馬文君 由芳田 彭洪尚 黃世華

(北京交通大學(xué),發(fā)光與光信息教育部重點(diǎn)實(shí)驗(yàn)室,北京交通大學(xué)光電子技術(shù)研究所,北京 100044)

小粒徑同質(zhì)/異質(zhì)殼層結(jié)構(gòu)NaGdF4∶3%Nd3+納米顆粒的近紅外發(fā)光特性?

馬文君 由芳田?彭洪尚 黃世華

(北京交通大學(xué),發(fā)光與光信息教育部重點(diǎn)實(shí)驗(yàn)室,北京交通大學(xué)光電子技術(shù)研究所,北京 100044)

(2017年1月16日收到;2017年3月13日收到修改稿)

采用共沉淀法制備了粒徑小于5 nm的六方相NaGdF4∶3%Nd3+納米顆粒.納米顆粒表面缺陷會(huì)使發(fā)光中心產(chǎn)生嚴(yán)重的淬滅,對(duì)其表面包覆適當(dāng)厚度的殼層可以有效地減少發(fā)光淬滅,提高發(fā)光性能.對(duì)NaGdF4∶3%Nd3+核心納米顆粒分別進(jìn)行同質(zhì)和異質(zhì)包覆并且通過(guò)調(diào)節(jié)核殼比制備了不同殼層厚度的NaGdF4∶3%Nd3+@NaGdF4和NaGdF4∶3%Nd3+@NaYF4納米顆粒,研究了不同的殼層厚度對(duì)核心納米顆粒發(fā)光的影響,并對(duì)兩種不同核殼結(jié)構(gòu)納米顆粒的發(fā)光性能進(jìn)行了對(duì)比.在808 nm近紅外光激發(fā)下,NaGdF4∶3%Nd3+納米顆粒發(fā)射出位于約866,893,1060 nm的近紅外發(fā)射.與核心納米顆粒相比,核殼結(jié)構(gòu)的納米顆粒的熒光強(qiáng)度增強(qiáng),熒光壽命增長(zhǎng),并且隨著殼厚的增加,熒光強(qiáng)度出現(xiàn)先增強(qiáng)后減弱、熒光壽命逐步增長(zhǎng)的趨勢(shì).與相同條件下同質(zhì)包覆的NaGdF4∶3%Nd3+@NaGdF4納米顆粒相比,異質(zhì)包覆的NaGdF4∶3%Nd3+@NaYF4納米顆粒光譜熒光強(qiáng)度增強(qiáng),壽命增長(zhǎng).

∶近紅外發(fā)光,同質(zhì)核殼結(jié)構(gòu),異質(zhì)核殼結(jié)構(gòu),NaGdF4∶Nd3+

PACS∶78.55.—m,78.67.Bf,76.30.Kg,42.70.HjDOI∶10.7498/aps.66.107801

1 引 言

稀土摻雜納米發(fā)光材料由于其發(fā)射光譜穩(wěn)定、譜帶窄、化學(xué)穩(wěn)定性高等特點(diǎn),正逐步成為一種新興的重要材料,并廣泛應(yīng)用于生物熒光成像、免疫分析、光動(dòng)力治療等醫(yī)學(xué)領(lǐng)域[1,2].近些年來(lái),越來(lái)越多的研究集中在上轉(zhuǎn)換發(fā)光領(lǐng)域.上轉(zhuǎn)換發(fā)光通過(guò)多光子吸收或能量傳遞將長(zhǎng)波長(zhǎng)光轉(zhuǎn)化為短波長(zhǎng)光,通常是將近紅外光轉(zhuǎn)化為可見(jiàn)或紫外光[3?5],在生物熒光探針應(yīng)用方面雖然有著譜帶窄、探測(cè)靈敏度高、背景干擾低等優(yōu)點(diǎn),卻受到生物組織穿透深度低、成像質(zhì)量不高等缺點(diǎn)的限制.Nd3+摻雜發(fā)光材料能夠吸收和發(fā)射出位于700—1100 nm的近紅外光,這一光譜范圍被稱為“近紅外組織透明窗口”.在此窗口,生物組織熒光吸收和散射被極大程度地降低,從而有助于提高信噪比,加深光在生物組織的穿透深度,同時(shí)也能有效減少組織損傷[6,7].在生物應(yīng)用領(lǐng)域,小粒徑納米顆粒具有易吸收和排泄、血液循環(huán)時(shí)間長(zhǎng)的優(yōu)點(diǎn),是一種理想的生物成像材料[8,9].但由于稀土摻雜納米顆粒存在表面效應(yīng),容易產(chǎn)生發(fā)光淬滅,降低發(fā)光效率.采取表面包覆惰性殼層的辦法可以有效地將發(fā)光中心與外界隔離開(kāi)來(lái),從而有助于提高發(fā)光性能[10,11].目前大量研究工作都圍繞上轉(zhuǎn)換核殼結(jié)構(gòu)納米顆粒的發(fā)光展開(kāi),在上轉(zhuǎn)換發(fā)光體系中,通過(guò)構(gòu)建核殼結(jié)構(gòu)可以減少敏化離子與發(fā)光離子的濃度淬滅效應(yīng)和交叉弛豫現(xiàn)象,并且可以實(shí)現(xiàn)多色發(fā)光[12,13].然而少有研究涉及稀土離子單摻體系中殼層材料對(duì)發(fā)光的影響,因此研究同質(zhì)包覆和異質(zhì)包覆納米顆粒對(duì)理解核殼結(jié)構(gòu)納米顆粒發(fā)光有一定的意義.

本文以共沉淀法制備了粒徑小于5 nm的六方相NaGdF4∶3%Nd3+核心納米顆粒,并且對(duì)其表面分別包覆了一層NaGdF4同質(zhì)殼層和NaYF4異質(zhì)殼層,通過(guò)調(diào)節(jié)不同的核殼摩爾比控制殼層厚度.測(cè)試了發(fā)射光譜與衰減曲線,具體分析了兩種不同的殼層材料以及殼層厚度對(duì)熒光性能的影響.

2 實(shí) 驗(yàn)

2.1 核心納米顆粒NaGdF4∶3%Nd3+的合成

采用共沉淀法制備1 mmol NaGdF4∶3%Nd3+納米顆粒過(guò)程∶在室溫狀態(tài)下,將0.97 mmol GdCl3,0.03 mmol NdCl3,4.0 mL油酸,15.0 mL十八烯混合加入三口燒瓶中,在氮?dú)猸h(huán)境下加熱至150?C直至形成透明澄清的混合溶液;分散完成后,混合溶液冷卻至60?C;將溶有4.0 mmol NH4F和2.5 mmol NaOH的10.0 mL甲醇溶液緩慢滴入三口燒瓶?jī)?nèi),快速攪拌30 min;待甲醇完全揮發(fā)后,升溫至280?C,快速攪拌,反應(yīng)40 min;反應(yīng)結(jié)束后,將溶液溫度降至室溫,加入10.0 mL乙醇,超聲沉淀,離心后加入乙醇與環(huán)己烷混合溶液清洗數(shù)次;分散至10.0 mL環(huán)己烷中備用[14].

2.2 核殼結(jié)構(gòu)納米顆粒的合成

核殼摩爾比為1∶2的NaGdF4∶3%Nd3+@Na-GdF4(G@2G)納米顆粒制備過(guò)程∶在室溫狀態(tài)下,將2.0 mmol GdCl3,12.0 mL油酸,30.0 mL十八烯混合加入三口燒瓶中,在氮?dú)猸h(huán)境下加熱至150?C直至形成透明澄清的混合溶液;分散完成后,混合溶液冷卻至80?C;將分散在10.0 mL環(huán)己烷中的1.0 mmol NaGdF4∶3%Nd3+核心納米顆粒緩慢加入,快速攪拌30 min,待環(huán)己烷揮發(fā)干凈,降溫至60?C,將溶有8.0 mmol NH4F和5.0 mmol NaOH的15 mL甲醇溶液緩慢滴入三口燒瓶?jī)?nèi),快速攪拌30 min;待甲醇揮發(fā)干凈,升溫至290?C,快速攪拌,反應(yīng)1 h;反應(yīng)結(jié)束后將溶液溫度降至室溫,倒入離心管,加入10.0 mL乙醇,超聲沉淀,離心后加入乙醇與環(huán)己烷混合溶液清洗數(shù)次;分散至適量環(huán)己烷中.核殼摩爾比為1∶4(G@4G)和1∶6(G@6G)同質(zhì)包覆納米顆粒以及核殼摩爾比為1 ∶2,1 ∶4,和1 ∶6的NaGdF4∶3%Nd3+@NaYF4(G@2Y,G@4Y和G@6Y)異質(zhì)包覆納米顆粒的制備過(guò)程與上述方法相同.

2.3 性能表征

X射線粉末衍射測(cè)試表征采用D8ADVANCE型X射線衍射儀 (XRD)(德國(guó)Braker公司),儀器測(cè)試參數(shù)∶Cu-Kα輻射,λ=0.15418 nm,管電壓40 kV,管電流100 mA,掃描范圍10?—65?. 納米顆粒形貌表征采用JEM-2100 F透射電鏡(日本JEOL公司),儀器測(cè)試參數(shù)∶加速電壓80 kV,點(diǎn)分辨率0.19 nm,線分辨率0.14 nm,傾斜角25?,放大倍數(shù)20000.近紅外發(fā)射光譜表征采用卓立漢光Omni-λ 300熒光光譜儀(北京卓立漢光有限公司),激發(fā)光源為808 nm半導(dǎo)體激光器;掃描速度為5次/nm,狹縫為2 nm(光柵)/2 nm(光電倍增管).衰減曲線表征采用FLS920(英國(guó)Edinburgh Instruments公司),激發(fā)光源為400 W連續(xù)氙燈Xe900,光電倍增管,狹縫為2 nm,壽命范圍為100 ps到10 s,波長(zhǎng)范圍190—1700 nm.

3 結(jié)果與討論

3.1 物相與形貌

圖1(a)為NaGdF4∶3%Nd3+核心納米顆粒XRD圖,樣品衍射峰與六方相NaGdF4標(biāo)準(zhǔn)卡片(JCPDS 27-0699)相符合,未有雜峰出現(xiàn),表明所制備的樣品為六方相NaGdF4結(jié)構(gòu).由于Nd3+與Gd3+離子半徑相差不大,且摻雜濃度較小,少量的Nd3+取代Gd3+格位未明顯影響晶格結(jié)構(gòu).利用謝樂(lè)公式D=kλ/β cosθ可計(jì)算樣品的平均粒徑約為4.7 nm.

圖1(b)—(f)為NaGdF4∶3%Nd3+納米顆粒與核殼結(jié)構(gòu)納米顆粒透射電鏡(TEM)圖片.由TEM圖片可知,NaGdF4∶3%Nd3+核心納米顆粒呈單分散狀態(tài),顆粒大小均勻,形貌規(guī)則沒(méi)有團(tuán)聚現(xiàn)象,納米顆粒呈一定程度的規(guī)則排列.對(duì)圖1(b)納米顆粒進(jìn)行統(tǒng)計(jì),得出核心納米顆粒粒徑約為4.5 nm,與謝樂(lè)公式計(jì)算結(jié)果相近.核殼結(jié)構(gòu)納米顆粒形貌與核心納米顆粒一致,但包覆惰性殼層的納米顆粒的粒徑增大,G@2Y,G@4Y,G@6Y和G@6G納米顆粒粒徑分別約為8,9,10和10 nm,殼層厚度分別為1.5,2,2.5和2.5 nm.在反應(yīng)條件完全一致的情況下,與同質(zhì)包覆納米顆粒相比,異質(zhì)包覆的納米顆粒表面形貌更為均勻,這表明在對(duì)納米顆粒進(jìn)行表面包覆時(shí),異質(zhì)殼層結(jié)構(gòu)更有利于納米顆粒的結(jié)晶.

圖1 (a)NaGdF4:3%Nd3+核心納米顆粒XRD圖與六方相NaGdF4標(biāo)準(zhǔn)卡片(JCPDS NO.27-0699);(b)—(f)NaGdF4:3%Nd3+核心納米顆粒與G@2Y,G@4Y,G@6Y,G@6G納米顆粒TEM圖Fig.1.(a)XRD pattern of NaGdF4:3%Nd3+core nanoparticles and standard data for hexagonal phase NaGdF4(JCPDS NO.27-0699);(b)–(f)TEM images of NaGdF4:3%Nd3+core nanoparticles and G@2 Y,G@4 Y,G@6 Y,G@6 G nanoparticles.

3.2 近紅外發(fā)射光譜

圖2為NaGdF4∶3%Nd3+納米顆粒和不同核殼結(jié)構(gòu)納米顆粒在808 nm近紅外光激發(fā)下的發(fā)射光譜.圖中在866,893和1060 nm處的發(fā)射峰分別對(duì)應(yīng)的躍遷[7],該發(fā)光過(guò)程是單光子過(guò)程.在808 nm光源激發(fā)下,電子從基態(tài)4I9/2能級(jí)被激發(fā)躍遷到4F5/2能級(jí),后無(wú)輻射弛豫到4F3/2能級(jí),從4F3/2能級(jí)輻射發(fā)光躍遷至4I9/2,4I11/2能級(jí),發(fā)射出位于約866,893,1060 nm的近紅外光.

由圖2可知,核殼結(jié)構(gòu)納米顆粒與NaGdF4∶3%Nd3+納米顆發(fā)射峰位置相同,未出現(xiàn)明顯的峰位位移,但發(fā)光強(qiáng)度明顯增強(qiáng).這是由于NaGdF4∶3%Nd3+納米顆粒的粒徑較小,比表面積較大,納米顆粒會(huì)產(chǎn)生晶格缺陷,這些晶格缺陷極易成為發(fā)光陷阱,例如發(fā)生高能量聲子耦合,增大無(wú)輻射躍遷概率等,從而降低發(fā)光效率.殼層結(jié)構(gòu)能夠有效地將Nd3+發(fā)光中心與外界環(huán)境隔絕開(kāi)來(lái),消除表面缺陷(懸空鍵、不飽和鍵等),增加表面原子的穩(wěn)定性,從而降低表面淬滅現(xiàn)象的發(fā)生[15].通過(guò)對(duì)圖2中不同核殼比例的NaGdF4∶3%Nd3+@NaYF4納米顆粒的熒光強(qiáng)度對(duì)比發(fā)現(xiàn),隨著核殼摩爾比例的增加,發(fā)光強(qiáng)度呈先增強(qiáng)后減弱的趨勢(shì).這表明在一定殼層厚度范圍內(nèi),當(dāng)殼層厚度增加時(shí),惰性殼層對(duì)Nd3+發(fā)光中心的保護(hù)作用逐漸增強(qiáng).當(dāng)核殼比達(dá)到1∶4時(shí),此時(shí)的殼層厚度約為2 nm,發(fā)光強(qiáng)度最強(qiáng),對(duì)Nd3+的保護(hù)作用達(dá)到頂峰.當(dāng)殼層厚度進(jìn)一步增加時(shí),Nd3+發(fā)光中心密度下降,同時(shí)過(guò)厚的殼層對(duì)入射光存在一定程度的反射和散射,不利于發(fā)光性能的進(jìn)一步提升.由于同質(zhì)包覆的納米顆粒與異質(zhì)包覆納米顆粒熒光強(qiáng)度變化趨勢(shì)類似,因此本文只給出了G@6G納米顆粒的熒光光譜.G@6G納米顆粒平均粒徑與G@6Y納米顆粒相同,但熒光強(qiáng)度比G@6Y納米顆粒弱,這表明異質(zhì)核殼結(jié)構(gòu)的納米顆粒更有利于提高Nd3+發(fā)光性能.

圖2 (網(wǎng)刊彩色)NaGdF4:3%Nd3+納米顆粒與G@2Y,G@4Y,G@6Y,G@6G納米顆粒近紅外發(fā)射光譜Fig.2.(color online)Emission spectra of NaGdF4:3%Nd3+ corenanoparticlesandG@2Y,G@4Y,G@6Y,G@6G nanoparticles.

圖3 (網(wǎng)刊彩色)NaGdF4:3%Nd3+核心納米顆粒與G@2Y,G@4Y,G@6Y,G@6G納米顆粒熒光衰減曲線Fig.3.(color online)Decay curves at 866 nm for core NaGdF4:3%Nd3+nanoparticles and G@2Y,G@4Y,G@6Y,G@6G nanoparticles

3.3 熒光衰減曲線

圖3給出了NaGdF4∶3%Nd3+核心納米顆粒與核殼結(jié)構(gòu)納米顆粒Nd3+離子4F3/2能級(jí)的衰減曲線(λex=808 nm, λem=866 nm).Nd3+發(fā)光中心在4F3/2能級(jí)處的衰減過(guò)程可以分為兩部分∶核心內(nèi)部Nd3+由于受外部殼層的保護(hù)作用,衰減過(guò)程相對(duì)緩慢,熒光壽命記為τ1;納米顆粒表面附近Nd3+由于表面缺陷的作用衰減過(guò)程相對(duì)較快,熒光壽命記為τ2.衰減曲線采用雙指數(shù)函數(shù) I=I0+(A1exp(?t/τ1)+A2exp(?t/τ2))進(jìn)行擬合,其中I為熒光強(qiáng)度,A1,A2為擬合參數(shù),代表τ1和τ2所占權(quán)重.Nd3+平均壽命可根據(jù)τav=(A1+A2)/(A1τ1+A2τ2)公式計(jì)算得到[13].擬合結(jié)果列在表1中.NaGdF4∶3%Nd3+納米顆粒的平均壽命τav=50μs,熒光壽命較短,這是由納米顆粒表面效應(yīng)所引起的.但核殼結(jié)構(gòu)納米顆粒熒光壽命明顯增長(zhǎng),表明惰性殼層能夠有效保護(hù)NaGdF4∶3%Nd3+納米顆粒,降低熒光猝滅作用,提高發(fā)光性能.相比較于NaGdF4∶3%Nd3+核心納米顆粒,核殼結(jié)構(gòu)納米顆?？焖偎p部分所占比例變小,表明包覆惰性殼層的納米顆粒表面猝滅中心減少.隨著殼層逐漸增厚,殼層對(duì)發(fā)光中心的保護(hù)作用逐漸增強(qiáng),快速衰減過(guò)程所占比例持續(xù)減小,熒光壽命明顯增長(zhǎng).

3.4 原因分析

與同質(zhì)核殼結(jié)構(gòu)納米顆粒相比,異質(zhì)核殼結(jié)構(gòu)納米顆粒熒光壽命更長(zhǎng).這也與上文中異質(zhì)核殼結(jié)構(gòu)納米顆粒的發(fā)光性能優(yōu)于同質(zhì)核殼結(jié)構(gòu)納米顆粒的現(xiàn)象相對(duì)應(yīng).一方面原因是在對(duì)NaGdF4核心納米顆粒進(jìn)行包覆時(shí),殼層生長(zhǎng)過(guò)程遵循奧斯瓦爾德熟化生長(zhǎng)機(jī)理[16?18],殼層前驅(qū)體首先自成核生長(zhǎng)為小顆粒,后溶解包覆在核心納米顆粒上.在同質(zhì)包覆過(guò)程中,由于六方相NaGdF4穩(wěn)定性更高[19],在較低的反應(yīng)溫度下就會(huì)自成核,迅速生長(zhǎng)成大小不一的納米顆粒,導(dǎo)致部分殼層前驅(qū)體不能外延生長(zhǎng)在核心納米晶上,從而出現(xiàn)部分殼層包覆不完整的現(xiàn)象[20].而在對(duì)核心納米顆粒表面包覆NaYF4殼層時(shí),由于六方相NaGdF4與NaYF4參數(shù)相差不大(NaGdF4∶a=6.02 ?,c=3.60 ?;NaYF4∶a=5.96 ?,c=3.53 ?)[19],NaYF4殼層比較容易生長(zhǎng),NaYF4穩(wěn)定性較NaGdF4低,有助于抑制NaYF4殼層前驅(qū)體的自成核現(xiàn)象,使得NaYF4能夠完全包覆在NaGdF4核心納米顆粒上[21].同時(shí)在加熱包覆過(guò)程中,核殼界面會(huì)發(fā)生陽(yáng)離子交換,NaYF4殼層中部分Y3+會(huì)通過(guò)擴(kuò)散作用進(jìn)入NaGdF4核心,取代Gd3+晶格格位,由于Y3+離子半徑比Gd3+小,使得納米顆粒表面負(fù)電荷數(shù)量較少,電荷吸引作用加快了溶液中F?離子向納米顆粒表面的擴(kuò)散速率,加快了殼層生長(zhǎng)[21],提高了納米顆粒結(jié)晶度,能夠有效地減少晶格缺陷和晶格破損,避免成為激發(fā)光能量陷阱,有助于提高納米顆粒的發(fā)光性能.另一方面,在808 nm光激發(fā)下,Nd3+之間存在交叉弛豫現(xiàn)象((4F3/2;4I9/2)→ (4I15/2;4I15/2)),NaYF4殼層有更低的聲子能量[22],能夠減少核殼界面處Nd3+的交叉弛豫現(xiàn)象[23],減少Nd3+在紅外區(qū)的發(fā)光,提高Nd3+在近紅外區(qū)的發(fā)光效率.

表1 NaGdF4:3%Nd3+核心納米顆粒與核殼結(jié)構(gòu)納米顆粒4F3/2能級(jí)壽命Table 1. Lifetimes at 866 nm for core NaGdF4:3%Nd3+nanoparticles and core/shell structured nanoparticles.

4 結(jié) 論

本文主要利用共沉淀法制備了粒徑小于5 nm的六方相NaGdF4∶3%Nd3+納米顆粒.在808 nm光激發(fā)下,發(fā)射出位于約866,893,1060 nm處的近紅外光.為減小納米顆粒表面效應(yīng)對(duì)Nd3+發(fā)光中心的影響,包覆了一定厚度的NaGdF4和NaYF4惰性殼層.通過(guò)對(duì)熒光光譜分析發(fā)現(xiàn),隨著殼層包覆厚度的增加,核殼結(jié)構(gòu)納米顆粒的發(fā)光呈現(xiàn)先增強(qiáng)后減弱的趨勢(shì),當(dāng)殼層厚度為2 nm時(shí),其發(fā)射光譜強(qiáng)度最高.同時(shí)通過(guò)對(duì)NaGdF4∶3%Nd3+核心納米顆粒和核殼結(jié)構(gòu)納米顆粒衰減曲線對(duì)比分析發(fā)現(xiàn),熒光壽命隨著殼層厚度的增加逐漸變長(zhǎng).兩者均表明惰性殼層的包覆能夠有效地減少納米顆粒表面缺陷對(duì)發(fā)光中心的影響,提高發(fā)光性能.同時(shí),異質(zhì)包覆NaYF4惰性殼層更有助于提高納米顆粒的發(fā)光性能.摻Nd3+核殼結(jié)構(gòu)納米顆粒能夠?qū)崿F(xiàn)近紅外激發(fā)和近紅外發(fā)射,在生物醫(yī)學(xué)等領(lǐng)域可能具有廣闊的應(yīng)用前景.

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PACS∶78.55.—m,78.67.Bf,76.30.Kg,42.70.HjDOI∶10.7498/aps.66.107801

*Project supported by the National Natural Science Foundation of China(Grant No.11274038)and the New Century Excellent Talents in University,China(Grant No.12-0177).

?Corresponding author.E-mail:ftyou@bjtu.edu.cn

Near-infrared luminescence properties of small-sized homogeneous/heterogeneous core/shell structured NaGdF4∶Nd3+nanoparticles?

Ma Wen-Jun You Fang-Tian?Peng Hong-Shang Huang Shi-Hua
(Key Laboratory of Luminescence and Optical Information,Ministry of Education,Institute of Optoelectronic Technology,Beijing Jiaotong University,Beijing 100044,China)

16 January 2017;revised manuscript

13 March 2017)

In recent years,considerable researches have focused on the upconversion phosphor nanoparticles in the application of biomedical imaging,which emit visible light.Nevertheless,these kinds of nanoparticles limit the light penetration depth and imaging quality.The Nd3+doped nanoparticles excited and emitted in a spectral range of 700—1100 nm can overcome those shortcomings.Furthermore,considering the applications of rare earth nanoparticles in biomedical imaging,smaller particle size is needed.However,the luminescence efficiencies of nano-structured materials are lower due to the inherent drawback of high sensitivity of Nd3+ions to the surface defects.So,it is of vital importance for introducing a shell with low phonon energy to be overgrown on the surface of nanoparticles.According to the ratio of core material to the shell,core/shell structured nanoparticles are separated into “homogeneous” and “heterogeneous” nanoparticles.And the shell material may influence the luminescence performance.In few reports there have been made the comparisons of luminescence performance of Nd3+between heterogeneous and homogeneous core/shell nanoparticles.In the present work,small-sized hexagonal NaGdF4∶3%Nd3+nanoparticles with an average size of sub-5 nm are synthesized by a coprecipitation method.To overcome the nanosize-induced surface defects and improve the luminous performance,the NaGdF4∶3%Nd3+nanoparticles are coated with homogeneous and heterogeneous shells,respectively.Core/shell structured nanoparticles with different values of shell thickness are synthesized by using the core/shell ratios of 1∶2,1∶4 and 1∶6.The luminescence properties of the prepared nanoparticles are characterized by photoluminescence spectra and fluorescence lifetimes.Under 808 nm excitation,the NaGdF4∶3%Nd3+nanoparticles exhibit nearinfrared emissions with sharp bands at～866 nm,～893 nm,～1060 nm,which can be assigned to the transitions of4F3/2to4I9/2,4F2/3to4I11/2,respectively.The locations of emission peaks of the core/shell nanoparticles are in accordance with the those of cores while the fluorescence intensity increases significantly.In addition,the average lifetimes of Nd3+ions at 866 nm of core/shell nanoparticles are longer than those of the cores,which indicates that the undoped shell can minimize the occurrence of unwanted surface—related deactivations.Notably,comparing with the homogeneous NaGdF4∶3%Nd3+@NaGdF4nanoparticles,the fluorescence intensity of heterogeneous NaGdF4∶3%Nd3+@NaYF4nanoparticles is enhanced and their lifetimes become longer.It is due to the low stability of hexagonal NaYF4,which suppresses the nucleation of the shell precursor and makes the shell able to be fully coated on the core.The decrease of electron charge density on the surface of core/shell nanoparticles is also beneficial to shell growth and crystallization.The high crystallinity of heterogeneous core/shell structured nanoparticles can eliminate negative influence of surface effect more efficiently.In addition,the phonon energy of NaYF4is lower than that of NaGdF4,which leads to low possibility of non-radiative cross-relaxation between Nd3+ions,thereby improving the luminescence efficiency in the near in frared emission.

∶near-infrared luminescence,homogeneous core/shell structure,heterogeneous core/shell structure,NaGdF4∶Nd3+

?國(guó)家自然科學(xué)基金(批準(zhǔn)號(hào):11274038)和教育部新世紀(jì)優(yōu)秀人才支持計(jì)劃(批準(zhǔn)號(hào):12-0177)資助的課題.

?通信作者.E-mail:ftyou@bjtu.edu.cn

?2017中國(guó)物理學(xué)會(huì)Chinese Physical Society