石墨烯的性能优异,在电子器件、导热材料和复合材料等领域有潜在的应用价值[1~6]。因此,近年来关于石墨烯材料的研究受到了高度重视。但是,石墨烯是零带隙半导体[6~8],没有发光特性。氧化石墨烯(Graphite oxide,GO)是石墨烯重要的衍生物之一,是规模化生产石墨烯的原料。GO和石墨烯的结构差异很大,GO内部有羟基、羧基和环氧基等大量氧化官能团。氧化官能团破坏了石墨烯片层的 π 共轭体系,使其电学性质和光学性质发生了巨大变化,由导电（石墨烯）变为绝缘(GO)[9~12]并具有光催化活性[13~15]。特别是sp2C/sp3C的交替分布打开了石墨烯的带隙,使其具有发光性能。GO发光分布在可见光和近红外波段,可用于生物检测[16,17]和荧光标记[18]。目前对氧化石墨烯光学性质的研究刚刚展开,对其能带结构的认识和发光机理的理解还很不深入。本文根据光致发光光谱、变温发光光谱和吸收光谱,研究GO的发光机制和不同激发波长与变温条件下的发光光谱,以揭示不同局域态的发光行为。
1 实验方法
1.1 实验用材料和仪器
天然鳞片石墨(325目)；微孔滤膜(醋酸纤维酯,直径50 mm,孔径0.22 μm)。
inVia型发光光谱仪(PL)；Lambda900型紫外-可见吸收光谱仪(UV-Vis)。
1.2 氧化石墨烯的制备
用改进的Hummers法[19],将天然鳞片石墨通过超声辅助液相氧化法制备氧化石墨烯（GO）。用真空抽滤法制备GO薄膜,改变过滤GO溶液的量或浓度,调节薄膜的厚度。分别在488 nm、514 nm和830 nm激发条件下测试GO薄膜的荧光光谱。在514 nm和830 nm激发条件下研究GO薄膜的原位变温发光。
2 结果和讨论
2.1 氧化石墨的吸收特性
其中E0为urbach能,表征带尾态的宽度,与材料的无序程度相关。根据
图1
图1
GO的紫外-可见吸收光谱和GO吸收光谱的Tauc拟合图
Fig.1
UV-Vis absorption spectra of GO (a) and corresponding Tauc plot (b)
2.2 常温条件下GO的发光特性
图2给出了GO分别在Eex =488 nm(2.53 eV>Eopt),Eex=514 nm(2.41 eV>Eopt)和Eex =830 nm(1.49 eV<Eopt)激发条件下的光致发光光谱(PL)。GO的PL发射峰都具有非常宽的谱带；随着激发光能量的降低发射峰中心波长逐渐红移,半峰宽(FWHM)变窄。
图2
图2
GO在488 nm,514 nm和830 nm激发波长下的发光光谱和GO 发射峰的中心波长(左)和半峰宽（右）与激发光波长的关
Fig.2
PL 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)
图3
用多量子阱的能带结构可解释GO的发光特征。sp2C团簇π-π*间的带隙与尺寸相关,碳团簇的尺寸越小带隙越宽。GO的片层结构中存在不同尺寸的sp2C区域,因此分布着非常多的局域状态,可由吸收光谱中较大的urbach能证明。由于共振吸收效应,激发光能量与发光中心带隙能量相同时吸收最强。在不同激发条件下,因共振吸收效应参与的发光中心不同,因此随着激发能量降低发光的主峰位置红移。同时,随着激发能量的降低可被激发的发光中心减少,使发光峰的半峰宽变窄。
2.3 GO的变温发光性质
图4给出了GO发光强度与温度的关系。在514.5 nm和830 nm激发条件下和80 K~300 K范围内,GO的发光强度降低一个数量级,强度变化的转折点分别出现在220 K(Eex =514.5 nm)和160 K(Eex =830 nm)。GO的发光强度随温度的变化在一个数量级内,与非晶碳发光随温度的变化关系相似[23,30,31]。以往的研究表明,在非晶碳材料体系中,即使具有较高的缺陷密度(发光猝灭中心密度大),温度对发光强度的影响仍然较小[31]。其原因是,电子-空穴对被sp3C区域的高势垒限制在局域的碳团簇中,使电子-空穴的波函数交叠变大,大大提高了发生辐射跃迁的几率[30,32]。相反,sp3C势垒层被破坏后,由于体系缺陷的密度较高,在声子辅助下电子快速转移缺陷处复合,在室温下很难观察到发光现象,如a-Si1-x Cx:H(x<0.09)[32]。制备过程中的强氧化作用,使GO中存在大量缺陷态(如碳空位)。但是,高势垒sp3C区域对sp2C团簇的限域效应,限制了电子到缺陷态的复合,使辐射跃迁几率提高,在室温下即可观察到发光现象。氧化官能团(高势垒区)被破坏后,如氧化石墨烯被还原,虽然GO内仍然存在小尺寸的sp2C团簇(发光中心)仍然能观察到发光现象[33]。
图4
图4
两种不同激发条件下GO相对发光强度的温度依赖性：(a) Eex =514 nm; (b) Eex =830 nm
Fig.4
Temperature dependence of the relative PL intensity of GO at two different exciting conditions: (a) Eex =514 nm; (b) Eex =830 nm
在不同激发波长条件下,变温发光随着温度的变化呈现不同的变化趋势。在514 nm激发下在80~220 K,GO的发光强度受温度的影响较小,温度≥220 K时发光强度明显降低；在830 nm激发下在80~300 K,GO的发光强度随着温度的降低而降低,温度≥160 K时发光强度降低的速率变大。利用Arrhenius方程[34],可拟合得到发光猝灭的热激活能
其中T为温度,kB为波尔兹曼常数,I0为0 K附近的PL谱积分强度,A为常数,Ea 为热激活能。拟合结果为：Eex =514 nm,热激活能Ea =119 MeV(图5a)；Eex =830 nm,热激活能Ea =63 MeV(图5b)。这表明,与Eex =830 nm相比,在Eex =514 nm激发下参与发光的sp2C团簇的热稳定性更高,发生温度猝灭所需的热激活能高达56 MeV。根据GO的多量子阱结构,可解释不同激发波长下的变温发光。由于共振效应,不同能量的光激发不同尺寸的局域态(sp2C团簇)。与Eex =830 nm相比,Eex =514 nm激发的sp2C团簇尺寸较小。在多量子阱结构中,由于量子尺寸效应,sp2C团簇尺寸越小能级间隔越大,热猝灭需要的参与的声子数目增加,因此尺寸越小发光强度受温度的影响越小。
图5
图5
光致发光强度与1/KBT之间的关系
Fig.5
Relationship between relative PL intensity and 1/KBT, exciting at Eex =514 nm (a) and Eex =830 nm (b)
3 结论
用多量子阱结构能解释GO的发光特性。GO的发光来自于片层内的sp2C团簇。sp2C团簇被高势垒的氧化官能团（sp3C）包围,形成了多量子阱结构。GO内有不同尺寸的sp2C团簇,由于量子尺寸效应sp2C团簇的带隙与尺寸相关。因此GO的发光呈现出对激发波长的依赖,尺寸越小带隙越宽,温度对发光强度的影响越小。
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