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The emission properties of organic field ion emitters
SURFACE
SCIENCE 40 (1973) 264-281 0 North-Holland
THE EMISSION ORGANIC A COMBINED
PROPERTIES
FIELD FIM AND
Publishing Co.
OF
ION EMITTERS FI-MS
D. M. TAYLOR, F. W. RBLLGEN
INVESTIGATION
and H. D. BECKEY
Institut fiir Physikalische Chemie der Universitiit Bonn, 53 Bonn, Germany Received 26 April 1973; revised manuscript
received 13 June 1973
The emission properties of tungsten emitters activated with benzonitrile, such as are used in analytical organic field ion mass spectrometry, have been investigated using n-heptane as test gas. It was discovered that these emission properties are influenced principally by the morphological structure and electronic state of the surfaces of the microneedles; the bulk structure of these organic agglomerates has practically no effect on their ionization behaviour. Field ion microscope investigation of emitters activated at high temperature (HT) revealed a similar surface structure to that of pyrolytic graphite; emitters activated at room temperature (RT) showed less order. The emission regions of highest electric field strength on the tips of the needles are projecting points, ledges, and in the case of HT-emitters, structure planes. A decrease in the mean emission field strength results from the formation of solid deposits on the needle surfaces. A field corrosion of the needle tips occurs in the presence of high pressures of water and leads also to an irreversible decrease in the mean emission field strength. The effect of physically adsorbed layers on the emission properties of the emitter surface is discussed in terms of the influences of the electric field penetration on the electronic properties of the needles.
1. Introduction In analytical field ionization (FI) mass spectrometry, activated emitters are most frequently used for field ionization of organic molecules, because their application allows particularly large ion yields to be attainedl). The ionization at these activated emitters occurs on organic micro-needles grown substrate, usually by field polymerisation 2l 3) on an electrically conducting thin tungsten wire of 3 - 10 urn diameter. Of the substances with the requisite benzonitrile has proved particularly field polymerisation properties, advantageous since it requires only short activation times, that is, at fields of several megavolts per centimetre a rapid needle growth occurs after a short induction period. Activation of an emitter may be carried out at room temperature (RT) or at an emitter temperature of about 1200°C (high temperature= HT)4). The HT emitters differ from the RT ones in that they have a higher mechan264
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ical and thermal stability (stable up to about 2000°C) and a greater resistance to attack by chemically aggressive substances. Higher field strengths may also be achieved with HT emitters. Mass spectrometric observations show that the emission properties of field anodes are subject to changes which are dependent on the test substance. In the case of organic emitters, these variations affect also the average field strength and the field strength distribution over the emitting surface regions. For instance, benzene causes an increase in the field strength at the emission centres which disappears when the benzene is pumped out of the ion sources). The present investigation of the emission properties of organic emitters immediately after activation and also after exposure to several simple gases reveals a complex dependence of the emission field strength on the morphological and chemical structure of the needle surface and on the coverage and chemical nature of any adsorbate. Several different surface types may be distinguished, which may be partly interconverted by appropriated treatments. In addition, the investigation enables conclusions to be drawn about the structure of the organic needles and the electronic state of their surfaces under field ionization conditions. The investigation was carried out by complementary FI-MS and FIM studies. Whereas under the usual conditions of a field ion mass spectrometric study only integrated emission properties may be observed, the FIM allows the investigation of ionization phenomena in small surface regions. For the characterization of the emission properties in both the mass spectrometric and the FIM investigations, n-heptane was used as the test gas. This substance is relatively inert in comparison to most other organic materials. Its ionization potential of about 10 eV is only a little higher than the common values for organic substances, so that measurements under the same conditions lead to ion yields comparable with those of other gases. The ionization potentials of the inert gases, including that of xenon, are too high for use with many types of emitter. A measure of the field strength in the emitting regions may be obtained from the ratio 29/100 of the fragment ion current at m/e= 29 (C,H:) to the molecular ion current at m/e= lOO(C,H:,) using n-heptane. The C,Hl fragment arises from a field-induced decomposition of the molecular ion6). Experimentally, the relationship 29/100 = I (C,H:)/Z
(C,H:,)
= a exp (bF)
was found for the dependence of the ratio 29/100 on the field strength Fat room temperature, using a platinum tip of known radius of curvature with a=1.6x 10m3 and b=21.6 when F is given in V/AT). Under these
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conditions, the intensity ratio 29/100 is equal to the ratio of the ion current densities i2.Ji,,,,. With activated emitters, however, emission regions of different field strengths contribute to the C,H: and C,HT6 ion currents so that 29/100 is only a measure of the average of these field strengths. For activated emitters, 29/100 has values between 10e4 and 0.4. Using eq. (1) these values correspond to average field strengths between N 0.1 and 0.4 V/A. The average field strength obtained in this way gives no indication of the field strength distribution. However, according to common experience, emitters with 29/100>0.1 possess a measurable ability to ionize nitrogen, i.e. with the same partial pressures in the ion source the ratio of the N: and C,HT6 ion current is greater than 10m3. A clearer indication of the presence of emission regions of high field strength is the detection of a small H30+/Hz0+ ratio (~0.1) from the background water. Since water is always present even on the surface of the emitter, this information may be easily obtained for a characterization of the emitter state. The investigation is confined to emitters activated with benzonitrile. 2. Experimental 2.1. THE FIELD ION MICROSCOPE The FIM used in the present investigation was a conventional all-metal machine equipped with a 50 mm diameter channel plate image converter. An additional inlet system for liquids was incorporated, following conventional mass spectrometer practice. A silicone rubber membrane some 3 mm thick, through which liquids could be injected using a miniature hypodermic syringe was isolated by a high vacuum valve from a small ballast chamber which acted as the image gas source for the microscope when using organic materials or water for this purpose. To this end, the ballast chamber was connected with the microscope through a UHV leak valve, through which the chamber could also be pre-evacuated and outgassed. The residual gas composition in the microscope and the level of impurities in the image gas were monitored by a Finnigan 400 quadrupole mass spectrometer. The organic liquids were of the purity normally used for spectrometric investigations, and the principal impurity was water, in general present at a level of ~0.2%. Because of the relatively high level of water vapour it was unnecessary to re-achieve UHV conditions in the microscope between measurements. A typical background pressure was 5 x lo-’ torr, composed largely of water. The present investigations were carried out with n-heptane as imaging gas, and mostly with the emitter at room temperature.
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The most frequently used image gas pressure was 2 x lo-’ torr. The use of high pressures of gases with low ionization potentials can easily lead to gas discharges in the microscope, and such discharges, which became more intense with rising emitter temperature, prevented the use of the FIM at emitter temperatures higher than a few hundred degrees centigrade. The presence of these discharges does not disturb measurements in the mass spectrometer, because of the extremely small acceptance angle of the instrument and the different focussing conditions for the two phenomena (field ions and gas discharge ions). 2.2. PREPARATION OF THEEMITTERS Emitters for the FIM were produced by activating electrolytically-etched tungsten tips at room temperature (RT) or at about 1200°C (HT) using the procedure described extensively elsewhere 4**), to produce a dense growth of organic microneedles on the tip cap. Fig. 1 shows a Stereoscan micrograph of such an RT emitter. It was found that tips which are somewhat blunt for normal FIM use produced the best results when activated. During field ionization the current originates on the tips of the needles, and the complex geometry of the emitting envelope, together with mutual
Fig. 1.
Stereoscan micrograph of an RT-activated tungsten emitter. Activation benzonitrile; radius of curvature of the W tip: about 2000 A.
gas:
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shielding effects of the densely-packed needles make an accurate determination of the emitting area corresponding to a single point on the microscope screen extremely difficult. Because of the overlapping of the projections of the emitting areas of neighbouring needles on the screen, the FI emission pattern may not be taken to represent the actual geometric arrangement of the emitting areas on the needle surface. It was estimated, however, from electron micrographs, that the FIM images were formed mainly by the overlapping projections of about 10 microneedles with lengths of some 1000 A and diameters of several 100 A. 2.3. THE MASSSPECTROMETER A conventional single-focusing mass spectrometer, equipped with an FI ion source was used. In place of an activated FIM emitter, a correspondingly activated 10 urn diameter tungsten wire was used, in order to increase the sensitivity of the ion detection. The density of the needles grown on a wire emitter amounted to about 108/cmZ as estimated from scanning electron micrographs, i.e. approximately lo4 needles contribute to the ion current. The length of the mostly dendritic needles was about 7 urn and the radii of curvature of the needle tips ranged from 100 to 1000 A. It was possible to heat the emitter electrically. 3. Results 3.1. EMISSIONPATTERNOF RT EMITTERS The value of 29/100 of n-heptane for the RT emitters usually lies between 0.01 and 0.05. Fig. 2a shows the FIM pattern obtained from an RT emitter at a low emitter voltage. The emission occurs largely from very small areas of the needle tips, as indicated by the bright spots on the screen. Since the effective radii of curvature of the needle tips are several hundred Angstroms, the emitting areas may be estimated to be lo-50 8, in diameter. The emission is very unstable at low emitter voltages, that is, the bright spots are repeatedly visible for a short time from several seconds to several minutes. The “switched off” time lies in the same order of magnitude. The frequency of this flickering and the number of spots participating is also influenced by the water background, increasing with increasing partial pressure of water. Increasing emitter voltage leads to a decrease of this instability, which, however, never fully disappears. The intensity fluctuations observed in the mass spectrometer, which are often of several orders of magnitude in the region of the cutoff voltage where an ion current is just detectable and remain at a level to 2 to 30% under normal measurement conditions, may be ascribed to this ionization phenomenon. .The mass
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(a)
(b) Fig. 2. (a) Field ion emission pattern of an RT-activated tungsten emitter at lower emitter voltage (7.0 kV); imaging gas: n-heptane; room temperature. (b) Field ion emission pattern of the same RT-activated tungsten emitter as in (a) at higher emitter voltage (9.0 kV); imaging gas: n-heptane; room temperature; the cellular structure is clearly visible.
D. M. TAYLOR,
F. W.
R6LLGEN
AND H. 0.
BECKEY
(a)
(b) Fig. 3. (a) Field ion emission pattern of an RT-activated emitter after a period of ionization at constant voltage (6.0 kV); imaging gas: n-heptane; room temperature. (b) Field ion emission pattern of the same RT-activated emitter as in (a), immediately after a “voltagejump”: the emitter voltage was raised to 9.0 kV, held there for a short time, then reduced rapidly to the original value of 6.0 kV. The picture was taken at this point. The marked increase in the number of emission centres may clearly be seen.
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spectrometer accepts ions from only a very small solid angle of the emitter surface, so that flicker effects are very clearly detectable. With increasing emitter voltage, the density of the emission centres grows until eventually a cellular type of structure appears (fig. 2b). The bright lines on the screen may be partly resolved into very closely-packed spots. The source of these bright lines may be taken to be projecting ledges on the surface from which an enhanced emission is obtained (see ref. 9). The projections of such ledges overlap each other and produce the impression of a cellular structure. Heating the emitter increases the definition of the cellular structure - the projection ledges become more prominent emitters. An increase in the effective emission field strength under these conditions can be measured in the mass spectrometer. It was also observed that the density of the bright spots is not instantaneously reduced to its low voltage equilibrium value after a sudden decrease in the emitter voltage. Immediately after the change in voltage, the number of emission centres remains relatively high and falls slowly to its equilibrium value with a time constant of several minutes (fig. 3). A complementary measurement in the mass spectrometer showed that 29/100 is somewhat higher immediately after the voltage reduction than later. 3.2.
THE
EMISSION PATTERN
OF
HT
EMITTERS
Immediately after activation HT emitters develop a very high field strength, in general much higher than that for RT emitters, once the voltage applied during the activation is exceeded. Values of 29/100 between 0.05 and 0.3 were measured, and the ratio H30+/HZO+ is usually less than 0.2. The ionization sensitivity for nitrogen and argon is sometimes unusually high, and can reach about 10% of that of n-heptane. These large NT and Ar+ currents suggest that the maximum attainable field strength with newlyactivated emitters is greater than 1 V/A. At the field strengths at which the emitter material field evaporates, even Ne and He ionization may be detected for a few seconds. Fig. 4 shows FIM patterns of the emission from an HT emitter at two emitter voltages. On first raising this voltage from zero, practically no bright spots become visible, and the emission comes principally from larger areas of the surface (fig. 4a). The fine, sometimes curved lines are taken to delineate graphite-type structure layers projecting from the surface, such as those observed with pure graphite emitters in the FIMiO~rl). At higher voltage, an increasing number of short lines is seen (fig. 4b) and bright spots, such as those observed with pyrolytic graphiteli), appear. The spots and lines all flicker strongly. The flickering becomes slower at lower emitter voltages. The integrating effect of the exposure time prevents the
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(b) Fig. 4. (a) Field ion emission pattern of an HT-activated tungsten emitter at low emitter volt age (2,5 kV); imaging gas: n-heptane; room temperature. (b) Field ion emission patt ern of the same I-IT-activated tungsten emitter as in (a), at high emitter voltage (3.3 kV); imaging gas: n-heptane; room temperature; the lines have become shorter and more numerous.
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small spots from appearing as such in the photograph. The spots and short lines are often arranged in chains. These phenomena are independent of the choice of image gas (n-heptane, nitrogen, or even water). With HT emitters, as with RT emitters, the equilibrium spot density corresponding to the new emitter voltage is not obtained imediately after a sudden voltage reduction, but the number of emission centres decreases exponentially with time. In agreement with observations in the mass spectrometer, the cutoff point for the emission moves to higher voltages once the activation voltage has been exceeded. This may be attributed to field evaporation of the emitter material in the emission regions at the highest field strengths. Heating the HT emitters causes no change in the structure ofthe needle material up to the activation temperature of about 1200°C. In the FIM an increase in the number of emission centres is observed, and in the mass spectrometer a rise in the emission field strength, from which it may be concluded that the emission regions of higher field strength grow with increasing temperature. 3.3.
EFFECT
OF WHEPTANE
ON THE EMISSION FIELD STRENGTH
A slow decrease of 29/100 with time is observed during the continuous field ionization of n-heptane. This decrease of the average field strength occurs more rapidly if the initial field strength is high, and especially for HT emitters. In confirmation of this field strength decrease, the ratio H30+/H20f of the background water rises and the ionization probabilities for Nz and Xe fall. In the nearly stable state reached after very long ionization times (up to several days) the ratio 29/100 is less than 0.01. Pumping out the test gas and subsequent readmission leads only to a small increase of 29/100, that is, the decrease of the average field strength is only slightly influenced by physically adsorbed layers under these’ conditions. Nor may the field strength decrease be attributed to the influence of the background water. It was discovered that the effect of water background alone on the emission field strength lies at least an order of magnitude lower than that of n-heptane at high pressures. A surface condition of higher field strength may be repeatedly reestablished by heating the emitter. However, the initial field strength of an HT emitter cannot usually be achieved again. In the FIM, an increase of the number of emission centres with temperature is observed, similar to that seen on increasing the emitter voltage. After turning off the heating current, the additional emission centres do not disappear immediately, but exponentially with time, the time constant being several minutes. Fig. 5 shows the typical time behavior of the M+ ion current (m/e= 100) of n-heptane observed with the mass spectrometer during such a thermal treatment of
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M. TAYLOR,
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R6LLGEN
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BECKEY
the emitter. Repeated heating showed that the emission behaviour is more strongly influenced by the temperature achieved than the heating time chosen. I’ (see fig. 5), and also the value of 29/100 measured after switching off the emitter heating current, increase with the heating temperature. So long as thermal/field evaporation of the emitter material is avoided, the presence of the electric field during heating is without effect on the subsequent emitter state. Emitter
Heating
on
off
I
I
---I At-
i
i
I
i
5
6
;
i
-
Time (min
1
Fig. 5. Schematic diagram of the response of the Mf ion intensity of n-heptane to a suddenly-imposed temperature increase of ca. 7OO”C, applied for a time hr. A peak intensity of I’ is achieved immediately after the heating is switched off.
Chemical changes in the surface may be caused in the regions of higher field strength by the neutral field dissociation products which remain on the surface in relatively high concentration, in particular CzH4 and C,H,. These unsaturated molecules can undergo field polymerisation and thereby release protonsrs). C,Hi and C&Hi ions are present in the n-heptane mass spectrum with only low intensity. An indication of the presence of such adsorption layers is given by the observation of a short pulse in the total emission on the first heating of the emitter after a long emission time. A reduction of the emission field strength also results from cooling the emitter. About the condensation temperature of n-heptane a rapid fall of 29/100 to 1% of its value at room temperature is observed, although the M+ ion current remains high. The original state, i.e. the same room tempera-
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ture value of 29/100, is obtained after thawing the emitter out. The effect of the emitter temperature on the field dissociation of heptane was measured at higher temperatures and is much smallerrs). This suggests that the emission field strength is also influenced by thick physically adsorbed layers. 3.4. THE EMISSION PATTERN AFTER THE FIELD IONIZATION OF WATER High partial pressures of water (about 10V4 torr) under field ionization conditions lead to rapid and marked changes in the emission properties of activated emitters. The 29/100 ratio of HT emitters for example, falls in the course of a few hours from over 0.1 to under 0.01. N2 ionization is subsequently impossible, and Xe ionization is only occasionally just detectable. Small partial pressures of water, however (about 10m6 torr), have hardly any effect on the emission field strength, so that it may be concluded that the de-activation phenomenon is due to the production of adsorbed multilayers of water. More precisely, the decrease in field strength may be attributed to corrosive reactions of OH radicals with. the needles. They are formed on the emitter surface by proton transfer reactions in such water layers. Typical field ionization products of the interaction between the material of the organic microneedles and water are the ions CHO+, CH,O+, CzH30+, CHO: and C3H50+ which are present in the water spectrum. Fig. 6a shows a typical FIM pattern of the emission of an RT emitter after ionization at high water pressure. Before this treatment, ionization occurs at small emission centres (fig. 2), but afterwards it proceeds mostly from large continuous areas. This ionization is usually very stable; no flickering is observed. The n-heptane M+ ion current, which approximates to the total ion current, is only slightly affected by the water treatment. A comparison of the needle shapes in the electron microscope before and after water treatment showed hardly any change. Only the needle tips were affected; some of them were very slightly flattened. The n-heptane emission pattern of an RT emitter after water treatment and a short heating is shown in fig. 6b. Heating the emitter produces a clearly observable emission pulse corresponding to the desorption of the thermally unstable parts of the surface, so that the cellular structure of the more stable parts remains. This cellular structure is only a secondary consequence of field corrosion. The primary effect is a thermally induced structural change in the needles and their surfaces. The 29/100 ratio is once more larger after the heating than before, however, the regions of very high field strength cannot be produced again by increasing the emitter voltage. The intensity fluctuations observed in the mass spectrometer are larger after the thermal treatment.
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(a)
(b) Fig. 6. (a) Field ion emission pattern of an RT-activated emitter after ionization of water at high partial pressure (2 x 1OP torr); the imaging gas is n-heptane at room temperature; emitter voltage is 6.0 kV. (b) Field ion emission pattern of an RT-activated emitter after ionization of water at high partial pressure (2 x 10m5torr) and subsequent heating to red heat after removal of the water; the imaging gas is n-heptane at room temperature; emitter voltage is 4.0 kV.
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4. Discussion 4.1. MORPHOLOGICALSTRUCTUREOF THE NEEDLESURFACES
The structure of the needle surface is not homogeneous. In both RT and HT emitters emitting ledges corresponding to the lines in the FIM patterns project from the needle surface. Judging from the lateral extent of the lines the ledges on RT emitters are coarser than those on HT emitters. The fine streaks obtained with HT emitters stem from graphite-like structure planes rO*ir) which project from the ends of the needles. Taking account of the projection conditions, it may be concluded that each needle possesses only a few such ledges. The bright spots on RT and HT emitters probably also arise from surface irregularities. This conclusion may be supported by two observations: first, their density is characteristic of the field strength, and second, they are frequently arranged in chains and may thus be regarded as parts of emitting ledges. The experiments indicate for both types of emitter a deeply cleft surface structure on an atomic scale. The extended emission areas visible after treatment with water are regions of low field strength. The clearly-defined cellular structure which appears after subsequent heating (fig. 6b) is principally the result of a graphitization of the surface, and this is an indication that the surface consisted of a more weakly-bound organic structure before the heating. The appearance of a cellular structure in RT emitters shows that even at room temperature the field polymerisation of benzonitrile does not produce a completely amorphous structure, but allows more firmly bound regions of short range order to grow. 4.2. THE FIELDENHANCEMENTOF THE MICRONEEDLES The emission regions of higher field strength which allow of N, and Ar ionization on HT emitters, are composed exclusively of the fine points and ledges projecting from the surface. With the disappearance of these protrusions, the ability to ionize molecules of high ionization potential is lost, i.e. the ratio of the Ar+ to the C,HT6 ion current becomes smaller than 10e4 at the same partial pressure in the ion source, and cannot be compensated for by increasing the emitter-cathode voltage. It may be concluded from this that the field strength distribution of activated emitters is less dependent on the radii of curvature of the needle tips, as previously assumedi4), than on the morphological state of the surface and on the electrical properties of the surface material of the needles, i.e. on the x-electron states of the surface since only these states, which occupy the upper levels of the band structure, are of importance for the acceptance of the positive charges. For the local emission field strength on the tips of the microneedles of an
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activated wire emitter, the following approximate expression may be used:
(2) The first term describes the so-called “basic field” which results from the geometry of the wire emitter14). V. is the emitter-cathode potential difference and R the separation of these electrodes. reff is not the wire radius but a somewhat greater value, which also takes the mutual shielding of the needles into account. The second term, a function A of the parameter (h - reff + r,)/r, takes into account the field enhancement of a needle of length h and radius r,. r. is the wire radius. Only the ratio of the distance (h - reff + ro), by which the needle projects beyond reff, to its radius r, is effective for this field enhancement. The term /3 describes the influence of the surface roughness on the field enhancement, and q represents the extent of the departure of the electronic properties of the needles in the region of the surface from those of a metal; it is therefore a measure of the ideal n-electron density and is less than unity. q achieves its maximum value for a graphite surface. The present investigation of RT and HT emitters produced no evidence for a dependence of the field enhancement on the different bulk electrical conductivities of the various needle materials. The observed changes of field enhancement are concerned exclusively with surface effects, that is with fl and q. The relationship between the local field strength F= F(s) and the average effective field strength characterized by the ratio 29/100 is: Z 29 1 ---=--iloo (s) a exp [b F @)I ds, 1 100 ZIOOs
(3)
s
taking eq. (1) into account. S is the total emitting area and i,,,(s) local ion current density of Mf . 4.3. REDUCTION OF THE EMISSION FIELD
STRENGTH
BY FIELD
the
CORROSION
The water-induced field etching leads to corrosion of the needles in the emission regions of high field strength, which in turn results in a reduction of the surface roughness. The changes in the surface structure between fig. 6a and fig. 6b also suggest that the water corrosion extends also to the interior of the needles, producing a less dense structure of the needle. A destruction of the carbon skeleton of the needles is to be expected in the space charge regions of the needle tips if water molecules can succeed in penetrating the surface, since the positively-charged radicalic x-electron
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systems react very readily with waterrb). The result of such a process would be a denudation of the regions near the surface of x-electrons, i.e. a decrease of the field enhancement factor q. The M+ ion current of n-heptane is hardly affected by water corrosion. This allows the conclusion to be drawn that under normal conditions the field enhancement of the needles is sufficient to allow ionization of n-heptane without taking the effects described by p and q into account. 4.4. EFFECT OFSOLID DEPOSITS AND ADSORPTION LAYERS ON THEION EMISSION
Covering the emitter surface with secondary products of field dissociation processes or adsorption layers (for instance, by cooling the emitter as described above) leads to a fall in the emission field strength. The reduction of the emission field strength by such covering of the surface results not only from a smaller #I value, but also from a reduced value of q, since the x-electron density of these insulating layers is very low and this prevents the formation of high charge densities. The o-electrons, which lie energetically much lower, play no part on the mechanism of field enhancement. It is to be expected that the removal of a-electrons from the surface will lead to a breaking of C-C bonds and result in a corrosion of the surface material. The positive effect on the emission field strength of heating the emitter results first from the desorption of a part of the surface adsorbate and second from graphitization of the more firmly-bound surface layers. After a sharp reduction of the emitter voltage the gradual fall in the number of emission centres at a constant field is the result of an increasing coverage of adsorbate; the lower ionization probability after the voltage reduction enables the formation of surface layers. The same phenomenon, an exponential decrease in the number of emission centres, also appears after a rapid reduction of the emitter temperature. Under these conditions the decrease of emission current shown in fig. 5 is observed. The ion emission is thus also affected by the formation of adsorbed layers. A sensitive dependence of the ion emission on the adsorbate coverage may be expected when the penetration of the high external field into the surface affects the local ionization capacity. Such a situation exists when a barrier to electron transmission between the surface and the bulk of the needles is present, and may be removed under the influence of the penetrating electric field. This will arise when the n-electrons of the surface, which lie energetically deeper than those of the bulk, are not in resonance with the latter in the absence of an external electric field. The external field must then suffice to raise these levels enough to allow electron transition from the surface to the bulk to take place. Adsorption layers hinder this process
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of field penetration and thereby effect a reduction in the local ionization probability. Molecules adsorbed closely enough to such n-electron acceptor states in the surface are not ionized since the ionization zone is some 8, farther away. The high emission current after heating indicates an adsorbatefree surface state. The flickering of the emission centres in the low field strength regime, and particularly that of RT emitters is most probably also a result of a timedependent adsorbate coverage which affects the field penetration at the surface. The flickering of the HT emitters, on the other hand, seems to be less affected by the formation of adsorption layers; similar phenomena were observed at graphite surfaces under different ionization conditionsrr). Possibly the flickering is caused in this case by space charge fluctuations resulting from the emission itself. The stability of the emission from parts of the surface after treatment with water originates in all probability in the production of an insulating solid layer on the surface which reaches into the ionization zone of the image gas molecules. The electron transition probability from this region to the needles is small, but not subject to large local variations. 5. Conclusion
The surface structure of the micro-needles is principally responsible for the emission properties of activated emitters. The maximum field strength which can be achieved with the variously-activated emitters is determined by the field evaporation properties of the fine ledges and points produced on the surface during the activation process. However, under normal conditions a rapid de-activation of the emitter surface at high field strengths by field corrosion and the formation of organic adsorption layers is to be expected. The understanding of the observed phenomena is at present largely qualitative; further more quantitative investigations are necessary in order to present a more complete picture of field ionization at organic microneedles. Acknowledgement
The authors are grateful to the Deutsche Forschungsgemeinschaft for the financial support of this work, and one of them (D.M.T.) expresses his thanks to the Alexander von Humboldt-Stiftung for the award of a Research Scholarship, during the tenure of which this study was carried out. Thanks are also due to Dr. H.-R. Schulten for his kind assistance in the preparation of the activated emitters.
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References 1) H. D. Beckey, Field Ionization Mass Spectrometry (Pergamon, Oxford, 1971). 2) F. W. Riillgen and H. D. Beckey, Surface Sci. 23 (1970) 69. 3) H. D. Beckey, M. D. Migahed and F. W. RSllgen, in: Advances in Mass Spectrometry, Vol. V (The Institute of Petroleum, London, 1971). 4) H.-R. Schulten and H. D. Beckey, Organ. Mass Spectrom. 6 (1972) 885; Messtechnik 81 (1973) 121. 5) F. W. RGllgen and H. D. Beckey, Ber. Bunsenges. Physik, Chem. 76 (1972) 661. 6) H. D. Beckey, Z. Naturforsch. 19a (1964) 71. 7) F. Speier, H. J. Heinen and H. D. Beckey, Messtechnik 80 (1972) 147. 8) H. D. Beckey, A. Heindrichs, E. Hilt, H.-R. Schulten and H. U. Winkler, Messtechnik 79 (1971) 196. 9) L. E. Murr and 0. T. Inal, Phys. Status Solidi 4 (1971) 159. 10) W. S. Williams, J. Appl. Phys. 39 (1968) 2131. 11) L. E. Murr and 0. T. Inal, J. Appl. Phys. 42 (1971) 3487. 12) F. Okuyama, F. W. Riillgen and H. D. Beckey, Z. Naturforsch. 28a (1973) 61. 13) H. Knijppel and H. D. Beckey, Z. Naturforsch. 2la (1966) 1930. 14) H. D. Beckey, H. Krone and F. W. R8lIgen, J. Sci. Instr. 1 (1968) 118. 15) F. W. RGllgen and H. D. Beckey, Surface Sci. 27 (1971) 321.
SCIENCE 40 (1973) 264-281 0 North-Holland
THE EMISSION ORGANIC A COMBINED
PROPERTIES
FIELD FIM AND
Publishing Co.
OF
ION EMITTERS FI-MS
D. M. TAYLOR, F. W. RBLLGEN
INVESTIGATION
and H. D. BECKEY
Institut fiir Physikalische Chemie der Universitiit Bonn, 53 Bonn, Germany Received 26 April 1973; revised manuscript
received 13 June 1973
The emission properties of tungsten emitters activated with benzonitrile, such as are used in analytical organic field ion mass spectrometry, have been investigated using n-heptane as test gas. It was discovered that these emission properties are influenced principally by the morphological structure and electronic state of the surfaces of the microneedles; the bulk structure of these organic agglomerates has practically no effect on their ionization behaviour. Field ion microscope investigation of emitters activated at high temperature (HT) revealed a similar surface structure to that of pyrolytic graphite; emitters activated at room temperature (RT) showed less order. The emission regions of highest electric field strength on the tips of the needles are projecting points, ledges, and in the case of HT-emitters, structure planes. A decrease in the mean emission field strength results from the formation of solid deposits on the needle surfaces. A field corrosion of the needle tips occurs in the presence of high pressures of water and leads also to an irreversible decrease in the mean emission field strength. The effect of physically adsorbed layers on the emission properties of the emitter surface is discussed in terms of the influences of the electric field penetration on the electronic properties of the needles.
1. Introduction In analytical field ionization (FI) mass spectrometry, activated emitters are most frequently used for field ionization of organic molecules, because their application allows particularly large ion yields to be attainedl). The ionization at these activated emitters occurs on organic micro-needles grown substrate, usually by field polymerisation 2l 3) on an electrically conducting thin tungsten wire of 3 - 10 urn diameter. Of the substances with the requisite benzonitrile has proved particularly field polymerisation properties, advantageous since it requires only short activation times, that is, at fields of several megavolts per centimetre a rapid needle growth occurs after a short induction period. Activation of an emitter may be carried out at room temperature (RT) or at an emitter temperature of about 1200°C (high temperature= HT)4). The HT emitters differ from the RT ones in that they have a higher mechan264
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ical and thermal stability (stable up to about 2000°C) and a greater resistance to attack by chemically aggressive substances. Higher field strengths may also be achieved with HT emitters. Mass spectrometric observations show that the emission properties of field anodes are subject to changes which are dependent on the test substance. In the case of organic emitters, these variations affect also the average field strength and the field strength distribution over the emitting surface regions. For instance, benzene causes an increase in the field strength at the emission centres which disappears when the benzene is pumped out of the ion sources). The present investigation of the emission properties of organic emitters immediately after activation and also after exposure to several simple gases reveals a complex dependence of the emission field strength on the morphological and chemical structure of the needle surface and on the coverage and chemical nature of any adsorbate. Several different surface types may be distinguished, which may be partly interconverted by appropriated treatments. In addition, the investigation enables conclusions to be drawn about the structure of the organic needles and the electronic state of their surfaces under field ionization conditions. The investigation was carried out by complementary FI-MS and FIM studies. Whereas under the usual conditions of a field ion mass spectrometric study only integrated emission properties may be observed, the FIM allows the investigation of ionization phenomena in small surface regions. For the characterization of the emission properties in both the mass spectrometric and the FIM investigations, n-heptane was used as the test gas. This substance is relatively inert in comparison to most other organic materials. Its ionization potential of about 10 eV is only a little higher than the common values for organic substances, so that measurements under the same conditions lead to ion yields comparable with those of other gases. The ionization potentials of the inert gases, including that of xenon, are too high for use with many types of emitter. A measure of the field strength in the emitting regions may be obtained from the ratio 29/100 of the fragment ion current at m/e= 29 (C,H:) to the molecular ion current at m/e= lOO(C,H:,) using n-heptane. The C,Hl fragment arises from a field-induced decomposition of the molecular ion6). Experimentally, the relationship 29/100 = I (C,H:)/Z
(C,H:,)
= a exp (bF)
was found for the dependence of the ratio 29/100 on the field strength Fat room temperature, using a platinum tip of known radius of curvature with a=1.6x 10m3 and b=21.6 when F is given in V/AT). Under these
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conditions, the intensity ratio 29/100 is equal to the ratio of the ion current densities i2.Ji,,,,. With activated emitters, however, emission regions of different field strengths contribute to the C,H: and C,HT6 ion currents so that 29/100 is only a measure of the average of these field strengths. For activated emitters, 29/100 has values between 10e4 and 0.4. Using eq. (1) these values correspond to average field strengths between N 0.1 and 0.4 V/A. The average field strength obtained in this way gives no indication of the field strength distribution. However, according to common experience, emitters with 29/100>0.1 possess a measurable ability to ionize nitrogen, i.e. with the same partial pressures in the ion source the ratio of the N: and C,HT6 ion current is greater than 10m3. A clearer indication of the presence of emission regions of high field strength is the detection of a small H30+/Hz0+ ratio (~0.1) from the background water. Since water is always present even on the surface of the emitter, this information may be easily obtained for a characterization of the emitter state. The investigation is confined to emitters activated with benzonitrile. 2. Experimental 2.1. THE FIELD ION MICROSCOPE The FIM used in the present investigation was a conventional all-metal machine equipped with a 50 mm diameter channel plate image converter. An additional inlet system for liquids was incorporated, following conventional mass spectrometer practice. A silicone rubber membrane some 3 mm thick, through which liquids could be injected using a miniature hypodermic syringe was isolated by a high vacuum valve from a small ballast chamber which acted as the image gas source for the microscope when using organic materials or water for this purpose. To this end, the ballast chamber was connected with the microscope through a UHV leak valve, through which the chamber could also be pre-evacuated and outgassed. The residual gas composition in the microscope and the level of impurities in the image gas were monitored by a Finnigan 400 quadrupole mass spectrometer. The organic liquids were of the purity normally used for spectrometric investigations, and the principal impurity was water, in general present at a level of ~0.2%. Because of the relatively high level of water vapour it was unnecessary to re-achieve UHV conditions in the microscope between measurements. A typical background pressure was 5 x lo-’ torr, composed largely of water. The present investigations were carried out with n-heptane as imaging gas, and mostly with the emitter at room temperature.
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The most frequently used image gas pressure was 2 x lo-’ torr. The use of high pressures of gases with low ionization potentials can easily lead to gas discharges in the microscope, and such discharges, which became more intense with rising emitter temperature, prevented the use of the FIM at emitter temperatures higher than a few hundred degrees centigrade. The presence of these discharges does not disturb measurements in the mass spectrometer, because of the extremely small acceptance angle of the instrument and the different focussing conditions for the two phenomena (field ions and gas discharge ions). 2.2. PREPARATION OF THEEMITTERS Emitters for the FIM were produced by activating electrolytically-etched tungsten tips at room temperature (RT) or at about 1200°C (HT) using the procedure described extensively elsewhere 4**), to produce a dense growth of organic microneedles on the tip cap. Fig. 1 shows a Stereoscan micrograph of such an RT emitter. It was found that tips which are somewhat blunt for normal FIM use produced the best results when activated. During field ionization the current originates on the tips of the needles, and the complex geometry of the emitting envelope, together with mutual
Fig. 1.
Stereoscan micrograph of an RT-activated tungsten emitter. Activation benzonitrile; radius of curvature of the W tip: about 2000 A.
gas:
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F. W.
RijLLGEN
AND
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shielding effects of the densely-packed needles make an accurate determination of the emitting area corresponding to a single point on the microscope screen extremely difficult. Because of the overlapping of the projections of the emitting areas of neighbouring needles on the screen, the FI emission pattern may not be taken to represent the actual geometric arrangement of the emitting areas on the needle surface. It was estimated, however, from electron micrographs, that the FIM images were formed mainly by the overlapping projections of about 10 microneedles with lengths of some 1000 A and diameters of several 100 A. 2.3. THE MASSSPECTROMETER A conventional single-focusing mass spectrometer, equipped with an FI ion source was used. In place of an activated FIM emitter, a correspondingly activated 10 urn diameter tungsten wire was used, in order to increase the sensitivity of the ion detection. The density of the needles grown on a wire emitter amounted to about 108/cmZ as estimated from scanning electron micrographs, i.e. approximately lo4 needles contribute to the ion current. The length of the mostly dendritic needles was about 7 urn and the radii of curvature of the needle tips ranged from 100 to 1000 A. It was possible to heat the emitter electrically. 3. Results 3.1. EMISSIONPATTERNOF RT EMITTERS The value of 29/100 of n-heptane for the RT emitters usually lies between 0.01 and 0.05. Fig. 2a shows the FIM pattern obtained from an RT emitter at a low emitter voltage. The emission occurs largely from very small areas of the needle tips, as indicated by the bright spots on the screen. Since the effective radii of curvature of the needle tips are several hundred Angstroms, the emitting areas may be estimated to be lo-50 8, in diameter. The emission is very unstable at low emitter voltages, that is, the bright spots are repeatedly visible for a short time from several seconds to several minutes. The “switched off” time lies in the same order of magnitude. The frequency of this flickering and the number of spots participating is also influenced by the water background, increasing with increasing partial pressure of water. Increasing emitter voltage leads to a decrease of this instability, which, however, never fully disappears. The intensity fluctuations observed in the mass spectrometer, which are often of several orders of magnitude in the region of the cutoff voltage where an ion current is just detectable and remain at a level to 2 to 30% under normal measurement conditions, may be ascribed to this ionization phenomenon. .The mass
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(a)
(b) Fig. 2. (a) Field ion emission pattern of an RT-activated tungsten emitter at lower emitter voltage (7.0 kV); imaging gas: n-heptane; room temperature. (b) Field ion emission pattern of the same RT-activated tungsten emitter as in (a) at higher emitter voltage (9.0 kV); imaging gas: n-heptane; room temperature; the cellular structure is clearly visible.
D. M. TAYLOR,
F. W.
R6LLGEN
AND H. 0.
BECKEY
(a)
(b) Fig. 3. (a) Field ion emission pattern of an RT-activated emitter after a period of ionization at constant voltage (6.0 kV); imaging gas: n-heptane; room temperature. (b) Field ion emission pattern of the same RT-activated emitter as in (a), immediately after a “voltagejump”: the emitter voltage was raised to 9.0 kV, held there for a short time, then reduced rapidly to the original value of 6.0 kV. The picture was taken at this point. The marked increase in the number of emission centres may clearly be seen.
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spectrometer accepts ions from only a very small solid angle of the emitter surface, so that flicker effects are very clearly detectable. With increasing emitter voltage, the density of the emission centres grows until eventually a cellular type of structure appears (fig. 2b). The bright lines on the screen may be partly resolved into very closely-packed spots. The source of these bright lines may be taken to be projecting ledges on the surface from which an enhanced emission is obtained (see ref. 9). The projections of such ledges overlap each other and produce the impression of a cellular structure. Heating the emitter increases the definition of the cellular structure - the projection ledges become more prominent emitters. An increase in the effective emission field strength under these conditions can be measured in the mass spectrometer. It was also observed that the density of the bright spots is not instantaneously reduced to its low voltage equilibrium value after a sudden decrease in the emitter voltage. Immediately after the change in voltage, the number of emission centres remains relatively high and falls slowly to its equilibrium value with a time constant of several minutes (fig. 3). A complementary measurement in the mass spectrometer showed that 29/100 is somewhat higher immediately after the voltage reduction than later. 3.2.
THE
EMISSION PATTERN
OF
HT
EMITTERS
Immediately after activation HT emitters develop a very high field strength, in general much higher than that for RT emitters, once the voltage applied during the activation is exceeded. Values of 29/100 between 0.05 and 0.3 were measured, and the ratio H30+/HZO+ is usually less than 0.2. The ionization sensitivity for nitrogen and argon is sometimes unusually high, and can reach about 10% of that of n-heptane. These large NT and Ar+ currents suggest that the maximum attainable field strength with newlyactivated emitters is greater than 1 V/A. At the field strengths at which the emitter material field evaporates, even Ne and He ionization may be detected for a few seconds. Fig. 4 shows FIM patterns of the emission from an HT emitter at two emitter voltages. On first raising this voltage from zero, practically no bright spots become visible, and the emission comes principally from larger areas of the surface (fig. 4a). The fine, sometimes curved lines are taken to delineate graphite-type structure layers projecting from the surface, such as those observed with pure graphite emitters in the FIMiO~rl). At higher voltage, an increasing number of short lines is seen (fig. 4b) and bright spots, such as those observed with pyrolytic graphiteli), appear. The spots and lines all flicker strongly. The flickering becomes slower at lower emitter voltages. The integrating effect of the exposure time prevents the
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F. W. RiiLLGEN
AND
H. D. BECKEY
(b) Fig. 4. (a) Field ion emission pattern of an HT-activated tungsten emitter at low emitter volt age (2,5 kV); imaging gas: n-heptane; room temperature. (b) Field ion emission patt ern of the same I-IT-activated tungsten emitter as in (a), at high emitter voltage (3.3 kV); imaging gas: n-heptane; room temperature; the lines have become shorter and more numerous.
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small spots from appearing as such in the photograph. The spots and short lines are often arranged in chains. These phenomena are independent of the choice of image gas (n-heptane, nitrogen, or even water). With HT emitters, as with RT emitters, the equilibrium spot density corresponding to the new emitter voltage is not obtained imediately after a sudden voltage reduction, but the number of emission centres decreases exponentially with time. In agreement with observations in the mass spectrometer, the cutoff point for the emission moves to higher voltages once the activation voltage has been exceeded. This may be attributed to field evaporation of the emitter material in the emission regions at the highest field strengths. Heating the HT emitters causes no change in the structure ofthe needle material up to the activation temperature of about 1200°C. In the FIM an increase in the number of emission centres is observed, and in the mass spectrometer a rise in the emission field strength, from which it may be concluded that the emission regions of higher field strength grow with increasing temperature. 3.3.
EFFECT
OF WHEPTANE
ON THE EMISSION FIELD STRENGTH
A slow decrease of 29/100 with time is observed during the continuous field ionization of n-heptane. This decrease of the average field strength occurs more rapidly if the initial field strength is high, and especially for HT emitters. In confirmation of this field strength decrease, the ratio H30+/H20f of the background water rises and the ionization probabilities for Nz and Xe fall. In the nearly stable state reached after very long ionization times (up to several days) the ratio 29/100 is less than 0.01. Pumping out the test gas and subsequent readmission leads only to a small increase of 29/100, that is, the decrease of the average field strength is only slightly influenced by physically adsorbed layers under these’ conditions. Nor may the field strength decrease be attributed to the influence of the background water. It was discovered that the effect of water background alone on the emission field strength lies at least an order of magnitude lower than that of n-heptane at high pressures. A surface condition of higher field strength may be repeatedly reestablished by heating the emitter. However, the initial field strength of an HT emitter cannot usually be achieved again. In the FIM, an increase of the number of emission centres with temperature is observed, similar to that seen on increasing the emitter voltage. After turning off the heating current, the additional emission centres do not disappear immediately, but exponentially with time, the time constant being several minutes. Fig. 5 shows the typical time behavior of the M+ ion current (m/e= 100) of n-heptane observed with the mass spectrometer during such a thermal treatment of
214
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M. TAYLOR,
F. W.
R6LLGEN
AND
H. D.
BECKEY
the emitter. Repeated heating showed that the emission behaviour is more strongly influenced by the temperature achieved than the heating time chosen. I’ (see fig. 5), and also the value of 29/100 measured after switching off the emitter heating current, increase with the heating temperature. So long as thermal/field evaporation of the emitter material is avoided, the presence of the electric field during heating is without effect on the subsequent emitter state. Emitter
Heating
on
off
I
I
---I At-
i
i
I
i
5
6
;
i
-
Time (min
1
Fig. 5. Schematic diagram of the response of the Mf ion intensity of n-heptane to a suddenly-imposed temperature increase of ca. 7OO”C, applied for a time hr. A peak intensity of I’ is achieved immediately after the heating is switched off.
Chemical changes in the surface may be caused in the regions of higher field strength by the neutral field dissociation products which remain on the surface in relatively high concentration, in particular CzH4 and C,H,. These unsaturated molecules can undergo field polymerisation and thereby release protonsrs). C,Hi and C&Hi ions are present in the n-heptane mass spectrum with only low intensity. An indication of the presence of such adsorption layers is given by the observation of a short pulse in the total emission on the first heating of the emitter after a long emission time. A reduction of the emission field strength also results from cooling the emitter. About the condensation temperature of n-heptane a rapid fall of 29/100 to 1% of its value at room temperature is observed, although the M+ ion current remains high. The original state, i.e. the same room tempera-
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ION EMITTERS
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ture value of 29/100, is obtained after thawing the emitter out. The effect of the emitter temperature on the field dissociation of heptane was measured at higher temperatures and is much smallerrs). This suggests that the emission field strength is also influenced by thick physically adsorbed layers. 3.4. THE EMISSION PATTERN AFTER THE FIELD IONIZATION OF WATER High partial pressures of water (about 10V4 torr) under field ionization conditions lead to rapid and marked changes in the emission properties of activated emitters. The 29/100 ratio of HT emitters for example, falls in the course of a few hours from over 0.1 to under 0.01. N2 ionization is subsequently impossible, and Xe ionization is only occasionally just detectable. Small partial pressures of water, however (about 10m6 torr), have hardly any effect on the emission field strength, so that it may be concluded that the de-activation phenomenon is due to the production of adsorbed multilayers of water. More precisely, the decrease in field strength may be attributed to corrosive reactions of OH radicals with. the needles. They are formed on the emitter surface by proton transfer reactions in such water layers. Typical field ionization products of the interaction between the material of the organic microneedles and water are the ions CHO+, CH,O+, CzH30+, CHO: and C3H50+ which are present in the water spectrum. Fig. 6a shows a typical FIM pattern of the emission of an RT emitter after ionization at high water pressure. Before this treatment, ionization occurs at small emission centres (fig. 2), but afterwards it proceeds mostly from large continuous areas. This ionization is usually very stable; no flickering is observed. The n-heptane M+ ion current, which approximates to the total ion current, is only slightly affected by the water treatment. A comparison of the needle shapes in the electron microscope before and after water treatment showed hardly any change. Only the needle tips were affected; some of them were very slightly flattened. The n-heptane emission pattern of an RT emitter after water treatment and a short heating is shown in fig. 6b. Heating the emitter produces a clearly observable emission pulse corresponding to the desorption of the thermally unstable parts of the surface, so that the cellular structure of the more stable parts remains. This cellular structure is only a secondary consequence of field corrosion. The primary effect is a thermally induced structural change in the needles and their surfaces. The 29/100 ratio is once more larger after the heating than before, however, the regions of very high field strength cannot be produced again by increasing the emitter voltage. The intensity fluctuations observed in the mass spectrometer are larger after the thermal treatment.
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D. M. TAYLOR,
F. W.
R6LLGEN
AND H. D. BECKEY
(a)
(b) Fig. 6. (a) Field ion emission pattern of an RT-activated emitter after ionization of water at high partial pressure (2 x 1OP torr); the imaging gas is n-heptane at room temperature; emitter voltage is 6.0 kV. (b) Field ion emission pattern of an RT-activated emitter after ionization of water at high partial pressure (2 x 10m5torr) and subsequent heating to red heat after removal of the water; the imaging gas is n-heptane at room temperature; emitter voltage is 4.0 kV.
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4. Discussion 4.1. MORPHOLOGICALSTRUCTUREOF THE NEEDLESURFACES
The structure of the needle surface is not homogeneous. In both RT and HT emitters emitting ledges corresponding to the lines in the FIM patterns project from the needle surface. Judging from the lateral extent of the lines the ledges on RT emitters are coarser than those on HT emitters. The fine streaks obtained with HT emitters stem from graphite-like structure planes rO*ir) which project from the ends of the needles. Taking account of the projection conditions, it may be concluded that each needle possesses only a few such ledges. The bright spots on RT and HT emitters probably also arise from surface irregularities. This conclusion may be supported by two observations: first, their density is characteristic of the field strength, and second, they are frequently arranged in chains and may thus be regarded as parts of emitting ledges. The experiments indicate for both types of emitter a deeply cleft surface structure on an atomic scale. The extended emission areas visible after treatment with water are regions of low field strength. The clearly-defined cellular structure which appears after subsequent heating (fig. 6b) is principally the result of a graphitization of the surface, and this is an indication that the surface consisted of a more weakly-bound organic structure before the heating. The appearance of a cellular structure in RT emitters shows that even at room temperature the field polymerisation of benzonitrile does not produce a completely amorphous structure, but allows more firmly bound regions of short range order to grow. 4.2. THE FIELDENHANCEMENTOF THE MICRONEEDLES The emission regions of higher field strength which allow of N, and Ar ionization on HT emitters, are composed exclusively of the fine points and ledges projecting from the surface. With the disappearance of these protrusions, the ability to ionize molecules of high ionization potential is lost, i.e. the ratio of the Ar+ to the C,HT6 ion current becomes smaller than 10e4 at the same partial pressure in the ion source, and cannot be compensated for by increasing the emitter-cathode voltage. It may be concluded from this that the field strength distribution of activated emitters is less dependent on the radii of curvature of the needle tips, as previously assumedi4), than on the morphological state of the surface and on the electrical properties of the surface material of the needles, i.e. on the x-electron states of the surface since only these states, which occupy the upper levels of the band structure, are of importance for the acceptance of the positive charges. For the local emission field strength on the tips of the microneedles of an
218
D.M.TAYLOR,F.W.RijLLGENANDH.D.BECKEY
activated wire emitter, the following approximate expression may be used:
(2) The first term describes the so-called “basic field” which results from the geometry of the wire emitter14). V. is the emitter-cathode potential difference and R the separation of these electrodes. reff is not the wire radius but a somewhat greater value, which also takes the mutual shielding of the needles into account. The second term, a function A of the parameter (h - reff + r,)/r, takes into account the field enhancement of a needle of length h and radius r,. r. is the wire radius. Only the ratio of the distance (h - reff + ro), by which the needle projects beyond reff, to its radius r, is effective for this field enhancement. The term /3 describes the influence of the surface roughness on the field enhancement, and q represents the extent of the departure of the electronic properties of the needles in the region of the surface from those of a metal; it is therefore a measure of the ideal n-electron density and is less than unity. q achieves its maximum value for a graphite surface. The present investigation of RT and HT emitters produced no evidence for a dependence of the field enhancement on the different bulk electrical conductivities of the various needle materials. The observed changes of field enhancement are concerned exclusively with surface effects, that is with fl and q. The relationship between the local field strength F= F(s) and the average effective field strength characterized by the ratio 29/100 is: Z 29 1 ---=--iloo (s) a exp [b F @)I ds, 1 100 ZIOOs
(3)
s
taking eq. (1) into account. S is the total emitting area and i,,,(s) local ion current density of Mf . 4.3. REDUCTION OF THE EMISSION FIELD
STRENGTH
BY FIELD
the
CORROSION
The water-induced field etching leads to corrosion of the needles in the emission regions of high field strength, which in turn results in a reduction of the surface roughness. The changes in the surface structure between fig. 6a and fig. 6b also suggest that the water corrosion extends also to the interior of the needles, producing a less dense structure of the needle. A destruction of the carbon skeleton of the needles is to be expected in the space charge regions of the needle tips if water molecules can succeed in penetrating the surface, since the positively-charged radicalic x-electron
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279
systems react very readily with waterrb). The result of such a process would be a denudation of the regions near the surface of x-electrons, i.e. a decrease of the field enhancement factor q. The M+ ion current of n-heptane is hardly affected by water corrosion. This allows the conclusion to be drawn that under normal conditions the field enhancement of the needles is sufficient to allow ionization of n-heptane without taking the effects described by p and q into account. 4.4. EFFECT OFSOLID DEPOSITS AND ADSORPTION LAYERS ON THEION EMISSION
Covering the emitter surface with secondary products of field dissociation processes or adsorption layers (for instance, by cooling the emitter as described above) leads to a fall in the emission field strength. The reduction of the emission field strength by such covering of the surface results not only from a smaller #I value, but also from a reduced value of q, since the x-electron density of these insulating layers is very low and this prevents the formation of high charge densities. The o-electrons, which lie energetically much lower, play no part on the mechanism of field enhancement. It is to be expected that the removal of a-electrons from the surface will lead to a breaking of C-C bonds and result in a corrosion of the surface material. The positive effect on the emission field strength of heating the emitter results first from the desorption of a part of the surface adsorbate and second from graphitization of the more firmly-bound surface layers. After a sharp reduction of the emitter voltage the gradual fall in the number of emission centres at a constant field is the result of an increasing coverage of adsorbate; the lower ionization probability after the voltage reduction enables the formation of surface layers. The same phenomenon, an exponential decrease in the number of emission centres, also appears after a rapid reduction of the emitter temperature. Under these conditions the decrease of emission current shown in fig. 5 is observed. The ion emission is thus also affected by the formation of adsorbed layers. A sensitive dependence of the ion emission on the adsorbate coverage may be expected when the penetration of the high external field into the surface affects the local ionization capacity. Such a situation exists when a barrier to electron transmission between the surface and the bulk of the needles is present, and may be removed under the influence of the penetrating electric field. This will arise when the n-electrons of the surface, which lie energetically deeper than those of the bulk, are not in resonance with the latter in the absence of an external electric field. The external field must then suffice to raise these levels enough to allow electron transition from the surface to the bulk to take place. Adsorption layers hinder this process
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of field penetration and thereby effect a reduction in the local ionization probability. Molecules adsorbed closely enough to such n-electron acceptor states in the surface are not ionized since the ionization zone is some 8, farther away. The high emission current after heating indicates an adsorbatefree surface state. The flickering of the emission centres in the low field strength regime, and particularly that of RT emitters is most probably also a result of a timedependent adsorbate coverage which affects the field penetration at the surface. The flickering of the HT emitters, on the other hand, seems to be less affected by the formation of adsorption layers; similar phenomena were observed at graphite surfaces under different ionization conditionsrr). Possibly the flickering is caused in this case by space charge fluctuations resulting from the emission itself. The stability of the emission from parts of the surface after treatment with water originates in all probability in the production of an insulating solid layer on the surface which reaches into the ionization zone of the image gas molecules. The electron transition probability from this region to the needles is small, but not subject to large local variations. 5. Conclusion
The surface structure of the micro-needles is principally responsible for the emission properties of activated emitters. The maximum field strength which can be achieved with the variously-activated emitters is determined by the field evaporation properties of the fine ledges and points produced on the surface during the activation process. However, under normal conditions a rapid de-activation of the emitter surface at high field strengths by field corrosion and the formation of organic adsorption layers is to be expected. The understanding of the observed phenomena is at present largely qualitative; further more quantitative investigations are necessary in order to present a more complete picture of field ionization at organic microneedles. Acknowledgement
The authors are grateful to the Deutsche Forschungsgemeinschaft for the financial support of this work, and one of them (D.M.T.) expresses his thanks to the Alexander von Humboldt-Stiftung for the award of a Research Scholarship, during the tenure of which this study was carried out. Thanks are also due to Dr. H.-R. Schulten for his kind assistance in the preparation of the activated emitters.
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References 1) H. D. Beckey, Field Ionization Mass Spectrometry (Pergamon, Oxford, 1971). 2) F. W. Riillgen and H. D. Beckey, Surface Sci. 23 (1970) 69. 3) H. D. Beckey, M. D. Migahed and F. W. RSllgen, in: Advances in Mass Spectrometry, Vol. V (The Institute of Petroleum, London, 1971). 4) H.-R. Schulten and H. D. Beckey, Organ. Mass Spectrom. 6 (1972) 885; Messtechnik 81 (1973) 121. 5) F. W. RGllgen and H. D. Beckey, Ber. Bunsenges. Physik, Chem. 76 (1972) 661. 6) H. D. Beckey, Z. Naturforsch. 19a (1964) 71. 7) F. Speier, H. J. Heinen and H. D. Beckey, Messtechnik 80 (1972) 147. 8) H. D. Beckey, A. Heindrichs, E. Hilt, H.-R. Schulten and H. U. Winkler, Messtechnik 79 (1971) 196. 9) L. E. Murr and 0. T. Inal, Phys. Status Solidi 4 (1971) 159. 10) W. S. Williams, J. Appl. Phys. 39 (1968) 2131. 11) L. E. Murr and 0. T. Inal, J. Appl. Phys. 42 (1971) 3487. 12) F. Okuyama, F. W. Riillgen and H. D. Beckey, Z. Naturforsch. 28a (1973) 61. 13) H. Knijppel and H. D. Beckey, Z. Naturforsch. 2la (1966) 1930. 14) H. D. Beckey, H. Krone and F. W. R8lIgen, J. Sci. Instr. 1 (1968) 118. 15) F. W. RGllgen and H. D. Beckey, Surface Sci. 27 (1971) 321.