Semiconductor-based direct emitter, and liquid ionizer with the same, method for generating free electrons

Abstract

Semiconductor-based direct emitter (33) for use in surface chemistry, comprising: an electron emitter structure (11), comprising: a) an atmosphere-stable electron emission layer (17) made of a mixture of metals, wherein the atmosphere-stable electron emission layer has a first side and a second side; and b) an electron acceleration structure (18) on the first side of the electron emission layer (17) with at least one electrode (22) electrically insulated from the electron acceleration structure (18) to form an acceleration section configured to allow electrons (26) released from the electron emission layer (17) to be selectively accelerated by generating an adjustable electric field, wherein the acceleration section has a length l in a range from 10 nm to 1 µm, an insulating layer (30) arranged on the second side of the electron emission layer (17) and having a tunnelable layer thickness, and a semiconductor (34) arranged on one side of the insulating layer (30) facing away from the electron emission layer (17), which has alternating n-doped regions (36) and p-doped regions (38), wherein the n-doped regions (36) and the p-doped regions (38) have boundary regions (39) adjacent to the insulating layer (30).

Description

[0001] The present invention relates to a semiconductor-based direct emitter, a liquid ionizer with the same, and a method for generating free electrons using the same.

[0002] Low-energy electrons are used, for example, in air purification and trace gas analysis. The formation of ions in air requires charges. These charges are generated by irradiating air molecules with high-energy electromagnetic radiation, by radioactive sources, or by electron sources. The charges then bond with the air molecules. When charges are generated using high-energy electromagnetic radiation or radioactive sources, a variety of ion types are formed due to the high and unpredictable energies involved in charge generation or charge transfer. To form ions in air using electron sources, electrons are released into the air molecules. Low-energy electrons can ionize these molecules. The ions formed in this way can be trapped in an electric field and thus removed from the surrounding air.This allows, for example, dust particles, harmful substances, contaminants, as well as mites, pollen, odors, pathogens, and bacteria to be bound. Such electron sources can therefore be used in air purifiers. Whether the electrons can be captured by the molecules depends primarily on the energy of the electrons. Different molecules can capture electrons of different energies. Charging only specific molecules is possible using electrons with a very sharp energy distribution in a low energy range. Such electrons are thus also suitable for the ionization of molecules for trace gas analysis, for example, by mass spectrometry. State of the art

[0003] Ion formation with UV sources is known in photoionization detectors (PID sensors), as is ionization with 63Ni sources are used in analytical instruments, and electron impact ionization in a vacuum is used in mass spectrometers. UV sources are also used for ionization in air purifiers. This process also produces ozone, which contributes to air purification. However, the breakdown products of nicotine and cigarette smoke, along with the ozone itself, pose significant health risks. Ozone-generating air purifiers are therefore disadvantageous.

[0004] UV sources in PID sensors ionize all molecules via the Compton effect whose ionization energy E is below the photon energy hv. This results in a broad spectrum of molecules with varying degrees of ionization, which is less suitable for precision applications such as the trace gas analysis mentioned above.

[0005] Free electrons in air can be generated by field emission or thermal emission. In field emission, electrons are released from a negatively charged electrode by a strong electric field, utilizing the tunneling effect. Thermal emission refers to the thermally induced flow of electrons from a surface or across a potential barrier. Here, the electrons in the carrier overcome the work function of the material by means of supplied heat energy. Thermal emission is more difficult to generate under atmospheric conditions than field emission, but could be produced with high-temperature-resistant, oxidation-stable materials such as SnO₂ or antimony-doped SnO₂.

[0006] Furthermore, electrons can be generated by the photoelectric effect. In this process, an electron is released from a bond in a solid by the absorption of a photon. A so-called electron ionization detector (ePID) is known from the prior art. Based on the photoelectric effect, it generates very low-energy electrons from an emitter material and accelerates them in an electric field to path lengths on the order of some mean free paths of molecules under standard conditions. Theoretically, this allows the electron energy to be freely adjusted, enabling the identification of different trace gases in air not only by their total quantity but also, to a certain extent, by their ionization energies. UV light-emitting sources, such as diodes (UV LEDs), are used to release the electrons.

[0007] In principle, emissive materials with low work functions are best suited for generating free electrons in air. These materials exhibit a high density of emittable electrons, meaning they have a high electron density at the Fermi level or at the valence band edge. However, the materials with low work functions of around 1-2 eV used for this purpose, such as Ba, BaO, Cs (alkali and alkaline earth metals) and their semiconducting oxides, are highly reactive in air, meaning they oxidize exothermically or form a hydroxide. Therefore, they can only be used under high vacuum conditions. Lanthanum boride, which is used, for example, in the field emitter tips of electron microscopes for electron emission, is also unsuitable for applications under atmospheric conditions, as it must be freed from its surface oxide at high temperatures in a vacuum before it can be used as an electron emitter.Application to air leads to re-oxidation and would therefore not be possible, or only for a very short time (approximately a few µs).

[0008] Precious metals such as gold, platinum, and silver, while stable against oxidation or forming only very thin oxide or sulfide layers that can be easily tunneled, have very high work functions in the range of 4.8 to 5.6 eV. This energy range is currently inaccessible to UV LEDs. The situation is similar with metals that occur in their "native" form, i.e., as pure metals with only thin oxide coatings. This group includes arsenic, antimony, tellurium, lead, bismuth, indium, and tin. They usually have work functions of more than 4 eV, and their oxides exhibit comparable values. Transition metals, especially valve metals, whose work functions are about 1 eV lower than those of native metals, usually form insulating, electron-deficient oxides with thicknesses exceeding the typical tunneling distance of 5 nm.The work functions of their oxides are usually not significantly lower than those of the pure metals, although some are semiconductors (NiO, CuO). x , CoO x , Cr2O3, SnO2) have at least conduction electrons.

[0009] Lanthanum and similar rare earth metals of the same group, due to their proximity to the alkaline earth metals, exhibit a lower and therefore more suitable work function range of 2.5 to 3.6 eV, as do their oxides. However, these materials are not stable when exposed to atmospheric moisture (they form hydroxides and oxidize completely throughout their entire volume).

[0010] Thorium is an exception with unique properties, possessing a work function of 2.2–2.4 eV, an oxide work function of 2.8 eV, and stability towards the constituents of air. However, due to its radioactivity, and especially the high energy of its emitted alpha particles (54 MeV), which ionize non-specifically, this material is unsuitable for applications requiring low, sharp electron energies. Cerium, samarium, and ytterbium (and the common transition metals yttrium and strontium) are used as thorium substitutes in other applications. As pure metals, however, they are subject to the limitations of the lanthanides in that they oxidize rapidly and react with water, and are therefore also unsuitable.

[0011] The ePID designs known from the prior art cannot be operated stably under atmospheric conditions over the long term. This precludes their use in electrostatically operated air purifiers that operate without ozone. Analytical methods such as mass spectrometry are also not possible with the ePID designs known from the prior art under atmospheric conditions. For this, we need free electrons in the air with the most precise energies possible, for example, in the range of < 25 eV. Likewise, an atmospherically stable, i.e., long-term stable with respect to oxygen and humidity, highly emissive material with the lowest possible work function is lacking. Such a material would allow the use of cost-effective light sources (e.g., UV LEDs) for the external photoelectric effect.

[0012] DE 10 2011 013 262 A1 discloses an electron ionization source, wherein photoelectrons from an atmosphere-stable layer are generated by a UV source and accelerated to the ionization energies of the gases with an acceleration distance on the order of the mean free path of the gases or smaller.

[0013] An electron emission material is disclosed in JP H04-152 296 A.

[0014] JP H04-152 296 A relates to a photoelectron-emitting material with a photoelectric effect.

[0015] In JP H03-29259A a photoelectric surface structure of a semiconductor is disclosed.

[0016] JP H03- 29 259 A relates to a semiconductor-based photoelectric surface structure that is arranged in a vacuum and converts light into photoelectrons and emits them.

[0017] WO 02 / 005 305 A1 discloses a device for the emission of electrons at addressable positions.

[0018] WO 02 / 015 223 A1 discloses a method and system for electron beam lithography with high throughput, shortened electron beam column length, reduced electron-electron interactions and higher beam current.

[0019] DE 690 09 357 T2 relates to an electron-emitting semiconductor device.

[0020] US patent 6 023 124 A discloses an electron emission device which has an electron supply layer made of metal, as well as an insulating layer formed on the electron supply layer and a thin layer metal electrode formed on the insulating layer.

[0021] US Patent 2016 / 0172144A1 discloses a treatment device for reducing electron affinity, wherein the treatment device has an activation chamber which is used in a treatment device for reducing electron affinity by evaporating a surface treatment material and using the evaporated surface treatment material.

[0022] An electron beam source is disclosed in JP 6 192 097 B2. The electron beam source is of the photocathonic type.

[0023] The present invention is therefore based on the objective of providing a semiconductor-based direct emitter with an efficient emitter for low-energy electrons in the range below 25 eV with a narrow energy bandwidth, which can be operated in a long-term stable manner. Description of the invention

[0024] According to the invention, this problem is solved by a semiconductor-based direct emitter, in particular for use in surface chemistry, comprising an electron emitter structure comprising: a) an atmosphere-stable electron emission layer made of a mixture of metals, wherein the atmosphere-stable electron emission layer has a first side and a second side;and b) an electron acceleration structure on the first side of the electron emission layer with at least one electrode electrically insulated from the electron acceleration structure for forming an acceleration section configured to allow electrons released from the electron emission layer to be selectively accelerated by generating an adjustable electric field, wherein the acceleration section has a length l in a range from 10 nm to 1 µm, an insulating layer arranged on the second side of the electron emission layer having a tunnelable thickness, and a semiconductor arranged on a side of the insulating layer facing away from the electron emission layer, the semiconductor having alternating adjacent n-doped regions and p-doped regions, wherein the n-doped regions (36) and the p-doped regions have boundary regions adjacent to the insulating layer.

[0025] The term "metals" in the mixture of metals refers exclusively to mixtures of elemental metals, i.e., metals in the oxidation state 0. In particular, metal oxides are therefore not considered metals according to the invention.

[0026] Advantageously, the mixture of metals is an alloy, preferably comprising a compound selected from InCe, AsCeSm, AgSm, AgCe, AgSmCe and AgAsCeSm and / or two or more elements from As, Ag, Zn, Au, Pt, Ru Rh, Pd, Os, Ir, Ce, Zn, Bi, Te, Sm, Eu, Gd, Yt, Yb, Nd, Pr and La.

[0027] Alternatively or additionally, the mixture of metals comprises a eutectic comprising at least one noble metal, at least one lanthanide and at least one element selected from As, Te, Bi and Sn.

[0028] With regard to the mixture of metals, "comprise" can preferably mean "consist of".

[0029] Eutectics are understood to be material combinations at their eutectic points or in the vicinity of the eutectic points.

[0030] Advantageously, the electron accelerator structure further comprises at least one insulating structural element which, together with the at least one electrode, forms at least one electron accelerator element.

[0031] In a particular embodiment of the invention, it is provided that the at least one insulating structural element is arranged between the electron emission layer and the at least one electrode and is connected to it.

[0032] In another particular embodiment of the invention, the electron emitter structure comprises several electron accelerator elements arranged in an array, preferably laterally.

[0033] It is possible that the electron emitter structure is further developed as an external photoelectric effect emitter, comprising an electron emitter structure according to one of the previously mentioned embodiments, and one or more UV LEDs for extracting electrons from the electron emission layer, wherein the one or more UV LEDs is / are configured to emit electromagnetic radiation at least partially onto the electron emission layer.

[0034] In a further particular embodiment of the invention, the external photoelectric emitter further comprises a substrate arranged on the second side of the electron emission layer, wherein one or more UV LEDs are configured to emit electromagnetic radiation at least partially onto the second side of the electron emission layer and are arranged outside and / or inside the substrate, and the substrate is at least partially transparent in the emission wavelength range of the UV LEDs.

[0035] Advantageously, the external photoelectric emitter also includes a reflective layer located on a side of the substrate facing away from the electron emission layer.

[0036] Advantageously, in the external photoelectric emitter, the reflective layer comprises a material selected from platinum, mercury, nickel, palladium and iridium.

[0037] According to a further particular embodiment of the invention, in the external photoelectric emitter one or more UV LEDs are configured to emit electromagnetic radiation at least partially onto the first side of the electron emission layer.

[0038] In particular, it can be provided that the electron acceleration structure is at least partially transparent in the emission wavelength range of the UV LEDs.

[0039] Additionally or alternatively, the electron emitter structure can be further developed as a particle collection device, comprising an external photoelectric emitter according to one of the aforementioned embodiments, and two electrically chargeable plates of different polarity for generating an electric field, which are arranged on the first side of the electron emission layer at a distance from each other and each perpendicular to the electron emission layer.

[0040] Furthermore, the electron emitter structure can be further developed into a tunnel-area emitter, comprising an electron emitter structure according to one of the aforementioned embodiments, an insulator layer arranged on the second side of the electron emission layer, having a thickness between 0.5 nm and 6 nm, and a metal layer arranged on the insulator layer on one side facing away from the electron emission layer, having a thickness between 0.5 nm and 6 nm, wherein both the electron emission layer and the insulator layer have tunnelable thicknesses. Tunnelable thicknesses can, for example, be in the range between 0.5 nm and 5 nm.

[0041] Advantageously, the semiconductor-based direct emitter further comprises an insulator arranged on a side of the semiconductor facing away from the insulating layer, the insulator having fingers arranged at interfaces between the n-doped regions and the p-doped regions and projecting into the semiconductor.

[0042] The invention further relates to a semiconductor-based direct emitter comprising an electron emitter structure comprising: a) an atmosphere-stable electron emission layer made of a mixture of metals, wherein the atmosphere-stable electron emission layer has a first side and a second side;and b) an electron acceleration structure on the first side of the electron emission layer with at least one electrode electrically insulated from the electron acceleration structure for forming an acceleration section configured to allow electrons released from the electron emission layer to be selectively accelerated by generating an adjustable electric field, wherein the acceleration section has a length l in a range of 10 nm to 1 µm, an insulating layer arranged on the second side of the electron emission layer having a tunnelable thickness, and a light emission device arranged on a side of the insulating layer facing away from the electron emission layer, selected from a VCSEL, a side-emitting semiconductor laser or a light-emitting diode.

[0043] Furthermore, the invention provides a liquid ionizer comprising a semiconductor-based direct emitter according to the invention, and a protective layer arranged on a side of the electron acceleration structure facing away from the electron emission layer, wherein the protective layer has holes extending in an electron emission direction.

[0044] Advantageously, the holes have a diameter d between 10 µm and 100 µm.

[0045] Using an external photoelectric emitter, a method for generating free electrons of defined low energy can be realized, comprising the steps of: extracting electrons from the electron emission layer using one or more UV LEDs, and accelerating the extracted electrons by means of an electric field generated by applying a voltage between the electron emission layer and the at least one electrode.

[0046] Using a particle collection device, a method for collecting particles can be implemented, comprising the steps of: extracting electrons from the electron emission layer using one or more UV LEDs, accelerating the extracted electrons by means of an electric field generated by applying a voltage between the electron emission layer and the at least one electrode, electrically charging particles by the accelerated electrons, and accelerating the electrically charged particles by means of a second electric field by applying a voltage to the electrically chargeable plates.

[0047] A tunnel surface emitter can be used to implement a method for generating free electrons of defined low energy, comprising the steps of extracting electrons from the electron emission layer by applying a voltage between the electron emission layer and the metal layer, and accelerating the extracted electrons by means of an electric field generated by applying a voltage between the electron emission layer and the at least one electrode.

[0048] Finally, the invention relates to a method for generating free electrons of defined low energy using a semiconductor-based direct emitter according to the invention, comprising the steps of: generating electrons in an interface between an n-doped region and a p-doped region, accelerating the electrons in the interface by an externally applied electric field, generating emitted electrons by tunneling the electrons through a tunnelable insulating layer and through an electron emission layer, and accelerating the emitted electrons by means of a second electric field generated by applying a voltage between the electron emission layer and the at least one electrode.

[0049] Preferably, a preferred direction of emission is imposed on the emitted electrons, for example by an internal electric field.

[0050] The invention is based on the surprising finding that materials with low work functions can be produced by suitable metal alloys and that electrons released from these materials can be accelerated to low energies with a narrow energy range by a suitable electron accelerator structure. In this context, low energies are preferably understood to be energies in the range below 25 eV, particularly preferably below 10 eV. A narrow energy range is preferably understood to be below 1 eV, particularly preferably below 0.5 eV, and most preferably below 0.1 eV.

[0051] Further features and advantages of the invention will become apparent from the attached claims and the following description, in which an embodiment of the present invention is described with reference to the schematic drawings. These show: Fig. 1 a sectional view of an external photoelectric emitter according to a particular embodiment of the present invention; Fig. 2 a sectional view of a tunnel surface emitter according to a further particular embodiment of the present invention; Fig. 3 a sectional view of a semiconductor-based direct emitter according to a further particular embodiment of the present invention; Fig. 4 a sectional view of another semiconductor-based direct emitter according to a particular embodiment of the present invention; Fig. 5 a sectional view of a liquid ionizer according to a particular embodiment of the present invention Fig. 6 a sectional view of a particle collection device according to a particular embodiment of the present invention.

[0052] Fig. Figure 1 shows a sectional view of an external photoelectric emitter 10 according to a particular embodiment of the present invention. The external photoelectric emitter 10 comprises an electron emitter structure 11 according to a particular embodiment of the invention, wherein the electron emitter structure 11 in turn comprises an atmosphere-stable electron emission layer 17 made of a mixture of metals with a first side and a second side, and an electron accelerator structure 18 on the first side of the electron emission layer 17. The electron emission layer 17 has a thickness between 1 nm and 10 nm.

[0053] The electron accelerator structure 18 comprises several electron accelerator elements 20. The electron accelerator structure 18 is suitable for accelerating charged particles, in particular electrons. The electron accelerator elements 20 are located above the electron emission layer 17 and are connected to it. Each electron accelerator element 20 has an electrode 22 and an electrically insulating structural element 24. Preferably, the structural element 24 is in direct contact above the electron emission layer 17. During operation, the electrodes 22 are charged positively or negatively relative to the electron emission layer 17 via a voltage source (not shown). Optionally, the electron accelerator structure 18 can have cover electrodes (not shown) above the electrodes 22.By applying a zero potential or a repulsion potential, the area above the electrodes 22 can be set to be field-free, which ensures free movement of the electrons 26 in an ionization space adjacent to the electrodes 22 (not explicitly shown).

[0054] Furthermore, the electron accelerator structure could include a cylindrical control electrode (not shown). Such a cylindrical control electrode could be designed in the form of a Wehnelt cylinder. The cylindrical control electrode allows the electrons to be focused onto regions between the electron accelerator elements. This prevents, among other things, the electrically insulating structural elements 24 from becoming charged.

[0055] The external photoelectric emitter 10 further comprises a substrate 12 made of a UV-transparent material with a front and a back. UV LEDs 14 are arranged next to the substrate 12 such that they emit light, represented by dashed arrows, in the direction of the transparent substrate 12. Alternatively, the UV LEDs 14 can also be integrated into the substrate 12. The UV LEDs 14 preferably emit light with a wavelength between 240 nm and 400 nm. A reflective layer 16 is arranged on the back of the substrate 12, which is suitable for reflecting the light emitted by the UV LEDs 14. The reflective layer 16 preferably has a material with a very high work function. In this context, a very high work function preferably means a work function of more than 4.5 eV, and particularly preferably more than 5 eV. The material can contain, for example, platinum, mercury, nickel, palladium and iridium.Preferably, the side of the substrate 12 facing away from the electron emission layer 17 is structured such that the light emitted by the UV LEDs 14 can be reflected multiple times towards the electron emission layer 17. Surfaces angled to the horizontal are suitable for this purpose. Due to the aforementioned multiple reflection, unabsorbed photons strike the electron emission layer 17 multiple times, thereby increasing the absorption probability and enhancing the emission efficiency of the photoelectric emitter.

[0056] During operation of the external photoelectric emitter 10, the UV LEDs 14 emit high-energy photons in the UV range with wavelengths between approximately 240 nm and 400 nm. The photons strike the electron emission layer 17 either directly or after reflection from the reflective layer 16. The photons are absorbed by the electron emission layer 17 and release electrons 26 from it via the external photoelectric effect. Since the penetration depth of the photons is typically greater than the mean free path of the electrons 26, grazing incidence of the light is advantageous for generating a sufficient number of free electrons 26. Grazing incidence is favored by arranging the UV LEDs 14 laterally next to the substrate.The electron accelerator structure 18 generates an electric field above the electron emission layer 17 by means of a voltage applied between the electrodes 22 and the electron emission layer 17. This electric field is essentially perpendicular to the surface of the electron emission layer 17 and accelerates the electrons 26. In relation to the electrons 26, the term "acceleration" is intended to explicitly include both positive and negative acceleration. In general, the kinetic energy of the emitted electrons 26 can be adjusted by means of the electron accelerator structure 18 via an accelerating voltage applied to the electrodes 22. The electron emission layer 17 consists of a material suitable for emitting electrons via the photoelectric effect and preferably has a low work function, for example, in the range of 2.5 to 3.6 eV.The length l of the acceleration path is of the same order of magnitude as, or smaller than, the mean free path of the gas molecules. It is preferably 10 nm to 1 µm, and particularly preferably 30 nm to 300 nm.

[0057] The mixture of metals can, for example, comprise an alloy comprising two or more elements selected from As, Ag, Zn, Au, Pt, Ru, Rh, Pd, Os, Ir, Ce, Zn, Bi, Te, Sm, Eu, Gd, Yt, Yb, Nd, Pr, and La. In particular, alloys comprising InCe, AsCeSm, AgSm, AgCe, AgSmCe, and AgAsCeSm are suitable for forming the electron emission layer. Alternatively, the mixture of metals can include or consist of a eutectic. Suitable eutectics consist of at least one noble metal, at least one lanthanide, and at least one element selected from As, Te, Bi, and Sn.

[0058] The mixture of metals preferably has a low work function of less than 3.5 eV, particularly preferably less than 3.0 eV, and is produced according to the invention by suitable alloys. The exemplary compounds InCe, AsCeSm, AgSm, AgCe, AgSmeCe and AgAsCeSm exhibit permanently constant work functions between 1.2 eV and 3.2 eV (after 14 months of storage under normal conditions).

[0059] The general principle for this class of materials is that the noble metals and the native metals contribute to the alloy's properties by increasing its atmospheric stability, while the lanthanides reduce the alloy's work function. Furthermore, if the material has a uniform stoichiometry, it can be deposited with greater crystalline order than if it has a non-uniform stoichiometry. This allows the noble metal atoms to better protect nearby lanthanide atoms from oxidation in air or reaction with atmospheric moisture. This increased stability can be explained by the so-called spillover effect. In the material groups mentioned here, electrons are transferred from the f-orbitals of the noble metal atoms to the d-orbitals of the lanthanide atoms.In particular, a uniform stoichiometry of the AgCeSm material is advantageous for optimizing the low work function and stability towards the atmosphere. Ag2(Ce) compounds are especially suitable for this purpose. x Sm 1-x ) and Ag(Ce x Sm 1-x These are only examples of a large number of compounds based on these elements.

[0060] Alternatively or additionally, it is also possible to illuminate the electron emission layer 17 from the front to release electrons from it. For this purpose, an electron accelerator structure 18 made of optically transparent materials increases efficiency. The insulating structural elements 24 can, for example, consist of quartz or a fluoride material. The electrode can, for example, consist of transparent indium tin oxide (ITO). In this case, the UV LEDs are arranged above the electron emission layer 17.

[0061] Fig. Figure 2 shows a sectional view of a tunnel surface emitter 27 according to a further particular embodiment of the present invention. The tunnel surface emitter 27 has an electron emitter structure 11, for example, according to the one shown in Figure 2. Fig. In the embodiment of the invention shown in Figure 1, an insulating layer 30 is arranged on the second side of the electron emission layer 17, and a metal layer 28 is arranged on the insulating layer 30 on a side facing away from the electron emission layer 17. The thicknesses of the metal layer 28, the tunnelable insulating layer 30, and the tunnelable electron emission layer 17 are preferably between 0.5 nm and 6 nm, and particularly preferably between 1 nm and 3 nm. Both the metal layer 28 and the electron emission layer 17 are conductive. Strong electric fields can be generated within the thin insulating layer 30 between the two conductive layers by applying a voltage. Electrons located in the three layers mentioned above can absorb energy from the electric field thus generated, so that their average energy is above the equilibrium value corresponding to the lattice temperature.The so-called "hot electrons" generated in this way can emit from the tunnelable anode 32 and the electron emission layer 17 due to their high energy. The tunnel surface emitter 27 has an electron acceleration structure 18 above the tunnelable anode 32, corresponding to that described in connection with [reference missing]. Fig. 1 described in a particular embodiment of the invention. This electron acceleration structure 18 is constructed in the same way as in the external photoelectric emitter 10. Fig. 1 is able to accelerate or decelerate the emitted electrons 26.

[0062] Fig. Figure 3 shows a cross-section of a semiconductor-based direct emitter 33 according to a further particular embodiment of the invention. The semiconductor-based direct emitter 33 comprises an electron emitter structure 11, for example, according to the one shown in Figure 3. Fig. In the embodiment of the invention shown in Figure 1, an insulating layer 30 is arranged on the second side of the electron emission layer 17 and has a tunnelable thickness. A semiconductor 34 is arranged on the side of the insulating layer 30 facing away from the electron emission layer 17. The semiconductor 34 has horizontally adjacent n-doped regions 36 and p-doped regions 38. The semiconductor 34 has a direct, large band gap of at least 3.0 eV. For this purpose, the semiconductor can, for example, comprise AlGaN or InAlGaN. At the boundary regions (39) between the n- and p-doped regions 36, 38, shown by vertical dashed lines, there are pn junctions. The pn junctions are suitable for the emission of photons by recombination of electrons and holes, for example, in a quantum film (not explicitly shown here) that has a lower band gap than the surrounding semiconductor material 34.In this way, the semiconductor material 34 forms a light-emitting diode (LED) at each of the pn junctions. On one underside, the semiconductor material 34 borders an insulator 40. The insulator 40 has fingers 42 projecting into the semiconductor material 34 at the locations of the pn junctions. The fingers 42 reduce the size of the pn junctions in the LED while keeping the size of the n- and p-doped regions 36, 38 approximately the same. By thus limiting the size of the pn junctions, the charge carrier density at the pn junctions can be increased.

[0063] On a first side of the semiconductor material 34 are elements consisting of a tunnelable insulating layer 30 and an electron emission layer 17 in a similar or identical manner as in the Fig. 1 and Fig. 2. Furthermore, the semiconductor-based direct emitter 33 also exhibits, as in the Fig. 1 and Fig. Figure 2 shows an electron acceleration structure 18 with which electrons released from the electron emission layer 17 can be accelerated.

[0064] Both the tunnelable insulating layer 30 and the electron emission layer 17 can each have layer thicknesses between 0.5 nm and 5 nm. The interrupted electron emission layer 17 shown here represents only one particular embodiment of the invention. An uninterrupted insulating layer 30 and electron emission layer 17 can also be used.

[0065] When one of the light-emitting diodes is operated by applying a voltage, free electrons and holes are generated in the conduction band and valence band of the semiconductor material, respectively. If a positive voltage is also applied to the electron emission layer 17 located above the junction 39 of the operated light-emitting diode, an electron 26 generated in the junction 39 of the light-emitting diode is accelerated towards the electron emission layer 17. If the electron 26 is generated in the junction 39 sufficiently close to the insulating layer 17 and accelerated towards it, recombination may be prevented. Instead, due to the small layer thicknesses, the electron 26 can tunnel through both the insulating layer 17 and the electron emission layer 17. In this way, a current of free electrons can be generated outside the light-emitting diode.

[0066] Furthermore, the semiconductor-based direct emitter 33 can also be used as in the Fig. 1 and Fig. Figure 2 shows an electron acceleration structure 18, which allows electrons 26 emitted from the electron emission layer 17 to be accelerated. However, this is generally only optional for such a semiconductor-based direct emitter 33. Depending on the application of the semiconductor-based direct emitter 33, the electron acceleration structure 18 can also be omitted.

[0067] To limit the emitter current and prevent breakdown, the electron emission layer 17 in this embodiment preferably comprises semimetals, for example, arsenic, selenium, antimony, tellurium, and bismuth. For compounds of these elements, the space charge regions, which are helpful for acceleration, extend deeper than the first atomic layer. Alternatively, a perovskite material, i.e., a material with a preferred axis of the k-vector, can also be used for this purpose. In the emissive layer, the momentum distribution of the excited electrons is fundamentally unidirectional. A vertical preferred direction of the momentum distribution is generated by mechanical stress in the plane. A perovskite lattice structure is particularly suitable for this purpose.

[0068] Fig. Figure 4 shows a sectional view of another semiconductor-based direct emitter 33 according to a particular embodiment of the present invention. This embodiment of the invention incorporates many of the features associated with Fig. The elements described in section 2 are listed. Elements designated with the same reference symbols correspond to those described in the table. Fig. 2 shown. In particular, the semiconductor-based direct emitter 33 also has an electron emitter structure 11, for example according to the one shown in Fig. Figure 1 shows an embodiment of the invention and an insulating layer 30. A vertical cavity surface-emitting laser (VCSEL) 41 is arranged on one side of the insulating layer facing away from the electron emission layer 17. The VCSEL 41 has a cavity 43, an upper Bragg mirror 50, and a lower Bragg mirror 52. The cavity 43, in turn, has an n-doped layer 44, a p-doped layer 46, and a quantum film 48. The upper and lower Bragg mirrors 50 and 52 are each in direct contact with the cavity 43. The band gap of the quantum film 48 is smaller than that of the surrounding n- and p-doped layers 44 and 46.

[0069] In the operation of the semiconductor-based direct emitter 33, electrons from the n-doped layer 44 recombine with holes from the p-doped layer 46 in the quantum film 48, thus generating photons of suitable wavelengths. These photons are repeatedly reflected by the Bragg mirrors 50 and 52, triggering further photons from the quantum film 48 through stimulated emission. The upper Bragg mirror 50 has a lower reflectivity than the lower Bragg mirror 52. As a result, the VCSEL 41 emits photons towards the electron emission layer 17. The photoelectric effect triggers the emission of an electron 26 in the electron emission layer 17. This electron can then be accelerated or decelerated by the electron accelerator structure 18. The processes are schematically represented by the solid and dashed arrows. Alternatively, a side-emitting semiconductor laser or a light-emitting diode can be used instead of the VCSEL 41.

[0070] Fig. Figure 5 shows a sectional view of a liquid ionizer 53 according to a particular embodiment of the present invention. The liquid ionizer 53 serves to ionize liquids or to supply electrons 26 of a defined energy to liquids. For this purpose, the liquid ionizer 53 comprises a semiconductor-based direct emitter 33, for example according to the one described in Figure 5. Fig. 3. A particular embodiment of the invention is shown. Alternatively, any of the embodiments of the invention according to the Fig. 1, Fig. 2 or Fig. 4 serves as the basis for the liquid ionizer 53. The liquid initiator 53 has a protective layer 54 arranged on one side of the electron accelerator structure 18 facing away from the electron emission layer 17. The protective layer 54 allows the emission of electrons 26 through holes 56 located in the protective layer 54. The holes 56 have a diameter d that is selected such that a liquid 58 applied to the protective layer 54 does not penetrate the holes due to its surface tension, and electron emission is restricted as little as possible. In addition, the protective layer 54 serves to protect the electrodes 22 and the electron emission layer 17 from reactions with the liquid 58. The protective layer preferably consists of an insulating polymer, such as fluorocarbon or fluorosilicone layers. The latter two materials, among other things, enhance the liquid-repellent lotus effect.The diameter d of the holes 56 is preferably between 10 µm and 100 µm, particularly preferably between 20 µm and 50 µm. As described above, the electrons are emitted from the electron emission layer 17, accelerated or decelerated by the electron accelerator structure 18, pass through the holes 56 of the protective layer 54, and strike the surface of the liquid 58. Electron emission on liquid surfaces leads to significant local changes in surface energies. This means that electrons are no longer diffusively supplied with a broad energy spectrum, but rather with energies precisely required for a specific reaction. This is helpful, for example, in catalysis or electrochemical reactions. As a result, reaction pathways can be controlled more precisely than has previously been possible with catalysts or the morphologies of reactive surfaces.

[0071] Fig. Figure 6 shows a particle collection device 59 according to a particular embodiment of the invention and can, for example, be used in air containing particles 62, as shown here. The particle collection device 59 comprises an external photoelectric emitter 10, for example according to the one shown in Fig. 1. In its embodiment shown, the invention can also be used as an alternative embodiment. However, any of the embodiments of the invention according to the Fig. 2, Fig. 3 or Fig.4 serve as the basis for the liquid ionizer 53. On the first side of the electron emission layer 17, two plates 60 charged with opposite polarities are arranged at a distance from each other and perpendicular to the electron emission layer 17. The plates 60 are suitable for generating an electric field perpendicular to the emission direction of the electrons 26. Electrons 26 emitted by the external photoelectric emitter 10 can be captured by particles 62 located between the plates 60 during the operation of the particle collection device 59. In this way, the particles 62 themselves become negatively charged. In the electric field generated by the plates 60, the negatively charged particles 62 are accelerated towards the positively charged plate 60 and can be captured either by the plate 60 itself or by a filter (not shown) located in front of the plate 60. In this way, they are removed from the air.

[0072] The features of the invention disclosed in the foregoing description, in the drawings and in the claims can be essential for the realization of the invention in its various embodiments, both individually and in any combination. Reference symbol list 10 External photoelectric emitters 11 Electron emitter structure 12 Substrat 14 UV LEDs 16 reflective layers 17 Electron emission layer 18 Electron acceleration structure 20 electron accelerator elements 22 electrode 24 structural element 26 electrons 27 tunnel surface emitters 28 Metal layer 30 Insulator layer 32 tunnelable anodes 33 semiconductor-based direct emitters 34 semiconductors 36 n-doped area 38 p-doped area 39 Border area 40 Insulator 41 VCSEL 42 fingers 43 Cavity 44 n-doped layer 46 p-doped layer 48 Quantum film 50 upper Bragg mirror 52 lower Bragg mirror 53 Liquid ionizer 54 Protective layer 56 holes 58 Liquid 59 Particle collection device 60 plates 62 particles l Length of the acceleration section d diameter of the holes

Claims

[1] Semiconductor-based direct emitter (33) for use in surface chemistry, comprising: an electron emitter structure (11), comprising: a) an atmosphere-stable electron emission layer (17) made of a mixture of metals, wherein the atmosphere-stable electron emission layer has a first side and a second side; and b) an electron acceleration structure (18) on the first side of the electron emission layer (17) with at least one electrode (22) electrically insulated from the electron acceleration structure (18) to form an acceleration section configured to allow electrons (26) released from the electron emission layer (17) to be selectively accelerated by generating an adjustable electric field, wherein the acceleration section has a length l in a range from 10 nm to 1 µm, an insulating layer (30) arranged on the second side of the electron emission layer (17) and having a tunnelable layer thickness, and a semiconductor (34) arranged on one side of the insulating layer (30) facing away from the electron emission layer (17), which has alternating n-doped regions (36) and p-doped regions (38), wherein the n-doped regions (36) and the p-doped regions (38) have boundary regions (39) adjacent to the insulating layer (30). [2] Semiconductor-based direct emitter (33) according to claim 1, wherein the mixture of metals is an alloy comprising a compound selected from InCe, AsCeSm, AgSm, AgCe, AgSmCe and AgAsCeSm and / or two or more elements from As, Ag, Ru Rh, Pd, Os, Ir, Ce, Zn, Bi, Te, Sm, Eu, Gd, Yt, Yb, Nd, Pr and La [3] Semiconductor-based direct emitter (33) according to any one of the preceding claims, wherein the mixture of metals comprises a eutectic comprising at least one noble metal, at least one lanthanide and at least one element selected from As, Te, Bi and Sn. [4] Semiconductor-based direct emitter (33) according to one of the preceding claims, wherein the electron accelerator structure (18) further comprises at least one insulating structural element (24) which together with the at least one electrode (22) forms at least one electron accelerator element (20). [5] Semiconductor-based direct emitter (33) according to claim 4, wherein the at least one insulating structural element (24) is arranged between the electron emission layer (17) and the at least one electrode (22) and is connected thereto. [6] Semiconductor-based direct emitter (33) according to claim 4 or 5, wherein the electron accelerator structure (18) comprises several electron accelerator elements (20) arranged in an array, preferably laterally. [7] Semiconductor-based direct emitter (33) according to one of the preceding claims, further comprising an insulator (40) arranged on a side of the semiconductor (34) facing away from the insulator layer (30), wherein the insulator (40) has fingers (42) arranged at interfaces (39) between the n-doped regions (36) and the p-doped regions (38) and projecting into the semiconductor (34). [8] Semiconductor-based direct emitter (33), comprising: an electron emitter structure (11), comprising: a) an atmosphere-stable electron emission layer (17) made of a mixture of metals, wherein the atmosphere-stable electron emission layer (17) has a first side and a second side; and b) an electron acceleration structure (18) on the first side of the electron emission layer (17) with at least one electrode (22) electrically insulated from the electron acceleration structure (18) to form an acceleration section configured to allow electrons (26) released from the electron emission layer (17) to be selectively accelerated by generating an adjustable electric field, wherein the acceleration section has a length l in a range from 10 nm to 1 µm, an insulating layer (30) arranged on the second side of the electron emission layer (17) and having a tunnelable layer thickness, and a light emission device arranged on one side of the insulating layer (30) facing away from the electron emission layer (17), selected from a VCSEL (41), a side-emitting semiconductor laser or a light-emitting diode. [9] Liquid ionizer (53), comprising: a semiconductor-based direct emitter (33) according to any one of the preceding claims, and a protective layer (54) arranged on one side of the electron accelerator structure (18) facing away from the electron emission layer (17), wherein the protective layer (54) has holes (56) extending in an electron emission direction. [10] Liquid ionizer (53) according to claim 9, wherein the holes (56) have a diameter d between 10 µm and 100 µm. [11] Method for generating free electrons (26) of defined low energy using a semiconductor-based direct emitter (33) according to claims 1 to 7, comprising the steps: Generating electrons (26) in a boundary region (39) between an n-doped region and a p-doped region, Acceleration of the electrons (26) in the boundary region (39) by an externally applied electric field, Generating emitted electrons by tunneling the electrons through a tunnelable insulating layer (30) and through an electron emission layer (17), and Acceleration of the released electrons (26) by means of a second electric field, which is generated by applying a voltage between the electron emission layer (17) and the at least one electrode (22).

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