Adsorption Site-Dependent Mobility Behavior in Graphene Exposed to

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Adsorption Site-Dependent Mobility Behavior in Graphene Exposed to Gas Oxygen Vaclav Blechta, Karolina A. Drogowska, Vaclav Vales, and Martin Kalbac J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06906 • Publication Date (Web): 03 Sep 2018 Downloaded from http://pubs.acs.org on September 6, 2018

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The Journal of Physical Chemistry

Adsorption Site-Dependent Mobility Behavior in Graphene Exposed to Gas Oxygen

Vaclav Blechta a, b, Karolina A. Drogowska a, Vaclav Vales a, Martin Kalbac a, *

a

J. Heyrovsky Institute of Physical Chemistry of the ASCR, v. v. i., Dolejškova 2155/3,

Prague 8, 182 23, Czech Republic b

Department of Physical Chemistry, Faculty of Science, Palacky University Olomouc,

Tř. 17. Listopadu 12, Olomouc, 771 46, Czech Republic

ABSTRACT

Transport characteristics of graphene field-effect transistors were measured in-situ in oxygen/nitrogen atmospheres and at various temperatures. Mobilities of holes were extracted from transport characteristics as well as the doping level in dependence on the time of graphene exposure to oxygen/nitrogen atmosphere. The hole mobility showed significant decrease upon the oxygen adsorption to low-energy adsorption sites (sp2 carbon) however it remained unaffected by the oxygen adsorption to high-energy adsorption sites which are represented by

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defects, impurities, transfer residuals, edges and functional groups on graphene. Dirac point was upshifted for both the low- and high-energy adsorption events. Activation energy of oxygen adsorption/desorption was estimated from temperature dependent desorption rate coefficients as 215 meV and 450 meV for the low- and high-energy adsorption, respectively.

Introduction

Graphene exhibits specific electronic properties such as high mobility of charge carriers,1 ambipolar field-effect, minimum conductance at zero charge density and anomalous Hall effect. 2 Being 2-D material, graphene is very sensitive to its environment, which means that the substrate3–11 and adsorbed molecular species8,12–17 strongly affect graphene electronic properties. Graphene has zero band-gap nevertheless it was demonstrated that electronic structure could be altered by molecular adsorption and size of the band-gap depends on the adsorbed molecules and orientation with the respect to graphene. 18,19 Graphene based field-effect-transistors (GFETs)20 were suggested for several applications including gas sensors21 as one can directly observe changes in doping (charge carriers density) and field-effect mobility of charge carriers within graphene in dependence on the changes of the environment. Graphene also offers wide variety of chemical functionalizations,22–28 which could be beneficial for the application in sensors or electronics where the band-gap needs to be open or the interaction requires specific surface modification.


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Recently, graphene grown by chemical vapor deposition (CVD) 29 and microwave plasma enhanced chemical vapor deposition (MPECVD) 30 were also employed in oxygen sensing. It was shown that adsorption of oxygen leads both to a change in density of charge carriers and a change of their mobility.31 More detailed transport characteristics of graphene transistors studied upon exposure to oxygen showed that oxygen adsorbed to exfoliated graphene causes upshift of the Dirac point and mobility is either increased or decreased depending on the nature of substrate impurities (positively or negatively charged). 32 It was suggested that oxygen and water adsorbates interact with both positive and negative substrate impurities but only compensate for negatively-charged impurities. Similarly bilayer graphene showed upshift of the Dirac point and decrease in electron mobility upon exposure to oxygen. 33,34 It was concluded that the mobility of holes decreased because of the screening of long-range scatters by oxygen molecules trapped between the graphene and the substrate. 34

It was reported that adsorption of the gas to graphene occurs at two different types of adsorption sites.35–45 Different adsorption sites were reported for carbon nanotubes as well. 46,47 First, the low-energy adsorption sites correspond to pristine graphene surface, sp2 carbon. In this case the adsorption is relatively fast and the energy needed to bond a molecule to graphene is in the range from 39 to 150 meV according to the other studies.

48–54

Second, the high-energy adsorption sites

refer to edges, defects, impurities, oxygen containing functional groups and sp 3 carbon. Adsorption/desorption at these high-energy sites happens much slower and molecules are bonded with higher adsorption energy.


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Temperature also affects electronic performance of GFETs.55–57 The dependence of Hall mobility on temperature was shown to be decreasing for mono-layer of exfoliated graphene and increasing for bi- and tri-layer graphene in the whole range from 4.2 to 350 K while measured in high vacuum.58 The charge carrier mobility in graphene decreased with increasing temperature in the range from 300 to 500 K as shown elsewhere. 59 Similar trend was confirmed in another experimental60 and theoretical studies.61

In this work the electronic performance of CVD-grown graphene samples at various temperatures (from 25 °C to 180 °C) and in various atmospheres (N2, O2) was examined by the means of GFETs. We show that the oxygen is adsorbed on two energetically different sites, what was confirmed using the model of two exponentials that fitted well our experimental data. It is also shown that the mobility of holes is not influenced by the oxygen adsorption to high-energy adsorption.

Materials and Methods

Samples preparation

Graphene was grown by the chemical vapor deposition method (CVD) as described elsewhere.62,63 In brief, the polycrystalline copper foil was annealed in the hydrogen flow of 50 sccm at temperature of 1000 oC for 20 minutes when the methane precursor with the flow of 1 sccm was introduced into the chamber. Then the graphene growth was carried out for 30 minutes. The pressure of the gases in the chamber was 350 mTorr. After the process, graphene was kept under the hydrogen flow for 5 minutes to etch the layer 62 and then the sample was cooled down to the room temperature.


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Raman spectroscopy

Raman spectra were acquired using the HORIBA Jobin-Yvon spectrometer with excitation laser wavelength 532 nm and Raman maps were recorded using WITec Alpha 300 spectrometer with the 532 nm laser excitation (E = 2.33 eV), a 100 x objective (NA, 0.82) and grating of 1200 lines/mm. The power of the laser was kept approximately at 1 mW to avoid the heating of the sample. The spectra were recorded with the integration time of 5 s. The maps of the area of 30 x 30 µm2 were collected with the lateral step of about 0.75 µm in both directions. The spectra were fitted with the single pseudo-Voigt line shapes to obtain the positions (Raman shifts), intensity and half-width at half maximum (HWHM) of the main graphene modes.

Device preparation and measurement

Graphene was transferred from growth substrate (Cu foil) to Si/SiO 2 by standard PMMAsupported transfer.64,65 Sample was contacted by Cr/Au pads deposited by physical vapor deposition using shadow mask. The metal pads were contacted by Ag wires using silver/epoxy paste. Sample was then mounted to holder that enables placement into measurement cell. Also Pt100 temperature sensor was fixed from the bottom of substrate using Torrseal. The Pt100 was used as a heating to elevate and keep the temperature of the substrate. Samples were mounted to the holders and placed to the self-fabricated measurement cell for the mixing gases and electronic measurements. Before each measurement the cell was constantly flushed with flowing nitrogen and sample was heated up to 180 °C for at least 24 hours to initiate desorption of water and other pollution from atmosphere. During in-situ measurement of GFET transport characteristics the Id-Ug and Ig-Ug curves were recorded in time intervals of 1 minute for whole the time of the experiment (Id is a drain current, Ig is a gate current and Ug is a gate voltage). The


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drain-source voltage Uds was kept at – 50 mV, the gate voltage range and step was adjusted for each experiment, but usually the range was not wider then - 90 to 90 V and step was set to 1 V. The gate leakage current was in order of units of nA (at least 3 orders of magnitude lower than the drain current).

Results and Discussion

The graphene samples were prepared by CVD method and transferred to SiO 2 substrate. Typical Raman spectrum of the transferred graphene samples is shown in figure 1a. The spectrum shows characteristic Raman features of graphene: low-intensity D-mode - corresponds to defects and sp3 states (1350 cm-1), G-mode - corresponds to optical phonons (1582 cm-1), G*-mode corresponds to longitudinal acoustical and transversal optical phonons (2450 cm-1) and 2D-mode - corresponds to transversal optical vibrations of two-phonons (2700 cm-1). Lorentzian shape of 2D-band of half widths at half maximum (HWHM) of about 25 cm-1 denotes single-layer graphene. The map of the D/G-mode Raman intensities ratios shows reasonable quality of graphene without a significant variation of defect density (Figure 1b).


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Figure 1: a) Typical Raman spectrum of CVD-graphene sample transferred to SiO2 substrate. The laser excitation wavelength was 532 nm. b) The map of the D/ G-band intensities ratios of CVD-graphene sample transferred to SiO2 substrate.

GFET devices were exposed to the gas oxygen diluted in nitrogen at different temperatures and transport characteristics were measured in-situ (Figure 2). As the graphene sample is exposed to the gas molecules adsorption occurs. Oxygen molecules adsorbed to graphene causes the shift of Fermi level, which is represented by the upshift of the Dirac point of graphene (Figure 3a). We note that nitrogen can adsorb to graphene as well, nevertheless this adsorption does not causes significant charge transfer from/to graphene66,67 thus it does not affect the position Dirac point. When graphene is placed into the pure nitrogen atmosphere air pollution (oxygen, water) desorbs from graphene and a downshift of the Dirac point is observed. However, this downshift is not directly caused by adsorbed nitrogen but by de-doping – desorption of the dopant molecules from graphene. Response of the sensor, defined as (R – R0)/R0, where R is the measured resistance of the drain-source channel during exposure towards oxygen and R0 is initial resistance, shows opposite behavior then Dirac point shift (Figure 3b). As the Dirac point upshifts the mobility of holes decreases (figure 4a) which is a consequence of enhanced


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electronic scattering from adsorbed species as discussed by Jaaniso et al. 31 The mobility was calculated from the linear fits of Id-Ug curves as µ =

𝐿 𝑑𝐼𝑑 1

1

𝑊 𝑑𝑈𝑔 𝑈𝑑𝑠 𝐶𝑔

at carriers density of about n ~

2.2 x 1012 cm-2 (Ug ~ 30 V from the Dirac point). Gate voltage Ug is related to the carrier density n by the equation: 𝑈𝑔 =

ħ|𝑣𝐹 |√𝜋𝑛 𝑒

𝑛𝑒

+ 𝐶 , where ħ is reduced Planck constant, 𝑣𝐹 if Fermi velocity 𝑔

and 𝐶𝑔 is the gate insulator capacitance. The mobility of holes drops relatively fast from initial 1040 cm2/Vs to approximately 760 cm2/Vs in the time of about 5 hours. Afterwards, the Dirac point still shifts but with much slower rate while the hole mobility retains its value. Thus for a specific magnitude of the Dirac point shift reached by oxygen doping the mobility does not decrease any more as shown in the correlation plot of the mobility and the shift of Dirac point in figure 4b. The mobility saturates after approximately 5 hours of doping by adsorbed oxygen at the Dirac point position of 65 V (corresponding to charge carriers density of 4.8 x 10 12 cm-2). We attribute this behavior to the different mechanisms at low-energy and high-energy adsorption sites. This hypothesis is supported by the response of the GFET towards oxygen, which can be fitted well by the double exponential term with the constant: y(t) = A1exp(-K1(t-t0)) + A2exp(K2(t-t0)) + c, where y(t) is the position of the Dirac point, t is the time, A1 and A2 are amplitudes of two terms, K1 and K2 are adsorption rate coefficients, c is the constant term and t0 is initial time (figure 5a). The double exponential term model also fitted well to the desorption curves. Generally, the adsorption rate coefficient K2, that corresponds to the low-energy adsorption sites, is greater than K1 (high-energy adsorption sites) for all the temperatures (table 1), which means that low-energy adsorption occurs faster than high-energy adsorption if we assume that second term of exponential model corresponds to low-energy adsorption sites.


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It is generally expected, that defects, impurities, or edges structures are more reactive than the sp2 carbon. It can be expected that the highly reactive sites will be saturated as the samples were prepared at ambient conditions. For the adsorption of oxygen we consider only physical process, which is energetically in the order of tens to hundreds of meV.48–54 As the adsorption to high energy sides is relatively slow we expect high activation barrier. If the adsorption occurs, it is expected to be less reversible because it is thermodynamically more stable. Indeed our experiments show that there is reversible response (all Dirac point, mobility and resistance returned to the initial value) for the short time exposures of graphene to the oxygen, however for the long time exposures the response is irreversible and the conductance decreased consequently.

Figure 2: Development of transport characteristics of GFET during exposure to 2.1 % of oxygen in nitrogen at temperature of 150 °C. Arrow indicates the direction of time (about 24 hours). Green circles correspond to maxima of the curves (Dirac points).


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Figure 3: a) Response of the graphene towards 2.1 % of oxygen in nitrogen at the temperature of 150 °C observed via the position of the Dirac point. After 25 hours the oxygen flow was set off and sensor partially recovers at 180 °C upon constant flow of nitrogen. b) Response and recovery of the device towards 2.1 % of oxygen in nitrogen at temperature of 150 °C and 180 °C, respectively.

Figure 4: a) Response of GFET towards 2.1 % of oxygen in nitrogen at the temperature of 150 °C. The position of Dirac point (blue circles) is continuously increasing while the hole mobility (green circles) is firstly rapidly decreasing and then approximately retains its value. b) Correlation of the hole mobility and the position of the Dirac point of GFET exposed to 2.1 % of


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oxygen in nitrogen at the temperature of 150 °C. This plot corresponds to the data shown in figure 3. The hole mobility saturates at value of about 750 cm2/Vs at the certain level of doping of approximately 65 V of gate voltage (corresponding to charge carriers density of 4.8 x 10 12 cm2

).

During adsorption of oxygen to the low-energy adsorption sites, the Dirac point upshifts and mobility drops significantly, nevertheless this process is finished in approximately 5 hours in case of our experimental setup. Then adsorption and desorption to/from low-energy sites is in equilibrium and response from adsorption to high-energy adsorption sites becomes dominant. In the case of oxygen adsorption to the high-energy adsorption sites of CVD-graphene irreversible doping of graphene might occur.68 It was also observed that a short time exposure (several minutes) of graphene to oxygen leads to the completely reversible doping, which we have observed as well, however reversibility of adsorption process depends on other conditions such a temperature and humidity. The importance of substrate and water molecules presence for the oxygen doping of graphene was also reported by Piazza. 4 In the majority of our experiments the complete recovery of graphene was not possible. For long time doping by oxygen (over 24 hours) we have observed only irreversible response (figure 3a). The mobility response towards oxygen adsorption was fitted well with a single exponential term y(t) = Amexp(-Km(t-t0)) + c for all the temperatures, where Am is amplitude and Km is adsorption rate coefficient of mobility response (Figure 5b). Values of K2 and Km obtained from the fits are reasonably close (table 1) which means that mobility reduction process and low-energy adsorption process have the same kinetics and supports the assumption that mobility is predominantly affected by adsorption of oxygen to sp2 carbon (low-energy adsorption sites).


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Figure 5: a) The upshift of the Dirac point of the GFET exposed to 2.1 % of oxygen in nitrogen at different temperatures (blue circles = 150 °C, green circles = 75 °C, red circles = 25 °C, black lines = fits to the data). b) Decrease of the hole mobility upon exposure to 2.1 % of oxygen in nitrogen at the temperature of 150 °C. Black line corresponds to fit by single exponential function.

T [°C]

A1 [V] K1 [1/h]

A2 [V] K2 [1/h]

Km [1/h]

c [V]

150

-17.5

0.112 ± 0.003

-21.3

0.968 ± 0.035

0.921 ± 0.011 73.6

75

-23.8

0.104 ± 0.002

-29.0

0.885 ± 0.015

0.909 ± 0.013 71.6

25

-25.4

0.088 ± 0.002

-22.1

0.670 ± 0.014

0.601 ± 0.006 69.4

Table 1: The parameters of the fits of the Dirac point and the mobility response to adsorbed oxygen for different temperatures. The parameters correspond to the curves displayed in the figure 5a. After few hours, the low energy adsorption sites are occupied by oxygen and adsorption/desorption to processes are in equilibrium. Further adsorption/desorption to these sites does not cause any shift of Dirac point, only fluctuations around the present value. Consequently, upshift of Dirac point is caused by the adsorption to the high energy adsorption


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sites only and obviously for this process it takes longer to reach equilibrium (activation energy needed for molecule to adsorb is higher). This high-energy adsorption, does not influence the mobility. The rate coefficients of processes obtained from fits to the response exactly agrees with that. Low energy adsorption and mobility response evince same rate coefficient and follows the same shape of response curve. We assume that high-energy adsorption sites already represents scattering centers. The modification of these sites by adsorption of other species do not change the scattering process significantly and therefore the mobility is not affected. On the other hand the adsorbed species lead to additional charge transfer, hence the shift of the Dirac point can be rationalized.

We note that in the oxygen adsorption/desorption experiments the mobility of electrons could not be evaluated during doping by oxygen because the Dirac point upshifts too high, thus the linearlike range of Id-Ug curves lies is not possible to be measured. The CVD-grown graphene transferred to SiO2 usually exhibits p-doping originating from the underlying substrate even after annealing in inert atmosphere.10,69 The lowest doping achieved for our samples corresponded to the Dirac point located at approximately 20 V (corresponds to the charge density of about 1.5 x 1012 cm-2) after annealing at 180 °C in nitrogen.

The response of the graphene towards adsorbed oxygen becomes more significant for the higher temperatures (figure 5a) as also observed in other studies.34,70 It was also suggested that oxygen molecules can be adsorbed at the graphene/substrate interface as well. Moreover the mobility – temperature measurements were performed in the range from 25 °C to 180 °C upon the constant flow of nitrogen. Obtained results showed decreasing mobility with increasing temperature (Figure 6) which is in agreement with previous results, where the graphene samples were heated


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in vacuum.58–61 The decrease of the mobility is commonly attributed to the temperature-enhanced phonon activity of SiO2 substrate.

Figure 6: The temperature dependence of charge carriers mobility of graphene measured upon constant flow of nitrogen. Each point of the plot is taken as an average of 60 experimental values. Error bars are included. Measurement started at the temperature of 180 °C, then temperature was decreased to 25 °C and increased back to 180 °C. Prior to measurement the GFET sample was kept at the temperature of 180 °C for 12 hours to initiate complete desorption of water and gas molecules and to stabilize the mobility values and the Dirac point position.

Exponential components of the fitted response corresponding to the fast and slow adsorption are displayed separately in Figure 7a. The fast component of the adsorption curve shows saturation in less than 5 hours however the slow adsorption process needs much longer time to reach the saturation (more than 24 hours). This agrees with the reversibility of the response towards short time (about one hour) oxygen exposures.


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Desorption of the oxygen was fitted well with superposition of two exponential terms for all the temperatures thus similar two-site mechanism can rationalize both desorption and adsorption processes. Desorption rate coefficients from the fits to the desorption curves were analyzed in the respect to the different temperatures (according to references39,71,72) and desorption energies were calculated according to Kd = νexp(-Ed/kT) for the fast and slow desorption process separately, where ν is frequency of oscillation of adsorbed particle. From the linear fits to the plotted values of ln(Kd) vs 1000/T (Figure 7b) energy E d of fast and slow desorption process was obtained as 215 meV/molecule and 450 meV/molecule, respectively. As expected, the energy of desorption Ed is higher for slow desorption. The fast desorption happens much more likely at certain temperature (the probability of desorption rate is higher for the process with lower activation energy). The values of oxygen adsorption energy to pristine graphene or graphite surface ranging from 39 to 150 meV were reported in the several experimental and theoretical studies. 48–54 We assume that the values of desorption and the adsorption energy, Ea and Ed, are similar.48 It is reasonable to expect that values of Ed calculated here from the fits of log(K) vs 1000/T (where K values are also obtained from the fits) are not perfectly accurate. Therefore the obtained value of desorption energy of oxygen from fast adsorption site (sp 2 carbon) of 215 meV could be served as comparable despite it is higher than all the values reported in the other studies. The desorption energy Ed from high-energy sites of 450 meV agrees well with the concept that this is still physical adsorption, and not the chemisorption (Ea < 1000 meV), but it can not be excluded that a part of adsorbed oxygen molecules can chemisorb to either defects or sp2 carbon, that’s why irreversible response curves were observed for long-time exposures to oxygen (Figure 3a). However, the value of Ed for high-energy sites is more than twice greater than Ed for low-energy sites. Moreover, the distribution of activation energies for adsorption/desorption process might


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be very wide for the high-energy sites. The value of Ed that was estimated is nothing more than the mean of activation energy distribution. The character (shape and width) of activation energy distribution could not be validated in this experiment. It is also not clear if slow adsorption occurs on all of various defects, impurities and oxygen containing groups or just on some of them.

Figure 7: a) Fit of the double exponential function to data (black) and its individual components, high-energy adsorption (red) and low-energy adsorption (blue), where c is the constant. Fit corresponds to the adsorption of 2.1 % of oxygen in nitrogen at the temperature of 150 °C. In the case of the fast adsorption oxygen molecules adsorb directly to the sp 2 carbon, while they adsorb to defects in the case of slow adsorption. b) Arrhenius plot for determination of desorption energies. Linear fits to the logarithm of desorption rate coefficients vs. 1000/T (including error bars).


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Conclusions

Transport characteristics of graphene field-effect-transistors were measured in-situ upon exposure to oxygen at different temperatures. Obtained results indicate that the oxygen adsorbed to the graphene caused significant doping observed via the shift of the Dirac point. The mobility of holes decreased significantly during fast adsorption that could be attributed to the adsorption of oxygen to low-energy adsorption sites. However, during slow adsorption of oxygen to highenergy adsorption sites, which are represented by structural and chemical defects, edges, impurities and oxygen containing functional groups, the mobility of holes didn’t change regardless the temperature in the range 25 °C to 150 °C. The response towards oxygen adsorption/desorption was fitted well by superposition of two exponential terms for all the temperatures which supports the concept of two different adsorption sites at the graphene surface. From the temperature dependent desorption rate coefficients the activation energy of oxygen desorption was calculated as 215 meV and 450 meV for low-energy and high-energy sites, respectively. The mobility dependence on temperature showed the same trend in the nitrogen atmosphere as reported for the vacuum.

AUTHOR INFORMATION Corresponding Author ∗ Corresponding author: Tel.: +420 26605 3804; e-mail: [email protected]. Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGEMENT The work was supported by the Czech Science foundation (18-20357S) and projects of the Czech Ministry of Education, Youth and Sports: LTC18039, NanoEnviCZ infrastructure project No. 2016-11-57 and project No. CZ.02.1.01/0.0/0.0/15_003/0000485. The authors are also grateful to Dr. J. Rathousky for fruitful discussions.

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Adsorption Site-Dependent Mobility Behavior in Graphene Exposed to

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