Partial secondaryelectron-yield NEXAFSspectromicroscopy with anenergy-filtered X-PEEM

S. L. ChristensenB. M. HainesU. D. LankeM. F. Paige

S. G. Urquhart

The narrow energy band pass of an energy-filtered x-rayphotoemission electron microscope (X-PEEM) can lead to unusualartifacts when used for spatially resolved near-edge x-ray absorptionfine structure (NEXAFS) spectroscopy and imaging of organicsurfaces. Work-function differences and the rapid work-functionchange with radiation exposure can impair quantitative chemicalanalysis by NEXAFS and invert the expected image contrast. We alsofind that Bpartial-yield[ detection from the narrow energy band pass ofan energy-filtered X-PEEM can lead to distorted NEXAFS spectra.These observations not only are relevant for the analysis of organicsurfaces by energy-filtered X-PEEM but also call into question someassumptions about quantitative NEXAFS spectroscopy.

IntroductionX-ray photoemission electron microscopy (X-PEEM) is usedby many groups to study Bsoft[ surfaces, such as thoseassociated with bio-adhesion phenomena and the structure oforganic thin films [1–3]. X-PEEM can be used to extractnear-edge x-ray absorption fine structure (NEXAFS) spectraof the near surface region of a sample (�5 nm deep [4]) withthe high lateral spatial resolution of PEEM. NEXAFSspectroscopy provides direct chemical information(elemental composition, chemical bonding and speciation,and molecular orientation measurement) [5] with adequateradiation damage resistance in appropriately executedexperiments [4].The development of energy-filtered PEEM microscopes [6]

offers an excellent opportunity to combine photoemissionspectromicroscopy [ultraviolet photoemission spectroscopyand x-ray photoemission spectroscopy] of organic thin filmswith NEXAFS chemical analysis. This combination would beideal for correlating chemical and structural information withelectronic structure for the study of organic electronicmaterials. This paper describes how energy-filtered X-PEEMcan compromise the acquisition of the NEXAFS spectra ofsome organic materials, in ways that are at times surprising.Unfiltered PEEM microscopes accept all of the

photoelectrons ejected by the sample surface, subject to the

1=E transmission function of the cathode lensVwhere Erefers to the kinetic energy of the photoelectrons. Therefore,low-energy secondary electrons dominate the imageintensity. These secondary electrons are created by inelasticscattering of the primary photoelectrons generated by x-rayabsorption within the solid. The energy distribution of theejected secondary electrons reflects the nature of the inelasticscattering processes within the solid [7], as well as thesurface work function.When NEXAFS spectra are recorded in an X-PEEM

microscope, electron emission is recorded by the imagedetector on a pixel-by-pixel basis, as a function of photonenergy. This provides a 4-D data set (intensity, x, y, andhv), which can be considered as a NEXAFS spectrum(intensity, hv) for each image pixel ðx; yÞ. Here, h is Planck’sconstant, and v is frequency. This method is similar to totalelectron yield detection of the NEXAFS spectra of solids,where a low-current amplifier is used to record the sampledrain current as a function of photon energy (current, hv) [8].This sample current neutralizes the loss of all photoemittedelectrons (primary, secondary, etc.) by the sample and isconsidered to be approximately proportional to the NEXAFScross section [9]. In comparison, the electron-yield signal inan X-PEEM is modified by the 1=E transmission function ofthe cathode lens. In an energy-filtered X-PEEM microscope,the energy filter further modifies this electron-yield signal.We describe this detection method as partial secondary

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electron-yield NEXAFS detection, when the secondaryemission band is used for NEXAFS spectromicroscopy.There are several types of energy-filtered PEEM

microscopes, with high-pass energy filters based on a retardingmesh [10], and bandpass energy filters based on a single [6]or paired hemispherical analyzers [11], an omega filter [12], ora prism array [13]. The energy filter adds a variable to theseX-PEEM experiments, i.e., imaging with a fixed photonenergy and a variable electron kinetic energy (intensity, x, y,Ekin; hv), e.g., photoemission spectromicroscopy, or byimaging with a fixed electron kinetic energy and a variablephoton energy (intensity, x, y, hv; Ekin), e.g., partial-yieldNEXAFS spectromicroscopy. This paper focuses on howthe bandpass energy filter modifies the NEXAFS signal oforganic surfaces, based on our experience with an ElmitecPEEM III microscope with an imaging energy filter [6].A schematic drawing of this energy-filtered PEEM is shownin Figure 1. Similar phenomena may be observed in ahigh-pass energy-filtered PEEM, but these effects are notexplored in this work.

ExperimentalExperiments for this work were performed on the CanadianPhotoemission Electron Research Spectromicroscope(CaPERS) on various beamlines at the Canadian LightSource (CLS) (VLS-PGM beamline 11ID-2, SGM beamline11ID-1, spectromicroscopy beamline 10ID-1). Here,VLS-PGM stands for variable line-spacing plane-grating

monochromator, and SGM stands for spherical gratingmonochromator. The CaPERS PEEM microscope is anElmitec PEEM-III microscope fitted with a hemisphericalimaging energy filter [6]. This microscope was operated ina Bmobile[ mode during the early development of beamlinesat the CLS but is now fixed on the PEEM branch of thespectromicroscopy beamline [15].Samples examined in this work include ultrathin films

prepared by Langmuir–Blodgett deposition and spin casting.Phase-separated Langmuir–Blodgett thin films were madefrom a 2 : 1mixture of arachidic acid (AA,C19H39COOH) andperfluorotetradecanoic acid (PA, C13F27COOH) (2AA:1PA).Full preparation details have been published previously[1, 16]. A thin film (�50–100 nm thick) of polystyrene(polymer source, P2343-St) was prepared by spin casting thepolymer dissolved in toluene onto cleaned silicon wafers.

Experimental results and discussionWe use X-PEEM microscopy and spectromicroscopy tostudy the composition and structure of phase-separatedorganic thin films, such as those prepared byLangmuir–Blodgett deposition [1, 17]. Spatially resolvedNEXAFS provides a direct measurement of surface chemicalcomposition and can also provide information on molecularorientation. However, when spatially resolved NEXAFSof surfaces is acquired with an energy-filtered PEEMmicroscope, the effect of work-function differences on theNEXAFS spectra must be considered.

Figure 1

Schematic of the energy-filtered X-PEEM microscope. Arrows indicate the images of the surface, and dots indicate the images of the back focalplane [14].

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Figure 2 presents X-PEEM images of a phase-separatedLangmuir–Blodgett film formed by a 2 : 1 ratio of arachidicacid (AA) and perfluorodecanoic acid (PA), denoted as2AA:1PA, at two different energy filter kinetic energysettings. Previous atomic force microcopy and X-PEEMstudies have shown that the continuous phase is PA-rich andthat the discontinuous phase is AA-rich [1, 16]. The imagecontrast differences between the left and right images aredescribed here.

Effect of work-function variationFigure 3(a) presents the secondary electron emissionspectra of the PA-rich continuous phase and the AA-richdiscontinuous phase. These spectra are recorded byilluminating the sample with a fixed photon energy [62 eVfor Figure 3(a)] and scanning the kinetic energy of theelectrons used to generate the image, in an energy-filteredX-PEEM microscope. This provides, on a pixel-by-pixelbasis, the photoemission spectrum of the sample; for lowkinetic energy electrons, this provides the secondary electronemission spectrum. Using image-masking techniques, thespectra of all continuous and discontinuous pixels areaveraged, and the two spectra are presented here.It is immediately obvious that the discontinuous phase

(AA-rich) emits secondary electrons with a lower kineticenergy than the fluorinated continuous domains (PA-rich).This can be attributed to work-function differences.Alkanethiol self-assembled monolayers are known todecrease the work function of metals such as Ag and Au,whereas perfluorinated alkanethiol self-assembledmonolayers will increase the work function of these surfaces[18, 19]. While the structure and formation of theLangmuir–Blodgett films differ, the effect of fluorination onthe relative work function is expected to be similar.

The nature of inelastic scattering processes within theAA- and PA-rich domains may also differ, but the energyof electrons that can escape is determined by the workfunction of each domain.Figure 2 illustrates how these work-function differences

and the secondary electron emission properties affect theenergy-filtered X-PEEM image contrast. At lower secondaryelectron kinetic energies (left image), the lowerwork-function dispersed AA domains are bright (morephotoemission), whereas, at higher secondary electronkinetic energies (right image), the higher work-function

Figure 3

Emission spectra. (a) Secondary electron emission spectra of thediscontinuous domains (blue; arachidic acid-rich) and continuousdomains (red trace; perfluorodecanoic acid-rich), showing work functioncontrast. Data were obtained by image sequence methods by scanningthe electron kinetic energy (start voltage) at a photon energy of 62 eV(CLS PGM beamline 11ID-2; 20-�m field of view, 300-ms dwell perimage, average of 15 scans, each from fresh areas). (b) Sequentiallyrecorded secondary electron emission spectra of the discontinuousdomains (blue; arachidic acid-rich), showing a rapid change in workfunction with radiation exposure. Data were obtained by image sequencemethods by scanning the electron kinetic energy (start voltage) at aphoton energy of 282 eV (CLS SM beamline 10ID-1, 25-�m field ofview, 300-ms dwell per image, repeat scans from the same area).

Figure 2

Energy-filtered X-PEEM images of a 2AA:1PA Langmuir–Blodgettthin film, which were recorded at different values of the secondaryelectron kinetic energy (Left: �0.5 eV; Right: 0.5 eV; these energiesare not calibrated). The photon energy was 695 eV. See text for sampledetail.

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continuous PA domains are bright. This can be directlycorrelated with the secondary electron emission spectra inFigure 3(a). Other contrast mechanisms contribute: Thethicker AA-rich regions (25 Å thick) will have a strongerNEXAFS cross section than the PA-rich film regions (18 Åthick) [1], but the work-function effects are very strongand will dominate at all x-ray energies. Similar contrastinversion effects have been observed for phase-separatedpolystyrene/polymethylmethacrylate thin films [5].This work-function contrast is unsurprising, but one

must also consider how this phenomenon will affect thequantitative measurement of NEXAFS spectra in anX-PEEM microscope. For example, NEXAFSspectromicroscopy is used to study the localization of proteinadsorption on phase-separated polymer surfaces, withunfiltered X-PEEM microscopes [2, 20]. If partial secondaryelectron-yield NEXAFS spectra are acquired in anenergy-filtered X-PEEM microscope at an electron kineticenergy other than the isobestic point, the signal from onephase will be enhanced relative to the signal of the otherphase (e.g., signal / NEXAFS cross section � secondaryelectron emission propensity). Therefore, the signal is nolonger directly proportional to the chemical composition.For complex organic surfaces, it is not easily possibleto know the work-function variation in advance of theexperiment.

Time stability of surface work functionOur results also indicate that the work function of organicthin films is not stable in time. Figure 3(b) presents thesecondary electron emission spectra of AA-rich domains in aphase-separated 2AA:1PA Langmuir–Blodgett thin film as afunction of dose; these traces are acquired from the samesample area, in sequence. We observe that the secondaryelectron emission band shifts to higher energy with sampleexposure, converging after a few scans. The initial changesare quite rapid. It appears that photon illumination increasesthe work function of the sample. We note that the initialstate of the sample surface is not pristine: theseLangmuir–Blodgett films are prepared on a water surfaceand transferred and stored in air. These rapid work functionchanges could be attributed to desorption of species fromthe surface with radiation exposure.Partial secondary electron-yield NEXAFS spectra of

organic surfaces recorded with an energy-filtered X-PEEMcan be unstable as the secondary emission rapidly changesduring the initial exposure. This effect is not necessarilylimited to NEXAFS acquired in an X-PEEM microscope. Wehave previously observed that the first NEXAFS scan ofan organic sample, which is recorded by a sample currenttotal electron yield, often has a higher background thansubsequent scans. This observation is consistent to a rapidincrease in work function with initial radiation exposure.A comparison to pristine in vacuo prepared organic samples

and photon-stimulated desorption studies would be needed toproperly characterize this effect.

Partial secondary electron-yield NEXAFS detectionWe have seen unusual effects in the partial secondaryelectron-yield NEXAFS spectra of organic thin films whenrecorded by energy-filtered PEEM microscopy. We findthat the NEXAFS spectrum of the organic surfaces differswhen recorded at different secondary electron emissionenergies. To explore this effect, we have recorded thesecondary electron emission spectrum, in dispersion mode, ata series of photon energies typical for a NEXAFS scan.We have averaged the signal for a series of electron kineticenergy Bbins,[ as illustrated in Figure 4(a). The NEXAFSspectra of each bin are shown in Figures 4(c) and 4(d).Because of the finite bandpass of the energy filter, NEXAFSspectra recorded in an energy-filtered X-PEEM are alwayspartial electron yield to some extent.

Figure 4

Emission spectra. (a) Sample secondary electron emission spectrumidentifying the energy range bins used to form the partial secondaryelectron emission NEXAFS spectra. (b) Sample secondary electronemission spectra of polystyrene, which were recorded at 284-, 285-,286-, and 330-eV photon illumination energies. The absolute energyscale is not calibrated. (c) and (d) Carbon 1s NEXAFS spectra ofpolystyrene, which were recorded in partial secondary electron-yieldmode, for a series of bins identified in panel (a). NEXAFS spectra are notnormalized for the Io flux, where Io is the incident photon flux.

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It is immediately obvious that the shape of the NEXAFSspectra differs when extracted from different secondaryelectron-yield bins; the Carbon 1s! ��ðC ¼ CÞ transitionat 285 eV in the NEXAFS spectra extracted from lowelectron kinetic energy bins (1, 2, and 3) is considerablysharper than that from higher bins. The higher energy bins(6–10) show dispersion from a valence photoemission peakthat shifts to higher energy as a function of photon energy.Upon close examination of the secondary emission

spectrum [Figure 4(b)], we observe that the secondaryelectron emission shifts to lower energy at theCarbon 1s! ��ðC ¼ CÞ transition energy (285 eV). Whenthis electron emission band shifts to lower kinetic energy, thesignal level in the low energy bins (1–3) increases, andthe intensity of the Carbon 1s! ��ðC ¼ CÞ transition isartificially enhanced. Identical effects are observed insubsequent scans of the same sample region; this effect isnot associated with radiation damage.It appears as if the sample work function changes with

strong photon absorption and photon energy. As thesecondary electron emission onset energy decreases, moresecondary electrons escape into the low-energy channels,distorting the strength of strongly absorbing features. Thiseffect appears to be related to the x-ray absorption crosssection; the secondary electron emission band is also shiftedto higher energy with 330-eV x-ray illumination, wherepolystyrene also absorbs strongly but not as strong as at285 eV.The origin of this effect is not clear, as sample

charging should have the opposite effect than observed.A nonconductive sample will tend to become more positivelycharged when it absorbs x-rays and photoelectrons areejected. This positive charge should retard the emissionof low-energy electrons, shifting the onset of the secondaryelectron emission spectrum to higher energy, not lower.This partial secondary electron-yield effect has been seen

in a series of similar experiments on energy-filteredX-PEEM, but the secondary electron emission shift was onlyobserved when the energy analyzer dispersion functionwas measured as a function of photon energy. The shiftinghigher energy peaks in Figure 4(d) also illustrates that aprimary photoelectron band from valence photoemission issuperimposed on the partial secondary electron emissionspectra. Depending on the magnitude of the apparentwork-function shift or valence photoemission contribution,the energy filter can distort the shape of NEXAFS spectraof organic surfaces.

ConclusionResults have shown that one must use caution whenmeasuring NEXAFS spectra of organic thin films in anenergy-filtered X-PEEM. Observations have also suggestedthe need for concern in traditional total electron-yieldNEXAFS analysis. As lower work-function samples will

have a stronger electron-yield signal, quantitative analysisof materials with greatly different work functionsVor anunstable work functionVshould be questioned. The originof the apparent shift in work function with strong photonabsorption is unknown and deserves further study.

AcknowledgmentsThe research described in this paper was performed at theCanadian Light Source, which is supported by the NaturalSciences and Engineering Research Council of Canada, theNational Research Council Canada, the Canadian Institutesof Health Research, the Province of Saskatchewan, WesternEconomic Diversification Canada, and the University ofSaskatchewan. The work of S. G. Urquhart was supported bythe Natural Sciences and Engineering Research Council ofCanada.

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Received November 1, 2010; accepted for publicationFebruary 1, 2011

Stephen L.Christensen Department of Chemistry, University ofSaskatchewan, Saskatoon, SK, Canada, S7N 0G6.Current affiliation: Department of Chemistry, Dalhousie University,Halifax, NS (s.christensen@dal.ca).

Brian M. Haines Department of Chemistry, University ofSaskatchewan, Saskatoon, SK, Canada, S7N 0G6(brian.haines@ymail.com).

Uday D. Lanke Department of Chemistry University ofSaskatchewan, Saskatoon, SK, Canada, S7N 0G6.Current affiliation: Canadian Light Source, University of Saskatchewan(uday.lanke@lightsource.ca).

Matthew F. Paige Department of Chemistry University ofSaskatchewan, Saskatoon, SK, Canada, S7N 0G6(matthew.paige@usask.ca).

Stephen G. Urquhart Department of Chemistry University ofSaskatchewan, Saskatoon, SK, Canada, S7N 0G6(stephen.urquhart@usask.ca).

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