GB2568754A

GB2568754A - Resonance enhanced work function reduction

Info

Publication number: GB2568754A
Application number: GB1719728.6A
Authority: GB
Inventors: Fox Neil; Nash'at Farhan Sahawneh Faris
Original assignee: University of Bristol
Current assignee: University of Bristol
Priority date: 2017-11-28
Filing date: 2017-11-28
Publication date: 2019-05-29
Also published as: GB201719728D0; WO2019106362A1

Abstract

A material, having a surface, wherein the surface has a natural frequency of electron oscillations at the surface, and the material comprises at least one surface layer species of atom or molecule, and wherein the vibrational frequency of the surface layer species matches said natural frequency of electron oscillations at the surface. Also disclosed is a method of reducing an effective work function of the surface. Said natural frequency of electron oscillations at the surface may be within a range given by the vibrational frequency of the surface layer species ±5% to ±70%. The material may be nanoparticles 11 of a semiconductor such as diamond, indium tin oxide, aluminium-doped zinc oxide, and gallium doped zinc oxide. The surface layer species may comprise a metal-oxygen dipole of LiO, CsO, RbO, BeO, NiO, or aluminium-doped zinc oxide or alternatively adsorbed gas molecules of hydrogen, methane, or ammonia. The material may be used in a thermionic emission device with a laser light source matching the vibrational frequency of the surface layer species and the surface plasmon resonance frequency. An example is lithium doped diamond nanoparticles 11 having a free carrier density between 1019 to 1021 per cm3.

Description

RESONANCE ENHANCED WORK FUNCTION REDUCTION

This invention relates to a method of reduction of the work function of a surface, for example for use in thermionic energy conversion.

More specifically, embodiments relate to a method of reducing the effective work function of a surface of a material by means of a resonance effect between adsorbate atoms or molecules and electrons at the surface of the material. Further embodiments relate to devices formed in this way. This resonance effect is expected to enhance the thermionic emission current from the cathode. The method can therefore be utilized to enhance the performance and conversion efficiency in thermionic energy conversion devices.

Thermionic energy conversion devices, which convert heat energy into useful electrical energy, are well known and described. For example, the articles “Measured Thermal Efficiencies of a Diode Configuration of a Thermo Electron Engine”, Hatsopoulos and Kaye, MIT, Journal of Applied Physics, 1958, pages 1124 to 1125, and “Theoretical Efficiency of the Thermionic Energy Converter”, Houston, GE Research Laboratory, NY, Journal of Applied Physics, vol. 30, No. 4, April 1959 both describe thermionic emission devices having a cathode emitter spaced apart from an anode collector. The cathode emitter emits electrons when heated, and these electrons are collected by the collector, thereby giving an electrical current. However, such devices have been found to be relatively inefficient in their energy conversion due in part to poor cathode emissivity. Furthermore, such devices only operate at high temperatures, typically from 1000 °C to 1500 °C, due to the high work function of the cathode material.

An emitter having a high work function means that electron emission at temperatures below 1000 °C is undesirably low. It is desirable to provide an energy conversion device based on thermionic electron emission techniques, but which has higher, and, hence, more useful, conversion efficiency. It is also desirable to provide a device which is able to operate successfully at lower temperatures than previous devices, for example, at 500 °C or lower.

AH thermionic devices are based upon the Richardson equation, which gives the current per unit area, J_H, emitted by a meta! surface with a work function <φ_β, at a temperature T_e, as:

where A, in turn, is a constant given by:

A typical thermionic generator might operate at T = 1500 K with a caesium metal surface that has a work function φ_βοί about 2 eV. Using these values gives

The efficiency of a thermionic diode converter is given by the relation:

where 4>_eand rp..are respectively the work functions of the emitter and the collector, T, and T_rare respectively the temperatures of the emitter and collector, a is the StefanBoltzmann constant, Q,_B is the thermal energy loss per second from the emitter via the electrical conductors, and is the Richardson saturation current of the emitter. The efficiency according to this equation is reduced due to radiation loss and thermal conductivity of the conductors, as described in US Patent No. 8,617,65162.

Thus, it can be seen that the work function and temperature of the emitter are the key parameters determining the operational performance of a thermionic emitter, while using a collector with a lower work function than the emitter can significantly boost efficiency.

Adsorbates have been widely studied for their role in changing the chemical, physical and electrical properties of diamond. In particular, certain surface terminations, or deposited surface layers, on diamond induce a negative electron affinity (NEA) property, in which the conduction band sits above the vacuum level. These offer a number of potential device applications, including in low threshold electron emission, photodetection and electrochemical cells.

Lithium monolayer on oxygen-terminated diamond appears to be a very promising material for low work function cathodes that are very stable [see K. M. O’Donnell, et al., Phys. Rev. B82, 115303 (2010)]. Density functional theory calculations show that a lithium monolayer on oxygen-terminated diamond can form an NEA comparable to CsO in strength, but with a much higher theoretical binding energy of around 4.7 eV per lithium atom adsorbed on to the C (100) surface, and a work function shift of — 4.52 eV.

According to the present invention, there is provided a material, having a surface, wherein the surface has a natural frequency of electron oscillations at the surface, and the material comprises at least one surface layer species of atom or molecule, and wherein the vibrational frequency of the surface layer species matches said natural frequency of electron oscillations at the surface.

The natural frequency of electron oscillations at the surface may be within a range given by the vibrational frequency of the surface layer species ±70%, or ±50%, or ±25%, or ±15%, or ±10%, or ±5%.

The surface layer species may be used to functionalise the surface.

The material may comprise nanoparticles of a semiconductor material, more specifically a wide band gap semiconductor material. For example, the semiconductor material may be a metal oxide material, such as indium tin oxide, aluminium-doped zinc oxide, and galiium-doped zinc oxide. Alternatively, the semiconductor material may be a lll-V semiconductor material, such as InGaN. The semiconductor material may be diamond.

The surface layer species may comprise a metal-oxygen dipole, for example a dipole of LiO, CsO, RbO, BeO, NIO, or aluminium-doped zinc oxide.

The surface layer species may comprise adsorbed gas molecules, for example adsorbed molecules of hydrogen, methane, or ammonia.

The material may comprise two surface layer species of atom or molecule, having respective first and second vibrational frequencies, wherein said natural frequency of electron oscillations at the surface is between the first and second vibrational frequencies. The first and second vibrational frequencies may differ by less than 20%.

According to another aspect, there is provided a thermionic emission device, comprising a material according to the first aspect. In such a device, the vibrational frequency of the surface layer species matches said natural frequency of electron oscillations at the surface, at least at an operating temperature of the device.

According to another aspect, there is provided a system comprising: a material according to the first aspect; and a laser light source, arranged such that light from the laser light source is absorbed by the surface of the material. The frequency of the laser light source may match the vibrational frequency of the surface layer species and the surface plasmon resonance frequency. Specifically, the frequency of the laser light source may be within ±70%, or within ±50%, or within ±25%, or within ±15%, or within ±10%, or within ±5% of the vibrational frequency of the surface layer species and the surface plasmon resonance frequency.

According to another aspect of the present invention, there is provided a method of reducing an effective work function of a surface, wherein the surface has a natural frequency of electron oscillations at the surface, the method comprising functionalizing the surface with a surface layer species, such that the vibrational frequency of the surface layer species matches said natural frequency of electron oscillations at the surface.

Functionalizing the surface may comprise adsorbing atoms or molecules onto the surface.

The material may comprise nanoparticles of a semiconductor material, more specifically a wide band gap semiconductor material. For example, the semiconductor material may be a metal oxide material, such as indium tin oxide, aluminium-doped zinc oxide, and gallium-dopsd zinc oxide. Alternatively, the semiconductor material may be a lll-V semiconductor material, such as InGaN. The semiconductor material may be diamond.

The method may comprise functionalizing the surface with two surface layer species of atom or molecule, having respective firstand second vibrational frequencies, wherein said natural frequency of electron oscillations at the surface is between the first and second vibrational frequencies. The first and second vibrational frequencies may differ by less than 20%.

The method may comprise arranging a laser light source, such that light from the laser light source is absorbed by the surface of the material. The frequency of the laser light source may match the vibrational frequency of the surface layer species and the surface plasmon resonance frequency. Specifically, the frequency of the laser light source may be within ±70%, or within +50%, or within ±25%, or within ±15%, or within ±10%, or within ±5% of the vibrational frequency of the surface layer species and the surface plasmon resonance frequency.

Thus, embodiments of the present invention involve a method of enhancing thermionic emission, or other effects dependent on a low work function, by utilizing a resonance induced by atoms or molecules on the emitter surface. The vibration of the surface species on the emitter surface at a frequency that is close to the surface plasmon resonance frequency of the emitter material achieves the resonance effect.

BRIEF DESCRIPTION OF DRAWINGS

For a better understanding of the present invention, and to show how it may be put into effect, reference will now be made, by way of example, to the accompanying drawings, in which:Figure 1 is a simplified diagram showing a thermionic energy converter.

Figure 2 shows a simplified diagram of a cathode.

Figure 3 illustrates the resonance effect in nanoparticles.

Figure 4 illustrates the oscillation of a work function.

Figure 5 illustrates the thermionic current enhancement in one example.

Figure 6 illustrates an equivalent work function.

Figure 7 is a simplified diagram showing an alternative thermionic energy converter.

DESCRIPTION OF DRAWINGS

Embodiments of this invention relate to a method of forming a surface with desirable properties, for example a reduced effective work function, and to a device with such a surface.

One use of such a surface is in a thermionic emitter, in which electrons are emitted from the surface when the surface is heated. More specifically, such a surface can be used in a thermionic energy converter, in which heat energy is applied to the surface, and a useful electric current is generated.

The principles of the invention will be described with reference to an example, in which the surface with the reduced effective work function is used in a thermionic energy converter, but it is pointed out that this is only an example, and that the methods and devices described in this example can be used in other applications.

Figure 1 is a simplified diagram showing a thermionic energy converter. An emitter (cathode) is formed of a substrate material 1, with a suitable emitter surface material 2.

The device also has a collector (anode) 3, which is placed parallel to the emitter surface 2. Between the emitter surface 2 and the collector 3, there is a gap 4 in the form of a space which may for example contain a vacuum or a low-pressure gas.

The emitter 1 and collector 3 are contained in a package 5. In this example, the package 5 is hermetically sealed and the whole package 5 contains a vacuum or a lowpressure gas.

A first electrical feed-through 6 is connected to the emitter 1, and a second electrical feed-through 7 is connected to the collector 3.

As shown schematically by the arrow 8, thermal energy is input into the thermionic energy converter. The thermal energy may for example be input by conduction from a heat transfer medium, or through solar or other radiation passing through a window 5a in the package 5. For example, solar radiation may be concentrated by lenses and/or mirrors onto the emitter 1.

As shown by the arrow 9, waste heat may be extracted from the thermionic energy converter, for example to a heat sink.

Figure 2 is a simplified diagram showing the form of a cathode 1. As mentioned above, such a cathode may be used in other applications, apart from in a thermionic energy converter.

The cathode 1 includes a substrate 1a. The substrate 1a may for example be formed of an electrically insulating layer with a high thermal conductivity (such as a chemical vapour deposition (CVD) diamond wafer, a ceramic layer, or a glass plate).

As shown in Figure 1, the substrate 1a is provided with an emitter surface.

In general, the emitter surface may be formed from any suitable material, for example: a CVD diamond layer, diamond nanoparticles, a metal-diamond composite, diamond thin films, diamond-like carbon materials, graphene, fullerenes, carbon nanotubes, degenerately doped semiconducting material layers or nanoparticles, or other wide band gap or 2-D materials, semimetals, or any material that can be designed to utilize the resonance effect.

Metals may be used if the surface plasmon resonance frequency is made to match the vibrational frequency of the adsorbed species. Some example methods that may enable the use of metals include nano-structuring (e.g. changing the shape and aspect ratio of metal nanoparticles), using different structured plasmonic surfaces, using metal alloys, using several layers of different materials, or using meta-materials. Furthermore, the use of quantum dots may provide further enhancements to matching the resonance condition.

Many semiconductor materials, with relatively low free carrier densities, can be designed to have surface plasmon resonance frequencies in the order of 10¹³ Hz, which makes them useful in methods described herein, as discussed in more detail below.

In the example shown in Figure 2, the emitter surface comprises nanoparticles 11 that are mechanically and electrically connected to the substrate 1a by means of a thin conductive metallic layer 10.

The metallic layer 10, may for example have a thickness of about 100nm. The metallic layer 10 can be made from conductive metal oxides (e.g. ITO, ZnO, ZnO:AI, etc.), or from metal alloys (e.g. Li:AI, Li:Ag, Ni-Cr), or from metals (e.g. Ag, Au, Pt, Ni), as described in WO 2009/004345 A2.

The nanoparticles 11 may for example be lithium processed diamond nanoparticles as described in WO 2009/004345 A2.

In other examples, the emitter surface material comprises nanoparticles of a semiconductor material, specifically, a wide band gap semiconductor material, for example a metal oxide material such as indium tin oxide, aluminium-doped zinc oxide, or gallium-doped zinc oxide; or a lll-V semiconductor material such as InGaN; or diamond.

In embodiments of the invention, the emitter surface is functionalized by a surface layer species of atoms or molecules, which may for example be adsorbed onto the emitter surface.

Figure 3 is a schematic illustration of this, in the case where the emitter surface is formed of nanoparticles 11 (for example formed of diamond as described in WO 2009/004345 A2) in a metallic layer 10 that mechanically and electrically connects the nanoparticles 11.

Thus, Figure 3 shows atoms 12 of a surface layer species on the surface of the nanoparticles 11. Specifically, in this example, where lithium has been diffused into the nanoparticle and the adsorbate atoms comprise oxygen as described in WO 2009/004345 A2, the surface is functionalized by a thermal processing step that causes the lithium to diffuse out of the diamond and coordinate with the oxygen to form a bond-centred (delocalized) lithium-oxygen dipole exhibiting NEA. This dipole acts as the surface layer species that functionalizes the emitter surface.

In other examples, the surface layer species comprises other metal-oxygen dipoles, such as CsO, RbO, BeO, NiO, or aluminium-doped zinc oxide.

The nanoparticles 11 can also have a surface plasmon resonance frequency that depends on various features of the surface, including the free carrier density.

in addition, the surface layer species, in this case the atoms 12, have a certain vibrational frequency. In the example of the lithium coordinating with the oxygen to form a strong dipole, which acts as the surface layer species, this has a specific vibrational frequency.

When the vibrational frequency of the atoms 12 or molecules in the surface layer species matches the surface plasmon resonance frequency of the nanoparticles, there is a resonance effect. The vibrations of the atoms or molecules in the surface layer species synchronize with surface charge density oscillations in the nanoparticles 11. The vibrational frequency of the atoms or molecules in the surface layer species can in principle differ from the surface plasmon resonance frequency of the nanoparticles by several orders of magnitude. Therefore, the vibrational frequency of the atoms or molecules in the surface layer species can for example be regarded as matching the surface plasmon resonance frequency of the nanoparticles if the natural frequency of electron oscillations at the surface is within a range given by the vibrational frequency of the surface layer species ±70%, or ±50%, or ±25%, or ±15%, or ±10%, or ±5%. This degree of matching is enough to produce a significant resonance effect.

This is illustrated, in greatly exaggerated form, in Figure 3.

Thus, Figure 3 shows the oscillations of the electron density cloud around each of the nanoparticles, creating a negative region 13 of the oscillating surface electric charge, and a positive region 14 of the surface electric charge. In this example, adjacent nanoparticles have opposite electric charge phases, and so the upper surfaces of the first and third nanoparticles from the left in Figure 3 have negative charges while the upper surfaces of the second and fourth nanoparticles from the left have positive charges. However, this is only one example, for illustrative purposes, and the actual effect in any particular system will depend on the geometry, the materials, and the distance between the adjacent nanoparticles.

The adsorbate atoms 12 that are used to functionalise the emitter surface may form a monolayer on the surface of the nanoparticles 11 or other emitter surface.

For some materials, the optimal adsorbate coverage may be less than a monolayer this may be the case if adjacent adsorbate atoms or molecules have significant interaction forces on each other.

In either case, the layer of adsorbate atoms may then have a negative electron affinity (NEA).

In this example, and in other embodiments, the bi-layer of atoms that functionalise the emitter surface forms an array of atomic dipoles disposed on the semiconducting cathode emitter surface to lower the effective work function.

It is known that the presence of adsorbates can reduce the work function of an emitter. However, the resonance effect, that occurs when the emitter has a natural frequency of electron oscillations at the surface (that is, a surface plasmon resonance frequency) that matches the vibrational frequency of the surface layer species (for example an adsorbate), provides a significantly greater reduction in the effective work function of the surface. This makes the material particularly suitable for applications where a low work function is advantageous, such as in thermionic energy converters. The resonant coupling therefore enhances the thermionic emission in this case. The work function of the material would be set into a resonant oscillating mode, giving a higher average thermionic emission current.

For example, in the case described above where the emitter surface comprises diamond nanoparticles, the surface layer species can be lithium atoms. The lithium doping in the diamond nanoparticles can be designed to be at a level that gives a free carrier density that causes the surface plasmon resonance frequency in the diamond nanoparticles to match the vibrational frequency of the lithium atoms adsorbed on the surface. The collective oscillation of the electrons in the nanoparticles at a frequency that matches the vibrational frequency of adsorbed atoms enables the adsorbate atoms to vibrate in a synchronised mode.

As an alternative example, in the case described above where the emitter surface comprises oxygen-functionalised diamond nanoparticles, the surface layer species atoms can be boron atoms or magnesium atoms. Subsequent thermal annealing would cause the oxygen and magnesium to co-ordinate on the surface to form a dipole that allowed the surface to exhibit NEA.

In general, in some embodiments, an adsorbate material can be chosen based on its stability in forming an NEA layer, and based also on its vibrational frequency. That is, it should have a vibrational frequency that lies within the range of surface plasmon resonance frequencies of the chosen emitter surface material, although this condition can be met for many possible adsorbates.

Several methods can then be utilized to engineer the emitter material to have a surface plasmon resonance frequency that matches the adsorbate vibrational frequency, as described in more detail below.

Adsorbate atoms or molecules may be chosen on their ability to create a negative electron affinity (NEA) on the emitter material as well as their ability to have a high binding energy.

The choice of materials for utilizing the resonance effect should take into consideration the free carrier density range required to achieve resonance. Diamond materials have properties that make them suitable candidates for utilizing the resonance effect.

Embodiments may use chemisorbed adsorbates and/or physisorbed adsorbates, where the former has stronger bond strengths (the adsorbate forms a chemical bond with the emitter material) and the latter involves a relatively weaker interaction between the adsorbate and the emitter material (mainly involving Van der Waals forces).

The use of chemisorbed adsorbates would involve a cathode with a monolayer (or less than a monolayer) of adsorbate, for example forming an NEA.

Several methods can be used to apply the adsorbate on the surface. For example, this can be achieved by physical vapor deposition (with the excess removed), or by electrochemical or chemical vapor deposition.

US Patent No. 8,617,651 describes one specific method of applying a lithium adsorbate onto an oxygen-terminated diamond surface.

An adsorbate can be physisorbed, by maintaining the emitter surface in a suitable atmosphere. For example, in the case of the device shown in Figure 1, or any similar device, the emitter surface can be placed in a hermetically sealed chamber, with a suitable low-pressure gas. As examples, suitable gases include hydrogen, methane, and ammonia. Suitable pressures can for example be in the range of about 1 to 1500 mTorr (i.e. about 0.1 to 200 Pa), depending on the specific system design.

The range of applicable pressures can enable control of the amount of adsorbed gas molecules per nanoparticle.

The adsorbed gas molecules then form the surface layer species, and the vibrational frequency of the adsorbed gas molecules can then be made to match the surface plasmon resonance frequency of the emitter material.

The incorporation of a gas in a thermionic energy converter has been shown to enhance the thermionic emission - see for example Nemanich RJ, Koeck FAM, et al. “Enhanced thermionic energy conversion and thermionic emission from doped diamond films through methane exposure”, Diamond & Related Materials 20 (2011) 1229-1233. If the material is designed to meet the resonance condition described above, then this may significantly further enhance the thermionic emission current.

It is also possible to utilize both chemisorbed and physisorbed adsorbates in the same thermionic energy converter.

The use of diamond materials for thermionic cathodes would allow several possible options for using chemisorbed adsorbates that form an NEA with a high binding energy. A method of forming a lithium monolayer on oxygen-terminated diamond is described in US Patent No. 8,617,651. This material can be engineered to have a free carrier density that gives a surface plasmon resonance frequency that is close to the vibrational frequency of the adsorbed atoms or molecules - typical free carrier densities in doped CVD diamond are in the range of 10¹⁹ to 10²¹ per cm³, which give a surface plasmon resonance frequency in the range required to match the vibrational frequency of the adsorbate. This can be used in the case of a cathode composed of single crystal diamond, CVD grown diamond or diamond-like layers on a substrate (e.g. microcrystalline diamond), or in the case of a cathode comprising diamond nanoparticles attached to a substrate. The diamond is of n-type material, which is typically needed for efficient thermionic emitter operation at low temperatures. However, it should also be noted that a p-type diamond layer can be used if it is incorporated into a Schottky emitter structure, such as a metal/intrinsic diamond/p++diamond diode. The p-diamond (with oxygen termination and a lithium adsorbate, say) would be the output surface and emission would occur under negative bias by tunneling from the NEA surface.

When using diamond for the emitter material, it may be noted that isotopically pure diamond, i.e. 100% pure material, has double the thermal conductivity of natural diamond, and this is beneficial to emitter operation. In addition, carbon-13 has a larger band gap than carbon-12 by 17meV. Therefore, using isotopically pure carbon-13 diamond may increase the effectiveness of the dipole set up on the diamond surface.

The localized surface plasmon resonance frequency w_sp of a nanoparticle can be estimated by the following equation [see J.M. Luther, et al., “Localized surface plasmon resonances arising from free carriers in doped quantum dots”, Nature Materials (2011). DOI: 10.1038/NMAT3004.] _ 1 j

Where N is the free carrier density; e is the electron charge; is the permittivity of free space; ni_s is the effective mass of free carrier (assumed equal to that of free electron);

is the high frequency dielectric constant; e^ is the medium dielectric constant.

For example, for a certain sample with a high frequency dielectric constant of = io and a medium dielectric constant in the range of 2 to 20 (depending on the choice of material and the design requirements) the material can be easily designed to have a surface plasmon resonance frequency in the FIR region of the spectrum, in the range of 10¹³ Hz (i.e. 10 THz) - which matches the vibrational frequency range of many adsorbates. The specific values of vibrational frequencies of different adsorbates on different emitter material surfaces can be determined by Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy.

Having determined the vibrational frequency of a selected adsorbate on a selected emitter material surface, the properties of the emitter material surface (for example the free carrier density) can then be controlled in order to produce the surface plasmon resonance frequency of the emitter material that matches the vibrational frequency of the adsorbate. Also, designing different geometries of nanostructures provides a further means of fine-tuning the surface plasmon resonance frequency.

When reference is made here to the surface plasmon resonance frequency of the emitter material matching the vibrational frequency of the adsorbate, this means that the two frequencies are close enough that there is some resonant coupling between them. For example, the surface plasmon resonance frequency should be within a range given by the adsorbate vibrational frequency ±70%, and more preferably should be within a range equal to the adsorbate vibrational frequency ±50%, and more preferably should be within a range equal to the adsorbate vibrational frequency ±25%, and more preferably should be within a range equal to the adsorbate vibrational frequency ±15%, and more preferably should be within a range equal to the adsorbate vibrational frequency ±10%, and more preferably still should be within a range equal to the adsorbate vibrational frequency ±5% or less. In general, the range is material dependent and is determined by a margin that yields a significant reduction in the effective work function due to the resonance effect.

As stated previously the condition where the resonant coupling is maximized is when = where is the vibrational frequency of the adsorbate and is the surface plasmon resonance frequency of the emitter material.

The vibrational frequency of the adsorbate is a function of the materials being used, and so it is effectively fixed when these materials have been chosen.

Therefore, the emitter material has to be designed to achieve that condition by matching the frequencies. There are several ways in which the surface plasmon resonance frequency of the emitter material can be controlled, mainly involving controlling the free carrier density of the material (achieved by doping) and by nanostructuring of the cathode surface.

The relationship between the localized surface plasmon resonance frequency ω_3ρ of a nanoparticle and the free carrier density N was given above, and so the free carrier density required to achieve resonance (i.e. giving, ¢^=^) may be calculated by:

A further point that may need to be considered is the shift in the resonance frequency after the adsorbate is deposited. The design of the cathode will take into account the matching of the adsorbate vibrational frequency to the surface plasmon resonance frequency (i.e. the amount of doping is chosen such that the resonance frequency after depositing the adsorbate would sufficiently closely match the vibrational frequency of the adsorbate). However, the shift in the resonance frequency after depositing the adsorbate will in general be small.

In some embodiments, there is one surface layer species. For example, the surface layer species may comprise one particular type of atom, or one particular type of molecule, or the lithium-oxygen bond-centred dipole described above. In other embodiments, there are two or more different surface layer species (adsorbates) that are not bonded to each other (i.e. are not functionalised and do not form a dipole) and have two different vibrational frequencies. Where the vibrational frequencies of two surface layer species are close enough together (for example with the one vibrational frequency being within about 20% of the other), the surface plasmon resonance frequency can be designed to be somewhere between the two vibrational frequencies.

This possibility has the advantage that, if the surface plasmon resonance frequency shift is significant for a specific material, the effect of setting the surface plasmon resonance frequency between the two vibrational frequencies of the two surface layer species is that the frequency shift in this case will be trapped and the surface plasmon resonance frequency remains between the two vibrational frequencies of the two surface layer species. This locking of the plasmon resonance frequency between the two vibrational frequencies of the surface layer species may give an additional enhancement to the resonance in some cases.

in these types of material, if there are significant interaction forces between different and adjacent adsorbate atoms or molecules on adjacent adsorption sites, then it may be required to have a surface patterning that provides enough separation between different types of surface layer species to make interaction forces insignificant.

An adsorbate atom or molecule that creates an attractive force on the surface electrons of the material lowers the effective work function. If the material is designed to have the resonance condition (i.e. = a^), then the amplitude of electron oscillations at the surface and the fluctuation of the force created by the adsorbate would increase in magnitude. The magnitude of the work function oscillation amplitude can be estimated by a model, which involves oscillating electrons at the surface of a material. A series of experiments can determine the magnitude of the oscillation and the resonance frequency for a specific set of emitter material and adsorbate.

For example, the experiments may involve placing different sample emitters in a thermionic energy conversion vacuum apparatus and measuring the thermionic current. The magnitude of the oscillation can be determined by comparing data from different samples with different parameters. The resonance frequency can be determined by several methods, including the extinction spectra.

The resonance frequency after depositing the adsorbate can be predicted in advance from the equation:

ΔΛ»™·. = ϊτϊΔϊϊ·|1 —«φ ϊ;---ίΐ, where the wavelength shift, m is the refractive index sensitivity,

A® is the change in refractive index induced by an adsorbate, d is the effective adsorbate layer thickness, and Id is the characteristic electromagnetic field decay length.

The mechanism by which the effective work function of the material is reduced is due to an increased magnitude of charge density oscillations in the conduction band caused by the adsorbate vibrating at a frequency that matches the surface plasmon resonance frequency. In a simplified sense, the surface plasmon resonance frequency of a material represents the natural frequency of electron oscillations at the surface against the restoring force of positive charge nuclei (the restoring force is absent in the Drude model for metals, however, for the case of nanoparticles the curved surface of the nanoparticle provides an effective restoring force on the electrons; also inter-band transitions can provide a restoring force). For resonance induced by electromagnetic radiation, the resonance condition is achieved when the frequency of incident photons matches the natural frequency of electron oscillations at the surface. Electromagnetic radiation induced excitation of surface plasmon resonance would require a special system configuration to match the momentum of the photon to the plasmon (e.g. Otto or Kretschmann configuration or localized surface plasmon resonances (LSPRs) in nanoparticles much smaller than the wavelength of the electromagnetic radiation).

In the case of an adsorbed atom or molecule, the positive charge of the vibrating adsorbate (e.g. a Lithium atom) will exert an oscillating attractive force on the electrons at the surface. If the oscillation frequency of the force caused by adsorbates is matched to the natural oscillation frequency of surface electrons (i.e. the surface plasmon resonance frequency), then this can increase the magnitude of the charge density oscillations at the surface. This would increase the magnitude of electric field oscillations at the surface, and thus due to the Schottky barrier lowering effect with electric fields it will cause the work function of the material to oscillate (or increase the magnitude of oscillations of an originally oscillating work function). The oscillating work function would give a lower effective work function.

The wavelength of charge density oscillations created by an adsorbate atom may stretch across a surface region that contains many adsorbate atoms (this would normally be the case for LSPR in nanoparticles with surface plasmon resonance frequencies in the far infrared range). The charge density oscillation may interact with the adsorbate atoms in a manner that would couple the adsorbate atoms to vibrate in a fixed phase relationship to each other. Adsorbate atom vibration phases will tend to match the phase of the charge density oscillation. Adsorbates at different phases than the charge density oscillation phase would either gain or lose energy to the charge density wave and tend to synchronize their oscillations with the charge density oscillation (if the resonance condition is matched).

The synchronisation of adsorbate atoms on the surface is analogous to Huygens synchronisation. The adsorbate atoms, which vibrate at the same frequency, have a substrate surface, which contains electrons in the conduction band that are free to move. In a similar way that acoustic pulses between oscillators synchronise the oscillators to oscillate in phase, electric charge pulses between adsorbate atoms can synchronise the atoms to vibrate in phase on the surface. The surface plasmon case, however, may have a restoring force. If the adsorbate vibrational frequency is equal to the surface plasmon resonance frequency, then synchronisation of atom vibrations may occur. The surface charge oscillation will be in a state of resonance, due to the coupling with adsorbate atoms oscillating at the natural frequency of the surface plasmon. The extent of the synchronisation, however, would be dependent on the material and surface properties.

A further mechanism that may affect the synchronisation of adsorbate vibrations is when the substrate material has a surface phonon frequency that matches the adsorbate vibrational frequency (i.e. the vibrational frequency of the adsorbate is less than the lattice cut-off frequency and matches a certain phonon mode frequency). Under certain circumstances this may either enhance or interfere with the synchronisation. In the case of a flat surface, where LSPR does not apply, it can be seen from the plasmon dispersion relation that the wavevector increases with frequency and reaches an asymptote near the surface plasma frequency. Small variations of the material doping concentrations would slightly change the surface plasma frequency, yet it can make significant changes to the wavevector near the asymptote at a chosen frequency - this may enable matching the plasmon wavevector to the corresponding phonon mode wavevector at the chosen frequency. In this case, the adsorbate vibrational frequency would be slightly further away from the chosen plasmon resonance frequency, however, it will be close enough to have a resonance effect, and it will enable matching and coupling the plasmon to the phonon mode at the chosen frequency. This coupling of the plasmon to the phonon may enhance the extent of the synchronisation and the resonance effect. It should be noted that for some cases the charge density oscillation may follow a dispersion relation that is different from the dispersion relation for an ordinary plasmon.

Optionally, an arrangement may utilize further methods of surface patterning and structuring that enhance the coupling and the resonance effect.

Given a material, having a surface onto which atoms or molecules are adsorbed, it can be determined whether the vibrational frequency of the surface layer atoms or molecules matches the natural frequency of electron oscillations at the surface. One way of measuring the localized surface plasmon resonance (LSPR) frequency would be through measurement of the extinction spectra in nanoparticles. In general, far-field extinction spectroscopy is sufficient to get reasonably accurate measurements. However, for higher accuracy, single nanoparticle measurement techniques may be employed, such as near-field optical microscopy, which involves placing a fiber tip into the near field of the particle under study. In situations, where it is difficult to use a fiber optical probe, dark-field optical microscopy can be used. Further techniques are available.

With regard to measuring the vibrational frequency of the surface layer species such as the adsorbate, there are many methods that can be used, for example Fourier transform infrared (FTIR) spectroscopy, or Raman spectroscopy, or alternative forms of IR spectroscopy. A further alternative is Electron Energy Loss Spectroscopy (EELS), which utilizes inelastic scattering of low energy electrons. Alternative measurement techniques are also available.

When the material is intended to be used in a specific device, the frequencies may be measured at an operating temperature of the device. Otherwise, the frequencies may be measured at normal room temperatures.

Figure 4 illustrates the oscillation of the work function for a material with an oscillating work function. In Figure 4, this is assumed to be oscillating in a sinusoidal fashion with an amplitude Αψ* and having an instantaneous work function given by = φ_β, in this case, the enhanced thermionic emission current would be given by the following equation, which is the time average of the thermionic emission current from an oscillating work function surface:

Where ]_s is the original thermionic emission current of the emitter material, i.e.

The table below shows the ratios of the enhanced current to the original current for different values of and for temperatures of 600 K, 700 K, 800 K, 900 K, and

1000 K. This is for a material with a work function = X4 W.

T * e	600 K	700 K	800 K	900 K	1000 K

0.1	3.869	2.876	2.331	1.998	1.778
0.2	39.65	19.85	11.94	8.110	5.994
0.3	481.6	163.4	73.35	39.65	24.39
0.4	6235	1435	481.6	207.5	106.5
0.5	83518	13050	3274	1125	481.6

Figure 5 shows a graph of the equation for these different temperatures.

The resonance enhanced thermionic emission current has a value that is equivalent to the thermionic emission current from a material with a lower work function. Thus, the resonant enhancement on the cathode can be equated to a cathode with an equivalent work function by the following equation,

The table below shows the equivalent work function for different values of and for temperatures of 600 K, 700 K, 800 K, 900 K, and 1000 K. This is for a material with a work function = It should be noted that effective work function and equivalent work function are the same, and may be used interchangeably.

	600 K	700 K	800 K	900 K	1000 K

0.1	1.330	1.336	1.342	1.347	1.350
0.2	1.210	1.220	1.229	1.238	1.246
0.3	1.081	1.093	1.104	1.115	1.125
0.4	0.9484	0.9617	0.9744	0.9864	0.9980
0.5	0.8143	0.8286	0.8423	0.8554	0.8680

Figure 6 shows a graph of the equation for these different temperatures.

It should be noted that the work function of the material may also be oscillating before the resonance condition. The amplitude of the oscillation significantly increases at resonance. It should also be noted that for the previous two equations to be valid, the average electron escape time from the surface should be significantly smaller than half the period of the oscillation of the work function. In general, for the range of materials of interest it would be significantly smaller and the equations will be valid. Furthermore, it should also be noted that these equations may be modified for certain materials. For semiconductors, an equivalent form of the equations may incorporate different parameters attributed to semiconductors.

The description above has focused on examples in which atoms or molecules are adsorbed onto an emitter surface, such that the vibrational frequency of the adsorbed atoms or molecules matches the natural frequency of electron oscillations at the surface to achieve a resonance condition and thereby lower the effective work function.

The same method can also be applied to a collector surface to lower the effective work function thereof. As in the case of the emitter, the surface plasmon resonance frequency of the collector is designed to match the vibrational frequency of adsorbates on the collector surface.

In order to produce an efficient thermionic energy converter, it is preferred that the collector should have a lower work function than the emitter.

The resonant enhancement to thermionic emission current can be utilized in many possible designs to enhance the conversion efficiency in thermionic energy converters. Further ideas may be incorporated to further enhance thermionic emission current. For example, nanostructuring pointed shapes such as, cones, pyramids, etc. may demonstrate field enhancement effects that can be exploited in thermionic emission.

Figure 7 is a simplified diagram showing a further possible form of a thermionic energy converter. An emitter (cathode) is formed of a substrate material 1, with a suitable emitter surface material 2.

As shown schematically by the arrow 8, thermal energy input is input into the thermionic energy converter. The thermal energy may for example be input by conduction from a heat transfer medium, or through solar or other radiation passing through a window 5a in the package 5. For example, solar radiation may be concentrated by lenses and/or mirrors onto the emitter 1.

As discussed above, the emitter surface 2 and/or the surface of the collector 3 has adsorbate atoms or molecules adsorbed onto the surface. The vibrational frequency of the adsorbed atoms or molecules matches the natural frequency of electron oscillations at said surface.

In the example shown in Figure 7, the thermionic energy converter includes a laser source with a frequency matching the resonance frequency (i.e. matching the vibrational frequency of the surface layer species and the surface plasmon resonance frequency, for example being within ±70%, or within ±50%, or within ±25%, or within ±15%, or within ±10%, or within ±5% of the. vibrational frequency of the surface layer species and the surface plasmon resonance frequency), and this is used to enhance the synchronisation of adsorbates and the magnitude of the resonance effect.

When reference is made here to the laser frequency matching the resonance frequency, i.e. the surface plasmon resonance frequency of the emitter material that itself matches the vibrational frequency of the adsorbate, this means that the frequencies are close enough that there is some resonant coupling between them. For example, the laser frequency may be within a range given by the adsorbate vibrational frequency ±70%, and more preferably should be within a range equal to the adsorbate vibrational frequency ±50%, and more preferably should be within a range equal to the adsorbate vibrational frequency ±25%, and more preferably should be within a range equal to the adsorbate vibrational frequency ±15%, and more preferably should be within a range equal to the adsorbate vibrational frequency ±10%, and more preferably still should be within a range equal to the adsorbate vibrational frequency ±5% or less.

Thus, laser light 15 from a laser source is focused by an optical system, which in this example comprises a primary lens 16 and a secondary lens 17.

The laser can be designed to pass through the gap between the emitter 1 and the collector 3 with some divergence, such that the laser light is mostly absorbed as it passes through the gap. Furthermore, the divergence and the angle of the beam can be designed to be controllable. For example, small adjustments can be made to the position and angle of the secondary lens (e.g. by a micromanipulator), or electro-optic or acousto-optic lenses and deflectors may be used to control the angle of the beam and the divergence, such that the extent of the laser absorbed by the cathode and anode is controllable. This may optimize the laser effect for a given operating condition. The gap length (that is, the emitter to collector distance) should be appreciably larger than the wavelength of the laser (this wavelength is about 30 micron for a frequency of

THz) to avoid large diffraction effects. The diffraction effects can be accounted for and used to increase the extent of the absorption of the beam in some designs. In some embodiments, a pulsed laser is used. In some embodiments, an alternating pulsed operation is used, with the laser entering the gap from opposite ends in turn, to provide more uniform absorption along the length of the gap. Furthermore, the use of an electro-optic modulator or an acousto-optic modulator (i.e. Bragg cell) may provide a means of controlling and fine-tuning the frequency of the laser to provide the best match to the resonance condition.

This arrangement may enhance the resonance effect on both the cathode and the anode, if both are provided with the adsorbed atoms or molecules as described above.

Many variations can be utilized to incorporate a laser in the device configuration. However, if the device is to be used for energy conversion applications, for example in a thermionic energy converter, the laser must consume an insignificant amount of energy to avoid reducing the efficiency of the system.

Although several of the embodiments described herein related to thermionic energy converters, the method comprising adsorbing atoms or molecules onto the surface, such that the vibrational frequency of the adsorbed atoms or molecules matches said natural frequency of electron oscillations at the surface, can equally be used in many other types of device that require electron emission.

For example, the method and devices can be used in applications such as radioisotope thermionic generators and thermionic solar energy converters (that can be utilized in parabolic dish concentrator designs); Scanning Electron Microscopes as a ‘cold’ thermionic field emission source; low power portable X-ray sources; high power thermionic valves and microwave amplifiers (Travelling Wave Tubes); ion-gated FETS or as a chemical sniffer; and other potential sensor applications. In any of these, the resonance effect may be enhanced by utilizing a laser as discussed with reference to Figure 7, with a frequency that matches the vibrational frequency of the adsorbate atoms.

Thus, there is disclosed a method of enhancing thermionic emission by utilizing a resonance, induced by atoms or molecules adsorbed on the cathode emitter surface to lower the effective work function.

Thus, there is disclosed a method of enhancing thermionic emission by utilizing a resonance, induced by a dipole layer composed of high and low electron affinity atoms disposed on the semiconducting cathode emitter surface to lower the effective work function.

Thus, there is disclosed a method of enhancing thermionic emission by utilizing a resonance, induced by plasmonic particles disposed on the semiconducting cathode emitter surface to lower the effective work function.

Thus, there is disclosed a method of lowering the effective work function of the collector by utilizing a resonance, induced by adsorbed atoms or molecules on the anode collector surface.

Thus, there is disclosed a method of lowering the effective work function by utilizing a resonance, induced by a dipole layer composed of high and low electron affinity atoms disposed on the semiconducting anode collector surface.

Thus, there is disclosed a method of lowering the effective work function by utilizing a resonance, induced by plasmonic particles disposed on the semiconducting anode collector surface.

Claims

1. A material, having a surface, wherein the surface has a natural frequency of electron oscillations at the surface, and the material comprises at least one surface layer species of atom or molecule, and wherein the vibrational frequency of the surface layer species matches said natural frequency of electron oscillations at the surface.

2. A material as claimed in claim 1, wherein said natural frequency of electron oscillations at the surface is within a range given by the vibrational frequency of the surface layer species ±70%.

3. A material as claimed in claim 2, wherein said natural frequency of electron oscillations at the surface is within a range given by the vibrational frequency of the surface layer species ±50%.

4. A material as claimed in claim 3, wherein said natural frequency of electron oscillations at the surface is within a range given by the vibrational frequency of the surface layer species ±25%.

5. A material as claimed in claim 4, wherein said natural frequency of electron oscillations at the surface is within a range given by the vibrational frequency of the surface layer species ±15%.

6. A material as claimed in claim 5, wherein said natural frequency of electron oscillations at the surface is within a range given by the vibrational frequency of the surface layer species ±10%.

7. A material as claimed in claim 6, wherein said natural frequency of electron oscillations at the surface is within a range given by the vibrational frequency of the surface layer species ±5%.

8. A material as claimed in any preceding claim, wherein the surface layer species is used to functionalise the surface.

9. A material as claimed in any preceding claim, wherein the material comprises nanoparticles of a semiconductor material.

10. A material as claimed in claim 9, wherein the semiconductor material is a wide band gap semiconductor material.

11. A material as claimed in claim 9, wherein the semiconductor material is a metal oxide material.

12. A material as claimed in claim 11, wherein the semiconductor material is selected from the group comprising indium tin oxide, aluminium-doped zinc oxide, and galliumdoped zinc oxide.

13. A material as claimed in claim 9, wherein the semiconductor material is a Ill-V semiconductor material.

14. A material as claimed in claim 13, wherein the semiconductor material is InGaN.

15. A material as claimed in claim 9, wherein the semiconductor material is diamond.

16. A material as claimed in any of claims 9 to 15, wherein the surface layer species comprises a metal-oxygen dipole.

17. A material as claimed in claim 16, wherein the surface layer species comprises a metal-oxygen dipole of LIO, CsO, RbO, BeO, NiO, or aluminium-doped zinc oxide.

18. A material as claimed in any preceding claim, wherein the surface layer species comprises adsorbed gas molecules.

19. A material as claimed in claim 18, wherein the surface layer species comprises adsorbed molecules of hydrogen, methane, or ammonia.

20. A material according to any preceding claim, comprising two surface layer species of atom or molecule, having respective first and second vibrational frequencies, wherein said natural frequency of electron oscillations at the surface is between the first and second vibrational frequencies.

21. A material according to claim 20, wherein said first and second vibrational frequencies differ by less than 20%.

22. A system comprising:

a material according to any preceding claim: and a laser light source, arranged such that light from the laser light source is absorbed by the surface of the material.

23. A system as claimed in claim 22, wherein the frequency of the laser light source matches the vibrational frequency of the surface layer species and the surface plasmon resonance frequency.

24. A system as claimed in claim 23, wherein the frequency of the laser light source is within ±70%, or within ±50%, or within ±25%, or within ±15%, or within ±10%, or within ±5% of the vibrational frequency of the surface layer species and the surface plasmon resonance frequency.

25. A method of reducing an effective work function of a surface, wherein the surface has a natural frequency of electron oscillations at the surface, the method comprising functionalizing the surface with a surface layer species, such that the vibrational frequency of the surface layer species matches said natural frequency of electron oscillations at the surface.

26. A method as claimed in claim 25, wherein functionalizing the surface comprises adsorbing atoms or molecules onto the surface.

27. A method as claimed in claim 25 or 26, wherein said natural frequency of electron oscillations at the surface is within a range given by the vibrational frequency of the surface layer species ±70%.

28. A method as claimed in claim 27, wherein said natural frequency of electron oscillations at the surface is within a range given by the vibrational frequency of the surface layer species +50%.

29. A method as claimed in claim 28, wherein said natural frequency of electron oscillations at the surface is within a range given by the vibrational frequency of the surface layer species ±25%.

30. A method as claimed in claim 29, wherein said natural frequency of electron oscillations at the surface is within a range given by the vibrational frequency of the surface layer species ±15%.

31. A method as claimed in claim 30, wherein said natural frequency of electron oscillations at the surface is within a range given by the vibrational frequency of the surface layer species ±10%.

32. A method as claimed in claim 31, wherein said natural frequency of electron oscillations at the surface is within a range given by the vibrational frequency of the surface layer species ±5%.

33. A method as claimed in any of claims 26 to 32, wherein the material comprises nanoparticles of a semiconductor material.

34. A method as claimed in claim 33, wherein the semiconductor material is a wide band gap semiconductor material.

35. A method as claimed in claim 33, wherein the semiconductor material is a metal oxide material.

36. A method as claimed in claim 35, wherein the semiconductor material is selected from the group comprising indium tin oxide, aluminium-doped zinc oxide, and gallium·· doped zinc oxide.

37. A method as claimed in claim 33, wherein the semiconductor material is a IIl-V semiconductor material.

38. A method as claimed in claim 37, wherein the semiconductor material is InGaN.

39. A method as claimed in claim 33, wherein the semiconductor material is diamond.

40. A method as claimed in any of claims 33 to 39, wherein the surface layer species comprises a metal-oxygen dipole.

41. A method as claimed in claim 40, wherein the surface layer species comprises a metal-oxygen dipole of LIO, CsO, RbO, BeO, NiO, or aluminium-doped zinc oxide.

42. A method as claimed in any of claims 25 to 41, wherein the surface layer species comprises adsorbed gas molecules.

43. A method as claimed in claim 42, wherein the surface layer species comprises adsorbed molecules of hydrogen, methane, or ammonia.

44. A method according to any of claims 25 to 43, further comprising:

arranging a laser light source, such that light from the laser light source is absorbed by the surface of the material.

45. A method as claimed in claim 44, wherein the frequency of the laser light source matches the vibrational frequency of the surface layer species and the surface plasmon resonance frequency.

46. A method as claimed in claim 45, wherein the frequency of the laser light source is within ±70%, or within ±50%, or within ±25%, or within ±15%, or within ±10%, or within ±5% of the vibrational frequency of the surface layer species and the surface plasmon resonance frequency.

47. A thermionic emission device, comprising a material as claimed in any one of claims 1 to 20.