WO2018116048A1

WO2018116048A1 - Method for integrating two-dimensional materials on a nanostructured substrate, suspended thin film of two-dimensional materials and uses thereof

Info

Publication number: WO2018116048A1
Application number: PCT/IB2017/057764
Authority: WO
Inventors: Gilles Lerondel; Hyun Jeong; Anisha Gokarna
Original assignee: Universite De Technologie De Troyes
Priority date: 2016-12-09
Filing date: 2017-12-09
Publication date: 2018-06-28
Also published as: EP3551788A1; FR3060023A1

Abstract

A method of integrating two-dimensional materials onto a nanostructured substrate, characterised in that it comprises the following steps: manufacturing the two-dimensional materials by means of a vapour deposition or an exfoliation method; and transferring the two-dimensional materials obtained in the previous step onto a synthesised nanostructured substrate that is selected such that a contact surface between the two-dimensional materials and the nanostructured substrate is minimised. The invention further relates to fully suspended thin films obtained by the above method, and to the use of the suspended thin films in various fields such as electronics, optoelectronics, photovoltaics, catalysis, ultrasensitive surfaces, integrated circuits etc.

Description

METHOD FOR INTEGRATING 2D MATERIALS ON A NANOSTRUCTURE SUBSTRATE, THIN FILM SUSPENDED FROM 2D MATERIALS AND

ASSOCIATED USES

TECHNICAL FIELD OF THE INVENTION

The invention relates to the field of integration of 2D materials. More particularly, the invention relates to a method for integrating 2D materials onto a nanostructured substrate to obtain a monolayer of 2D materials entirely suspended. The invention also relates to fully suspended films and the use of said suspended films in different technologies.

[0002] A two-dimensional material, sometimes called a monolayer material or 2D material, is a material consisting of a single (or some) layer of atoms or molecules. Due to their unusual characteristics and for potential use in applications such as semiconductors, photovoltaics, ...

STATE OF THE PRIOR ART

The 2D materials of atomic thickness have unique properties (absorption, electrical conduction and thermal) and allow to consider a set of ultrafine devices, ultralight flexible etc .. however the thickness of these materials give them a very large sensitivity to the environment. The simple fact of depositing these materials on a substrate modifies their intrinsic properties (ex: exchange of charges). This is related to the contact surface that makes each atom of the 2D material in contact with the substrate. [0004] In recent years, the new two-dimensional materials: dichalcogenides of transition metals are the subject of important studies. The spectacular progress in controlling the electronic properties of graphene has indeed powerfully stimulated the search for new two-dimensional (2D) materials. Monolayers of transition metal Dichalcogenides such as M0S2 (and its cousins MoSe2, WS2, WSe2 ...) have appeared very recently as very promising nanostructures for various applications in both the field of optics and optical fiber. 'electronic. In parallel, knowledge about Boron Nitride and the Van der Waals Heterostructures, constituting the stacking of different 2D materials, are progressing very rapidly.

[0005] Document "Nanoscale Integration of Two-Dimensional Materials by Lateral Heteroepitaxy, Nano Lett., 2014 .." teaches the ascending production of 2D nanostructures. The integration of materials into heterostructures with new properties different from those of constituents. The results are only obtained on a suitable substrate. This paper gives a method for building nano and hetero structures from a wide range of 2D materials. [0006] Concerning the exaltation of optical properties of 2D materials after integration on photonic structures, the document "Two-dimensional material nanophotonics. Nat Photon 2014, 8, 899-907 "describes two approaches to improve the interactions of two-dimensional materials with light: - by their integration with external photonic structures and - by intrinsic polaritonic resonances. Black phosphorus has been presented as a narrow-banded laminate material, which punctually links the energy gap between zero-band graphene and wide bandgap transition metal dichalcogenides.

[0007] Document "Parallel Stitching of 2D Materials. Adv Mater 2016, 28, 2322-2329 "describes the large scale integration of 2D materials by selective growth, in which, various parallel stretching 2D heterostructures, including metal-semiconductor, semiconductor-semiconductor and the semiconductor insulator, are synthesized directly by selective growth. The methodology allows the large scale manufacture of lateral heterostructures. With regard to the integration of 2D materials on lithographed substrates at the microscopic scale: In this case, relatively small suspended areas are obtained, but the whole of the layer is not suspended. "Local Strain Engineering in Atomically Thin MoS2, Nano Lett., 2013, 13 (1 1), pp 5361-5366" as well as "Exciton Dynamics in Suspended Monolayer and Few-Layer MoS2 2D Crystals, ACS Nano, 2013, 7 (2), pp 1072-1080.

[0009] No documents or references on the possibility of obtaining fully suspended films are taught. None of the documents gives a method of transferring 2D materials to nanostructures so that the intrinsic characteristics of said 2D materials are not modified.

SUMMARY OF THE INVENTION

The object of the present invention is to obtain a monolayer of 2D material completely suspended, without stress, that is to say the layer does not interact with the substrate. The present invention provides a solution to this problem by minimizing the contact areas of the 2D materials by transferring them to a nanostructured substrate.

Therefore, and in this context, the present invention relates to a method of integrating two-dimensional materials on a nanostructured substrate characterized in that it comprises the following steps:

A) manufacturing two-dimensional materials by a vapor deposition method or by an exfoliation method;

B) transferring said two-dimensional materials, obtained in the preceding step, onto a synthesized nanostructured substrate, said nanostructured substrate is chosen so that a contact surface between said two-dimensional materials and said nanostructured substrate is minimized.

According to particular features, the manufacture of two-dimensional materials by the vapor deposition method consists of:

A1) growing two-dimensional materials by chemical vapor deposition on an SiO 2 substrate;

A2) depositing a layer of PMMA on said two-dimensional materials obtained in the previous step;

A3) etching the SI02 substrate in a dilute solution of hydrofluoric acid HF; and in that

the transfer of said PMMA-coated materials obtained in a previous step onto a nanostructured substrate further comprises the following steps:

B1) removal of PMMA with acetone solution;

B2) drying said two-dimensional materials to remove residual PMMA layers; or

B2 ') sublimation of said two-dimensional materials to remove residual PMMA layers. Advantageously, the use of the sublimation technique, to eliminate the residual layers of PMMA, avoids the mechanical stresses associated with drying.

According to a variant of the invention, the two-dimensional materials are manufactured by the exfoliation method in an inert environment and the transfer of said two-dimensional materials obtained by exfoliation on a synthesized nanostructured substrate consists of a single step of depositing said two-dimensional materials. on said synthesized nanostructured substrate.

According to a variant of the invention, the nanostructured substrate is ZnO nanowires, zinc oxide, said nanostructured substrate is synthesized by a liquid phase chemical deposition method, CBD, on an SiO 2 substrate or by all other growth technologies of ZnO nanowires.

According to particular characteristics, the synthesized ZnO nanowires are disordered and of variable sizes so as to minimize the contact area with the two-dimensional materials obtained.

[0017] Preferably, the ZnO nanowires have a diameter of less than 100 nm.

According to one variant of the invention, the two-dimensional materials are either molybdenum sulphide MoS 2, or tungsten sulphide WS 2 or diselenide Tungsten WSe 2.

According to particular features, two-dimensional materials are any two-dimensional rigid materials.

The invention also relates to a suspended thin film obtained by the transfer of the 2D materials onto a ZnO nanowire substrate according to the above method. ZnO nanowires are disordered and of varying sizes.

According to particular features, the thin film suspended obtained by the above method is characterized in that the two-dimensional materials are either molybdenum sulphide MoS2, or tungsten sulphide WS2 or diselenide tungsten WSe2. In addition, said two-dimensional materials are Rigid and are obtained by chemical vapor deposition on SiO 2 substrate or by exfoliation deposit on an SiO 2 substrate.

The invention also relates to the use of thin films suspended from 2D materials in the fields of electronics and / or optoelectronics and / or thermal and / or photonic.

The invention also relates to the use of thin films suspended from 2D materials in catalysis domains and / or in ultrasensitive surfaces.

BRIEF DESCRIPTION OF THE FIGURES

Other features, details and advantages of the invention will become apparent on reading the description which follows, with reference to the appended figures, which illustrate: FIG. 1 illustrates the concept and manufacture of 2D materials suspended according to the method object of the present invention;

FIG. 2 illustrates the exalted optical properties and associated band structures;

FIG. 3 illustrates the image obtained by SEM of the suspended integrated layers;

Figures 4A and 4B illustrate the concept of active substrate. For clarity, identical or similar elements are identified by identical reference signs throughout the figures.

DETAILED DESCRIPTION OF AN EMBODIMENT

The two-dimensional materials (2D) are atomically thin semiconductors made of transition metals m- (Mo, W, Sn, etc.) covalently bound to chalcogen X- (S, Se, Te). . The monolayer group of two-dimensional materials with MX2 chemical formula. (M = Mo, W, X = S, se) are promising materials for the fabrication of ultrathin photodetectors, photovoltaic systems, etc ... However, the optical and crystalline properties of these 2D materials integrated on flat substrates are still not satisfactory for an application The simple fact of depositing these materials on a substrate modifies their intrinsic properties (ex: exchange of loads). This is related to the contact surface that makes each atom of the 2D material in contact with the substrate.

The present invention aims to circumvent this problem of modifying intrinsic characteristics of said 2D materials when they are transferred to a substrate, by proposing to minimize the contact surface using nanostructured substrates (carpet fakir). To do this, a method for integrating 2D materials onto a nanostuctured substrate is proposed. The first step of the process involves the development or growth of 2D materials by chemical vapor deposition on SiO 2 / Si substrates. The 2D materials can also be obtained by simple exfoliation and in this case simply deposited on a SiO 2 / Si substrate. The 2D materials are either molybdenum sulfide, MoS2, or tungsten sulfide, WS2 or tungsten diselenide, WSe2. The next step is to deposit a layer of PMMA on said two-dimensional raw materials in the previous step. The PMMA layer is used to support the single layer during chemical etching of SiO2. Then the SiO 2 substrate is etched in a hydrofluoric acid (HF) solution, diluted. The next step consists in transferring said 2D materials, covered with PMMA, onto nanowires of ZnO synthesized on a substrate (Si, SiO2, etc.). To remove PMMA, an acetone solution was used. Then the 2D materials were dried at 80 ° C to remove the residual layer of PMMA. Advantageously, the sublimation technique can be used to avoid the mechanical stresses associated with drying.

Note that the growth of ZnO nanowires on the silicon substrate was carried out by adopting a chemical deposition technique in liquid phase, CBD. First, 0.025 M zinc acetate was dissolved in 250 ml of water; Then 0.3 ml of ammonium hydroxide was added to the solution and stirred at room temperature. The synthesis was carried out in the absence of catalysts or metal additives. The mixture was heated to 87 ° C. The sample consisting of a ZnO layer previously deposited on a silicon substrate was immersed in the solution for 30 min. Then the sample was washed with water and air dried for 1 hour.

Note that to obtain nanowires, all other techniques of growth or structuring, top-down approach, can be used from the moment we obtain son diameters quite small. Figure 1 shows the three-dimensional schemas of the process of simplified transfer of two-dimensional materials on ZnO nanowires. Fig. 1a, 1b and 1c illustrate MoS2, WS2 and WSe2 on ZnO nanowires respectively. The two-dimensional materials were transferred to the ZnO nanowires by a method of vapor transfer. Fig.ld, fig. 1 (e) and fig. 1 f are the 3D enlarged diagrams of materials, MoS2, WS2 and WSe2 on the ZnO nanowires respectively. SEM images in fig. 1 g, Fig. 1 h and 1 i, show that said 2D materials are partially supported by nanowires. Fig.1 j shows the contact between 2D materials and nanowires. Note that the contact occurs only at the edge of the ZnO nanowires. Fig 1k shows the spectrum of the photoluminescence intensity of said 2D materials on ZnO nanowires at room temperature.

Raman analysis shows that the layer of 2D material is free of stress and is fully relaxed. This is due to the limited number of contact points, which is partly explained by the fact that the nanowires are not all exactly the same length and slightly disoriented with respect to the vertical axis (Figure 1).

The goal is to integrate the 2D material thin layer without changing its electronic structure. The emission factor observed in emission is explained by the direct nature of the electronic transition. The same 2D material deposited on a SiO 2 plane substrate becomes optically inefficient and this is explained by the indirect nature of the electronic transitions. We are talking here about the emission but the absorption is also modified.

The final material integrated on the ZnO nanowire substrate has preserved optical properties as shown in FIG. 2.

[0034] Figure 2 shows the optical properties and structures of the associated bands. To verify the optical properties, the photoluminescence intensities were measured at room temperature. The position of the PL signal (photoluminescence) is closely related to the nature of the forbidden band, ie, it reveals a direct or indirect transition. The PL spectra of MoS2, WS2 and WSe2 on the ZnO nanowires are shown in Figures 2a, 2b and 2c, respectively, together with the PL spectra of the same 2D materials deposited on them. SiO2 substrates. Said PL spectra of the 2D materials on the ZnO nanowires are represented by the gray color and on the SiO 2 substrate it is represented by the black color. The energy spectra of the MoS2, WS2 and WSe2 materials on the ZnO nanowires and SiO2 substrate are shown in Figures 2d, 2e and 2f. Figures 2g, 2h and 2i show the energy band diagrams showing the optical transitions of the 2D materials on the nanowires and on the SiO2 substrate. Solid and dotted lines indicate 2D material on both nanowires and SiO2, respectively. The maximum of the valence band for MoS2 and WS2 on the Nanowires is the minimum of the conduction band, which results in a higher PL intensity at an increase in the bandwidth when the layer is relaxed, as shown in FIG. 2g and 2h. From PL analysis of 2D materials on ZnO nanowires, we deduce that 2D materials on nanowires behave almost like a suspended film. It is deduced that the transfer of 2D materials on ZnO nanowires minimizes the contact surface. To further minimize the contact surface some disorder on the orientation and the size of the pads or wires is introduced. In this way, we obtain lines of contact points in very limited numbers whose effect on the properties of 2D materials can be neglected. The SEM analysis reveals that the contact is made at the edges of the nanowires (white lines in Figure 3).

As shown in Figure 3, the effect is observed regardless of the 2D material. Already demonstrated on a number of materials such as MoS2 the method proposed in the present invention can be extended to any type of materials provided that it is sufficiently rigid. Being a purely geometric effect of other substrates can also be considered.

This is called exaltation factor with respect to the same constrained material deposited on a thin layer of SiO2.

PL intensities of 2D materials can be affected by several factors such as doping and crystalline quality. In the present invention the influence of crystalline quality can be excluded because we have deposited identical 2D materials on SiO2 substrate and on ZnO nanowires. By against possible effect of doping 2D materials by contact with ZnO can not be ruled out because the work function of ZnO is greater than the electronic affinity of all 2D materials. Therefore, the photoexcited electrons of 2D materials could be transferred to ZnO at the point of contact. However, we have noticed that the PL intensities of 2D materials such as MoS2 and WS2 are not more than 3 times increased charge transfer. On the other hand, the intensity PL of WSe2 is decreased by photoexcited electron transfer. This means that the charge transfer occurring in 2D materials on ZnO nanowires has a minor effect. The following process can be imagined: Modification of the properties of the deposited thin film (see the analogy with doped semiconductors). The growth process makes it possible to obtain a pure ingot and dopants are then added to control the type and level of doping.

This leads to the concept of active substrate. Through the wires it is possible to inject or trap electrons and thus modify the doping of the 2D material layer. The nature of the dopant is controlled by the substrate; example of networks of nanowires of material type p or n or metallization of ZnO nanowires with a gold layer. The idea of the active substrate is illustrated in FIGS. 4A and 4B, the doping type being connected to the difference in energy level between the 2D material and that of the nanowires. FIG. 4A illustrates the doping n: electron injection of the nanowire to the 2D material. FIG. 4B illustrates the p-doping: electron transfer from the 2D material to the nanowire.

The solution proposed by the present invention is simply limited by the size of the 2D material layer. Indeed, the ZnO nanowire substrates can be obtained on centimeter or metric surfaces (chemical growth in solution).

Advantage of the combination of 2D material with ZnO nanowires is promising for integrated optoelectronics. Indeed, zinc oxide (ZnO) is a semiconductor of the direct forbidden band with a wide band interval of 3.2 eV.

It should be noted that the method which is the subject of the present invention can be extended to any sufficiently rigid 2D materials. The invention thus relates to a thin film suspended obtained by the transfer of 2D materials on Zno nanowires. The suspended thin film is obtained by the method object of the present invention described above. The 2D materials used for the transfer on the ZnO nanowires are of the molybdenum sulfide type, MoS2, or tungsten sulphide, WS2 or tungsten diselenide, WSe2. Said 2D materials are obtained by chemical vapor deposition on an SiO 2 substrate or simply exfoliated.

The development of this low-cost technology compatible with mass production of 2D materials (we now obtain centimeter surfaces see metrics) and on a very large scale opens many perspectives of applications in several areas such as (electronic , optoelectronics and photonics, thermal ...).

With the method of the present invention by minimizing contact areas by transferring the 2D materials on a nanostructured substrate, a monolayer of 2D material completely suspended (without constraint) is obtained. In other words, the layer does not see the substrate (does not interact with the substrate). The 2D materials of the present invention are rigid.

Another problem that can be solved by the present invention is the control of the contact surface so as to locally be able to inject carriers (contact engineering). This is particularly interesting in that we have shown that self-supporting layers can be obtained.

The only technique proposed today is to deposit the 2D material on a micro-perforated substrate obtained by lithography. In this case, suspended and unsprung areas are obtained. We can not speak of fully suspended layers.

The invention furthermore relates to the use of thin films suspended from 2D materials in electronic and / or optoelectronic and / or thermal and / or photonic domains.

The invention also relates to the use of thin films suspended from 2D materials in catalysis fields and / or in ultrasensitive surfaces.

Many combinations can be envisaged without departing from the scope of the invention; the skilled person will choose one or the other depending on the economic, ergonomic, dimensional or other constraints that must be respected.

Claims

1. A method of integrating two-dimensional materials on a nanostructured substrate characterized in that it comprises the following steps:

2. Method of integrating two-dimensional materials on a nanostructured substrate according to claim 1, characterized in that the manufacture of two-dimensional materials by the vapor deposition method consists of:

the transfer of said two-dimensional, PMMA-coated materials obtained in a previous step onto a nanostructured substrate comprises the following steps:

B1) removal of PMMA with acetone solution;

B2) drying said two-dimensional materials to remove residual PMMA layers; or

B2 ') sublimation of said two-dimensional materials to remove residual PMMA layers.

3. A method of integrating two-dimensional materials on a nanostructured substrate according to claim 1 characterized in that the two-dimensional materials are further manufactured by the exfoliation method in an inert environment; and in that the transfer of said two-dimensional materials obtained by exfoliation onto a synthesized nanostructured substrate consists of a single step of depositing said two-dimensional materials on said synthesized nanostructured substrate.

4. Method of integrating two-dimensional materials according to claim 1 wherein the nanostructured substrate is ZnO nanowires, zinc oxide, said substrate is synthesized by a method of deposition in the liquid phase on an SiO 2 substrate or structuring approach 'top-down' or any other ZnO nanowire growth techniques.

5. Integration method according to claim 4 wherein the ZnO nanowires are disordered and of variable sizes so as to minimize the contact area with the two-dimensional materials obtained.

6. Method of integrating two-dimensional materials according to claim 4 or 5, wherein the ZnO nanowires have a diameter less than 100 nm.

7. A method of integrating two-dimensional materials according to one of the preceding claims characterized in that the two-dimensional materials are either molybdenum sulfide MoS2, or tungsten sulfide WS2 or diselenide Tungsten WSe2.

The method of integration of claim 1 wherein the two-dimensional materials are any two-dimensional rigid materials.

9. suspended thin film obtained by the 2D material transfer method on a nanostructured substrate according to one of claims 1 to 8 characterized in that the nanostaructured substrate is a ZnO nanowire substrate; and in that the ZnO nanowires are disordered and of varying sizes.

1. thin film suspended 2D material according to claim 9 characterized in that the two-dimensional materials are either molybdenum sulfide MoS2, or tungsten sulfide WS2 or WSe2 tungsten diselenide, said two-dimensional materials are rigid; and in that they are obtained by the vapor deposition on an SiO2 substrate or by exfoliation.

1 1. Use of the suspended thin film of 2D material according to claims 9 and 10 in electronic and / or optoelectronic and / or thermal and / or photonic fields.

2. Use of the thin film suspended from 2D materials according to claims 9 and 10 in catalysis fields and / or in ultrasensitive surfaces.