• Fe/W System
• Co/W System

Adsorption Studies
Growth of nickel on tungsten
Equilibration Studies
Current Stability measurements and Noise analysis

Field emission from Amorphous Silicon films on tungsten

Light Induced effects
Effects of thermal annealing

Field emission from Diamond coated tungsten tips

Field Emission from Carbon Nanotubes deposited on tungsten

Adlayers of 3-d transition metals (Fe, Co, Ni) studied using Probe-hole field emission microscopy

        First transition series of metals (Fe, Co, Ni etc) deposited on tungsten form important bimetallic catalyst systems having extensive applications in reactions involving hydrocarbons, like methanation. Ni/W(110) and Ni/W(100)  enables CO methanation  because nickel forms an active metal on the relatively inactive tungsten surfaces. An excellent review of the catalytic reactions over single crystal metal surfaces  and supported metal layers is presented by J A Rodriguez and D W Gooman [84]. The dissociation of carbon monoxide (CO) by the well known disproportionation reaction (Boudouard Reaction), 2CO --. C +CO2  has been studied on suported nickel catalysts and on  nickel films [85-87].

        Supported metallic overlayers catalyze most of these reactions and it is often found in most of the cases that the catalysis is “structure sensitive” i.e., the morphology of a catalyst is found to change its catalytic activity.  Chemically modified surfaces and mixed metal catalyst pauses the long-standing question of the role of electronic in comparison with the geometric (structural) effects in changing the surface reactivity. The answers to these questions can be arrived at by using models of bimetallic systems that have been made by deposition of one metal onto another single crystal faces of a substrate metal in UHV environment. The overlayers are generally strained due to the lattice misfit with the underlying metals and give rise to drastic change in properties in comparison with their freestanding surfaces.  The studies on these strained overlayers provide deep insight into the problems in bimetallic catalysis and thus enhancing the ability to control the catalytic activity and selectivity by tailoring the catalysts composition.

Probe-hole field emission microscopy has been used to study submonolayer and monolayer amounts of iron on various tungsten planes. The adsorption is found to be insensitive to the substrate depostion temperatures of 300, 600 and 1100K as seen by the variation of the workfunction and the pre exponential factor with the adsorbate doses. The workfunction of the adsorbate covered surfaces reduces in case of  the (100), (110) and (112) tungsten planes, while it increases in case of (111) and (116) planes, becoming coverage independant with a workfunction of 4.6 eV. This has been attributed to surface aloying of iron atoms with tungsten.

(Adsorption studies of iron on tungsten by probe hole field emission microscopy, A D Adsool, Rajeev Pande, R B sharma, M A  More, and D S Joag. Appl..Surf Sci. 87/88, (1995) 37)

The changes in workfunction, and in the Fowler Nordheim Pre-exponential factor, have been investigated for  adsorbtion on tungsten (110), (100) and (111) planes by probe-hole field emission microscopy. The workfunction variations have been found to be strongly dependent on the temperature in the range 300 - 580K  on these planes. Workfunction decreases with submonolayer coverages of cobalt on the W(110) and W(100) planes. On the W(111) plane, the workfunction initially decreases and then exhibits a jump towards positive values at relatively higher doses. The abrupt increase in workfunction and the corresponding zero cross over of the pre exponential factor have been attributed to surface reconstruction. The results are
interpreted in terms of the electronic structure and the degree of strain produced in the pseudomorphically grown cobalt on various crystallographic planes of tungsten.

Adsorption studies of cobalt on tungsten (110), (100), (111) planes by probe hole field emission microscopy , R B Sharma, A D Adsool, N. Pradeep and D S Joag,  Appl. Surf. Sci. 94/95, 177 (1996).

Adsorption Studies

Adsorption in the submonolayer coverages of nickel on tungsten has been studied at 300K, 500K, 630K, 730K, on average W, W(110), W(121), W(111) and W(100) via workfunction variations with coverage. At higher temperatures, apart from adsorption, there are other processes like surface diffusion, surface alloying, bulk diffusion comes into picture. The surface rearrangement of the substrate itself will play a major role on the ad layers. The important results and findings of this study are as follows:

1. The average workfunction of tungsten with nickel coverage is found to increase and this can be explained on the basis of the electronic charge transfer from tungsten to nickel due to the higher electron affinity of nickel as compared to tungsten.
2. Probe-hole measurements indicated that the behaviour on individual single crystal planes are controlled more by the geometry of the substrate surface. As a general observation, it was found that, the workfunction of the rough plane W(111) increases upon adsorption of nickel. This is because the adatoms in the submonolayer coverage can be accommodated in the open substrate structure because of their comparable atomic sizes. The workfunction of the rough planes was thus found to increase. Similarly, a decrease in the workfunction has been observed for relatively smoother planes viz. W(110), W(121), W(100) because of the roughening effects.
3. The average workfunction at all three temperatures was found to increase with nickel coverage, though the magnitude of saturation was different. This observation is similar to that observed at 300K. The difference in the saturation value may be due to the various processes taking place on different substrate planes at various temperatures. The observations on the total tip surface are a gross effect of all these process contributions coming from different emitting planes.
4. The variations in workfunctions of W(110) and W(111) follow same pattern at the higher temperature range. A clear saturation was observed in the case of 500K for W(110) and W(111). This is an indication of surface alloying effects.
5. A  decrease in workfunction was observed for all three temperatures studied, in the case of W(121) and W(100).

Few monolayers of nickel were deposited onto a tungsten emitter to study the growth features. The effect of equilibration temperature on the growth of nickel has been studied. The  main results are as follows:

1. It was found that the temperature plays a major role in deciding the morphology of the growth.  A “well spread” pattern was observed with an increase in workfunction when the doses were annealed at 760K. This corresponds to a pseudomorphic growth and there was no overgrowth observed on any particular plane.
2. For 960K anneal, nickel was found to grow on W(111) giving rise to a “two spot” pattern. The reduction in the workfunction is found to be apparent and is attributed to the field enhancement at the overgrowth.
3. Probe hole field emission microscopy studies for growth of nickel on W(121) and W(100) planes  shows that the workfunction of these planes decreases initially at low coverages and increases at higher coverages. This reversal in trend in the workfunction change  is attributed to the surface restructuring on these planes
4. The activation energy of nickel on W(100) has been estimated to be 0.78 eV.

                       Well Spread pattern                                       Two spot pattern

Growth of Nickel on a tip terminated with a Grain Boundary has been studied. The temperature dependent
growth has been observed. Click Here for the Patterns

Equilibration Studies
 

1. The results obtained indicated that nickel atoms on the W(121) plane cause reduction in the work function and  reconstruction upon annealing at 1150K.
2. At the annealing temperature of 1150K the diffusion of nickel atoms on the W(111) plane reduces the work function by ~1.0 eV.  A steady saturation was observed for W(111) indicative of surface alloying.

Long-term current stability observations were carried out on the nickel deposited tungsten field emitter.

1. It was found that the current stability recorded from the Ni/W emitters prepared under different conditions, were different.  The total field emission current drawn from the “two spot” state of the emitter showed current stability over a long period of time. It is indicative of the resistance of the emitter to the ion bombardment effects.   Formation of an alloy phase of nickel with tungsten (viz. Ni4W) is expected at the annealing temperature of 960K.
2. The total emission current from the “well spread” state of the emitter did not give stability. The fluctuations observed were substantially larger than those observed in the case of  “two spot” pattern. This is due to the ion bombardment damages of the tip.
3. It is concluded that only the emitter with the “two spot” state has the desired properties and more stability observations were performed using this state.  Fluctuations in current were found to increase with increased current levels.  At higher current levels,  (1 ?A onwards) the current was found to decrease over a short duration.  The “two spot” pattern disappeared while drawing a current of 1.2 ?A.  The overgrown nickel is destroyed to the increased ion bombardment and also due to the resistive heating and melting of the surface.
4. Noise analysis of the  “two spot” state has been carried out. Flicker type noise was observed upto 100KHz frequency. Auto power spectrum upto 100KHz showed 1/f type behaviour of the noise. This has been attributed to the ion bombardment due to the ambient gas present in the system.

Field emission from nickel adsorbed tungsten tip: temperature dependent growth and its effect on current stability N. Pradeep, R B sharma and D S Joag : Ultramicroscopy 73 (1998) 59-66.

Diffusion studies of nickel on W(121) and W(111) by using probe hole field emission microscopy N. Pradeep, R B Sharma and D S Joag : Mat. Sci. and Engg A270,107, 1999.

Field emission from Amorphous Silicon films on tungsten

Field electron emission from a hydrogenated amorphous silicon deposited tungsten tip has been studied. The effect of prolonged (hourly) exposure to light on the field electron emission current has been investigated. The current voltage characteristics of the as deposited, light exposed, and annealed states of the emitter showed semi conducting behavious in accordance with the Fowler Nordheim law. In case of an initially annealed emitter tip, the field emission current was found to increase monotonically with the light exposure, reversibily for shorter durations of a few minutes. the effect of long term (hourly) exposure of light resulted in the increase in the field emission current  levels non-linearly with the exposure time. The current level did not come down to the origibal level even after switching of the light in contrast to the effect of short duration light exposure.The enhancement in the field emission current has been attributed to tunneling through the dangling bond states created by light exposure. The results are discussed in the light of Stabler Wronski effect.

Field emission from hydrogenated amorphous silicon films on tungsten : Light induced effects, D S Joag, A K Sharma, M A More, R D Dusane, and M G Takwale, Appl. Phys. Lett. 64, 3437 (1994)

Effects of thermal annealing

Field electron emission from an amorphous silicon deposited tungsten tip has been studied. The current-voltage (I-V) characteristics corresponding to the as-deposited and annealed states of the emitter were recorded.
When the deposited tip was annealed at 450K for 120 seconds, the emission current which could be drawn from this annealed tip state was considerably higher (5uA) than that from the un-annealed as deposited state (0.5uA) for same tip voltage. This enhancement in the current level with reduction in spikes shows an improvement in overall stability. When the emission current from the tip is drawn for longer periods of time, it was found that the current decreases. But as the tip was annealed at 450K-500K intermittently, the current was found to recover its original levels Field emission current fluctuations from the two states were recorded. As-deposited tip shows more spikes in low field region and a mixture of spikes and steps at higher fields. Fast Fourier Transform (FFT) analysis of the current signal shows Lorentzian-type of power spectrum which is indicative of bistable traps in a-Si:H. Total emission current level remained almost constant  for the entire duration of recording.

Field emission from amorphous silicon deposited tungsten tips, R B Sharma, N. Pradeep, G C Dubey, Surendra Pal and D S Joag, Ultramicroscopy 79, 131, 1999.

Field emission from Diamond coated tungsten tips

Field emission properties of diamond coated tungsten tips have been studied using a glass field emission (FE) microscope. Diamond film was deposited on a tungsten field emitter by Hot Filament Chemical Vapor Deposition  (HFCVD) method.  Stable emission was found to occur from discrete locations as seen from the FE patterns. Field emission current - voltage (I-V) characteristics at 300K and 78K have been studied. The difference in I-V characteristics at 300K and 78K has been attributed to the reduction in conductivity at 78K. The emission current is found to be remarkably stable over a long period of more than three hours. Spectral analysis of the FE current shows noise proportional to 1/f; f being the frequency. The overall behaviour of the diamond coated tungsten emitter is consistent with the subband model with the emission being controlled by the conductivity of the coating.
 

Diamond deposition work carried out in Collaboration with
Dr. V. P. Godbole, Department of Physics, University of Pune

The closed tubular structures of carbon atoms called Carbon Nanotubes descovered by Iijima in 1991 showed interesting properties.  These are found suitable for tips in scanning probe microscopes and they posses excellent field electron emission properties.  Field emission studies from carbon nanotubed deposited on tungsten is being studied at present in the Laboratory. Reproducible Fowler Nordheim Characteristics (350 points) are recorded over a wide range of emission currents (0.1nA -1mA). The field emission current satbility measurements showed reduction in the noise levels with increasing emission currents. Field emission induced luminescense with visible intensities was observed.