Graphene oxide windows for in situ environmental cell photoelectron spectroscopy

Graphene oxide windows for in situ environmentalcell photoelectron spectroscopyAndrei Kolmakov1*, Dmitriy A. Dikin2, Laura J. Cote2, Jiaxing Huang2, Majid Kazemian Abyaneh3,

Matteo Amati3, Luca Gregoratti3, Sebastian Gunther4 and Maya Kiskinova3

The performance of new materials and devices often depends on processes taking place at the interface between an activesolid element and the environment (such as air, water or other fluids). Understanding and controlling such interfacialprocesses require surface-specific spectroscopic information acquired under real-world operating conditions, which can bechallenging because standard approaches such as X-ray photoelectron spectroscopy generally require high-vacuumconditions. The state-of-the-art approach to this problem relies on unique and expensive apparatus including electronanalysers coupled with sophisticated differentially pumped lenses. Here, we develop a simple environmental cell withgraphene oxide windows that are transparent to low-energy electrons (down to 400 eV), and demonstrate the feasibilityof X-ray photoelectron spectroscopy measurements on model samples such as gold nanoparticles and aqueous saltsolution placed on the back side of a window. These proof-of-principle results show the potential of using graphene oxide,graphene and other emerging ultrathin membrane windows for the fabrication of low-cost, single-use environmental cellscompatible with commercial X-ray and Auger microprobes as well as scanning or transmission electron microscopes.

The requirement for high-vacuum conditions in conventionalelectron spectroscopy studies such as X-ray photoelectronspectroscopy (XPS), Auger electron spectroscopy (AES) and

electron energy loss spectroscopy (EELS) imposes a major obstaclefor analysing interfacial physicochemical processes under ambientconditions. Consequently, there has been a persistent trend,mainly in the field of heterogeneous catalysis and environmentalremediation, to overcome this so-called ‘pressure gap’ and observeprocesses taking place at surfaces and interfaces under operandoconditions, namely elevated or ambient gas pressure or liquidenvironments (see recent highlights in refs 1 and 2, and referencestherein). During the last three decades, tremendous progresshas been made regarding high-resolution transmission electronmicroscopy (HRTEM) coupled with the analytical power of EELSand X-ray fluorescence spectroscopy, using so-called open environ-mental cells (E-cells) (see reviews in refs 3–5). Since the 1930s,elevated pressure has been used in environmental studies usingTEM and scanning electron microscopy (SEM) by implementingnon-permeable windows (originally made from aluminium6,collodion7,8 or amorphous carbon9). Later, microfabricated SiO2,Si3N4 or 10–100-nm-thick polymer membranes were used,which are highly transparent to the commonly used 10–300 keVelectrons and 1 × 103 to 1 × 104 eV X-rays. This simplifiedclosed-cell design, in combination with a variety of commerciallyavailable microfabricated membranes, have boosted activity inenvironmental electron microscopy10–13 and X-ray spectromicro-scopy research14,15.

However, it still remains a great experimental challenge to adaptpowerful, surface sensitive tools such as XPS or AES—the action ofwhich is based on the detection of emitted electrons with relativelylow kinetic energy (1 × 102 to 1 × 103 eV) to explore objects atambient conditions. This is mainly due to the rather short electroninelastic mean free path (IMFP), lIMFP, in dense media, which is ofthe order of a nanometre in condensed matter (Fig. 1) and 1 mm ingases at atmospheric pressure.

These main impediments have been successfully resolvedthrough the development of an electron energy analyser with differ-entially pumped lens systems, thus decoupling the spectrometerfrom the high-pressure ambient conditions of a sample placedvery close to the analyser entrance slit (Fig. 1a; see recent reviewin ref. 16 for details, and references therein). Alternatively, liquidmicrojets17 and droplet ‘trains’ methods18 have also beenimplemented to probe analytes inside highly volatile solvents. Inspite of their tremendous success, these state-of-the-art instrumentsare currently accessible only in a few laboratories and synchrotronfacilities16,17,19, which limits the span of the research to a smallnumber of users.

Very recently, protocols for the high-yield fabrication of ultrathin(�1–5 nm) suspended membranes made of graphene, grapheneoxide (GO) and other two-dimensional materials have been devel-oped20–25. Owing to their superior mechanical stability, grapheneand GO membranes appear to show great promise as low-atomic-number Z support materials for HRTEM studies26,27. Togetherwith the pioneering gas impermeability and elastic studies of gra-phene membranes by Bunch and colleagues28, more recent reportsin graphene29,30 and similar tests made on GO31,32 have demon-strated that graphene derivatives (such as GO) are robust enoughto be used as membranes for environmental cells, the interior ofwhich can be filled with gas or liquid at atmospheric pressure.Provided such ultrathin membranes are sufficiently transparent to1 × 102 to 1 × 103 eV electrons, this will provide a new route tothe fabrication of the next generation of E-cells, allowing XPS,AES and possibly photoemission electron microscopy (PEEM)and spectroscopy of fully hydrated samples, liquid–graphene (orGO) interfaces or samples under ambient pressure, as illustratedin Fig. 1b,c. To the best of our knowledge, this Article reportsthe first experimental demonstration that even few-monolayer(ML)-thick GO membranes are transparent enough to relativelylow-energy (1 × 102 to 1 × 103 eV) electrons, and can therefore beused as windows for environmental in situ XPS studies.

1Southern Illinois University, Carbondale, Illinois 62901, USA, 2Northwestern University, Evanston, Illinois 60208, USA, 3Sincrotrone Trieste 34012 Trieste,Italy, 4TU Munchen, Chemie Department, Lichtenbergstr. 4, D-85748 Garching, Germany. *e-mail: [email protected]

ARTICLESPUBLISHED ONLINE: 28 AUGUST 2011 | DOI: 10.1038/NNANO.2011.130

NATURE NANOTECHNOLOGY | VOL 6 | OCTOBER 2011 | www.nature.com/naturenanotechnology 651

© 2011 Macmillan Publishers Limited. All rights reserved.

mailto:[email protected]

http://www.nature.com/doifinder/10.1038/nnano.2011.130

www.nature.com/naturenanotechnology

Fabrication and characterization approachesDetails of GO fabrication, E-cell design, tests and XPS measure-ments are described in the Methods and Supplementary SectionS1. Briefly, the GO sheets were produced by a modifiedHummer’s synthesis33 and purified by multiple centrifugationsteps. For effective attenuation length (EAL) measurements, GOsheets were deposited on a silicon wafer pre-covered with a 20 nmgold conducting layer. For fabrication of the front window of theE-cells, commercial 50–100-nm-thick Si3N4 or SiO2 primary mem-branes were used (Fig. 2).

To ensure the mechanical stability of the suspended GOwindows, small 3–10 mm holes were milled in the primary mem-branes using a focused ion beam (FIB, Fig. 2a), and individual�100–1,000 mm2 GO sheets were deposited over these orificesusing a Langmuir–Blodgett technique (Fig. 2b,c)34. To carry outfeasibility tests for the XPS measurements through GO membranes,we used the following samples: (1) 50 nm gold unconjugated colloidnanoparticles drop-cast from a water solution onto the back side ofthe GO membrane (Fig. 2d,e); (2) a thin 5–10 nm gold film depos-ited via d.c. sputtering directly on the back-side surface of the free-standing GO membrane; (3) a 3 M NaI aqueous solution filling theback side of the membrane. To obtain a vacuum-compatible E-cell,in the third sample, a second silicon wafer was placed over the GOwindow containing the droplet of NaI solution. The entire assemblywas sealed with ultraviolet-curable adhesive (Fig. 2f ). Before

ultrahigh-vacuum (UHV) measurements, the mechanical integrityand vacuum compatibility of the sealed cells were tested in an inter-mediate-vacuum interlock chamber. Owing to the few-micrometresize of the GO windows (Fig. 2g,h), in situ photoelectron spec-troscopy measurements with such a cell require microprobe XPSand a sufficiently intense photon source provided by the scanningphotoelectron microscope (SPEM) operated at ELETTRA35. InSPEM, the X-ray beam is focused onto a spot with a diameter of�100 nm using Fresnel zone plate optics. The local XPS spectra(as well as chemical, topology and conductivity maps) of the(nano-) mesoscopic sample can be obtained by raster-scanningthe sample with respect to the focused beam while simultaneouslymonitoring emitted photoelectrons, transmitted photons and/orthe beam-induced sample photocurrent36.

Electron transparency tests of GO layersFor spectroscopic and signal attenuation tests, we selected Au 4fcore levels. These levels provide the advantageous combination ofappreciable photo-ionization cross-section (�1–4 Mbarn) for exci-tation with soft X-rays (650 and 777 eV were used in the presentstudy) and relatively low binding energy (Eb¼ 83.9 and 87.6 eVfor 4f5/2 and 4f7/2, respectively), which means that the kineticenergy of the emitted Au 4f photoelectrons is high enough for pen-etration through the GO membrane. The principle and geometry ofthe experimental setup using GO layers on a gold/silicon substrate

a b

X-rays

Ultrathinmembrane

Sample

Sample

Pum

ping

X-rays

Gas

Photoelectrons

Electronenergyanalyser

λ IM

FP (n

m)

1

100 1,000Energy (eV)

c

Figure 1 | Design of ambient-pressure XPS systems. a, Combination of differential pumping stages and advanced transfer electron optics. For details see the

recent review in ref. 16. b, An alternative design uses an E-cell with a membrane that is transparent to electrons, but cannot be penetrated by molecules.

c, Dependence of inelastic mean free path (IMFP) for electrons in carbon on their kinetic energy, calculated using the NIST SRD-71 database38. The shaded

light-yellow area corresponds to the kinetic energy of photoelectrons capable of escaping 1-nm-thick membranes (analogous to GO single-layer membranes).

ARTICLES NATURE NANOTECHNOLOGY DOI: 10.1038/NNANO.2011.130

NATURE NANOTECHNOLOGY | VOL 6 | OCTOBER 2011 | www.nature.com/naturenanotechnology652




is presented in Fig. 3a. The general morphology of the Langmuir–Blodgett-deposited GO layers on the gold/silicon substrate wasinspected optically, with SEM and atomic force microscopy(AFM). The average lateral dimension of the individual GO flakeswas of the order of 20 mm, and folding, overlapping and wrinkle for-mation were commonly observed. Analysis of the AFM images(similar to Fig. 3b) indicates that the majority of the flakes aresingle-layer-thick individual GO sheets. From the Z-height histo-gram (Fig. 3c) taken for the representative square area in Fig. 3b,the thickness of the individual GO layer can be inferred to bed1 ≈ 1.0+0.07 nm. The discrete electron transparency for GOsheets of different thicknesses is illustrated by the three-dimensional(3D) Au 4f intensity map in Fig. 3d, in which patches of single,double, triple (and so on) GO layers can be identified easily.

Figure 4a depicts the same 25.6 × 25.6 mm2 Au 4f image in atwo-dimensional (2D) representation, where the variations of thegrey scale reflect the Au 4f signal attenuation by the multiplyfolded and overlapping GO layers covering the gold thin film. TheAu 4f spectra in Fig. 4b, collected at selected locations withknown thickness (number of GO overlayers, n¼ 0, 1, 2, 3. . .),quantitatively support the information obtained by the Au 4fimages. The Au 4f spectra line shape corresponds to metallic goldand, as expected, the Au 4f intensity drops exponentially withincreasing number of covering GO sheets due to inelastic photo-electron scattering. The quantification of the attenuation effect is

commonly obtained from the ratio In/I0, where In and I0 are theAu 4f intensity attenuated by n GO layers and from a bare goldsubstrate (n¼ 0), respectively. The log In/I0 plot versus thenumber of GO layers n on top of the gold substrate in Fig. 4cshows an exponential dependence with rather small standarddeviation. Using the standard relationship of the straight-lineapproximation (SLA)37 for the attenuated Au 4f signal in n GOlayers due to inelastic electron scattering we obtain

In

I0

= exp −d/lIMFP · cos u( )

(1)

where d is the thickness of the GO overlayer, lIMFP is the inelasticmean free path of the Au 4f electrons in GO and u is the emissionangle with respect to the sample normal. By substituting lIMFP withthe effective attenuation length lEAL to account for elastic scatteringeffects (see definitions in ref. 38), one can obtain the lEAL valuefor GO. However, unlike graphene, the reported experimentaldata for the GO monolayer thickness dGO scatter significantlyfrom 0.35 nm to 1.7 nm. The major reasons for this scattering aredGO variations likely resulting from different degrees of thermal orchemical reduction of the film25,39 and intrinsic surface roughness40.In addition, the X-ray beam could partially reduce GO as it canundergo a de-oxygenation reaction upon gentle heating41.Therefore, instead of dGO, it is more feasible to measure the ratio

h

GO

b c

f

Si

Sealant

g Sealed membrane

i

GO

SiO2

aFIB

Si

SiO2GO

H2Od

e

100

Au

AuC

O

50

02 4 6 8 10

(keV)

Inte

nsity

(a.u

.)

SiO2 membrane

GOmembrane

AuNP

3 µm

Figure 2 | Fabrication of GO windows for E-cells. a, Ion-beam milling of the microhole in the primary Si3N4 (or SiO2) membrane. b, Deposition of the GO

suspended secondary membrane over the ion-milled hole using the Langmuir–Blodgett approach. c, SEM image of the resultant deposit (scale bar, 20 mm).

d, Deposition of the metal colloid nanoparticles on the back side of the membrane. e, SEM image and EDX of gold nanoparticle aggregates made through the

GO membrane (scale bar, 3 mm). f, Enclosed E-cell with ultraviolet-curable sealant placed along the perimeter of the silicon wafer containing the primary

Si3N4 and secondary GO membrane. g, Magnified (×50) optical image of the front membrane sealing the E-cell filled with water solution. h, Magnified

(×1,000) optical image showing the GO membrane sealing the window in the filled E-cell. i, TEM image (before sealing) of the same GO membrane as in g

and h. Areas with increased numbers of GO sheets can be seen as darker patches (scale bar, 1 mm).

NATURE NANOTECHNOLOGY DOI: 10.1038/NNANO.2011.130 ARTICLES





lEAL/dGO, which can be obtained from the experimental values ofI0/In from the SLA-derived expression

lEAL

dGO= n · cos u · ln

I0

In

( )( )−1

(2)

The best fit gives lEAL/dGO¼ 2.53+0.05, where only the unam-biguous number of the GO layers (n , 5) is used (see red line inFig. 3c). Taking the AFM estimated single GO layer thickness,dGO ≈ 1 nm, the effective attenuation length for 567 eV photo-electrons and u¼ 308 is estimated as lEAL ≈ 2.5 nm.

Probing the model samples through the GO membraneThe enhanced transparency of the GO for photoelectrons has beenconfirmed by the ability to collect XPS spectra and images fromsamples placed on the back side of suspended GO membranes.Owing to the appreciable lEAL value, this shows that GO membranewindows consisting of one to four layers can be used for quantitative

in situ XPS. Comparing the data obtained with the two types of goldsamples, the photoelectron signal from the percolating gold nano-particle layer evaporated directly on the back side of the GO mem-brane is much higher than one from the drop-cast colloidal gold,most likely due to the presence of citrate molecular layers at theinterface between the gold colloid nanoparticles and membrane inthe latter case. These comparative experiments indicate that forthis technique the intimate proximity of the tested sample to themembrane surface is critical. The SEM image in Fig. 5a shows anindividual GO flake covering a hole in a SiO2 primary membrane.The TEM/HRTEM images in Fig. 5b,c depict the same membraneafter deposition of a few nanometres of gold onto the back side ofthe membrane. The morphological electron microscopy data werecomplemented with SPEM chemical mapping of the same mem-brane, shown in Fig. 5d,e. Comparing the TEM/HRTEM andSPEM images before and after gold deposition indicates that (1)similar to graphene, a few-ML-thick GO membrane can be con-sidered a promising sample support for TEM/HRTEM imaging;

0

1

3

2

XPS Au 4f map

a b

d

AFM map

XPSanalyser

X-rays

GO

C 1s

Au 4fAu

30°

30°

c

−2 −1 0

200

100

01

1.86

1.040.00

−1.02

2 3Z height (nm)

Num

ber o

f eve

nts

Au

subs

trat

e

1 lay

er

2 la

yers

3 la

yers

Figure 3 | Experimental setup and GO overlayer characterization. a, Sketch of the experimental setup for photoelectron imaging and electron attenuation

experiments. b, Non-contact AFM topology image of GO deposit on the gold/silicon substrate. Scale bar, 2.5 mm. c, Z-height histogram taken for the square

area in b. d, 3D Au 4f map of the gold sample, where discrete colour (Au 4f signal intensity) changes correspond to different local GO thicknesses.

a b c120 × 103

100Au

13

280

60

40

20

0

Inte

nsity

(cou

nts

s–1)

90 88 86 84 82Binding energy (eV)

Au 4f5/2 4f7/2

−2

Slope: −0.457ln(In/I 0

)

−1

0

−3

−4

0 2 4 6 8 10GO thickness (ML)

0

1

2

3

9L

Figure 4 | Principle of effective attenuation length measurements of GO sheets. a, 2D Au 4f map of the gold sample, with the grey scale contrast (Au 4f

signal intensity) changes corresponding to different local GO thicknesses. b, Microspot Au 4f spectra taken at different locations depicted as white circles

in a. The spot corresponding to 9-ML-thick GO is beyond the perimeter of a. c, Log plot of Au 4f intensity attenuation as a function of the number of GO

overlayers. Data point sets were taken from different areas of independently prepared samples, and the slope of the fitting line defines –(lEAL.cosu)21.






(2) the GO membrane preserves its mechanical integrity even afterbeing loaded by up to few-nanometre-thick metal layers orprefabricated nanostructures.

The TEM (Fig. 5b) and HRTEM (Fig. 5c) images reveal the mor-phology of a gold deposit composed of percolating single-crystalnanoparticles, typical for a Volmer–Weber-type growth mode ofmetals on weakly interacting substrates, such as graphite or metaloxides. The HRTEM image and selected area electron diffraction(SAED) pattern of the GO/gold ‘sandwich’ in Fig. 5c are typicalfor randomly oriented gold nanoparticles with major latticefringes indexed to a polycrystalline gold sample with face-centredcubic (fcc) crystallites. SPEM data in Fig. 5d–f provide complemen-tary information about the chemical status of the gold specimensurface and the interface with the GO window. For this membrane,an effective thickness of �3–4 ML was estimated, based on theIC1s/IAu4f ratio recorded for GO layers with known thickness. Thequality of the XPS survey and Au 4f and C 1s spectra taken at differentlocations (Fig. 5f) is sufficient to identify the chemical status of thesample on the back side of the membrane. As expected, the fit ofthe Au 4f doublet corresponds to unreacted metallic gold. The decon-voluted C 1s spectrum of the membrane contains low weight of theC¼O, C–OH components. They resemble the C 1s spectra of ther-mally reduced GO42, presumably as a result of oxygen desorptioninduced by prolonged electron or X-ray irradiation during themeasurements. The Au 4f7/2 (Fig. 5d) and C 1s (Fig. 5e) chemicalmaps of the membrane indicate certain inhomogeneities in the

gold and carbon distribution across the membrane area, presumablydue to radiation damage. As is shown in Supplementary Section S2,the long-term radiation effects are not of crucial importance providedthe ultrathin membranes are designed as inexpensive single-useE-cells for in situ XPS characterization.

The feasibility of conducting photoelectron spectromicroscopyof liquid media was tested by filling the E-cell compartment witha 3 M NaI aqueous solution, and subsequently vacuum sealing itusing the procedure described in the Methods. It was found thatfew-nanometre-thick membranes are capable of maintaining theliquid and/or elevated-pressure water (vapour) for an extendedperiod of time, sufficient for routine XPS studies (SupplementarySection S1). Some interfacial diffusion between the GO sheets andthe primary silicon nitride membrane was observed in only a fewcells after long experiments. Similar phenomena, namely interfacialmass transport and trapping of water layers, have been reportedrecently30,43. Therefore, the vacuum tightness of the E-cell frontside is determined mainly by the initial adhesion of the GO sheetto the Si3N4 window, which can be improved by moderatethermal annealing. These samples sustained UHV conditions forat least a few hours of continuous measurements. Figure 6 showsrepresentative O 1s spectra taken at the points indicated in theAu 4f7/2 images, namely in the centre of the ‘wet’ hole (point A),where the GO membrane isolates the 3 M NaI aqueous solutionfrom the vacuum, and in the ‘dry’ GO covered area �30 mm awayfrom the hole (point B). The O 1s spectrum from the ‘dry’ GO on

a b

d

f

e

c

90 88 86Binding energy (eV)

Binding energy (eV)Binding energy (eV)

Inte

nsity

(s−1

)84 82

Au 4fAu 4f7/2

7/2

5/2

290 288 286 284

ABC

C 1s

C 1s

B

A

C

O=C

−OH

C=O

C−O

H

C−C

, C=C

3.0 × 104

2.5

2.0

1.5

1.0

0.5

0.0300 250 200 150 100

GO

hν = 650 eV

ABC

ABC

10 µm200 nm

10 1/nm

1 µm

Au

Figure 5 | Suspended GO membranes as windows for an E-cell. a, SEM image of the suspended membrane before gold deposition. b,c, TEM/HRTEM

images after deposition of a few nanometres of gold on the membrane back side. The SAED pattern in c shows a typical diffraction pattern from gold

deposit, characteristic for randomly oriented fcc gold nanoparticles. d,e, Au 4f7/2 and C 1s chemical maps of the GO membrane with percolating gold

nanoparticles on the back side. f, Survey (bottom right), C 1s (bottom left) and Au 4f (top) photoelectron spectra taken at locations A, B and C in e.






gold-covered SiN is similar to the standard O 1s XPS spectrum ofpartially reduced GO39,42, with a small contribution from H2O pre-sumably trapped at the interface or between the GO sheets and thesubstrate43. The O 1s spectrum of the ‘wet’ area filled with the sol-ution is very different, apparently due to pronounced componentscorresponding to the liquid and vapour phases of water. The conco-mitant XPS features in the region of iodine 4d, sodium 2s andsodium 2p corresponding to the presence of I2 and Naþ solvatedions (not shown here) appears to be too weak to be unambiguouslyassigned, presumably due to the low ion concentrations and a rela-tively thick GO membrane. However, the presented data are a proofof concept for environmental electron spectroscopy in membrane-based E-cells. Further developments are under consideration toimplement the proposed technique to study the interfaces underoperando conditions.

The feasibility of using the thinnest (5 nm) commercially avail-able SiN membranes as windows for XPS-compatible E-cells isdescribed in Supplementary Section S3.

ConclusionsIn conclusion, we have demonstrated that GO membranes are a verypromising window material for E-cells to enable XPS studies ofsamples immersed in liquid or dense gaseous media. Our proof-of-concept experiments proved that up to several layers suspendedGO membranes have sufficient transparency for photoemittedelectrons with kinetic energies .450–500 eV. Good-quality XPSspectra can be obtained from aqueous solutions and nanoparticlesdeposited on the back side of the GO membranes. Monitoring theattenuation in the Au 4f electron emission (567 eV electron

kinetic energy) due to one- to nine-layered GO membranes coveringthe evaporated gold film, the effective attenuation length in the GOmatrix has been estimated as �2.5 nm for an emission angle of 308.The results in this Article indicate that, in addition to their uniqueelectronic, thermal, magnetic, mechanical and other properties, therecently demonstrated 2D and quasi-2D crystals, such as graphene,GO, BN, MoS2, NbSe2, Bi2Sr2CaCu2Ox , Bi2Te3 (and many others),appear to be an ideal platform for experimental XPS tests of inter-facial, electron transport and inelastic phenomena in solids,because the morphology and the thickness of such an overlayercan be precisely determined.

From the broader application prospective, ultra-transparent androbust graphene/GO windows open the door to unprecedentedopportunities for in situ electron spectroscopy in operando con-ditions. The development of large-yield protocols for the fabricationof suspended graphene and GO membranes is a current agenda formany groups44 and has already reached a commercialization stage45.In contrast to the current environmental XPS methodology, basedon sophisticated instrumentation using synchrotron radiationsources, these low-cost single-use E-cells can potentially be usedwith any commercial micro-XPS or AES instrument, routineSEM/TEM or optical microscopes.

MethodsGO synthesis and purification. GO was prepared from graphite powders (Baycarbon, SP-1) using a modification of Hummers method33. Graphite (2 g), NaNO3(1 g) and 46 ml of H2SO4 were stirred together in an ice bath. KMnO4 (6 g) was thenadded slowly. All chemicals were purchased from Sigma-Aldrich and were used asreceived. Once mixed, the solution was transferred to a 35 8C water bath and stirred for�1 h, forming a thick paste. Water (80 ml) was then added, and the solution wasstirred for 1 h while the temperature was raised to 90 8C. Finally, 200 ml of water wasadded, followed by the slow addition of 6 ml H2O2 (30%), causing the solution to turnfrom dark brown to yellow. The warm solution was then filtered and washed with threealiquots of 200 ml 10% HCl, followed by 200 ml water. The filter cake was dispersedin water by mechanical agitation and stirred overnight. The dispersion was allowed tosettle and the top clear, yellow dispersion was subjected to dialysis for 1 month.

Langmuir–Blodgett assembly of GO. See ref. 34. The as-prepared GO was diluted toa 5:1 methanol/water solution. The trough (Nima Technology, model 116) was carefullycleaned with chloroform and then filled with deionized water (DI). GO solution wasspread dropwise onto the water surface using a glass syringe, to a total volume of10–15 ml, and the film was then allowed to equilibrate for at least 20 min. The substrateswere vertically dipped into the trough and slowly pulled up (2 mm min21) aftercompressing the film to form a dense GO monolayer at the air–water interface.

Samples and cells preparation. Most of the experiments were carried out usingcommercial (SiMPore Inc/TEMwindows) 200-mm-thick frames with 50-nm-thick100 × 100 mm2 Si3N4 (or 40 nm SiO2) windows as primary membranes. Windowsfor GO deposition were fabricated by ion beam milling of the primary membraneusing an FEI Helios NanoLab Dual Beam SEM/FIB. To eliminate charging, thesurface of the primary membranes was coated with �10 nm of gold before GOdeposition. Three different types of samples were used in the study. The first hadunconjugated monodispersed 50 nm gold colloids (Tedd Pella) drop-cast onto theback of the membrane and dried. In the second, to evaluate the transparency of thefreestanding GO membranes, 5–10 nm of gold was d.c. sputtered onto the backsideof the GO membrane. To protect the front side of the membrane from unintentionaldeposition of gold during sputtering, the periphery of the front side of themembrane was ‘sealed’ using a ring made of Gel-Film. In the third sample, 3 M NaIaqueous solution in double-distilled (DD) water: �1 × 1022 ml of analyte was filledin the compartment of the primary membrane and a second silicon wafer was placedover the filled compartment to create a sealed E-cell. The excess water was removedand the entire assembly sealed using ultraviolet-curable Norland optical adhesiveNOA-81. A strip of conducting silver paste was drawn between the front side of thecell and the metal support to ground the front part of the primary membrane. Such acell was compatible with the typical vacuum (1 × 1029 torr) of the main chamber.

Imaging and spectroscopy. SEM imaging was performed with an FEI Nova 600NanoSEM in reflection mode by collecting secondary electrons, and also intransmission mode using a two-segment solid-state STEM detector with eitherbright- or dark-field electron collection. The routine imaging conditions were anacceleration voltage of 10 kV, beam current of �1 × 10211 A, and an electron doseof �1 mC cm22. TEM imaging was carried out with a JEOL JEM-2100F FAST TEMusing an acceleration voltage of 200 kV.

AFM images were recorded using a Park-XE100 (Park Systems Corp)microscope. The scanning probe microscope (SPM) was equipped with a silicon

Inte

nsity

(a.u

.)

−540 −535 −530Binding energy (eV)

Point A

Point B

O 1s

C=O,C−OH, SiO2O=C−OH

H2OLIQH2OVAP

B

Au 4f

A

Figure 6 | XPS on wet samples. Top inset: 40 × 50 mm2 SPEM image of the

GO membrane with 3 M NaI aqueous solution on the back side taken with

Au 4f7/2 photoelectrons. The bottom inset is an enlarged 12× 12mm2 area

around the membrane. Point A is at the centre of the GO membrane, and

point B is �30mm away. Top and bottom spectra were taken at points B

and A, respectively. In addition to the common O 1s spectrum from the dry

area, the spectrum from the membrane contains H2O vapour and liquid

contributions from the enclosed cell compartment. The red dots indicate

the raw data points of the O 1s spectra, and the small dark spots represent

the interpolated spectrum used for peak deconvolution analysis. The green

curve depicts a sum of the fitted peaks. The fitting details can be found

in the Methods.






cantilever (DP15 from MikroMasch) with a force constant k¼ 40 N m21 and acurvature radius r¼ 10 nm. The AFM was operated in non-contact mode at aresonance frequency of f¼ 270 kHz. Image scanning was performed with a speed of0.5 Hz, which corresponds to one line per second. SPEM measurements were carriedout at the ESCA microscopy beamline at the ELETTRA synchrotron facilities. Theincident soft X-ray photon beam (200–1,400 eV) was focused to a small spot with adiameter of �100 nm at the sample surface. The sample could be scanned with anaccuracy of 10 nm in front of the beam. A Fresnel zone plate was used to demagnifythe source into a microprobe. The measured flux in the probe was 1 × 109 to1 × 1010 ph s21. For XPS spectra and imaging, a hemispherical 100 mm energy analyserwith a 48-channel detector and an energy resolution better than 200 meV was used.XPS spectra analysis and image data were processed using IGOR Pro software. Forquantitative intensity comparisons, peak areas were determined after Shirleybackground subtraction. The O 1s spectra were fitted assuming a Doniach Sunjic lineshape of low asymmetry (a¼ 0.02), a Lorentzian linewidth of 0.25 eV and a Gaussianlinewidth ranging between 1.8 and 2.6 eV and subtracting a linear background.The peak positions of the indicated species were 532.0–532.5 eV (C¼O, C–OH, SiO2,O¼C–OH; that is, species belonging to the membrane), 533.6 eV (H2O liquid) and536.8 eV (H2O vapour).

Received 21 April 2011; accepted 14 July 2011;published online 28 August 2011

References1. Bluhm, H. et al. In situ X-ray photoelectron spectroscopy studies of gas–solid

interfaces at near-ambient conditions. MRS Bull. 32, 1022–1030 (2007).2. Somorjai, G. A. & Park, J. Y. Molecular surface chemistry by metal single

crystals and nanoparticles from vacuum to high pressure. Chem. Soc. Rev. 37,2155–2162 (2008).

3. Parsons, D. F. Structure of wet specimens in electron microscopy. Science186, 407–414 (1974).

4. Boyes, E. D. & Gai, P. L. Environmental high resolution electron microscopy andapplications to chemical science. Ultramicroscopy 67, 219–232 (1997).

5. Sharma, R. & Weiss, K. Development of a TEM to study in situ structural andchemical changes at an atomic level during gas–solid interactions at elevatedtemperatures. Microsc. Res. Tech. 42, 270–280 (1998).

6. Marton, L. La microscopie electronique des objects biologiques. Bull. de L’Acad.Royale de Belgique 21, 553–560 (1935).

7. Abrams, I. & McBain, J. A closed cell for electron microscopy. J. Appl. Phys.15, 607–609 (1944).

8. Swift, J. & Brown, A. An environmental cell for the examination of wet biologicalspecimens at atmospheric pressure by transmission scanning electronmicroscopy. J. Phys. E 3, 924–926 (1970).

9. Kohyama, N., Fukushima, K. & Fukami, A. Observation of hydrated form oftubular halloysite by an electron microscope equipped with an environmentalcell. Clays Clay Miner. 26, 25–40 (1978).

10. Daulton, T. L., Little, B. J., Lowe, K. & Jones-Meehan, J. In situ environmentalcell transmission electron microscopy study of microbial reduction ofchromium(VI) using electron energy loss spectroscopy. Microsc. Microanal.7, 470–485 (2001).

11. Gai, P. L. & Calvino, J. J. Electron microscopy in the catalysis of alkane oxidation,environmental control, and alternative energy sources. Ann. Rev. Mater. Res. 35,465–504 (2005).

12. Thiberge, S., Zik, O. & Moses, E. An apparatus for imaging liquids, cells, andother wet samples in the scanning electron microscopy. Rev. Sci. Instrum.75, 2280–2289 (2004).

13. de Jonge, N., Peckys, D. B., Kremers, G. J. & Piston, D. W. Electron microscopyof whole cells in liquid with nanometer resolution. Proc. Natl Acad. Sci. USA106, 2159–2164 (2009).

14. Hitchcock, A. P., Morin, C., Heng, Y. M., Cornelius, R. M. & Brash, J. L. Towardspractical soft X-ray spectromicroscopy of biomaterials. J. Biomat. Sci. PolymerEdition 13, 919–937 (2002).

15. Yoon, T. H. Applications of soft X-ray spectromicroscopy in material andenvironmental sciences. Appl. Spectrosc. Rev. 44, 91–122 (2009).

16. Salmeron, M. & Schlogl, R. Ambient pressure photoelectron spectroscopy: a newtool for surface science and nanotechnology. Surf. Sci. Rep. 63, 169–199 (2008).

17. Winter, B. et al. Full valence band photoemission from liquid water using EUVsynchrotron radiation. J. Phys. Chem. A 108, 2625–2632 (2004).

18. Starr, D. E., Wong, E. K., Worsnop, D. R., Wilson, K. R. & Bluhm, H.A combined droplet train and ambient pressure photoemission spectrometerfor the investigation of liquid/vapor interfaces. Phys. Chem. Chem. Phys.10, 3093–3098 (2008).

19. Gunther, S. et al. In situ X-ray photoelectron spectroscopy of catalytic ammoniaoxidation over a Pt(533) surface. J. Phys. Chem. C 112, 15382–15393 (2008).

20. Novoselov, K. S. et al. Two-dimensional atomic crystals. Proc. Natl Acad. Sci.USA 102, 10451–10453 (2005).

21. Pacile, D., Meyer, J. C., Girit, C. O. & Zettl, A. The two-dimensional phase ofboron nitride: few-atomic-layer sheets and suspended membranes. Appl. Phys.Lett. 92, 133107 (2008).

22. Hernandez, Y. et al. High-yield production of graphene by liquid-phaseexfoliation of graphite. Nature Nanotech. 3, 563–568 (2008).

23. Emtsev, K. V. et al. Towards wafer-size graphene layers by atmospheric pressuregraphitization of silicon carbide. Nature Mater. 8, 203–207 (2009).

24. Kim, K. S. et al. Large-scale pattern growth of graphene films for stretchabletransparent electrodes. Nature 475, 706–710 (2009).

25. Stankovich, S. et al. Synthesis of graphene-based nanosheets via chemicalreduction of exfoliated graphite oxide. Carbon 45, 1558–1565 (2007).

26. Meyer, J. C., Girit, C. O., Crommie, M. F. & Zettl, A. Imaging and dynamics oflight atoms and molecules on graphene. Nature 454, 319–322 (2008).

27. Wilson, N. R. et al. Graphene oxide: structural analysis and application as ahighly transparent support for electron microscopy. ACS Nano 3,2547–2556 (2009).

28. Bunch, J. S. et al. Impermeable atomic membranes from graphene sheets.Nano Lett. 8, 2458–2462 (2008).

29. Booth, T. J. et al. Macroscopic graphene membranes and their extraordinarystiffness. Nano Lett. 8, 2442–2446 (2008).

30. Stolyarova, E. et al. Observation of graphene bubbles and effective masstransport under graphene films. Nano Lett. 9, 332–337 (2009).

31. Dikin, D. A. et al. Preparation and characterization of graphene oxide paper.Nature 448, 457–460 (2007).

32. Chen, C. M. et al. Self-assembled free-standing graphite oxide membrane.Adv. Mater. 21, 3007–3011 (2009).

33. Hummers, W. S. & Offeman, R. E. Preparation of graphitic oxide. J. Am. Chem.Soc. 80, 1339 (1958).

34. Cote, L. J., Kim, F. & Huang, J. X. Langmuir–Blodgett assembly of graphite oxidesingle layers. J. Am. Chem. Soc. 131, 1043–1049 (2009).

35. Gregoratti, L. et al. 48-Channel electron detector for photoemissionspectroscopy and microscopy. Rev. Sci. Instrum. 75, 64–69 (2004).

36. Kolmakov, A. et al. Spectromicroscopy for addressing the surface and electrontransport properties of individual 1-D nanostructures and their networks.ACS Nano 2, 1993–2000 (2008).

37. Cumpson, P. J. & Seah, M. P. Elastic scattering corrections in AES and XPS. 2.Estimating attenuation lengths and conditions required for their valid use inoverlayer/substrate experiments. Surf. Interface Anal. 25, 430–446 (1997).

38. Powell, C. J. & Jablonski, A. Surface sensitivity of X-ray photoelectronspectroscopy. Nucl. Instrum. Methods A 601, 54–65 (2009).

39. Akhavan, O. The effect of heat treatment on formation of graphene thin filmsfrom graphene oxide nanosheets. Carbon 48, 509–519 (2010).

40. Mkhoyan, K. A. et al. Atomic and electronic structure of graphene-oxide.Nano Lett. 9, 1058–1063 (2009).

41. Cote, L. J., Cruz-Silva, R. & Huang, J. X. Flash reduction and patterningof graphite oxide and its polymer composite. J. Am. Chem. Soc. 131,11027–11032 (2009).

42. Yang, D. et al. Chemical analysis of graphene oxide films after heat and chemicaltreatments by X-ray photoelectron and micro-Raman spectroscopy. Carbon47, 145–152 (2009).

43. Xu, K., Cao, P. G. & Heath, J. R. Graphene visualizes the first water adlayers onmica at ambient conditions. Science 329, 1188–1191 (2010).

44. Aleman, B. et al. Transfer-free batch fabrication of large-area suspendedgraphene membranes. ACS Nano 4, 4762–4768 (2010).

45. EMS & GF1200 Graphene Support Films for TEM (Electron MicroscopySupplies, 2010); available via www.emsdiasum.com.

AcknowledgementsA.K. thanks E. Strelcov, C. Watts and J. Bozzola (SIUC) for their help in the preparationof the experiment. The work at ELETTRA was partly supported by AMBIOSEN FriuliVenezia-Giulia regional grant 47/78. M.K.A. thanks P. Parisse for AFM measurements.The TEM and FIB work was performed in the EPIC facility of the NUANCE Center atNorthwestern University. The NUANCE Center is supported by NSF-NSEC,NSF-MRSEC, the Keck Foundation, the State of Illinois and Northwestern University. TheSIUC part of the research was supported by a NSF ECCS-0925837 grant. L.J.C and J.H.were supported by the NSF through a CAREER award (DMR 0955612).

Author contributionsA.K. conceived the project, designed and tested the E-cell prototypes, and assembled themanuscript, with contributions from all co-authors. D.D., L.C. and J.H. developed themethods of GO synthesis, processing and Langmuir–Blodgett deposition onto SiO2/Si3N4

membrane samples. D.D. performed all micromachining and carried out SEM, TEM andHRTEM characterization of the GO overlayers and suspended membranes. M.K.A., M.A.,L.G., S.G. and M.K conducted the SPEM experiments and the corresponding data analysisof the photoelectron images and spectra. A.K. and S.G. participated in spectromicroscopytests as users of the ELETTRA ESCA microscopy beamline.

Additional informationThe authors declare no competing financial interests. Supplementary informationaccompanies this paper at www.nature.com/naturenanotechnology. Reprints andpermission information is available online at http://www.nature.com/reprints. Correspondenceand requests for materials should be addressed to A.K.




www.emsdiasum.com


http://www.nature.com/reprints

mailto:[email protected]



Date post:	03-Oct-2016
Category:	Documents
Upload:	maya
View:	214 times
Download:	0 times

Graphene oxide windows for in situ environmental cell photoelectron spectroscopy

Documents