李丹 梁君武 劉華偉 張學(xué)紅 萬強 張清林?潘安練?

1)(湖南大學(xué)物理與微電子科學(xué)學(xué)院,微納結(jié)構(gòu)物理與應(yīng)用技術(shù)湖南省重點實驗室,長沙 410082)

2)(玉林師范學(xué)院物理科學(xué)與工程技術(shù)學(xué)院,玉林 537000)

(2016年11月6日收到;2016年12月17日收到修改稿)

CdS/CdS0.48Se0.52軸向異質(zhì)結(jié)納米線的非對稱光波導(dǎo)及雙波長激射?

李丹1)梁君武2)劉華偉1)張學(xué)紅1)萬強1)張清林1)?潘安練1)?

1)(湖南大學(xué)物理與微電子科學(xué)學(xué)院,微納結(jié)構(gòu)物理與應(yīng)用技術(shù)湖南省重點實驗室,長沙 410082)

2)(玉林師范學(xué)院物理科學(xué)與工程技術(shù)學(xué)院,玉林 537000)

(2016年11月6日收到;2016年12月17日收到修改稿)

本文利用可控的化學(xué)氣相沉積法合成了高質(zhì)量的軸向CdS/CdS0.48Se0.52異質(zhì)結(jié)納米線.掃描電子顯微鏡表征發(fā)現(xiàn)這些納米線具有光滑的表面結(jié)構(gòu);熒光顯微圖像表明納米線由CdS和CdSSe兩部分沿軸向構(gòu)成;微區(qū)熒光光譜研究表明界面區(qū)具有高的結(jié)晶質(zhì)量;光波導(dǎo)研究表明異質(zhì)結(jié)納米線具有非對稱光傳輸特性;進一步的激發(fā)功率依賴的熒光光譜研究表明此結(jié)構(gòu)可以實現(xiàn)紅和綠雙波長激射,并且紅色激射閾值低于綠色激射.理論模擬表明波導(dǎo)光可以兩相間有效傳輸.

納米線,異質(zhì)結(jié),光波導(dǎo),激光器

1 引 言

半導(dǎo)體異質(zhì)結(jié)構(gòu)是各種傳統(tǒng)電學(xué)和光電器件的重要構(gòu)成部分,例如:半導(dǎo)體激光器[1]、諧振隧道二極管[2]、高電子遷移率晶體管[3]、光電探測器[4]等.隨著光電器件對集成度和性能的要求日益提高,微型化的半導(dǎo)體異質(zhì)結(jié)越來越受到關(guān)注.由于納米線具有高效的應(yīng)力弛豫能力,使得在晶格失配率大的不同相間形成無缺陷的異質(zhì)結(jié),所以一維半導(dǎo)體納米線被認為是構(gòu)成納米異質(zhì)結(jié)構(gòu)的最理想體系[5,6].一維半導(dǎo)體納米線具有優(yōu)異的光學(xué)、電學(xué)、力學(xué)物理性質(zhì),被認為是下一代納米光電器件和集成系統(tǒng)的構(gòu)成單元.自從十年前第一次成功制備軸向半導(dǎo)體納米線異質(zhì)結(jié)構(gòu),各國科學(xué)家一直在探索各種材料體系的納米線異質(zhì)結(jié)的制備方法,但是截至目前只在有限的材料體系中實現(xiàn)了納米線軸向異質(zhì)結(jié)的制備,如砷化鎵/磷化鎵[7]、硅/鍺硅[8,9]、砷化銦/磷化銦[10]、氮化鎵/氮化鋁和砷化銦/磷砷銦異質(zhì)結(jié)構(gòu)[5]等.

II-VI族半導(dǎo)體,如CdS,CdSe及其合金化合物,由于其直接帶隙等特性,具有高吸收和高發(fā)射效率等優(yōu)點.它們除了在光電器件領(lǐng)域的應(yīng)用[11?18],還在納米光子學(xué)如光波導(dǎo)[19,20]、納米激光器[21?24]、光互聯(lián)[25]等方面有潛在的應(yīng)用.盡管II-VI族半導(dǎo)體納米線的制備及應(yīng)用的研究已經(jīng)取得了很多進展,但是大部分報道是關(guān)于組分均一的納米線的研究,而對II-VI族半導(dǎo)體納米線軸向異質(zhì)結(jié)構(gòu)還鮮有報道.

本文利用分步控溫的化學(xué)氣相沉積(CVD)方法成功制備了高質(zhì)量的CdS/CdSSe納米線軸向異質(zhì)結(jié)構(gòu).微區(qū)熒光顯微圖像結(jié)果表明所獲得的納米線具有沿軸向界限分明的紅色和綠色發(fā)光部分,并且界面區(qū)的熒光光譜表明異質(zhì)界面沒有缺陷;單根納米線的光波導(dǎo)研究發(fā)現(xiàn)此異質(zhì)結(jié)構(gòu)具有非對稱波導(dǎo)特性.這是由于CdS的發(fā)光在波導(dǎo)過程中將被CdSSe吸收再發(fā)射,而CdSSe的發(fā)光可以在CdS中實現(xiàn)無源波導(dǎo).在此基礎(chǔ)上,我們實現(xiàn)了基于單根納米線的紅、綠雙色納米線激光器,并且紅色激射閾值小于綠色激射閾值.

2 CdS/CdSSe異質(zhì)結(jié)納米線的制備

利用分步控溫CVD方法制備異質(zhì)結(jié)納米線,圖1為生長裝置示意圖.將表面沉積有10 nm厚金膜的硅片(1 cm×1 cm)放在石英管的氣流下游區(qū)沉積樣品.將兩個裝有適量CdS(A lfa Aesar, 99.999%)粉末和一個CdSe(A lfa Aesar,99.999%)粉末的瓷舟放入石英管中,其中一個裝有CdS的瓷舟(1號)位于管式爐的中央高溫區(qū),而另一個裝有CdSe的瓷舟(2號)和裝有CdS的瓷舟(3號)位于氣流上游加熱區(qū)外,且1號和2,3號瓷舟間用石英棒隔開(見step 1).另一根石英棒通過磁鐵與步進電機相連,在步進電機推動下控制反應(yīng)源依次進出管式爐的高溫區(qū).在管式爐升溫之前,先以150 sccm(1 sccm=1m L/m in)流速通入高純氮氣(99.99%)40 m in以排空石英管內(nèi)的空氣,然后以28?C/m in的速率升溫至800?C.恒溫50 m in后,得到CdS納米線.隨后以25?C/m in降溫到580?C終止CdS源的蒸發(fā),并恒溫15 m in(間隔時間),氮氣將管內(nèi)殘留的蒸氣排出石英管,為下一階段的生長提供一個潔凈的環(huán)境.在隔離時間結(jié)束之前, 2號和3號瓷舟通過步進電機以8 cm/m in的速率推進蒸發(fā)區(qū)域,取代之前的1號瓷舟(step 2).然后,中心溫度以25?C/m in的速率升高到830?C.恒溫30m in后關(guān)閉電源,爐溫自然冷卻至室溫.最后,我們得到軸向組分突變的CdS/CdSSe異質(zhì)結(jié)納米線.

圖1 (網(wǎng)刊彩色)生長CdS/CdSSe軸向異質(zhì)結(jié)納米線樣品的實驗過程示意圖Fig.1.(color on line)The setup schem atic for the grow th of CdS/CdSSe axial nanow ire heterostructures.

3 實驗結(jié)果與分析

圖2(a)是所得樣品的掃描電子顯微鏡(SEM)圖像,由圖可知樣品形貌均為線狀,且線的長度可達數(shù)十微米.單根納米線高倍SEM表征(見插圖)進一步表明納米線的直徑大約200 nm,并且納米線具有光滑的表面.頂端的納米顆粒為催化劑Au顆粒,這表明此納米線以氣-液-固(VLS)機制生長[26].圖2(b)為原基片上的納米線在405 nm激光照射下的微區(qū)熒光顯微圖像,由圖可知每根納米線具有綠色和紅色兩種發(fā)光部分.為了進一步研究納米線組分分布,我們利用微探針操控技術(shù)將納米線分散在透明石英襯底上.圖2(c)為分散后的納米線在405 nm激光照射下的微區(qū)熒光顯微圖像,由圖可以清晰地觀察到每根納米線均具有界限清晰的綠色和紅色兩部分.圖2(d)是對應(yīng)于圖2(c)中所標注位置1—5的微區(qū)熒光光譜.對應(yīng)于紅色部分的位置1和2的發(fā)光為峰值在605 nm的單一發(fā)光峰光譜;對應(yīng)于綠色部分的位置4和5的發(fā)光為峰值在515 nm的單一發(fā)光峰光譜;對應(yīng)于結(jié)區(qū)的位置3的發(fā)光同時具有605 nm和515 nm兩個發(fā)光峰.515 nm和605 nm的發(fā)光峰分別與CdS和CdS0.48Se0.52的近帶邊發(fā)光相一致[27,28].結(jié)果表明這些納米線是由CdS和CdSSe沿軸向方向構(gòu)成的異質(zhì)結(jié),并且界面清晰.這些熒光光譜中沒有明顯的缺陷發(fā)光帶,只有近帶邊發(fā)光峰,說明制備的CdS/CdS0.48Se0.52軸向異質(zhì)結(jié)納米線具有高的結(jié)晶質(zhì)量.

圖2 (網(wǎng)刊彩色)(a)CdS/CdSSe異質(zhì)結(jié)納米線的SEM圖,插圖為單根納米線的高倍SEM圖(標尺為200 nm); (b)在405 nm激光激發(fā)下原襯底上納米線異質(zhì)結(jié)的微區(qū)熒光顯微圖像;(c)分散在石英襯底上的納米線在405 nm激光激發(fā)下的遠場顯微圖像;(d)對應(yīng)(c)中單異質(zhì)結(jié)納米線不同位置(位置1—5)的微區(qū)光致發(fā)光光譜Fig.2.(color on line)(a)SEM im age of the as-grown heterostructure nanow ires,the inset is the en larged SEM im age of an single nanow ire w ith catalyst(scale bar:200 nm);(b)top-view of the real-color m icroscope photograph of the nanow ires illum inated w ith 405 nm laser;(c)the real-color m icroscope photograph of several dispersed nanow ire excited w ith d iff used 405 nm laser illum ination;(d)m icro-photolum inescence spectra for the position 1–5 denoted in(c).

圖3 (網(wǎng)刊彩色)(a)在405 nm激光激發(fā)下納米線微區(qū)熒光顯微圖像;(b),(c)488 nm激光聚焦在納米線兩端點激發(fā)時的顯微圖像,E1,E2表示激發(fā)端點,O 1,O 2表示波導(dǎo)光出射端點;(d)O 1,O 2的發(fā)射光譜,插圖為E1,E2的發(fā)射光譜Fig.3.(color on line)(a)Real-color m icroscope photograph of the rep resentative heterostructure nanow ire under the illum ination of a diff used 405 nm laser;(b),(c)real-color m icroscope im ages of the nanow ire in (a)excited at the end of CdSSe and CdS,respectively,by a focused laser of 488 nm,and O 1,O 2 denote the rem ote ends for light guiding ou t;(d)the spectra of the light em itted from O 1 and O 2,the inset is the em ission spectra for the excitation positions of E1 and E2.

為了研究CdS/CdSSe軸向異質(zhì)結(jié)納米線光波導(dǎo)特性,我們選擇其中一根納米線為研究對象(見圖3(a)).利用光斑直徑大約為300 nm的488 nm激光分別激發(fā)在納米線的CdSSe部分的端點(激發(fā)端點E1,見圖3(b))及CdS部分的端點(E2,見圖3(c)).當光激發(fā)點在E1時,遠端O1(出射端點)發(fā)出與激發(fā)點相同顏色的紅光.這說明CdSSe的發(fā)光可以被波導(dǎo)至遠端(正向波導(dǎo)).而當光激發(fā)點在E2時,遠端O2發(fā)出紅色光,而E2為綠色.這是因為CdS的發(fā)光在波導(dǎo)過程中被CdSSe吸收,再發(fā)射出紅光,并被波導(dǎo)至O2(反向波導(dǎo)).圖3(d)為O1和O2對應(yīng)的發(fā)光光譜.為了便于比較兩種波導(dǎo)方向的波導(dǎo)效率,我們通過調(diào)控激發(fā)光的強度使激發(fā)點E1和E2的發(fā)光強度相同(見插圖).盡管E1和E2的發(fā)光波長不同(見插圖),但是O1和O2具有基本相同的發(fā)光波長.這與圖3(b)和圖3(c)相一致.另外,O1的發(fā)光峰值強度大約是O2的10倍,這表明CdS/CdSSe軸向異質(zhì)結(jié)納米線對紅光的波導(dǎo)效率要高于綠光,即正向波導(dǎo)效率高于反向波導(dǎo).在組分均一的納米線中,波導(dǎo)效率與方向無關(guān),而僅與波導(dǎo)距離有關(guān).而此軸向異質(zhì)結(jié)納米線由兩部分帶隙不同的材料構(gòu)成.當激發(fā)窄帶隙端(CdSSe)時,其發(fā)光的光子能量小于寬禁帶部分(CdS),故可以直接通過寬禁帶部分而不被吸收,形成無源波導(dǎo).而當激發(fā)寬帶隙端(CdS)時,其發(fā)光將被窄禁帶部分(CdSSe)吸收,然后再發(fā)射出低能量光子,形成有源波導(dǎo).對于無源波導(dǎo),其損耗只有傳輸損耗主要由結(jié)構(gòu)無序性所導(dǎo)致的界面和表面散射[25,29,30].而對于有源波導(dǎo),除了上述損耗外,主要損耗來自于吸收-再發(fā)射過程中的無輻射過程[19].由于這兩種不同的傳輸機制導(dǎo)致正向和反向傳輸具有不同的傳輸效率.

圖4 (網(wǎng)刊彩色)(a)在共聚焦近場顯微鏡下的光學(xué)測試實驗裝置示意圖;(b)—(e)納米線異質(zhì)結(jié)在抽運光激發(fā)下,不同激發(fā)強度下材料的光致發(fā)光光譜;(f)綠光(圓點)和紅光(五角星)發(fā)射帶的發(fā)光強度隨抽運光強度的變化Fig.4.(color on line)(a)Schem atic of the experim ental setup for the lasing m easurem ents based on single CdS/CdSSe nanow ire heterostructu re;(b)–(e)pum ping power-dependent photolum inescence spectra for a rep resentative CdS/CdSSe nanow ire heterostructu re;(f)pum p ing power-dependent em ission intensity of a representative CdS/CdSSe nanow ire heterostructure for green(dots)and red(red pentagram s)em ission band,respectively.

由于納米線具有平整的自然斷裂端面,這些端面可以形成天然的反光鏡,將部分到達端面的波導(dǎo)光反射回波導(dǎo)腔.由于半導(dǎo)體的自吸收效應(yīng),使得半導(dǎo)體納米線還可以作為增益介質(zhì)[21,24,31,32].根據(jù)激射產(chǎn)生的原理,當增益等于損耗時,就會產(chǎn)生受激發(fā)射[33?35].基于上述討論,CdS/CdSSe軸向異質(zhì)結(jié)納米線可以作為性能優(yōu)異的波導(dǎo)腔,并且同時具有對兩個不同波段光的不同波導(dǎo)效率.所以,對其不同激發(fā)功率下的納米線發(fā)射光譜的研究可以進一步理解波導(dǎo)光在異質(zhì)結(jié)納米線中與物質(zhì)的相互作用.圖4(a)是激射過程測試裝置圖,利用470 nm飛秒激光(spectra physics, 1 kHz)作為激發(fā)光源,通過透鏡聚焦至80μm直徑的光斑將納米線全部照明.發(fā)光信號經(jīng)過顯微鏡收集,進入CCD光譜儀檢測.圖4(b)—(e)為不同激發(fā)功率下的發(fā)光光譜,由圖可知,在低激發(fā)功率(20μJ/cm2,圖4(b))下,發(fā)射光譜為中心波長在520 nm和600 nm附近的寬帶發(fā)射帶,這是典型的自發(fā)輻射光譜[19?21].當激發(fā)功率增加到45μJ/cm2時(圖4(c)),在591.5 nm波長處開始出現(xiàn)了一個尖峰.當激發(fā)功率達到160μJ/cm2時(圖4(d)),在526.4 nm波長處同時出現(xiàn)了激射峰.并且隨著激發(fā)功率的進一步增強,尖峰的強度迅速增加,最終將自發(fā)輻射全部掩蓋(圖4(e)),這對應(yīng)著受激輻射的過程特征[17?19,36].圖4(f)是兩個發(fā)射帶峰值強度隨激發(fā)功率的變化.紅光發(fā)射帶和綠光發(fā)射帶發(fā)射強度與激發(fā)功率關(guān)系曲線的斜率分別在45μJ/cm2和160μJ/cm2處呈現(xiàn)了超線性關(guān)系,這表明紅光發(fā)射帶的激射閾值遠低于綠光發(fā)射帶.這是由于對于紅光發(fā)射帶,其自吸收損耗只發(fā)生在CdSSe部分,而對于綠光發(fā)射的自吸收則不僅發(fā)生在CdSSe部分,還發(fā)生在CdS部分,因為CdSSe的帶隙遠低于綠光發(fā)射光子能量.另一方面,綠光發(fā)射在CdSSe部分的自吸收還進一步增強了紅光發(fā)射帶的增益.基于這些原因,紅光發(fā)射的激射閾值遠低于綠光發(fā)射帶的激射閾值.

為了進一步證明波導(dǎo)光可以在納米線中不同介質(zhì)(CdS和CdSSe)間波導(dǎo),我們利用多物理場方法(COMSOL)對CdS/CdSSe軸向異質(zhì)結(jié)納米線中的光波導(dǎo)進行了模擬.圖5(a)是所建模型圖,其中左半部為CdS部分,右半部為CdSSe部分,長度均為2μm,直徑為200 nm;通過不同折射率區(qū)分兩部分介質(zhì),CdS折射率為2.5,CdSSe為3.0[37].圖5(b)和圖5(c)是分別從納米線的兩端入射526.4 nm和591.5 nm光的波導(dǎo)模擬結(jié)果.盡管CdSSe折射率要大于CdS折射率,從模擬結(jié)果可以清楚地看到無論是綠光從CdS進入CdSSe,還是紅光從CdSSe進入CdS,均可以進入另一介質(zhì),并基本不改變光場強度.這表明CdS發(fā)射的綠光可以高效地進入并激發(fā)CdSSe,這也是圖3(c)中CdS端(E2)激發(fā),在遠端(O2)出射紅光的原因,同時也是紅光激射閾值低于綠光的原因.尤其是CdSSe中的紅光發(fā)射從高折射率CdSSe進入低折射率CdS并沒有對光的強度有明顯的影響,表明紅光可以在整個納米線中波導(dǎo),這與圖3(b)中的結(jié)果一致.

圖5 (網(wǎng)刊彩色)不同波長光在CdS/CdSSe異質(zhì)結(jié)納米線中波導(dǎo)的理論模擬結(jié)果(COMSOL)(a)擬合建立的模型圖,左邊為CdS部分,右邊為CdSSe部分; (b)526.4 nm光從CdS端點入射和(c)591.5 nm光從CdSSe入射的波導(dǎo)模擬結(jié)果Fig.5. (color on line)Sim u lation resu lts of the waveguide in the CdS/CdSSe axial nanow ire heterostructu res: (a)The m odel for the sim u lation,CdS and CdSSe com ponents locate at left and right side,respectively;(b)the sim u lation resu lt of the waveguide for 526.4 nm light incident from the end of CdS com ponent;(c)the sim u lation resu lt of the waveguide for 591.5 nm light incident from end of CdSSe com ponent,respectively.The arrow s indicate the incidence direction of the light.

4 結(jié) 論

本文采用CVD法成功地制備出CdS/CdS0.48-Se0.52軸向異質(zhì)結(jié)納米線.單根納米線的光波導(dǎo)研究表明,該異質(zhì)結(jié)構(gòu)具有非對稱光波導(dǎo)特性.由于CdS/CdSSe軸向異質(zhì)結(jié)納米線可以作為性能優(yōu)異的波導(dǎo)腔,并且同時具有對兩個不同波段光的不同波導(dǎo)效率,利用此原理,我們實現(xiàn)了基于此異質(zhì)結(jié)構(gòu)的紅、綠雙色納米線激光器,并且紅光激射閾值遠低于綠光激射閾值.理論模擬結(jié)果表明,在此異質(zhì)結(jié)納米線結(jié)果中,盡管CdS和CdSSe折射率不同,但是波導(dǎo)光可以高效地在兩相間波導(dǎo).該納米線異質(zhì)結(jié)構(gòu)在雙色納米線激光器、光互聯(lián)等光子學(xué)方面有潛在的應(yīng)用.

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PACS:42.55.–f,42.55.Px,42.82.–m,42.82.EtDOI:10.7498/aps.66.064204

A symm etric w avegu ide and the dual-w avelength stim u lated em ission for Cd S/Cd S0.48Se0.52ax ial nanow ire heterostructu res?

Li Dan1)Liang Jun-Wu2)Liu Hua-Wei1)Zhang Xue-Hong1)Wan Qiang1)Zhang Qing-Lin1)?Pan An-Lian1)?

1)(Key Laboratory for M icro-Nano Physics and Technology of Hunan Province,School of Physics and Electronics, Hunan University,Changsha 410082,China)
2)(School of Physical Science and Technology Engineering,Yu lin Norm al University,Yu lin 537000,China)
(Received 6 Novem ber 2016;revised m anuscrip t received 17 Decem ber 2016)

Sem iconductor axial nanow ire heterostructures are im portant for realizing the high-perform ance nano-photonics and opto-electronics devices.A lthough diff erent IV and III-V sem iconductor axial nanow ire heterostructures have been successfully prepared in recent decade,few of them focused on the optical properties,such as thewaveguide,due to their low light em ission effi ciencies.The II-V I sem iconductor nanow ires grown by chem ical vapor deposition strategy,such as CdS,CdSe and their alloys,can act as nanoscale waveguide,nanolasers,etc.,because of their high op tical gains and atom ically sm ooth surfaces.However,it is stilla challenge to grow ing the high-quality II-V Isem iconductor axialnanow ire heterostructures,owning to the poor controllability of the vapor grow th techniques.Here,the CdS/CdSSe axialnanow ire heterostructures are prepared w ith well controlled CVD method under the catalysis of annealed Au nanoparticles.The scanning electron m icroscope characterization show s that the w ires have sm ooth surfaces w ith Au particles at the tips, indicating the vapor-liquid-solid grow th m echanism for the nanow ire heterostructures.The m icroscope im ages of the dispersed w ires illum inated w ith a 405 nm laser show that the red and the green segm ent align axially w ith a sharp interface,demonstrating the axial alignment of CdSand CdSSe segments.The position related m icro-photolum inescence spectra exhibit near band edge em issions of CdS and CdSSe w ithout obvious em ission from defect states,which suggests that the w ires have highly crystalline quality.The waveguide of the nanow ire heterostructures is studied through respectively locally exciting the two ends of the w ire w ith a focused 488 nm laser.The local illum inations at both the CdS end and the CdSSe end resu lt in red em ission at the corresponding remote ends of the w ires,w ith the em ission intensity of the former being one order lower than that of the later,which is caused by the reabsorption of the green light em ission(from CdS segm ent)in the CdSSe segm ent.This indicates the asymm etric waveguide in these heterosturctures, which im p lies that the CdS/CdSSe nanow ire heterostructures have the potential app lications in the photodiode.Under the pum ping of 470 nm fem tosecond laser,dual-color(red and green)lasing is realized based on these w ires,w ith the lasing threshold of red light lasing being lower than that of the green one,which results from the larger round-trip loss for the green light arising from the self-absorp tion in CdSSe segment.To prove that the light can be transfer between the two segm entsw ith diff erent refractivities,the waveguide of the nanow ire heterostructure is simulated by the COM SOL.The resu lt show s that the light can eff ectively propagate between CdS and CdSSe segm ents,which ensures the light-m atter interaction in the axial CdS/CdSSe nanow ire heterostructures as discussed above.These high-quality CdS/CdSSe axial nanow ire heterostructures can be found to have the potential app lications in photodiodes,dual-color nanolasers and photodetectors.

nanow ires,heterostructure,waveguide,lasers

10.7498/aps.66.064204

?國家自然科學(xué)基金(批準號:11374092,61474040,61574054,61505051)資助的課題.

?通信作者.E-m ail:qinglin.zhang@hnu.edu.cn

?通信作者.E-m ail:an lian.pan@hnu.edu.cn

*Pro ject supported by the National Natu ral Science Foundation of China(G rant Nos.11374092,61474040,61574054, 61505051).

?Corresponding au thor.E-m ail:qinglin.zhang@hnu.edu.cn

?Corresponding au thor.E-m ail:an lian.pan@hnu.edu.cn