WO2021105343A1 - Van der waals heterostructures - Google Patents

Van der waals heterostructures Download PDF

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WO2021105343A1
WO2021105343A1 PCT/EP2020/083611 EP2020083611W WO2021105343A1 WO 2021105343 A1 WO2021105343 A1 WO 2021105343A1 EP 2020083611 W EP2020083611 W EP 2020083611W WO 2021105343 A1 WO2021105343 A1 WO 2021105343A1
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substrate
layer
head
films
abrasion
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Freddie WITHERS
Darren NUTTING
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University Of Exeter
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    • C25B11/075—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of a single catalytic element or catalytic compound
    • C25B11/087—Photocatalytic compound
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Definitions

  • This invention relates to the fabrication of Van der Waals (vdW) heterostructures.
  • High quality vdW heterostructures offer many improvements over conventional materials as they are lightweight, transparent and are compatible with flexible substrates whilst at the same time display competitive performance to that of conventional compound semiconductor electronics.
  • the highest quality heterostructures are fabricated through mechanical exfoliation of bulk single crystals and built up layer-by-layer using standard mechanical transfer procedures. This method, however, is not scalable.
  • the production method is unlikely to be incompatible with many highly air sensitive vdW materials, and so the range of suitable materials with which the technique can be used is limited. Moreover, residue solvent in the printed films are also likely to degrade the electrical properties of the devices by reducing the quality of the interface between neighbouring nanocrystals.
  • a method for use in the fabrication of a van der Waals heterostructure comprising the steps of applying a mask to a substrate, rubbing a first material against the substrate to deposit and apply, by abrasion, a first layer of the first material to the substrate, applying a second mask over the first layer, and rubbing a second material against the substrate to apply, by abrasion, a second layer of the second material over at least part of the first layer. It will be appreciated that, in this manner, a multi-layered vdW heterostructure may be formed upon the substrate.
  • the second material is preferably less hard or less abrasive than the first material. In this manner, the risk of damaging the first layer during the application of the second layer is reduced.
  • the steps of rubbing the first and second materials against the substrate are preferably achieved using a rotatable head.
  • the material to be deposited may be applied to the rotatable head or to the substrate, and the material may be deposited onto the substrate by pressing the head against the substrate whilst rotating the head, and moving the head relative to the substrate to apply the material over an area of the substrate, thereby applying a layer of the material to the substrate.
  • the second layer is preferably of a different material to the first layer.
  • it may be of a less abrasive material than the material of the first layer. If there is a need to apply a layer of a harder or more abrasive material onto a layer of a softer or less abrasive material, then this may be achieved by transfer of a preformed layer onto the substrate. Rather than comprise a single preformed layer, a multi-layered element could be transferred in this manner.
  • a multi-layered structure fabricated using the afore described method.
  • the structure may comprise, for example, a photo detector or light emitting device. Alternatively, it could comprise a transistor, a capacitor or a resistor. It is further envisaged that the structure could take the form of a superconducting memory device.
  • the method may be used to introduce single photon emitters onto a film.
  • Other applications in which the invention may be employed include in the formation of triboelectric generators and hydrogen evolution catalysts.
  • Figure 1A illustrates steps the fabrication of a thin film produced through powder abrasion of vdW powders for the production heterostructures through mechanical abrasion
  • Figure IB illustrates typical optical transmission spectra for different materials deposited onto 0.5 mm thick PET substrates
  • Figure 2 illustrates the electronic properties of the bare films with (A) showing l Sd -V Sd of a graphitic film produced through mechanical abrasion (inset: optical photograph of a graphitic film produced on a 2.5 cm PET substrate thinned to three different thicknesses by back-peeling with adhesive tape); (B) showing gate dependence of the channel resistivity for the device shown in the bottom-right inset using an liCI03 electrolyte.
  • Top left inset Contour map of the I sd -V sd for different applied gate voltages; (C) showing typical l Sd -V Sd for a 5 mm x 0.025 mm two terminal device based on M0S2 powder abrasion with an average film thickness of 100 nm for different levels of applied uniaxial strain (inset: V Sd is held at 0.5V and the device is subjected to reversible uniaxial strain in time); and (D) showing impedance spectroscopy for a hBN dielectric capacitor produced using a 5mm thick hBN film;
  • Figure 4 illustrates heterostructures produced solely from mechanical abrasion of 2D powders with (A) showing photoresponse of a diode structure consisting of Gr-WS2-Gr (inset: Temporal response of the open circuit photovoltage); and (B) showing multi-layer type-ll band alignment diode consisting of Gr-WS2-MoS2-Gr in dark and under optical excitation of lOOmW/cm 2 (Device area 2 x 3 mm);
  • Figure 5 illustrates WS2 films as a catalyst for hydrogen evolution and photocurrent generation with (A) showing photocurrent power generation for increasing resistive load (inset: temporal response of the generated photocurrent measured through a IMOhm resistor); (B) showing spectral dependence of the responsivity of the generated photocurrent; (C) showing polarization curves measured in 0.5 M FI2SO4 with a scan rate of 2mV/s at room temperature (inset: shows the onset HER potential of WS2; and (D) showing linear portions of the Tafel plots for WS2 sample exfoliated by mechanical abrasion on an Au thin film;
  • Figure 6 illustrates High Energy Radiation detection with (A) showing the response of the current through the device for a high 30Gy gamma radiation dose; and (B) showing the temporal response of the conductivity for 5 successive exposures to 5Gy's; and
  • Figure 7 illustrates a triboelectric nanogenerator based on abraded films with (A) showing a schematic of the device and electrical setup; and (B) showing the time response for three successive release cycles.
  • Figure 1A shows the general approach used in accordance with an embodiment of the invention to produce thin films of 2D nanocrystals on S1O2 and flexible PET substrates.
  • a soft polymer, Polydimethylsiloxane (PDMS) rotatable writing pad 10 is coated in a 2D material powder film 12.
  • PDMS Polydimethylsiloxane
  • the PDMS pad 10 is then oscillated back and forth against a substrate 14, whilst being pressed against the substrate and whilst the writing pad 10 is being rotated, with various 2D materials embedded or located between the writing pad 10 and the substrate 14 to cause the build-up of a layer of material upon the substrate 14.
  • a tape mask 16 is applied to the substrate 14 before writing. After the material has been written or deposited in this manner, the tape mask 16 is removed leaving only the unmasked region coated in the 2D material.
  • heterostructures of relatively complex form may be built up.
  • subsequent layers are preferably of materials that are softer or are less abrasive that the layer(s) already deposited.
  • Figure IB shows the optical transmission spectra of several different 2D materials fabricated by mechanical abrasion on a 0.5 mm thick PET substrate, the resultant films displaying some level of transparency which can also be tuned to higher levels by back peeling the films with an adhesive tape. Examples of the transmission spectra of various films are also shown in Figure IB and show the characteristic increases in absorption associated with the A and B excitons indicating that the films still display similar optical properties compared to the bulk pristine materials.
  • Devices fabricated by mechanical abrasion in this manner may have a CVD graphene layer applied thereto, for example by spin coating PMMA onto CVD graphene deposited onto copper, applying a tape window thereto and etching away the copper from selected parts thereof using a 0.1 M aqueous solution of ammonium persulfate, for example over a period of around 6 hrs.
  • the CVD graphene may then be transferred onto the target device completing the heterostructure.
  • To finish the device it may be baked for lhr at 150 degrees to improve the mechanical contact of the CVD graphene with the abraded nano-crystal films. It will be appreciated that in this manner, a harder or more abrasive layer can be applied to an already deposited layer. The method thus allows relatively greater flexibility of design, and so allows heterostructures for use in a wide range of applications to be fabricated.
  • Raman spectroscopy was carried out using 532 nm excitation at 1 mW laser power focused onto a 1 mm spot.
  • AFM was performed using a Bruker Innova system operating in the tapping mode to ensure minimal damage to the sample's surface.
  • the tips used were Nanosensors PPP-NCHR, which have a radius of curvature smaller than 10 nm and operate in a nominal frequency of 330 kHz.
  • Optical measurements Optical transmission spectra were recorded using an Andor Shamrock 500i spectrograph with 3001/mm grating resolution and iDus 420 CCD.
  • a fibre coupled halogen white light source was used to excite the photo-active samples which generates 1.4 W at the fibre tip.
  • the white light is collimated to give uniform excitation of 70-100 mW/cm 2 .
  • the white light source was blocked for the time response using a mechanical shutter with a response time of 10 ms.
  • the spectral dependence of the photocurrent was carried out using lOnm band pass filters to filter the halogen white light source with the power at each wavelength calibrated using a Thorlabs photodiode S120C.
  • Electron transport measurements were carried out using a KE2400 source-meter for both source and gate electrodes.
  • An Agilent 34410A multi meter was used to record the voltage drop over a variable resistor in order to determine the drain current and also photo response for different load resistances.
  • Electrochemical data was obtained using an Ivium-stat potentiostat/galvanostat.
  • Linear sweep voltammetry (LSV) experiments were carryout in 0.5 M FI2S04 with a scan rate of 2 mV/s.
  • LSV Linear sweep voltammetry
  • a three-electrode electrochemical cell was used, i.e., saturated calomel electrode (SCE) (reference), platinum foil electrode (counter), and WS2/Au (working).
  • SCE saturated calomel electrode
  • the work electrode area used was 0.196 cm 2 ( Figure 1).
  • the reference electrode was stored in KCI solution and rinsed with deionized water before use.
  • high-purity N2 gas was bubbled into the solution for at least 60 min before the electrochemical measurements.
  • Figure 2 highlights some basic electrical characterisation measurements of the materials.
  • Figure 2A displays an l sc r V sd curve for a thin graphite film with transparency of 20% which gives a characteristic resistance per square of 2 KOhm, similar to levels achieved with structures fabricated using other techniques.
  • the thickness of the film can be controlled by thinning with specialist tapes, allowing for improved transparency but increased resistance.
  • the resistance of the few layer graphitic channel can be controlled by application of a gate-voltage (indicating ⁇ 10 layer individual crystallite thicknesses).
  • a gate-voltage indicating ⁇ 10 layer individual crystallite thicknesses.
  • an electrolyte gate Li + :CI03 may be employed, drop cast over the channel region and contacted using an abraded graphitic gate electrode.
  • Figure 2B shows the typical resistance vs gate voltage for a tape thinned graphitic channel region, with the inset showing a contour map of the I sd -V sd for different applied gate voltages. It has been found that the electro-neutrality region is at large positive gate voltages, indicating a strong p-type doping, likely due to ambient water or oxygen doping.
  • FIG. 1C shows some typical I sd -V sd curves for a M0S2 channel. The different curves are for increasing (red to blue) uniaxial strain generated by bending the 0.5mm PET substrate in a custom-built bending rig. It was found that the device resistance increased for increased levels of strain, and that the resistance changes are reversible as shown in the inset of Figure 2C, indicating that these films are suitable for strain sensing applications.
  • TMDC's are semiconductor materials with an indirect bandgap in the bulk and have already shown great promise for future flexible photovoltaic and photodetection applications.
  • Fleterostructures based on liquid phase exfoliated nanocrystals typically display poor photo detectivity in the order of 10-1000pA/W restricting their use in certain applications.
  • Devices manufactured in accordance with the invention display a significant improvement in the photo response of simple photodetector devices.
  • Figure 3 shows a variety of different photodetection architectures, including lateral as well as vertical.
  • an aluminium foil shadow mask may be employed, but this may have the effect of strongly increasing the vertical junction resistance, likely explained by the partial oxidation of the WS2 film during the flame deposition process (expected as the WS2 film is being passed across a naked flame).
  • the responsivity for those structures is still found to be ⁇ 0.5mA/W, the reduction is likely explained because of the lower transparency of the thick highly absorbing a- C top electrode and degraded WS2 film.
  • the l Sd -V Sd curve of a typical device is shown in Figure 3C and shows a greater conductivity compared to the a-C device shown in Figure 3B. This is more representative of the true vertical conductivity of the pristine abraded WS2 films. Indeed, similar vertical devices based on n and p-type silicon show similar conductivities with CVD graphene top electrodes.
  • the asymmetry in the l Sd -V Sd curves here is down to the difference in the work functions of the graphene (4.6-4.9 eV) and Au ( ⁇ 5.2 eV) with the conduction band edge of the WS2 closely aligned with the neutrality point of graphene.
  • More complex vertical heterostructures may be fabricated by repeating steps (1-3) shown in Figure 1A with all the layers deposited through abrasion methods.
  • Figure 4A shows the l Sd -V Sd curves for a single vertical diode structure consisting of Gr-WS2-Gr in dark and under a white light exposure of lOOmW/cm 2 .
  • the top inset shows a photo of ten such diode structures connected in series on a 0.5mm PET substrate highlighting the up scalability of the technology, and the bottom inset shows the time response of the open circuit voltage for one representative junction.
  • FIG. 4B A more complex type-ll band alignment heterostructure consisting of Gr-WS2-MoS2- Gr is shown in Figure 4B, all layers of which are again deposited through the above described mechanical abrasion technique.
  • the asymmetric l Sd -V Sd black curve
  • the vertical conductivity increases significantly and exhibits a photo response of a few mA/W and an open circuit voltage of 14 mV and a short circuit current of 100 nA under 110 mW/cm 2 white light illumination. This leads to a power conversion efficiency of 10 5 %.
  • FIG. 5A displays the photocurrent results for a 1 cm 2 WS2 electro catalyst produced through mechanical abrasion on an Au substrate (we also demonstrate the large area compatibility with commercial stainless steel in the supplementary materials).
  • Figure 5A shows the photocurrent power generation of a WS2 film abraded onto a thermally evaporated Au substrate and placed in Dl water. The photocurrent is measured between a Pt wire in the Dl water and the Au substrate for various resistive loads.
  • the HER polarization curves of the current density are plotted as a function of potential for a representative WS2 sample and shown in Figure 5C.
  • a current density 8.5 mA/cm 2 at an overpotential of 200 mV, comparable to the values observed elsewhere.
  • the inset of Figure 5C shows the onset potential of -97 mV (vs RH E). This shows that WS2 films produced through mechanical abrasion are suitable for HER catalyst applications.
  • the overpotential is plotted in Figure 5D and the absolute value of the current density within a cathodic potential window and the corresponding Tafel fit is shown, red curve.
  • b represents the Tafel slope
  • b represents the Tafel slope
  • Figure 5D The reported Tafel slopes vary significantly for different studies depending strongly on the synthesis route of the WS2 films. For example, Bonde et al. reported the HER activity on carbon supported WS2 nanoparticles with Tafel slopes of 135 mV/dec. Xiao et al.
  • High energy photon-matter interactions including, the photoelectric effect (PE), Rayleigh scattering (RS) and Compton scattering (CS) depend strongly of the atomic mass, Z. Specifically the cross-section of such processes depends on the atomic mass as Z as ⁇ Z 45 , Z 2 and Z for the PE, RS and CS processes respectively.
  • High energy gamma-ray radiation detectors have practical uses in various sectors from medical to space exploration. Ideally, they should be highly sensitive while being radiation hard (That is the response should not change greatly after prolonged exposure).
  • the high atomic mass of tungsten in the compound WS2 makes it a stand-out choice for high energy radiation detection applications.
  • Triboelectric nanogenerator The triboelectric effect in 2D materials has recently been reported. Those previous devices were mainly based on thin films based on liquid phase exfoliated. Here we demonstrate the use of mechanically abraded thin films for the use as a triboelectric effect.
  • Figure 7A shows the schematic of the setup with a thin PET substrate with the abraded film or multilayer stack stuck to a large metallic plate which was kept grounded. A PDMS pad was then pressed onto the 2D material, which has the effect of charging the film. This sets a voltage relative to ground and this voltage drop is measured over a fixed resistor to calculate the current.
  • complex functional heterostructure devices can be built up from 2D nanocrystals through a simple mechanical abrasion method.
  • This technology allows for the rapid up-scaling of heterostructures and it is demonstrated that it has practical use in several simple device applications including conductive few layer graphene coatings, photodetectors.
  • abraded WS2 coatings can be used directly as electro catalysts for the HER reaction. The ease with which the films can be applied, and simplicity of up-scalability makes this technology significantly attractive for a large variety of applications.

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Abstract

A method is described for use in the fabrication of a van der Waals heterostructure, the method comprising the steps of applying a mask to a substrate, rubbing a first material against the substrate to deposit and apply, by abrasion, a first layer of the first material to the substrate, applying a second mask over the first layer, and rubbing a second material against the substrate to apply, by abrasion, a second layer of the second material over at least part of the first layer. Structures fabricated using the method are also described.

Description

VAN DER WAALS HETEROSTRUCTURES
This invention relates to the fabrication of Van der Waals (vdW) heterostructures. High quality vdW heterostructures offer many improvements over conventional materials as they are lightweight, transparent and are compatible with flexible substrates whilst at the same time display competitive performance to that of conventional compound semiconductor electronics. The highest quality heterostructures are fabricated through mechanical exfoliation of bulk single crystals and built up layer-by-layer using standard mechanical transfer procedures. This method, however, is not scalable.
Chemical vapour deposition techniques, where monolayer films are grown layer by layer at high temperatures have shown promising results. However, the energy cost of growth is high for a given quantity of material, the growth of multi-layer heterostructures is complex and is also confined to a small number of material combinations. Furthermore, transfer of the films formed in this manner from catalyst substrates often introduces contamination, tears and/or cracks which may prevent the formation of high quality vertical heterostructure devices.
An alternative approach for use in the mass-scalable production of nanocrystal heterostructures is through the ink-jet printing of liquid-phase exfoliated dispersions. In this technique, 2D dispersions are produced through either ultra-sonication or shear force exfoliation of bulk vdW microcrystals in suitable solvents. This leads to stable dispersions which can then subsequently be printed on a variety of substrates. By mixing the dispersions with specialist binders the heterostructures can also be built-up layer-by-layer through ink-jet printing. However, strong disorder in the crystals caused by both solvent induced oxidation and poor interface quality can lead to severe performance degradation compared to devices based on single crystals. Furthermore, the production method is unlikely to be incompatible with many highly air sensitive vdW materials, and so the range of suitable materials with which the technique can be used is limited. Moreover, residue solvent in the printed films are also likely to degrade the electrical properties of the devices by reducing the quality of the interface between neighbouring nanocrystals.
It is an object of the invention to provide a technique that allows the building up of a semi- transparent and flexible vdW nanocrystal heterostructure in a simple and convenient manner, using a wide range of materials.
According to a first aspect of the invention there is provided a method for use in the fabrication of a van der Waals heterostructure, the method comprising the steps of applying a mask to a substrate, rubbing a first material against the substrate to deposit and apply, by abrasion, a first layer of the first material to the substrate, applying a second mask over the first layer, and rubbing a second material against the substrate to apply, by abrasion, a second layer of the second material over at least part of the first layer. It will be appreciated that, in this manner, a multi-layered vdW heterostructure may be formed upon the substrate.
The second material is preferably less hard or less abrasive than the first material. In this manner, the risk of damaging the first layer during the application of the second layer is reduced.
The steps of rubbing the first and second materials against the substrate are preferably achieved using a rotatable head. By way of example, the material to be deposited may be applied to the rotatable head or to the substrate, and the material may be deposited onto the substrate by pressing the head against the substrate whilst rotating the head, and moving the head relative to the substrate to apply the material over an area of the substrate, thereby applying a layer of the material to the substrate.
Further layers may be applied using this technique. As mentioned above, the second layer is preferably of a different material to the first layer. By way of example, it may be of a less abrasive material than the material of the first layer. If there is a need to apply a layer of a harder or more abrasive material onto a layer of a softer or less abrasive material, then this may be achieved by transfer of a preformed layer onto the substrate. Rather than comprise a single preformed layer, a multi-layered element could be transferred in this manner.
According to another aspect of the invention there is provided a multi-layered structure fabricated using the afore described method. The structure may comprise, for example, a photo detector or light emitting device. Alternatively, it could comprise a transistor, a capacitor or a resistor. It is further envisaged that the structure could take the form of a superconducting memory device. The method may be used to introduce single photon emitters onto a film. Other applications in which the invention may be employed include in the formation of triboelectric generators and hydrogen evolution catalysts.
The invention will further be described, by way of example, with reference to the accompanying drawings, in which:
Figure 1A illustrates steps the fabrication of a thin film produced through powder abrasion of vdW powders for the production heterostructures through mechanical abrasion;
Figure IB illustrates typical optical transmission spectra for different materials deposited onto 0.5 mm thick PET substrates;
Figure 2 illustrates the electronic properties of the bare films with (A) showing lSd-VSd of a graphitic film produced through mechanical abrasion (inset: optical photograph of a graphitic film produced on a 2.5 cm PET substrate thinned to three different thicknesses by back-peeling with adhesive tape); (B) showing gate dependence of the channel resistivity for the device shown in the bottom-right inset using an liCI03 electrolyte. Top left inset: Contour map of the Isd-Vsd for different applied gate voltages; (C) showing typical lSd-VSd for a 5 mm x 0.025 mm two terminal device based on M0S2 powder abrasion with an average film thickness of 100 nm for different levels of applied uniaxial strain (inset: VSd is held at 0.5V and the device is subjected to reversible uniaxial strain in time); and (D) showing impedance spectroscopy for a hBN dielectric capacitor produced using a 5mm thick hBN film;
Figure 3 illustrates mechanically abraded films for photodetection applications with (A) showing Isd-Vsd for a planer geometry WS2 channel without (black curve) and with (red curve) 111 mW/cm2 incoherent optical excitation; (B) showing lSd-VSd with and without optical excitation for a vertical WS2 device with a candle flame deposited amorphous carbon top electrode; (C) showing similar vertical heterostructure as in (B) but with a CVD graphene top electrode with device area of 1mm x 1mm and thickness of 300nm (inset: a zoomed in region around VSCF+/- IV with and without optical excitation and with power density of 74 mW/cm2); and (D) showing spectral responsivity of the diode structure shown in (C) (inset: temporal dependence of the photocurrent when opening and closing the shutter on the incident white light of power 74mW/cm2 and VSd=-l V);
Figure 4 illustrates heterostructures produced solely from mechanical abrasion of 2D powders with (A) showing photoresponse of a diode structure consisting of Gr-WS2-Gr (inset: Temporal response of the open circuit photovoltage); and (B) showing multi-layer type-ll band alignment diode consisting of Gr-WS2-MoS2-Gr in dark and under optical excitation of lOOmW/cm2 (Device area 2 x 3 mm);
Figure 5 illustrates WS2 films as a catalyst for hydrogen evolution and photocurrent generation with (A) showing photocurrent power generation for increasing resistive load (inset: temporal response of the generated photocurrent measured through a IMOhm resistor); (B) showing spectral dependence of the responsivity of the generated photocurrent; (C) showing polarization curves measured in 0.5 M FI2SO4 with a scan rate of 2mV/s at room temperature (inset: shows the onset HER potential of WS2; and (D) showing linear portions of the Tafel plots for WS2 sample exfoliated by mechanical abrasion on an Au thin film;
Figure 6 illustrates High Energy Radiation detection with (A) showing the response of the current through the device for a high 30Gy gamma radiation dose; and (B) showing the temporal response of the conductivity for 5 successive exposures to 5Gy's; and
Figure 7 illustrates a triboelectric nanogenerator based on abraded films with (A) showing a schematic of the device and electrical setup; and (B) showing the time response for three successive release cycles. Referring to the accompanying drawings, Figure 1A shows the general approach used in accordance with an embodiment of the invention to produce thin films of 2D nanocrystals on S1O2 and flexible PET substrates. As illustrated, a soft polymer, Polydimethylsiloxane (PDMS) rotatable writing pad 10 is coated in a 2D material powder film 12. The PDMS pad 10 is then oscillated back and forth against a substrate 14, whilst being pressed against the substrate and whilst the writing pad 10 is being rotated, with various 2D materials embedded or located between the writing pad 10 and the substrate 14 to cause the build-up of a layer of material upon the substrate 14.
To ensure that the 2D material is only written at selected locations on the substrate 14, a tape mask 16 is applied to the substrate 14 before writing. After the material has been written or deposited in this manner, the tape mask 16 is removed leaving only the unmasked region coated in the 2D material. These steps are illustrated diagrammatically in Figure 1A, steps 1 to 3. As shown in step 4 of Figure 1A, this process can then be repeated to build up multi-layered heterostructures such as capacitors or photovoltaics as described below. By way of example, by using a different tape mask 16 leaving different unmasked regions, heterostructures of relatively complex form may be built up.
In order to avoid damage to a layer that has already been deposited, subsequent layers are preferably of materials that are softer or are less abrasive that the layer(s) already deposited.
The films deposited or fabricated using the technique described hereinbefore can be studied using Raman spectroscopy and atomic force microscopy. Figure IB shows the optical transmission spectra of several different 2D materials fabricated by mechanical abrasion on a 0.5 mm thick PET substrate, the resultant films displaying some level of transparency which can also be tuned to higher levels by back peeling the films with an adhesive tape. Examples of the transmission spectra of various films are also shown in Figure IB and show the characteristic increases in absorption associated with the A and B excitons indicating that the films still display similar optical properties compared to the bulk pristine materials. Devices fabricated by mechanical abrasion in this manner may have a CVD graphene layer applied thereto, for example by spin coating PMMA onto CVD graphene deposited onto copper, applying a tape window thereto and etching away the copper from selected parts thereof using a 0.1 M aqueous solution of ammonium persulfate, for example over a period of around 6 hrs. The CVD graphene may then be transferred onto the target device completing the heterostructure. To finish the device, it may be baked for lhr at 150 degrees to improve the mechanical contact of the CVD graphene with the abraded nano-crystal films. It will be appreciated that in this manner, a harder or more abrasive layer can be applied to an already deposited layer. The method thus allows relatively greater flexibility of design, and so allows heterostructures for use in a wide range of applications to be fabricated.
Materials characterisation
Raman spectroscopy was carried out using 532 nm excitation at 1 mW laser power focused onto a 1 mm spot. AFM was performed using a Bruker Innova system operating in the tapping mode to ensure minimal damage to the sample's surface. The tips used were Nanosensors PPP-NCHR, which have a radius of curvature smaller than 10 nm and operate in a nominal frequency of 330 kHz.
Optical measurements Optical transmission spectra were recorded using an Andor Shamrock 500i spectrograph with 3001/mm grating resolution and iDus 420 CCD. A fibre coupled halogen white light source was used to excite the photo-active samples which generates 1.4 W at the fibre tip. The white light is collimated to give uniform excitation of 70-100 mW/cm2. The white light source was blocked for the time response using a mechanical shutter with a response time of 10 ms. The spectral dependence of the photocurrent was carried out using lOnm band pass filters to filter the halogen white light source with the power at each wavelength calibrated using a Thorlabs photodiode S120C.
Electrical measurements Electron transport measurements were carried out using a KE2400 source-meter for both source and gate electrodes. An Agilent 34410A multi meter was used to record the voltage drop over a variable resistor in order to determine the drain current and also photo response for different load resistances.
Electrochemical data was obtained using an Ivium-stat potentiostat/galvanostat. Linear sweep voltammetry (LSV) experiments were carryout in 0.5 M FI2S04 with a scan rate of 2 mV/s. For determination of activity of hydrogen evolution reaction (HER), a three-electrode electrochemical cell was used, i.e., saturated calomel electrode (SCE) (reference), platinum foil electrode (counter), and WS2/Au (working). The work electrode area used was 0.196 cm2 (Figure 1). The reference electrode was stored in KCI solution and rinsed with deionized water before use. For the measurements, high-purity N2 gas was bubbled into the solution for at least 60 min before the electrochemical measurements. The potentials reported here are with respect to reversible hydrogen electrode (E (RHE) = E (SCE) + 0.273 V [ref5].)
In many applications, the electronic performance of the films is important. Figure 2 highlights some basic electrical characterisation measurements of the materials. Figure 2A displays an lscr Vsd curve for a thin graphite film with transparency of 20% which gives a characteristic resistance per square of 2 KOhm, similar to levels achieved with structures fabricated using other techniques. Importantly, the thickness of the film can be controlled by thinning with specialist tapes, allowing for improved transparency but increased resistance.
It has been found that the resistance of the few layer graphitic channel can be controlled by application of a gate-voltage (indicating <10 layer individual crystallite thicknesses). By way of example, an electrolyte gate Li+:CI03 may be employed, drop cast over the channel region and contacted using an abraded graphitic gate electrode. Figure 2B shows the typical resistance vs gate voltage for a tape thinned graphitic channel region, with the inset showing a contour map of the Isd-Vsd for different applied gate voltages. It has been found that the electro-neutrality region is at large positive gate voltages, indicating a strong p-type doping, likely due to ambient water or oxygen doping. It should be noted that not all substrates are compatible with direct abrasion of graphite. Success was found with PET but not with S1O2, but all other 2D materials explored were found to be compatible with S1O2 substrates also. Figure 1C shows some typical Isd-Vsd curves for a M0S2 channel. The different curves are for increasing (red to blue) uniaxial strain generated by bending the 0.5mm PET substrate in a custom-built bending rig. It was found that the device resistance increased for increased levels of strain, and that the resistance changes are reversible as shown in the inset of Figure 2C, indicating that these films are suitable for strain sensing applications.
Important for any integrated electronic application is the development of capacitors. By making use of hBN dielectrics produced through mechanical abrasion over evaporated gold electrodes, films of thickness 5±lpm (estimated from AFM and surface profile measurements) were produced. Following the deposition of the hBN dielectric, a sheet of CVD graphene was transferred onto the hBN film with two Au electrodes which act as the source and drain contacts for the graphene channel. An optical image of the device is shown in the inset of Figure 2D. The total area of the capacitor in this instance was approximately 2xl0 sm2. The impedance spectrum is presented in Figure 2D and can be well described by the capacitive contribution, \Zr I = (27 r/C)_1. The gradient to the linear fit, therefore, gives the capacitance which we find to be C=9.8pF. If we assume a plane plate capacitor model, then the capacitance is related to the relative permittivity by the following relation C =
Figure imgf000009_0001
which allows us to make an
Figure imgf000009_0002
estimate of the dielectric constant, which we find to be eG~2.8. The dielectric constants for typical nanocrystal hBN dielectrics vary wildly, with values ranging from 1.5 up to 200, whilst single crystal hBN is known to display a relative permittivity of ~4. The lower value found in the material manufactured in accordance with the invention could be due to air voids in the films effectively lowering the effective capacitance of the whole barrier. Photodetection and Photovoltaic Devices
TMDC's are semiconductor materials with an indirect bandgap in the bulk and have already shown great promise for future flexible photovoltaic and photodetection applications. Fleterostructures based on liquid phase exfoliated nanocrystals typically display poor photo detectivity in the order of 10-1000pA/W restricting their use in certain applications. Devices manufactured in accordance with the invention display a significant improvement in the photo response of simple photodetector devices. Figure 3 shows a variety of different photodetection architectures, including lateral as well as vertical. The most basic focusses on simple in-plane channel of WS2 on Si-SiC substrates fabricated by mechanically abrading WS2 nanocrystals over a 1cm substrate followed by shadow mask evaporation of Cr/Au electrodes using the technique described hereinbefore. This leaves a 25pm channel length with a channel width of 1mm and a typical film thickness of ~1 pm. The lSd-VSd curves are shown in Figure 3A with (red curve) and without (black curve) excitation with an incoherent white light source with a power density of lllmW/cm2. The asymmetry in the lSd-VSd curves is likely due to either asymmetric doping of the different regions of the abraded films or differences in the mechanical contact of the electrodes after deposition. Such simple planar photodetectors already show improved photo- responsivity compared with liquid phase exfoliated materials with responsivity up to 18mA/W at Vsd=-2V. A simple and quick route to construct vertical heterostructures is to deposit amorphous carbon top electrodes, deposited from a candle flame. To selectively deposit the material an aluminium foil shadow mask may be employed, but this may have the effect of strongly increasing the vertical junction resistance, likely explained by the partial oxidation of the WS2 film during the flame deposition process (expected as the WS2 film is being passed across a naked flame). The responsivity for those structures is still found to be ~0.5mA/W, the reduction is likely explained because of the lower transparency of the thick highly absorbing a- C top electrode and degraded WS2 film. By making use of a CVD graphene top electrode, chosen because of its high conductivity and transparency, the optoelectronic properties the abraded WS2 films can be addressed without using highly absorbing top electrodes. The lSd-VSd curve of a typical device is shown in Figure 3C and shows a greater conductivity compared to the a-C device shown in Figure 3B. This is more representative of the true vertical conductivity of the pristine abraded WS2 films. Indeed, similar vertical devices based on n and p-type silicon show similar conductivities with CVD graphene top electrodes. The asymmetry in the lSd-VSd curves here is down to the difference in the work functions of the graphene (4.6-4.9 eV) and Au (~5.2 eV) with the conduction band edge of the WS2 closely aligned with the neutrality point of graphene. This means that the conductivity is high at zero bias as electron transport occurs through the conduction band of the WS2 while at negative voltages the energy difference between the chemical potential of graphene and the conduction band of WS2 increases therefore increasing the barrier for conduction which is seen as a reduction in the conductivity. The spectral dependence of the photo responsivity for this device is plotted in Figure 3D with a peak responsivity at 2.0 eV consistent with the peak in absorption associated with the A-exciton, and maximal responsivities of 0.15 A/W at VSd=-lV. The time response to the incident white light source is also shown in the inset with peak photocurrent values of IOOmA at VSd = -1 V under uniform white light illumination of 74mW/cm2.
More complex vertical heterostructures may be fabricated by repeating steps (1-3) shown in Figure 1A with all the layers deposited through abrasion methods. Figure 4A shows the lSd-VSd curves for a single vertical diode structure consisting of Gr-WS2-Gr in dark and under a white light exposure of lOOmW/cm2. The top inset shows a photo of ten such diode structures connected in series on a 0.5mm PET substrate highlighting the up scalability of the technology, and the bottom inset shows the time response of the open circuit voltage for one representative junction. A more complex type-ll band alignment heterostructure consisting of Gr-WS2-MoS2- Gr is shown in Figure 4B, all layers of which are again deposited through the above described mechanical abrasion technique. The asymmetric lSd-VSd (black curve) is likely a result of the band misalignment between the WS2 and M0S2 materials. Upon illumination the vertical conductivity increases significantly and exhibits a photo response of a few mA/W and an open circuit voltage of 14 mV and a short circuit current of 100 nA under 110 mW/cm 2 white light illumination. This leads to a power conversion efficiency of 105%. The low value likely due to the highly absorbing top graphene electrode and also the thick M0S2 layer, which was necessary to prevent short circuit, this means most of the light is absorbed before reaching the p-n junction and the charge separation is likely taking place at the Gr-MoS2 interface.
Hydrogen Evolution and Photocurrent Generation
Mono and few layer TMDC's have been widely considered for their potential as electro catalysts for the hydrogen evolution reaction. Here, we select WS2 as its the most thermoneutral, therefore minimizing the barrier for the photo-excited splitting of water. A useful bi product of the light assisted HER reaction is the generation of photocurrent. Figure 5A displays the photocurrent results for a 1 cm 2 WS2 electro catalyst produced through mechanical abrasion on an Au substrate (we also demonstrate the large area compatibility with commercial stainless steel in the supplementary materials). Figure 5A shows the photocurrent power generation of a WS2 film abraded onto a thermally evaporated Au substrate and placed in Dl water. The photocurrent is measured between a Pt wire in the Dl water and the Au substrate for various resistive loads. The maximum power generated is found to reach 100 pW/cm2 at a load of 1 MOhm (comparable to the resistance of the cell). The inset to Figure 5A shows the time response of the photocurrent determined from the voltage drop over a IMOhm resistor. To show that the photoresponse is due to WS2 film we also perform a spectral study of the short circuit photocurrent, Figure 5B.
We observe that there is a small peak at E~2 eV consistent with the direct exciton in WS2. The responsivity is then found to increase rapidly for higher excitation energy consistent with the increased joint density of states in WS2 at higher energies. Ultimately, we find peak values of 75 mA/W at 400 nm excitation. The inset of Figure 5B displays the power dependence for 550 nm excitation, we observe that the responsivity saturates at very low powers (70 mW/cm2) and then drops rapidly to l/3rd of the peak value at maximum power available. The electrochemical performance of our WS2 films have been characterised 0.5 M FhSC via linear sweep voltammetry (LSV). The HER polarization curves of the current density are plotted as a function of potential for a representative WS2 sample and shown in Figure 5C. We find a current density of 8.5 mA/cm2 at an overpotential of 200 mV, comparable to the values observed elsewhere. The inset of Figure 5C shows the onset potential of -97 mV (vs RH E). This shows that WS2 films produced through mechanical abrasion are suitable for HER catalyst applications. The overpotential is plotted in Figure 5D and the absolute value of the current density within a cathodic potential window and the corresponding Tafel fit is shown, red curve. Thus, the polarization curve shows an exponential behaviour, with the Tafel equation overpotential= a+b log | j | (where b represents the Tafel slope) and j is the current density. The better the catalyst material gives the highest currents at the smallest overpotential. For our WS2 films we find a Tafel slope of 111 mV/dec, Figure 5D. The reported Tafel slopes vary significantly for different studies depending strongly on the synthesis route of the WS2 films. For example, Bonde et al. reported the HER activity on carbon supported WS2 nanoparticles with Tafel slopes of 135 mV/dec. Xiao et al. used an electrochemical route to obtain the amorphous tungsten sulphide thin films onto nonporousgold, which the Tafel slope was 74 mV/dec. Chen et al. found a similar value (78 mV/dec) for the WS2 prepared at 1000 °C. Flowever, the synthesis route of these works involves high temperature processes and/or several steps to obtain the WS2 catalysts. While the WS2 catalysts exfoliated by mechanical abrasion are rapidly produced through a single low-cost step.
WS? Based Ionising Radiation Detector High energy photon-matter interactions including, the photoelectric effect (PE), Rayleigh scattering (RS) and Compton scattering (CS) depend strongly of the atomic mass, Z. Specifically the cross-section of such processes depends on the atomic mass as Z as ~Z45, Z2 and Z for the PE, RS and CS processes respectively. High energy gamma-ray radiation detectors have practical uses in various sectors from medical to space exploration. Ideally, they should be highly sensitive while being radiation hard (That is the response should not change greatly after prolonged exposure). The high atomic mass of tungsten in the compound WS2 makes it a stand-out choice for high energy radiation detection applications. This is because, high energy photon- matter interactions including, the photoelectric effect (PE), Rayleigh scattering (RS) and Compton scattering (CS) depend strongly of the atomic mass, Z. Specifically the cross-section of such processes depends on the atomic mass as Z as ~Z45, Z2 and Z for the PE, RS and CS processes respectively. To this end we fabricate a simple vertical heterostructure consisting of Au-WS2-aC as depicted in the inset to Figure 6A. A large bias of 5 V is then applied across the WS2 whilst monitoring the current. We then expose them to a high energy gamma radiation source; in this case we used a 6OC0 source which emits photons at 1.17 and 1.33 MeV. As soon as the sample is exposed to the radiation, we observe a rapid increase of the current as a result of photo-excited carriers, see the middle inset of Figure 6A. The sample is then exposed to a high dose amounting to 30 KGy. Initially the conductivity of the sample increases which we attribute to doping due to an increase of sulphur vacancies in the WS2 which are known to n-type dope the WS2. After ~10Gry's this increase saturates and the conductivity drops proportional to the log of the dose, loc-log(dose). This decrease is attributed to irreversible crystallographic damage of the WS2 at high exposure levels. After the prolonged exposure the device was removed from the radiation source and the temporal response once again measured, Figure 6B displays 5 successive 5 Gy exposures. These results indicate that abraded WS2 films could be useful for future high energy radiation detector applications, especially suited for high dose environments such as nuclear reactors or space probe applications.
Triboelectric nanogenerator The triboelectric effect in 2D materials has recently been reported. Those previous devices were mainly based on thin films based on liquid phase exfoliated. Here we demonstrate the use of mechanically abraded thin films for the use as a triboelectric effect. Figure 7A shows the schematic of the setup with a thin PET substrate with the abraded film or multilayer stack stuck to a large metallic plate which was kept grounded. A PDMS pad was then pressed onto the 2D material, which has the effect of charging the film. This sets a voltage relative to ground and this voltage drop is measured over a fixed resistor to calculate the current. We find that simple graphitic films can lead directly to a triboelectric effect with a power output of 0.36 mW when the pad is released from the 2D material. Furthermore, when a multilayer stack is produced such as Graphite/MoS2 then the effect is significantly increased with a max power output of 1.44 mW. Given the simplicity that the films can be produced makes mechanically abraded films and heterostructures suitable for future triboelectric generator devices.
As described hereinbefore, complex functional heterostructure devices can be built up from 2D nanocrystals through a simple mechanical abrasion method. This technology allows for the rapid up-scaling of heterostructures and it is demonstrated that it has practical use in several simple device applications including conductive few layer graphene coatings, photodetectors. We have extended the technology and demonstrated the successful creation of various more complex vertical heterostructure devices including, multi-layer photovoltaics. We also show that abraded WS2 coatings can be used directly as electro catalysts for the HER reaction. The ease with which the films can be applied, and simplicity of up-scalability makes this technology significantly attractive for a large variety of applications.
Whilst specific embodiments of the invention are described hereinbefore, it will be appreciated that a wide range of modifications and alterations may be made thereto without departing from the scope of the invention as defined by the appended claims.

Claims

CLAIMS:
1. A method for use in the fabrication of a van der Waals heterostructure, the method comprising the steps of applying a mask to a substrate, rubbing a first material against the substrate to deposit and apply, by abrasion, a first layer of the first material to the substrate, applying a second mask over the first layer, and rubbing a second material against the substrate to apply, by abrasion, a second layer of the second material over at least part of the first layer.
2. A method according to Claim 1, wherein the second material is less hard or less abrasive than the first material.
3. A method according to Claim 1 or Claim 2, wherein the steps of rubbing the first and second materials against the substrate are achieved using a rotatable head.
4. A method according to Claim 3, wherein the material to be deposited is applied to the rotatable head or between the substrate and the head, and the material is deposited onto the substrate by pressing the head against the substrate whilst rotating the head, and moving the head relative to the substrate to apply the material over an area of the substrate, thereby applying a layer of the material to the substrate.
5. A method according to any of the preceding claims, and further comprising adding one or more further layers using this technique.
6. A method according to any of the preceding claims, and further comprising transferring a prefabricated element to the substrate.
7. A method according to Claim 6, wherein the prefabricated element comprises a layer fabricated using a CVD technique.
8. A multi-layered structure fabricated using the method of any of the preceding claims.
9. A structure according to Claim 8 and comprising at least one of a photo detector, a light emitting device, a transistor, a capacitor, a resistor, a superconducting memory device, a single photon emitter located on a film, a triboelectric generators and a hydrogen evolution catalyst.
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