Patent Publication Number: US-2020303447-A1

Title: Apparatus for photodetection and manufacturing method thereof

Description:
TECHNOLOGICAL FIELD 
     Examples of the disclosure relate to an apparatus and method of forming an apparatus for photo-detection. In particular examples of the disclosure relate to an apparatus and method of forming an apparatus for photo-detection comprising graphene. 
     BACKGROUND 
     Apparatus comprising photodetectors have a wide range of uses. For instance they can be used in cameras, in infrared detectors, in x-ray detectors and in other suitable devices. 
     It is useful to provide an apparatus comprising photodetectors which provides an improved array of photodetectors for such apparatus. This may provide improved images and/or may provide a more versatile device. 
     BRIEF SUMMARY 
     According to various, but not necessarily all, examples of the disclosure there is provided an apparatus comprising: a substrate comprising a first surface, a second surface and a plurality of apertures extending through the substrate from the first surface to the second surface, wherein the plurality of apertures respectively have a first end and a second end and wherein the substrate is arranged such that one or more photons incident on the first end of a respective aperture pass through the respective aperture to the second end; and a plurality of photodetectors positioned overlaying the second end of the plurality of apertures, wherein the plurality of photodetectors comprises graphene functionalised with quantum dots, and wherein at least some of the plurality of apertures extend perpendicular to the plane of the second surface and the plane of the plurality of photodetectors. 
     The substrate may be arranged to absorb photons so that photons incident on an inner wall of an aperture are absorbed. 
     The graphene overlaying the second end of a respective aperture may be arranged as a channel within a field effect transistor. Contact electrodes of the field effect transistor may be formed on the substrate adjacent to the second end of a respective aperture. 
     The substrate may be flexible. 
     The substrate may be curved. 
     At least one of the plurality of apertures may be filled. 
     At least one of the plurality of apertures may be non-filled. The apparatus may comprise a support layer arranged to support the graphene over the second end of the aperture. The support layer may comprise hexagonal boron nitride. 
     The diameter of at least some of the plurality of apertures may be smaller than the length of those aperture. 
     The apparatus may comprise a scintillator overlaying the substrate and arranged to convert incident x-rays into visible photons. 
     The apparatus may be configured so that each of the plurality of apertures extend perpendicular to the plane of the second surface and the plane of the plurality of photodetectors. 
     According to various, but not necessarily all, examples of the disclosure there may be provided an x-ray detector comprising an apparatus as claimed in any preceding claim. 
     According to various, but not necessarily all, examples of the disclosure there may be provided a camera comprising an apparatus as claimed in any of claims  1  to  13 . 
     According to various, but not necessarily all, examples of the disclosure there may be provided a method comprising: providing a substrate comprising a first surface, a second surface and a plurality of apertures extending through the substrate from the first surface to the second surface, wherein the plurality of apertures have a first end and a second end and wherein the substrate is arranged such that one or more photons incident on the first end of a respective aperture pass through the respective aperture to the second end; and positioning graphene functionalised with quantum dots over the second end of the plurality of apertures to form a plurality of photodetectors, wherein at least some of the plurality of apertures extend perpendicular to the plane of the second surface and the plane of the plurality of photodetectors. 
     The substrate may be arranged to absorb photons so that photons incident on an inner wall of an aperture are absorbed. 
     The method may comprise arranging the graphene overlaying the second end of a respective aperture as a channel within a field effect transistor. The method may comprise forming contact electrodes of the field effect transistor on the substrate adjacent to the second end of a respective aperture. 
     The substrate may be flexible. 
     The substrate may be curved. 
     At least one of the plurality of apertures may be filled. 
     At least one of the plurality of apertures may be non-filled. The method may comprise providing a support layer arranged to support the graphene over the second end of the aperture. The support layer may comprise hexagonal boron nitride. 
     The diameter of at least some of the plurality of the apertures may be smaller than the length of those aperture. 
     The method may comprise providing a scintillator overlaying the substrate and arranged to convert incident x-rays into visible photons. 
     The method may be configured so that each of the plurality of apertures extend perpendicular to the plane of the second surface and the plane of the plurality of photodetectors. 
     According to various, but not necessarily all, examples of the disclosure, there are provided examples as claimed in the appended claims. 
    
    
     
       BRIEF DESCRIPTION 
       For a better understanding of various examples that are useful for understanding the detailed description, reference will now be made by way of example only to the accompanying drawings in which: 
         FIG. 1  illustrates an apparatus: 
         FIGS. 2A to 2D  illustrate an apparatus; 
         FIGS. 3A to 3B  illustrate an apparatus; 
         FIG. 4  illustrates an apparatus in use as a camera; 
         FIG. 5  illustrates an apparatus with a curved substrate; 
         FIG. 6  illustrates an apparatus in use as an x-ray detector; 
         FIG. 7  illustrates another apparatus with a curved substrate; 
         FIG. 8  illustrates an example x-ray detector in use with a curved substrate; 
         FIG. 9  is a geometric representation of the field of view for each photodetector pixel within an apparatus; 
         FIG. 10  is a geometric representation of the field of view for each photodetector pixel within an apparatus with a curved substrate; 
         FIGS. 11A and 11B  illustrate an example apparatus with a curved substrate; and 
         FIG. 12  illustrates an example method. 
     
    
    
     DETAILED DESCRIPTION 
     The Figures illustrate an apparatus  1  comprising: a substrate  3  comprising at least one aperture  5  extending through the substrate  3  wherein the aperture  5  has a first end  7  and a second end  9  and wherein the substrate  3  is arranged such that one or more photons  17  incident on the first end  7  of the aperture  5  pass through the aperture  5  to the second end  9 ; and a photodetector  11  positioned overlaying the second end  9  of the aperture  5  wherein the photodetector  11  comprises graphene  13  functionalised with quantum dots  15 . 
     The apparatus  1  may be for detecting photons. A plurality of photodetectors  11  may be arranged in an array on the substrate  3  to enable an image to be obtained. The apertures  5  within the substrate  3  may be arranged so that the images that are obtained by the array of photodetectors  11  are in focus. The apparatus  1  may be used in a lensless camera, an x-ray detector, an infrared detector or any other suitable device. 
       FIG. 1  schematically illustrates an apparatus  1  according to an example of the disclosure. The apparatus  1  comprises a substrate  3  and a photodetector  11 . 
     The substrate  3  comprises a support layer upon which one or more photodetectors can be provided. In the example of  FIG. 1  the substrate  3  is arranged in a flat configuration, in other examples the substrate  3  could be curved. In some examples the substrate  3  may be flexible so that the curvature of the substrate  3  may be controlled by a user of the apparatus  1 . In such examples the material and/or the thickness of the substrate  3  may be selected to ensure that a user can bend and/or flex and/or otherwise deform the substrate  3 . 
     The substrate  3  may be formed from an insulating material so that photodetectors  11  and/or other electronic components can be provided on the substrate  3 . In some examples the substrate  3  may be formed from a material such as silicon, polymer or any other suitable material. In some examples the substrate  3  may comprise a thin film metal substrate  3  or any other suitable material. 
     The substrate  3  has a first surface  6  and a second surface  8 . The surfaces  6 ,  8  are provided on opposite sides of the substrate  3 . The surfaces  6 ,  8  may provide surfaces upon which electronic components may be provided. The electronic components may be formed on the surfaces  6 ,  8 . For instance electrical contacts and conductive traces could be printed onto the surfaces  6 ,  8  as needed. In the example of  FIG. 1  the photodetector  11  is provided on the second face  8 . 
     In examples of the disclosure the substrate  3  comprises at least one aperture  5 . The aperture  5  comprises a thru hole which extends all the way through the thickness of the substrate  3 . The aperture  5  has a first end  7  which is provided on the first surface  6  of the substrate  3  and a second end  9  which is provided on the second surface  8  of the substrate  3 . 
     The aperture  5  may be formed within the substrate  3  using any suitable process. The aperture  5  may be formed using a laser drill or any suitable means. 
     The length of the aperture  5  is the distance between the first end  7  and the second end  9 . In the example of  FIG. 1  the length of the aperture  5  corresponds to the thickness of the substrate  3 . 
     In examples of the disclosure the length of the aperture  5  may be greater than the diameter of the aperture  5 . The length of the aperture  5  may be multiple times larger than the diameter of the aperture  5  so that an elongate aperture  5  is provided. 
     The aperture  5  may be any suitable size. In some examples the aperture  5  may have a diameter of around 50 micrometers. The size of the aperture  5  may be determined by the process that is used to form the aperture  5 . 
     The aperture  5  within the substrate  3  is arranged so that only photons  17  arriving from a particular direction pass through the aperture  5  from the first end  7  to second end  9 . The aperture  5  within the substrate  3  is arranged so that only photons  17  aligned with the length of the aperture  5  pass through the aperture from the first end  7  to the second end  9 . The aperture  5  is arranged so that photons  17  that are aligned with the length of the aperture  5  pass through the aperture  5  without being incident on a side wall of the aperture  5 . 
     The aperture  5  may be arranged so that photons  17  that are not aligned with the length of the aperture  5  are absorbed before they reach the second end  9  of the aperture  5 . The photons  17  that are not aligned with the length of aperture  5  are incident on an inner wall of the aperture  5 . The substrate  3  may comprise a material which is arranged to absorb the photons  17  that are incident on the inner wall of the aperture  5 . In some examples a coating may be provided on the inner wall of the aperture  5  which may be arranged to absorb the incident photons  17 . 
     The apparatus  1  also comprises a photodetector  11  comprising graphene  13  functionalised with quantum dots  15 . 
     The graphene  13  may be provided between source and drain electrodes. The graphene  13  may be provided as a channel in a field effect transistor. The graphene  13  may be provided in a thin layer. In some examples the graphene  13  may have a thickness in the range of nanometers. In some examples the graphene  13  may comprise an atomic monolayer. 
     In the example apparatus  1  of  FIG. 1  quantum dots  15  are coupled to the graphene  13 . In the example of  FIG. 1  the quantum dots  15  are provided overlaying the graphene  13 . In other examples the quantum dots  15  may be provided within and/or adjacent to the graphene channel  13 . 
     The quantum dots  15  may comprise a nanocrystal in which there is quantum confinement in all three dimensions. The quantum dots  15  may comprise any suitable material. The material that is used for the quantum dots  15  may be chosen to enable photons  17  to be detected. In some examples the quantum dots  15  may comprise lead sulphide, cadmium sulphide, cadmium selenide, or any other suitable material. 
     The quantum dots  15  are arranged so that photons  17  which pass through the length of the aperture  5  are incident on the quantum dots  15 . The quantum dots  15  may be provided on one side of the graphene  13 . In the example of  FIG. 1  the quantum dots  15  are provide on the side of the graphene  13  which faces the substrate  3 . In other examples the quantum dots  15  could be provided on the side of the graphene  13  which faces away from the substrate  3 . 
     The quantum dots  15  convert incident photons  17  into electrical charge. The changes in charge distribution caused by the incident photons  17  may be detected by the graphene  13  which produces a measurable electronic response. The electronic response provides an indication of photons  17  which pass through the aperture  5 . This enables the apparatus  5  to be arranged to detect photons  17  which are aligned with the aperture  5 . An array of apertures  5  and photodetectors  11  may be arranged to obtain an image. As only photons  17  that are aligned with the apertures  5  are detected the image is in focus. 
       FIGS. 2A to 2D  illustrate an example apparatus  1  which comprises a plurality of apertures  5 . A photodetector  11  is positioned overlaying each of the apertures  5 . 
       FIG. 2A  illustrates a second surface  8  of a substrate  3  before the graphene  13  is provided. The substrate  3  comprises a plurality of apertures  5  arranged in an array  21 . The apertures  5  may extend through the substrate  3  as described in relation to  FIG. 1 . The second ends  9  of the apertures  5  are provided in the second surface  8  of the substrate  3 . 
     In the example of  FIG. 2A  the array  21  comprises eighteen apertures  5  arranged in a plurality of perpendicular rows and columns. It is to be appreciated that any number of apertures  5  may be provided in other examples of the disclosure. The plurality of apertures  5  may be arranged in any suitable arrangement in other examples of the disclosure. 
     In the example of  FIG. 2A  each of the apertures  5  has a circular cross section. The shape of the apertures  5  may be determined by the process that is used to form the aperture  5 . It is to be appreciated that other shapes may be used in other examples of the disclosure. 
     In the example of  FIG. 2A  a pair of contact electrodes  23  are provided adjacent to each of the apertures  5 . The contact electrodes  23  may be arranged to provide a source and drain electrode for a field effect transistor. 
     The contact electrodes  23  may be made of any suitable material such as gold, silver, nickel, aluminium, copper or any other conductive material including alloys such as aluminium-copper (Al—Cu). The contact electrodes  23  may be formed directly onto the second surface  8  of the substrate  3 . In some examples the contact electrodes  23  may be printed onto the second surface  8  of the substrate  3 . 
     In some examples other circuitry may also be provided on the second surface  8  of the substrate  3 . The other circuitry may comprise read-out circuitry, data processing circuitry or any other suitable circuitry. 
       FIG. 2B  illustrates the first surface  6  of the substrate  3  before the graphene  13  is provided. The substrate  3  comprises the array  21  of apertures  5  as illustrated in  FIG. 2A . The first ends  7  of the apertures  5  are provided in the first side  6  of the substrate  8 . 
       FIG. 2C  illustrates the second surface  8  of a substrate  3  after the graphene  13  is provided. The graphene  13  may be formed by any suitable method and transferred to the second surface  8  of the substrate  3 . 
     The graphene  13  is provided overlaying the contact electrodes  23  so that each of the pairs of contact electrodes  23  and the layers of graphene  13  form a field effect transistor. Each of the field effect transistors forms a photodetector  11  so that the apparatus  1  comprises a plurality of photodetectors  11 . 
     The graphene  13  is provided overlaying the second end  9  of the aperture  5 . The graphene  13  may be provided overlaying the second end  9  of the aperture  5  so that the graphene  13  is suspended over the aperture  5 . In some examples a support layer may be provided to support the graphene  13  over the aperture  5 . 
     In the example of  FIG. 2C  the graphene  13  completely covers the second end  9  of the aperture  5  so that any photons  17  that pass through the aperture  5  are incident on the graphene  13 . The aperture  5  may be arranged as described in relation to  FIG. 1  so that only photons  17  that are aligned with the length of the aperture  5  are incident on the graphene  13 . 
     The graphene  13  is functionalised with quantum dots  15 , which may be as described above, so that photons  17  that pass through the aperture  5  are converted into an electrical charge. 
       FIG. 2D  illustrates the first surface  6  of the substrate  3  after the graphene  13  is positioned overlaying the contact electrodes  23 . The graphene  13  can be seen through the apertures  5 . 
     Each of the photodetectors  11  in the apparatus  1  acts as a pixel. This may enable an image to be formed from the signals provided by each of the photodetectors  11  in response to incident photons  17 . As only photons  17  that are aligned with the aperture  5  are incident on the photodetector  11  this ensures that the image obtained by the array of photodetectors  11  is in focus. 
       FIGS. 3A and 3B  illustrate a cross section of an example apparatus  1 . Apparatus  1  could be a section of the apparatus  1  as illustrated in  FIGS. 2A to 2D . 
       FIG. 3A  illustrates a cross section of a substrate  3  before the graphene  13  is provided. The substrate  3  comprises a plurality of apertures  5  which extend from the first surface  6  of the substrate  3  to the second surface  8 . 
     In the example of  FIG. 3A  the substrate  3  is flat and each of the apertures  5  are parallel to each other. The apertures  5  are parallel to each other such that each of the lengths of the apertures  5  extend parallel to the lengths of the other apertures  5 . The apertures  5  extend perpendicular to the plane of the second surface  8 . Once the photodetectors  11  are formed the apertures  5  extend perpendicular to the plane of the photodetectors  11 . 
     In the example of  FIG. 3A  a pair of contact electrodes  23  are provided on the second surface  8  of substrate  3  adjacent to each of the apertures  5 . The contact electrodes  23  may be arranged to provide a source and drain electrode for a field effect transistor as described above. 
       FIG. 3B  illustrates a cross section of the substrate  3  after the graphene  13  is provided. The graphene  13  is provided overlaying the contact electrodes  23  so that each of the pairs of contact electrodes  23  and the layers of graphene  13  form a field effect transistor which may be as described above. 
     The graphene  13  is functionalised with quantum dots  15  so that photons  17  that pass through the aperture  5  are converted into an electrical charge. The quantum dots  15  may be patterned on to the graphene  13  after the graphene  13  has been transferred to the substrate  3 . 
     The quantum dots  15  may be provided on either side of the graphene  13 . In some examples the quantum dots  15  may be provided on the side of the graphene  13  which faces towards the substrate  3 . In other examples the quantum dots  15  may be provided on the side of the graphene  13  which faces away from the substrate  3 . 
     If the quantum dots  15  are provided on the side of the graphene  13  which faces towards the substrate  3  the quantum dots  15  may be patterned through onto the graphene  13  through the apertures  5 . This may ensure that the quantum dots  15  are provided in the areas where photons  17  can be detected. 
     In the examples of  FIGS. 2A to 3B  the graphene  13  and contact electrodes  23  are formed on the substrate  3  around the apertures  5 . This ensures that the photodetectors  11  are aligned with the apertures  5 . This method of forming the photodetectors  11  may enable alignment of the photodetectors  11  and the apertures  5  even when the apertures are very small. This may also enable a small separation between the photodetectors  11 . This may allow for a small pixel pitch for the array of photodetectors  11 . Using a small pixel pitch may increase the resolution of images obtained using the apparatus  1 . 
     In the example of  FIGS. 2A to 3B  the graphene  13  is suspended over the second end  9  of the aperture  5 . The apertures  5  are unfilled so that there is nothing within the aperture  5  which supports the graphene  13 . In some examples the apparatus  1  may comprise a support layer to support the graphene  13 . In some examples the support layer could be provided on the second surface  8  of the substrate  3  between the substrate  3  and the graphene  13 . In such examples the support layer is transparent to enable photons  17  to pass through the support layer to the graphene  13 . In some examples the support layer may comprise hexagonal boron nitride. Hexagonal boron nitride may be suitable for use as a support layer as it has a structure very similar to the structure of graphene  13 . This may reduce any defects introduced into the graphene  13  by the support layer. 
     In some examples the support layer may be provided on the outer side of the graphene  13  so that the graphene  13  is sandwiched between the substrate  3  and the support layer. In such examples the support layer does not need to be transparent. In some examples the support layer may be opaque to prevent photons  17  passing through the support layer onto the graphene  13 . This may ensure that only photons  17  that have passed through the apertures  5  are detected by the photodetectors  11 . In some examples the support layer may form part of the encapsulation of the apparatus  1 . 
     In some examples the apertures  5  may be filled or at least partially filled. The apertures  5  may be filled with a material which is transparent to photons  17 . The material that is used to fill the apertures may also support the graphene  13  overlaying the second end  9  of the aperture  5 . This optically transparent material may also form part of the encapsulation of the apparatus  1 . 
     In the example of  FIGS. 2A to 3B  the photodetectors  11  comprise two contact electrodes  23  formed on the second surface  8  of the substrate  3 . In other examples other arrangements of electrodes may be used. 
       FIG. 4  illustrates an apparatus  1  in use as camera. The camera may be a lensless camera as the substrate  3  and apertures  5  focus the image obtained by the camera by filtering photons which are not aligned with the apertures  5 . 
     In the example of  FIG. 4  the apparatus  1  comprises a flat substrate  3  with a plurality of apertures  5  and photodetectors  11  which may be as described above. In the example of  FIG. 4  only three apertures  5  and photodetectors  11  are illustrated. It is to be appreciated that in other examples other numbers of apertures  5  and photodetectors  11  may be used. 
     The object  41  which is to be imaged is positioned in front of the apparatus  1 . Photons  17  from the object  41  are directed in different directions. Photons  17  that are aligned with the apertures  5  are indicated by solid lines  43  and photons  17  that are not aligned with the apertures  5  are indicated by dotted lines  45 . It is to be appreciated that photons  17  directed in a particular direction may also be referred to as light rays directed in a particular direction. 
     As indicated by the dotted lines  45  photons  17  that are not aligned with the apertures  5  are incident on the substrate  3 . The photons  17  are incident on the inner walls of the substrate  3 . The substrate  3  may be made of a material which absorbs the photons  17  so that these photons  17  are absorbed and prevented from passing through the aperture  5  to the photodetector  11 . In some examples a coating may be provided on the inner walls of the apertures  5  to absorb the photons  17  that are not aligned with the aperture  5 . The coating may comprise a matt black material such as a carbon coating or any other suitable material. 
     In some examples the inner walls of the aperture  5  may have a rough surface so that any photons  17  incident on the inner walls are redirected. This may attenuate the photons  17  before they are incident on the photodetector  11  and/or may reflect the photons  17  back out of the aperture  5 . 
     Photons  17  which are aligned with the apertures  5  pass through the apertures  5  and are incident on the photodetector  11 . This enables a focussed image of the object  41  to be obtained by the array of photodetectors  11 . 
     The diameter of the aperture  5  and the thickness of the substrate  3  affects the number of photons  17  that are absorbed before they are incident on the photodetector  11 . The longer the length of the aperture  5  the fewer photons  17  will be adequately aligned with the length of the aperture  5  so that they will be detected by the photodetector  13 . Similarly the narrower the diameter of the aperture  5  the fewer photons  17  will be adequately aligned with the length of the aperture  5 . The thickness of the substrate  3  and/or the diameter of the apertures  5  may be selected so as to provide a desired level of sensitivity for the apparatus  1 . 
     In the example of  FIG. 4  the substrate  3  is flat and the apertures  5  are perpendicular to the array of photodetectors  11 . The resolution d obj  of an image of an object obtained by the apparatus is equal to the pixel pitch d pix : 
       d obj =d pix . 
     This may enable a lower pixel pitch to be used to achieve the same level of resolution than would be used in a camera which uses lenses to focus the image. This may be useful if the apparatus  1  is formed on a large area substrate  3 . 
       FIG. 5  illustrates another apparatus  1  in use as a lensless camera. In the example of  FIG. 5  the apparatus  1  comprises a curved substrate  3 . 
     In the example of  FIG. 5  the substrate  3  is curved so that the first surface  6  of the substrate  3  is convex and the second surface  8  of the substrate  3  is concave. The photodetectors  11  are provided on the concave second surface  8 . 
     In some examples the substrate  3  may comprise a rigid substrate that has a fixed radius of curvature. This may enable the radius of curvature of the substrate  3  to be optimised for imaging a particular type of objects  41 . 
     In some examples the substrate  3  may comprise a flexible substrate  3  which may be arranged to enable a user to adjust the radius of curvature of the substrate  3 . This may enable the apparatus  1  to be adjusted for use with different objects  41 . The substrate  3  may be arranged to enable a user to bend the substrate to obtain the desired field of view for the apparatus  1 . 
     In the example of  FIG. 5  the curvature of the substrate  3  increase the field of view compared to the flat substrate  3  used in the example of  FIG. 4 . The resolution d obj  of an image of an object obtained by the apparatus is greater than the pixel pitch d pix : 
       d obj &gt;d pix    
       FIG. 6  illustrates another apparatus  1  in use as an x-ray detector. The x-ray detector may be used for medical imaging, security purposes or any other suitable applications. 
     In the example of  FIG. 6  an x-ray source  61  is provided. The x-ray source  61  may comprise any suitable means for providing x-rays  67 . 
     An object  41  that is to be imaged is positioned between the x-ray source  61  and the apparatus  1 . The object  41  is positioned between the x-ray source  61  and the apparatus  1  so that x-rays  67  that are not absorbed by the object  41  pass through the object  41  and are incident on the apparatus  1 . 
     The apparatus  1  may be positioned relative to the x-ray source  61  so that the x-rays  67  are parallel or substantially parallel when they are incident on the apparatus  1 . 
     The example apparatus  1  in  FIG. 6  comprises a substrate  3  and a plurality of photodetectors  11  which may be as described above. In the example of  FIG. 6  the substrate  3  is a flat or substantially flat substrate  3 . 
     In the example of  FIG. 6  the apparatus also comprises a scintillator  63 . The scintillator  63  may comprise any means which may be arranged to convert  65  incident x-rays  67  into visible photons  17  that can be detected by the photodetectors  11 . The scintillator  63  is provided overlaying the substrate  3  so that x-rays  67  incident on the apparatus  1  are incident on the scintillator  63 . The photons that are emitted by the scintillator  63  may then be detected by the photodetectors  11  of the apparatus  1 . 
     The x-rays  67  that are incident on the scintillator  63  may be perpendicular to the surface of the scintillator  63 . When the x-rays  67  are converted  65  to visible photons  17  the photons  17  may be emitted omnidirectionally. However as only photons  17  that are aligned with the aperture  5  pass through the aperture  5  this means that only the photons  17  in a direction aligned with apertures  5  will be detected by the photodetectors  11 . 
     In some examples the scintillator  63  may be a thick layer. The thick layer may be several millimeters thick. The scattering of photons  17  within the scintillator  63  will normally degrade the resolution of images obtained by such devices. Such thick scintillators typically require a columnar structure to reduce scattering and guide photons  17  towards a photodetector  11 . In the example of the disclosure this is not necessary as the apertures  5  in the substrate  3  act to filter out the photons  17  that have been scattered. 
     As the graphene  13  based photodetectors  11  are very sensitive to incident photons  17  the x-ray imaging device can still obtain high quality images even with the substrate  3  filtering out photons  17  that are not aligned correctly. 
     In some examples the scintillator  63  may comprise a reflective upper surface. The reflective upper surface may act to reflect photons  17  generated within the scintillator  63  back towards the substrate  3  and the photodetectors  11 . This may improve the efficiency of the apparatus  1 . 
       FIG. 7  illustrates another apparatus  1  in use as an x-ray detector. In the example of  FIG. 7  the apparatus  1  comprises a curved substrate  3  and a curved scintillator  63 . 
     In the example of  FIG. 7  the substrate  3  and the scintillator  63  are curved so that the first surface  6  of the substrate  3  is convex and the second surface  8  of the substrate  3  is concave. The photodetectors  11  are provided on the concave second surface  8 . 
     The scintillator  63  may comprise a flexible material which may enable the scintillator  63  to be flexed and/or deformed with the substrate  3 . In other examples the scintillator  63  and substrate  3  could have a fixed radius of curvature. 
     In the example of  FIG. 7  the curvature of the substrate  3  increases the field of view compared to the flat substrate  3  used in the example of  FIG. 6 . The resolution d obj , of an image of an object obtained by the apparatus is greater than the pixel pitch d pix : 
       d obj &gt;d pix    
     It is to be appreciated that in other examples the apparatus  1  could be arranged into any suitable shape.  FIG. 8  illustrates another example apparatus  1  in which the substrate  3  and the scintillator  63  are curved so that the first surface  6  of the substrate  3  is concave and the second surface  8  of the substrate  3  is convex. In such an arrangement the photodetectors  11  would be provided on the convex second surface  8 . This may enable the apparatus to be curved around an object  41 . This may enable the apparatus  1  to be used to partially or fully enclose the object being imaged. 
     Having a flexible apparatus  1  for photo-detection may enable the shape of the apparatus  1  to be adapted to correspond to the shape of the object  41  being imaged. This may improve the efficiency of the imaging device which may reduce the x-ray dosage required. A reduction in the x-ray dosage required is particularly beneficial for medical applications. 
     In examples of the disclosure each photodetector  11  provides a pixel within an imaging array.  FIG. 9  is a geometric representation of the field of view for each photodetector pixel within an example apparatus  1 . 
     In the example of  FIG. 9  the image plane  81  is indicated by the dashed line. The image plane  81  corresponds to the distance L opt  from the apparatus  1  at which the paths of photons  17  that are incident on adjacent photodetectors  11  intersect. Any source of scattered omnidirectional photons  17  located beyond L opt  could have photons  17  detected by more than one photodetector  11 . This reduces the resolution for images positioned beyond L opt . 
       FIG. 9  illustrates the angle of view α view  for each photodetector  11 . The angle of view α view  can be expressed as: 
     
       
         
           
             
               α 
               view 
             
             = 
             
               2 
                
               
                 arctan 
                  
                 
                   ( 
                   
                     
                       d 
                       via 
                     
                     
                       d 
                       subs 
                     
                   
                   ) 
                 
               
             
           
         
       
     
     Where d via  is the diameter of the aperture  5  and d subs  is the thickness of the substrate  3 . The thickness of the substrate  3  is the same as the length of the aperture  5 . 
     At the image plane  81  the field of view of two adjacent photodetectors  11  overlaps. The distance between the points at which the paths of photons  17  intersect is the same as the pixel pitch d pix . This gives: 
     
       
         
           
             
               
                 
                   ( 
                   
                     
                       L 
                       opt 
                     
                     + 
                     
                       
                         d 
                         
                           s 
                            
                           u 
                            
                           b 
                            
                           s 
                         
                       
                       2 
                     
                   
                   ) 
                 
                 · 
                 
                   tan 
                    
                   
                     ( 
                     
                       
                         α 
                         view 
                       
                       2 
                     
                     ) 
                   
                 
               
               = 
               
                 
                   d 
                   pix 
                 
                 2 
               
             
              
             
                 
             
           
         
       
       
         
           hence 
         
       
       
         
           
             
               L 
               opt 
             
             = 
             
               
                 
                   d 
                   
                     s 
                      
                     u 
                      
                     b 
                      
                     s 
                   
                 
                 2 
               
                
               
                 ( 
                 
                   
                     
                       d 
                       pix 
                     
                     
                       d 
                       via 
                     
                   
                   - 
                   1 
                 
                 ) 
               
             
           
         
       
     
     Therefore the optimal distance between the image plane  81  and the apparatus  1  can be selected by choosing appropriate dimensions for the thickness of the substrate  3  and the diameter of the apertures  5 . This may enable the apparatus  1  to be optimized for a particular application. 
     The distance that is selected for L opt  may depend on the application that the apparatus  1  is being used for. For instance, if the apparatus  1  is being used for x-ray imaging a large L opt  is not needed because the photons  17  that are detected are emitted from the scintillator  63  which is positioned overlaying the substrate  3 . 
     In such examples typical values for the parameters of the apparatus  1  could be: d pix =100 μm, d via =25 μm, d subs =300 μm. This gives L opt =450 μm. A different value of L opt  could be obtained using different parameter values. For instance if d pix =300 μm, d via =30 μm, d subs =500 μm this gives L opt =2250 μm. It is to be appreciated that any suitable value for L opt  could be obtained by using any suitable parameters for the apparatus  1 . 
     Apparatus  1  having a flat substrate  3  may be suitable for use in near field imaging applications such as x-ray detection. Other near field imaging applications could be medical applications such as vein mapping where an apparatus  1  may be placed in contact with a patient&#39;s skin and the area is illuminated with a light source such as one or more light emitting diodes (LEDs). 
       FIG. 10  is a geometric representation of the field of view for each photodetector pixel within an example apparatus  1  with a curved substrate  3 . In the example of  FIG. 10  the substrate  3  is curved so that the second surface  8  on which the photodetectors  11  is concave. The substrate  3  has radius of curvature R bend . 
     As the substrate  3  is now curved the image plane  81  is replaced by a curved image surface  91 . The curved image surface  91  is indicated by the dashed line in  FIG. 10 . The curved image surface  91  corresponds to the distance L opt  from the apparatus  1  at which the paths of photons  17  that are incident on adjacent photodetectors  11  intersect. As, in the example of  FIG. 9  any source of scattered of omnidirectional photons  17  located beyond L opt  could have photons  17  detected by more than one photodetector  11  so this reduces the resolution for images positioned beyond L opt . 
     In examples where the substrate  3  is curved, the thickness d subs  of the substrate  3 , the diameter d via  of the apertures  5 , the pixel pitch d pix  and the radius of curvature R bend  of the substrate  3  can all be selected to adjust the value of L opt . 
     As L opt  increases this introduces areas  93  on the curved image surface  91  where no photons  17  are emitted in alignment with the apertures  5 . This means that no photons  17  from these areas  93  will be detected by the apparatus  1 . This means that portions of the field of view will be missing from the detected image. 
     In the example arrangement of  FIG. 10  the angle θ between two adjacent photodetectors  11  is given by 
     
       
         
           
             θ 
             = 
             
               
                 d 
                 pix 
               
               
                 R 
                 bend 
               
             
           
         
       
     
     The curved image surface  91  at a distance L opt  which provides a non-overlapping field of view for each photodetector  11  satisfies: 
     
       
         
           
             
               
                 α 
                 view 
               
               · 
               
                 ( 
                 
                   
                     L 
                     opt 
                   
                   + 
                   
                     
                       d 
                       subs 
                     
                     2 
                   
                 
                 ) 
               
             
             = 
             
               θ 
               · 
               
                 
                   ( 
                   
                     
                       R 
                       bend 
                     
                     + 
                     
                       d 
                       subs 
                     
                     + 
                     
                       L 
                       opt 
                     
                   
                   ) 
                 
                 . 
               
             
           
         
       
     
     This can be re-written as 
     
       
         
           
             
               L 
               opt 
             
             = 
             
               
                 
                   
                     
                       d 
                       subs 
                     
                      
                     
                       ( 
                       
                         θ 
                         - 
                         
                           
                             α 
                             view 
                           
                           2 
                         
                       
                       ) 
                     
                   
                   + 
                   
                     θ 
                     · 
                     
                       R 
                       bend 
                     
                   
                 
                 
                   ( 
                   
                     
                       α 
                       view 
                     
                     - 
                     θ 
                   
                   ) 
                 
               
               = 
               
                 
                   
                     
                       
                         d 
                         subs 
                       
                        
                       
                         ( 
                         
                           
                             
                               d 
                               pix 
                             
                             
                               R 
                               bend 
                             
                           
                           - 
                           
                             
                               α 
                               view 
                             
                             2 
                           
                         
                         ) 
                       
                     
                     + 
                     
                       d 
                       pix 
                     
                   
                   
                     ( 
                     
                       
                         α 
                         view 
                       
                       - 
                       
                         
                           d 
                           pix 
                         
                         
                           R 
                           bend 
                         
                       
                     
                     ) 
                   
                 
                 . 
               
             
           
         
       
     
     By approximating 
     
       
         
           
             
               
                 α 
                 view 
               
               ≈ 
               
                 2 
                 · 
                 
                   
                     d 
                     via 
                   
                   
                     d 
                     subs 
                   
                 
               
             
             , 
           
         
       
     
     L opt  can be expressed as 
     
       
         
           
             
               L 
               opt 
             
             = 
             
               
                 
                   
                     d 
                     
                       s 
                        
                       u 
                        
                       b 
                        
                       s 
                     
                   
                    
                   
                     
                       d 
                       pix 
                     
                     
                       R 
                       bend 
                     
                   
                 
                 + 
                 
                   d 
                   via 
                 
                 + 
                 
                   d 
                   pix 
                 
               
               
                 ( 
                 
                   
                     2 
                      
                     
                       
                         d 
                         via 
                       
                       
                         d 
                         subs 
                       
                     
                   
                   - 
                   
                     
                       d 
                       pix 
                     
                     
                       R 
                       bend 
                     
                   
                 
                 ) 
               
             
           
         
       
     
     From the above equation, it is evident that 
     
       
         
           
             
               
                 L 
                 opt 
               
               → 
               ∞ 
             
             , 
             
               
                 when 
                  
                 
                     
                 
                  
                 
                   
                     R 
                     bend 
                   
                   
                     d 
                     subs 
                   
                 
               
               = 
               
                 
                   1 
                   2 
                 
                  
                 
                   
                     
                       d 
                       pix 
                     
                     
                       d 
                       via 
                     
                   
                   . 
                 
               
             
           
         
       
     
     Therefore the optimal distance between the curved image surface  91  and the apparatus  1  can be selected by choosing appropriate dimensions for the thickness of the substrate  3  and the diameter of the apertures  5  and the radius of curvature of the substrate  3 . This may enable the apparatus  1  to be optimized for a particular application. 
     The distance that is selected for L opt  may depend on the application that the apparatus  1  is being used for. Some example figures for various cases are listed below:
         (i) L opt →∞ with d pix =200 μm, d via =30 μm, d subs =300 μm, R bend =1 mm. With these dimensions, a full circle of 2πR bend  would comprise 31 pixels.   (ii) L opt →∞ with d pix =400 μm, d via =30 μm, d subs =375 μm, R bend =2.5 mm. With these dimensions, the full circle of 2πR bend  would comprise 39 pixels.   (iii) L opt →∞ with d pix =1 mm, d via =100 μm, d subs =20 mm, R ben =10 cm.       

     It is to be appreciated that the listed examples are for illustrative purpose and that other values could be used in other examples of the disclosure. 
     These different examples could be used for different applications. For instance, if the values of example (ii) were used, the apparatus  1  would be suitable for use as a pill type camera. The pill type camera would be small enough to be swallowed by a patient without discomfort. In such cases the camera may comprise a light source to enable internal images of the patient to be obtained. In such cases the apparatus  1  may be arranged to enable three dimensional images to be obtained. 
     The values of example (iii) may be used to provide a spherical camera. In such examples a spherical substrate  3  may be moulded with the graphene  13  photodetectors  11  on the inner surface of the sphere. The apertures  5  could then be drilled into the surface of the sphere. With the values of example (iii) such an apparatus would comprise 628 photodetectors  11  around the surface of the sphere. 
       FIGS. 11A and 11B  illustrate an example apparatus  1  with a curved substrate. 
       FIG. 11A  schematically illustrates an apparatus  1  where the substrate  3  is provided in a cylindrical shape. This may enable the apparatus  1  to be used to detect photons arriving from any direction to provide a full 360° field of view. The cylindrical shape may be formed by rolling a flat substrate  3  into a cylinder. In other examples the substrate  3  may be formed in a cylindrical arrangement. 
       FIG. 11B  illustrates the projection of the image obtained by the three dimensional camera into a two dimensional format. Once the image has been converted into a two dimensional format it may be displayed on a display. 
       FIG. 12  illustrates an example method which may be used to form example apparatus  1  such as those described above. 
     The method comprises, at block  121  providing a substrate  3  comprising at least one aperture  5  extending through the substrate  3  wherein the aperture  5  has a first end  7  and a second end  9  and wherein the substrate  3  is arranged such that one or more photons  17  incident on the first end  7  of the aperture  5  pass through the aperture  5  to the second end  9 . The method also comprises, at block  123 , positioning graphene  13  functionalised with quantum dots  15  over the second end  9  of the aperture  5  to form a photodetector  11 . 
     Examples of the disclosure provide an improved photodetector  11  that may be used as an imaging device. 
     In some examples the apparatus  1  may be provided within a lensless camera. As the camera is lensless this reduces the number of components in the apparatus  1 . The field of view of the camera may be adjusted by adjusting the radius of curvature of the substrate  3 . 
     In examples of the disclosure the pixel size is determined by the size of the photodetectors  11 . The arrangements of the apparatus  1  in examples of the disclosure may enable larger pixel sizes and pixel pitches to be used in current CMOS (Complementary metal-oxide semiconductor) detectors which would be used in cameras with a lens system. This may enable lower cost technologies such as flexible printed circuit boards and quantum dot-graphene field effect transistor panels to be used. 
     In examples of the disclosure the graphene  13  may be suspended over the second end  9  of the aperture  5 . As the graphene  13  is suspended this may reduce any defects or discontinuities that could be introduced into the graphene. This could improve the quality of the graphene channel and reduce noise in the signals obtained by the photodetectors  11 . 
     The use of graphene  13  also provides the advantage that it is flexible and suitable for use in flexible circuits. This makes the quantum dot-graphene field effect transistor suitable for use in flexible and curved apparatus  1 . 
     The quantum dot-graphene field effect transistors are also very sensitive photodetectors  11  and may enable images to be provided even for low levels of incident photons  17 . 
     The use of the apertures  5  within the substrate  3  allows photons  17  that are not aligned with the apertures  5  to be filtered out. This makes the apparatus  1  suitable for use with an x-ray scintillator and enables high spatial resolution to be obtained without the need for a columnar scintillator. 
     The term “comprise” is used in this document with an inclusive not an exclusive meaning. That is any reference to X comprising Y indicates that X may comprise only one Y or may comprise more than one Y. If it is intended to use “comprise” with an exclusive meaning then it will be made clear in the context by referring to “comprising only one . . . ” or by using “consisting”. 
     In this brief description, reference has been made to various examples. The description of features or functions in relation to an example indicates that those features or functions are present in that example. The use of the term “example” or “for example” or “may” in the text denotes, whether explicitly stated or not, that such features or functions are present in at least the described example, whether described as an example or not, and that they can be, but are not necessarily, present in some of or all other examples. Thus “example”, “for example” or “may” refers to a particular instance in a class of examples. A property of the instance can be a property of only that instance or a property of the class or a property of a sub-class of the class that includes some but not all of the instances in the class. It is therefore implicitly disclosed that a features described with reference to one example but not with reference to another example, can where possible be used in that other example but does not necessarily have to be used in that other example. 
     Although embodiments of the present invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the invention as claimed. For instance in the above the described examples all of the microphones used are real microphones. In some examples the one or more of the microphones used for obtaining a beamforming signal could be a virtual microphone, that is, an arithmetic combination of at least two real microphone signals. 
     Features described in the preceding description may be used in combinations other than the combinations explicitly described. 
     Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not. 
     Although features have been described with reference to certain embodiments, those features may also be present in other embodiments whether described or not. 
     Whilst endeavoring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.