石墨烯的性能優(yōu)異,在電子器件、導(dǎo)熱材料和復(fù)合材料等領(lǐng)域有潛在的應(yīng)用價值[1~6] 因此,近年來關(guān)于石墨烯材料的研究受到了高度重視 但是,石墨烯是零帶隙半導(dǎo)體[6~8],沒有發(fā)光特性 氧化石墨烯(Graphite oxide,GO)是石墨烯重要的衍生物之一,是規(guī)?；a(chǎn)石墨烯的原料 GO和石墨烯的結(jié)構(gòu)差異很大,GO內(nèi)部有羥基、羧基和環(huán)氧基等大量氧化官能團(tuán) 氧化官能團(tuán)破壞了石墨烯片層的 π 共軛體系,使其電學(xué)性質(zhì)和光學(xué)性質(zhì)發(fā)生了巨大變化,由導(dǎo)電（石墨烯）變?yōu)榻^緣(GO)[9~12]并具有光催化活性[13~15] 特別是sp2C/sp3C的交替分布打開了石墨烯的帶隙,使其具有發(fā)光性能 GO發(fā)光分布在可見光和近紅外波段,可用于生物檢測[16,17]和熒光標(biāo)記[18] 目前對氧化石墨烯光學(xué)性質(zhì)的研究剛剛展開,對其能帶結(jié)構(gòu)的認(rèn)識和發(fā)光機(jī)理的理解還很不深入 本文根據(jù)光致發(fā)光光譜、變溫發(fā)光光譜和吸收光譜,研究GO的發(fā)光機(jī)制和不同激發(fā)波長與變溫條件下的發(fā)光光譜,以揭示不同局域態(tài)的發(fā)光行為

1 實驗方法1.1 實驗用材料和儀器

天然鱗片石墨(325目)；微孔濾膜(醋酸纖維酯,直徑50 mm,孔徑0.22 μm)

inVia型發(fā)光光譜儀(PL)；Lambda900型紫外-可見吸收光譜儀(UV-Vis)

1.2 氧化石墨烯的制備

用改進(jìn)的Hummers法[19],將天然鱗片石墨通過超聲輔助液相氧化法制備氧化石墨烯（GO） 用真空抽濾法制備GO薄膜,改變過濾GO溶液的量或濃度,調(diào)節(jié)薄膜的厚度 分別在488 nm、514 nm和830 nm激發(fā)條件下測試GO薄膜的熒光光譜 在514 nm和830 nm激發(fā)條件下研究GO薄膜的原位變溫發(fā)光

2 結(jié)果和討論2.1 氧化石墨的吸收特性

圖1a給出了GO的紫外-可見吸收光譜 可以看出,吸收譜有227 nm和300 nm兩個峰,分別來自于C=C鍵的π-π*電子躍遷和C=O鍵的n-π*電子躍遷[20,21] 在吸收光譜上沒有發(fā)現(xiàn)清晰可辨的吸收邊,表明GO內(nèi)分布著很多局域態(tài)[22] 在強(qiáng)吸收區(qū)α≥104 cm-1范圍(Tauc region),吸收系數(shù)與光學(xué)帶隙之間滿足Tauc方程

(αE)1/2=B(E-Eopt)

(1)

其中B為常數(shù),與材料性質(zhì)有關(guān)[23]；E為光子能量；Eopt為光學(xué)帶隙[24~26],擬合可得到GO的Eopt=1.58 eV 在弱吸收區(qū)2×102 cm-1<α<5×103 cm-1范圍內(nèi)(Urbach tail)[27],吸收系數(shù)和E滿足e指數(shù)關(guān)系

α=α0exp(E/E0)

(2)

其中E0為urbach能,表征帶尾態(tài)的寬度,與材料的無序程度相關(guān) 根據(jù) 式(2)擬合得到E0=1.06 eV,表明GO具有較高的無序度且有較廣的局域態(tài)

圖1



圖1GO的紫外-可見吸收光譜和GO吸收光譜的Tauc擬合圖

Fig.1UV-Vis absorption spectra of GO (a) and corresponding Tauc plot (b)

2.2 常溫條件下GO的發(fā)光特性

圖2給出了GO分別在Eex =488 nm(2.53 eV>Eopt),Eex=514 nm(2.41 eV>Eopt)和Eex =830 nm(1.49 eV<Eopt)激發(fā)條件下的光致發(fā)光光譜(PL) GO的PL發(fā)射峰都具有非常寬的譜帶；隨著激發(fā)光能量的降低發(fā)射峰中心波長逐漸紅移,半峰寬(FWHM)變窄

圖2



圖2GO在488 nm,514 nm和830 nm激發(fā)波長下的發(fā)光光譜和GO 發(fā)射峰的中心波長(左)和半峰寬（右）與激發(fā)光波長的關(guān)

Fig.2PL spectra of GO for excitation at Eex =488 nm, Eex =514 nm and Eex =830 nm (a) and the dependences as the function of excitation wavelengths: (left) peak wavelength and (right) FWHM of PL spectra (b)

石墨烯是一種零帶隙半導(dǎo)體,無發(fā)光特性[26~28] 但是當(dāng)其尺寸減小至納米級別時,由于量子限域效應(yīng)帶隙被打開,具有發(fā)光特性[28] GO的發(fā)光來自于量子限域效應(yīng),其發(fā)光中心由小尺寸的sp2C團(tuán)簇組成 sp2C團(tuán)簇的π-π*間的帶隙被高勢壘的氧化官能團(tuán)(sp3C區(qū)域)包圍形成量子阱結(jié)構(gòu),如圖3所示,此能帶結(jié)構(gòu)與非晶碳相似[30]

圖3



圖3非晶碳的能帶結(jié)構(gòu)

Fig.3Schematic diagram of band structure of GO

用多量子阱的能帶結(jié)構(gòu)可解釋GO的發(fā)光特征 sp2C團(tuán)簇π-π*間的帶隙與尺寸相關(guān),碳團(tuán)簇的尺寸越小帶隙越寬 GO的片層結(jié)構(gòu)中存在不同尺寸的sp2C區(qū)域,因此分布著非常多的局域狀態(tài),可由吸收光譜中較大的urbach能證明 由于共振吸收效應(yīng),激發(fā)光能量與發(fā)光中心帶隙能量相同時吸收最強(qiáng) 在不同激發(fā)條件下,因共振吸收效應(yīng)參與的發(fā)光中心不同,因此隨著激發(fā)能量降低發(fā)光的主峰位置紅移 同時,隨著激發(fā)能量的降低可被激發(fā)的發(fā)光中心減少,使發(fā)光峰的半峰寬變窄

2.3 GO的變溫發(fā)光性質(zhì)

圖4給出了GO發(fā)光強(qiáng)度與溫度的關(guān)系 在514.5 nm和830 nm激發(fā)條件下和80 K~300 K范圍內(nèi),GO的發(fā)光強(qiáng)度降低一個數(shù)量級,強(qiáng)度變化的轉(zhuǎn)折點分別出現(xiàn)在220 K(Eex =514.5 nm)和160 K(Eex =830 nm) GO的發(fā)光強(qiáng)度隨溫度的變化在一個數(shù)量級內(nèi),與非晶碳發(fā)光隨溫度的變化關(guān)系相似[23,30,31] 以往的研究表明,在非晶碳材料體系中,即使具有較高的缺陷密度(發(fā)光猝滅中心密度大),溫度對發(fā)光強(qiáng)度的影響仍然較小[31] 其原因是,電子-空穴對被sp3C區(qū)域的高勢壘限制在局域的碳團(tuán)簇中,使電子-空穴的波函數(shù)交疊變大,大大提高了發(fā)生輻射躍遷的幾率[30,32] 相反,sp3C勢壘層被破壞后,由于體系缺陷的密度較高,在聲子輔助下電子快速轉(zhuǎn)移缺陷處復(fù)合,在室溫下很難觀察到發(fā)光現(xiàn)象,如a-Si1-x Cx:H(x<0.09)[32] 制備過程中的強(qiáng)氧化作用,使GO中存在大量缺陷態(tài)(如碳空位) 但是,高勢壘sp3C區(qū)域?qū)p2C團(tuán)簇的限域效應(yīng),限制了電子到缺陷態(tài)的復(fù)合,使輻射躍遷幾率提高,在室溫下即可觀察到發(fā)光現(xiàn)象 氧化官能團(tuán)(高勢壘區(qū))被破壞后,如氧化石墨烯被還原,雖然GO內(nèi)仍然存在小尺寸的sp2C團(tuán)簇(發(fā)光中心)仍然能觀察到發(fā)光現(xiàn)象[33]

圖4



圖4兩種不同激發(fā)條件下GO相對發(fā)光強(qiáng)度的溫度依賴性：(a) Eex =514 nm; (b) Eex =830 nm

Fig.4Temperature dependence of the relative PL intensity of GO at two different exciting conditions: (a) Eex =514 nm; (b) Eex =830 nm

在不同激發(fā)波長條件下,變溫發(fā)光隨著溫度的變化呈現(xiàn)不同的變化趨勢 在514 nm激發(fā)下在80~220 K,GO的發(fā)光強(qiáng)度受溫度的影響較小,溫度≥220 K時發(fā)光強(qiáng)度明顯降低；在830 nm激發(fā)下在80~300 K,GO的發(fā)光強(qiáng)度隨著溫度的降低而降低,溫度≥160 K時發(fā)光強(qiáng)度降低的速率變大 利用Arrhenius方程[34],可擬合得到發(fā)光猝滅的熱激活能

IPL(T)=I0/(1+Aexp(-Ea/kBT))

(3)

其中T為溫度,kB為波爾茲曼常數(shù),I0為0 K附近的PL譜積分強(qiáng)度,A為常數(shù),Ea 為熱激活能 擬合結(jié)果為：Eex =514 nm,熱激活能Ea =119 MeV(圖5a)；Eex =830 nm,熱激活能Ea =63 MeV(圖5b) 這表明,與Eex =830 nm相比,在Eex =514 nm激發(fā)下參與發(fā)光的sp2C團(tuán)簇的熱穩(wěn)定性更高,發(fā)生溫度猝滅所需的熱激活能高達(dá)56 MeV 根據(jù)GO的多量子阱結(jié)構(gòu),可解釋不同激發(fā)波長下的變溫發(fā)光 由于共振效應(yīng),不同能量的光激發(fā)不同尺寸的局域態(tài)(sp2C團(tuán)簇) 與Eex =830 nm相比,Eex =514 nm激發(fā)的sp2C團(tuán)簇尺寸較小 在多量子阱結(jié)構(gòu)中,由于量子尺寸效應(yīng),sp2C團(tuán)簇尺寸越小能級間隔越大,熱猝滅需要的參與的聲子數(shù)目增加,因此尺寸越小發(fā)光強(qiáng)度受溫度的影響越小

圖5



圖5光致發(fā)光強(qiáng)度與1/KBT之間的關(guān)系

Fig.5Relationship between relative PL intensity and 1/KBT, exciting at Eex =514 nm (a) and Eex =830 nm (b)

3 結(jié)論

用多量子阱結(jié)構(gòu)能解釋GO的發(fā)光特性 GO的發(fā)光來自于片層內(nèi)的sp2C團(tuán)簇 sp2C團(tuán)簇被高勢壘的氧化官能團(tuán)（sp3C）包圍,形成了多量子阱結(jié)構(gòu) GO內(nèi)有不同尺寸的sp2C團(tuán)簇,由于量子尺寸效應(yīng)sp2C團(tuán)簇的帶隙與尺寸相關(guān) 因此GO的發(fā)光呈現(xiàn)出對激發(fā)波長的依賴,尺寸越小帶隙越寬,溫度對發(fā)光強(qiáng)度的影響越小

參考文獻(xiàn)

View Option 原文順序文獻(xiàn)年度倒序文中引用次數(shù)倒序被引期刊影響因子

[1]

Stoller M D, Park S J, Zhu Y W, et al.

Graphene-based ultracapacitors

[J]. Nano. Lett., 2008, 8: 3498

DOIPMID [本文引用: 1] " />

The exceptional electronic properties of graphene, with its charge carriers mimicking relativistic quantum particles and its formidable potential in various applications, have ensured a rapid growth of interest in this new material. We report on electron transport in quantum dot devices carved entirely from graphene. At large sizes (>100 nanometers), they behave as conventional single-electron transistors, exhibiting periodic Coulomb blockade peaks. For quantum dots smaller than 100 nanometers, the peaks become strongly nonperiodic, indicating a major contribution of quantum confinement. Random peak spacing and its statistics are well described by the theory of chaotic neutrino billiards. Short constrictions of only a few nanometers in width remain conductive and reveal a confinement gap of up to 0.5 electron volt, demonstrating the possibility of molecular-scale electronics based on graphene.

[4]

Said A R, Said K, Awwad F, et al.

Design, fabrication, and characterization of Hg2+ sensorbased on graphite oxide and metallic nanoclusters

[J]. Sensor Actuat. A. Phys., 2018, 271: 270

DOIURL

[5]

Yang J K, Zhang X T, Li B, et al.

Photocatalytic activities of heterostructured TiO2-graphene porous microspheres prepared by ultrasonic spray pyrolysis

[J]. J. Alloys Compd., 2014, 584: 180

DOIURL

[6]

Li B, Zhang X T, Li X H, et al.

Photo-assisted preparation and patterning of large-area reduced graphene oxide-TiO2 conductive thin film

[J]. Chem. Commun., 2010, 46: 3499

DOIURL [本文引用: 2]

[7]

Ma F, Guo Z K, Xu K W, et al.

First-principle study of energy band structure of armchair graphene nanoribbons

[J]. Solid State Commu., 2012, 152: 1089

DOIURL

[8]

Zagonel L F, Mazzucco S, Tence M, et al.

Nanometer scale spectral imaging of quantum emitters in nanowires and its correlation to their atomically resolved structure

[J]. Nano. Lett., 2011, 11: 568

DOIURL [本文引用: 1]

[9]

Zhang Y B, Tan Y W, Stormer H L, et al.

Experimental observation of the quantum Hall effect and Berry's phase in graphene

[J]. Nature, 2005, 438: 201

DOIURL [本文引用: 1]

[10]

Luo Z T, Vora P M, Mele E J, et al.

Photoluminescence and band gap modulation in graphene oxide

[J]. Appl. Phys. Lett., 2009, 94: 111909

DOIURL

[11]

Eda G, Lin Y Y, Mattevi C, et al.

Blue photoluminescence from chemically derived graphene oxide

[J]. Adv. Mater., 2010, 22: 505

DOIURL

[12]

Pan D Y, Zhang J C, Li Z, et al.

Hydrothermal route for cutting graphene sheets into blue-luminescent graphene quantum dots

[J]. Adv. Mater., 2010, 22: 734

DOIURL [本文引用: 1]

[13]

Li B, Gao F, Yang G M, et al.

Synthesis and characterization of graphene film via photo-chemical reduction of graphene oxide

[J]. Chem. J. Chinese Universities, 2014, 35(12): 2612

[本文引用: 1]

[14]

Li B, Zhang X T, Chen P, et al.

Waveband-dependent photochemical processing of graphene oxide in fabricating reduced graphene oxide film and graphene oxide-Ag nanoparticles film

[J]. RSC. Adv., 2014, 4: 2404

DOIURL

[15]

Zhao M X, Meng Z, Li H P, et al.

Photodegradation of antibiotic in environmental water by graphene oxide modulation bismuth molybdate under visible light irradiation

[J]. Chem. J. Chinese Universities, 2020, 41(11): 2479

[本文引用: 1]

[16]

Lee J, Kim JG, Kim S C, et al.

Biosensors based on graphene oxide and its biomedical application

[J]. Adv. Drug. Deliv. Rev., 2016, 105: 275

DOIURL [本文引用: 1]

[17]

Shen X Q, Li Z, Wang G, et al.

Logic and reversible dual DNA detection based on the assembly of graphene oxide and DNA-templated quantum dots

[J]. Chem. J. Chinese Universities, 2017, 38(12): 2176

[本文引用: 1]

[18]

Sun X M, Liu Z, Welsher K, et al.

Nano-graphene oxide for cellular imaging and drug delivery

[J]. Nano. Res., 2008, 1: 203

DOIURL [本文引用: 1]

[19]

Hununers W S, Offeman R E.

Preparation of graphitic oxide

[J]. J. Am. Chem. Soc., 1958, 80: 1339

DOIURL [本文引用: 1]

[20]

Li D, Müller M B, Gilje S, et al.

Processable aqueous dispersions of graphene nanosheets

[J]. Nat. Nanotechnol., 2008, 3: 101

DOIURL [本文引用: 1]

[21]

Li X Y, Zhang G Y, Bai X M, et al.

Highly conducting graphene sheets and Langmuir-Blodgett films

[J]. Nat. Nanotechnol., 2008, 3: 538

DOIURL [本文引用: 1]

[22]

Urbaeh F.

The long-wavelength edge of photographic sensitivity and of the electronic absorption of solids

[J]. Phys.Rev., 1953, 92: 1324

[本文引用: 1]

[23]

Wu J X, Xu F, Ye P, et al.

The effects of nitrogen partial pressure on the microstructure of amorphous carbon nitride films

[J]. Integr. Ferroelectr., 2017, 180(1): 139

DOIURL [本文引用: 2]

[24]

Prasad N, Tanwar S, Mukhopadhyay S, et al.

Spatio-temporal analysis of the electric field-induced solid-state reduction dynamics of graphene oxide thin films for controlled band-gap modulation

[J]. J. Phys. Chem. C., 2020, 124: 21874

DOIURL [本文引用: 1]

[25]

Adhikary S, Tian X M, Adhikari S, et al.

Bonding defects and optical band gaps of DLC films deposited by microwave surface-wave plasma CVD

[J]. Diam. Relat. Mater., 2005, 14: 1832

DOIURL

[26]

Essig S, Marquardt C W, Vijayaraghavan A, et al.

Phonon-Assisted Electroluminescence from Metallic Carbon Nanotubes and Graphene

[J]. Nano. Lett., 2010, 10: 1589

DOIPMID [本文引用: 2] " />

We show that strong photoluminescence (PL) can be induced in single-layer graphene using an oxygen plasma treatment. The PL is spatially uniform across the flakes and connected to elastic scattering spectra distinctly different from those of gapless pristine graphene. Oxygen plasma can be used to selectively convert the topmost layer when multilayer samples are treated.

[29]

Guo H L, Wang X F, Qian Q Y, et al.

A Green Approach to the Synthesis of Graphene Nanosheets

[J]. ACS. Nano., 2009, 3: 2653

DOIURL

[30]

Robertson J.

π-bonded clusters in amorphous carbon materials

[J]. Philos. Mag. B., 1992, 66: 199

DOIURL [本文引用: 3]

[31]

Robertson J.

Diamond-like amorphous carbon

[J]. Mater. Sci. Eng. R., 2002, 37: 129

DOIURL [本文引用: 2]

[32]

Vassilyev V A, Volkov A S, Musabekov E U, et al.

Photoluminescence of amorphous hydrogenated silicon-carbon (a-SiC:H) films

[J]. J.Non. Cryst.Solids, 1989, 114: 507

[本文引用: 2]

[33]

Lou Q, Qu S N, Jing P T, et al.

Water-triggered luminescent "nano-bombs" based on supra-(carbon nanodots)

[J]. Adv. Mater., 2015, 27: 1389

DOIURL [本文引用: 1]

[34]

Chambers M D, Clarke D R.

Doped oxides for high-temperature luminescence and lifetime thermometry

[J]. Annu. Rev. Mater. Res., 2009, 39: 325

DOIURL [本文引用: 1]

Graphene-based ultracapacitors

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2008