructures，such as pulsed laser deposition，thermal evaporation， electrochemical deposition，chemical or physical vapor deposi— tion，solution—phase approach，etc[ 。一 ．But，most of the ZnO nanostructures in these published field emission articles were synthesized with the high—temperature syn thesizing techniques． The high—temperature techniques，including pulsed laser deposi— tion chemical vapor deposition(CVD)[18】’and thermal evapo— rationf 一 ．are energy—consuming and expensive．In most of these studies，the 1D ZnO nanostructures are deposited on the higher resistance silicon and sapphire substrates．Therefore，un— desirable，defective contact resistance can be caused between the ZnO nanostructures and substrates．This result is unfavorable to the enhancement in the field emission current density of 1 D ZnO nanostructure field emitters． Recently,the solution-·phase approaches to produce high·· quality 1 D aligned ZnO nanostructures have attracted extensive interest on account of their low growth temperature(<1 00℃)， low cost，no metal catalyst needed，easy to control，and good po— tential for scale-up with general substrates[10-161．In addition，this solution-phase controlled fabricating approach can grow 1D ZnO nanostructures directly on various metal foils．Then，robust electrical contact can be formed in the growth processes．This better electrical contact is beneficial to rational design s With dif- ferent sizes for raising the field emission current density of the 1D ZnO nanostructure field emitters．Meanwhile，the controlled fabrication of high-·quality ZnO nanostructures with low temper-- ature，facile manipulation，and potential for scale—up can enable the straightforward integration ofZnO nanostructures into nano— electronic devices，such as field emission displays and mi— cro／nanosensors． In this article，well—align ed 1D ZnO nanowire arrays were fab— ricated on the zinc foil using a very simple hydrotherm al reac— tion method at a low temperature(95℃)．The comparative in— vestigation on the field emission properties of different ZnO nanowire array samples was carried out．Th e influence factors of field emission property were analyzed． 1 Experimental Align ed ZnO nanowire arrays were directly prepared on zinc foils(99．99％．0．2 mnl thick)reacting in aqueous ammonia solu． tion．Beforehand zinc foils r10 mmxl0 mm)were ultrasonically washed in analytical grade acetone，ethanol，and deionized water for 20 min，successively．The effects of solution concentration and growth time on the microstructure of ZnO nanowire arrays had been studied．f l 1 Some zinc foils were dipped into corre— sponding reactive aqueous ammonia for 3 h at room temperature to forlTl a ZnO—seed film．and then the treated zinc foils were vertically immersed into 20 mL aqueous ammonia solution of 4％，7％，10％，and 15％(V／ for 24 h，seperatively．(2)Some treated zinc foils were reacted in 1 5％ aqueous ammonia for 6． 12．24．and 48 h．All ofthe reacting processes were performed in a sealed Teflon reaction kettle(25 mL)heating at a constant temperature of 95℃． The obtained ZnO nanowire products were then rinsed with deionized water and dried in air for further characterization．Th e morphology and microstmcture of syn thesized nanowires were characterized by X—ray diffraction(XRj)，Rigaku DMAX PSPC MDG 2000)，scanning electron microscopy(SEM，KYKY— l01 o)，andtransmission electronmicroscopy(TEM，JEOL一2010)． Field emission properties ofthe dilyerent samples were carried out inside a vacuum chamber，which was pumped down to about 3．1 xlO Pa at room temperature．The tests were measured using a simple diode configuration．The cathode was the as—grown ZnO nanowires and the zinc foil was used as a cathode—conduct． ing layer．The anode was polished pure copper rod．The gap be— tw een cathode and anode was controlled by the thickness of a mica spacer containing a 2 mm circular hole in the center．VoIt． ages up t0 2．5 kV were applied to the anode with a step of 100 V．And the emission current(n was detected with a micro am— perometer．The testing electric field( was estimated from di- viding the applied voltage( by the anode—cathode distance( ． The emission current density r was calculated from the ob— rained emission current and the area of the rounded hole in the mica．The emission current—voltage characteristics were ana． 1yzed by using the Fowler—Nordheim equation． 2 Results and discussion Fig．1(a一(I)show low—and high—magnification SEM and TEM images of the ZnO nanowire arrays synthesized in different pre． cursor concentrations of aqueous ammonia solution at 95℃ for 24 h．From the images．1arge—scale well—oriented ZnO nanowire arrays were observed with uniforill and dense arrays．They are approximately of the same length，8-10 m．The TEM inset in Fig．1 fd)testifies that the ZnO nanowires have a smooth surface without catalytical growt h droplets at their growth tips．In each sample．the diameter of ZnO nanowires has liale variation from bottom to top．As increase of precursor concentrations from 4％ to 1 5％。the mean diameter of the nanowires altered from 600． 400，250，to 150 nm，the growt h density changedfrom 2，6，9to 25 txm ．And the diameter sizes of the grown ZnO nanowires were reduced and the growth density in a unit area was en． hanced．According to these basic va1ues f_rom the SEM ima es in Fig．1， the specific surface area of the syntl1esized Zln0 No．9 ZHANG Huan et a1．：Field Emission Properties of Aligned ZnO Nanowire Arrays 2565 Fig．1 Typical SEM images of ZnO nanowire arrays fabricated in various aqueous ammonia concentrations for 24 h at 95℃ The insets are low—and high—magnification SEM and TEM pictures of the ZnO arrays and individual nanowires．ammonia concentration：(a)4％，(b)7％，(c)10％，(d)15％ nanowlre arrays in a unit area was calculated．The calculated ra— tio of specific surface area is about 6．86：30．6：72．9：333．So the specific surface areas of synthesized ZnO samples increased with the increase of precursor concentration from 4％ to 1 5％ ． Fig．2 shows the corresponding XRD pattern of ZnO nanowire arrays in different aqueous ammonia concentrations for 24 h． The three diffraction peaks of(002)，(1OO)，and(1O1)show good agreement with those of the JCPDS(36—1451)data of the ZnO (a=0．325 nm，c=0．521 nm)．The sharp and narrow(002)diffrac— tion peaks at 34．2。exist in every product．With the increase of aqueous ammonia concentration，the intensity of the(002)diff- raction peaks is enhanced．It indicates that the synthesized ZnO materials are highly aligned perpendicular to the substrate with a c—axial growth direction． The SEM images in Fig．3(a—d)show four different ZnO nan— owire arrays prepared at different reacting time．These re—suits represent that growt h length of ZnO nanowire arrays are a func— tion ofthegrowthtimefrom 6to 48 hin 15％ aqueous am—mo— nia．As the reaction time was changed from 6 to 48 h，the length— wise growt h has experienced from nucleation to short nanowires．to long nanowires．From the initial time to 6 h growth stage，a layer of ZnO growth nucleation with higher dis— tributed density was observed in Fig．3 fa)．The diameter of the growth nucleation distribution is in the range of 240—300 nm， and the average growth length is about 350 nm．When the reae— tlon time was lncreased from 12 to 24 h in Fig．3(b—c)，ZnO nanowires began to grow along the(002)direction obviously． And the length varied from 3 to 10 txm．When the response time was increased to 48 h，an interesting aspect of the as—grown sam— pie in Fig．3(d)is gained．Some longer and sparse ZnO nanowires with high aspect ratio and thinness of the tips are showed on the top surface of the nanowire arrays．The average length is 40 um． The inset in Fig．3(d)presents the high magn ification image of the samples．We think that the diameter of ZnO nanowires on the growing tips will appear sign ificantly different as the in- crease of growth time．The result is that a hi gh diversity in 2o／(。) Fig．2 XRD patterns of ZnO nanowire arrays synthesized at 95℃ and 4％，7％，10％，and 15％ aqueous ammonia for 24 h 2566 Acta Phys．-Chim．Sin．，2010 V01．26 Fig．3 SEM images of ZnO nanowire arrays prepared in 15％ aqueous ammonia at different growth time growth time：(a)6，(b)12，(c)24，(d)48 h；The inset in Fig 3(d)presents the high magnification image ofthe samples． growth rate would be present．The nanowwes with small—diame— ter tips have a larger growth rate and the nanowires with large— diameter tips have a lower growt h rate．In the latter stage of the Zn0 nanowire growth processes．these nanowire arrays appear to form mostly discrete and sparse morphology on the top sur- face of as—grown ZnO nanowire arrays．W hen the growt h time iS long enough，sparse ZnO nanowires with high aspect ratio and thinness ofthe tips can be represented． The field emission performances of Zn0 nanowire samples Fig．4 _E behaviors of ZnO nanowire arrays synthesized in different ammonia solu廿ons for 24 h at 95℃ ammonia concentration：(a)4％，(b)7％，(c)10％，(d)l5％； The inset is corresponding Fowler-Nordheim plots of four differentZnO nanow~e arrays samples synthesized under different conditions have been invest／gated． Firstly．the Cffect of diverse ZnO nanowire diameters on their field emission characteristics was studied．Fig．4 gives the J-E characteristic CHIVe of four kinds ofZnO nanowire samples cot— responding to that of Fig．1(a—d)．It shows that the field emission current density(J3 from the different samples is a function ofthe applied electrical field(E)．For all the samples，as the E is in- creased，the emission current density J iS also elevated。and no saturation of J iS evident under the highest E．The turn．on field (点 )and threshold field( )values of all the samples were evaluated．Generally，E and E are arbitrarily defined as the electrica1 fields underwhich aJof1O0 LLA·Cm严and l mA·cm--2 can be observed．respectively．Table l lists the measured values o{E and Eh for all the samples．It shows that the larger the mean diameter of the ZnO nanowires iS，the higher the values of Table 1 El删Em andflvaluesofZnOnanowirearrays synthesized in different aqueous ammonia at 95℃ for different tlme No．9 ZHANG Huan et a1．：Field Emission Properties of Aligned ZnO Nanowire Arrays 2567 E／fV tam 。、 Fig．5 -E behaviors of ZnO nanowire arrays prepar ed in 15％ aqueous ammonia for different reaction time reaction time：(a)6，(b)12，(c)24，(d)48 h． The inset is corresponding Fowler—Nordheim plots their Em and E are attained． The published article show that electrical contact．emitter ge— ometry and screening effect are three prerequisites for field emission property ．The direct growth of aligned ZnO 1D nan— owires on conducting metal substrates via a simple solution— phase approach may facilitate their electrical contact with the externa1 circuit．These arrays will，therefore，have smaller and values．Meanwhile，the inset in Fig．4 shows approximate linear relations between ln(J／E2)and 1 ，suggesting that the ele— ctron emission could be well formulated by the Fowler—Nordheim theo ， ln(J／EO=(-B~b +ln(A／32／~b) where， is the field enhancement factor， is the work function of the emitter which is 5．3 eV for ZnO materia1『24]．A and B are constants with the value of 1．56×10 A ·V ·eV and 6．83xlO。 V ‘eV ‘Ixm～，respectively．The could be derived from the slope of In( )一(1 )，and the values of／3 are estimated and listed in Table 1．It clearly demonstrates that the sample in Fig． 1a has the smallest／3 value of 1．1xlO and the sample in Fig．1d has the largest／3 value of2．4xlO ． Field emission characteristic is correlated with the morpholo- gY ofZnO nanowires．In these field emission experiments in Fig． 4，four ZnO nanowire samples are approximately of the same length，8-10 m．W e can see from the SEM images that ZnO nanowiresinthefour samples showhighgrowthdensity，andthe distances betw een single ZnO nanowire is in the range of 20 nm to tens of nanometer in space．So strong field screening effect will be generated．Then，the worse field emission property with the higher E and E as well as a relative small／3 value is pro— duced．These results come from the screening effect of dense growth of 1D ZnO nanowires． Ku et a1． demonstrated thatⅡ1e geometric construction such as tip cone angle( )and tip radius( 6p)are two important fac— tors for the field emission property of 1 D nanostmcture emitter， i，e．，small 0 and are beneficiaJ to field emission Ramgir et a1． had reportedthetheoreticalfield enhancementfactor／3o=1／kr． where k is a constant known as the geometrical factor．and r is the radius．Without doubt，The sample in Fig．1(a)has the largest diameter，so it has the smallest value．And the sample in Fig．1 (d)has the smallest diameter，so it has the largest／3 value．On the other hand，the high aspect ratio of the nanowires can generate a high electric field，which decreases the field emission potential barrier and so increases the field emission current[ ．As a result， ZnO nanowires in Fig．1(d)with smaller diameter and higher as- pect ratio have largerB value(2．4x10 )and thus smaller turn．on field(10 V·txm )． Secondly，the field emission behaviors of ZnO nanowire sam— ples with four different reaction time in 1 5％ aqueous ammonia were measured．which correspond to the SEM images of Fig．3 (a-d)．The reaction times are 6，12，24 and 48 h and the growth lengths of these ZnO nanowires are about 350 nm，3，1 0 and 40 m，respectively．Fig．5 gives the J-E curve from these ZnO nanowire samples．The inset is corresponding Fowler-Nordheim plots．Tab1e 1 shows the measured values of E ，and for these samples．It can be seen that the ZnO nanowire arrays fabri— cated for 6 and 1 2 h have the worse field emission property with the highest and values，and their／3 values are about 1．5x 10。and 2．OxlO ．Meanwhile．the samples fabricated for 24 h has the better field emission property with the lower Elm and E val— ues，and B is about 2．4x10 ．But the samples fabricated for 48 h has the best field emission property with the lowest E and Eh values，and its value is about 3．2×10 ．The applied field re— quired to 1 mA ·cm-2 is about l1 V·LLm～．This result is smaller than the corresponding E value of 22 V ‘ixm—in the samples fabricatedfor 6 h． It is well known that the field enhancement factor卢reflects the enh anced electron emission due to the 1ocalized electronic states and the value of／3 can be modified by other e~cts such as the field—screening eriect from the proximity of emitters．After obtaining the values of／3 of the four different samples，we can analyze quantitatively the field—screening effect on the field emission property from the ZnO nanowire arrays with different coverage densities on the top growth surface．In the sample of Fig．3(d)，the thinness ofthe tip，high aspect ratio and lower COV- erage densities on the top growth surface are appeared．111e dis— tance between the adjacent emission sites is enlarged compared with the other samples，which can decrease the electrostatic screening effect and increase the effective emission sites．There- fore，this growth morphology will substantially reduce the result of the screening efrect．So the excellent field emi ssion property and the higher B value are gained．But the ZnO nanowire emit— ters in Fig．3(a，b)are main consisted of a layer of spherical ZnO growth nuclei with high coverage density．An d some growth nu。 clei have not clear growth direction．The loca1 field produced from hJ gher coverage of ZnO emitters will decrease the口value owing to the screening effect【1JJ At the same time．many st