WO2019110502A1

WO2019110502A1 - Electron source with improved optical control

Info

Publication number: WO2019110502A1
Application number: PCT/EP2018/083342
Authority: WO
Inventors: Pierre Legagneux; Laurent Gangloff; Didier Pribat; Stephan PURCELL; Anthony Ayari
Original assignee: Thales; Centre National De La Recherche Scientifique; Ecole Polytechnique; Universite Claude Bernard Lyon 1
Priority date: 2017-12-07
Filing date: 2018-12-03
Publication date: 2019-06-13
Also published as: EP3721464A1; FR3074955B1; FR3074955A1

Abstract

The invention relates to an optically controlled electron source (100) comprising at least one tip (P) which is made of a conductive material (Cond) and of which the surface of the apex and the vicinity thereof is covered with at least one monolayer (L) of a two-dimensional semiconductor (semi2D), a monolayer comprising between 1 and 5 atomic planes.

Description

Electron source with improved optical control

FIELD OF THE INVENTION

The invention relates to optically controlled electron sources for applications such as time-resolved electron microscopy / spectroscopy / electron diffraction, pulsed X-ray sources, or pulsed sources for accelerators.

STATE OF THE ART

Electron emission by tunneling effect, by a very fine metal tip P, is used in electron microscopes, according to the schematic diagram described in FIG. 1. The emitter, in the form of an FO wire, one end of which is cut in the form of PO point, is disposed in a vacuum chamber and is a Cath cathode.

The electrons are extracted from the tip by the application of an electric field Eext by applying a potential difference Vext (typically 1 to 5 kV) between the Cath cathode and the extrector Extra, then are accelerated to an Anode A by a electric field E0 obtained by applying a potential difference V0 between the Cath cathode and the anode A, V0 being typically between 10kV and 500 kV. A SF system is used to focus the electron beam on a sample Ech. A diaphragm can be used to select a small beam angular extent.

In a scanning microscope the electron beam scans the sample line by line and a secondary electron detector measures the secondary electron current. In a transmission electron microscope as described in FIG. 1, the thickness of the sample is small (<100 nm) and the beam transmitted is analyzed with an Ana analyzer either by imaging or by electron energy spectroscopy. either by diffraction spectrum analysis. In an X-ray tube, in one mode the electrons are braked when they enter the anode, creating a continuous braking radiation or Bremsstrahlung effect. The RX sources are based on the impact of an energy electron beam of 10 to 500 keV. According to another mode, the energetic electrons interact with the core electrons of the target atoms (anode). The induced electron reorganization is accompanied by emission of photons of characteristic energy.

Micro / nano focus X-ray sources are based on a single source of electrons.

To obtain a source generating a strong X-ray flux, a set of PO peaks is conventionally used on a substrate S in the form of a network, which impact an anode A as shown schematically in FIG. 2.

Typically the metal used for the point or points is tungsten W, well known for its chemical stability, good conductivity and its resistance properties to strong electric fields and high temperatures. The radius of curvature at the top of the tip is then 20 to 120 nm. It is also possible to use molybdenum as the metal.

It is also possible to use carbon nanotubes (CNT for Carbon Nano Tube in English).

The principle of field emission is recalled Figure 3.

Figure 3a shows the probability f (E) for an electron to occupy an energy state E in the metal Met of the tip. All levels are occupied to the level of fermi E _F.

NV level is Vac vacuum level. The output work of the electrons of the metal is equal to NV - E _F.

Under the action of a field E0, the curve 32 represents the potential V (x) seen by the electron of the metal subjected to an electric field E0, as a function of its position (the effect of the image load is neglected)

V (x) = NV-e.EO.x

This curve 32 illustrates the width of the barrier that must cross the electrons to tear the metal, depending on their energy. When the electric field at the surface of the tip reaches 2 V / nm, the thickness of the tunnel barrier is of the order of 2 nm and electrons 30 are emitted by tunnel effect from the Fermi level in the vacuum, while their energy level E _F is lower than the vacuum level NV. The typical electric field at the top of a tip in a microscope is 5 V / nm.

Figure 3b illustrates the intensity 1 (E) of electrons emitted as a function of their energy E (in eV). The width of the energy distribution 1 (E) measures the quality of the electron source. The finer it is, the more electrons are homogeneous in energy, and the lower the chromatic aberrations. In electron microscopy, the resolution of the images is even better than the chromatic aberrations are weak.

In the case of the field emission of FIG. 3a, the width at half height of the energy distribution 1 (E) of the emitted electrons FWHM is 0.2 eV at a low current and reaches approximately 0.35 eV for a current of the order 10 mA. Note that the emission is highly localized: it comes from a quarter of the surface of the apex.

Recently, optically controlled electron (through a tip) emission has appeared (see for example H. Yanagisawa Publication, PRL 107, 087601, 201 1). Its implementation principle and the method of characterization of the effect are illustrated in Figure 4.

A pulsed laser beam F _L is focused on a tungsten tip PM subjected to an electric field E0. The polarization of the laser is controlled. The laser beam focused on the tip induces an optical electric field in the vicinity thereof. Depending on the amplitude of this oscillating electric field with respect to the electric field E0, the PM tip emits electrons according to various mechanisms described below. The analysis of electron energy emitted in the axis of the tip is carried out with a hemispheric analyzer AE.

The electron beam impacts a pierced plate covered with a phosphorescent material to visualize the different emitting zones of the tip. The positioning of the plate makes it possible to select the electron beam emitted by a given zone (part of this beam passes through an opening) and to study it with the electron energy analyzer. The wavelength of the laser used today, mainly for reasons of laser source availability, is typically equal to A0 = 800 nm. The pulses of duration 80 fs and more induce so-called weak optical fields, that is to say less than 2 V / nm. For 7 fs pulses and high laser powers, so-called strong optical fields, that is to say higher than 3 V / nm, are obtained.

In the electron emission with optical control, we use the absorption by electrons of one or more photons.

In a first type of optically controlled electron emission, illustrated in FIG. 5, the electric field E0 is relatively weak (less than 2 V / nm), that is to say that it is not possible with this E0 field only to have the field emission from the Fermi E _{F level} . An electron acquires energy by absorption of one or more photons

When the electron acquires enough energy to pass over the barrier, typically by a multi-photon absorption (three photons, 3PE, in the example of Figure 5) is called photoemission (first mechanism).

When the energy acquired by the electrons does not allow them to be above the barrier, but is high enough to be compatible with the tunnel emission (emission of electron through the barrier) we speak of emission of photo-assisted field (second mechanism).

In both cases, the optical field is said to be "weak", ie less than 3V / nm at the top of the tip.

Figure 6 illustrates the energy distribution of electrons emitted solely by photoemission. The field E0 is small enough here that a field emission from the Fermi level is not possible. The electric field E0 is small enough that the photo-assisted field emission mechanism is negligible. Only the electrons excited by several photons (3 photons "3PE" in the example of Figure 6) acquire enough energy to be emitted above the barrier. The half-height width of the energy distribution of the emitted electrons 1 (E) is of the order of 0.7 eV. Because of the high density of electrons in the metal (typically tungsten), the many collisions Electron-electron elastics induce a very rapid spread (a few fs) of the energy distribution of the electrons in the material and thus of the electrons emitted.

FIG. 7 illustrates the energy distribution of the electrons emitted according to the second mechanism, namely photo-assisted field emission. The field E0 is such that one is at the limit of the field emission, that is to say that a weak or very weak emission of field is possible starting from the level of Fermi (E0 typically around 2 V / nm). In this regime, the energy distribution of the electrons having absorbed 2 photons and emitted by photo-assisted field effect is of the order of 0.5 eV. Because of the high density of electrons in tungsten, the very numerous electron-electron elastic collisions induce a very rapid spread (a few fs) of the energy distribution of the electrons in the material and therefore of the electrons emitted.

Note that the longer the wavelength, the more photons are required for an electron to acquire the required energy level.

Another type of electron emission, illustrated in FIG. 8, is observed when the electric field E0 is strong, typically at least equal to 2V / nm, and the optical field (oscillating electric field induced by the focused beam) is also strong. at least equal to 3V / nm. In this case for which the two fields are of the same order of magnitude, the optical field adding to the field E0, we obtain a relatively large oscillation of the potential, that is to say the potential barrier seen by the electrons , schematized by the arrow 80 of FIG.

In this regime called optical emission regime (electric field and strong optical) illustrated in Figure 9, according to H Yanagisawa previously cited, a plateau of width between a few eV and 20 eV appears in the energy spectrum of electrons initially emitted . It is due to the strong space charge effects in the emitted electron pack (arrow 90). Electrons are also emitted with energy close to the Fermi level. Some electrons after emission are re-accelerated to the surface by the optical field and then re-emitted after diffusion (mechanism described by the arrows 91). The energy distribution of these electrons emitted with an average delay of 18 fs is wide (some eV), as illustrated in Figure 9. Thus, for the various types of optical electron emission of the state of the art, the energy distribution of the electrons is relatively wide (greater than or equal to 0.5 eV). An example is a distribution of 0.7 eV wide for recent experiments with a tungsten tip irradiated with a laser of wavelength 400 nm and pulse duration 50 fs, subjected to an electric field of 0.1 V / nm (Feist et al., Nature 521, 200 (2015)). In this case, the mechanism is a two-photon photoemission

An object of the present invention is to overcome the aforementioned drawbacks by providing an optically controlled electron source having an improved electron energy distribution emitted.

DESCRIPTION OF THE INVENTION

The present invention relates to an optically controlled electron source comprising at least one tip made of a conductive material whose surface of the apex and its vicinity is covered with at least one monolayer of a two-dimensional semiconductor, a monolayer comprising between one and five atomic planes.

According to a variant, the conductive material is a metal chosen from tungsten and molybdenum and the peak radius of curvature is between 20 nm and 150 nm.

According to another variant the conductive material is graphitic, the tip comprising a carbon nanotube.

Preferably, the number of monolayers of the two-dimensional semiconductor is between 1 and 5.

According to one embodiment, the two-dimensional semiconductor is a transition metal dichalcogenide, of chemical formula MX ₂ , with M metal and X chalcogen, comprising three atomic planes.

Preferably, the transition metal dichalcogenide is chosen from: MOS ₂ WS ₂ s MoSe ₂ , WSe ₂ ; MoTe ₂ , WTe ₂ . According to one embodiment, the two-dimensional semiconductor has a band gap of between 0.8 eV and 3 eV.

According to one embodiment, the difference between the fermi level and the valence band of the two-dimensional semiconductor is less than 20% of the value of the forbidden band (E _G ) of said two-dimensional semiconductor. Preferably, the two-dimensional semiconductor is p-doped. .

According to another aspect, the invention relates to a scanning microscope comprising an optically controlled electron source according to the invention, the tip being obtained from a wire whose one end is cut in the form of a peak or consisting of a raw carbon nanotube on a tip.

According to another aspect the invention relates to an X-ray source comprising an electronically controlled electron source according to the invention, the source comprising a plurality of points arranged in a network on a substrate.

According to yet another aspect the invention relates to a method of manufacturing an optically controlled electron source comprising the steps of:

at least one point made of conductive material,

-recouvrir the surface of the apex and its vicinity of at least one monolayer of a two-dimensional semiconductor, a monolayer comprising between one and five atomic planes.

The invention also relates to a method for generating electrons with low energy dispersion from an electronically controlled electron source comprising at least one tip of conductive material configured to be able to emit electrons by emission of field and whose surface of the apex and its vicinity is covered with at least one monolayer of a two-dimensional semiconductor, a monolayer comprising between one and five atomic planes, and comprising the steps of: -position the at least one tip in a vacuum chamber and apply an electric field so that said tip is configured to be at the limit of a generation of electrons by field emission,

illuminating the apex of the tip by an optical wave such that the elementary energy of a photon or the total energy (Etph) consisting of the sum of the elementary energies of a determined number (n) of photons is substantially equal at the value of a forbidden band of the two-dimensional semiconductor, so that an electron of the valence band of the two-dimensional semiconductor passes into the conduction band by absorption of said photon or said determined number of photons,

a fraction of the electrons of the conduction band then being emitted by said tip by photo-assisted field emission.

The invention also relates to a method for generating electrons with low energy dispersion from an optically controlled electron source comprising at least one tip of conductive material configured to be able to emit electrons by emission of electrons. field and whose surface of the apex and its vicinity is covered by at least one monolayer of a two-dimensional semiconductor, a monolayer comprising between one and five atomic planes, and comprising the steps of:

-position the at least one tip in a vacuum chamber and apply an electric field so that said tip is configured to be at the limit of a generation of electrons by field emission,

illuminating the apex of the tip by an optical wave such that the elementary energy of a photon or the total energy (Etph) consisting of the sum of the elementary energies of a determined number (n) of photons is substantially equal the difference in energy between the conduction band of the two-dimensional semiconductor and the fermi level of the conductor, so that an electron of the metal passes into the conduction band by absorption of said photon or said determined number of photons,

Preferably the electric field at the apex of the tip is less than 2V / nm. Preferentially, the optical wave which illuminates the apex creates an electric field in the vicinity of the tip of less than 2 V / nm.

More preferably, the method according to the invention further comprises a step of cooling the at least one tip.

Other features, objects and advantages of the present invention will appear on reading the detailed description which follows and with reference to the appended drawings given by way of non-limiting examples and in which:

Figure 1 describes the principle of electron emission by tunnel effect, applied to a transmission electron microscope.

Figure 2 describes the principle of field emission applied to a strong flux X-ray source (based on an electron source consisting of a set of spikes).

Figure 3 schematizes the principle of field emission. Figure 3a shows the probability of occupation of a state of energy by an electron in the metal M of the tip. Figure 3b illustrates the intensity of electrons emitted as a function of their respective energies.

Figure 4 illustrates the implementation and characterization of the optically controlled field emission.

FIG. 5 illustrates a type of electron emission in the so-called weak electric field, by photoemission (first mechanism) and / or assisted photo field emission (second mechanism).

Figure 6 illustrates the energy distribution of electrons emitted by photoemission.

Figure 7 illustrates the energy distribution of electrons emitted by photoassisted field emission.

FIG. 8 illustrates another type of electron emission under strong electric field and strong optical field, called optical emission regime.

Figure 9 describes the energy distribution of emitted electrons in the optical emission regime.

Figure 10 illustrates an optically controlled electron source according to the invention.

Figure 11a illustrates a monolayer of a transition metal dichalcogenide. FIG. 11b illustrates a two-dimensional semiconductor structure consisting of 3 atomic planes.

FIG. 12 schematizes the emission of electrons by a point of a source according to a first variant of the invention.

FIG. 13 schematizes the emission of electron by a point of a source according to a second variant of the invention.

Figure 14 illustrates the electron emission by a tip of an electron source according to the invention made from a metal tip.

FIG. 15 illustrates the emission of electron by a tip of an electron source according to the invention made from a carbon nanotube tip.

DETAILED DESCRIPTION OF THE INVENTION

An electronically controlled electron source 10 according to the invention is illustrated in FIG.

The source 10 comprises at least one tip P made of a conductive material whose surface of the apex and the surface in the vicinity of the apex is covered with at least one monolayer ("monolayer" in English) L of a semiconductor two-dimensional, semi-2D A monolayer L of two-dimensional semiconductor comprises one to five atomic planes AL, the number of planes being a function of its atomic structure, that is to say of the elementary cell of the two-dimensional semiconductor.

The term "optically controlled electron source" means an electron source comprising at least one emitting tip, this point being subjected to an electric field E0 and illuminated by a focused laser beam (see FIGS. 1, 2 and 4).

At the tip P two variants are possible.

According to a first variant, the conductive material is a metal Me. Preferably, the metal Me of the tip P is Tungsten W or Molybdenum Mo because of their very good electrical and thermal properties, as explained above. Preferably the radius of curvature of the tip is between 20 nm and 150 nm. According to a second variant, the material is graphitic, the tip P has a tubular shape and is a CNT carbon nanotube mono or multi-wall.

2D semiconductors (semi2D) have been developed recently. They have a planar structure and are composed of one to a few monolayers L, each monolayer comprising some atomic planes, the number of planes being determined by the structure of the elementary cell of the 2D semiconductor. The chemical bonds within a monolayer are covalent.

A well-known example of this type of material are the transition metal dichalcogenides called DCMT (Metal Transition Dichalcogenides or TMDCs in English) of general chemical formula MX ₂ with M metal and chalcogenous X such as S, Te or Se, having a structure 2D.

To date, there are about 40 known distinct MX ₂ lamellar compounds which crystallize in monolayers.

FIG. 11a illustrates a monolayer L of a DMCT composed of 3 atomic planes, a plane of metal atoms M (clear) sandwiched between two atomic planes X (dark).

FIG. 11b illustrates a stack of three monolayers L of a semi-2D material of DMCT type MS ₂ (S suffers). For multi-layer semi2D, monolayers stack up and are bonded by van der Waals forces.

The electronic properties of DCMTs depend on their chemical formula, they can be metallic or semiconductors.

Examples of semiconductor DCMTs applicable to the invention are: MOS ₂ ; WS ₂ ; MoSe ₂ , WSe ₂ ; MoTe ₂ , WTe ₂ .

It is known that the properties of DCMTs vary according to the number of monolayers (value and structure, direct or indirect, band gap for example). For example, the value of the band gap increases as the number of monolayers decreases.

In general, several families of 2D materials have been identified and studied and apply to the invention such as:

Elemental Semiconductors: Tellurene, Black Phosphorus, and Selenine, Monochalcogenides: MX; M = Cu, Ga, Ge, In, Sb, Sn, T1, ...; X = S, Se, and Te Dichalcogenides: MX ₂ ; M = Co, Mo, Pt, Re, Ti, Ta, Hf, Sn, W, Zr, ...; X = S, Se, and Te

Trichalcogenides: MX ₃ ; M = Nb, Ti, Ta, Zr; X = S, Se, ... and Te

2D Phosphides: BPs, FePS3, FePSe3, MnPSe3, GeP, ...

2D Arsenides: As2S3, As2Se3, As2Te3, GeAs, ...

2D Oxides: Mn02 and Mn03, ...

2D lodides: Pbl2, Cdl2, SbSI, ...

In what follows certain aspects of the invention are described with DCMT materials, but the invention is also applicable to any 2D material of which examples are given above.

The number of monolayers deposited on the apex of the tip is typically between 1 and 5.

Two-dimensional semiconductor materials (semi2D) have the particularity of having a stable surface. These 2D materials are different from conventional solid 3D materials for which, on the surface, the atoms have unsatisfied chemical bonds, called pendent bonds. Conventionally, the properties of the surface of the 2D materials are different from the properties of the bulk materials. For the 2D semiconductors deposited on a plane surface, all the atoms of a monolayer are connected to each other by covalent bonds, there are no bonds hanging on its surface.

The emission of electrons by the tip P according to the invention is carried out by a photo-assisted field emission, as described below.

The combination metal tip (Tungsten, Molybdenum, ..) or carbon nanotube type with an ultra thin 2D semiconductor (1 to a few monolayers) makes it possible to benefit, on the one hand, from the very good electrical and thermal conductivity of the metal tip and, secondly, to optically control the emission of electrons from the conduction band BC of the 2D semiconductor, according to a mechanism explained below.

As explained above, the surface of a 2D semiconductor does not have dangling bonds. This property makes it possible to obtain a very good structural stability of the apex of the tip P which is subjected to a strong field electrical and heating during the emission of electrons. In these extreme conditions, the monolayer (s) of 2D material provide (s) the chemical stability of the tip P.

For the same reasons, after vacuum desorbing the adsorbates, a 2D semiconductor has no or very few surface states. The surface curvature of the emitter is then very low, which reduces the energy dispersion of the emitted electrons.

In addition, the quantum efficiency of a 2D semiconductor is relatively high (a few% to 10% for a monolayer), the optical absorption can be improved by plasmonic effects at the top of the tip.

According to one option, the metal Me of the tip and the metal M of the DCMT are identical, for example Me = W and semi2D = WS ₂ . The 2D monolayer is typically obtained by sulfurization of the tungsten tip.

According to another option they are different, for example a peak in W and the semiconductor 2D in MoSe ₂ . The monolayer of semi2D is then deposited on the surface of the tip independently.

Preferentially, the semi-semiconductor Semi2D has a band gap E _G of between 0.8 eV and 3 eV. For example, the WS ₂ and MOS ₂ have a gap of approximately 2eV.

Given the known manufacturing processes, the semi2D monolayer on the tip P covers a large part of it, but only the part deposited in the vicinity of the apex is useful for the emission of electrons.

The electron emission mechanism of the tip P of the source according to a first variant of the invention is described in FIG. 12, for the case of a mono photon absorption.

One places oneself in a situation in which the applied field E0 is less than or equal to the electric field allowing the emission by field effect "pure" of a weak current of electrons (typically <1 pA) from the band of BV valence of semi2D semiconductor.

We focus on the tip a pulsed F _L laser beam energy equal to or slightly greater than the gap of the semiconductor (case of mono absorption photon). Electrons of the valence band BV then populate the bottom of the conduction band BC which was empty. Since the potential barrier at the bottom of the conduction band is of small thickness, the photoexcited electrons passed from the valence band BV to the conduction band BC then have a high probability of being emitted by tunnel effect. . The electron current emitted is proportional to the light power absorbed at the top of the tip. The optical field induced near the tip is "weak" is less than 3V / nm. At equilibrium, as electrons move from the valence band BV to the conduction band BC and are emitted, electrons of the metal are injected into the valence band of the 2D semiconductor.

Figure 12 illustrates in a nonlimiting manner the case in which the electrons pass from BV to BC by a single-photon absorption, but a transition from BV to BC by multiphoton absorption is also possible.

We call Eph the elementary energy of the photon of F _L.

For the case of a single-photon absorption, the elementary energy of the photon Eph (1 PE) of FL is greater than the gap E _G , but must not be too much greater than the gap E _G to obtain a peak of energy (electrons emitted) very fine.

Eph (1 EP)> E _G

For the case of a two-photon absorption, the elementary energy of the photon Eph (2PE) of FL is greater than half of the gap E _G , but must not be too much greater than this half gap E _G / 2 to obtain a peak of energy (emitted electrons) very fine.

Eph (2PE)> E _g / 2

By generalizing to the case of an absorption at n photons:

Eph (nPE)> E _G / n

The choice of the number of monolayers makes it possible to adjust the forbidden band of the DCMT and thus to optimize the absorption as a function of the wavelength of the laser used to illuminate the tip. In order to cover all the absorption cases (mono and multiphotons), total energy of the Etph photons is called the sum of the n elementary energies Eph of the photons making it possible to reach the n-photon absorption conduction band.

Absorption at 1 photon: Etph = Eph

2-photon absorption: Etph = 2. Eph

Absorption at n photons: Etph = n.Eph

The difference in energy between the total energy of the photons Et _ph and the gap E _G of the Semi2D makes it possible to fix the energy distribution of the electrons emitted as well as the number of electrons emitted. Indeed the number of accessible states in the conduction band depends on (Et _ph - E _G ). To obtain a sufficiently fine peak of energy, typically:

Etph - E _G <200 meV.

The electronic affinity E _G for MoS ₂ and WS ₂ is between 3.9 and 4.5 eV.

The transmission mechanism according to a second variant of the invention is described in FIG. 13, for the case of a mono photon absorption. Here the energy of the photons Eph delivered by the laser is equal to or slightly greater than the energy difference D between the conduction band of the semi2D and the Fermi level of the conductive material E _F (Cond).

D = BC - E _F (Cond) and Eph> D

Here also Figure 13 illustrates in a non-limiting manner the case of a single-photon absorption, but a multi-photon absorption is covered by the invention.

The condition is expressed as a function of total energy:

Etph> A

In this case, the total energy Etph consisting of the sum of the elementary energies of a determined number n of photons is substantially equal to

The electrons of the metal go directly into the conduction band. This mechanism is called internal photoemission. It has the advantage of emitting electrons for a lower photon energy than for the first one. variant, which makes it possible to use a laser having a longer wavelength.

To obtain a peak of sufficiently fine energy, preferably the energy of the photon Eph is not too much greater than the difference in energy between the BC and the Fermi level of the conductive material. The excess energy is low, preferentially less than 200 meV:

Eph - D <200 meV

Preferably, for both variants, the polarization of the laser is controlled so as to better control the emitting sites.

For both variants, to obtain a low dark current, it is preferable that the semi2D BC conduction band is sparsely populated, ie its fermi E _F (semi2D) level is close to BV , such as:

E _F (semi2D) - BV <20% E _G.

For example, for a 2eV gap, the Fermi level is close to the BV at less than 0.4 eV.

To obtain this localization of the fermi level close to the valence band, advantageously Sem2D semiconductor is p-doped.

DCMTs such as MoS2 and WS2 are generally p-doped, presumably because of sulfur vacancies that are electron donor type. MoS2 can be p-doped, for example, by substitution of molybdenum atoms by niobium atoms with a hole concentration of ~ 3 c ¹⁹ cm-3.

With this configuration we obtain a tip that emits no or very few electrons when it is not lit, and then emits electrons with a low energy dispersion under illumination. The 2D semiconductor with its gap acts as an optically controlled switch, allowing only the emission of photo-assisted field and making difficult remission of "pure" field.

On the contrary, for an n-doped semiconductor, the Fermi level is close to the BC, and it is difficult in this case to have only a photoassisted emission and not (or very little) pure field emission. In the case of a single-photon absorption, the overall emission efficiency is much greater than that of photoemission or photo-assisted field emission mechanisms that rely on multi-photon absorption.

According to another aspect, the invention relates to an electron microscope comprising an electron source 100 according to the invention. According to a variant, the tip P is obtained from a metal wire FM, one end of which is cut into the shape of a point P, as illustrated in FIG. 14. According to another variant, the tip P consists of a carbon nanotube CNT that is grown on a metal tip, as shown in Figure 15.

For example, the source of FIGS. 14 or 15 photoassisted with F _L laser pulses can be used in a transmission electron microscope for the analysis of the crystal structure of proteins or protein-based submicron crystals. The use of laser pulses and thus the emission of electron packets allows the analysis of such structures by minimizing the deleterious thermal effects.

According to another aspect, the invention relates to an X-ray source comprising an optically controlled electron source 100, the source comprising one or a plurality of points P arrayed on a substrate, which can be used for imaging or time-resolved fluorescence (pulsed RX source, source 100 illuminated by a pulsed laser). The use of multiple pulsed RX sources, which is activated one by one, allows images to be obtained from different angles and to reconstruct a 3D RX image (tomosynthesis).

In another aspect the invention relates to a method of manufacturing an optically controlled electron source comprising the steps of:

-making at least one tip P made of conductive material Cond, so that it is able to emit electrons by field emission, -recover the surface at least in the vicinity of the apex of at least one monolayer of a semidimensional semiconductor semiconductor, this monolayer comprising between 1 and 5 atomic planes. Thus the manufacture of the electron source 100 according to the invention comprises two main stages, the first consisting in manufacturing the metal point or nanotubes or carbon nanotubes, the second consisting in depositing on this or these points one or several semiconductor monolayers. 2D semi.

The manufacture of individual metal tips, can be performed by electrochemical etching of a wire (tungsten, molybdenum, ...) typically of diameter 0.1 to 0.5 mm in a chemical liquid (eg soda). For tungsten, this technique is the same as that used for the manufacture of STM tips. During etching, an oxide layer can be created at the tip. It can then be removed by vacuum annealing at 850 ° C.

To manufacture a network of metal tips, the method proposed by Spindt et al (Spindt et al., J. Appl Phys 47, 5248 (1976)), or any other method known to those skilled in the art, can be used. art.

For the production of carbon nanotubes, it is possible to use the growth of these nanotubes by plasma-assisted chemical vapor deposition (Teo et al., Appl Phys Lett 79, 1534 (2001)).

For the deposition of the 2D semiconductor on the surface of metallic tips or nanotubes, a first method is the chalcogenation of metal tips in a sealed ampoule. For this purpose, the metal tips (Mo, W, Zr, Hf, Pt, Nb, Ta, etc.) and the chalcogen (sulfur, selenium or tellurium) are placed in a vacuum sealed ampoule. The ampoule is placed in an oven so as to evaporate the chalcogen. It reacts with the tips to form a transition metal dichalcogenide (DCMT). A DCMT is chosen and also the thickness of the DCMT so as to obtain a 2D semiconductor material. Preferably an ampoule is used in a 2-zone furnace, a first part of the ampoule comprising the chalcogen and being in the first zone heated to 100-200 ° C, a second part of the bulb comprising the tips and is found in the second zone heated from 700 to 1000 ° C. A second method involves the chemical vapor deposition (CVD) of transition metal dichalcogenides (DCMTs).

The deposition of DCMT can be obtained by gas phase transport techniques, close to chemical vapor deposition (CVD). The main approaches are 3 in number:

(a) The "chalcogenation" of a metal deposit or metal oxide: this approach, the simplest, consists in the sulfurization (or selenization) of metal tips possibly oxidized on the surface (for example M0O ₃ / M0O ₂ ). Annealing temperatures of the order of 1000 ° C. in a sulphurised atmosphere are necessary to complete the reduction of MoO ₂ (initiated at 500 ° C.) and to displace the oxygen.

(b) The use of precursors (usually in the form of powder) which are vaporized and transported by a neutral gas in a cooler zone of the furnace where they condense in the MX ₂ form. For example, for MoS ₂ , M0O ₃ and S present in each of the crucibles are vaporized. The M0O ₃ decomposes partially during the evaporation and the radicals thus formed (MOO _3-x ) then react with S on the substrate to complete the decomposition and generate MoS ₂ .

(c) The high temperature vaporization of massive DCMT, followed by its condensation in an area of the oven where the temperature is lower ("vapor-solid growth"). For example MoS ₂ (in the form of powder) can be directly evaporated at ~ 900 ° C under low pressure (20 Torr) and be condensed on the tips downstream of the tube in an area where the temperature is maintained at ~ 650 ° C.

For the approaches (b) and (c), the growth tends to progress laterally, because the surface of the DCMT does not comprise dangling bonds likely to fix the atoms of M or X. On the other hand, the edges of march of the clusters 2D have unsatisfied bonds, which effectively trap deposited atoms, thereby promoting 2D lateral growth. It should be noted that other synthetic methods can be used, such as the decomposition of sulfur salts such as ammonium thiomolybdate, (NH ₄ ) ₂ MOS ₄ , which has the particularity of containing the two elements of the compound MX ₂ . In order to obtain the good stoichiometry, a second heat treatment (~ 800 ° C.) under a sulphurized atmosphere is necessary, after a first at 450 ° C. during which the thiomolybdate is decomposed.

A third method consists of CVD growth from metallo-organic precursors (MOCVD).

"Chalcogenation" of the metal tips is carried out using a metallo-organic precursor, for example (C ₂ H ₅ ) ₂ X. X being sulfur, selenium or tellurium.

Deposition can also be carried out by supplying the two metallo-organic precursors, for example Mo (CO) ₆ for molybdenum and W (CO) ₆ for tungsten with (C ₂ H ₅ ) ₂ X (X being sulfur, selenium or tellurium).

A fourth method is to use ALD ("atomic layer deposition") synthesis techniques, which conceptually lend themselves to deposition of materials of very low thickness. ALD is a deposition technique developed in the 1970s and resembles the CVD in that it also uses gaseous sources. However, the operating principle is different, and an ALD growth cycle (for example for a binary material consisting of 2 elements) consists of 4 steps, namely:

(i) injection of the precursor of the element 1,

(ii) purging the reactor with a neutral gas, in order to evacuate the excess of precursor 1 as well as the reaction products,

(iii) injection of the precursor of element 2, followed by

(iv) a second purge of the reactor.

It is therefore necessary for the precursors to be adsorbed on the surface and for the adsorption to be self-limited to a single monolayer of precursor of element 1 (i) and then element 2 (iii). This monolayer is generally decomposed thermally (pyrolyzed) so as to deliver the element that one seeks to deposit. The excess precursor and the decomposition products are removed during the purge phases of the reactor. The ALD deposition of MoS ₂ can use as precursor MoCI ₅ and H ₂ S.

According to another aspect, the invention relates to a method for generating electrons with low energy dispersion, from an optically controlled electron source comprising at least one tip P of conductive material, configured to be able to emit electrons by field emission and whose surface in the vicinity of the apex is covered with at least one monolayer of a semidimensional semiconductor, this monolayer comprising between one to five atomic planes.

The method has a first step of positioning at least one tip in a vacuum chamber and applying an electric field E0 so that said tip is configured to be at the boundary of an electron generation by field emission.

According to a first variant, the method comprises a second step of illuminating the apex of the tip with an optical wave F _L such that the elementary energy Eph of a photon or the total energy Etph consisting of the sum of the elementary energies d a determined number n of photons (Etph = n.Eph) is substantially equal to the value of a forbidden band E _G of the semiconductor 2D, so that an electron of the valence band BV of the semiconductor 2D passes in the band BC conduction by absorption of said photon or said determined number of photons. A fraction of the electrons of the conduction band BC is then emitted by said tip P by photo-assisted field emission.

According to a second variant, the method comprises a second step of illuminating the apex of the tip with an optical wave F _L such that the elementary energy of a photon Eph or the total energy Etph consisting of the sum of the elementary energies of a determined number of photons (n) is substantially equal to the energy difference D between the conduction band BC of the semi2D and the Fermi level of the conducting material E _F (Cond), so that an electron of the metal passes in the band of conduction BC of semi2D by absorption of said photon or said determined number of photons. A fraction of the electrons of the conduction band BC is then emitted by said tip P by emission of photo-assisted field.

Advantageously, the applied electric field E0 is less than 2V / nm. Advantageously, the optical wave which illuminates the apex creates an electric field in the vicinity of the tip less than 2 V / nm.

Advantageously, the electron generation method further comprises a step of cooling the at least one tip, to further reduce the energy dispersion of the emitted electrons.

In electron microscopy and for electron interference experiments, brightness B and beam degeneracy are important parameters. Degeneracy d can be defined as the average number of electrons in the same state of the phase space. Because of the Pauli exclusion principle, d is necessarily less than or equal to 1. For a continuous field emission source, it is possible to show that the degeneracy is proportional to the transparency of the tunnel barrier (Kodama et al. Physics Rev. A 57 2781 (1998), Eq.1 1) and that the brightness of the source is proportional to the product of the degeneracy by the energy dispersion (Kodama et al., Phys Rev A 57 2781 (1998), Eq.8). The best value of degeneracy in the literature for a field emission source is 10 ⁴ .

The present invention proposes that the electron emission is done by tunneling assisted by the absorption of the energy of 1 or more photons. The emission of electrons is therefore done for energies where the tunnel barrier is thinner, that is to say where the probability of emission is higher, which leads to an exponential increase in degeneracy.

The combination of this strong degeneracy, the high efficiency of photon-electron conversion and low energy dispersion leads to a higher brightness electron source.

Claims

An optically controlled electron source (100) comprising at least one tip (P) made of a conductive material (Cond), the apex surface of which is surrounded by at least one monolayer (L) of a two-dimensional semiconductor (semi2D), a monolayer (L) comprising between one and five atomic planes (AL).

Electron source according to claim 1, in which the conductive material is a metal (Me) chosen from tungsten (W) and molybdenum (Mo) and in which the peak radius of curvature is between 20 nm and 150 nm. nm.

3. Electron source according to claim 1 wherein the conductive material is graphitic, the tip (P) comprising a carbon nanotube (CNT).

The electron source according to claim 1, wherein the number of monolayers of the two-dimensional semiconductor is between 1 and 5.

5. Electron source according to one of the preceding claims wherein the two-dimensional semiconductor (semi2D) is a transition metal dichalcogenide (DCMT), of chemical formula MX ₂ , with M metal and X chalcogen, comprising three plan atomic.

The electron source of claim 5 wherein the transition metal dichalcogenide (DCMT) is selected from: MoS ₂ ; WS ₂ s MoSe ₂ , WSe ₂ ; MoTe ₂ , WTe ₂ .

7. Electron source according to one of the preceding claims wherein the two-dimensional semiconductor (Semi2D) has a band gap (E _G ) between 0.8 eV and 3 eV

8. electron source according to one of the preceding claims wherein the difference (D) between the fermi level (E _F (semi2D)) and the valence band (BV) of the two-dimensional semiconductor (Semi2D) is lower than at 20% of the value of the band gap (E _G ) of said two-dimensional semiconductor

9. Electron source according to one of the preceding claims wherein the two-dimensional semiconductor (semi2D) is p-doped.

10. A scanning microscope comprising an optically controlled electron source according to one of claims 1 to 9, said tip (P) being obtained from a wire (FM) having one end cut into a peak shape. or consisting of a carbon nanotube (CNT) raw on a tip.

An X-ray source comprising an optically controlled electron source according to one of claims 1 to 9, the source comprising a plurality of peaks arrayed on a substrate.

A method of manufacturing an optically controlled electron source comprising the steps of:

-making at least one tip (P) made of conducting material (Cond), -recovering the surface of the apex and its vicinity of at least one monolayer of a two-dimensional semiconductor (semi2D), a monolayer comprising between one and five atomic planes.

A method for generating electrons with low energy dispersion from an optically controlled electron source comprising at least one tip (P) of conductive material configured to be able to emit electrons by field emission and whose surface of the apex and its vicinity is covered with at least one monolayer of a two-dimensional semiconductor (semi2D), a monolayer comprising between one and five atomic planes, and comprising the steps of:

-positioning the at least one tip (P) in a vacuum chamber and applying an electric field (E0) so that said tip is configured to be at the limit of a generation of electrons by field emission, illuminating the apex of the tip by an optical wave (F _L ) such as the elementary energy (Eph) of a photon or the total energy (Etph) consisting of the sum of the elementary energies of a number (n ) determined from photons is substantially equal to the value of a forbidden band of the two-dimensional semiconductor (E _G ), so that an electron of the valence band (BV) of the two-dimensional semiconductor passes in the conduction band (BC) by absorption of said photon or said determined number of photons,

A method of generating electrons with low energy dispersion from an optically controlled electron source comprising at least one tip (P) of conductive material configured to be able to emit electrons by field emission and whose surface of the apex and its vicinity is covered with at least one monolayer of a two-dimensional semiconductor (semi2D), a monolayer comprising between one and five atomic planes, and comprising the steps of:

-positioning the at least one tip (P) in a vacuum chamber and applying an electric field (E0) so that said tip is configured to be at the limit of a generation of electrons by field emission, illuminate the apex of the tip with an optical wave (F _L ) such as the elementary energy of a photon (Eph) or the total energy (Etph) consisting of the sum of the elementary energies of a number (n ) determined from photons is substantially equal to the energy difference (D) between the two-dimensional semiconductor conduction band (Semi2D) and the driver's fermi level (E _F (Cond)), so that an electron of the metal passes into the conduction band (BC) by absorption of said photon or said determined number of photons,

15. The method of claim 13 or 14 wherein the electric field at the apex of the tip (E0) is less than 2V / nm.

16. The method of claim 13 or 15 wherein the optical wave which illuminates the apex creates an electric field in the vicinity of the tip less than 2 V / nm.

17. Method according to one of claims 12 to 16 further comprising a step of cooling the at least one tip.