紅外探測器可用于軍事探測、航空航天、生命科學(xué)和環(huán)境監(jiān)測等領(lǐng)域[1~4] 基于窄帶隙半導(dǎo)體的紅外探測器,由于其結(jié)構(gòu)簡單、性能穩(wěn)定和易于制備,已經(jīng)成為當(dāng)前的研究熱點(diǎn)[5,6] SnSe是一種重要的窄帶隙半導(dǎo)體材料,其具有電導(dǎo)率和化學(xué)穩(wěn)定性高和成本較低等優(yōu)點(diǎn),是制造紅外探測器的理想材料[7~9] 但是,光照后SnSe的電子-空穴對復(fù)合極快,使其載流子濃度降低,嚴(yán)重影響SnSe紅外探測器的效率[10,11] 抑制光生載流子復(fù)合提高單一半導(dǎo)體光電探測效率的方法,有元素?fù)诫s、構(gòu)建半導(dǎo)體異質(zhì)結(jié)和貴金屬修飾[12~15] 其中,用貴金屬納米粒子修飾半導(dǎo)體,具有成本低、促進(jìn)電子-空穴對分離快且操作簡單的優(yōu)點(diǎn)

與其它貴金屬(Au、Pt、Pd)相比,Ag具備無毒且價(jià)格較低、易制備、化學(xué)性質(zhì)穩(wěn)定等優(yōu)點(diǎn)[16,17] 同時(shí),光沉積法成本較低和工藝簡單,通過調(diào)整沉積時(shí)間、光強(qiáng)和前驅(qū)體溶液配比即可在室溫下實(shí)現(xiàn)Ag納米粒子的可控制備[18~20] 目前,關(guān)于用金屬Ag修飾半導(dǎo)體以加快電子-空穴對分離已有大量的研究工作 Liu等[21]通過將化學(xué)氣相沉積與熱蒸發(fā)相結(jié)合制備了Ag修飾ZnO陣列,能有效分離電子-空穴對 且與未修飾ZnO相比,其紫外探測性能顯著提高 Devi等[22]合成了一種Ag修飾CeO2納米棒光電探測器,Ag納米粒子修飾能顯著抑制CeO2納米棒電子-空穴對的復(fù)合并提升其捕獲電子的能力 Joshna等[23]制備的Ag修飾TiO2納米管(TiO2 NTs),金屬Ag顯著減少了電子-空穴對的復(fù)合,Ag納米粒子修飾的TiO2 NTs的光電流是純TiO2 NTs的120倍 同時(shí),在各種SnSe納米結(jié)構(gòu)材料中一維SnSe納米管具有高電子傳輸效率、幾何異向性和量子限域效應(yīng),更有利于提高紅外探測性能[24~26] 因此,使用Ag修飾的SnSe納米管有望制備出高性能紅外探測器 本文用光沉積法在SnSe納米管表面修飾金屬Ag納米粒子,在室溫下合成Ag修飾SnSe(Ag/SnSe)納米管并以Pt為對電極組裝紅外探測器,研究其在模擬紅外光(830 nm)照射下的紅外探測性能、光響應(yīng)速度和循環(huán)穩(wěn)定性,并討論其機(jī)理

1 實(shí)驗(yàn)方法1.1 Ag/SnSe納米管的制備

以Se納米線為模板,用溶液法制備SnSe納米管[26],然后將0.02 g的SnSe納米管加到30 mL的0.05 mol/L 硝酸銀溶液中并磁力攪拌使其分散均勻；用波長為365 nm的紫外光照射溶液,光沉積15 min后自然沉降5 min 然后將得到的樣品用去離子水清洗并離心分離出Ag/SnSe納米管,重復(fù)2次后將其在烘箱中低溫干燥

1.2 Ag/SnSe納米管紅外探測器的制備

將制備出的Ag/SnSe納米管分散在無水乙醇中,然后旋涂到FTO導(dǎo)電面后烘干,將這一過程重復(fù)三次,獲得Ag/SnSe納米管薄膜 以負(fù)載在FTO表面的Ag/SnSe納米管薄膜為工作電極,以鍍Pt的FTO為對電極,通過熱封膜將工作電極和對電極相連接,中間注入聚硫電解質(zhì)溶液后密封

1.3 性能表征

使用掃描電子顯微鏡(SEM,Hitachi SU-70)及附帶的X射線能譜儀(EDS)表征樣品形貌和化學(xué)成分 用透射電子顯微鏡(TEM,FEI,Tecnai G2 F20)和高分辨透射電子顯微鏡(HRTEM)觀測樣品的形貌 用X射線衍射儀(XRD,Bruker D8 Advance)表征樣品的晶體結(jié)構(gòu) 將組裝好的Ag/SnSe納米管探測器與Keithley 2400數(shù)字源表連接,使用830 nm的光作為模擬紅外光源,測試其紅外探測性能

2 結(jié)果與討論

圖1a給出了SnSe納米管的SEM照片 可以觀察到,SnSe納米管生長均勻且其外表面包覆著細(xì)小的納米片 從高倍照片(圖1b)可見SnSe納米管的管口有明顯的開口,表明是中空結(jié)構(gòu) 光沉積Ag納米粒子(圖1c)的Ag/SnSe納米管形貌沒有明顯的變化,整體為魚鱗狀結(jié)構(gòu),直徑為100~200 nm 圖1d給出了Ag/SnSe納米管的高倍SEM照片,可見納米管的表面粗糙,包覆著分布緊密的魚鱗狀納米薄片 但是,在納米管表面沒有明顯的Ag納米顆粒,其原因可能是SnSe納米管表面較粗糙且Ag納米粒子的尺寸較小

圖1



圖1SnSe納米管和Ag/SnSe納米管的SEM照片

Fig.1SEM images of SnSe nanotubes (a, b) and Ag/SnSe nanotubes (c, d)

用EDS分析Ag/SnSe納米管的元素組成,結(jié)果如圖2所示 圖中有源于SnSe納米管的Se和Sn元素的特征峰,最強(qiáng)峰來自測試支撐Si襯底(用于樣品形貌觀察) 在2.99 keV處出現(xiàn)了Ag元素的特征峰,表明在SnSe納米管表面沉積了Ag納米粒子

圖2



圖2Ag/SnSe納米管的EDS能譜

Fig.2EDS pattern of Ag/SnSe nanotube

用TEM進(jìn)一步表征了Ag/SnSe納米管的微觀結(jié)構(gòu),結(jié)果如圖3a所示 從圖3a中的單根Ag/SnSe納米管TEM照片可觀察到納米管表面包裹了大量層次分明的魚鱗狀納米片,邊緣兩側(cè)比中間顏色更深,表明其為中空管狀結(jié)構(gòu) 同時(shí),還明顯可見Ag納米顆粒均勻地負(fù)載在魚鱗狀納米片表面 Ag/SnSe納米管的高分辨TEM照片,如圖3b所示 在照片中觀察到的0.208 nm的晶格條紋對應(yīng)SnSe的(141)晶面,而0.123 nm的晶格條紋則與Ag(311)晶面對應(yīng)[27]

圖3



圖3Ag/SnSe納米管的TEM和高分辨TEM照片

Fig.3TEM (a) and high resolution TEM (b) images of Ag/SnSe nanotube

圖4給出了樣品的XRD譜 從圖4中,位于2θ為26.5°、29.4°、30.4°、31°和51°處的衍射峰對應(yīng)于斜方晶系SnSe(JCPDS No.65-3767)的(021)、(101)、(111)、(040)和(122)晶面 在33°附近的衍射峰源于測試支撐Si襯底的(200)晶面 在38.1°和44.2°處的衍射峰較好地匹配立方晶系A(chǔ)g(JCPDS No.04-0783)的(111)和(200)晶面[28],進(jìn)一步證明Ag納米粒子成功地沉積在SnSe表面

圖4



圖4Ag/SnSe納米管的XRD譜

Fig.4XRD pattern of Ag/SnSe nanotubes

在無外加偏壓條件下用830 nm的光作為紅外光模擬光源,開啟紅外光照射10 s后關(guān)閉紅外光10 s作為一個(gè)測試周期,研究了Ag/SnSe納米管探測器對紅外光的探測性能,其結(jié)果如圖5所示 在無紅外光照射時(shí)Ag/SnSe納米管紅外探測器為靜默狀態(tài),光電流密度幾乎為零；開啟紅外光后器件瞬間產(chǎn)生光電流并快速攀升至最大值120 nA/cm2,然后逐漸穩(wěn)定 關(guān)閉紅外光后,光電流迅速衰減并恢復(fù)到初始狀態(tài) 經(jīng)過6次開關(guān)循環(huán),電流密度曲線沒有明顯的變化,這表明,所組裝的探測器具備高電流密度和較好的耐用性 同時(shí),器件能在無偏壓條件下穩(wěn)定工作,表明其具有自供能特性 在相同的實(shí)驗(yàn)條件下測試了SnSe納米管紅外探測器的性能,其光電流密度只有46 nA/cm2,比Ag/SnSe納米管紅外探測器的光電流密度降低了61% 這表明,Ag納米粒子修飾明顯提高了SnSe納米管探測器對紅外光的探測性能

圖5



圖5SnSe納米管和Ag/SnSe納米管紅外探測器在開/關(guān)紅外光照射下的電流密度曲線

Fig.5Time dependent current response of the SnSe and Ag/SnSe nanotubes infrared photodetector (IRPD) measured under on/off of IR light illumination

探測器的響應(yīng)時(shí)間,是評價(jià)其探測性能的一個(gè)關(guān)鍵參數(shù) 光電流從初始值升至峰值的63%所用的時(shí)間定義為上升時(shí)間,光電流從峰值降至峰值的37%所用的時(shí)間定義為下降時(shí)間[29] 圖6給出了器件的單周期光電響應(yīng)特征曲線 從圖6可見,SnSe納米管紅外探測器的上升時(shí)間和下降時(shí)間分別為0.174和0.349 s 用Ag納米粒子修飾后,上升時(shí)間和下降時(shí)間分別縮短至0.109和0.086 s 這表明,Ag納米粒子修飾SnSe納米管不僅增強(qiáng)了SnSe納米管探測器的光電流,也提高了對紅外光的響應(yīng)速度

圖6



圖6SnSe納米管和Ag/SnSe納米管紅外探測器單個(gè)周期的光電響應(yīng)特征曲線

Fig.6Single-cycle photocurrent response of the IRPD based on SnSe and Ag/SnSe nanotubes

Ag/SnSe納米管紅外探測器的機(jī)理如圖7所示 Ag/SnSe納米管獨(dú)特的魚鱗狀中空結(jié)構(gòu)和大比表面積,顯著提高了對紅外光的吸收能力,進(jìn)而提高了對紅外光的利用效率 用紅外光照射時(shí)SnSe納米管吸收的光子能量高于其帶隙,因此光生電子激發(fā)后從價(jià)帶躍遷至導(dǎo)帶并在價(jià)帶留下空穴 電子-空穴對的快速復(fù)合嚴(yán)重影響了SnSe納米管探測器的效率 而在SnSe表面負(fù)載Ag納米粒子后,金屬Ag與SnSe納米管表面發(fā)生肖特基接觸產(chǎn)生肖特基勢壘,進(jìn)而出現(xiàn)內(nèi)建電場 在光生電子從SnSe表面遷至單質(zhì)Ag的過程中促進(jìn)載流子的分離,抑制了光生電子-空穴對的復(fù)合 同時(shí),隨著SnSe納米管和電解液間電子-空穴對的分離[30],電子通過外部電路遷移至Pt電極并與電解液中的S x2-反應(yīng)生成S2-和S x-12- 生成的S2-在電解液中擴(kuò)散至SnSe納米管與其表面的空穴反應(yīng)生成S單質(zhì),S與S x-12-進(jìn)一步反應(yīng)生成S x2-[31] S x2-和S2-沒有被消耗而持續(xù)循環(huán),使光生電子通過外電路產(chǎn)生電流 因此,Ag/SnSe納米管紅外探測器能實(shí)現(xiàn)對紅外光的自供能探測 關(guān)閉紅外光后沒有光生電子產(chǎn)生,Ag/SnSe納米管紅外探測器迅速恢復(fù)到初始狀態(tài)

圖7



圖7Ag/SnSe納米管紅外探測器的原理

Fig.7Schematic illustration of the possible IR detection mechanism of the Ag/SnSe nanotubes IRPD

3 結(jié)論

用光沉積法將Ag納米顆粒沉積在魚鱗狀中空SnSe納米管表面,在室溫下制備Ag/SnSe納米管 Ag/SnSe納米管表面粗糙致密,在表面能觀察到微小的Ag納米粒子 與SnSe納米管探測器相比,用Ag修飾的SnSe納米管紅外探測器的光電流密度提高了約160%(達(dá)到120 nA/cm2)、光響應(yīng)速度也明顯改善、上升時(shí)間縮短至0.109 s、下降時(shí)間縮短至0.086 s,且具有較高的循環(huán)穩(wěn)定性

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PMID

Solution-processed semiconductor quantum dot solar cells offer a path towards both reduced fabrication cost and higher efficiency enabled by novel processes such as hot-electron extraction and carrier multiplication. Here we use a new class of low-cost, low-toxicity CuInSexS2-x quantum dots to demonstrate sensitized solar cells with certified efficiencies exceeding 5%. Among other material and device design improvements studied, use of a methanol-based polysulfide electrolyte results in a particularly dramatic enhancement in photocurrent and reduced series resistance. Despite the high vapour pressure of methanol, the solar cells are stable for months under ambient conditions, which is much longer than any previously reported quantum dot sensitized solar cell. This study demonstrates the large potential of CuInSexS2-x quantum dots as active materials for the realization of low-cost, robust and efficient photovoltaics as well as a platform for investigating various advanced concepts derived from the unique physics of the nanoscale size regime.

Si基微絕熱結(jié)構(gòu)PLZT厚膜紅外探測器陣列

1

2004