Photoinduced Terahertz Conductivity and Carrier Relaxation in

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Photo-Induced Terahertz Conductivity and Carrier Relaxation in Thermal-Reduced Multilayer Graphene Oxide Films Xiao Xing, Litao Zhao, Zeyu Zhang, Liang Fang, Zhengfu Fan, Xiumei Liu, Xian Lin, Jianhua Xu, Jinquan Chen, Xinluo Zhao, Zuanming Jin, and Guo-Hong Ma J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10580 • Publication Date (Web): 11 Jan 2017 Downloaded from http://pubs.acs.org on January 18, 2017

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The Journal of Physical Chemistry

Photo-induced Terahertz Conductivity and Carrier Relaxation in Thermal-reduced Multilayer Graphene Oxide Films Xiao Xing1, Litao Zhao2, Zeyu Zhang1, Liang Fang1, Zhengfu Fan1, Xiumei Liu1, Xian Lin1, Jianhua Xu2, Jinquan Chen2, Xinluo Zhao1, Zuanming Jin1* and Guohong Ma1* 1

Department of physics, Shanghai University, 99 Shangda Road, Shanghai 200444, P. R. China 2

State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai, 200062, China

* Corresponding Author: Guohong Ma and Zuanming Jin Email: [email protected], and [email protected] Tel: 86-21-66132513

1

ACS Paragon Plus Environment


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Abstract

Graphene oxide (GO) is an attractive option for large scale production of graphene. On the other hand, the graphene obtained by the reduction of GO has inevitable structural defects, and the vacant lattice sites will significantly restrict its conductivity. It has been demonstrated that thermal annealing in hydrogen is an efficient method to reduce defects and heal the lattice in GO samples. However, it is still not clear that how the defects and/or disordering influence on the photoelectric conversion efficiency and the carrier relaxation pathway in GO. Herein, the time-domain terahertz (THz) spectroscopy is employed to characterize the properties of the multilayer GO films which were annealed in hydrogen at various temperatures. Upon photo excitation, a transient increase of the conductivity was observed for the reduced graphene oxide (RGO) samples. The ultrafast carrier relaxation process can be well assigned to the carrier-carrier scattering and carrier-phonon coupling. Our results demonstrated that the RGO films with fewer defects and better lattice structure is successfully manufactured. In addition, by fitting to the Drude model, several electron transport parameters, such as the carrier scattering time, carrier plasma frequency and photoinduced conductivity, are obtained in our multilayer RGO films.

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INTRODUCTION Graphene is a two-dimensional carbon material with honeycomb lattice. Due to its unique band structure, graphene exhibits unusual optical and electronic properties1-4. In recent years, graphene oxide (GO) which is an excellent candidate derivant of graphene has attracted great research attentions due to its potential application in mass production of graphene5-8. Chemical exfoliation method producing graphene oxide from graphite presents a promising scheme to achieve solution-processed large-scale graphene 9-14. As-synthesized GO is insulating, but after controlled de-oxidation, it becomes reduced graphene oxide (RGO) which is not only transparent but also conductive15. The properties of RGO were systematically altered by thermal annealing or chemical reduction in order to reform the defect-rich GO characteristics into the graphene-like ones16. RGO film has been widely investigated for transparent conductor applications17-21. It is a substitution of ITO in devices such as organic solar cells22,23, electrochemical supercapacitors24,25 and organic light-emitting diodes26. Electronically, RGO is a semi-metal with finite density of states at Fermi level and it is similar to disordered monolayer and multilayer graphene27,28. So far, the characterizations of GO and RGO films on the ground state have been performed

using

X-ray

photoelectron

spectroscopy,

Raman

spectroscopy,

transmission electron microscope, scanning electron microscope and so on29,30,31. All these measurements provide useful structural and optical information, but it is still unclear that if the photoelectric conversion efficiency and the relaxation dynamics of photoexcited carriers are closely related to the lattice structure. As a matter of fact, the characterizations of GO and RGO films on the excited state can be investigated by ultrafast spectroscopy. While only few literatures studied the relaxation dynamics of photoinduced carriers of RGO in solution or film following different reduction methods using ultrafast transient absorption (TA) spectroscopy in the near infrared and terahertz region16,32-38. The fast ( < 1ps ) non-equilibrium carrier relaxation dynamics is still under debate as it might be carrier-carrier scattering39, carrier-phonon 3



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scattering40,41 or Auger recombination16. To our best knowledge, a better lattice structure and fewer defects can be acquired after thermal annealing in hydrogen13, however, the photocarrier response has not been studied yet. As the terahertz spectroscopy is high sensitive to the free carriers, the ultrafast terahertz spectroscopy was used to study the relaxation dynamics of photoinduced free carriers in RGO film. The primary aim is to investigate the role of the lattice structure, defects and the Fermi energy on the ultrafast carrier dynamics in RGO film thermally reduced in hydrogen. Specifically, we presented the pump fluence dependence of the carrier relaxation ways in RGO with different reduction degrees. The THz peak conductivity in RGO samples shows a square root dependence on pump fluence. The transient conductivity after photo-excitation of RGO samples is found to exhibit a Drude response instead of a Drude-Smith response of chemical reduced GO samples.32 Our study suggests that the thermal-reduced multilayer GO films are produced with fewer defects and better lattice structure. Ultrafast THz conductivity studies offer alternative ways to assess the carrier relaxation pathways, and contribute to the further development of efficient RGO-based optoelectronic devices. EXPERIMENT GO Sample Preparation. GO nanosheets were synthesized from natural graphite with a modified Hummer’s method.42 Briefly, 1-gram graphite was added to 23 ml H2SO4 (98 %) cooled in ice-water bath. The mixture was stirred for 20 minutes, before 3 g KMnO4 was added into the solution at 0ºC. Then the mixture was stirred at 35 ºC for 90 min. After 46 ml DI water was added, the solution was heated at 98 ºC for 30 min. Then 140 ml DI water was added to further dilute the solution, before 10 mL 30% H2O2 was injected into the solution to completely react with the excess KMnO4. The resulting mixture was washed, using 5% HCl solution and DI water for dozens of times. The GO dispersion solution was coated onto a fused silica substrate by spin-coating method. The pristine GO film was thermally reduced in a furnace with an Ar and H2 mixed gas (Ar: 50%, H2: 50%) at the typical temperature of 200 °C, 400 °C, 600 °C, 800 °C, respectively. 4


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THz Time-domain spectroscopy (THz-TDS). The intrinsic THz conductivity of RGO samples without photoexcitation was obtained from THz-TDS measurements performed in a transmission configuration. A pair of photoconductive antennas was used as an emitter and a detector, respectively. A femtosecond laser pulse with the wavelength of 800 nm, a repetition rate of 80 MHz and pulse duration of 100 fs was used for generating and sampling of THz pulse. RGO films were placed at the focus position (with the spot size of ~1 mm) of THz radiation. The THz beam path was purged with dry nitrogen, to prevent the THz absorption by atmospheric water. Time-resolved THz spectroscopy. The laser pulse for both optical pumping and THz probing (OPTP) was generated by a regenerative Ti: sapphire amplifier system, which produced pulses with energy of 2 mJ, duration of 120 fs, and repetition rate of 1 kHz and central wavelength of 800 nm. The standard OPTP measurements were carried out in a transmission configuration43. The laser beam was divided into three parts: (1) THz generation pulse, (2) the sampling pulse and (3) the pump pulse. THz pulses were generated from the femtosecond laser pulses by optical rectification in a 1-mm thick ZnTe (110) crystal. The emitted THz radiation was collimated and focused onto the RGO films by a pair of off-axis parabolic mirrors. After propagating through the sample, the diverging THz radiation was collimated and focused onto another 1-mm thick ZnTe (110) crystal by one pair of off-axis parabolic mirrors. The sampling beam was scanned using an optical delay stage (td). The THz field was detected by free-space electro-optic sampling (EOS). The pump beam (tp) was scanned using a second optical delay stage enabled the variation of the time delay of the THz probe pulse with respect to the optical excitation pulse. The signal was collected with a lock-in amplifier phase locked to an optical chopper that modulated either the THz generation arm or the pump beam at a frequency of 500 Hz. While measuring the terahertz field passing through unexcited samples, the chopper was placed in the terahertz generation path, when the first delay line (td) was used. In case of the photo induced transmission measurements, the chopper was placed in the path of the pump beam exciting the RGO films, when the pump delay line (tp) was used. The spot size of the terahertz beam on the sample position was 2 mm, whereas the 5



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spot size of pump beam on the sample was 5 mm. All measurements were performed at room temperature. The THz beam was enclosed and purged with dry nitrogen to avoid water vapor absorption. RESULTS AND DISSCUSIONS Figure 1(a) shows the UV-visible absorption spectra of four GO films annealed at various temperatures. The absorption coefficient increases and the excitonic resonance peak shifts from 4.6 eV to 4.5 eV, when the annealing temperature varies from 200 to 800 °C16. Figure 1(b) shows the Raman spectra of the four RGO films recorded at room temperature in the backscattering geometry using λ=532 nm laser light (HORIBA Jobin Yvon T64000). The Raman bands are observed at about 1335 cm-1 (D band) and 1590 cm-1 (G band) and 2656 cm-1 (2D band). From Raman spectroscopy, it is noted that the peak of D bands firstly decreases then increases with the annealing temperature. The D bands indicate the disorder degree of the lattice. The G bands are corresponding to the G mode optical phonon emission. The size of defect-free domain, , can be estimated using the intensity ratios of the D and G bands 44: = 1.8 ± 0.5 × 10 ⁄

(1)

So is approximately 14 ± 7.5 nm , 16 ±8.5 nm , 14.9 ± 7.8 nm and 14.8 ± 7.7 nm for the four samples with annealing temperature from 200, 400, 600 and 800

ºC, respectively. The average size of the sp2 clusters suggests an increase below 400 ºC and a possible decrease above 400 ºC. The size of sp2 cluster is consistent with the degree of lattice disorder. This can be explained, the size of sp2 domains becomes smaller but the number of sp2 domains gets larger after annealing45, so the average size of these domains does not significantly increases with the annealing temperature. In addition, the thickness of around 20 nm in average for all typical RGO films in this study is measured using a Step Profiler.

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2.0

1.2

G (1590

1.0

D 0.8 (1335

)

)

200℃ 400℃ 600℃ 800℃

0.6

1.0

300 250

2D (2656 cm-1)

0.2

50

0.0 400

500

600

700

Wavelengthnm)

800

150 100

0.0 300

(c)

200℃ 400℃ 600℃ 800℃

200

D+G (2913 cm-1)

0.4

0.5

350

(b)

S/cm

1.5

(a)

Intensitya.u.

200℃ 400℃ 600℃ 800℃

)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1000

1500

Raman shift

3000

0 0.4

)

0.6

0.8

1.0

FrequencyTHz

1.2

Figure 1 (a) UV-visible absorption spectra from 240 to 850 nm for the four RGO samples. (b) Raman spectra of the four RGO films. (c) The real parts of the static conductivity of the multilayer RGO films at THz frequencies are extracted from the amplitude and phase of relative THz-TDS transmission. Here, we first measure the THz-TDS response for the four RGO samples with different annealing temperature. We recorded the time-dependent THz wave transmittance through the quartz substrate, E0(t) as a reference, and through the multilayer RGO films on the quartz substrate E(t). Fourier transformation then yields the corresponding frequency-domain fields, E0(ω) and E(ω). By the standard thin-film approximation,46 the frequency-dependent complex sheet conductivity σ(ω) can be obtained from the measured transmission spectra by applying47: !"

!# "

=

$%

$%%&# '"

(2)

where Z0 =377 Ω is the impedance of the free space, and n is the refractive index of the substrate. The THz refractive index (n) of the fused silica substrate is 1.96.48,49 Figure 1 (c) shows the real parts of the complex conductivities of the four RGO samples. Before reduction, the GO multilayers behave insulator-like properties. However, thermal annealing results in a pronounced increase of real conductivity. The largest real conductivity of RGO films annealing at 800 ºC is approaching to 200 S/cm. This is found on the same order as the value reported by J. T. Hong et.al.50 The intrinsic conductivity of all samples increases gradually with the rise of the annealing temperature. The Fermi level of RGO is around 8~32 meV, which was roughly estimated by the intrinsic conductivity in Fig. 1(c). (See the supporting information) The findings suggest that the insulating GO is transformed to conductive RGO films as the oxygen-function groups are reduced and new sp2 clusters are created29. 7



Fluence

10

(%)

173 316 469 601 744 928 1080

8 6

6

(b)

316

5

1.0

3

0.8

2 1

200℃ 800℃

0.6

0 0

4

20

40

60

80 100 120

ps)

0.4

400 ℃ annealing

316

0.2

2 0 -1

(c)

200℃ 800℃

4

Norm.

(a) 12

0.35

Pump delay 0

1

ps

2

3

400 annealing

0.0 -2

4

4.0

(d)

3.5

Pump delay

0

0.36

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ps

8

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0.33

3.0 2.5

ps

3.0 0.30

ps

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0.30 2.5

0.27 0.25

2.0 0.24

2.0

1.5

600

0.21 0.20

1.0 200

400

600

Fluency

800

1.5

1000

200

400

600

800

Annealing Temperature ℃

Figure 2 (a) Photo-excited transient THz transmission response of multilayer RGO film annealed at 400 ºC with different fluence (F). The inset shows the long decay time up to 130 ps. (b) The normalized THz transient dynamics of GO films annealed at 200 ºC and 800 ºC with F of 316 μJ⁄cm . The solid lines are fitted to

tri-exponential models. (c) The relaxation components A and A of tri-exponential models are displayed as a function of F for GO films annealed at 400 ºC. (d) The relaxation components A and A plotted as a function of annealing temperatures at

the pump fluence of 600 μJ/B .

In order to investigate the relaxation dynamics of hot carriers in RGO films, the peak of the pump-induced differential signal (|ΔEt/Et| as a function of the pump delay, was recorded. Figure 2 (a) shows the typical time-resolved THz photoconductivity dynamics of GO annealed at 400 ºC with different pump fluence. The photoinduced transmission change (|FGH/GH|) in the peak of the terahertz electric field is proportional to the real part of the photoconductivity:32 ∆σH ≈ −

$% ∆!M &#

!M

(3)

The THz transmission decreases rapidly after photo-excitation corresponding to the 8


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positive photoconductivity, ∆σ. The inset of Fig. 2(a) shows ∆σt as a function of the pump-probe delay time with the delay time up to 130 ps. Momchil T. Mihnev et al. summarize that the sign of the THz conductivity in a wide variety of graphene samples39. The positive photoconductivity was observed in RGO films 16,32 and lightly doped graphene39. Figure 2 (b) shows the normalized transient THz photoconductivity dynamics of RGO annealed at 200 and 800 ºC with the pump fluence of 316 TU⁄B . The peak normalized kinetics of the photoconductivity with tri-exponential fit reveals that the relaxation dynamics consists of three carrier relaxation pathways: a fast relaxation with time constant A (a few hundred femtoseconds), a slower relaxation with time constant A (a few picoseconds) and a small third component with time

constant AV (~hundreds of picoseconds). The three relaxation components were also

observed in the chemical reducing GO membrane by S. Kar et al.32 To understand the role of three relaxation components, Fig. 2 (c) shows A and

A as the function of the pump pulse fluence, taking the RGO film annealed at 400 ºC

as an example. The fast relaxation time A had been previously assigned to Auger

recombination or optical phonon emission. The extracted value of A is

independently with the pump fluence, as shown in Fig. 2 (c). In contrast to the Auger recombination observed in graphene oxide,16 in which A decreases with the pump

fluence increases. Thus A can mainly be assigned to the optical phonon emission32,33.

The initial cooling of hot thermalized carriers proceeds via the emission of high-energy optical phonons (~ 200 meV), once the carrier temperature is below the optical phonon energy, the cooling should proceed via the emission of low-energy acoustic phonons (~ 4 meV).39 During each scattering event, very small energy (~4 meV) is dissipated and thus the relaxation process can last as long as 300 ps.51 However, this obviously is not the case observed in our RGO samples. As shown in Fig. 2 (c), the relaxation time A shows no pump fluence dependence with the decay constant of around 2.5 ps. Recently, a proposal for the relaxation process with time constant of several ps is the emission of high energy (~XY Z[ ) and high momentum

(~ XY Z[ ⁄ℏ]^ ) acoustic phonons mediated by disorder, which is referred to the

supercollision cooling.52 The relaxation time of super collision model (SC) is around a 9



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few picoseconds, which is consistent with our slower decay constant A . Therefore,

the second component A in the context may be originated from disorder-assisted carrier-phonon scattering. (See the supporting information) The third component AV

(hundreds of picoseconds) in the context is related to the relaxation of electrons in deep trap states in RGO

32,33

and/or interlayer thermal coupling effects 39. Our pump

fluence dependent OPTP measurements show similar relaxation dynamics for RGO films with different annealing temperature. In order to further ascertain the relaxation dynamics of RGO films with different reduction temperatures at fixed pump fluence, we investigated the A and A as a function of the annealing temperature, as shown in Fig. 2 (d). After photo-excitation, the hot-carrier distribution is maintained by efficient carrier-carrier thermalization (A_` ). And then the optical phonon emission (Aab ) removes the energy from the high-energy tail of the hot-carrier distribution. As mentioned by a microscopic modeling, the dynamics of THz photoconductivity is dominated by the combined effect of carrier-carrier scattering and carrier-optical-phonon scatting. The magnitude of A can be given by 1/τ =1/τde + 1/τgh . According to the thermal dynamics model, as |EF| rises with the increase of the annealing temperature (See the supporting information), the carrier-carrier scattering efficiency increases with larger |EF|, leading to a faster scattering process (A_` ). This could be the reason for A slightly decreases at high annealing temperature. The slow component (A ) remain almost identical (i.e. within the errors) as compared to the GO films with different annealing temperature. This observation can be explained as follows. There are two types of disorder were reported in the literature. One includes some oxygen-functional groups on the graphene basal plane consist of epoxy and hydroxyl molecules, these impurities are sp3 hybridized domains. Another one includes ripples, misorientation in layer stacking, lattice deformation (epoxy groups are substantially distort the graphene lattice on desorption) as well as newly formed carbonyl and ether groups.13 These defects are generated together with the newly-formed sp2 domains, and may form new deep trap states. With increasing annealing temperature, the former impurities are reduced,

10


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while

the

latter

defects

increased54.

are

Therefore,

the

disorder-assisted

electron-phonon scattering is weakly dependent of the annealing temperature. 12

8

Fit ~ = 0 ps

8

Fit ~ = 1 ps

6

Annealed temp. 0.6

β

10

(%)

(%)

6

Annealed temp. 200

4

400

600

4

200

400

600

800

0.5

0.4

800

0.5

2

η

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

(a)

0.4

(b)

0

0 0

200

400

Fluency

600

Figure 3 The THz signals (−

800

1000

0

200

400

600

Fluency

800

1000

∆ij !M

) measured at two pump-porbe delay times (a)

Hb = 0 kl and (b) Hb = 1 kl, as a function of pump fluence. Solid lines are fitted

with power function. The inset summarizes the fitting indices η and β with different annealing temperatures. Figure 3 (a) and (b) show the THz transient conductivity signals −

∆iM !M

collected at delay time Hb = 0 kl and 1 kl, respectively, as a function of pump fluence. The photoinduced transient conductivity of the RGO samples shows a sub-linear dependence on the pump fluence. The THz conductivity value in Fig. 3 (a) and (b) are saturated as function of o p q r os , respectively, as shown by the solid

fitting curves. The fitting indices of η and β for four GO samples with different annealing temperature are shown in the inset of Figure 3 (a) and (b), respectively. The fitting indices η and β are close to 1/2. The observed THz peak signal is scaling as square-root dependence of pump fluence has also been observed in the single-layer graphene grown by chemical vapor deposition47. Such observation can be explained in term of the pump-induced increase in scattering rate, that varies as F/ or with the

peak transient electronic temperature is scaling as F/ . For the positive

photoconductivity in our RGO samples, the pump fluence dependence of sublinear response of photoconductivity may come from many things. Firstly, electron temperature in graphene has a square-root dependence on the pumping fluence. 11



Secondly, two competition processes also contribute to the nonlinear THz photoconductivity: one the one hand, higher pump fluence produces more photo-carriers which give rise to an increase in THz conductivity. On the other hand, higher carrier concentration leads to an increase of the carrier scattering rate, which causes the decrease in photoconductivity of the graphene. The pump fluence dependence of THz photoconductivity is the balanced results of the two issues. In addition, the charge trapping under low pump fluence and Auger recombination under high pump fluence should also be taken into account in the GO samples. Notably, our results are different from the THz peak conductivity saturated as a function of 1/3 order of F in GO with defect.16 This difference further suggests that Auger recombination mediated by three-particle interaction is found not to be important in present work.

20

(a)

200℃ 400℃ 600℃ 800℃

15

24

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(b)

20

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8 0 -2

-1

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1

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ps

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3 200

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800

Figure 4 (a) THz photoconductivity scaled to the absorbed photons density (N) of RGO films excited at 316 μJ/cm with sheet excitation density of 2.7×1018, 2.5×1018, 2.56×1018, 2.3×1018 photons/m2 at the annealing temperature of 200 ºC, 400 ºC, 600 ºC, and 800 ºC, respectively. (b) The peak of the numerical value of ∆t ⁄ u for RGO samples with four different annealing temperatures. The change tendency is similar to the rising trend of the proportion of sp2 carbon as the annealing temperature increases. The line is a guide to the eyes. Now we focus on the effective mobility of RGO samples with different reduction degree. The photoconductivity (∆σ) is determined by the product of the carrier density

12


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(n), the elementary (e), and the intrinsic effective carrier mobility (T)： ∆σ = v × × T. Figure 4 (a) shows the measured conductivity normalized by the excited charge density (∆σ⁄N), as a function of pump-probe delay for the four RGO samples. Figure

4 (b) presents the peak value of ∆σ⁄N for RGO films annealed at four temperatures. The ∆σ⁄N corresponds to the intrinsic material response parameter, the product of the quantum efficiency of charge generation (η) multiplied by the carrier mobility (μ = eτ⁄m∗ ). As shown in Fig. 4 (b), the increasingly large effective mobility is obtained by increasing the annealing temperature. It is noted that the photoexcited effective mobility increases rapidly between 200 and 400 ºC, while increases slightly the annealing temperature is higher than 400 ºC. The results are consistent with the work by Cecilia Mattevi et.al. They reported that the proportion of sp2 carbon shows increase with the annealing temperatures in the RGO films29. This consistency suggests that the photoinduced effective mobility is closely related to the percentage composition of the sp2 domain. It is worth noting that the numerical value of ∆t ⁄ u in the multilayer RGO sample is on the same order of magnitude as that of the carbon nanotube film reported by Søren A. Jensen55. While it is not as good as the monolayer graphene41, as the relative rotation of multilayer RGO film is not a negligible issue in comparison to the monolayer graphene38. In addition, the remaining oxygen cannot be completely removed from GO even at the annealing temperature of up to 1100 ºC. The residual impurities limit the RGO’s electronic quality which is often characterized by carrier effective mobility. Thus, a better lattice structure of RGO films is still acquired under thermal annealing in hydrogen.

13



Real

Imaginary

4 2 0

200℃

8

400℃

4 0

/N*

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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8

600℃

4 0 8 4

800℃

0 0.5

1.0

1.5

FrequencyTHz

2.0

Figure 5 The frequency-dependence of the real (blue squares) and imaginary (red circles) photoconductivity scaled to the number of photons measured at t b =1.4 ps with F of 316 μJ/cm , for the four RGO with different annealing temperature. The solid lines are Drude-fittings as explained in the main text. Finally, we consider the frequency dependence of photoinduced conductivity (∆σω scaled to the number of photons for the four RGO samples. Figure 5 shows the photoconductivity spectra (∆σω/u) of the four RGO samples at the pump delay

of Hb =1.4 ps. Overall, the shape of photoconductivity spectrum changes slightly with

the annealing temperature. The nonzero value of the real conductivity suggests that the response in RGO sample is dominated by the mobile free carriers. The real part decreases while the imaginary part increases with the rise of the frequency. They are all positive over the THz frequency range from 0.2 to 2 THz. The observed photo-carrier responses of RGO samples are fitted to the Drude conductivity model51. The Drude model describes the conductivity of free charge carriers in bulk crystalline semiconductors and metals with the assumption of exclusive momentum randomizing scattering events on either lattice defects or phonons56:

14


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


t`z{[ ω = t| ⁄1 − }ωA = ωb ~ A⁄1 − }ωA

(4)

where ~ is the vacuum permittivity, b is the plasma frequency, and A is the average scattering time. The plasma frequency is related to the density of excited charge carriers (N) by b =

where ∗ is the carrier effective mass.

[

(5)

# ∗

Table 1 Parameters of the Drude Model fitted to the data of the probe Frequency-dependent RGO films (Hb = 1.4 ps )

200

1.9±0.55

17±4.2

0.7±0.28

400

1.5±0.24

59±17

1.5±0.64

600

1.6±0.3

47±10

1.4±0.69

800

3.3±0.37

14±3.3

1.75±0.57

Annealed Temperature (ºC)

Table 2 Parameters of the Drude Model fitted to the data of the THz probe frequency-dependent RGO films annealed at 400 ºC with various pump fluence and delay time Hb = 1.4 ps and Hb = 1.8 ps. =1.4 ps

316 /2

1.5±0.24

59±17

1.5±0.64

600 /2

2.1±0.17

44±10

2.2±0.61

928 /2

3.3±0.07

20±10.5

2.5±1.3

=1.8 ps

F

316 /2

1.42±0.08

56±13

1.29±0.33

600 /2

1.9±0.13

43±9.3

1.76±0.45

928 /2

3.2±0.06

19±11.3

2.23±1.29

15



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Page 16 of 25

Equation (4) is used to fit the measured real and imaginary parts of ∆t ⁄ u, and a very good agreement between the data and the model is observed. Unlike previously reported THz spectroscopy of RGO, where the multiple oscillators suppressed DC conductivity

32

16,32

and the

indicate the presence of defect states, our RGO

samples show Drude response. The obtained τ and b from the Drude model are listed in Table 1 and Table 2, respectively. The Table 1 presents the fitting parameters for the frequency-resolved conductivities of RGO films with different annealing temperatures at pump fluence of 316 μJ/B . It is found that b is in the range of 1.5 - 3.3× 10 Hz and τ ranges from 14 to 59 femtoseconds for different annealing

temperatures. Firstly, the electron scattering time of 14 ~ 59 fs is found to be longer than those RGO samples thermally annealed in vacuum or inert gases

16

, which

indicates that the defects in our samples are reduced more significantly and the lattice structure is healed better through reduction in a hydrogen atmosphere. Furthermore, the longest electron momentum scattering time is observed in the GO film annealed at 400 ºC. This is consistent with the previous result of Raman spectrum, in which the size of sp2 cluster for GO film annealed at 400 ºC is the largest among all the samples. The finding suggests that the mean value of scattering time τ is related to the size of the sp2 cluster. The shorter scattering time may be the result of the formation of carbonyl and ether groups at higher annealing temperature. Table 2 shows the fitting parameters for the THz frequency-dependent RGO films annealed at 400 ºC with various pump fluence (316~928 μJ/B ) at two delay times (Hb = 1.4 and 1.8 ps). It is seen that the DC conductivity t| ) increases, while the

scattering time (τ) decreases with the increase of pump pulse. The plasma frequency b is proportional to √u as described in equation (5). In the plasma frequency

range (1.5-3.3× 10 ) for this study, τ decreases drastically from ~60 to ~20 fs.

The inversely proportional correlation between the electron scattering rate and the carrier density has also been observed in conjugated polymer and semiconductors such as GaAs due to carrier-carrier scattering57 and the density-dependent phase-space filling effect, respectively.56,58

16


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CONCLUSIONS In summary, this paper reports the charge carrier dynamics of multilayer GO films thermally annealed in hydrogen with time-resolved THz spectroscopy. Quantitative analysis based on tri-exponential fitting reveals that photoelectric conversion efficiency increases with the raise of the annealing temperature, predominantly as a consequence of lattice healing and more sp2 domains during reduction process at the high annealing temperature. Our study shows that the photoinduced conductivity of samples annealed at 800 ºC is the highest at the same excited pump fluence. The frequency-dependent complex conductivity can be well described with Drude response. The RGO films show fewer defects and vacant lattice sites after annealing in hydrogen. The defects, lattice structure and the Fermi energy can be controlled by adjusting the annealing temperature. The present study will help in evaluating the application potential of RGO for ultrafast optoelectronic applications. Acknowledgements This work is supported by the National Natural Science Foundation of China (NSFC, Nos. 11674213, 11604202) and the Research Innovation Fund of the Shanghai Education Committee (14ZZ101). Z.J. thanks the Young Eastern Scholar (QD2015020) at Shanghai Institutions of Higher Learning.

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23



TOC Graphic

Annealed Temperature 25

Photo-electric conversion efficiency

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 25

200℃ 400℃ 600℃ 800℃

20 15 10 5 0

0

2 0 2 0 2 0 2 Pump-probe delay (ps)

24


Page 25 of 25

Annealed Temperature 25

Photo-electric conversion efficiency

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


200℃ 400℃ 600℃ 800℃

20 15 10 5 0

0

2 0 2 0 2 0 2 Pump-probe delay (ps)


Photoinduced Terahertz Conductivity and Carrier Relaxation in

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