Olivier M. Küttel

Field emission properties of carbon nanotubes

We have investigated the field emission properties of nanotube thin films deposited by a plasma enhanced chemical vapor deposition process from 2% CH4 in H2 atmosphere. Depending on the deposition of the metallic catalyst [Fe(NO3)3 in an ethanol solution or sputtered Ni] the nanotube films showed a nested or continuous dense distribution of tubes. The films consisted of multiwalled nanotubes (MWNTs) with diameters ranging from 40 down to 5 nm, with a large fraction of the tubes having open ends. The nanotube thin film emitters showed a turn-on field of less than 2 V μm−1 for an emission current of 1 nA. An emission site density of 10 000 emitters per cm−2 is achieved at fields around 4 V μm−1. The emission spots, observed on a phosphorous screen, show various irregular structures, which we attribute to open ended tubes. A combined measurement of the field emitted electron energy distribution (FEED) and the current-voltage characteristic allowed us to determine the work function at the field emission site....

Electron field emission from phase pure nanotube films grown in a methane/hydrogen plasma

Phase pure nanotube films were grown on silicon substrates by a microwave plasma under conditions which normally are used for the growth of chemical vapor deposited diamond films. However, instead of using any pretreatment leading to diamond nucleation we deposited metal clusters on the silicon substrate. The resulting films contain only nanotubes and also onion-like structures. However, no other carbon allotropes like graphite or amorphous clustered material could be found. The nanotubes adhere very well to the substrates and do not need any further purification step. Electron field emission was observed at fields above 1.5 V/μm and we observed an emission site density up to 104/cm2 at 3 V/μm. Alternatively, we have grown nanotube films by the hot filament technique, which allows to uniformly cover a two inch wafer.

Vacuum arc discharges preceding high electron field emission from carbon films

Field emission measurements on chemical vapor deposition diamond and laser ablated a‐C films show an activation step after reaching a certain critical electric field. At this field a vacuum arc of some hundred ns duration initiates. While high current arcing leads to the evaporation of the spot surface melting, amorphization or cracking of the film is encountered for lower currents. In any case, much higher electron emission can be observed after this activation procedure due possibly to tip formation resulting in an electric field enhancement. By using a 1 GΩ resistance the discharge current can be limited nevertheless, an activation is observed.

Field emission from DLC films

Abstract Field emission measurements on diamond like carbon (DLC) films with different amounts of sp3 and sp2 carbon were carried out. Depending on the amount of sp2 carbon in the film, activated and non-activated Fowler-Nordheim like emission could be observed. The emission spots were investigated using a combination of AFM and STM, by simultaneously measuring the topography and the conductivity of the samples. In the case of sp2 rich DLC films we could observe that the emission originates from highly conducting inclusions of sp2 carbon in a matrix of insulating sp3 carbon. These inclusions are already existing on the sp2 sample by the deposition process itself and are formed by the activation on the sp3 rich sample.

Field emitted electron energy distribution from nitrogen-containing diamondlike carbon

We report on energy resolved field emission measurements of 0.3 μm thick nitrogen containing diamondlike carbon (DLC). The film used in this study was deposited by filtered arc deposition onto a highly p-doped Si(100) wafer. The film showed homogeneous field emission over the entire wafer area with an onset field for 1 nA emission current of 20–25 V/μm. The energy resolved field emission measurements were carried out with an applied electric field of 20–22 V/μm. The field emitted electrons originate from the Fermi energy, indicating that no field penetration occurs. The energy distribution has a FWHM of 0.63 eV at an applied field of 21 V/μm. The spectra could be deconvoluted using standard tunneling theory. The results of the deconvolution indicate an electric field strength of 6500 V/μm  at the emission site.

Field emission from diamond, diamond-like and nanostructured carbon films

Abstract We have deposited nanotube films on silicon via a chemical vapor deposition (CVD) growth process known from the deposition of diamond. We used a metallic catalyst which was deposited onto the silicon surface prior to the CVD deposition. The films are very pure, adhere well and are very well suited for electron field emission. We measured emission at 2.6 V/μm (for 1 nA emission current) and an emission site density reaching 104/cm2 at 3–4 V/μm as measured on a phosphor screen. Electrons originate at the Fermi level and the high local fields at the emission site is produced by the geometry of the nanotube. The results obtained on these films are comparable to those from differently prepared CVD diamond films. So far, we have no evidence that electron injection occurs. The emission process is governed by field amplification at protrusions and tips. In a second experiment we have measured emission from a metallic micrometer sized grain fixed on a diamond (100) surface, with different surface termination (hydrogen, oxygen, sp2 carbon). The field emitted electron energy distribution (FEED) spectra show large energy shifts which are due to the surface resistivity and not due to injection of electrons in the conduction band. Hence, energy shifts in FEED spectra do not necessarily reflect an injection mechanism.

Photoemission from the negative electron affinity (100) natural hydrogen terminated diamond surface

Abstract We reported recently about the cleaning and polishing of natural doped (100) and (111) diamond surfaces in a hydrogen plasma at 870°C and 40 mbar [Kuttel et al., Surf. Sci. 337 (1995) L812]. This smooth (100) surface shows a sharp negative electron affinity (NEA) peak for the 2 × 1 monohydride terminated surface upon annealing to 300°C, experimentally observed by ultraviolet photoemission spectroscopy (UPS). The effect of annealing the crystal up to 1000°C in an UHV environment ( p −9 mbar) results in a positive electron affinity (PEA) whereas a subsequent atomic hydrogen adsorption from a heated filament leads to an 1 × 1 reconstruction and to the reappearance of the NEA peak with a lower intensity than the plasma exposed surface. Upon annealing the surface up to 1000°C at a pressure of 5 × 10 −8 mbar, a NEA is observed which probably is caused by remaining hydrogen at the surface. The hydride terminated surface seems to be responsible for the NEA property. The link between the NEA property and the band bending of the (100) surface are elucidated by photoemission spectroscopy of the core level and the valence band and we measured the spatial distribution of the NEA peak for the plasma exposed surface by angle resolved UPS. Finally, we show photoelectron current measurements and the influence of the bias applied to the surface.

The preparation and characterization of low surface roughness (111) and (100) natural diamonds by hydrogen plasma

Abstract Natural doped (100) and (111) diamond surfaces were polished in a hydrogen plasma at 870°C and 40 mbar and analyzed by AFM, LEED and X-ray photoelectron diffraction (XPD). The initial surface roughness of 7 nm (rms) is decreased to 1 nm on the (111) surface and a 2 × 1 LEED pattern is observed after annealing at 1000°. After etching the (100) surface has a roughness of 0.8 nm and shows a sharp 2 × 1 LEED pattern which is even stable in air without annealing. XPD measurements indicate that the quality of the top surface layer (30 A) is increased considerably by the etching and annealing process. Exposing the surfaces to atomic hydrogen produced by a heated filament leads to an increase of the surface roughness comparable to what was observed on the as received sample. The often reported 1 × 1 reconstruction seems to be a consequence of a large surface roughness and is absent on smooth surfaces.

X-ray photoelectron and Auger electron diffraction study of diamond and graphite surfaces

Abstract Natural diamond (001) and (111) surfaces as well as highly oriented pyrolithic graphite (HOPG) (0001) have been analyzed by X-ray induced photoelectron diffraction (XPD). The measured 2π patterns of C Is emission (964 and 1450 eV) and of KVV Auger emission (260 eV) are compared to single scattering cluster (SCC calculations, and excellent agreement is found. The comparisons show that photoelectron forward focusing is much less prominent in carbon solids than in all previously studied, heavier elements, and a direct interpretation of the data is therefore more difficult. The highly textured nature of HOPG presents itself as a circular pattern with concentric rings centered at the surface normal (c-axis). The aim of this work is to show the advantage of the XPD technique as a diagnostic tool for the investigation of epitaxial diamond growth with monolayer sensitivity.

Nanotube electronic states observed with thermal field emission electron spectroscopy

We observe nonmetallic electronic states above the Fermi level in single-walled carbon nanotubes by measuring the energy distribution of thermal-field-emitted electrons. This measurement method examines electronic states associated with the nanotube cap or end termination, and with it, we resolve electronic states greater than 3 eV above the Fermi level. The observed emitting states are broad at high temperatures (0.7–1.5 eV full width at half maximum), and the peak positions shift linearly with applied voltage. We present possible mechanisms responsible for these states.