WO2015144972A1

WO2015144972A1 - An apparatus with nanoparticle reference pixel and associated methods

Info

Publication number: WO2015144972A1
Application number: PCT/FI2014/050225
Authority: WO
Inventors: Michael Astley; Elisabetta Spigone
Original assignee: Nokia Technologies Oy
Priority date: 2014-03-27
Filing date: 2014-03-27
Publication date: 2015-10-01

Abstract

An apparatus comprising an active pixel (201, 301) and a reference pixel (202, 302), the active pixel comprising (201, 301 ) one or more first nanoparticles (203a, 303a) supported on a first layer (204a), the reference pixel (202, 302) representative of the active pixel active pixel (201, 301) and comprising one or more second nanoparticles (203b) supported on a second layer of material (204b), the first (203a) and second (203b) nanoparticles configured such that electromagnetic radiation incident thereon causes excitation of electrons within the respective nanoparticles resulting in the generation of photo-excited charge, wherein the first nanoparticles comprise a first coating (207a), the first coating (207a) configured to allow the transfer of photo-excited charge (206), generated from the first nanoparticles (203a) by incident electromagnetic radiation, to the first layer of material (204a) to leave photo-excited charge (206) of opposite polarity on the first nanoparticles (203a), the photo-excited charge (206) of opposite polarity generating an electric field which causes a change in an electrical property of the first layer of material (204a), and wherein the apparatus is configured to substantially suppress the transfer of photo-excited charge (206), generated from the second nanoparticles (203b) by the incident electromagnetic radiation, to the second layer of material (204b), suppression of the charge transfer hindering a corresponding change in the electrical property of the second layer of material (204b) resulting in a difference in the respective electrical property of the first (204a) and second (204b) layers of material, the difference in the respective electrical property indicative of one or more of the presence and magnitude of the incident electromagnetic radiation.

Description

AN APPARATUS WITH NANOPARTICLE REFERENCE PIXEL AND

ASSOCIATED METHODS

Technical Field The present disclosure relates to the field of image sensors, associated methods and apparatus, and in particular concerns an apparatus for use in detecting one or more of the presence and magnitude of incident electromagnetic radiation whilst accounting for common- mode noise. Certain disclosed example aspects/embodiments relate to portable electronic devices, in particular, so-called hand-portable electronic devices which may be hand-held in use (although they may be placed in a cradle in use). Such hand-portable electronic devices include so-called Personal Digital Assistants (PDAs) and tablet PCs.

The portable electronic devices/apparatus according to one or more disclosed example aspects/embodiments may provide one or more audio/text/video communication functions (e.g. tele-communication, video-communication, and/or text transmission, Short Message Service (SMS)/ Multimedia Message Service (MMS)/emailing functions, interactive/non- interactive viewing functions (e.g. web-browsing, navigation, TV/program viewing functions), music recording/playing functions (e.g. MP3 or other format and/or (FM/AM) radio broadcast recording/playing), downloading/sending of data functions, image capture function (e.g. using a (e.g. in-built) digital camera), and gaming functions.

Background

Research is being done to develop image sensors which can account for common-mode noise. One or more aspects/embodiments of the present disclosure may or may not address this issue.

The listing or discussion of a prior-published document or any background in this specification should not necessarily be taken as an acknowledgement that the document or background is part of the state of the art or is common general knowledge.

Summary

According to a first aspect, there is provided an apparatus comprising an active pixel and a reference pixel, the active pixel comprising one or more first nanoparticles supported on a first layer of material, the reference pixel representative of the active pixel and comprising one or more second nanoparticles supported on a second layer of material, the first and second nanoparticles configured such that electromagnetic radiation incident thereon causes excitation of electrons within the respective nanoparticles resulting in the generation of photo- excited charge,

wherein the first nanoparticles comprise a first coating, the first coating configured to allow the transfer of photo-excited charge, generated from the first nanoparticles by incident electromagnetic radiation, to the first layer of material to leave photo-excited charge of opposite polarity on the first nanoparticles, the photo-excited charge of opposite polarity generating an electric field which causes a change in an electrical property of the first layer of material, and

wherein the apparatus is configured to substantially suppress the transfer of photo- excited charge, generated from the second nanoparticles by the incident electromagnetic radiation, to the second layer of material, suppression of the charge transfer hindering a corresponding change in the electrical property of the second layer of material resulting in a difference in the respective electrical property of the first and second layers of material, the difference in the respective electrical property indicative of one or more of the presence and magnitude of the incident electromagnetic radiation.

The reference pixel may comprise a plurality of second nanoparticles. The apparatus may be configured to substantially suppress the transfer of photo-excited charge by the plurality of second nanoparticles being in physical contact with one another such that the exited electrons relax to an unexcited state before charge transfer to the second layer of material can occur.

The apparatus may be configured to substantially suppress the transfer of photo-excited charge by the second nanoparticles comprising a second coating configured to suppress the transfer of photo-excited charge to the second layer of material.

The apparatus may be configured to substantially suppress the transfer of photo-excited charge by preventing or reducing the transfer of photo-excited charge to the second layer of material.

The reference pixel may be representative of the active pixel by the second nanoparticles being configured to exhibit substantially the same photoresponse to the incident electromagnetic radiation as the first nanoparticles. The reference pixel may be representative of the active pixel by the second nanoparticles being formed with one or more of substantially the same size, shape and material as the first nanoparticles. The reference pixel may be representative of the active pixel by the second layer of material being configured to have substantially the same electrical properties under the same conditions as the first layer of material.

The reference pixel may be representative of the active pixel by the second layer of material being formed with one or more of substantially the same size, shape and material as the first layer of material.

One or more of the first and second nanoparticles may be configured such that the generated photo-excited charge comprises the electrons excited by the incident electromagnetic radiation, and the photo-excited charge of opposite polarity comprises electron-holes left by the excited electrons.

One or more of the first and second nanoparticles may be configured such that the photo- excited charge of opposite polarity comprises the electrons excited by the incident electromagnetic radiation, and the generated photo-excited charge comprises electron-holes left by the excited electrons.

The first coating may be configured such that the transfer of photo-excited charge comprises tunnelling of the electrons excited by the incident electromagnetic radiation, or electron-holes left by the excited electrons, through the first coating to the first layer of material.

The first coating may be configured such that the transfer of photo-excited charge comprises thermally-activated hopping of the electrons excited by the incident electromagnetic radiation, or electron-holes left by the excited electrons, through the first coating to the first layer of material.

The second coating may be configured such that the transfer of photo-excited charge comprises tunnelling of the electrons excited by the incident electromagnetic radiation, or electron-holes left by the excited electrons, through the second coating to the second layer of material. The second coating may be configured such that the transfer of photo-excited charge comprises thermally-activated hopping of the electrons excited by the incident electromagnetic radiation, or electron-holes left by the excited electrons, through the second coating to the second layer of material.

One or more of the first and second nanoparticles may comprise one or more of quantum dots, quantum wires, nanorods, nanostars and nanotetrapods.

One or more of the first and second nanoparticles may comprise one or more of PbS, CdSe, CdS, PbSe, ZnO, ZnS, CZTS, Cu₂S, Bi₂S₃, Ag₂S, Ag₂S, HgTe, CdHgTe, InAs and InSb.

The second coating may comprise one or more primary ligands attached to the surface of the second nanoparticles. The first coating may comprise one or more secondary ligands attached to the surface of the first nanoparticles.

The primary ligands may comprise one or more of oleate, trioctylphosphine oxide, alkylphosphonic acid, fatty acid and long-chain alkylamine. The secondary ligands may comprise one or more of 1 ,2-ethanedithiol, pyridine, butylamine and 1 ,3-benzenedithiol. One or more of the first and second layers of material may comprise one or more of graphene, reduced graphene oxide and carbon nanotubes. The first and second layers of material may comprise different regions of the same layer of material.

The incident electromagnetic radiation may comprise one or more of radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, gamma rays and thermal radiation.

The apparatus may comprise source and drain electrodes configured to cause a flow of charge from a source electrode through the respective first and second layers of material to a drain electrode to enable the difference in the respective electrical property of the first and second layers of material to be determined.

The difference in the respective electrical property may comprise a difference in one or more of electrical conductivity and electrical conductance of the first and second layers of material. The apparatus may be one or more of an electronic device, a portable electronic device, a portable telecommunications device, a camera, an image sensor, a photodetector, a phototransistor and a module for one or more of the same. According to a further aspect, there is provided a method of making an apparatus, the method comprising:

forming an active pixel and a reference pixel, the active pixel comprising one or more first nanoparticles supported on a first layer of material, the reference pixel representative of the active pixel and comprising one or more second nanoparticles supported on a second layer of material, the first and second nanoparticles configured such that electromagnetic radiation incident thereon causes excitation of electrons within the respective nanoparticles resulting in the generation of photo-excited charge,

Forming the active pixel may comprise forming the first coating on the first nanoparticles. Forming the reference pixel may comprise forming a second coating on the second nanoparticles which is configured to suppress the transfer of photo-excited charge to the second layer of material. The second coating may comprise one or more primary ligands attached to the surface of the second nanoparticles. The first coating may comprise one or more secondary ligands attached to the surface of the first nanoparticles.

The method may comprise forming the respective first and second coatings on the first and second nanoparticles simultaneously by:

attaching the one or more primary ligands to the surface of the first and second nanoparticles, the one or more primary ligands configured to suppress the transfer of photo- excited charge to the respective first and second layers of material; and selectively replacing the one or more primary ligands at the surface of the first nanoparticles with the one or more secondary ligands, the one or more secondary ligands configured to allow the transfer of photo-excited charge to the first layer of material. The method may comprise forming the respective first and second coatings on the first and second nanoparticles simultaneously by:

attaching the one or more primary ligands to the surface of the first and second nanoparticles, the one or more primary ligands configured to suppress the transfer of photo- excited charge to the respective first and second layers of material;

replacing the one or more primary ligands at the surface of the first and second nanoparticles with the one or more secondary ligands, the one or more secondary ligands configured to allow the transfer of photo-excited charge to the respective first and second layers of material; and

selectively replacing the one or more secondary ligands at the surface of the second nanoparticles with one or more tertiary ligands, the one or more tertiary ligands configured to suppress the transfer of photo-excited charge to the second layer of material.

The one or more tertiary ligands may be the same as the one or more primary ligands. Forming the active pixel may comprise forming the first coating on the first nanoparticles. The first coating may comprise one or more secondary ligands attached to the surface of the first nanoparticles. Forming the reference pixel may comprise forming a plurality of second nanoparticles in physical contact with one another such that the exited electrons relax to an unexcited state before charge transfer to the second layer of material can occur.

The method may comprise forming the active and reference pixels simultaneously by:

attaching one or more primary ligands to the surface of the first and second nanoparticles, the one or more primary ligands configured to suppress the transfer of photo- excited charge to the respective first and second layers of material;

selectively removing the one or more secondary ligands from the surface of the second nanoparticles to cause physical contact between the plurality of second nanoparticles. The method may comprise forming the active and reference pixels simultaneously by:

selectively replacing the one or more primary ligands at the surface of the first nanoparticles with the one or more secondary ligands, the one or more secondary ligands configured to allow the transfer of photo-excited charge to the first layer of material; and

removing the one or more primary ligands from the surface of the second nanoparticles to cause physical contact between the plurality of second nanoparticles.

One or more of the selective replacement and removal of ligands may be performed using one or more lithographic processes.

According to a further aspect, there is provided a method of using an apparatus,

the apparatus comprising an active pixel and a reference pixel, the active pixel comprising one or more first nanoparticles supported on a first layer of material, the reference pixel representative of the active pixel and comprising one or more second nanoparticles supported on a second layer of material, the first and second nanoparticles configured such that electromagnetic radiation incident thereon causes excitation of electrons within the respective nanoparticles resulting in the generation of photo-excited charge,

wherein the apparatus is configured to substantially suppress the transfer of photo- excited charge, generated from the second nanoparticles by the incident electromagnetic radiation, to the second layer of material, suppression of the charge transfer hindering a corresponding change in the electrical property of the second layer of material resulting in a difference in the respective electrical property of the first and second layers of material, the difference in the respective electrical property indicative of one or more of the presence and magnitude of the incident electromagnetic radiation,

the method comprising:

determining the difference in the respective electrical property of the first and second layers of material; and determining one or more of the presence and magnitude of the incident electromagnetic radiation based on the determined difference in respective electrical property. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated or understood by the skilled person.

Corresponding computer programs (which may or may not be recorded on a carrier) for implementing one or more of the methods disclosed herein are also within the present disclosure and encompassed by one or more of the described example embodiments.

The present disclosure includes one or more corresponding aspects, example embodiments or features in isolation or in various combinations whether or not specifically stated (including claimed) in that combination or in isolation. Corresponding means for performing one or more of the discussed functions are also within the present disclosure.

The above summary is intended to be merely exemplary and non-limiting.

Brief Description of the Figures

A description is now given, by way of example only, with reference to the accompanying drawings, in which:-

Figure 1 illustrates schematically an apparatus configured to detect incident electromagnetic radiation whilst accounting for common-mode noise;

Figure 2a illustrates schematically an active pixel which is suitable for use in the apparatus of Figure 1 ;

Figure 2b illustrates schematically a reference pixel which is suitable for use in the apparatus of Figure 1 ;

Figure 2c illustrates schematically another reference pixel which is suitable for use in the apparatus of Figure 1 ;

Figure 3a illustrates schematically an apparatus according to one embodiment of the present disclosure;

Figure 3b illustrates schematically an apparatus according to another embodiment of the present disclosure;

Figure 4 illustrates schematically an apparatus according to another embodiment of the present disclosure; Figure 5a shows the main steps of a method of making the apparatus described herein;

Figure 5b shows the main steps of a method of using the apparatus described herein; and

Figure 6 shows a computer-readable medium comprising a computer program configured to perform, control or enable one or more of the method steps of Figure 5a or 5b.

Description of Specific Aspects/Embodiments

As illustrated in Figure 1 , image sensors may make use of reference pixels 102 to produce a reference signal R. Rather than processing the absolute value of the signal 1 -5 from an active pixel 101 , the signal 1 -5 is compared with that of the reference pixel 102 to produce a difference signal (e.g. signal 1 minus signal R). This technique removes any common noise, such as signals caused by thermal drift, electrical pickup, contact potentials, etc, and therefore improves the sensor image quality. The ideal reference pixel 102 would be one that is as similar to the active pixels 101 as possible, but which exhibits no optoelectrical response.

Current methods of creating reference pixels 102 have their disadvantages. A window which allows light to fall onto the image sensor may be aligned to prevent light falling onto one or more pixels. But this requires extremely precise alignment (to the micron) between the chip and the carrier, which is demanding, increases manufacturing costs and results in poorer yields due to misalignments. Also, light from multiply reflected paths may still fall onto the dark area of the sensor, leading to systematic errors in the dark level reading and reduced sensor performance.

Individual pixels may be covered by depositing a light-absorbing or light-reflecting material, but this involves placing materials directly over the pixels. Both the thermal and electrical behavior of such pixels differs compared to the active pixels which can result in artifacts in the reference signal which are not present in the active signal. For example, the material may prevent thermal radiation leading to heating of the pixel, and reflective materials are usually conductive and will therefore couple to any local noise-generating electrical fields.

There will now be described an apparatus and associated methods that may or may not provide a solution to one or more of these issues. Figure 2a illustrates schematically an active pixel 201 which is suitable for use in the apparatus of Figure 1. The active pixel 201 comprises one or more first nanoparticles 203a (only one nanoparticle 203a is shown) supported on a first layer of material 204a, the first nanoparticles 203a configured such that electromagnetic radiation 205 incident thereon causes excitation of electrons within the nanoparticles 203a resulting in the generation of photo-excited charge 206. The incident electromagnetic radiation 205 may comprise one or more of radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, gamma rays and thermal radiation.

The first nanoparticles 203a comprise a first coating 207a which is configured to allow the transfer of photo-excited charge 206, generated from the first nanoparticles 203a by incident electromagnetic radiation 205, to the first layer of material 204a to leave photo-excited charge 208 of opposite polarity on the first nanoparticles 203a. The photo-excited charge 208 of opposite polarity generates an electric field causing a change in an electrical property (e.g. conductivity or conductance) of the first layer of material 204a which can be used to detect one or more of the presence and magnitude of the incident electromagnetic radiation 205.

In order to enable the electrical property of the first layer of material 204a to be determined, the apparatus may also comprise source 210 and drain 21 1 electrodes configured to cause a flow of charge from the source electrode 210 through the first layer of material 204a to the drain electrode 21 1. The change in electrical property may be determined using a voltmeter or ammeter 209, for example.

In this example, the first nanoparticles 203a are configured such that the generated photo- excited charge 206 comprises the electrons excited by the incident electromagnetic radiation 205, and the photo-excited charge 208 of opposite polarity comprises electron-holes left by the excited electrons. Furthermore, the first coating 207a is configured such that the transfer of photo-excited charge 206 comprises tunnelling of the electrons excited by the incident electromagnetic radiation 205 through the first coating 207a to the first layer of material 204a.

In other examples, the first nanoparticles 203a may be configured such that the photo- excited charge 208 of opposite polarity comprises the electrons excited by the incident electromagnetic radiation 205, and the generated photo-excited charge 206 comprises the electron-holes left by the excited electrons. In this scenario, the first coating 207a may be configured such that the transfer of photo-excited charge 206 comprises tunnelling of the electron-holes through the first coating 207a to the first layer of material 204a. The charge transfer mechanism is not limited solely to electron/hole tunnelling, however. In some examples, electron thermally-activated electron/hole hopping may be used. Transfer of the photo-excited charge 206 is not only governed by the first coating 207a, but also by the band structure between the first nanoparticles 203a and the first layer of material 204a. This may be affected by doping or bias voltages, which could be used to tune the optoelectrical response of the apparatus.

Figure 2b illustrates schematically a reference pixel 202 which is suitable for use in the apparatus of Figure 1 . The reference pixel 202 is representative of the active pixel 201 and comprises one or more second nanoparticles 203b (only one nanoparticle 203b is shown) supported on a second layer of material 204b. As with the first nanoparticles 203a of the active pixel 201 in Figure 2a, the second nanoparticles 203b are configured such that electromagnetic radiation 205 incident thereon causes excitation of electrons within the nanoparticles 203b resulting in the generation of photo-excited charge 206. Also like the first nanoparticles 203a, the second nanoparticles 203b in the example shown in Figure 2b are configured such that the generated photo-excited charge 206 comprises the electrons excited by the incident electromagnetic radiation 205, and the photo-excited charge 208 of opposite polarity comprises electron-holes left by the excited electrons.

The reference pixel 202 is representative of the active pixel 201 in the sense that it is configured to exhibit substantially the same photoresponse to the incident electromagnetic radiation 205 as the first nanoparticles 203a. This may be achieved by the second nanoparticles 203b being configured to exhibit substantially the same photoresponse to the incident electromagnetic radiation 205 as the first nanoparticles 203a, e.g. by being formed with one or more of substantially the same size, shape and material as the first nanoparticles 203a. Additionally or alternatively, the representative behaviour may be achieved by the second layer of material 204b being configured to have substantially the same electrical properties under the same conditions as the first layer of material 204a, e.g. by being formed with one or more of substantially the same size, shape and material as the first layer of material 204a.

An important aspect of the present apparatus is that it is configured to substantially suppress (e.g. prevent altogether or substantially reduce) the transfer of photo-excited charge 206, generated from the second nanoparticles 203b by the incident electromagnetic radiation 205, to the second layer of material 204b (as illustrated by the "X" in Figure 2b). Suppression of the charge transfer hinders a corresponding change in the electrical property of the second layer of material 204b (i.e. the same electrical property as the first layer of material 204a) resulting in a difference in the respective measured electrical property of the first 204a and second 204b layers of material. In order to enable the electrical property of the second layer of material 204b to be determined, the apparatus may also comprise source 210 and drain 21 1 electrodes configured to cause a flow of charge from the source electrode 210 through the second layer of material 204b to the drain electrode 21 1. The change in electrical property may be determined using a voltmeter or ammeter 209, for example.

The difference in the respective electrical property is indicative of one or more of the presence and magnitude of the incident electromagnetic radiation 205. This is because any common-mode noise within the apparatus is cancelled out by subtracting (or combining depending on the responses of the active 201 and reference 202 pixels) the signals from the active 201 and reference 202 pixels, each of which contains a common-mode noise component. Since the reference pixel 202 is substantially unresponsive to the incident electromagnetic radiation 205 (i.e. does not exhibit an optoelectrical response), only the noise component is subtracted from the active signal leaving a signal which is associated with the incident electromagnetic radiation 205.

The apparatus may be configured in different ways to substantially suppress the transfer of photo-excited charge 206, generated from the second nanoparticles 203b by the incident electromagnetic radiation 205, to the second layer of material 204b. In the example shown in Figure 2b, the second nanoparticles 203b comprise a second coating 207b configured to suppress the transfer of photo-excited charge 206 to the second layer of material 204b by preventing altogether or substantially reducing tunnelling of the excited electrons through the second coating 207b.

Figure 2c illustrates schematically another reference pixel 202 which is suitable for use in the apparatus of Figure 1. In this example, the reference pixel 202 comprises a plurality of second nanoparticles 203b, and the apparatus is configured to substantially suppress the transfer of photo-excited charge 206, generated from the second nanoparticles 203b by the incident electromagnetic radiation 205, to the second layer of material 204b. Rather than the use of a second coating 207b, the second nanoparticles 203b are in physical contact with one another such that the excited electrons relax to an unexcited state before charge transfer to the second layer of material 204b can occur. In this scenario, the second nanoparticles 203b are effectively bound together and behave as a bulk material. Therefore, the excited electrons are delocalized and quickly scatter back into the valence band 224 of the bulk material.

Figure 3a illustrates schematically an apparatus comprising the active pixel 201/301 of Figure 2a and the reference pixel 202/302 of Figure 2b. In this example, the reference pixel 302 has a thicker coating than the active pixel 301 . The first and second coatings may, however, comprise substantially the same material composition, density and shape. In other examples, the first and second coatings may comprise different material compositions but have substantially the same thickness, shape and density.

Similarly, Figure 3b illustrates schematically an apparatus comprising the active pixel 202/302 of Figure 2a and the reference pixel 202/302 of Figure 2c. In the examples of Figures 3a and 3b, a difference signal based on the active signal 1 and the reference signal R could be used to indicate one or more of the presence and magnitude of the incident electromagnetic radiation 305.

In each of the examples described herein, one or more of the first 203a and second 203b nanoparticles may comprise quantum dots, quantum wires, nanorods, nanostars or nanotetrapods, which may be formed from one or more of PbS, CdSe, CdS, PbSe, ZnO, ZnS, CZTS, Cu₂S, Bi₂S₃, Ag₂S, Ag₂S, HgTe, CdHgTe, InAs and InSb. Furthermore, one or more of the first 204a and second 204b layers of material may comprise one or more of graphene, reduced graphene oxide and carbon nanotubes.

The first 207a and second 207b coatings could also be formed from a number of different materials. For example, in some embodiments, the second coating 207b may comprise one or more primary ligands attached to the surface of the second nanoparticles 203b, and the first coating 207a may comprise one or more secondary ligands attached to the surface of the first nanoparticles 203a. The primary ligands would typically be relatively long ligands such as oleate, trioctylphosphine oxide, alkylphosphonic acid, fatty acid or long-chain alkylamine ligands, and the secondary ligands would typically be relatively short ligands such as 1 ,2-ethanedithiol, pyridine, butylamine or 1 ,3-benzenedithiol ligands.

Figure 4 illustrates schematically an apparatus 412 according to another embodiment of the present disclosure. The apparatus 412 may be one or more of an electronic device, a portable electronic device, a portable telecommunications device, a camera, an image sensor, a photodetector, a phototransistor and a module for one or more of the same. In the example shown, the apparatus 412 is an image sensor comprising the active 401 and reference 402 pixels, a power source 413, an ammeter 409, a voltmeter 414, a processor 415 and a storage medium 416, which are electrically connected to one another by a data bus 417. In practice, the apparatus 412 would normally comprise a plurality of active pixels 401 and at least one reference pixel 402 (possibly more than one). Furthermore, the active 401 and reference 402 pixels would typically be arranged to form a one, two or three dimensional array configured to receive the incident electromagnetic radiation, the specific configuration of the pixel array depending on the particular application of the apparatus 412.

The power source 413 is configured to apply a voltage between the source and drain electrodes, the voltmeter 414 is configured to measure the applied voltage, and the ammeter 409 is configured to measure the resulting current flowing through the first and second layers of material.

The processor 416 is configured for general operation of the apparatus 412 by providing signalling to, and receiving signalling from, the other components to manage their operation. In addition, the processor 415 is configured to receive the voltage and current measurements from the voltmeter 414 and ammeter 409, respectively, determine an electrical property of the first and second layers of material using the voltage and current measurements, determine a difference in the electrical property of the first and second layers of material, and determine one or more of the presence and magnitude of the incident electromagnetic radiation using the difference in the electrical property. In another embodiment, the processor 415 may be configured to determine a difference in the electrical property of the first and second layers of material without measuring the absolute electrical property of the first and second layers of material (i.e. it simply receives a difference signal). In this scenario, the apparatus 412 may comprise circuitry configured to subtract or combine the signals from the active 401 and reference 402 pixels to produce the differential signal which is then passed to the processor 415. Of course, electrical properties other than voltage, current, conductivity and conductance may be assessed. For example, differences in the resistance or resistivity of the first and second layers of material may be assessed to determine one or more of the presence and amount of incident electromagnetic radiation.

The storage medium 416 is configured to store computer code configured to perform, control or enable operation of the apparatus 412. The storage medium 416 may also be configured to store settings for the other components. The processor 415 may access the storage medium 416 to retrieve the component settings in order to manage the operation of the other components. The storage medium 416 may also be configured to store calibration data (e.g. predetermined measurements of intensity levels of incident electromagnetic radiation versus a corresponding electrical property) for use by the processor 415 in determining one or more of the presence and magnitude of the incident electromagnetic radiation.

The processor 415 may be a microprocessor, including an Application Specific Integrated Circuit (ASIC). The storage medium 416 may be a temporary storage medium such as a volatile random access memory. On the other hand, the storage medium 416 may be a permanent storage medium such as a hard disk drive, a flash memory, or a non-volatile random access memory. The main steps 518-519 of a method of making the apparatus are shown schematically in Figure 5a.

A number of different fabrication processes may be used to form the active and reference pixels. One step of the method is the formation of the first and second nanoparticles. As one example, the first and second nanoparticles may comprise oleate-coated lead sulfide (PbS) nanoparticles which can be synthesized in the following way.

First, 2mmol of lead oxide (PbO) are dissolved in a mixture of oleic acid (2ml_) and octadecene (20ml_). The size of the nanoparticles can be varied by adjusting the ratio of oleic acid to PbO. The mixture is then heated at 150°C for one hour in a nitrogen environment to form lead oleate (indicated by a colour change of the solution from yellow to clear). The temperature of the solution is then lowered to 120°C before 0.2ml of bis (tri methylsilyl) sulfide (TMS) dissolved in 10ml octadecene (ODE) is rapidly injected into the lead oleate solution. After the rapid injection the colour of the reaction mixture changes from colourless to deep brown. The reaction is continued for 2 minutes until oleate-coated PbS nanocrystals are formed. The solution is then dissolved in 10ml of toluene and precipitated with methanol and acetone, followed by centrifugation. The size of the nanoparticles can be increased by increasing the injection temperature (e.g. between 90, 100, 1 10 and 120°C). The nanocrystals are then dispersed in chloroform and the isolation process is repeated to remove any unreacted materials. Finally the nanocrystals are dried and dispersed in chloroform suitable for film formation.

The active and reference pixels also comprise first and second layers of material on which the respective first and second nanoparticles are supported. As one example, the first and second layers of material may comprise graphene, which can be prepared as single or double layers of graphene flakes using mechanical cleavage or as a graphene film using chemical vapour deposition. The graphene flakes can then be deposited onto a substrate to form the first and second layers of material.

In addition, source and drain electrodes may be formed on top of the first and second layers of material to enable a flow of charge from the source electrode through the respective first and second layers of material to the drain electrode so that the difference in the respective electrical property of the first and second layers of material can be determined. The source and drain electrodes may be formed using one or more lithographic techniques, e.g. photolithography in combination with a lift-off or etching process followed by deposition of the electrode materials. When metals are used to form the source and drain electrodes, evaporation or sputtering may be used for deposition.

Once the first and second layers of material have been formed, the respective first and second nanoparticles can be deposited thereon. This may be achieved by spin coating a solution of nanoparticles on top of the layers of material. In the case of oleate-coated PbS nanoparticles, chloroform may be used as a suitable solvent.

Another step of the fabrication process is the formation of the first and second coatings on the respective first and second nanoparticles. As mentioned previously, the second coating may comprise one or more primary ligands, and the first coating may comprise one or more secondary ligands. In this scenario, the first and second coatings may be formed on the first and second nanoparticles simultaneously by:

Process A

(i) Attaching the one or more primary ligands to the surface of the first and second nanoparticles, the one or more primary ligands configured to suppress the transfer of photo- excited charge to the respective first and second layers of material; and

(ii) Selectively replacing the one or more primary ligands at the surface of the first nanoparticles with the one or more secondary ligands, the one or more secondary ligands configured to allow the transfer of photo-excited charge to the first layer of material.

When the oleate-coated PbS nanoparticles are used as the first and second nanoparticles, the oleate coating may constitute the primary ligands and 1 ,2-ethanedithol (EDT) may be used as the secondary ligands. In this scenario, step (ii) of Process A may be performed using three different selective approaches. These approaches are described below in which the oleate-coated PbS nanoparticle solution is deposited onto a layer of graphene. In the first approach, before depositing the nanoparticle solution onto the graphene, a layer of EDT is deposited selectively on top of the active pixel region by covering the reference pixel region in a masking material and then soaking the entire graphene surface in a solution of EDT in acetronile. The masking material is then removed from the reference pixel region before the graphene is soaked in the nanoparticle solution. Thiols possess a strong affinity for Pb atoms, leading to effective competition with the existing oleate agent for ligand-binding sites. As a result, the nanoparticles in the active pixel region (first nanoparticles) bind to the graphene surface (first layer of material) through the EDT molecules (secondary ligands). Since the reference pixel region is not functionalized with EDT, these nanoparticles (second nanoparticles) bind to the graphene surface (second layer of material) via the oleate molecules (primary ligands).

In the second approach, a monolayer of nanoparticles is first formed on top of the active and reference pixel regions by spin-coating the nanoparticle solution onto the graphene surface. A masking material is then deposited on top of the nanoparticles in the reference pixel region (second nanoparticles) before the entire graphene surface is washed in EDT in acetonitrile to replace the oleate molecules (primary ligands) attached to the nanoparticles in the active pixel region (first nanoparticles) with EDT molecules (secondary ligands). Quantitatively, the removal of the oleate ligands from the surface of the nanoparticles occurs via the nucleophilic attack of oleate by EDT, followed by Pb-S bond formation and desorption of oleic acid, or by the dissociative adsorption of EDT onto the nanoparticle surface to yield adsorbed HS(CH2)2S and hydrogen, again followed by desorption of oleic acid. As a result of oleate removal, the nanoparticles move closer together allowing charge transfer to the underlying graphene (first layer of material). The reference pixel is prevented from being washed by the EDT solution and therefore the oleate coating remains. In this case, the quantum confinement is still present but the photo-excited charge carrier will not be transferred to the underlying graphene surface (second layer of material). The masking material may then be removed from the reference pixel region.

In the third approach, the exchange of ligands happens in solution before the nanoparticles are deposited onto the graphene surface. To achieve this, the solution of oleate-coated PbS nanoparticles (second nanoparticles) is separated into two volumes, one of which is mixed with EDT in acetonitrile to form the first nanoparticles. The first and second nanoparticles are then selectively deposited within the respective active and reference pixel regions of the graphene surface. The selective deposition may be performed by depositing a masking material onto the reference pixel region, exposing the graphene surface to the first nanoparticle solution, removing the masking material, and then repeating the process to deposit the second nanoparticle solution.

Rather than using Process A, the first and second coatings may be formed on the first and second nanoparticles simultaneously by:

Process B

(i) Attaching the one or more primary ligands to the surface of the first and second nanoparticles, the one or more primary ligands configured to suppress the transfer of photo- excited charge to the respective first and second layers of material;

(ii) Replacing the one or more primary ligands at the surface of the first and second nanoparticles with the one or more secondary ligands, the one or more secondary ligands configured to allow the transfer of photo-excited charge to the respective first and second layers of material; and

(iii) Selectively replacing the one or more secondary ligands at the surface of the second nanoparticles with one or more tertiary ligands, the one or more tertiary ligands configured to suppress the transfer of photo-excited charge to the second layer of material. The three different selective approaches described above with reference to step (ii) of process A may also be used to perform step (iii) of process B. Furthermore, the one or more tertiary ligands may or may not be the same as the one or more primary ligands.

Instead of depositing a second coating on the surface of the second nanoparticles, the reference pixel may comprise a plurality of second nanoparticles in physical contact with one another such that the excited electrons relax to an unexcited state before charge transfer to the second layer of material can occur. In this scenario, the active and reference pixels may be formed simultaneously by: Process C

(i) Attaching one or more primary ligands to the surface of the first and second nanoparticles, the one or more primary ligands configured to suppress the transfer of photo- excited charge to the respective first and second layers of material;

(iii) Selectively removing the one or more secondary ligands from the surface of the second nanoparticles to cause physical contact between the plurality of second nanoparticles.

In a solution of oleate-coated PbS nanoparticles, the oleate molecules constitute the one or more primary ligands which may be exchanged with EDT molecules as described above to form the one or more secondary ligands. Selective removal of the secondary ligands can then be performed by depositing a masking material on top of the nanoparticles in the active region (first nanoparticles) followed by exposure of the graphene surface to a ligand-stripping solution. After removal of the secondary ligands, the masking material may be removed.

An alternative process is as follows:

Process D

(ii) Selectively replacing the one or more primary ligands at the surface of the first nanoparticles with the one or more secondary ligands, the one or more secondary ligands configured to allow the transfer of photo-excited charge to the first layer of material; and

(iii) Removing the one or more primary ligands from the surface of the second nanoparticles to cause physical contact between the plurality of second nanoparticles.

The techniques described above in respect of steps (i) and (ii) of Process A may also be used to perform steps (i) and (ii) of Process D. Once the oleate ligands at the surface of the first nanoparticles have been replaced with EDT ligands, the graphene surface can be immersed in a 1 M aqueous solution of potassium hydroxide for approximately 20mins at room temperature to neutralize the oleic acid and cause removal of the oleate ligands.

Although masking layers have been described above to prevent exposure of the active or reference pixel region to a particular solution, the skilled person will appreciate that other lithographic processes may be used for the selective deposition, replacement and removal of ligands. The main steps 520-522 of a method of using the apparatus to determine one or more of the presence and magnitude of incident electromagnetic radiation are shown schematically in Figure 5b. Step 520 shown in the dashed box is optional. The key measurement is the difference between the active and reference pixels (known as a differential technique). The active and reference pixels will both be subjected to the same common-mode noise sources, such as electrical pickup and thermal drift. If the transfer functions of both pixels are identical, these common-mode signals will have no effect on the final signal. More generally, if the transfer functions of the pixels are given by S_active and Sreference, then the common-mode rejection ratio (CMMR) is given by

CMMR = 0.5 (Sactive ⁺ Sreference)/(S_active— Sreference) Equation 1

Using the fabrication processes described herein, the transfer functions of the resulting active and reference pixels in response to common-mode signals are virtually identical, so the CMMR is very large.

Figure 6 illustrates schematically a computer/processor readable medium 623 providing a computer program according to one embodiment. In this example, the computer/processor readable medium 623 is a disc such as a digital versatile disc (DVD) or a compact disc (CD). In other embodiments, the computer/processor readable medium 623 may be any medium that has been programmed in such a way as to carry out an inventive function. The computer/processor readable medium 623 may be a removable memory device such as a memory stick or memory card (SD, mini SD, micro SD or nano SD).

The computer program may comprise computer code configured to perform, control or enable one or more of the method steps 518-519, 520-522 of Figure 5a or 5b. In particular, the computer program may be configured to measure/determine the difference in the electrical property (e.g. conductivity, conductance, resistance, resistivity, etc) of the first and second layers of material, and determine one or more of the presence and magnitude of the incident electromagnetic radiation. Additionally or alternatively, the computer program may be configured to control the above-mentioned fabrication processes to form the active and reference pixels of the apparatus. Other embodiments depicted in the figures have been provided with reference numerals that correspond to similar features of earlier described embodiments. For example, feature number 1 can also correspond to numbers 101 , 201 , 301 etc. These numbered features may appear in the figures but may not have been directly referred to within the description of these particular embodiments. These have still been provided in the figures to aid understanding of the further embodiments, particularly in relation to the features of similar earlier described embodiments.

It will be appreciated to the skilled reader that any mentioned apparatus/device and/or other features of particular mentioned apparatus/device may be provided by apparatus arranged such that they become configured to carry out the desired operations only when enabled, e.g. switched on, or the like. In such cases, they may not necessarily have the appropriate software loaded into the active memory in the non-enabled (e.g. switched off state) and only load the appropriate software in the enabled (e.g. on state). The apparatus may comprise hardware circuitry and/or firmware. The apparatus may comprise software loaded onto memory. Such software/computer programs may be recorded on the same memory/processor/functional units and/or on one or more memories/processors/functional units.

In some embodiments, a particular mentioned apparatus/device may be pre-programmed with the appropriate software to carry out desired operations, and wherein the appropriate software can be enabled for use by a user downloading a "key", for example, to unlock/enable the software and its associated functionality. Advantages associated with such embodiments can include a reduced requirement to download data when further functionality is required for a device, and this can be useful in examples where a device is perceived to have sufficient capacity to store such pre-programmed software for functionality that may not be enabled by a user.

It will be appreciated that any mentioned apparatus/circuitry/elements/processor may have other functions in addition to the mentioned functions, and that these functions may be performed by the same apparatus/circuitry/elements/processor. One or more disclosed aspects may encompass the electronic distribution of associated computer programs and computer programs (which may be source/transport encoded) recorded on an appropriate carrier (e.g. memory, signal).

It will be appreciated that any "computer" described herein can comprise a collection of one or more individual processors/processing elements that may or may not be located on the same circuit board, or the same region/position of a circuit board or even the same device. In some embodiments one or more of any mentioned processors may be distributed over a plurality of devices. The same or different processor/processing elements may perform one or more functions described herein.

It will be appreciated that the term "signalling" may refer to one or more signals transmitted as a series of transmitted and/or received signals. The series of signals may comprise one, two, three, four or even more individual signal components or distinct signals to make up said signalling. Some or all of these individual signals may be transmitted/received simultaneously, in sequence, and/or such that they temporally overlap one another. With reference to any discussion of any mentioned computer and/or processor and memory (e.g. including ROM, CD-ROM etc), these may comprise a computer processor, Application Specific Integrated Circuit (ASIC), field-programmable gate array (FPGA), and/or other hardware components that have been programmed in such a way to carry out the inventive function.

The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole, in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that the disclosed aspects/embodiments may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the disclosure.

While there have been shown and described and pointed out fundamental novel features as applied to different embodiments thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices and methods described may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. Furthermore, in the claims means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures.

Claims

1 . An apparatus comprising an active pixel and a reference pixel, the active pixel comprising one or more first nanoparticles supported on a first layer of material, the reference pixel representative of the active pixel and comprising one or more second nanoparticles supported on a second layer of material, the first and second nanoparticles configured such that electromagnetic radiation incident thereon causes excitation of electrons within the respective nanoparticles resulting in the generation of photo-excited charge,

2. The apparatus of claim 1 , wherein the reference pixel comprises a plurality of second nanoparticles, and wherein the apparatus is configured to substantially suppress the transfer of photo-excited charge by the plurality of second nanoparticles being in physical contact with one another such that the exited electrons relax to an unexcited state before charge transfer to the second layer of material can occur.

3. The apparatus of claim 1 , wherein the apparatus is configured to substantially suppress the transfer of photo-excited charge by the second nanoparticles comprising a second coating configured to suppress the transfer of photo-excited charge to the second layer of material.

4. The apparatus of any preceding claim, wherein the apparatus is configured to substantially suppress the transfer of photo-excited charge by preventing or reducing the transfer of photo-excited charge to the second layer of material.

5. The apparatus of any preceding claim, wherein the reference pixel is representative of the active pixel by the second nanoparticles being configured to exhibit substantially the same photoresponse to the incident electromagnetic radiation as the first nanoparticles.

6. The apparatus of any preceding claim, wherein the reference pixel is representative of the active pixel by the second nanoparticles being formed with one or more of substantially the same size, shape and material as the first nanoparticles.

7. The apparatus of any preceding claim, wherein the reference pixel is representative of the active pixel by the second layer of material being configured to have substantially the same electrical properties under the same conditions as the first layer of material.

8. The apparatus of any preceding claim, wherein the reference pixel is representative of the active pixel by the second layer of material being formed with one or more of substantially the same size, shape and material as the first layer of material.

9. The apparatus of any preceding claim, wherein one or more of the first and second nanoparticles are configured such that the generated photo-excited charge comprises the electrons excited by the incident electromagnetic radiation, and the photo-excited charge of opposite polarity comprises electron-holes left by the excited electrons, or vice-versa.

10. The apparatus of any preceding claim, wherein the first coating is configured such that the transfer of photo-excited charge comprises tunnelling of the electrons excited by the incident electromagnetic radiation, or electron holes left by the excited electrons, through the first coating to the first layer of material.

1 1 . The apparatus of any of claims 3 to 10, wherein the second coating is configured such that the transfer of photo-excited charge comprises tunnelling of the electrons excited by the incident electromagnetic radiation, or electron holes left by the excited electrons, through the second coating to the second layer of material.

12. The apparatus of any preceding claim, wherein one or more of the first and second nanoparticles comprise one or more of quantum dots, quantum wires, nanorods, nanostars and nanotetrapods.

13. The apparatus of any preceding claim, wherein one or more of the first and second nanoparticles comprise one or more of PbS, CdSe, CdS, PbSe, ZnO, ZnS, CZTS, Cu₂S, Bi₂S₃, Ag₂S, Ag₂S, HgTe, CdHgTe, InAs and InSb.

14. The apparatus of any of claims 3 to 13, wherein the second coating comprises one or more primary ligands attached to the surface of the second nanoparticles, and the first coating comprises one or more secondary ligands attached to the surface of the first nanoparticles.

15. The apparatus of claim 14, wherein the primary ligands comprise one or more of oleate, trioctylphosphine oxide, alkylphosphonic acid, fatty acid and long-chain alkylamine, and the secondary ligands comprise one or more of 1 ,2-ethanedithiol, pyridine, butylamine and 1 ,3-benzenedithiol.

16. The apparatus of any preceding claim, wherein one or more of the first and second layers of material comprise one or more of graphene, reduced graphene oxide and carbon nanotubes.

17. The apparatus of any preceding claim, wherein the first and second layers of material are different regions of the same layer of material.

18. The apparatus of any preceding claim, wherein the incident electromagnetic radiation comprises one or more of radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, gamma rays and thermal radiation.

19. The apparatus of any preceding claim, wherein the apparatus comprises source and drain electrodes configured to cause a flow of charge from a source electrode through the respective first and second layers of material to a drain electrode to enable the difference in the respective electrical property of the first and second layers of material to be determined.

20. The apparatus of any preceding claim, wherein the difference in the respective electrical property comprises a difference in one or more of electrical conductivity and electrical conductance of the first and second layers of material.

21 . The apparatus of any preceding claim, wherein the apparatus is one or more of an electronic device, a portable electronic device, a portable telecommunications device, a camera, an image sensor, a photodetector, a phototransistor and a module for one or more of the same.

22. A method of making an apparatus, the method comprising:

23. The method of claim 22, wherein forming the active pixel comprises forming the first coating on the first nanoparticles, and forming the reference pixel comprises forming a second coating on the second nanoparticles which is configured to suppress the transfer of photo-excited charge to the second layer of material, the second coating comprising one or more primary or tertiary ligands attached to the surface of the second nanoparticles, the first coating comprising one or more secondary ligands attached to the surface of the first nanoparticles.

24. The method of claim 23, wherein the method comprises forming the respective first and second coatings on the first and second nanoparticles simultaneously by:

attaching the one or more primary ligands to the surface of the first and second nanoparticles, the one or more primary ligands configured to suppress the transfer of photo- excited charge to the respective first and second layers of material; and selectively replacing the one or more primary ligands at the surface of the first nanoparticles with the one or more secondary ligands, the one or more secondary ligands configured to allow the transfer of photo-excited charge to the first layer of material.

25. The method of claim 23, wherein the method comprises forming the respective first and second coatings on the first and second nanoparticles simultaneously by:

26. The method of claim 25, wherein the one or more tertiary ligands are the same as the one or more primary ligands.

27. The method of claim 22, wherein forming the active pixel comprises forming the first coating on the first nanoparticles, the first coating comprising one or more secondary ligands attached to the surface of the first nanoparticles, and forming the reference pixel comprises forming a plurality of second nanoparticles in physical contact with one another such that the exited electrons relax to an unexcited state before charge transfer to the second layer of material can occur.

28. The method of claim 27, wherein the method comprises forming the active and reference pixels simultaneously by:

replacing the one or more primary ligands at the surface of the first and second nanoparticles with the one or more secondary ligands, the one or more secondary ligands configured to allow the transfer of photo-excited charge to the respective first and second layers of material; and selectively removing the one or more secondary ligands from the surface of the second nanoparticles to cause physical contact between the plurality of second nanoparticles.

29. The method of claim 27, wherein the method comprises forming the active and reference pixels simultaneously by:

30. The method of any of claims 24 to 26, 28 and 29, wherein one or more of the selective replacement and removal of ligands is performed using one or more lithographic processes.

31 . A method of using an apparatus,

the method comprising:

determining the difference in the respective electrical property of the first and second layers of material; and

determining one or more of the presence and magnitude of the incident electromagnetic radiation based on the determined difference in respective electrical property.

32. A computer program comprising computer code configured to perform the method of any of claims 22 to 31 .