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Field emission through single strontium atoms adsorbed on a tungsten surface
SURFACE
SCIENCE 12 (1968) 385-389 0 North-Holland
LETTERS FIELD EMISSION
TO THE
THROUGH
ADSORBED
Publishing
Co., Amsterdam
EDITOR
SINGLE
ON A TUNGSTEN
STRONTIUM
ATOMS
SURFACE*
Received 25 June 1968
The theory of tunnel resonance recently developed by Duke and Alferieffr) has stimulated interest in the field emission properties of single atoms adsorbed on metal surfaces2). The present study indicates that by using probe-hole techniques with the field emission microscope one may not only observe the arrival of single adatomss) and measure their current-voltage characteristics4) (Fowler-Nordheim curves), but also measure the totalenergy distribution for electrons emitted “through” the adsorbed atom. Measurements show substantial agreement with predictions of the DukeAlferieff theory. A schematic diagram of the apparatus used for this experiment appears in fig. 1. The evaporation source was prepared by spraying a mixture of spectroscopically pure SrCO, in a nitrocellulose binder upon a 0.15 mm W wire. Activation in the vacuum system followed customary procedures for oxide cathodess) except for the slightly higher temperatures desired for evaporation of Sr rather than the usual emission of electrons. Prepared in this way the coil provides a supply of approximately 99.7% pure single Sr atoms6). Typical evaporation times of 20 set at 1400 K were needed to provide a single atom in the region under study. For our tip of 60 nm (600 A) radius, the region from which electrons are accepted by the probe hole has an area of about 4 nm2 (400 A’). All data reported here were obtained under ultrahigh vacuum conditions (p< lo-i1 Torr). The evaporation coil and support leads were thoroughly outgassed prior to each measurement sequence. Electrode insulation in the energy analyzer placed an uncomfortably low (2 kV) upper limit on anode potential. Thus, in order to avoid excessive blunting of the emitter, field desorption was abandoned and tip flashing temperatures were restricted to about 1500 K. While it is realized that one cannot completely clean a surface by such gentle heating, it should be noted that this temperature is completely adequate to remove any Sr, SrO, CO, H and N. The pattern, as viewed on the * The information presented herein is taken from a thesis by the first author offered in partial fulfillment of the requirements for the degree M.S. in Physics, University of Maryand, College Park, Md., U.S.A. 385
386
HOWARD
E. CLARK
AND
RUSSELL
D. YOUNG
second anode, very closely resembled that reported by Gomer and Hulm 7) for an oxygen covered tip heated to this temperature. The region of the tip used in these experiments was slightly off the central (110) plane in the direction of a (111) plane and had a work function of about 6 eV as determined from the slope of the Fowler-Nordheim curve and the slope of the logarithmic total-energy distributions). By comparing the slope of the F-N curve for the overall tip with that for the small region mentioned above, we arrived at a value of work function for the overall tip of about 4.7 eV.
ANODESTRONTIUMSOURCE
/ -ELECTRONS OR IONS
PHOSPHOR’ SCREEN ENERGY ANALYZER
“TO
Fig. 1.
ELECTROMETER
Schematic diagram of field emission energy analyzer.
Fig. 2 is an example of the differentiated total-energy distributions which were obtained at 21 K from the region consisting of about 30 substrate atoms described above. The arrival of a single Sr atom is clearly demonstrated in curve (A) which represents a plot of total current from the region as a function of time. Note the five-fold increase in current. If this additional current is interpreted as tunneling “through” the adsorbed atom, then, about 120 times as many electrons are emitted through this atom as are emitted per atom in the surface. Curve (B) is the total-energy distribution for electrons tunneling out of the W surface, while (C) includes contributions from the W substrate (curve B) as well as current through the adsorbed Sr atom. Normalization was accomplished by reducing the gain of the X-Y recorder.
FIELD
Note the absence
of any additional
EMISSION
peaks or shoulders
387
in curve (C) as well
as the reduced width at half-maximum. By subtracting the background current (curve B) from curve (C) we were able to study the total energy distribution of electrons emitted “through” the adsorbate. In a similar fashion we obtained F-N curves for the W, the W + Sr and for the Sr alone. Results of these measurements indicate that upon adsorption of a single Sr adatom (a) The tunneling current from the region of about 30 atoms increases by a factor of 3 to 5 so that the emission “through” the adsorbed atom was about 100 times greater than for the surface atoms. (b) The additional current “through” this adsorbed atom has the same F-N slope (to within about 3%) as the current from the substrate, and (c) An increased value for the slope of the logarithmic total-energy distribution (10 to 25% larger).
-60
-5.5 TOTAL
-5.0 ENERGY
(ev)-
Fig. 2. Curve (A) is a plot of total current from small region of tip versus time. Note current increase caused by the arrival of a single Sr atom. (B) and (C) are differential total-energy distributions obtained before and after the arrival of the Sr adatom.
It remains, now, to interpret these results. Adopting the customary, classical, point of view we first note that Sr is an electropositive atom which decreases the work function of the surface upon which it is adsorbed. Also, since the atom protrudes from the surface one would expect the electric field to increase. Now, the slope of the F-N curve, S,,cc(p3/F while the slope of a logarithmic total-energy distribution S,,cccp*/F. Upon adsorption of Sr one should, therefore, expect to find (a) an increase in emitted current, (b) a decrease in the slope of the F-N curve with an even, (c) smaller decrease in the slope of the energy distribution and a corre-
388
HOWARD
E. CLARK
AND
RUSSELL
D. YOUNG
sponding increase in the half-width of the differential curves shown in fig. 2. Clearly, the measurements do not agree with these predictions. The inability of the classical model to explain these results leads us to consider the possibility of quantum mechanical tunneling through the adsorbed atom. This problem has been treated by Duke and Alferieff 1) who obtained an exact solution for a simplified one dimensional model. They first calculated R(W), the factor by which an adsorbate atom increases the F-N transmission probability, and then concluded that metallic adsorbates should lead to wide (d- 1 eV) resonances in the tunneling probability, which would cause: (a) large (102-104) en h ancements in emission current, (b) no changes in the slopes of F-N curves at typical fields of 3-5 x lo7 V/cm, (c) increases in the slope of the logarithmic total-energy distributions In [I, -Z(E)] versus E or In [dZ(E)/dE] versus E, and (d) additional peaks and shoulders in the energy distributions (provided the valence level of the adsorbate lies below the Fermi energy). Predictions (a), (b) and (c) are clearly in qualitative agreement with the measurements reported above. Furthermore, if one uses a straight line approximation for R(W) in the region of energy below the resonance (dotted curve in fig. 5 of the Duke-Alferieff paper’)), then it is possible to calculate the increase in slope expected for a logarithmic plot of the totalenergy distribution. For the specific model considered in that example, viz. a metal atom 0.2 nm above the surface and approximated by a square well potential 0.128 nm wide, the model predicts a change in slope of +0.20. For a square well of width 0.103 nm it suggests an increase of about 0.40. Measured changes for Sr on W (width of about 0.14 nm for a Thomas-Fermi potential) fell within the range 0.20 to 0.44 providing additional quantitative agreement with this theory. Regarding prediction (d) we point out that the surface upon which the Sr atoms were deposited had a measured work function of about 6 eV. For an isolated Sr atom, the first ionization potential is 5.7 eV. When such an atom is moved to within a fraction of a nanometer of a metal surface, however, this valence level is broadened and shifted upward by a few tenths of an electron volts) (to the range of 5.0 to 5.4 eV). The valence level is, therefore, expected to lie above the Fermi energy so that the peak of the transmission probability lies outside the range of observation, and we do not expect to find a peak of the type predicted in (d). Only the effects (a, b and c) of the broad tail of the resonance have been observed. Future measurements will be performed on clean, well characterized surfaces such as field-desorbed single crystal planes; additional adsorbate-
FIELD
EMISSION
389
substrate systems will be examined, and measurements will probe deeper into the Fermi sea. The authors are indebted to Dr. Ward Plummer for many fruitful discussions, particularly regarding the interpretation of data, as well as for his suggestions and assistance in performing the experiment. HOWARD E. CLARK
and RUSSELL D. YOUNG
National Bureau of Standards, Washington, D.C. 20234, U.S.A.
References 1) C. B. Duke and M. E. Alferieff, J. Chem. Phys. 46 (1967) 923. 2) The visibility of individual adatoms in the field emission microscope was first reported by E. W. Mullet-, Z. Physik 108 (1938) 668. 3) Observations of the arrival of single adatoms were first reported by A. A. Holscher in his doctoral dissertation, Adsorption Studies with the Field-Emission and Field-Ion Microscope (Univ. of Leiden, 1967). 4) E. W. Plummer used the field ion microscope to locate single tungsten atoms adsorbed on a tungsten tip and then measured their current-voltage characteristics but not their energy distributions, cf. Appl. Phys. Letters 11 (1967) 194. 5) F. Rosebury, Handbook of Electron Tube and Vacuum Techniques (Addison-Wesley, Reading, Mass., 1965). 6) G. E. Moore, H. W. Allison and J. Morrison, J. Chem. Phys. 18 (1950) 1579. 7) R. Gomer and J. K. Hulm, J. Chem. Phys. 27 (1957) 1363. 8) The procedure for measuring single crystal work functions with this instrument is described in Appl. Phys. Letters 9 (1966) 265. 9) J. W. Gadzuk, Surface Sci. 6 (1967) 133.
SCIENCE 12 (1968) 385-389 0 North-Holland
LETTERS FIELD EMISSION
TO THE
THROUGH
ADSORBED
Publishing
Co., Amsterdam
EDITOR
SINGLE
ON A TUNGSTEN
STRONTIUM
ATOMS
SURFACE*
Received 25 June 1968
The theory of tunnel resonance recently developed by Duke and Alferieffr) has stimulated interest in the field emission properties of single atoms adsorbed on metal surfaces2). The present study indicates that by using probe-hole techniques with the field emission microscope one may not only observe the arrival of single adatomss) and measure their current-voltage characteristics4) (Fowler-Nordheim curves), but also measure the totalenergy distribution for electrons emitted “through” the adsorbed atom. Measurements show substantial agreement with predictions of the DukeAlferieff theory. A schematic diagram of the apparatus used for this experiment appears in fig. 1. The evaporation source was prepared by spraying a mixture of spectroscopically pure SrCO, in a nitrocellulose binder upon a 0.15 mm W wire. Activation in the vacuum system followed customary procedures for oxide cathodess) except for the slightly higher temperatures desired for evaporation of Sr rather than the usual emission of electrons. Prepared in this way the coil provides a supply of approximately 99.7% pure single Sr atoms6). Typical evaporation times of 20 set at 1400 K were needed to provide a single atom in the region under study. For our tip of 60 nm (600 A) radius, the region from which electrons are accepted by the probe hole has an area of about 4 nm2 (400 A’). All data reported here were obtained under ultrahigh vacuum conditions (p< lo-i1 Torr). The evaporation coil and support leads were thoroughly outgassed prior to each measurement sequence. Electrode insulation in the energy analyzer placed an uncomfortably low (2 kV) upper limit on anode potential. Thus, in order to avoid excessive blunting of the emitter, field desorption was abandoned and tip flashing temperatures were restricted to about 1500 K. While it is realized that one cannot completely clean a surface by such gentle heating, it should be noted that this temperature is completely adequate to remove any Sr, SrO, CO, H and N. The pattern, as viewed on the * The information presented herein is taken from a thesis by the first author offered in partial fulfillment of the requirements for the degree M.S. in Physics, University of Maryand, College Park, Md., U.S.A. 385
386
HOWARD
E. CLARK
AND
RUSSELL
D. YOUNG
second anode, very closely resembled that reported by Gomer and Hulm 7) for an oxygen covered tip heated to this temperature. The region of the tip used in these experiments was slightly off the central (110) plane in the direction of a (111) plane and had a work function of about 6 eV as determined from the slope of the Fowler-Nordheim curve and the slope of the logarithmic total-energy distributions). By comparing the slope of the F-N curve for the overall tip with that for the small region mentioned above, we arrived at a value of work function for the overall tip of about 4.7 eV.
ANODESTRONTIUMSOURCE
/ -ELECTRONS OR IONS
PHOSPHOR’ SCREEN ENERGY ANALYZER
“TO
Fig. 1.
ELECTROMETER
Schematic diagram of field emission energy analyzer.
Fig. 2 is an example of the differentiated total-energy distributions which were obtained at 21 K from the region consisting of about 30 substrate atoms described above. The arrival of a single Sr atom is clearly demonstrated in curve (A) which represents a plot of total current from the region as a function of time. Note the five-fold increase in current. If this additional current is interpreted as tunneling “through” the adsorbed atom, then, about 120 times as many electrons are emitted through this atom as are emitted per atom in the surface. Curve (B) is the total-energy distribution for electrons tunneling out of the W surface, while (C) includes contributions from the W substrate (curve B) as well as current through the adsorbed Sr atom. Normalization was accomplished by reducing the gain of the X-Y recorder.
FIELD
Note the absence
of any additional
EMISSION
peaks or shoulders
387
in curve (C) as well
as the reduced width at half-maximum. By subtracting the background current (curve B) from curve (C) we were able to study the total energy distribution of electrons emitted “through” the adsorbate. In a similar fashion we obtained F-N curves for the W, the W + Sr and for the Sr alone. Results of these measurements indicate that upon adsorption of a single Sr adatom (a) The tunneling current from the region of about 30 atoms increases by a factor of 3 to 5 so that the emission “through” the adsorbed atom was about 100 times greater than for the surface atoms. (b) The additional current “through” this adsorbed atom has the same F-N slope (to within about 3%) as the current from the substrate, and (c) An increased value for the slope of the logarithmic total-energy distribution (10 to 25% larger).
-60
-5.5 TOTAL
-5.0 ENERGY
(ev)-
Fig. 2. Curve (A) is a plot of total current from small region of tip versus time. Note current increase caused by the arrival of a single Sr atom. (B) and (C) are differential total-energy distributions obtained before and after the arrival of the Sr adatom.
It remains, now, to interpret these results. Adopting the customary, classical, point of view we first note that Sr is an electropositive atom which decreases the work function of the surface upon which it is adsorbed. Also, since the atom protrudes from the surface one would expect the electric field to increase. Now, the slope of the F-N curve, S,,cc(p3/F while the slope of a logarithmic total-energy distribution S,,cccp*/F. Upon adsorption of Sr one should, therefore, expect to find (a) an increase in emitted current, (b) a decrease in the slope of the F-N curve with an even, (c) smaller decrease in the slope of the energy distribution and a corre-
388
HOWARD
E. CLARK
AND
RUSSELL
D. YOUNG
sponding increase in the half-width of the differential curves shown in fig. 2. Clearly, the measurements do not agree with these predictions. The inability of the classical model to explain these results leads us to consider the possibility of quantum mechanical tunneling through the adsorbed atom. This problem has been treated by Duke and Alferieff 1) who obtained an exact solution for a simplified one dimensional model. They first calculated R(W), the factor by which an adsorbate atom increases the F-N transmission probability, and then concluded that metallic adsorbates should lead to wide (d- 1 eV) resonances in the tunneling probability, which would cause: (a) large (102-104) en h ancements in emission current, (b) no changes in the slopes of F-N curves at typical fields of 3-5 x lo7 V/cm, (c) increases in the slope of the logarithmic total-energy distributions In [I, -Z(E)] versus E or In [dZ(E)/dE] versus E, and (d) additional peaks and shoulders in the energy distributions (provided the valence level of the adsorbate lies below the Fermi energy). Predictions (a), (b) and (c) are clearly in qualitative agreement with the measurements reported above. Furthermore, if one uses a straight line approximation for R(W) in the region of energy below the resonance (dotted curve in fig. 5 of the Duke-Alferieff paper’)), then it is possible to calculate the increase in slope expected for a logarithmic plot of the totalenergy distribution. For the specific model considered in that example, viz. a metal atom 0.2 nm above the surface and approximated by a square well potential 0.128 nm wide, the model predicts a change in slope of +0.20. For a square well of width 0.103 nm it suggests an increase of about 0.40. Measured changes for Sr on W (width of about 0.14 nm for a Thomas-Fermi potential) fell within the range 0.20 to 0.44 providing additional quantitative agreement with this theory. Regarding prediction (d) we point out that the surface upon which the Sr atoms were deposited had a measured work function of about 6 eV. For an isolated Sr atom, the first ionization potential is 5.7 eV. When such an atom is moved to within a fraction of a nanometer of a metal surface, however, this valence level is broadened and shifted upward by a few tenths of an electron volts) (to the range of 5.0 to 5.4 eV). The valence level is, therefore, expected to lie above the Fermi energy so that the peak of the transmission probability lies outside the range of observation, and we do not expect to find a peak of the type predicted in (d). Only the effects (a, b and c) of the broad tail of the resonance have been observed. Future measurements will be performed on clean, well characterized surfaces such as field-desorbed single crystal planes; additional adsorbate-
FIELD
EMISSION
389
substrate systems will be examined, and measurements will probe deeper into the Fermi sea. The authors are indebted to Dr. Ward Plummer for many fruitful discussions, particularly regarding the interpretation of data, as well as for his suggestions and assistance in performing the experiment. HOWARD E. CLARK
and RUSSELL D. YOUNG
National Bureau of Standards, Washington, D.C. 20234, U.S.A.
References 1) C. B. Duke and M. E. Alferieff, J. Chem. Phys. 46 (1967) 923. 2) The visibility of individual adatoms in the field emission microscope was first reported by E. W. Mullet-, Z. Physik 108 (1938) 668. 3) Observations of the arrival of single adatoms were first reported by A. A. Holscher in his doctoral dissertation, Adsorption Studies with the Field-Emission and Field-Ion Microscope (Univ. of Leiden, 1967). 4) E. W. Plummer used the field ion microscope to locate single tungsten atoms adsorbed on a tungsten tip and then measured their current-voltage characteristics but not their energy distributions, cf. Appl. Phys. Letters 11 (1967) 194. 5) F. Rosebury, Handbook of Electron Tube and Vacuum Techniques (Addison-Wesley, Reading, Mass., 1965). 6) G. E. Moore, H. W. Allison and J. Morrison, J. Chem. Phys. 18 (1950) 1579. 7) R. Gomer and J. K. Hulm, J. Chem. Phys. 27 (1957) 1363. 8) The procedure for measuring single crystal work functions with this instrument is described in Appl. Phys. Letters 9 (1966) 265. 9) J. W. Gadzuk, Surface Sci. 6 (1967) 133.