Ultrafast Processes in Graphene Oxide during Femtosecond Laser

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Ultrafast Processes in Graphene Oxide during Femtosecond Laser Excitation Nikos Liaros, Stelios Couris, Emmanuel Koudoumas, and Panagiotis A. Loukakos J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11943 • Publication Date (Web): 02 Feb 2016 Downloaded from http://pubs.acs.org on February 8, 2016

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The Journal of Physical Chemistry

Ultrafast Processes in Graphene Oxide during Femtosecond Laser Excitation

N. Liaros1,2 and S. Couris1,2,† 1

Department of Physics, University of Patras, 26504 Patras, Greece

2

Foundation for Research and Technology-Hellas, Institute of Chemical Engineering Sciences, 26504 Patras, Greece E. Koudoumas Center of Materials Technology & Photonics and Department of Electrical Engineering, School of Engineering, Technological Educational Institute of Crete, 710 04 Heraklion, Crete, Greece P. A. Loukakos‡

Foundation for Research and Technology Hellas, Institute of Electronic Structure and Laser, 71110 Heraklion, Greece

Corresponding authors’ contact information: †

[email protected], Tel.: +30 2610 996086

‡


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Abstract Ultrafast pump-probe spectroscopy on as-grown Graphene Oxide single sheet thin layers was employed in order to study their response following ultrafast excitation of electrons in the sp3 hybridized domains. The study of the transient spectra showed the existence of two wavelength regions with distinct responses: a saturable absorption region and a reverse saturable absorption region. The study of the competing responses on these two regions revealed for the first time that these wavelength regions are not stable following excitation but instead they present a dynamic redshift resulting in a crossover wavelength. This marks the transition from saturable to reverse saturable absorption and is dynamic in nature. The ultrafast dynamics of this reported effect may be of crucial importance to the application of graphene-oxide based elements in optoelectronics.

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1. Introduction

Graphene,1 the two dimensional lattice of carbon atoms in a honeycomb arrangement, has attracted significant attention by the scientific community because of its unique structural,2 electronic,3-4 thermal,5 optical6-7 etc. properties. Owing to these properties it has opened new pathways in photonics and optoelectronics.6, 8 As a result, there was a rapid growth of novel graphene-based two dimensional counterparts; the most famous among them being graphene oxide (GO).9-10 GO is a key functionalized analogue of graphene showing much more efficient dispersion in solvents and therefore much easier handling and applicability. Reduction of GO has proven to be an effective method towards the development of graphene-modified materials and devices.11 The attachment of epoxides, carboxyl and hydroxyl bonds, etc. on graphenes’ sheet, perturb significantly the density of states at the carbon atoms directly linked to oxygen groups, which results to a bandgap opening on graphene’s band structure.12 In fact, GO features conducting π-states (from the sp2 carbon sites) and σ-states (from its sp3-bonded carbons) exhibiting a large energy gap between them. Hitherto, GO has been already exploited for applications ranging from thin film transistors,13 environmental catalysis,14 capacitors,15 and sensors16 to optical switchers and optical limiters.17-22 In this way, towards the implementation of graphene-oxide based nanosystems in the realization and the development of integrated photonic devices, the investigation of optical properties of GO is of crucial importance, for understanding the fundamental properties of the structure and the electronic transitions as well. One of the techniques that correlate structural characteristics with the optical response is ultrafast transient absorption spectroscopy. Following a number of reports relating to transient absorption/transmission measurements on graphene,23-29 detailed studies regarding the ultrafast dynamics of GO are thus currently a hot subject of research.30-32 The first pump-probe experiments studying GO suspensions have been performed only recently24, 30, 32-36 and have revealed the significant differentiation of the ultrafast mechanisms occurring following excitation between graphene and GO, which has been generally attributed to the existence of a band gap owing to the existence of C-O bonds, the interplay between sp2 and sp3 domains30, 34 and the interactions of the exited carriers with defects and the relative trapping mechanisms.24, 32, 37 Additionally, preparation and exfoliation of GO frequently results is specimens with different oxidization levels, a parameter which needs further investigation because of its relevance to the ultrafast excitation and relaxation mechanisms in GO. Therefore, ultrafast pump-probe experiments are a key method and a powerful tool to determine the influence of the specimens’ details on its fundamental interaction mechanisms and provide the characterization needed for its potential applications and implementation in innovative functional structures and devices. Herein, the transient absorption of GO is reported, as studied by means of femtosecond time-resolved pump-probe spectroscopy in the entire visible spectral region using white light (WL) probe. From these measurements, the transient decay kinetics were determined and associated with the electronic transitions occurring following ultrafast optical excitation in GO. Two clearly distinguishable wavelength regions were observed where GO exhibited saturable absorption (SA) and reverse saturable absorption (RSA) respectively. It has been found that these wavelength regions mark a transition point where tha pump-induced absorption changes sign from negative to positive. This transition point, the crossover Page 3 of 22 ACS Paragon Plus Environment


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wavelength, is found to be dynamic and to shift towards red parts of the spectrum in a time window of several ps thus showing the potential of the GO system to be considered as an ultrafast optical switch. The ultrafast dynamics of these wavelength regions are discussed with respect to the electronic interactions. 2. Experimental

The studied GO samples were prepared according to the Brodie method38 as previously described.39 For the preparation of highly stable aqueous suspensions, 2 mg of GO were dissolved in 0.8 mL of H2O which, had previously been regulated at high pH by adding NaOH. 17 The prepared samples were sonicated for about 20 min and were centrifuged for 4 min at 5000 rpm. Then, the supernatant colloid was isolated and used at different concentrations for the pump-probe measurements. The ultrafast transient absorption spectra were recorded using a pump-probe technique. In particular, a Ti:sapphire laser multipass amplifier produced laser pulses of < 30 fs duration (FWHM) and < 1 mJ pulse energy at center wavelength of 785 nm. A small part (ca. 5%) of the fundamental laser beam was used to generate white light continuum in a 2 cm quartz cell containing distilled water, while the remaining energy was frequency-doubled by means of a BBO crystal. The last was the pump beam since its photon energy (3.91 eV – 392 nm) could ensure near-resonance above band gap excitation of the studied samples. The white light supercontinuum beam was used as probe and could be temporally delayed with respect to the pump by reflectance on a retroreflecting set of mirrors that were mounted on a computer-controlled motorized delay stage. The probe beam was then split into reference and signal beams, the signal one passing through the excited by the pump beam spot of the sample, while the reference beam was vertically displaced by ~ 2 mm and it was separately and concurrently measured for calibration purposes. The beam sizes (at 1/e2) of the pump and the probe beams inside the sample were measured to be about 410 μm and 165 μm, respectively using a CMOS camera. After having passed through the sample, the signal and reference probe beams were imaged at the entrance slit of a 300 mm focal length imaging monochromator. The images of the signal and reference probe beams were concurrently recorded as a function of delay time between the pump and the probe, by means of a CCD camera mounted at the exit of a spectrograph. So, the differential transmission ΔT of the probe beam could thus be readily obtained. Neglecting the small reflectance of the liquid samples the induced change in the absorbance ΔA was given by ΔA = - ΔT/T. The accuracy of the experimental measurements is limited in the time domain by the cross correlation of the pump and probe pulses estimated by the data to be ~ 150 fs while in the wavelength domain is limited by the resolution of the spectrometer (~ few nm) and the band width of the excitation laser pulses (~ 7 nm). Several pump-probe scans were typically collected and averaged for every individual measurement thus resulting to a resolution of the order of ΔA = 10-3 for the minimum ΔA that could be accurately measured. 3. Results

In Fig. 1, the absorption spectra for the exfoliated GO single layer sheets dispersed in water at various concentrations are presented. The monotonous increase of the absorbance with concentration is characteristic of a homogeneous dispersion of the GO sheets in the aqueous medium.17 Several absorption regions of interest can be seen and analyzed below, Page 4 of 22 ACS Paragon Plus Environment

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and their center wavelengths are marked with vertical dashed lines. In particular, the observed absorption bands are: 1. A strong absorption peak centered at about 236 nm (5.26 eV), which is attributed to the π-π* transitions of C-C and C=C bonds in the sp2 hybrid regions.40-41 2. An absorption peak at about 300 nm (4.14 eV) originating from the transitions of the sp3 hybridized regions, due to the attachment of Oxygen containing bonds.12, 40-42 3. Several additional and weaker spectral features at about 346 nm (3.59 eV), 365 nm (3.40 eV), 325 nm (3.82 eV) and 394 nm (3.15 eV) that can possibly be a result of the fine dispersion of the GO sheets in water which makes evident the fine absorption features and structure of the material due to the non-interacting state of the dispersed sheets.17 4. A very broad and featureless region extending to wavelengths longer than 400 nm of relatively low absorbance. In order to resolve the temporal dynamics of the excited states and the relaxation dynamics towards equilibrium, pump-probe spectroscopy was employed. A typical result is shown in Fig. 2, where the changes of the absorption spectrum of the aqueous GO dispersion induced by the laser excitation at 392 nm are presented as a function of the probe wavelength λ and the pump-probe delay time τ, i.e. ΔΑ(λ,τ). In the pseudo-color representation, red/blue indicates an increase/decrease of the absorption. The presented time-resolved spectra extend in the wavelength region from 400 nm to 700 nm where a measurable response for the induced absorbance ΔA was recorded. Moreover, the presented time-resolved spectra were collected in the temporal window from about -5 ps to 40 ps, in order to restrict the data acquisition time and eliminate signal instabilities, keeping at the same time a reasonable temporal resolution. From the time-resolved spectrum of Fig. 2 three different areas of interest can be clearly observed: 1. λ < 500 nm: here the pump-induced transient absorbance ΔA is negative, indicating reduction of GOs’ absorption due to the excitation (SA). 2. 500 nm < λ < 550 nm: a wavelength region with a mixed transient response; for early time delays (few ps after excitation) the induced change in absorbance ΔA is positive while for later time delays τ, the induced change in absorbance reverses its signal becoming negative. 3. λ > 550 nm: here, the induced change in absorbance is always positive indicating an increase of GOs’ absorption at this region (RSA). In order to evaluate the spectral response of the system following the excitation at 392 nm, cross sections were perform on the two-dimensional plots, so that induced absorbance spectra ΔΑ(λ,τi) of the GO single sheets were obtained at specific time delay instances. These spectral cross sections are shown in Fig. 3 for the spectral region (400 nm - 700 nm). Upon excitation, an instantaneous decrease of the absorption was observed from 400 nm to 510 nm. The maximum decrease in ΔA is observed at τ = 0 and for the spectral region from 450 nm to 475 nm centered at ~ 460 nm (2.70 eV). The negative ΔA partially recovers for increasing τ and persists for more than 40 ps, which was the delay limit in our presented measurements. For longer wavelengths the induced absorbance becomes positive, up to 700 nm, the change of the sign of ΔA appearing at about 510 nm. The positive ΔΑ for τ = 0 extends to a broad wavelength region and drops back to ΔA = 0 at about 700 nm. It exhibits Page 5 of 22 ACS Paragon Plus Environment


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an almost symmetric shape with a weak shoulder (with a rough center at 535 nm) on its left side. This shoulder is evident only for the very first moments of excitation (here shown only for τ = 0). For increasing temporal delays the symmetric induced absorption (ΔΑ > 0) band changes its shape: It loses weight on the left side (which eventually drops to negative ΔΑ < 0 values) and gains on the right side, i.e. towards longer wavelengths. The maximum of the ΔA, i.e. ΔA = 0.0165 is observed around 675 nm at τ = 0.5 ps. For even larger temporal delays τ, the overall positive absorbance change ΔA is uniformly and monotonously decreasing, but a small positive ΔA still persists for λ > 600 nm and τ > 40 ps. This temporal behavior of the spectral cross sections essentially results in a redshift of the crossover wavelength, λc, i.e. the spectral point at which the sign of the pump-induced absorbance changes from negative (reduced absorption) to positive (increased absorption). This spectral redshift in time is observed for the first time here and is depicted in Fig. 4. As can be seen, starting from λc = 509 nm, there is an abrupt small increase to λc = 522 nm within the first ps after excitation and then the crossover wavelength increases monotonically and quasi-linearly for time delays extending up to 40 ps, which was the maximum measured delay in the present experiments. This implies a multi-ps process which starts from the very first moments following excitation and covers a very broad temporal regime. Unfortunately, in the present experiments the temporal delay could not be further extended to larger values, in order to study the longer temporal behavior of the crossover wavelength, because it has been sacrificed in favor of higher number of accumulated pumpprobe scans and therefore higher statistical average. In order to clarify the mixed temporal behavior mentioned above, three temporal cross sections have been plotted, i.e. the induced absorbance change ΔA(λi,τ) at specific wavelengths λi, which are representative of the above mentioned three spectral regions. This is shown in Fig. 5 at probe wavelengths 440 nm, 530 nm and 620 nm. As can be seen, for the probe wavelength λ1 = 440 nm, ΔΑ becomes negative attaining maximum ΔA = 0.0064 within a few 100 fs, which essentially means a quasi-instantaneous response of the absorbance. Then, within about 10 ps, it has partially recovered to about half the maximum of the initial change and reaches a plateau with a value of about ΔA = - 0.003, which persists for delays longer than 40 ps. For the intermediate wavelength of λ2 = 530 nm, an abrupt and mainly positive change ΔΑ is observed, which decays fast within few ps to ΔA = 0, then continues to slowly reduce and becomes negative, continues with a downward fashion almost linearly until the end of the measured time window of 40 ps and shows a continuance trend beyond that. Lastly, for probe wavelength of λ3 = 620 nm, a fast quasiinstantaneous increase in ΔA was observed to a maximum value of ΔA = 0.012, which then relaxes in an exponential-like fashion and reaches a quasi-plateau value of about 0.0013 at 40 ps, showing a continuing slow drop of the ΔΑ for longer times that were not measured in the presented experiments. Therefore, to summarize the current observations, we observe a region of probe wavelengths with a clear negative temporal response of ΔΑ, a probe wavelength region with a mixed temporal response and a third probe wavelength region with a clearly positive response of ΔA. 4. Discussion In order to extract some information about the value of the band gap of the prepared samples, a Tauc analysis was performed (not shown here) on the data presented in Fig. 1. From this analysis it became evident that it was not possible to determine a unique band Page 6 of 22 ACS Paragon Plus Environment

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gap value for all the samples. In fact, the heterogeneous atomic and electronic structure of GO sheets arising from the mixture of electronically conducting sp2 hybridized graphitic domains and insulating sp3 hybridized diamond-like matrix gives a semiconducting behavior with a variable optical bandgap depending on the degree of oxidation and extending from the UV to the NIR spectral region, as discussed by Robertson.43-44 Moreover, the optical properties of the GO sheets, including the absorption edges and the optical band gap may be modified by localized states which are situated in the forbidden energy gap.45 Additionally they can change the local symmetry due to interactions with phonons in anharmonic approximation and thus may lead to discrepancies in the effective energy gap. In alignment with the above, from the Tauc analysis of the UV-VIS absorption spectra of the prepared GO acqueous dispersions, two band gap values, Eg, were extracted, one with Eg < 1 eV and another one with 2.6 eV < Eg < 3.0 eV. The former band gap value is in relatively poor agreement with the value of 0.6 eV reported previously in the literature,35, 46 and associated with π-electrons of the sp2 nano-domains that are apparent on the GO sheet. The latter band gap value Is in good agreement with the observations and estimation made by Liu et al.30 and is related with transitions that take place among the σ states of the sp3 carbon matrix. This signifies the choice of making use of the second harmonic of the laser fundamental photon energy, at about 3.17 eV (392 nm) for the pump beam in order to ensure near-resonance above band gap excitation conditions. It is important to notice at this point that based on this analysis which gives band gap values in accordance with the electronic structure recently introduced by Liu et al.30 and by Shang et al.35-36 a schematic model has been developed and presented in Fig. 6. According to this model, the system can be roughly considered as a semiconductor and presents a dual band gap: Firstly, a small band gap of ~ 0.6 eV originating from the HUMOLUMO energy difference resulting from the non-oxidized C-C and C=C regions;30, 35-36 And secondly a larger band gap ranging from 2.6 eV to 3 eV originating from sp2 domains in GO that are predominantly amorphous as a result of sp3 C-O bonds therein.30, 42, 47 Taking into account the above, the sketch of Fig. 6 was employed in order to explain the experimentally observed time-resolved pump-probe spectra that are shown in Fig. 2 and the relevant mechanisms occurring following excitation. This simplified band structure model is based on a semiconductor-like band structure with a band gap of 2.6 eV - 3.0 eV, as was discussed above. In this simplified model, the valence band maximum is marked as VB, the conduction band minimum is marked as CB1 while the energy states in the conduction band are marked as CB2 and CB3. These have energy values of about 4.1 eV and 5.4 eV respectively. The CB2 and CB3 energy states correspond to the 300 nm and 236 nm peaks respectively as observed in Fig. 1. These two states are primarily indicated in the conduction band as these are the two most prominent features observed in the absorbance spectra (Fig. 1). In between these sharp static absorption features a quasi-continuum of energy states is assumed, attributed to the non-zero exponential-like background absorption of our system. The zero point of energy was set at VB for simplicity reasons. Moreover, the low band gap of the system is considered to be at around 0.6 eV, similar to the energetic difference of the HUMO-LUMO resonance. The HUMO and LUMO states are presented with dashed lines in Fig. 6. The arrows denote possible transitions with a solid arrow denoting a strong and probable transition while a dotted arrow indicates a weak transition. Our considerations for the excitation and probing schemes and the corresponding mechanisms are thus as following: Page 7 of 22 ACS Paragon Plus Environment


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1. At negative time delays, the excitation of the system has not occurred yet and therefore ΔA(τ < 0, λ) exhibits minimum variations along the wavelength axis. 2. At time zero (τ = 0) excitation of the system occurs mainly by promoting electrons from the vicinity of VB to 3.1 eV higher (arrows 1 and 2 in Fig. 6) i.e. towards CB1 and this occurs primarily in the vicinity of sp3 hybridized regions (i.e.the oxygen-rich regions) which exhibit the relevant energetic resonance. Here, optical excitations intermixing sp2 with sp3 hybridized regions are considered less probable to occur and therefore are ignored as a first approximation. For example, transitions from VBLUMO or HUMOCB2 are less likely to occur because the electrons will be either in the sp2 or in the sp3 region immediately before and immediately after excitation. In Fig. 3 the buildup of a weak shoulder peaked at about 535 nm (2.32 eV) is shown, denoting a positive change in the absorption of the probe beam around this wavelength region. The energy difference of 2.32 eV corresponds to the opening of the transition probability from CB1 to CB3 by the probe beam (arrow 6) due to the excitation of electronic population at CB1 as a result of the action of the pump beam. Therefore it can be concluded that the observed increase in the absorption ΔA(τ = 0 , λ = 535 nm) is an evidence for transitions from CB1 to the state marked as CB3 that lies at around 5.4 eV (i.e. ~ 2.5 eV higher than the center of CB1) which is responsible for further promotion caused by the probe wavelength of 535 nm of the initially excited by the pump laser beam electrons. CB3 corresponds to the strong characteristic absorption feature centered at 236 nm as observed in Fig. 1. 3. Immediately upon and following the excitation, the initial electron distribution at CB1 will relax and broaden energetically and phase-filling effects will come into play via electron-electron and electron-phonon collisions, which have been shown to be very strong in this system.32, 48-49 This will result in population of all states within the broad (0.4 eV) definition of CB1. At the same time (shortly after τ = 0), population of states within CB1 (mainly towards the lower energy parts of CB1 possibly due to very fast electronic scattering and broadening of the electron distribution which was initially very narrow and towards the higher energetically part of CB1) will contribute to bleaching effects leading to reduction of the transition probability from VB to the bottom of CB1, which corresponds to probe wavelengths lower than 510 nm, or equivalently, probe photon energies between larger than 2.43 eV, as shown in Fig. 6 (arrow 4 is dashed to indicate the cutoff of bleaching effects). This effect will cause the negative change in absorbance ΔA(τ = 0, λ < 515 nm) that is observed at this probe wavelength regime at and following τ = 0. Therefore, the system behaves as an efficient saturable absorber for λ < 515 nm and under the above conditions. 4. Due to the state filling effects of CB1 described above, all probe transitions ranging from the very bottom of CB1 towards CB2 (CB1minCB2) up to the very maximum of CB1 to CB3 (CB1maxCB3) will become possible, due to a continuum of states covering all the spectral region from CB2 to CB3. This will result in a continuous positive change of ΔA, i.e. ΔA > 0, for probe wavelengths from 510 nm to 700 nm (example transition shown with arrow 7). This implies the opening-up of a multitude of possible induced absorption transitions thus making the system behave like a reverse saturable absorber for λ > 510 nm. The establishment of the RSA regime occurs shortly after excitation and the redder spectral regions still exhibit ΔA > 0 well after 40 ps which was the time delay limit during the present experiments. It


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continues to drop for higher delays but a small percentage still persists even for delays as long as 40 ps and beyond (ΔΑ(λ = 700 nm, τ = 40 ps) = 0.003). 5. After sufficient time, as the energy states close to CB1 bottom are being populated by electrons through the above mentioned relaxation mechanisms, the transition from VB to CB1-bottom is gradually blocked due to population of the final states (this contributes to bleaching similarly to point No. 3 above, with the exception that mechanisms mentioned in point No. 3 are occurring shortly after τ = 0, while here only the slower mechanisms are considered and discussed). This is in agreement with the broad negative dip in the spectral cross sections that persists for these time delays and for λ < 530 nm and which corresponds to a reduced transition probability. i.e. reduced absorption (ΔA < 0). The negative ΔΑ at this wavelength region drops very fast around τ = 0 (as explained above in point No. 3) to about half of its maximum value but remains constant for 40 ps and beyond, which implies that the energetically relaxed electrons at energetic regions close to CB1-bottom remain there for time delays larger than the 40 ps upper limit of the present experiments. Therefore future experiments with larger pump-probe time windows would be required to estimate the relaxation time of these processes. These very slow processes can be attributed to carrier trapping effects.32, 36, 49 6. A very interesting observation that comes about is related to the “crossover” wavelength, λc, that marks the sign change of ΔA from negative to positive, i.e. from the SA to the RSA regime. This is gradually red shifted with increasing the pumpprobe delay time and this is a consequence of the fact that the reverse SA region with ΔA > 0 becomes narrower with increasing pump-probe delay time, while at the same time the SA region, where ΔA < 0, becomes broader with increasing pumpprobe delay time. This is very interesting for a potential saturable absorber which exhibits not only an absorption modulation with intensity but also exhibits a temporal variation of its absorption spectral region thus increasing its application potential by an additional path. The behavior of the crossover wavelength with pump-probe delay time, λc(τ) is shown in Fig. 4 and is discussed in the next. Usually, the RSA (SA) mechanism refers to excitation by light and to increase (decrease) of the absoprption due to e.g. multi-photon absorption processes.50 Here, the use of the term RSA (SA) is related to the induced increase (decrease) of the absorption, yet to a different nonlinear effect i.e. the opening up (or blockade) of absorption channels of the excited state, as a result of the excitation by the pump laser beam. Since, these channels strongly depend on the population dynamics of the excited state, the RSA (SA) effects are transient and their strengths depends on the pump-probe time delay. In order to analyze and estimate the temporal dynamics of the processes discussed above (points 1 through 6), the temporal cross sections obtained from Fig. 2 and shown in Fig. 5 are further discussed. A typical temporal trace for the negative ΔA < 0 wavelength region is shown in Fig. 5a centered for λ = 460 nm. ΔA decreases quasi-instantaneously upon excitation to ΔΑ = - 0.0064 and recovers with an exponential-like decay until it reaches a quasi-plateau value of about ΔΑ = - 0.003 within 5 - 10 ps. For longer time delays, the decay is very slow and the signal stays practically constant for the time window (40 ps) used in the present experiments. At 620 nm, shown in Fig. 5c, which is well into the positive ΔA > 0 wavelength region, the situation is generally reversed: ΔA increases quasi-instantaneously to ΔA = 0.012. It decays with an exponential-like decay for the whole time window of the Page 9 of 22 ACS Paragon Plus Environment


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experiments reducing its value to about ΔA = 0.0013 and it exhibits a continuously decaying trend even τ > 40 ps. In order to evaluate the time constants of the observed decay rates, the temporal behavior of ΔΑ(λ,τ) was considered to be given by the convolution of the cross correlation of the pump and probe pulses with the response function of the system. To that purpose, the response function of the system was assumed to consist of the sum of three exponents with varying amplitudes and characteristic decay times τ1, τ2, τ3 respectively. This assumption represents the fact that the decay slopes of the temporal traces continuously change as a result of the various mechanisms contributing to the electronic relaxation. Nevertheless, in this way the relaxation processes can be categorized in early (0 ps - 2 ps), intermediate (3 ps - 7 ps) and long ( > 10 ps) temporal windows. The obtained results of the fitting procedures for two probe wavelengths λ1 = 460 nm and λ2 = 620 nm were the following: [τ1, τ2, τ3]460nm = [1.6, 20, ∞] ps and [τ1, τ2, τ3]620nm = [3.3, 10.5, 32] ps. The comparison of the trends for τ1 suggests that τ1 is larger for λ = 620 nm and generally for all wavelengths exhibiting positive ΔΑ > 0. In contrast, for the slowest mechanisms represented by τ2 and τ3 it was found that for λ = 460 nm (and generally for all wavelengths exhibiting negative ΔΑ < 0), the decay is clearly much slower than for the case of λ = 620 nm and for all wavelengths with positive ΔA. Therefore, although the SA region (ΔΑ > 0) exhibits somewhat slower dynamics during the first few ps, in total it decays much faster than the corresponding mechanisms that contribute to the bleaching dynamics, which are represented by the negative ΔA < 0 traces and which exhibit a much slower overall response. This very slow (τ >> 40 ps, see Fig. 5a) relaxation dynamics for the remaining negative ΔA can be possibly associated with electron-hole recombination that takes place in larger time windows (i.e. ns or even ms). This is indeed expected since GO has been reported to exhibit strong fluorescence from the visible to the near infrared range10, 51-52 which can be therefore attributed to the very slow dynamics of the excited electrons due to radiative recombination at this wavelength region. The faster relaxation in the RSA wavelength regime can be attributed in part to the diffusion of the electrons, initially excited in sp3 regions diffusing into sp2 regions and decaying energetically much faster because of the strong graphene-like electron-phonon coupling which is responsible for a very fast, ps electron decay, mediated by emission of optical and acoustic phonons.53 Therefore, it can be concluded that a SA wavelength regime exists for the shorter wavelengths (λ < 510 nm), with a very slow overall temporal response, persisting for much more than 40 ps, while a considerably faster RSA regime exists for the longer wavelengths (λ > 530 nm), lasting for a time period that does not extend much more than 40 ps. In the intermediate wavelength region, two intermixed contributions are occuring, one contributing towards ΔA > 0 and another one which contributes towards ΔA < 0. This is the result of the competition between SA and RSA mechanisms exhibiting a different dependence on the time delay τ, especially at early pump-probe delay times. Therefore, the resulting temporal trace (Fig. 5b) becomes too complicated to be easily deconvoluted to its separate contributions. The first competing mechanism is the excitation of electrons as shown in Fig. 6 with arrow-3 from the top of VB towards slightly above the bottom of CB1. This probes the bleaching dynamics, i.e. explains the reduction of absorption. The second competing mechanism is depicted in Fig. 6 with arrow-5 which is responsible for further promotion of the already excited electrons near the bottom of CB1 towards CB3. This probes the dynamics of the exlectrons out of their position towards lower energetically Page 10 of 22 ACS Paragon Plus Environment

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positions within CB1 and it explains the increase of the absorption. Thus, two mechanisms are competing with each other due to their opposite contribution to the absorption change. So, it can be summarized that both mechanisms co-exist and contribute differently to the total signal. Still, the important conclusion is that it a very interesting effect has been demonstrated: this is the redshift of the crossover wavelength, λc, at which the induced absorption change, ΔΑ, changes its sign from negative to positive. λc shifts monotonically and quasi-linearly with increasing time delay towards longer values. The shift continues for the whole temporal window of the present experiments and it looks as if it tends to reach a plateau value for τ > 40 ps. This is indeed expected since, at some point in delay time, when the system has finally relaxed, ΔA is expected to become uniformly ΔA = 0 for the whole spectral region. However, in future experiments it would still be interesting to extend the delay time in order to simulate the saturation dynamics of the crossover wavelength, λc. This behavior of λc could in principle be attributed to a temporal change and evolution of the band structure upon excitation and relaxation of the electron population. However, a more detailed knowledge of the band structure shape in both energy and momentum space both experimentally and theoretically would be required in order to further elaborate on this effect. An alternative explanation can be an ultrafast reversible change in the oxidization state of the system where solvation dynamics within the environment of a polar solvent may play a crucial role.54 Note here that similar trends were observed when performing the same experiments with reducing the excitation strength of the system to about half its nominal value thus showing independence on the pump fluence. Nevertheless, this effect may need further investigation in order to obtain a more detailed elaboration. Reversion of the absorption characteristics, i.e. from the RSA to the SA state (or vice versa) has been shown to be highly desirable for nonlinear optics.50 Here an additional parameter in the absorption state has been highligted, namely the transient reversion of the absorption in separate wavelength regions thus combining competing desirable optical responses of nonlinear optical materials highly suitable for photonic applications. 5. Conclusions Employing time-resolved pump-probe spectroscopy, the response of as-grown Graphene Oxide single sheet aqueous suspensions was investigated under ultrafast excitation in the sp3 hybridized domains. A transient crossover wavelength, λc, was revealed, where the pump-induced absorbance ΔA changes sign, from negative to positive, which in addition was found to exhibit a dynamic redshift with increasing time delay in a temporal window of 40 ps following excitation, due to presence of competitive mechanisms. This transition from SA to RSA is dynamic in nature and can be potentially exploited for graphene-oxide-based ultrafast optical switches suitable for various applications in photonics. The effects and the competing mechanisms leading to this response in the absorption spectrum of GO have been discussed on the basis of a simplified semiconductor-like band structure model. 5. Acknowledgments This project is implemented through the Operational Program "Education and Lifelong Learning" action Archimedes III and is co-financed by the European Union (European Social Fund) and Greek national funds (National Strategic Reference Framework 2007 - 2013). The pump-probe experiments were kindly offered at the facilities of FORTH-IESL, which are supported in part by the Integrated Initiative of European Laser Research Infrastructures Page 11 of 22 ACS Paragon Plus Environment


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LASERLAB EUROPE III (Grant agreement 284464). We thank Dr. A. Bakandritsos for the preparation of the GO samples. The contribution of Dr. P. Aloukos in the data analysis is kindly acknowledged.


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7. Figures and captions

Fig. 1 UV-VIS absorption spectra of some GO acqueous dispersions of different concentrations.


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Fig. 2 Pump-probe two-dimensional pseudo-color representation of the time-resolved absorbance change around the visible optical spectrum vs. pump-probe time delay for single-layer GO. The excitation photon energy is at 3.1 eV and the probing is achieved with a white light supercontinuum probe beam.



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Fig. 3 Spectral cross sections obtained by applying vertical cuts at the pump-probe matrix of Fig. 2 at various pump-probe time delays.


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Fig. 4 Temporal drift of the characteristic crossover wavelength at which ΔΑ changes sign from negative to positive.



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Fig. 5 Temporal cross sections at specific probe wavelengths, (a) 460 nm , (b) 530 nm and (c) 620 nm respectively.


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Fig. 6 Schematic of the assumed energetic band model showing the excitation with the pump beam at 3.17 eV (392 nm) and probing schemes with photons at 2.82 eV (440 nm), 2.43 eV (510 nm) and at 2.00 eV (620 nm). The arrows represent possible transitions discussed in the text. Full solid lines represent strong transitions and dotted line represents transitions assumed to be weak.



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TOC Graphic


Ultrafast Processes in Graphene Oxide during Femtosecond Laser

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