Charge correction by gold deposition onto non-conducting samples in X-ray photoelectron spectroscopy

Journal of Electron Spectroscopy and Related Phenomena, 23 (1981) 55-62 Elsevler Sclentlfic Pubhshmg Company, Amsterdam - Pnnted m The Netherlands

CHARGE CORRECTION BY GOLD DEPOSITION ONTO NON-CONDUCTING SAMPLES IN X-RAY PHOTOELECTRON SPECTROSCOPY

Y UWAMINO

and T ISHIZUKA

Government Industraal Research Institute, Nagoya, Hwate-cho, Nagoya 462 (Japan)

Klta-ku,

H YAMATERA Department of Chemistry, Faculty of Science, Nagoya Unrversity, Furo-cho, Chrkusa-ku, Nagoya 464 (Japan) (First recenred 10 June 1980,

In final form 7 November 1980)

ABSTRACT Gold deposited onto sample materrals has been used as a cahbrant for determmmg their absolute bmdmg energies, and the optlmum thickness of the gold decoration mvestlgated Polytetiafluoroethylene flhn and NaF and graphite pellets were used as the substrate mater&s A serxes of deposltlons of gold layers up to “60 A thick was performed on each sample surface The thickness of gold deposited Influences the apparent bmdmg energies, the preclslon of the chargecorrected binding energies, and the FWHM’s of the Au 4f, C Is and F Is peaks It seems that, for the mstrument used m this study (which records photoelectrons at 45O to the sample surface), the optimum thxkness of gold deposited onto a non-conductmg sample surface 1s “6 a (1 e , “9 A path-length through the layer)

INTRODUCTION

When the absolute bmdmg enermes (BE’s) of non-conductmg matenals are measured by X-ray photoelectron spectroscopy (XPS), an appropriate cabbrant 1s requved for charge-correction of the mated. Many mvestlgators have dzscussed several cahbrants [l-12]. The use of the C Is peak from mountmg tape as a cahbrant 1s unsatisfactory [2] . Wagner has reported that chargecorrectlon cannot be accomphshed by usmg an adnuxed powdered cahbrant [3]. The C 1s lme from pump-oil deposits on a sample surface has given good results m some mstruments 13-7 J , but m the sample chamber of a high-vacuum mstrument (below 10 -8 torr) whose vacuum system m con0368-2048/81/0000-0000/$02

50

0 1981

Elsevler Sclentlfic Pubhshmg Company

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strutted with dry vacuum-pumps, the C Is line from hydrocarbon deposits on the sample 1s too weak to allow BE determmatlon with high precision Several reports have stated that the gold deposltron technique 1s unsuitable for the calibration of BE’s, because evaporated gold reacts with compounds studied, such as cyanide or hahde [ 4-6,8] . However, as gold deposited onto most materials remams m the metallic state, the technique has been employed by many mvestlgators Hnatowlch et al [9] showed that gold or palladium layers 10-80 a thick on barium sulfate followed the changes m the potential applied to the sample Ebel and Ebel [ll] reported that theu measurement on polytetrafluoroethylene (PTFE) film coated with a silver film 50 ii thick indicated complete agreement m the relative shifts of the C 1s (contammatlon and PTFE), Ag 3d and F 1s lme posltlons on changmg the mstrumental parameters These deposlted cahbrant metal films, several tens of angstroms thick, were of thickness far greater than the mean free path of electrons eJected from the samples [13] As gold decoration of thickness greater than the mean free path of the ejected electrons would snmlarly influence XPS measurements, it 1s required to be as thm as possible while still giving a signal adequate for cahbratlon purposes. In addition, Gmnard and Riggs [ 121 found that control of the thickness of a deposited gold film 1s necessary if this technique 1s to be used to correct the charge with high preclslon It 1s thus reasonable to examme progressmely-deposlted gold films of thickness less than the mean free path of ejected electrons This paper discusses results obtamed using PTFE film and a sodium fluonde (NaF) pellet as non-conductmg substrate materials For companson, a graphite pellet with gold decoration was used as a conducting matenal

EXPERIMENTAL

The photoelectron spectra were obtamed using a JEOL JESCA-4 photoelectron spectrometer equipped mth an Mg Kar X-ray source The X-ray power-supply was operated at 6 kV and 50 mA The pressure inside the sample chamber was below 12 x 10-8torr durmg the measurements The instrumental resolution expressed by the half-width of the Au 4f,,2 lme was 1.1 eV. All samples conslsted of circular disks - 1 mm thrck and 10 mm m dlameter The PTFE f&n was cut, and the powder samples (NaF and graphite) were pressed at 200 kg cm-’ using a pellet die (JASCO Ltd.) to this size Gold evaporation was carned out m the reaction chamber (at a pressure below 2 x lo-‘torr) of the mstrument by heating a tungsten basket-heater mto which a suitable amount of gold forl(2 mg cms2 ) had been placed The thickness of the gold film deposited onto a sample surface was calculated from the amount of gold foil evaporated and the dW,a.nce between the heater

57

and the sample In our mstrument, because the mput lens of the energy analyzer 1s set at 45” to the sample surface, photoelectrons eJected from the sample and traversing the gold layer have a path-length mthm it fi times the thickness of the deposited film Each sample was given a number of exposures to the evaporator, and an XPS measurement was performed after each deposltlon All samples were fixed to the sample-holder by means of double-sided adhesive tape, to Isolate them from ground

RESULTS

AND DISCUSSION

Figure 1 shows the Au 4f and F Is spectra obtained from PTFE film decorated successively mth gold The resolution of the doublet peak of the Au 4f spectrum was helghtened for an increase of gold thickness up to 6 ii, but then dunmlshed for subsequent increases over the range 6-24 A Shoulders were observed on the higher-energy sides of the mam peaks for a layer of thickness 16 Bi When 24 ii of gold was deposited onto the PTFE film, the shoulders observed at 16 A grew to mam peaks, and the mam peaks observed at 16 a turned to shoulders The shoulders observed at 24 i% disappeared on

~,nd,ng

Energy

eV

Fig 1 Au 4f (right-hand side) and F Is (left-hand side) peaks from PTFE film covered with vacuum-deposited gold Each numerical value refers to the thickness of gold deposited onto the surface of the sample

58

Fig 2 Apparent bmdmg energies versus thickness of gold deposited onto a graphite pellet, PTFE film and an NaF pellet Graphite *, Au 4fm, A, C Is, PTFE n, Au 4fm, 0, F ls, NaF 0, Au 4fm, 0, F 1s

further deposltlon of gold, and the doublet-peak was obtamed with good resolution. The resolution obtamed at 63 a was almost the same as that for bulk gold metal Ebel and Ebel [lo] reported that a contmuous phase IS formed when the thickness of the deposited gold film exceeds 90 a As mentioned m the preceding Expernnental Section, our mstrument measures photoelectrons eJected at an angle of 45” to the sample surface Therefore, a thickness of 63 a corresponds to a path-length of 89 a, which IS nearly equal to the value obtamed by Ebel and Ebel When a gold layer of thickness greater than -45 A (path-length 64 a) was deposited onto the PTFE film, the F 1s peak was no longer observed This 1s due to the fact that gold was deposited too thickly for F Is electrons to traverse the layer Figure 2 shows the apparent BE’s of the Au 4f,,2 peak from gold deposited onto graphite, PTFE and NaF samples versus the thickness of gold decoration m each case, together with the BE’s of the C Is and F Is peaks as appropmate For the graphite pellet sample, the apparent BE of the 4f7,2 electrons from gold mcreased steadily with mcrease m the thickness of the deposited layer For the PTFE and NaF samples, the behavlour differed The

59

BE of the 4f,,2 peak from gold deposited onto PTFE decreased mth mcreasmg thickness of gold decoratlon up to 6 A, where It showed a mmlmum, and then mcreased gradually mth additional deposltlon For NaF, the Au 4f,,2 BE decreased mth mcreasmg thickness of gold decoratIon up to 6 A, then showed a broad mmunum over the range 6-16 a, followed fmally by a slight mcrease for ad&tlonal deposltlon The behavlour of the BE’s of C Is electrons from graphite and F 1s electrons from PTFE and NaF was slmllar to that for the Au 4fT12 electrons for each sample A gold film of thickness greater than -45 ii (path-length 64 a) concealed the F Is lme from the PTFE film, as mentioned above, but did not conceal the F 1s and C 1s lmes from the NaF and graphite pellets respectively It seems that, as the surfaces of the NaF and graphite pellets (made from powders) have a greater roughness, the gold films deposIted onto the pellets remam as Elands Therefore, the fact that F 1s and C Is electrons can still be observed m these cases may be traced back to still mcomplete closure of the gold film The apparent BE’s of electrons eJected from a non-conductmg sample are related to the degree of sample chargmg As shown m Fig 2, sample chargmg versus gold thickness shows complex patterns for the PTFE and NaF samples. Wagner [3] has described the factors nnportant m estabhshmg steady-state chargmg The behavlour of sample chargmg as a function of different thlcknesses of deposited gold may be attnbuted to the different photoelectic cross-sections of the samples, different electron affmltles of their surfaces, and differences m their bulk or photon-induced surfaceconductlvrtles, and so on Figure 3 shows the behamour of the FWHM’s of each spectrum for dlffer-

s LL O

0

20 Thlcknces

40 01

Gold

Oecorotlon

60 i

Fig 3 FWHM versus thickness of gold decoration (a) ., AU 4f7/2 on graphl% I, AU 4f7/2 on PTFE, 0, Au 4f7/2 on NaF (b) a, C 1s of graphite, 0, F 1s of PTFE, 0, F 1s of NaF

60

ent gold thlckneaaea For the graphite pellet sample, the FWHM of the Au 4f7,2 peak was constant for gold thicknesses up to 63 a and was the same as the matrumental reaolutlon (1 1 eV) For the PTFE film, the FWHM of the Au 4f7,2 peak decreased urlth increasing gold thlcknesa up to 6 a, where its value (1 5 eV) was about half that (2 7 eV) at 2 a thickness, and then mcreased ‘with additional gold depoaltlon up to 31 a, at which pomt It reached 2.9 eV The FWHM decreased agam with increase m gold thickness above 31 a, and an FWHM the same as the matrumental resolution was obtamed at 63 a thickness. For the NaF pellet sample, the FWHM of the Au 4f7,2 peak decreased mth mcrease m gold thlcknesa up to 6 i-i, and remamed nearly constant for additional decoration The behavlour of the FWHM’s of the C 1s peak from graphite and the F 1s peaks from the PTFE and NaF samples was smular to that for the Au 4fTj2 peak for each sample Ebel and Ebel [lo] reported that for the non-contmuous phase, the hnemdth of the Au 4f 7,2 peak (from gold deposited onto glass) was -lo-30% mder than for the contmuous phase They mentloned the cause of this as a local, ahghtly varymg chargmg of the sample surface Both gold-rich and depleted islands are formed on a sample surface, the gold-rich islands havmg large chargmg and the depleted reaons, small chargmg For the graphite sample, all the gold islands deposited have equivalent chargmg, because the sample 1s a good conductor For the PTFE sample, the behavlour of the FWHM of the Au 4f 7,2 peak can be explained as follows The senes of gold deposltlons up to a thlcknesa of 6 ii result m islands that are uniform With ad&tlonal gold deposltlon above 6 Ii thickness, the gold islands become more vaned m size, mth mterconnectmg islands Consequently, as 1s seen m Fig. 1, the Au 4f doubletcpeak 1s obtamed as a sum of several overlapping spectra With further gold deposltlon above 31 ii thickness, the islands become completely mterconnectmg, and the Au 4f spectrum shows a smgle doublet-peak with narrow FWHM For the NaF sample, the behavlour of the Au 4fm FWHM up to 6 ii thickness can be explamed m a slmllar manner as for the PTFE sample The constancy of the FWHM for thicknesses above 6 i% for the NaF sample may be explamed by the difference of electrlcal conductlvlty between the NaF pellet and the PTFE film, by the creation of current-carrymg centers on the NaF sample surface due to radlatlon damage [143, or by reaction between gold and NaF [5,6] Figure 4 shows the charge-corrected BE’s of C 1s electrons from the graphite pellet and F 1s electrons fi-om the PTFE film and NaF pellet for different gold thicknesses Each sample’s charging was corrected by regardmg the 4f7,2 peak from each gold deposit as bemg located at 83 80 eV Each plot m Fig 4 represents the average of values obtamed from five measurements, the vertical lines represent then standard devlatlona The BE of C Is electrons from the graphite sample, which 1s a good conductor, was constant (284 25 eV) for gold thlckneases from 2 to 63 ii For the NaF sample, the BE of F 1s electrons was nearly constant (685 7 eV) over the same range of

61

I

284.

0

LO Thickness

of

Gold

40 Dccoratron

Fig 4 Charge-corrected bmdmg energies graphite, 0, F Is of PTFE, 0, F 1s of NaF

60 A

versus thickness

of gold decoratlon

*,

C 1s of

thicknesses The reproduabrllty of the F Is BE for thx sample was good for the range 6-9 a gold thickness, havmg a preclslon (standard devlatlon) of 0 07+.12 eV, nearly equal to the mstrumental reproducrblhty (
62 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

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