ARTICLE pubs.acs.org/JPCC
Ultrafast Relaxation Dynamics in Graphene Oxide: Evidence of Electron Trapping Sreejith Kaniyankandy,*,† S. N. Achary,‡ Sachin Rawalekar,† and Hirendra N. Ghosh*,† †
Radiation and Photochemistry Division and ‡Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India
bS Supporting Information ABSTRACT: We have employed ultrafast optical pump probe spectroscopy to study excited dynamics of graphene oxide (GO) and reduced graphene oxide (RGO) dispersed in water by exciting the samples at 400 nm laser light. We have observed multiexponential relaxation dynamics of nonequilibrium photogenerated carriers for both GO and RGO with time constants varying from 0.8 to >400 ps. Our analysis on excited dynamics of charge carriers revealed the involvement of phonon and trap states in relaxation dynamics. Transient absorption in the visible region on the longer time scale is attributed mainly due to the electron trap states which have been confirmed by quenching studies. Dynamics of RGO after chemical reduction was found to have reduced trap state contributions. Although the reduction procedure was found to be effective, still we have observed minor contribution due the trap states of charge carriers in transient absorption.
1. INTRODUCTION Graphene is a 2-D array consisting of sp2 hybridized carbon atoms in a honeycomb lattice. This distinctive structure imparts graphene with a unique linear electronic dispersion near the K-point of the Brillouin zone1 which leads to velocity of ∼106m/s and mobility of ∼15 000 cm2/(V s) for the electrons.2 These unique properties are ideally suited for application in fast electronic devices like transistors, diodes, and oscillators.3 Apart from these applications, graphene has been exploited for verifying table top quantum electrodynamics experiments due to its unique electronic structure. Previously, several groups have verified a number of exotic behaviors like the observation of room temperature quantum Hall effect,4 Klein paradox,5 etc. These applications and interesting novel properties of graphene have given impetus to their synthesis by different routes. The first report on synthesis of graphene was by Geim et al.4 who produced graphene by micromechanical cleavage from highly oriented pyrolitic graphite (HOPG) using scotch tape.2 Graphene prepared by micromechanical cleavage was used to verify several novel attributes of graphene like observation of the Klein paradox, quantum Hall effect, high mobility, etc. at room temperature. However, from the application point of view, the scotch tape method is not very conducive due to nonscalability. Recently several methods like chemical vapor deposition (CVD),6 decomposition of SiC at higher temperature,7 and chemical methods like oxidation promoted exfoliation8 have been proposed as possible routes to synthesis of graphene. Decomposition of SiC to form hexagonal graphene lattice has shown promising behavior similar to the ones prepared by micromechanical cleavage used by Geim et al.4 However, this method suffers from the formation of multilayers along the surface, leading to inhomogeneity along the films of graphene. Furthermore, strong r 2011 American Chemical Society
interaction with the SiC substrate led to breakdown of AB sublattice symmetry leading to band gap opening at the K-point.9 Klaus Mullen et al. have used the bottom up approach to synthesize graphene sheets which are in principle defect-free; however, these methods are economically nonviable.10On the other hand, oxidation induced exfoliation has captured the attention of several groups due to their ability to produce these layers of graphene oxide (GO) on a large scale.11 Furthermore, these samples can be processed by chemical reduction to obtain monolayer graphene.8 However, an inherent drawback of this method is incorporation of a considerable amount of oxygen in the graphene layer, making it a semiconducting material.12 Therefore, the oxidation induced exfoliation produces GO in which there is an incorporation of COOH groups, epoxides, hydroxyl bonds, etc., which are produced at the cost of the CdC sp2 linkage of graphene.13 These linkages have shown the alteration of the density of states (DOS) significantly at the carbon atoms directly linked to oxygen groups.13 Furthermore, changes in hybridization of carbon bonds in these groups have been shown to lead to significant distortion or buckling of the hexagonal lattice.14 Formation of endoperoxide or hydroperoxides on semiconducting carbon nanotubes (CNT) was found to shift electron density toward oxygen due to its electronegativity and strongly hole doped CNT.15 Similar effects may be prevalent in GO too due to the similarity in CC bonding in CNT and graphene. Moreover, oxygen incorporation could scatter the carriers thereby affecting the electronic properties like mobility and conductivity significantly. Improvement in electronic characteristics have been Received: July 20, 2011 Revised: August 19, 2011 Published: August 23, 2011 19110
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The Journal of Physical Chemistry C achieved by reducing of GO chemically; however, the conductivities were found to be still less compared to pristine graphene.16,17 This indicates that oxygen incorporated in graphene is not completely removed. One of the techniques that helps in understanding the role of electrically active defects is ultrafast transient absorption (TA) spectroscopy. Previously, ultrafast TA technique in the terahertz and near-infrared region was successfully used in studying the carrier dynamics in graphene.1820 These studies revealed the role of both carriercarrier and carrierphonon scattering processes in the relaxation of photoexcited charge carriers. However, the relaxation dynamics that was measured for graphene on SiC might not be the correct dynamics. Graphene on SiC can lead to AB symmetry breaking and opening a gap at the K-point of the Brillouin zone.21 In addition to that, interaction with SiC could adversely affect the relaxation dynamics by allowing alternate relaxation pathways. Ultrafast studies on graphene oxide prepared by chemical exfoliation procedure by Ruzicka et al.18 revealed that the relaxation dynamics was fast in reduced graphene oxide which resembles graphene where the excitation of the samples was carried out at 750 nm. Exciting graphene by near infrared (NIR) has also been carried out by Zhao et al.22 and Kumar et al.23 On NIR excitation one can excite larger fragments of graphene. Graphene oxide is believed to contain small fragments of sp2 clusters which are confined by edge decorated by oxygen. These fragments absorb in the visible region.24 A clear picture of true dynamics only emerges with a higher energy excitation. Therefore with an aim to understand the dynamics of single sheets of GO and its reduced form (RGO) after exfoliating and dispersing in water, we have carried out femtosecond pumpprobe spectroscopy with 400 nm excitation and detection of the transients in the visible to near IR region. In addition to that, different oxygen functional units in graphene oxide which are responsible for the luminescence of GO can create trap states. So it is very important to study excited dynamics of GO and its reduced form to find out the involvement of those oxygen related trap states by using femtosecond transient absorption spectroscopy.
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2. EXPERIMENTAL SECTION
c. Reduction of Graphene Oxide. To the above solution (dispersed in ethylene glycol), 1 mL of hydrazine hydrate was added, and the solution was refluxed for 1 and 24 h. The reduced samples were named RGO-1 and RGO-2 for 1 and 24 h reduced samples, respectively. d. Femtosecond Transient Absorption Studies. The femtosecond tunable visible spectrometer has been developed based on a multipass amplified femtosecond Ti:sapphire laser system supplied by Thales, France.26 The pulses of 20 fs duration and 4 nJ energy per pulse at 800 nm obtained from a self-modelocked Ti-sapphire laser oscillator (Synergy 20, Femtolaser, Austria) were amplified in a regenerative and two-pass amplifier pumped by a 20 W DPSS laser (Jade) to generate 40 fs laser pulses of about 1.2 mJ energy at a repetition rate of 1 kHz. The 800 nm output pulse from the multipass amplifier is split into two parts to generate pump and probe pulses. In the present investigation, we have used frequency doubled 400 nm as excitation source. To generate pump pulses at 400 nm, one part of 800 nm with 200 μJ/pulse is frequency doubled in BBO crystals. To generate visible probe pulses, about 3 μJ of the 800 nm beam is focused onto a 1.5 mm thick sapphire window. The intensity of the 800 nm beam is adjusted by iris size and ND filters to obtain a stable white light continuum in the 400 nm to over 1000 nm region. The probe pulses are split into the signal and reference beams and are detected by two matched photodiodes with variable gain. We have kept the spot sizes of the pump beam and probe beam at the crossing point at around 500 and 300 μm, respectively. The noise level of the white light is about ∼0.5% with occasional spikes due to oscillator fluctuation. We have noticed that most laser noise is low-frequency noise and can be eliminated by comparing the adjacent probe laser pulses (pump blocked vs unblocked using a mechanical chopper). The typical noise in the measured absorbance change is about 400 ps (28.2%). We have monitored transient kinetics at different wavelengths, but we did not observe any difference in dynamics. We have observed pulse-width limited rise of the transient signal which indicates the immediate formation of charge carriers on laser excitation. It is interesting to observe that a sizable contribution (26.7%) of transient absorption signal does not decay beyond 400 ps. This observation is in contrast to the transient absorption signal of pure graphene as observed earlier.18 Earlier studies on graphene or graphene oxide show ultrafast relaxation dynamics of photoexcited charge carriers with time constants ranging from ∼100 fs to 2 ps. So the slower decay time constants for transient signal observed in the present investigation might be due to slow recombination of the trapped charge carriers. Confirmation of this assignment is discussed in the later parts of this contribution. The dynamics at 670 nm for GO as shown in Figure 3 reveals a pulse width limited rise after excitation indicating the formation of the state within pulsewidth time scale (300 (28.2%)
GO-PY
0.9 ( 0.2 (33%)
9 ( 2 (39%)
>300 (28%)
GO-BQ
0.85 ( 0.2 (61.8%)
10 ( 2 (9.8%)
>300 (28.4%)
through π-ring of GO surface more effectively. In this condition it is expected that individual charge carriers will interact with the quenchers and will facilitate faster recombination with the complementary carriers. Figure 4 shows the transient decay for GO (trace a), GO in the presence of electron quencher (benzoquinone, BQ) (trace b), and GO in the presence of electron hole quencher (pyridine, Py) (trace c) at 670 nm. In the presence of BQ the kinetic trace decays faster as compared to that of pure GO. The kinetics can be fitted multiexponentially with time constants of τ1 = 0.85 ps (61.8%), τ2 = 10 ps (9.8%), and τ3 = >400 ps (28.4%) (Table 1). Additionally contribution of a 0.9 ps component increases from 33.3% to 61.8% in the presence of BQ. However, the 9 ps component shows considerable decrease in contribution while the >300 ps component shows negligible changes. From our experimental observation we can suggest that BQ can interact with the photoexcited electron before trapping and can facilitate faster recombination, and as a result we see the reduction of 9 ps and >400 ps components which are primarily attributed to trapped electrons in different trap depths. We have observed that the contribution of the >400 ps component has not been changed much from 28.2% (without BQ) to 28.4% (in the presence of BQ) which earlier was attributed to the component due to the deeper trapped states. So it is clear that BQ cannot quench the electrons effectively which are deeply trapped in GO. This observation clearly ascertains the significant roles of electron traps in the relaxation dynamics of photoexcited charge carriers in chemically synthesized GO. However, in the presence of hole quencher (Py) the kinetics at 670 nm of GO did not show any difference (Figure 4 c), which indicates that transient absorption is mostly dominated by electron trap states. From the above investigation it is evident that trapping dynamics of charge carrier governs the 19113
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Table 2. Lifetimes of the Transient Absorption Signal of Graphene Oxide (GO), Reduced (1 h) Graphene Oxide (RGO-1), and Reduced (for 24 h) Graphene Oxide (RGO-2) and at 670 nm after Exciting at 400 nm Laser Light sample
τ1 (ps)
τ2 (ps)
τ3 (ps) + τ4 (ps) >300 (28.2%)
GO
0.9 ( 0.2(33.3%)
9 ( 2 (38.5%)
RGO-1
0.7 ( 0.2 (37%)
10 ( 2 (36.5%)
RGO-2
0.26 ( 0.1 (65%)
2.5 ( 0.5 (24.5%)
>300 (26.5%) 30 ( 5 (6.5%) + >300 (4%)
recombination dynamics in GO, which in turn affects the suitability of the material as practical purposes. It is important to remove those defect states by reducing GO to reduced GO so that it can be made as a defect-free material. It is important to ask how the reduction procedure improves or, more precisely, changes the carrier dynamics of the material. Does the charge carrier recombination dynamics become faster? Does it decrease the concentration of trap states? To answer all the above questions, we have reduced graphene oxide (GO) by following the reported method29 as described in the Supporting Information. We have prepared two different samples named as RGO-1 and RGO-2 where we have reduced the sample for 1 and 24 h respectively as described in the Supporting Information. Previous studies on reduction revealed that the reduction process is efficient only if the reduction time is 24 h. To prove this we have prepared a 1 h reduction sample. To further confirm the role of defects in relaxation dynamics, we have carried out transient absorption studies of RGO-1 sample and compared the decay kinetics with GO at 670 nm as shown in Table 2. The decay kinetics was not influenced by intensity similar to GO. The kinetic data can be fitted multiexponentially with τ1 = 0.70 ps (37%), τ2 = 10 ps (36.5%), and τ3 = >400 ps (26.5%) (Table 2). The lifetime of RGO-1 and the contributions in RGO-1 remain almost identical (i.e., within the errors) as compared to that of GO. It is reported in the literature11,16 that on reduction oxygen related defects of GO can be removed by a larger reduction time. To further ascertain the role of reduction processes, we have reduced GO for 24 h, labeled RGO-2, which followed the reduction procedure generally used in the reduction of GO, and carried out femtosecond transient absorption studies to monitor the charge carrier relaxation dynamics. Figure 5 show the transient spectrum of RGO-2 at different time delays after exciting the samples at 400 nm laser pulse. The transient spectrum of RGO-2 looks significantly different as compared to that of GO. It is clearly seen that transient signal intensity increases dramatically in RGO-2 as compared that of GO. The transient spectrum broad absorption in the visible region (500700 nm) shows higher absorption in the blue region of the spectrum. Furthermore, the broad feature at 670 nm is significantly reduced indicating that this feature might come from oxygen related trap state absorptions. We have monitored the transient decay kinetics at 670 nm as shown in the inset of Figure 5. The transient data can be fitted multiexponentially with time constants of τ1 = 0.26 ps (65%), τ2 = 2.5 ps (24.5%), τ3 = 30 ps (6.5%), and τ4 = >400 ps (4%) (Table 2). It is clear that the transient signal decays significantly faster as compared to that of GO. The faster two lifetimes components (0.26 and 2.5 ps) contribute almost 90% to the relaxation process, which indicates a significant percolation in
Figure 5. Transient absorption spectra of reduced (24 h) graphene oxide (RGO-2) at different time delays after exciting at 400 nm laser pulse. Inset: Transient decay kinetics at 670 nm of RGO-2 (actual data, blue filled circles, and normalized data at 50% intensity, red empty circles).
GO from sp3 to sp2 carbon. The first two components match with excited dynamics of photoexcited graphene as measured in earlier reports. These fast components have been assigned to the relaxation by electronphonon interactions. It is reported in the literature20 that carrier relaxation processes in graphite are hindered by accumulation of hot phonons at higher carrier density which is called hot phonon bottleneck effect; as a result, one can see slower component in the relaxation dynamics. Kamfrath et al.31 measured the 2.5 ps component of carrier relaxation dynamics in photoexcited graphite phonon mediated relaxation. In the present studies the slower 0.26 and 2.5 ps can be attributed to interactions with optical and acoustic phonons, respectively. However, in the decay kinetics of RGO-2, we can still observe a ∼4% contribution >400 ps component. In the case of transient decay analysis of GO, we have explained that the >400 ps component arises due to deep trap states. We can see clearly on reduction of GO to RGO-2 the contribution of 30 ps and >400 ps components. These components could come from traps still present after reduction.28 Our ultrafast study clearly shows that RGO-2 still contains a reasonable amount of defect states. This conclusion is further supported by previous study on electrical and surface probe analysis28 of GO. Electrical measurement in reduced graphene oxide indicates that hopping and tunneling modes of conduction contribute to electrical conduction, which indicates disorder in the percolated region. Previous electrical conductivity studies demonstrate that the electrical behavior of graphene oxide is much poorer as compared to graphene prepared by micromechanical cleavage. Electron traps as detected from our measurements arise from epoxides, and carboxylic acids are responsible for these electrical properties. High resolution transmission electron microscopy (HRTEM) studies on CNT32 confirm the presence of oxygen related impurities introduced during the cleaning procedures with mineral acids mixtures like H2SO4 and HNO3 which introduces defects in CNT. These reports also have indicated that oxygen related defects like COOH and epoxides can act as electron traps. In our quenching studies it is confirmed that traps are associated mainly with electron traps and they have tremendous effect on relaxation and recombination dynamics of the photoexcited charge carriers in GO. The reduction study further confirmed that the most likely candidate for the electron traps are the oxygen related groups decorating GO which are incorporated during the chemical exfoliation procedure. This led us to 19114
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The Journal of Physical Chemistry C conclude that oxygen related electron traps control the relaxation dynamics in GO. From our transient studies it is confirmed that it is almost impossible to remove the defect states in graphene oxide (GO) by reducing it to reduced graphene oxide (RGO), by chemical reduction procedure, and this observation also supported by previous literature reports based on STM, XPS, and FTIR measurements.33 We have calculated the carrier density and reported it in the Supporting Information, and it has been found that carrier density in the present studies is ∼1013/cm2 which is lower as compared to the recently reported literature20 value for graphene layer of ∼4 1013/cm2. This is justifiable as our reduced graphene (RGO) still has some defect states as compared to pure graphene. Furthermore, comparison of recent transient absorption studies does show that the in the wavelengths measured the transient absorption shows a bleach for pure graphene.34,35 An estimation of trap density from the long relaxation time (>400 ps) by using the equation below Nt ¼
1 τσv
where Nt = trap density, τ = lifetime, σ = cross section, and v = Fermi velocity, showed the traps density were ∼3 1015/cm3 (see Supporting Information for the values used). This value is close to the trap densities observed by electrical measurements, which further ascertain the role of traps and complementary information obtained from both electrical and transient absorption measurements.36
4. CONCLUSION In conclusion, we state that relaxation dynamics in GO is governed by electron traps incorporated during the chemical exfoliation carried out by oxidizing graphite. Femtosecond transient absorption studies on GO shows that the trap states dominate the relaxation dynamics. Oxygen related impurities arise in the form of epoxides, and carboxylic acid groups are responsible for those trap states. Carrier quenching studies confirmed that defect states are mainly due to electron traps. Reduction of GO for 24 h led to reduced graphene oxide (RGO). Relaxation dynamics of photoexcited RGO were found to be significantly faster as compared to GO and matches that of pristine graphene. Our ultrafast studies confirmed that even after strong reduction a small fraction of defect states still exist in RGO. It is important to develop a chemical reduction process which can annihilate all the oxygen related traps in GO and produced RGO which can replace graphene as a material of choice for electronic applications. ’ ASSOCIATED CONTENT
bS
Supporting Information. Details of optical absorption and emission spectra of GO and reduced GO, selected transient kinetic decay trace of GO and reduced GO, calculation of carrier density in GO, calculation of trap density from the lifetimes for RGO-2. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (H.N.G.); Fax 00-91-22-2550-5151,
[email protected] (S.K.).
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’ ACKNOWLEDGMENT The authors thank Dr. D. K. Palit, Dr. S. K. Sarkar, and Dr. T. Mukherjee for their encouragement and continuous support. ’ REFERENCES (1) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183–191. (2) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Gregorieva, I. V.; Firsov, A. A. Science 2004, 306, 666–669. (3) Berger, C.; Song, Z.; Li, T.; Li, X.; Ogbazghi, A. Y.; Feng, R.; Dai, Z.; Marchenkov, A. N.; Conrad, E. H.; First, P. N.; Heer, W. A. De. J. Phys. Chem. B 2004, 108, 19912–19916. (4) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. Nature 2005, 438, 197–200. (5) Katsnelson, M. I.; Novoselov, K. S.; Geim, A. K. Nat. Phys. 2006, 2, 620–625. (6) Reina, A.; Jia, X.; Ho, J.; Nezich, D.; Son, H.; Bulovic, V.; Dresselhaus, M.; Kong, J. Nano Lett. 2009, 9, 30–35. (7) Hass, J.; Feng, R.; Millan-Otoya, J. E.; Li, X.; Sprinkle, M.; First, P. N.; de Heer, W. A.; Conrad, E. H. Phys. Rev. B 2007, 75, 214109 (18). (8) Li, D.; Mueller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Nat. Nanotechnol. 2008, 3, 101–105. (9) Zhou, S. Y.; G.-H. Gweon, A. V.; Fedorov, P. N.; First, W. A.; de Heer, D.-H.; Lee, F.; Guinea, A.; Castro Neto, H.; Lanzara, A. Nat. Mater. 2007, 6, 770–775. (10) Xiaoyin Yang, X.; Dou, A.; Rouhanipour, L.; Zhi, H.; Joachim, R.; Klaus, M. J. Am. Chem. Soc. 2008, 130, 4216–4217. (11) Hummers, W. S., Jr.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, 1339. (12) Jeong, H. K.; Jin, M. H.; So, K. P.; Lim, S. C.; Lee, Y. H. J. Phys. D: Appl. Phys. 2009, 42, 065418 (16). (13) Mkhoyan, K. A.; Contryman, A. W.; Silcox, J.; Stewart, D. A.; Eda, G.; Mattevi, C.; Miller, S.; Chhowalla, M. Nano. Lett 2009, 9, 1058–1063. (14) Schniepp, H. C.; Li, J.-L.; McAllister, M. J.; Sai, H.; Alonso, M. H.; Adamson, D. H.; Prud’homme, R. K.; Car, R.; Saville, D. A.; Aksay, I. A. J. Phys. Chem. B 2006, 110, 8535–8539. (15) Klinke, C.; Afzali, A.; Avouris, P. Chem. Phys. Lett. 2006, 430, 75–79. (16) Tung, V. C.; Allen, M. J.; Yang, Y.; Kaner, R. B. Nat. Nanotechnol. 2009, 4, 25–29. (17) Sasha Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Carbon 2007, 45, 1558–1565. (18) Wang, H.; Strait, J. H.; George, P. A.; Shivaraman, S.; Shields, V. B.; Chandrashekhar, M.; Hwang, J.; Rana, F.; Spencer, M. G.; Ruiz-Vargas, C. S.; Park, J. Appl. Phys. Lett. 2010, 96, 081917 (13). (19) Huang, L.; Hartland, G. V.; Chu, L.-Q.; Luxmi; Feenstra, R. M.; Lian, C.; Tahy, K.; Xing, H. Nano Lett. 2010, 10, 1308–1313. (20) Gao, B.; Hartland, G.; Fang, T.; Kelly, M.; Jena, D.; Xing, H. G.; Huang, L. Nano Lett. 2011, 11, 3184–3189. (21) Ruzicka, B. A.; Werake, L. K.; Zhao, H.; Wang, S.; Loh, K. P. Appl. Phys. Lett. 2010, 96, 173106 (13). (22) Zhao, X.; Liu, Z.-B.; Yan, W.-B.; Wu, Y.; Zhang, X.-L.; Chen, Y.; Tian, J.-G. Appl. Phys. Lett. 2011, 98, 121905 (13). (23) Kumar, S.; Anija, M.; Kamaraju, N.; Vasu, K. S.; Subrahmanyam, K. S.; Sood, A. K.; Rao, C. N. R. Appl. Phys. Lett. 2009, 95, 191911 (13). (24) Navarro, C.-G.; Meyer, J. C.; Sundaram, R. S.; Chuvilin, A.; Kurasch, S.; Burghard, M.; Kern, K.; Kaiser, U. Nano Lett. 2010, 10, 1144–1148. (25) Kovtyukhova, N. I.; Ollivier, P. J.; Martin, B. R.; Mallouk, T. E.; Chizhik, S. A.; Buzaneva, E. V.; Gorchinskiy, A. D. Chem. Mater. 1999, 11, 771–778. (26) Sreejith, K.; Rawalekar, S.; Verma, S.; Ghosh, H. N. Phys. Chem. Chem. Phys. 2010, 12, 4210–4216. 19115
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