NEGATIVE ION IMAGING IN FIELD ION MICROSCOPY

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1 NEGATIVE ION IMAGING IN FIELD ION MICROSCOPY R. Schmitz, L. Bütfering, F. Röllgen To cite this version: R. Schmitz, L. Bütfering, F. Röllgen. NEGATIVE ION IMAGING IN FIELD ION MICROSCOPY. Journal de Physique Colloques, 1986, 47 (C7), pp.c7-53-c7-57. < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1986 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE Colloque C7, suppl6ment au no 11, Tome 47, Novembre 1986 NEGATIVE ION IMAGING IN FIELD ION MICROSCOPY R. SCHMITZ, L. B~TFERING and F. W. R~LLGEN Institute of Physical Chemistry, University of Bonn. Wegelerstrasse 12, Bonn 1, F.R.G. Abstract - Field ion microscopy with negative ion imaging has been successfully performed using organic image gases of high electron affinity such as tetracyanoethylene (TCNE). Pure ion images without contributions from field electrons are obtained in a rather broad range of field strengths since field electron emission is suppressed by the field induced formation of a polymer layer from image gas molecules. Under condensation of TCNE on the field cathode surface, unusual ring structures are observed in the ion image. The origin of these structures is not yet clear. I - INTRODUCTION Since the introduction of field ion microscopy (FIM) only positive ions have been used for imaging. In addition to inorganic gases some organic gases have also been applied /I While inorganic image gases, such as helium, neon and hydrogen provide atomic resolution, organic gases of low ionization energy have the advantage of image formation at low field strengths causing low electrostatic stress to the surface. Since the FI of organic molecules may alter the chemical and morphological structure of surfaces substantially /6/, FIM with organic image gases has also been employed to study field ionization effects at high lateral resolution /1,2/. In previous mass spectrometric studies negative ion formation under inverted field conditions was accompanied by considerable field electron emission /7,8/. However, more recently it was found that molecular anions can be formed by field ionization from organic molecules of high electron affinity below the threshold of a detectable contribution of electrons to the total emission current /9,10/. This observation prompted us to examine the use of negative ions for imaging in a field ion microscope. This paper reports first results obtained with tetracyanoethylene (TCNE) and 2,3-dichloro-5,6-dicyanop-benzoquinone (DDQ) having electron affinities of 2.88 ev /11/ and 3.3 ev 1121 respectively. Article published online by EDP Sciences and available at

3 C7-54 JOURNAL DE PHYSIQUE I1 - EXPERIMENTAL The experiments were performed using a conventional field ion microscope equipped with a single channel plate ima e intensifier. The image gas pressure was varied between about lo5 and 3xl0-~ mbar, measured with a Bayert-Alpert gaug:. The gas inlet system and microscope had to be heated to about 80 C to achieve these vapour pressures for TCNE and DDQ. The chemical structure and the field ionization mass spectrum of TCNE i.n the negative ion mode is shown in Fig. 1. No fragment ion but a dimer anion is formed as known from previous experiments /lo/. Tungsten and iridium tips were prepared from wires by electrochemical etching without further treatment. The tip radii were estimated to be in the order of several hundred nm. Fig. 1 - Negative ion field ionization mass spectrum of tetracyanoethylene (TCNE). I11 - RESULTS AND DISCUSSION Pure negative field ion images could be obtained with TCNE and DDQ (electron affinity 3.3 ev /11/). The application of a strong manetic field perpendicular to the ion trajectories revealed that there was no contribution from field electon emission to the bright spots on the screen. Examples of TCNE-anion image of a W and Ir tip is shown in Fig. 2. The ion image provides a view of the local variation of the probability for electron attachment to molecules on the tip surface. A correlation between the image and the tip material (Ir, WO,) could not be established. Using TCNE, negative ion images without contribution from electron emission could be obtained in a rather broad range of field strengths (sometimes by more than a factor of 1.5 above the onset field strength of anion formation) which depends on the TCNE gas pressure, emission time, temperature etc.

4 Fiq. 2 - Negative field ion micrographs obtained with TCNE as image gas. The tip and gas temperature was 80 OC the gas pressure about 3x10-' mbar. A W tip was used in (a) and an Ir tip in (b). The applied tip potentials were -2 kv and -1.3 kv, respectively. Slightly above the onset field strength of electron emission the image is formed by ions and electrons. Keeping a constant emitter cathode potential, the electron emission was found to cease within seconds or minutes depending again on the gas pressure. This and the latter observations indicate the formation of a field induced polymer layer on the tip surface. Under condensation conditions of the imaging gas TCNE on the tip surface i.e. at slightly lower tip temperatures than the ambient gas temperature, unusual ring structures appear in the negative ion image. This effect was observed with W and Ir tips and is shown in Fig. 3 for a tungsten tip at different applied potentials to the tip. With increasing field strength more rings appear on the screen. The number of rings and their position on the screen also change with time and gas pressure. The images of Fig. 3 arise from anion formation on the surface of an electrically conducting thick polymer layer coating the tip. Evidence for the formation of such a deposit was among other things obtained from electron microscopy of "treated" tips and from an observed decrease of the onset voltage for field electron emission. This emission depends on the time of exposure of a tip to the TCNE gas supply under field ionization conditions. The polymerization of TCNE is caused by field induced chemistry, because at high fields the solid layer remained after the tip temperature had been raised above the ambient gas temperature while at zero field a condensed layer formed could be removed again by sublimation. The orgin of the ring structures in Fig. 3 has not been investigated in detail yet.. The bright rings can probably be attributed to the ion emission from the tip of field enhancing whiskers growing on the surface of the cathode by field polymerization of TCNE. Fig. 3d shows the overlap of ion emission from many protrusions at higher field strength which is in support of this assumption.

5 C JOURNAL DE PHYSIQUE Fis. 3 - Negative field ion micrographs obtained with TCNE under condensation conditions of TCNE on the tip surface at increasing tip potentials. Accordingly the temperature of the tungsten tip was slightly lower than the ambient gas pressure. The tip potentials were -2.3 kv (a), -2.7 kv (b), -3.3 kv (c) and -3.7 kv (dl. In tentative experiments with DDQ applying similar experimental conditions as in Fig. 3 no such ring structures were observed in the ion image. IV - CONCLUSION This study shows that molecular anions of organic molecules of high electron affinity can be used as imaging particles in a field ion microscope. In the case of TCNE and DDQ examined as image gases for negative FIM, field electron emission does not contribute to the image brightness in a broad range of field strengths. This effect arises from the formation of a polymer layer under field ionization conditions. This field effect and the ionization phenomena observed with TCNE give rise to many questions, for example for the chemical structure of the polymer formed, the mechanism of field polymerization, the electronic structure of the polymer layer, the mecha-

6 zation, the electronic structure of the polymer layer, the mechanism of electron emission from and anion formation on the surface of such a layer, etc. These and other questions, as for the effect of the polymer layer on the energy distribution and appearance energy of field anions 1131 have to be investigated and will be the subject of further research. The effect of lowering the onset field strength for field electrons by negative ion FI of TCNE has already been successfully utilized to improve the performance of field cathodes for negative ion field desorption mass spectrometry /I41. ACKNOWLEDGEMENT - The authors are grateful to the Wissenschaftsministerium des Landes Nordrhein-Westfalen for finacial support. REFERENCES 1 W. Schmidt and E. Krautz, Surface Sci., 32 (1972) D. M. Taylor, F.W. Rallgen and H.D. Beckey, Surface Sci., 40 (1973) J.A. Panitz, J. Vac. Sci, Technol., 16 (1979) M. Karas and F.W. Rallgen, 26th IFES Berlin 1979; M. Karas Dissertation, Bonn J.A. Panitz, Ultramicroscopy, 1 (1982) H.D. Beckey, Principles of Field Ionization and Field Desorption Mass Spectrometry, Pergamon Press, Oxford A.J.B. Robertson and P. Williams, Adv. Mass Spectrom., 4 (1968) J. van der Greef and N.M.M. Nibbering, Int. J. Mass Spectrom. Ion. Phys., 31 (1979) V.A. Nazarenko and V.D. Pokhodenko, Int. J. Mass Spectrom. Ion Phys., 31 (1979) G.F. Mes, J. van der Greef, N.M.M. Nibbering, K.H. Ott and F.W. Rallgen, Int. 3. Mass Spectrom. Ion Phys., 2 (1980) Handbook of Chemistry and Physics, 61 st edition, Ed. R.C. Weast, CRC Press, E.C.M. Chen and W.E. Wentworth, J. Chem. Phys., 63 (1975) K.H. Ott, R. Stoll and F.W. Rallgen, Proc. 27th IFES, Tokio R. Schmitz and F.W. Rallgen, Org. Mass Spectrom., in preparation.