BCSJ Award Article

Stability of Graphene Oxide Film to Electron Beam Irradiationand Possible Thickness Dependence of Electron Attenuation

Tatsuya Sugimoto and Keisaku Kimura*

Department of Material Science, Graduate School of Material Science, University of Hyogo,3-2-1 Koto, Kamigori-cho, Ako-gun, Hyogo 678-1297

Received October 2, 2012; E-mail: kimura@sci.u-hyogo.ac.jp

Graphene oxide (GO), one of the best transparent substrates for electron microscopy of biological substances, isknown to be not very stable to exposure to electron beams (e-beams). We present a method of the preparation of GO filmhighly resistive to e-beams by controlling layer-by-layer thickness and quantitatively examined the stability of GO filmthus prepared. Scanning transmission electron microscopic measurements were engaged in these films with 10-kVacceleration-voltage. A simple method is proposed to classify the layer structure of GO. As an application of the method,we determined the electron attenuation length through GO film in nm scale.

Graphene, a single-atom thick carbon film, shows undoubt-edly extraordinary properties such as electric and mechanicalproperties thanks to its unique structure.1­6 Chemical reductionof graphene oxide (GO) is one route toward the large-scalefabrication of graphene-based devices. GO monolayer is alsoa single-atom thick two-dimensional carbon film and is usefulfor the support of biological tissues due to hydrogen bondingand can be used as a highly transparent support for electronmicroscopy.7,8 There are however few reports on the stability ofGO thin film9 especially for electron beam (e-beam) tolerance.Some GO films have been found to be tough and others fragiletoward e-beam irradiation. The conventional method reportedgives a mixture of these films. In addition, multilayer structuresoccasionally encountered in large-scale production10 prohibitfurther application as a transparent support for many purposes.To determine the layer structure of graphene or GO, Ramananalysis (intensity ratio and the position of resonance lines),11

transmission electron diffraction (intensity change by samplerotation and {1100} and {2110} contrast ratio),12 and high-resolution transmission electron microscopy (contrast analysisof edges)13,14 are often engaged in classifying layer structureas a useful tool. However, these techniques are hardly appliedfor three, four, or more layers. Atomic force microscopy15 andscanning tunneling microscopy16 are generally accepted fordetermining multilayer structure of graphene or GO, becauseof the ease and simplicity of the interpretation of experimentalresults. For use as a substrate for electron microscopy, it isdesirable to complete the characterization of GO with anelectron microscopic method.

Here we report a simple and widely applicable methodfor the evaluation of multilayer structure of GO or graphenetogether with a protocol for preparation of relatively stable GO

layers. By this method, we can quantitatively evaluate thestability of GO sheets to electron beam irradiation. The basictool is a contrast analysis of a scanning transmission electronmicroscopic (STEM) image under low acceleration voltage. Asapplication of the method, we evaluated the electron attenuationlength of GO in nm precision. Throughout the experiment, theLambert­Beer law for electron attenuation in bulk crystals wasfound not to be appropriate for ultimately thin materials. Asimple model describing the experimental results is presented.

Experimental

The basic preparation method for GO film follows that ofWilson et al.,7 which is a modified method of Hummers andOffeman17 and is described in Supporting Information 1. Weused Madagascar natural graphite flake as starting material. Ittook around one month to get the final GO colloidal suspensionfollowing Wilson’s method. We largely modified the fraction-ation process. The GO concentrated dispersion thus preparedwas black and after centrifugation at 3000 rpm for 10mindiscarded the precipitate. This process is very important toexclude various admixtures. We prepared seven dispersionswith different concentrations. The supernatant was diluted(brownish yellow) at an appropriate amount with MilliQ waterand centrifuged at 8500 rpm for 10min. The precipitate wasrecovered and diluted to give 3mL. The density of as-preparedstock dispersion of GO thus obtained was 0.59mgmL¹1. Thisstock dispersion was further diluted five times (no color) butperceptible by the scattering light induced by laser irradiation.Occasionally, the stock GO dispersion (pH ca. 2) was neu-tralized by addition of aqueous NH4OH solution up to giveneutral pH for a long-term storage for the sake of suppressionof the instability of the film.

© 2013 The Chemical Society of Japan

Published on the web March 5, 2013; doi:10.1246/bcsj.20120267

Bull. Chem. Soc. Jpn. Vol. 86, No. 3, 333­338 (2013) 333

Preparation of GO covered microgrids for STEM observa-tion was as follows. 2¯L of GO suspension was placeddropwise on a commercially available holy carbon covered Cugrid and allowed to dry within several minutes giving good GOthin film on micrometer-diameter holes. Normally, six Cu gridswere prepared for the STEM examination, because some wereunstable to e-beam irradiation. The general chemical propertiesof GO are outlined in the literature.18,19 e-Beam resistive GOfilm was prepared by the following procedure. All purifiedwater (MilliQ water) for dilution was deaerated once justbefore use and evaporated under anaerobic conditions such asin an atmosphere of argon. A field emission scanning trans-mission electron microscope (FESTEM) [Hitachi S-4800] wasused with acceleration voltage of 10 kV and 10¯A. Theseconditions were fixed throughout all experiments.

We used Image64 software to analyze the contrast of STEMimage and black level of STEM photos. In the image analysis,black corresponds to level 0 and white is level 255.20

Results and Discussion

It is very difficult to observe GO or graphene film directlyby transmission electron microscopy (TEM)13 because carbonis almost transparent with a conventional 200 or 300 kV TEM.There is no large contrast difference by TEM bright fieldobservation between vacuum and the film or among differentlayers13 so that we employed field emission scanning trans-mission electron microscopy (FESTEM). In general, carbonstructures are easily damaged by high-energy electron-beamirradiation. We noticed that there were two kinds of filmresponding to e-beam irradiation, easily damaged and toughfilms even at 10 kV acceleration voltage, hence we must checkeach film prior to use. The presented data are solely on toughfilms other than stability experiments. Nine data sets weresuccessfully obtained from more than tens trials with newlyprepared films following a conventional preparation technique.When diluted and dried under anaerobic conditions, the yieldof e-beam resistive GO was improved up to around 50% withcoverage of 10% mostly mono- and bilayer cover using therevised method. Generally, GO film showed amorphous elec-tron diffraction patterns, but some parts of GO film showedclear sixfold electron diffraction (shown in Supporting In-formation 2a) in accordance with reports from the literature.7,12

Other samples (Supporting Information 2b) revealed multiplespots manifesting rotational misfit. The majority of sampleswere found to be amorphous and very unstable to beamirradiation. The sixfold diffraction spots of fresh GO graduallyvanished when kept as a stock suspension several months in aglass bottle.

Figure 1 shows the time course of STEM image of weak (a)and tough (b) films for e-beam at 0, 10, and 60 s after exposureto the beam. Note that it is difficult to perceive the contrastchange for tough film even after 60-s exposure. A vague spotis highlighted in the circle. In contrast, the e-beam effect isobvious in the weak film (a). It seems that the reaction rate ofbeam damage on GO film follows first-order rate kinetics. Thefollowing rate equation is assumed, in which I0, and It are theblack level of the square part of Figure 1a at time zero and t. I¨is the hypothetical black level at t = infinity. The rate constantexpressed by k and r is the ratio of I¨ to I0.

It=I0 ¼ r � ðr � 1Þ expð�ktÞ ð1ÞWhen kt � 1 (initial stage of a tough film), the equation can beexpanded as

It=I0 ¼ 1þ kðr � 1Þt ð2ÞFigure 2 shows the plots together with the best fit based onthe above equations. The rate constants derived from the fittingare 0.002 s¹1 for tough film and 0.045 s¹1 for weak film,respectively. The difference in the rate constant between weakand tough GO is more than 20-fold. The origin of this differ-ence is ascribable to the composition of GO. Two facts supportthis conjecture. One is from the preparation of GO in aerobic orin anaerobic conditions. In general, the latter gave very strongGO film. The second is from the chemical nature of GO. GO is

(a) (b)

Figure 1. STEM images of a GO film at different timeinterval. Electron beam conditions; 10¯A, 10 kV accel-eration voltage at time 0, 10 and 60 s. Note that (a) centralsquare dark spots on weak GO at 10 and 60 s and (b) a faintspot in a circle at 60 s at tough GO. In case (b), there wasalmost no perceptible trace at 10 s by naked eye.

1

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1.4

1.6

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2

2.2

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0 10 20 30 40 50 60 70

gray

rat

io ratio(weak)

ratio(tough)

t /s

Figure 2. Time development of GO film damage byelectron-beam irradiation. Ordinate is gray ratio definedby summation of black point within a square area. Circle,weak GO; rectangle, tough GO. Solid and broken lines arefrom best-fit rate kinetics.

BCSJ AWARD ARTICLEBull. Chem. Soc. Jpn. Vol. 86, No. 3 (2013)334

prepared by partial oxidation of graphene by destroying theconductive network. Hence it is considered to be a mixture ofconductive and insulating moiety.18,19 The component ratio canbe further tuned by film treatment. It is known that insulatingvery thin film is easily broken by electron beam irradiation orcontamination with carbonaceous fragments ubiquitously ex-isting in the chamber. This may be the origin of enhancementof contrast and differences. Hereafter, we examine only beamresistive GO films for further analysis.

In Figure 3, two typical examples of monolayer (a) andmultilayer (b) GO films are shown. One of our aims of thisresearch was to get a large holy carbon grid covered withmonolayer GO for TEM or SEM observation as an idealsupport. In some cases however, we have multilayer GO asshown in (b). Note that clear contrast images are seen in thisfigure as designated by m (mono-), d (di- or double-), t (triple-),and q (quadruple-) layered film. Qualitatively to say, thisclear contrast difference is a key issue to distinguish the layerstructure of GO or graphene. In the past, Raman, TED or othermethods are mainly applied to determine the layer structure ofthese materials with the help of theoretical treatment as statedin the introduction other than STM, which gives us straight-forward interpretation. Clear contrast difference shown inFigure 3b also indicates that STEM images can be used tosimply classify the layer structure of GO. It is easy to under-stand that this contrast difference can be directly related to theelectron attenuation process in GO film. As application of ourtechnique, we have made a quantitative analysis of multi-layered structures, which gives us information on the electronattenuation length as a function of layer thickness at nm scale.The analysis is shown in Figures 4a to 4c. Figure 4 presentsa simpler image than that of Figure 3, which is good forexplanation of the procedure. Another example is also shown inSupporting Information 3. In Figure 4b, the rectangle region inFigure 4a is enlarged for Image64 analysis. The white regionon the upper right in (a) is a small hole (absence of GO) in aholy carbon grid. The result of the analysis is given in (c). Thecontrast difference between mono to double, and double totriple is almost equal up to 11 in the gray level but the dif-ference of the first layer to vacuum is 27, much larger than thatof other parts. It seems that some of the incident electrons arescattered at the topmost surface in addition to absorption

amount to 11. Taking the position of vacuum value as an origin,we plot the contrast difference from the value of each layer,say mono, di, triple, and quadruple layer as a function oflayer position in Figure 5, in which ordinate is converted to1� I=I0, hence the gray level difference of direct beam is 0and that of black is 255 in the ordinate scale.

Several points should be remarked on Figure 5. 1) The slopeof the region between 0 and 1 is larger than that of multilayerregion. Note that only this region includes surface effects. 2)A slope of the region mentioned above has strong sampledependence. 3) The slope of the multilayer region is almostconstant and independent of samples. The item 3 shows that

(b)

m

dtq

d

d

t

t

(a)

d

m

Figure 3. Typical examples of STEM images of monolayerGO (a) and multilayer GO (b: sample A3). Abbreviations:m, monolayer; d, double layer; t, triple layer; q, quadruplelayer, scale bar: 1¯m. Acceleration voltage: 10 kV.

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l diff

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double

triple

double layer

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vacuumtriple layer

(a)

(b)

(c)

Figure 4. Contrast analysis of STEM image of a GO film(sample D2-2). (a) 10 kV STEM image with a rectangleanalysis region that covers vacuum, monolayer, doublelayer, and triple layer GO. (b) Enlarged rectangle partrotated by 30 degree. (c) Contrast analysis of rectangle(b) along horizontal line (x axis). Ordinate is a differenceof the contrast given by gray level and origin is arbitraryshifted for clarity. Abscissa is a distance along withrectangle (b) whose length is 5¯m. Clear step structuresappearing in the figure are for vacuum portion of the holycarbon film, monolayer, double layer, and triple layer GOfilms from right to left.

T. Sugimoto et al. Bull. Chem. Soc. Jpn. Vol. 86, No. 3 (2013) 335

each layer absorbs electrons at the same amount irrespectiveof the layer position. This is a common feature for all GOfilms examined. It is known that the transmission probability ofelectrons, p for solid obeys an exponential relationship as afunction of the thickness of the sample.21

p ¼ exp½�ðN=vÞ·L� ð3ÞHere, N is the number of atoms in a volume v, · is the crosssection of electron scattering, and L is the sample thickness. Thereference value of exponent in eq 3, (N/v)·L, is sometimesrepresented as and, in which n is the number of layer of thinfilm, d is thickness of layer, and a is a proportional index. Thesame kind of equation is reported for the remaining unscat-tered electron intensity through solid materials.22 A exponentialdependence of the intensity of transmitted quanta (electrons orphotons) on the substrate thickness is a ubiquitous phenomenon.However the result indicates in Figure 5 seems to contradict toabove equation. A similar law in optical absorption spectros-copy of solution or amorphous materials is known as Lambert­Beer law, I=I0 ¼ exp½�kcL�23 where I is the intensity of lightpassing through a sample, I0 is the intensity of incident light, kis the absorption coefficient, c is the concentration of absorbatemolecules and is given by (N/v) and L is again the samplethickness. In a solution or amorphous substance, moleculesrandomly distribute. Therefore, c is not a rational number norinteger, but a real number. A typical procedure for exponentialdependence is starting from the following differential equa-tion,23 dI = ¹kcIdx, here, dx is an arbitrary small distance alongwith the incident light, dI is the difference of intensity throughdx and other notations are already defined. If c is a discretevalue, the equation does not hold. We note the fact that

exponential dependence is valid for random systems and notapplicable to a regular lattice in principle. Hence, eq 3 is correctfor random systems but not for a lattice. In the latter case, wemust reconsider the intensity variation with a unit replacementalong the x axis as in our sample conditions. We will show thiswith a simple phenomenologicalmodel as shown in Figure 6, inwhich electrons are normally incident to a lattice plane. If weneglect inelastic scattering, the sum of the forward scatteringand the backward scattering is held constant and the forwardscattering probability is approximately unity under our exper-imental conditions. This is not the case. Of the incident electronintensity I0, fraction fb of electrons is back scattered and sofraction 1 ¹ fb of electrons enters the crystals. In Figure 6,d is spacing of the lattice and the surface is designated by 0,monolayer as 1, and n-th layer is n. The electron intensitythrough monolayer is then given by I0 (1 ¹ fb)(1 ¹ ad), inwhich a is an electron attenuation coefficient and is equivalentto the inverse of electron attenuation length. After n-steppenetration, the intensity of electron beam is given by eq 4, as aresult of multi-scattering.

I=I0 ¼ ð1� fbÞð1� adÞn ð4ÞThis is apparently different from the Lambert­Beer law as ineq 3. However, it is possible to show that eq 4 can agree witheq 3 in the case of a bulk limit. Putting aside a factor (1 ¹ fb),(1 ¹ ad)n term leads to exp[¹aL] as following the definitionof the Napier constant when taking a limit of d approaches 0keeping a sample thickness L = nd being constant. Hence,the conventional exponential dependence as seen in eq 3 isrecovered in the bulk limit even for crystal. Physically, thiscorresponds to the transformation from a discrete lattice modelto continuum medium appropriate for bulk crystal. Conse-quently, we can expect the deviation from exponential law topower series in the nm region. The monolayer thickness ofgraphene or GO is 0.34 nm and can be a precisely controlledlayer-by-layer sequence.10,13 It may be possible to detect thisdeviation using GO by STEM experiments.

From quantum mechanical calculation, the number ofelectrons for elastic and inelastic single scattering in a crystalhas been reported as in the next equation with the number ofincident electrons, N0.24­26

Figure 5. Gray level difference plot as for the position of alayer. The notation on the abscissa, 0: at surface, 1: aftermonolayer, 2: after double layer, 3: after triple layer, 4:after quadruple layer. Note that the ordinate is 1 ¹ I/I0 inFigure 5 in the unit of gray level so that white is 0 andblack is 255 in this plot. The samples from seven differentlots were used starting from A to G. From each lot, weusually prepared six Cu grids and examined several field inone microgrid. For example, D2-2 means forth lot, secondgrid, and second microgrid field. The lowest lying singlestraight line is B1-3. Total number of successful exami-nations is nine.

d

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2

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(1-fb)

(1-fb)(1-ad )

(1-fb)(1-ad )2

(1-fb)(1-ad )n

L

I0(1-fb )

Figure 6. A phenomenological model of electron penetra-tion through lattice plane. Electron beam, the intensity ofwhich is I0, is coming onto a topmost surface of GO, fb is abackscattered fraction of electrons, d is monolayer thick-ness, a is the inverse of electron attenuation length. Theinteger figure on the right side is the layer number.

BCSJ AWARD ARTICLEBull. Chem. Soc. Jpn. Vol. 86, No. 3 (2013)336

N=N0 ¼ aL ð5ÞThe present experiment on the electron scattering in sub-nmregion supports eq 5, that is, single scattering takes in place inthin GO film. Equation 5 can be obtained from eq 3 or 4 in thelimit of aL � 1. This is also consistent with the phenomenonobserved in the present study being a characteristic of very thinfilm made of several atomic layers. Since a, the value obtainedfrom a slope of lines in Figure 5 is 0.134 nm¹1, is inverse ofelectron attenuation length mentioned before, the average of theexperimentally obtained attenuation length, 7.5 nm should becompared with literature values. There is no report on grapheneoxide in the literature to the authors’ knowledge. Instead forgraphite, it was reported to be 3.7 nm at 1 kV27 and 4.5 nm at10 kV.22 For bulk sample, there are always multiple scatteringprocesses and it is reasonable to guess that the bulk attenuationlength is much shorter than that of a single scattering process.Transition from a single scattering (linear relation on d) tomultiple scattering (exponential or power series on d) mayappear in the region of L > 1/a, namely thicker than 7.5 nm,which corresponds to n = 22 in eq 4.

The initial slope in Figure 5 leads to an estimate of the(1 ¹ fb) term in eq 4. The obtained slopes were strongly sampledependent and were scattered in the range 1.4 to 3 times largerthan the value in the multilayer region (ad = 0.0456). Since fbcan be determined by the initial slope, experimental values offb range from 0.067 to 0.136 with d = 0.34 nm. The literaturevalue for carbon28 is approximately 0.07 so the obtained valueis equal to or somewhat larger than the reference value. This isreasonable because the present sample is not pure carbon butmixed with oxide, which causes much scattering probability.This offers good diagnosis for the quality of GO film. Inaddition, the preparation method cannot control the oxidationstate on an atomic level. Hence the experimental values mustfluctuate from sample to sample. An exceptional case was foundon B1-3 among nine samples. Only one sample B1-3 indicatesthat fb is almost zero, which suggests that the wave number ofincident electrons satisfies the quantum condition for unscat-tered transmission, i.e., 100% transparency to this sample.

We must discuss here on one of our current concerns to findthe relation between eqs 3 and 4. It is easy to show thatthe difference of eq 3 from 4, that is, F(n) = exp(¹bn) ¹(1 ¹ b)^n > 0 always holds for 0 < b < 1, here b = ad =0.0456. Since F(0) = F(infinity) = 0 and F(n) is continuous,maximum F(n) must exist for special value of n. At the givenvalue b, n is determined to be 22. This value coincidentallycoincides with the transition number from single scattering tomultiscattering. Maximum deviation between eq 3 and eq 4 atn = 22 is 0.009, about 0.9% of initial value 1. Beyond n = 22,the difference approaches zero with incremental increase of n.For example at n = 200, that corresponds to a thickness of68 nm, the disagreement comes to 2 © 10¹3% and practicallyno difference between exponential to power dependence, thatis to say, the system can be said to be bulk.

Conclusion

In this paper, we reported that graphene oxide thin filmscan be classified into two categories according to the stabilityto electron beam irradiation and presented a protocol for the

preparation of beam resistive films. We also determined theelectron attenuation length of graphene oxide by virtue ofSTEM method by lattice-to-lattice step. It was found that asingle scattering process was the major path for the electronscattering in the film whose thickness is less than ten layersunder 10 kV acceleration voltage. The crossover of single tomultiple scattering events was estimated to occur at 7.5 nm-thick (22 layers). A simple argument shows that we have powerdependence for the transmitted electron intensity as a functionof sample thickness instead of the conventional exponentialfunction for very thin film. The difference in the expressionfor electron penetration intensity by a power series or byexponential was estimated to be less than 1% at 7.5 nm.

Supporting Information

A protocol for the preparation of graphene oxide (GO) filmsuspension. Transmission electron diffraction of GO. Contrastanalysis for sample B2-2. This material is available free ofcharge on the web at http://www.csj.jp/journals/bcsj/.

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