Elsevier

Carbon

Volume 95, December 2015, Pages 228-238
Carbon

On the origin and tunability of blue and green photoluminescence from chemically derived graphene: Hydrogenation and oxygenation studies

https://doi.org/10.1016/j.carbon.2015.08.036Get rights and content

Abstract

We report on the identification of structural defects and oxygenated functional groups responsible for blue and green photoluminescence (PL) from the chemically derived graphene (CDG) thin films with the help of Raman imaging/spectroscopy, high resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FTIR) and PL analyses. In particular, we probe the role of in-plane and edge oxygenated functionalities on the evolution of visible PL emissions from CDGs after controlled hydrogenation and oxygenation studies. The assignments of the various PL bands were corroborated from thermogravimetric and FTIR analyses in the CDGs and are directly correlated with the Raman analysis. Our studies reveal that the PL emission spectrum in CDGs can be tuned by controlled hydrogen and oxygen treatments. Two green emission bands in the range of ∼497–502 nm and ∼534–551 nm are assigned to the COOH and Cdouble bondO sub-band energy states belonging to the edge sites, while the blue emission is attributed to the localised states of sp2/sp3 domains and epoxy related in-plane functional groups in the CDG materials. Our study demonstrates the tunability of PL spectrum from CDG materials through selective manipulation of the functional groups at the in-plane defects and edge sites.

Introduction

Chemically derived graphene (CDG) is a new class of functional carbon nanomaterial that exhibits interesting optical characteristics for the potential applications in the field of photovoltaics, photocatalysis, bioimaging etc. [1], [2], [3] Fabrication of CDG functional materials such as graphene oxide (GO), reduced-GO (rGO), graphene quantum dots (GQD) and graphene oxide nanoribbons (GONR) using chemical approaches and understanding their intriguing photoluminescence (PL) properties are important for the photovoltaic and photoconductive devices, which open up a new era of 2D nanomaterial research [4], [5]. The absence of band gap in graphene necessitates the use of CDGs grown by simple chemical methods, which show numerous interesting optical properties by tuning the in-plane and edge oxygen functionalities [6], [7]. In CDGs, plenty of oxygenated functionalities are covalently attached at the graphene basal planes as well as at the edges [1]. On the other hand, the graphene grown by conventional chemical vapour deposition (CVD) does not exhibit PL due to large coverage of sp2 carbon without any conjugated network of oxygenated functionalities, although it may possess structural defects.

The band gap engineering of CVD graphene and GO ultra-thin films has been explored through various means, such as reactive gas annealing and plasma treatments in certain doping environments (e.g., boron, nitrogen, oxygen and fluorine etc.) to induce visible to near infrared (NIR) PL for optoelectronic applications [8], [9], [10]. The CDGs can show enhanced PL emission due to the carbon and oxygen covalent and non-covalent functionalization [1], [11]. In fact, the electronic structure of graphene can be altered by chemical approaches depending upon the size distribution of sp2 and sp3 domains and by tuning the selective oxygenated surface along with the edge functionalities on pristine graphene layer [12]. The commonly observed oxygenated functional groups that are responsible for the PL in these kind of CDG materials are hydroxyl (–OH), epoxy (O–C–O) and (O–Cdouble bondC) groups on the basal plane of graphene and ring type functionalities, such as carboxylic (COOH), carbonyl (Cdouble bondO), phenol (C6H5OH), lactone, quinone groups attached at the edges [1]. Wide range of tunable visible−NIR PL can be achieved by manipulation of these oxygenated functionalities mostly at sp3 carbon sites depending on size and fraction of the adjacent sp2 domains [11]. The distribution and nature of oxygenated functionalities among the sp2 and sp3 carbon domains at the basal plane and edges play a vital role in the band gap opening as well as the formation of CO-related localized sub-band electronic states [1], [13]. Note that most of the PL studies on CDG functional materials have been carried out by dispersion of CDGs in aqueous and organic solvents and there is a lack of one to one correlation among different characterization techniques [6], [11], [14], [15], [16]. There are very few reports on the PL studies of CDG materials in solid state thin film form coated on inorganic inert substrates and there is a lack of understanding on the origin of sub-band gap energy PL emissions [1], [9], [14]. Note that the fabrication of CDG thin film is relatively challenging and the exact contributions of in-plane and edge functionalities in CDG thin films on the visible PL is not understood well [17]. A clear understanding on the classification and assignment of various functional groups and the role of intrinsic defects and extrinsic oxygenated functionalities for the evolution of PL are imperative for their future applications.

Herein, we attempt to provide a clear insight on the origin of visible PL from various CDG materials using thermogravimetry analysis (TGA), micro-Raman, Fourier transform infrared spectroscopy (FTIR) and PL spectroscopy. We primarily focus on the structural and optical characteristics of in-plane and edge functionalised graphene derivatives. The graphene and its derivatives have been fabricated using a thermal CVD system and simple chemical exfoliation methods, respectively [1], [18]. The evolution of structural defects and oxygenated functionalities on the basal plane and edges of CVD graphene and CDGs were systematically elucidated by micro-Raman spectroscopy, high resolution transmission electron microscopy (HRTEM) and TGA analyses. Further, the sub-band gap PL emissions originating from the in-plane defects and edge oxygen functionalities in various CDGs were identified from quantitative analysis of differential thermogravimetry (DTG) results, for the first time. These results are supported by the UV–visible absorption, FTIR and micro-Raman studies. Nature of in-plane and edge functionalities in CDGs, which are responsible for the distinct PL emissions, is distinguished on the basis of evolution of the FTIR and DTG peaks. Further, these results are correlated with the Lorentzian line shape features of Raman spectra of CDGs before and after hydrogenation. The evolution of blue and green PL emission due to the oxygenation and hydrogenation treatments reveals the critical role of in-plane and edge Cdouble bondO and COOH functionalities in the CDG samples in the observed PL. This study open up the possibility to engineer the specific PL emissions in the visible and NIR wavelength regions by controlling the oxygen functional groups and would enable the band gap engineering of functionalized graphene and CDG nanomaterials.

Section snippets

Growth of graphene by thermal CVD

Large area single layer graphene is synthesized by thermal CVD on a 25 μm thick Cu foil. The full experimental details on CVD growth of graphene is discussed in the supporting information (see section SI1).

Preparation of chemically derived graphene (CDG)

Simple chemical exfoliation techniques are employed for the preparation of GO based CDG samples with different carbon source materials. The processing steps and full details of the experimental parameters used for the preparation of CDGs are described in the supporting information (Sections

Characterizations

The morphology and crystal structure of CVD graphene and CDG samples are mainly characterized by HRTEM and micro-Raman spectroscopy. Raman measurements have been performed with a high resolution spectrometer (Horiba, LabRam HR) with excitation wavelengths (λex) 514.5 nm (Ar+ ion laser) and 632.8 nm (He–Ne laser). Excitation source was focused with 100X objective lens, spot size of 2 μm, laser power 1 mW to avoid laser heating and damage to the sample. The Raman signal was collected by a CCD in

Raman studies of CVD graphene and CDGs

Raman spectroscopy is an important non-destructive tool to evaluate the crystal structure and covalently attached surface moieties on the sp2 and sp3 carbon nanomaterials. From Raman finger prints and Lorentzian line shape analyses, we have probed the structural imperfections arising from the intrinsic defects and oxygenated functionalities on graphene basal plane and edges. The intrinsic defects are naturally grown during the CVD growth process and are unavoidable. In case of CDGs, various

Conclusion

The spatial distribution and identification of intrinsic and functional group defects at the in-plane and edge sites in graphene and CDG materials were established from the micro-Raman, FTIR and HRTEM analysis to elucidate the origin of blue and green PL emissions from the CDG thin films. The evidence for the specific functionalities at the edges and basal plane that gives rise to the blue and green emission in CDGs have been obtained from the combined analysis of micro-Raman and TGA/DTG

Acknowledgements

We acknowledge Central Instruments Facility (CIF) for providing micro-Raman and TGA facilities. We acknowledge the financial support from DEITY (No−5(9)/2012−NANO, Vol−II), Govt. of India, to carry out the part of this work. We thank DST (No SR/55/NM−01/2005) for providing the HRTEM facility at IIT Guwahati. We also thank Albert V. Tamashausky, Asbury Graphite Mills, USA, for providing the high purity expandable graphite flakes to synthesize GO, rGO and GQD.

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