Graphene has attracted remarkable interest, within both scientific and industrial sectors, as a result of its distinctive electrical, thermal and mechanical features. Graphene is predicted to be a disruptive technology with multiple possible functions, ranging from flexible electronics to water filtration. The relevance of this technology was formally acknowledged following the presentation of the 2010 Nobel Prize in Physics for “groundbreaking experiments regarding the two-dimensional material graphene”.1
Graphene possesses an unusual electronic band structure with the valence and conduction bands forming conical surfaces in momentum space and touching at Dirac Points (Figure 1). One consequence of this touching is that graphene has no band gap, and is thus referred to as a zero bandgap semiconductor.
Figure 1. Simplified chemical and band structures of graphene and graphene oxide. VB = valence band, CB = conduction band. Adapted from Sehrawat et al.2
As a result of this absence of a bandgap, pure graphene is somewhat uninteresting in terms of photoluminescence (PL) spectroscopy, because materials possessing zero bandgap do not display PL. Nevertheless, by oxidizing graphene to constitute graphene oxide, the π-electron configuration of the carbon atoms in graphene is disturbed. This generates the development of a gap between the valence and conduction bands, causing the material to become photoluminescent, and consequently opening the door to additional functions.
As the PL of graphene oxide is associated with the degree of oxidation of the graphene lattice, the PL features can be tuned through oxidation degree alterations. This article examines the PL features of graphene oxide, and how they can be tuned via photothermal reduction, utilizing the FLS1000 Photoluminescence Spectrometer.
Figure 2. FLS1000 Photoluminescence Spectrometer with double excitation and emission monochromators for the measurement of highly scattering samples.
An aqueous dispersion of graphene oxide was acquired from PlasmaChem GmbH, and was diluted with deionized water to a concentration of 0.02 mg cm-3. At this concentration, the absorbance of the graphene oxide dispersion is 0.15 at 350 nm (beginning of the PL emission), and consequently there is no consequent distortion of the PL spectra as a result of the inner filter effect.
The PL of the graphene oxide was calculated by the FLS1000 Photoluminescence Spectrometer, which was fitted with double excitation and emission monochromators and PMT-900 and PMT-1700 photomultiplier tube detectors. The graphene oxide dispersion was loaded into a 10 mm pathlength quartz cuvette and held with the cuvette sample holder.
While measurements were taken, the dispersion was stirred constantly by the integrated magnetic stirrer to stop the graphene oxide from falling out of suspension. Photothermal reduction of the graphene oxide dispersion was executed out by positioning the cuvette at the focal point of a 450 W xenon lamp.
Results and Discussion
As graphene oxide is insoluble in water, it forms a dispersion and is highly scattering, as a result. The synthesis of this high scatter with its low quantum yield makes graphene oxide a difficult sample for the majority of spectrometers to assess. This is because most monochromators are incapable of totally removing the scattered excitation light, causing the distortion of the PL spectra, which is called stray light.
In contrast, the FLS1000 possesses optional double monochromators. With these, the light is diffracted and the wavelength selected twice, which therefore considerably enhances the stray light rejection of the monochromator from 1:1x105 to 1:1x1010, and makes sure that the spectra of even the most weakly emitting and highly scattering samples can be examined precisely. The PL range of graphene oxide when excited at 300 nm, utilizing double excitation and emission monochromators, is displayed in Figure 3.
Figure 3. PL spectrum of graphene oxide.The spectrum was recorded using two detectors with a crossover point at 800 nm. Visible (PMT-900) Parameters: λex = 300 nm, Δλex = 5 nm, Δλem = 5 nm. NIR (PMT-1700) Parameters: λex = 300 nm, Δλex = 10 nm, Δλem = 20 nm.
The PL of graphene oxide was discovered to be distinctly broad, spanning 350 nm to 1250 nm. To calculate this wide spectrum of emission wavelengths, two photomultiplier tube detectors, that have been optimized for varying wavelength scales. The observable section of the spectrum was evaluated with the basic PMT-900 detector of the FLS1000, which is sensitive up to 900 nm, and the NIR area was evaluated with the PMT-1700, which is sensitive up to 1700 nm.
The PL was calculated using each detector, and the spectra merged at 800 nm (the crossover point of the detector quantum efficiencies), to provide the total spectrum, shown in Figure 3.
An FLS1000 can concurrently hold a maximum of five different detectors, and these can be easily switched between solely using the Fluoracle® software of the FLS1000 with no hardware changes required.
One of the more notable photoluminescent features of graphene oxide is that the emission wavelength is governed by its degree of oxidation. An easy means of regulating the degree of oxidation is by executing photothermal reduction, with which the aqueous graphene oxide dispersion is subjected to heat and light, thus initiating deoxygenation reactions.
To interrogate the impact that the degree of oxidation has on the PL, the graphene oxide dispersion was photothermally reduced with the use of a focused xenon lamp. Figure 4 exhibits the PL spectra calculated after various exposure times.
Figure 4. Change in the PL spectra of graphene oxide with increasing photothermal reduction. λex = 300 nm, Δλex = 5 nm, Δλem = 5 nm.
Considering this data alongside existing PL, XPS and AFM studies,2,3 the basis of these two PL peaks can be determined. In the molecular image of a pure graphene cluster, all of the carbons are sp2 hybridized, and the overlap of the sp2 carbons' p-orbitals constitute a π bonding orbital and a π* antibonding orbital, which are equivalent to the valence and conduction bands.
In graphene oxide, each cluster possesses a fixed quantity of carbons that have been oxidized to sp3. These sp3 carbons disrupt the organization of the graphene and generate disorder induced defect states. These disorder induced states, which embody a broad energy distribution, possess a lower energy than the π-π* gap, and are thus accountable for the broad PL peak centered on 710 nm (Figure 5 left).
Figure 5. Origin of the PL shift when graphene oxide is reduced. Adapted from Chien et al.3
When the graphene is reduced, the quantity of oxidized sp3 carbons consequently decreases, and therefore the quantity of disorder-induced defect states is also decreased. The reduction generates fresh clusters of pure sp2 carbon, these confined cluster states possess a higher energy than the disorder states, and are thus accountable for the PL at 450 nm (Figure 5 right).
The PL of a low quantum yield and highly scattering graphene oxide dispersion was examined utilizing the FLS1000 Photoluminescence Spectrometer, fitted with double monochromators and an infrared detector. The PL of graphene oxide ranged from 350 nm to 1250 nm, with a maximum at 710 nm, and is caused by radiative recombination within low lying disorder induced defect states.
By photothermally reducing the graphene oxide, the PL maximum was tuned from 710 nm to 450 nm through decreasing lattice disorder and the configuration of confined cluster states which lie higher in energy.
References and Further Reading
- Press release. NobelPrize.org. Nobel Media AB 2018. Mon. 10 Dec 2018.
- A. Sehrawat, P. Sehrawat, S. S. Islam, P. Mishra & S. Ahmad, Sci. Rep. 8 (2018).
- C. Chien, S. Li, W. Lai, Y. Yeh, H. Chen, I. Chen, L. Chen, K. Chen, T. Nemoto, S. Isoda, M. Chen, T. Fujita, G. Eda, H. Yamaguchi, M. Chhowalla, and C. Chen, Angew. Chem. Int. Ed. 51, (2012).
This information has been sourced, reviewed and adapted from materials provided by Edinburgh Instruments.
For more information on this source, please visit Edinburgh Instruments.