Photodetector composition

Abstract

Disclosed is a photodetector, preferably a mid-infrared photodetector, composition comprising semiconductor particles (101) comprising a band gap of less than 1.5 eV, preferably less than 1eV, and metal nanoparticles (103), wherein the semiconductor particles are interconnected via electrically conducting bridges (102) comprising the metal nanoparticles thereby forming an electrically conductive network. A photodetector comprising the composition and a method of manufacturing the composition are also described.

Description

[0001] Photodetector composition

[0002] INTRODUCTION

[0003] The invention relates to the field of detecting light radiation. Specifically, the invention relates to a photodetector composition comprising semiconductor particles and metal nanoparticles. A photodetector comprising the composition and a method of manufacturing the composition are also described.

[0004] BACKGROUND

[0005] Infrared (IR) detection and imaging technologies play a crucial role across diverse applications, encompassing security, healthcare, automotive, thermal imaging, and environmental monitoring. The increasing demand for compact, cost-effective, and energy-efficient IR detectors and cameras capable of operating at room temperature has driven significant advancements, particularly in Short-Wave Infrared (SWIR, 1-2.5 pm)1. Nevertheless, a notable gap remains in the availability of photodetectors and CMOS cameras suitable for room temperature operation in the Mid-Wave Infrared (MWIR, 3-5 pm) and Long-Wave Infrared (LWIR, 8-12 pm) regions2. MWIR and LWIR cameras, without a need for active illumination, can detect black-body radiation emitted by warm objects in the observed scene, enabling new applications (e.g., search-and-rescue, driving at night) that require capabilities unmet by SWIR devices.

[0006] The underlying pixel technology in state-of-the-art IR cameras relies primarily on two types of photodetectors: photon detectors and microbolometers. Photon detectors are based on the photoelectric effect, which is the generation of charge carriers (electron and holes) within a semiconductor material by absorbed photons. Photon detectors typical display high detectivities, fast response, and wide spectral range, making them suitable for high-speed and multi-spectral imaging applications. However, they suffer from several challenges including the requirement for cryogenic cooling, use of toxic materials and high cost and fabrication complexity. These factors prevent widespread use of these technologies. Microbolometers, on the other hand, are thermal detectors widely used in LWIR detection and imaging due to their sensitivity to temperature changes resulting from absorbed infrared radiation3. They operate on the principle that incident IR radiation heats a material (amorphous Si, VOX, etc.), leading to a change in its electrical resistance. Microbolometers can operate at room temperature without the need for cryogenic cooling systems, simplifying camera design and reducing power consumption, making them suitable for various applications such as night vision, thermal imaging, and gas detection. However, microbolometers typically have low sensitivity, slow response times, and a limited spectral range.

[0007] To bridge the technological gap between IR photon detectors and microbolometers, there has been significant effort to find alternative materials including the use of colloidal quantum dots (CQDs) such as PbS and PbSe CQDs. These solution- processable nanomaterials offer advantages such as facile fabrication at low temperature, tunable optical properties, scalability, and compatibility with monolithic CMOS integration, compared to vacuum-processed counterparts that require high-cost epitaxial growth and lattice matching for integration. However, while these have been shown to offer high sensitivity in SWIR, CQDs targeted at MWIR and LWIR detection (e.g., HgTe) face additional challenges due to the requirement to control the doping level and the size of the quantum dots in order to engineer the band gap and extend the operational spectral range to longer wavelengths4. In particular, the growth of nanocrystals with sizes > 15 nm can be very difficult. These materials also suffer from high Auger recombination, and the operational temperature for MWIR and LWIR technologies remains well below room temperature (<100K). Moreover, these materials typically contain toxic elements (Hg, Pb, Cd, etc.) that pose environmental and safety concerns.

[0008] Accordingly, there is a need for alternative mid-infrared (mid-IR) detection and imaging technologies to be developed based on solution-processable environmentally friendly nanomaterials.

[0009] SUMMARY OF INVENTION

[0010] In general, the present invention proposes a new photodetector composition suitable for mid-infrared detection. The composition is a photosensitive composite material comprising an interconnected network of semiconductor particles and metal nanoparticles. The semiconductor particles have a relatively small band gap suitable for absorbing infrared light, and are electrically connected through bridges that comprise the metal nanoparticles. As a result, charge carriers can be generated from absorption of IR radiation and are then able to travel through the composition to generate a detectable current.

[0011] Accordingly, in an aspect of the invention there is provided a photodetector, preferably a mid-infrared photodetector, composition comprising: semiconductor particles; and metal nanoparticles; wherein the semiconductor particles are interconnected via electrically conducting bridges comprising the metal nanoparticles thereby forming an electrically conductive network.

[0012] Preferably the semiconductor particles comprise a band gap of less than 1.5 eV.

[0013] More preferably the semiconductor particles comprise a band gap of less than 1 eV.

[0014] Advantageously, the composition of the invention has been shown (see the examples below) to detect mid-IR light at room temperature. It has also been shown that the network can be constructed using cheap solution-processable semiconductors. The composition may therefore be used to provide mid-IR photodetectors (and imagers) displaying desirable performance while still being cost-effective.

[0015] The composition has also been shown to detect mid-IR light without using any toxic semiconductor materials, offering a substantial improvement compared to the widely used semiconductors that include mercury, lead, and cadmium elements.

[0016] The metal nanoparticles can also act to protect the semiconductor particles against oxidation which may otherwise reduce the charge transfer through the material.

[0017] The described network structure can also provide further desirable optical and electrical functionalities resulting in improved IR sensitivity and overall device efficiency compared to other known detectors. In particular, the composition can provide enhanced optical absorption, better charge separation and collection, an internal gain mechanism for higher responsivity, higher response speeds, and reduced dark currents, all of which are key parameters for a photodetector. Without wishing to be bound by theory, these properties are described in further detail below.

[0018] Increased optical absorption: The network structure incorporating the small bandgap semiconductor and metal nanoparticles may reduce reflections of the incident light from the surface while increasing optical scattering and absorption within the nanocomposite. A composite with a thickness thinner than the wavelength may therefore still absorb all the light in mid-IR.

[0019] Increased charge separation: The metal / semiconductor interfaces may promote separation of photogenerated charge carriers (electron and holes) by the internal electric fields in Schottky barriers.

[0020] Increased charge collection: the network may form both serial and parallel connections of photoconductors with a size smaller than the charge diffusion length, enabling collection of charges before their recombination. Additional internal gain mechanism: While the majority charge carries are collected by the metal interfaces through the connected electrical pathways in the network, minority charge carries may be trapped on the nanoparticles’ uncovered surfaces using p-n junctions additionally formed by either oxidation or chemical doping treatments. This provides an internal gain mechanism to boost the responsivity.

[0021] Higher response speed: Distributed resistances (R) and capacitances (C) of the interconnected nanoparticles in the network can reduce the overall RC time constant enabling faster response than those of existing photodetector configurations.

[0022] Reduced dark currents: Schottky barriers in serially connected metal / semiconductor interfaces may impede the thermally generated current flowing through the network when there is no light.

[0023] In another aspect of the invention there is provided a layer of photoactive material comprising the photodetector composition described herein.

[0024] In another aspect of the invention there is provided a photodetector, preferably a mid-infrared photodetector, comprising the photodetector composition described herein, or the layer of photoactive material described herein.

[0025] In a further aspect of the invention there is provided a method of manufacturing a photodetector composition as described herein, the method comprising: contacting metal nanoparticles with semiconductor particles, the semiconductor particles preferably comprising a band gap less than 1 .5 eV and more preferably less than 1 eV; and forming electrically conductive bridges comprising the metal nanoparticles to interconnect the semiconductor particles and thereby form an electrically conductive network.

[0026] Advantageously this provides a method for manufacturing the beneficial photodetector composition described herein.

[0027] Advantageously, due to the relatively facile nature of the manufacturing method, the production method can allow for the composition to be easily integrated as pixels on CMOS imagers.

[0028] In another aspect of the invention there is provided a method of manufacturing a layer of photoactive material comprising depositing the photodetector composition described herein onto a substrate.

[0029] In another aspect of the invention there is provided a method of manufacturing a photodetector, preferably a mid-IR photodetector comprising manufacturing a composition as described herein, or layer of photoactive material as described herein, and incorporating said composition, or said layer, as a photo-detecting layer in the photodetector.

[0030] DEFINITIONS

[0031] As used herein the term “mid-IR” refers to infrared radiation with a wavelength in the range of 3 to 12 pm. It covers the mid-wave infrared (MWIR, 3-5 pm) and long-wave infrared (LWIR, 8-12 pm) regions of the spectrum. Analogously mid-IR photodetector (or photodetector composition) refers a detector (or composition) capable of detecting these wavelengths of light.

[0032] The term band gap is well known within the field and should be interpreted the standard way. For example, it is known to refer to the minimum energy required to excite an electron from the valance band up to an unoccupied state in the conduction band in semiconductors. The band gap may be estimated by measuring the absorption coefficient of the material as a function of photon energy (hv) using Fourier transform infrared spectroscopy (FTIR).

[0033] As used herein the term “mean diameter” refers to the mean average particle size approximating the particles as spherical particles. Irregular particles can be approximated as spherical by averaging the longest and shortest dimensions. This is described in more detail below.

[0034] As used herein the term “nanoparticles” refers to particles which have a mean diameter of 1000nm or less.

[0035] As used herein the term “capillary forces” refers to attractive forces that exist at solid-liquid interfaces (e.g., between molten metal nanoparticles and solid semiconductor particle surfaces, or from evaporating solvent within a network of particles (scaffold))

[0036] DETAILED DESCRIPTION OF THE INVENTION

[0037] Embodiments of the various aspects of the invention are described below. For the avoidance of doubt, it will be appreciated, where appropriate, that any embodiments as described herein in relation to one aspect of the present invention will also apply to the other aspects of the present invention.

[0038] An aspect of the present invention relates to a photodetector, preferably a midinfrared photodetector, composition comprising: semiconductor particles, preferably comprising a band gap of less than 1.5 eV and more preferably comprising a band gap of less than 1.0 eV; and metal nanoparticles; wherein the semiconductor particles are interconnected via electrically conducting bridges comprising the metal nanoparticles thereby forming an electrically conductive network.

[0039] Semiconductor particles

[0040] Preferably the mean diameter of the semiconductor particles is from 5 nm to 10000 nm. The mean diameter of the semiconductor particles may be from 5 nm to 500 nm. The mean diameter of the semiconductor particles may be from 5 nm to 300 nm. The mean diameter of the semiconductor particles may be from 50 nm to 500 nm. The mean diameter of the semiconductor particles may be from 100 nm to 300 nm. The semiconductor particles may be nanoparticles.

[0041] The particles may be monodisperse (e.g. may be of approximately uniform size) or may be polydisperse (e.g. having a range of sizes).

[0042] Preferably the semiconductor particles are selected from Ti2Os, InSb, InAs, InN, GaSb, Cd3As2, CdaP2, FeS2, Bi2Te3, Mg2Si, Cu2SnS3, silver chalcogenides (e.g., Ag2Se), SnTe, black phosphorus (BP), lead chalcogenides (e.g., PbSe), platinum chalcogenides (e.g., PtSe), tin chalcogenides (e.g., SnTe), binary or ternary alloys of Hg, Cd, Zn, and Te optionally with a stoichiometry specified by x (e.g. Hgi-xCdxTe or Hgi-xZnxTe), lead tin telluride (Pbi-xSnxTe), single or binary alloys of semiconducting elements (e.g., Ge, Te, Sii-xGex), binary or ternary alloys of lll-V group elements (e.g., InxGai-xAs, Alxlni-xAs, AlxGai-xSb, GaAsSb, InGaSb), metal oxides (e.g., VO2, CuO) or combinations thereof.

[0043] More preferably the semiconductor particles are selected from Ti2Os, InSb, InAs, InN, GaSb, Cd3As2, CdsP2, FeS2, Bi2Te3, Mg2Si, Cu2SnS3, Ag2Se, SnTe, Black Phosphorus (BP), PbSe, PtSe, and SnTe.

[0044] Most preferably the semiconductor particles are Ti2Os.

[0045] Optionally the semiconductor particles have either a crystalline structure or a polycrystalline structure.

[0046] Preferably the semiconductor particles have a band gap between 0.01 eV and 1 eV. The semiconductor particles may have a band gap of less than 0.5 eV. For example, the semiconductor particles may have a band gap between 0.01 eV and 0.5 eV. The semiconductor particles may have a band gap between 0.05 eV to 0.4 eV. The semiconductor particles may have a band gap of between 0.01 eV and 0.25 eV. The semiconductor particles may have a band gap of between 0.05 eV and 0.15 eV. Advantageously, using semiconductor particles with these band gaps allow for the detection of mid-IR light radiation, which is not possible with large band gap materials.

[0047] Optionally the semiconductor particles are irregularly shaped. Alternatively the semiconductor particles are spherical. The semiconductor particles may also have regular facets, for example, be polygonal or elongated (ellipsoidal) particles.

[0048] Metal nanoparticles

[0049] Preferably the metal nanoparticles are nanoparticles comprising, optionally consisting of, an inert metal. The nanoparticles may comprise, or may consist of, a combination of inert metals. More preferably the metal nanoparticles are selected from Gold (Au), Silver (Ag), Platinum (Pt), Palladium (Pd), Ruthenium (Ru), Osmium (Os), Iridium (Ir), Indium (In), or combinations thereof. Most preferably the metal nanoparticles are gold (Au) nanoparticles. Alternatively, or additionally, the metal nanoparticles may comprise Indium (In) nanoparticles.

[0050] Advantageously, metal nanoparticles made of low melting point metals (e.g. less than 500K), such as Indium, can be used to reduce the post-deposition sintering temperature.

[0051] Preferably the mean diameter of the metal nanoparticles is from 1 nm to 1000 nm. The mean diameter of the metal nanoparticles may be from 1 nm to 100 nm. The mean diameter of the metal nanoparticles may be from 2 nm to 50 nm. The mean diameter of the metal nanoparticles may be from 5 nm to 50 nm. The mean diameter of the metal nanoparticles may be from 2 nm to 10 nm.

[0052] Advantageously, using smaller metal nanoparticles may allow for connections between semiconductor particles without forming full metal conduction pathways through the composition which could create a short circuit. Further, smaller metal nanoparticles may have melting temperatures lower than that of their bulk forms, enabling more effective sintering of the nanoparticles within the network.

[0053] Preferably the composition comprises from 1 wt% to 65 wt% of the metal nanoparticles based on the total weight of the composition. The composition may comprise from 3 wt% to 60 wt% of the metal nanoparticles based on the total weight of the composition. The composition may comprise from 5 wt% to 55 wt% of the metal nanoparticles based on the total weight of the composition. The composition may comprise from 10 wt% to 40 wt% of the metal nanoparticles based on the total weight of the composition. The composition may comprise from 15 wt% to 30 wt% of the metal nanoparticles based on the total weight of the composition.

[0054] Preferably the amount of metal nanoparticles present is less than the percolation threshold. This can ensure that there is no all-metal electrical pathway in the composition, thereby avoiding short circuits.

[0055] Combination of semiconductor particles and metal nanoparticles

[0056] Preferably the semiconductor particles have a mean diameter that is larger than the mean diameter of the metal nanoparticles.

[0057] Preferably the ratio of the mean diameter of the semiconductor particles to the mean diameter of the metal nanoparticles is from 1000:1 to 2:1. The ratio of the mean diameter of the semiconductor particles to the mean diameter of the metal nanoparticles may be from 250:1 to 5:1. The ratio of the mean diameter of the semiconductor particles to the mean diameter of the metal nanoparticles may be from 250:1 to 10:1 . The ratio of the mean diameter of the semiconductor particles to the mean diameter of the metal nanoparticles may be from 20:1 to 5:1.

[0058] Preferably the weight ratio of the semiconductor particles to metal nanoparticles is from 100:1 to 2:3. The weight ratio of the semiconductor particles to metal nanoparticles may be from 20:1 to 1 :1. The weight ratio of the semiconductor particles to metal nanoparticles may be from 6:1 to 2:1.

[0059] Overall composition and network

[0060] In some compositions, the electrically conductive bridges comprise a plurality of metal nanoparticles. In some compositions, the electrically conductive bridges consist essentially of, preferably consist of, metal nanoparticles.

[0061] The electrically conductive bridges may comprise on average at least two metal nanoparticles. The electrically conductive bridges may comprise on average at least three metal nanoparticles. The electrically conductive bridges may comprise on average at least five metal nanoparticles per contact between semiconductor particles. The electrically conductive bridges may comprise on average at least 10 metal nanoparticles.

[0062] The electrically conductive bridges may have a length of 10nm or greater. The electrically conductive bridges may have a length of 15nm or greater. The electrically conductive bridges may have a length of 25nm or greater. The electrically conductive bridges may have a length of 50nm or greater. The electrically conductive bridges may be achieved by forming physical contacts between metal nanoparticles attached to different semiconductor particles. The electrically conductive bridges may be achieved by fully merging or sintering of metal nanoparticles into a single form by heat treatment.

[0063] It will be appreciated that the metal nanoparticles are generally not homogenously distributed over the semiconductor particles.

[0064] Preferably the metal nanoparticles are predominantly connecting the semiconductor particles (i.e. located in between adjacent semiconductor particles as conductive bridges). Preferably the metal nanoparticles are not randomly distributed throughout the composition.

[0065] In some compositions, the electrically conductive bridges comprise metal nanoparticles and conductive polymers. The conductive polymers may be either p-doped or n-doped. The conductive polymers may be selected from PEDOT:PSS (Poly(3,4- ethylenedioxythiophene) polystyrene sulfonate), PCBM (Phenyl-C61 -butyric acid methyl ester), MEH-PPV (Poly[2-methoxy-5-(2-ethylhexyloxy)-1 ,4-phenylenevinylene]), P3HT (Poly(3-hexylthiophene)), PPV (Poly(p-phenylene vinylene)), or combinations thereof. However, it will be appreciated that these are merely examples of suitable conductive polymers and other could also be used.

[0066] The composition may consist essentially of, preferably consist of, the semiconductor particles and the metal nanoparticles.

[0067] Preferably the composition is porous. The porosity may be of from 30% to 65%, for example 35% to 45%. Advantageously, a porous structure enables high thermal isolation from a substrate. This may allow the photodetector to function as a photoconductive microbolometer sensing temperature changes induced by photoabsorption through the temperature-dependent current in Schottky barriers.

[0068] Preferably the network is a 3-dimensional network. In other words, the semiconductor particles are connected to other semiconductor particles in all three dimensions by the electrically conducting bridges. This is in contrast with the structure of existing nanoparticle-based photodetectors, which are built by using either vertical or lateral stacks of nanoparticle films and metal contacts.

[0069] Preferably the metal nanoparticles are fused to the semiconductor particles. The network may have been thermally treated to fuse the metal nanoparticles to the semiconductor particles.

[0070] Preferably the semiconductor particles have been interconnected by assembling the metal nanoparticles using capillary forces. Photodetector

[0071] Preferably to composition is used as a layer in a photodetector, preferably a mid- IR photodetector. The layer may be positioned (e.g. deposited) on a substrate, such as a silica or silicon substrate, sapphire substrate, AI2O3 coated substrate, transparent conductive oxide (e.g, ITO) coated substrate., polymer (e.g., Pedot:PSS) coated substrate, or a metal oxide (e.g., TiO2, MoOa) coated substrate. The substrate may be a compound semiconductor (e.g., Ill-V group) substrate. The substrate may be rigid or may be flexible. The substrate may be flat. The substrate may be curved. The substrate may be porous (e.g., porous silicon) or not. The coatings on substrates may be made of organic (e.g,, polymer) material or inorganic material (e.g., metal oxide, metal, or glass like SisN4). The layer may be deposited on the substrate by drop casting, spin coating, doctor blading, or ink-jet printing. Preferably the layer is deposited on the substrate by drop casting.

[0072] Electrodes may be positioned in-between the substrate and the photodetecting layer. Alternatively, electrodes may be positioned on the photodetecting layer but not adjacent to the substrate. In other photodetectors, the photodetecting composition is positioned in-between the electrodes.

[0073] Method of manufacturing of a composition

[0074] An aspect of the present invention relates to a method of manufacturing a photodetector composition as described herein, the method comprising: contacting metal nanoparticles with semiconductor particles, the semiconductor particles preferably comprising a band gap less than 1.5 eV and more preferably comprising a band gap of less than 1.0 eV; and forming electrically conductive bridges comprising the metal nanoparticles to interconnect the semiconductor particles and thereby form an electrically conductive network.

[0075] Another aspect relates to a method of manufacturing a layer of photoactive material comprising depositing the photodetector composition described herein onto a substrate. This method may comprise forming the electrically conductive bridges to interconnect the semiconductor particles and thereby form an electrically conductive network, after the depositing the composition onto the substrate, or during depositing the composition onto the substrate. In some of these methods, the electrically conductive bridges are initially formed by inducing dewetting of the metal nanoparticles on the surface of the semiconductor particles. The bridges may then be formed by contacting the dewetted metal nanoparticles on adjacent semiconductor particles. The bridges may also be formed, or may be enhanced, by using capillary forces that control the motion of the metal nanoparticles, or the semiconductor particles decorated with the dewetted metal nanoparticles. In preferred methods, capillary forces may be created by evaporating solvents, such by drop casting film. In such methods the evaporating solvents may move metal nanoparticles (or the semiconductor particles decorated with dewetted metal nanoparticles) to form the bridges.

[0076] In some methods, contacting the metal nanoparticles with semiconductor particles comprises attaching the metal nanoparticles to the semiconductor particles with organic ligands. In such methods forming bridges may comprise thermally treating the semiconductor particles to remove the organic ligands and induce dewetting of the metal nanoparticles on the surface of the semiconductor particles. In such methods, the method may further comprise the step of exchanging the organic ligands with new organic ligands prior to removing the organic ligands by thermal treatment. Such methods may also comprise a chemical treatment step to remove the organic ligands. The chemical treatment step may be instead of the thermal treatment, or may be used subsequent to the thermal treatment to remove any residual attached organic ligands. The chemical treatment step may comprise washing the metal nanoparticles and semiconductor particles with an acidic (e.g. HCI) or an alkaline solution. The bridges may also be created, or enhanced, by capillary forces created by evaporating a solvent. Alternatively or additionally, the bridges may also be created, or enhanced, by capillary forces created from (re)melting the dewetted metal nanoparticles after deposition, for example via additional thermal annealing at high temperatures.

[0077] Advantageously, the use of capillary forces allows for self-assembly of the metal nanoparticles to form the bridges and the electrically conductive network.

[0078] In some methods of the invention the bridges are at least partially formed by the settling of the metal nanoparticles due to gravity. The settling may form a closed packed composition, for example, a close packed porous film.

[0079] In some methods of the invention, the method may comprise melting the metal nanoparticles to generate a surface tension of the molten metal nanoparticles between the faces of adjacent semiconductor particles and form capillary bridges between the adjacent semiconductor particles. In some methods of the invention, the semiconductor particles and metal nanoparticles may be dispersed in a polar solvent with a high surface tension, and capillary forces created by the evaporating the polar solvent.

[0080] In some methods, the method may comprise mixing of a minor phase solvent dispersion of the metal nanoparticles with an immiscible major phase solvent dispersion of the semiconductor particles, wherein capillary bridges form between adjacent semiconductor particles due the interfacial tension between immiscible solvents.

[0081] In some methods, the method forming bridges may comprises adding a conductive polymers with an affinity for the metal nanoparticles.

[0082] In some methods, the method may comprise producing the metal nanoparticles in situ on the semiconductor particles by reducing a metal precursor using a reducing agent. For example, gold nanoparticles may be produced by reduction of tetrachloroauric acid (HAuCk).

[0083] Some methods of the invention further comprise thermally annealing the electrically conductive network to fuse the metal nanoparticles to the semiconductor particles. Thermal annealing may comprise annealing in an oven, microwave heating, laser heating, and / or radio frequency heating.

[0084] Some methods of the invention further comprise milling, for example ball milling, the semiconductor particles prior to contacting the semiconductor particles with the metal nanoparticles. Milling may comprise milling in the presence of a solvent with or without a surfactant.

[0085] The method may comprise forming electrically conductive bridges comprising a plurality of the metal nanoparticles.

[0086] The method may comprise forming electrically conductive bridges that consist of the metal nanoparticles.

[0087] The method may comprise forming electrically conductive bridges that comprise metal nanoparticles and a conductive polymer.

[0088] The method may comprise forming the photodetector composition described herein. Accordingly, the properties of the semiconductor particles and metal nanoparticles may be as defined above in relation to the composition of the invention.

[0089] As described above, the invention also relates to a method of manufacturing a photodetector comprising incorporating the composition described herein as a photodetecting layer in the photodetector. The photodetector may be a photoconductor, photodiode or a phototransistor. The method may comprise depositing the composition on a substrate. The method may comprise depositing the composition on a substrate whereby the depositing of the composition forms the electrically conductive bridges that interconnect the semiconductor particles. The method may comprise depositing the composition on a substrate and then forming the electrically conductive bridges that interconnect the semiconductor particles. Depositing may comprise drop casting, spin coating, doctor blading, or ink-jet printing the composition. Preferably the layer is deposited on the substrate by drop casting.

[0090] The method may comprise pre-patterning (an) electrode(s) on the substrate prior to deposition. Alternatively, or additionally, the method may comprise patterning (an) electrode(s) on the deposited photodetecting layer.

[0091] BRIEF DESCRIPTION OF FIGURES

[0092] Figure 1 shows a schematic of a photodetector composition of the invention.

[0093] Figure 2 shows the photoresponse of a metal / semiconductor / metal interface, which forms the basic functional unit of the composition of the invention. Specifically, it shows the photoresponse of a single microparticle of Ti2Oa between gold contacts.

[0094] Figure 3 shows SEM images of a) Ti2Oa particles with Au nanoparticles homogenously covering the surface and b) Ti2Oa particles with dewetted Au nanoparticles forming electrically conductive bridges.

[0095] Figure 4 shows SEM images of Ti2Oa particles with differing loadings of Au nanoparticles.

[0096] Figure 5 shows EDX data for Ti2Oa particles with differing loadings of Au nanoparticles.

[0097] Figure 6 shows the photoresponse of a photodetector composition of the invention.

[0098] Figure 7 shows the photoresponse of a comparative sample without Au nanoparticles.

[0099] EXAMPLES

[0100] As described above, the present disclosure relates to a composition and photoactive material having a structure which is an electrically connected network comprising small band gap semiconductor particles interconnected via metal nanoparticles. The small band gap semiconductor particles in combination with the electrical connections provided by the metal nanoparticles allows for a photocurrent to be measured in response to illumination of mid-infrared light. The metal nanoparticles can be (self)assembled using e.g., capillary forces to provide the interconnections between the small band gap semiconductor particles. Thermal annealing may be used to fuse the metal / semiconductor interconnections. A schematic of this concept is shown in Figure 1. Figure 1a) illustrates the network of semiconductor particles 101 interconnected by electrically conductive bridges 102. Figure 1b) illustrates a bridge in more detail, showing it comprising a plurality of metal nanoparticles 103. Figure 1c) illustrates the optional fusing of the metal nanoparticles.

[0101] The examples below set out exemplary methods for preparation of the electrically connected network, along with the testing of the photoresponse of this network structure.

[0102] Some initial results showing the photoresponse of the metal / semiconductor interfaces is also provided to aid understanding of the invention.

[0103] The materials used in the examples were all commercially available, unless otherwise stated.

[0104] Preparation of T12O3 Particles

[0105] Ti2C>3 particles were purchased from Alfa Aesar (325 mesh powder, 99.8% purity) and were subjected to ball-milling in acetone to provide irregular shaped particles with smaller sizes (mean diameters) in the range of 20-500 nm. Surfactants such as oleylamine with a vol.1-2% can be added to the acetone solvent to increase the efficiency of the ball milling process, prevent cold welding of nanoparticles and reach sub-100 nm sizes. Surfactants may also protect against oxidation of nanoparticles during ball-milling due to the solvent-particle interaction or dissolved residual oxygen. The particles were first washed with fresh polar solvents by using an ultrasonicator and centrifuge. Then, they were dispersed in non-polar solvents like toluene or hexane as inks, which are compatible with photoresists used in optical lithography for patterning of nanoparticle films via lift-off. Ti2Oa exhibits a band gap of approximately 0.1 eV.

[0106] Preparation of Au nanoparticles

[0107] Dodecanethiol functionalized Gold (Au) nanoparticles were purchased from Nanoprobes, catalog number 3014. The gold particles were 3 to 5 nm in diameter and coated and stabilized with dodeane-1 -thiol, i.e. the gold particles comprised dodecanethiol (DDT) ligands attached to each particle.

[0108] Particle size measurements A mean particle size was determined using measurements obtained from SEM images. The size of each individual particle was determined by measuring and averaging the smallest and longest dimensions of said particle. Specifically, the size of the particles was measured by using SEM images of semiconductor particle films, which were drop- casted on either SEM sample stubs or silica / Si substrates. The magnification of the SEM was adjusted such that taken SEM images cover more than 50 particles. The average of the smallest and longest dimensions of each particle was measured with the ruler or length scale available in the SEM imaging and assigned as the size of the particle. The mean size was calculated by averaging over the measured sizes of many individual particles.

[0109] Drop casting of Au / Ti2O3 particles

[0110] Au / Ti2C>3 particles were dispersed in non-polar solvents, such as toluene, and stored as inks. Droplets (25 pl) from the inks (with concentrations up to 2g / 100ml) were then drop casted on flat substrates in a glove box (inert atmosphere), such that they spread over the surface of the substrate. The drop-casted wet particles were then dried naturally due to evaporation of the low boiling point solvent. Advantageously, drop casting can be used for film coating over large areas (1x1 cm2). It enables thin (< 3 pm) and uniform film (no holes) coating without wasting any material. Thicker films of nanoparticle can result in cracks, which may disrupt the electrical pathways.

[0111] SEM measurements

[0112] SEM measurements were conducted by using a secondary electron detector and a tube-in-lens detector (for higher resolution) in an SEM under ultrahigh vacuum (< 10'5Pa). Particle films were drop-casted on conductive substrates (e.g., SEM stubs) and then loaded into the SEM chamber. The top surface of the substrates was positioned at a working distance of 4 mm to increase spatial resolution in SEM imaging. An acceleration voltage of 5 kV and a current of 0.2 nA were chosen to minimize charging effects. Magnification and focus were adjusted accordingly to be able to see individual submicron particles.

[0113] Results

[0114] Photoresponse of T12O3 and Gold particle composite The basic functional unit of the network of the composition of the invention is a nanoparticle photoconductor with a metal / semiconductor / metal interface. In the following exemplary embodiments, the semiconductor used is titanium sesquioxide (titanium(lll) oxide, formula Ti2Oa) and the metal is gold (Au). The photoresponse of this basic unit was tested by positioning a single microparticle of Ti2Oa between two interdigitated Au metal contacts on a silica / Si substrate. A scanning electron microscope (SEM) image of the particle is shown in Figure 2 a). A bias voltage of 0.1 V was applied while the current was recorded using a semiconductor parameter analyser. Figure 2 b) shows the current measured in response to a modulated (on-off) laser beam of wavelength 642nm that was focused on the particle. As can be seen, the current changes in response to the on-off nature of the modulated laser. Figure 1 c) shows that a response could also be obtained from the infrared radiation emitted by a human hand. This IR is predominantly in the longwave infrared (LWIR, 8-12 pm) regime. The Au / Ti2Oa system is therefore clearly capable of functioning as a photodetector at single particle level.

[0115] Preparation of metal-interconnected semiconductor networks

[0116] To prepare the interconnected networks of semiconductor particles, Au nanoparticles were homogeneously attached to Ti2Oa particles by exchanging their original dodecanethiol (DDT) ligands with bifunctional mercaptopropionic acid (MPA) ligands and mixing the Au nanoparticles with the Ti2Oa particles in a solvent. The carboxylic acid and thiol groups of MPA ligands preferentially attached to Ti2Oa and Au particles, respectively. The process started with dispersing dry Au nanoparticles in the glass vial as received from the commercial source by adding 5 ml of toluene to form an Au nanoparticle ink. 1 ml of this Au nanoparticle ink was then added step wise to a mixture of 5 ml Ti2Oa nanoparticle ink in toluene with the desired concentration of particles and 50 pl pure MPA solution by using a pipettor in a glove box. At each step, 100 pl of the Au nanoparticle ink was used to ensure homogenous attachment and the resulting mixture was ultrasonicated for 1 hour and then centrifuged at 1000 rotation per minute (RPM) for 10 mins. Complete attachment is indicated by a clear remaining supernatant and precipitated particles. The precipitated Au / Ti2Oa particles in the mixture was redispersed by ultrasonication. The process steps were repeated until the total amount of 1 ml Au nanoparticle ink was used. Finally, the Au / Ti2Oa particle solution was washed a few times with fresh toluene and acetone to remove unattached Au nanoparticles and excess MPA ligands. The solution was evaporated in vacuum at room temperature for 1 hour to obtain dried Au / Ti20a particles, which were sealed in a glass vial and stored in a glove box.

[0117] Pre-deposition treatment for thermal activation

[0118] Thermal treatment of the dried Au / Ti20a particles was then conducted in vacuum within a temperature range of 300-1000 °C. The thermal treatment removed the organic ligands and induced dewetting of the Au nanoparticles on the surface of Ti2Oa particles due to high surface tension and interfacial energy between the Au and Ti2Oa, forming intimate contacts between Au and Ti2Oa and leading to metal / semiconductor interfaces. The thermal treatment was carried out in vacuum to minimize oxidization. It could also have been conducted in inert atmosphere (e.g. in an argon filled glovebox). Figure 3 a) shows an SEM image of a sample of Ti2Oa particles with Au nanoparticles homogenously covering the surface (i.e., before any thermal treatment). Figure 3 b) shows an SEM image after thermal annealing (and the subsequent postdeposition treatment described below), showing the dewetted Au nanoparticles 303 decorating the larger Ti2Oa particles. The Au nanoparticles provide physical contacts between the Ti2Oa particles, leading to an electrically conducting network after deposition. Figure 3c) shows a schematic for the process. Au particles 304 are attached to Ti2Oa particles 301 via ligands 305. Thermal treatment removes the ligands 305 and results in dewetted Au particles 303 on the surface of the Ti2Oa particles 301 , forming connections I bridges 302.

[0119] For semiconductor (nano)particles having sizes smaller than 100nm, the predeposition thermal treatment can be done at low temperatures (<300 °C). The use of lower temperatures can avoid unintentional sintering of the nanoparticles or phase change of the semiconductor material. To remove the organic ligands, room temperature chemical treatments based on washing of the nanoparticles with non-oxidizing acidic solutions such as hydrochloric acid (HCI) can also be applied. To remove the chemical residuals, the metal nanoparticle decorated semiconductor (nano)particles can be washed two to three times with fresh ethanol.

[0120] It will be appreciated that when Ti2Oa particles are first exposed to air, a thin shell (< 5nm) of TiC>2 forms around the particles 306. TiC>2 has a conductivity (o < =10“7S cm-1) several orders of magnitude lower than that of Ti2Oa (o ==160 S cm-1). TiC>2 also has a high band gap (3 eV). The TiC>2 thus behaves as an insulating barrier impeding charge transfer. However, the Au nanoparticles attached to the surface provide an electrical connection to the Ti2Oa core, bypassing the TiC>2 shells. This is another reason for the achievement of an electrically conducting network using the metal nanoparticles, which would otherwise not be possible due to the surface oxidation of semiconductor particles upon air exposure after deposition.

[0121] Once pre-deposition thermal treatment is over, the Au / Ti2Oa particles was dispersed in toluene solvent as a final ink, which can then be deposited (e.g., via drop casting or spin coating) onto a substrate to prepare a film of Au-interconnected Ti2Oa particle network. The thickness of the Au / Ti2Oa particle film can be adjusted in a range of 0.1-3 pm by controlling the concentration of the ink and the amount of ink volume taken for deposition. After the deposition on substrates and natural evaporation of the solvent, the Au / Ti2Oa particle films already showed very high conductance indicating establishment of an electrically connected network. Meanwhile deposited materials without any Au nanoparticles incorporated, i.e., films consisting of only Ti2Oa particles, result in an open circuit between contacts indicating high resistances above 109ohms.

[0122] Post deposition treatment

[0123] A post-deposition annealing of the Au-decorated Ti2Oa films could be applied at temperatures in the range of 100-300 C° (10 mins), which is relatively low compared to those in the pre-deposition thermal treatment (300-1000 C°, 30 mins). Post-deposition thermal annealing at these temperatures can remove residual solvents trapped between particles improving the electrical connectivity without causing any structural changes. As mentioned above, Figure 3b shows an SEM image after this post deposition treatment. For the avoidance of doubt, the predeposition treatment (described above) results in dewetted Au nanoparticles decorating the larger Ti2Oa particles, and post deposition treatment removes of solvent. Higher temperatures (>300 C°) and longer durations (>5 hours) can also be applied during the post deposition thermal treatment to induce sintering of Au nanoparticles, which decorate Ti2Oa particles. This effect could enable permanent capillary bridges forming between the touching faces of adjacent Audecorated Ti2C>3 particles providing an improved electrically connected network.

[0124] Different loadings of Gold Nanoparticles

[0125] Different loadings of Au nanoparticles on the Ti2Oa particles were achieved by changing the amount of Au nanoparticles added to the ball-milled Ti2Oa particle inks. Figures 4 a)-c) shows three SEM micrographs of Au / Ti2Oa particles with increasing amounts of Au nanoparticles, recorded in each case after the post-deposition thermal treatment step. Figure 4a) shows a very low loading of gold particles, Figure 4b) shows a low gold loading (11 wt % loading of Au particles) and Figure 4c) shows a high gold loading (53 wt% loading of Au particles). As can be seen, the number of dewetted gold particles and their size on the surface of the Ti2Oa semiconductor particles increases with the higher loading of the Au particles due to the merging of small Au nanoparticles (ripening effect) and their increasing surface coverage.

[0126] Energy dispersive X-ray (EDX) spectroscopy was used to identify and quantify the elements in the 2nd(11wt%) and 3rd(53wt%) samples, as shown in figure 4a, which is referred as the low and high Au loadings. A comparative sample consisting of only Ti2C>3 particles was also measured as reference to show the initial Ti and O atomic percentages (at.%). The EDX spectra is shown in figure 5 and the data is quantified in Table 1 below. Atomic percentages of the elements are provided, along with the wt% of Au. It will be appreciated that the silicon present is an impurity.

[0127] Table 1

[0128] Figure 4d), shows an image taken by a CMOS camera of the comparative film without any Au nanoparticles and the film with the high loading of Au nanoparticles. As can be seen, the film incorporating the Au nanoparticles exhibits a blacker appearance on the substrate. This is an indication of enhanced optical absorption and scattering due to the Au nanoparticles attached on the surfaces of the Ti2Oa particles.

[0129] Photo Detection

[0130] Photodetectors were made by drop casting the thermally treated Ti2Oa / Au particles (the low Au loading sample specified above) on a substrate comprising interdigitated electrodes of 5 pm separation. An SEM image of the Au / Ti2Oa particles after thermal activation and deposition is provided in figure 6 a). In this case, the post deposition thermal annealing was not conducted, as the solvent naturally evaporated at the room temperature resulting in a dry film deposited on substrates. The drop-casted film showed a photoresponse across a broad range of wavelengths of light, including visible, mid-wave infrared (MWIR) and long-wave infrared (LWIR). The photoresponse can be seen in figures 6 b) to d), which shows the current measured in response to on- off modulated laser light of different wavelengths. Figure 6 b) shows red visible light (642 nm), c) shows MWIR light (3.7 pm), and d) shows LWIR light (11 pm). MWIR and LWIR light were provided using a quantum cascade laser. All three graphs show distinct jumps in the current in response to the discrete periods of laser light illumination (on-off modulation), indicating that the film is suitable for detecting all three wavelengths of light.

[0131] A comparative Ti2Oa sample without any Au nanoparticles was also prepared and tested under same conditions, using interdigitated electrodes of varying separation (2 pm to 6 pm). These films exhibited very high resistances. A bias of 30V was therefore applied to read reasonable currents. Figure 7 a) shows the response to on-off modulated laser light of 11 pm and 8 pm wavelengths (LWIR light) of the comparative sample deposited on interdigitated electrodes of 2 pm separation. As can be seen, even with the bias voltage applied, and the reduced separation between the electrodes (and therefore reduced pathway for the charge carriers), the photocurrent values were at the nanoampere level, three orders of magnitude lower than the films with the Au nanoparticles. Using 5 pm electrode separation, in line with the Ti2Oa / Au example above, resulted in no appreciable photocurrent (See figure 7 b)). The comparative semiconductor particles without the Au interconnections are therefore unable to act as suitable photodetector.

[0132] Without wishing to be bound by theory, it is believed that a significant number of the charger carriers (electrons I holes) that are generated during the absorption of the infrared light in the comparative sample are unable to travel through the film to the electrodes. However, with the interconnected network of the present invention, a conductive pathway is present that allows for transport of the charge carriers and therefore higher current readings, leading to an appreciable photoresponse.

[0133] References

[0134] [1] V. Pejovic et al., “Infrared Colloidal Quantum Dot Image Sensors.” IEEE Transactions on Electron Devices 69, 2840-2850, 2022.

[0135] [2] A. Rogalski et al., “Infrared HOT Photodetectors: Status and Outlook.” Sensors 23(17), 7564, 2023.

[0136] [3] P. V. Karthik Yadav et al. “Advancements of uncooled infrared microbolometer materials: A review.” Sensors and Actuators A: Physical 342, 113611 , 2022.

[0137] [4] S. Bin Hafiz et al., “Colloidal quantum dots for thermal infrared sensing and imaging” Nano Convergence 6, 7, 2019.

Claims

CLAIMS:

1. A photodetector, preferably a mid-infrared photodetector, composition comprising: semiconductor particles comprising a band gap of less than 1.5 eV, preferably less than 1 eV; and metal nanoparticles; wherein the semiconductor particles are interconnected via electrically conducting bridges comprising the metal nanoparticles thereby forming an electrically conductive network.

2. A composition as claimed in claim 1 , wherein the semiconductor particles have a mean diameter that is larger than the mean diameter of the metal nanoparticles.

3. A composition as claimed in claim 1 or 2, wherein the ratio of mean diameter of the semiconductor particles to the mean diameter of the metal nanoparticles is from 1000:1 to 2: 1 , preferably from 20: 1 to 5: 1.

4. A composition as claimed in any preceding claim, wherein the mean diameter of the semiconductor particles is from 5 nm to 10000 nm, preferably from 5 nm to 300 nm.

5. A composition as claimed in any preceding claim, wherein the mean diameter of the metal nanoparticles is from 1 nm to 1000 nm, preferably from 5 nm to 50 nm.

6. A composition as claimed in any preceding claim, wherein the semiconductor particles are selected from Ti2Oa, InSb, InAs, InN, GaSb, Cd3As2, CdsP2, FeS2, Bi2Te3, Mg2Si, Cu2SnS3, silver chalcogenides (e.g., Ag2Se), SnTe, black phosphorus (BP), lead chalcogenides (e.g., PbSe), platinum chalcogenides (e.g., PtSe), tin chalcogenides (e.g., SnTe), binary or ternary alloys of Hg, Cd, Zn, and / or Te, lead tin telluride (Pbi-xSnxTe), single or binary alloys of semiconducting elements (e.g., Ge, Te, Sii-xGex), binary or ternary alloys of lll-V group elements (e.g., lnxGai-xAs, Alxlni-xAs, AlxGai-xSb, GaAsSb, InGaSb), metal oxides (e.g., VO2, CuO) or combinations thereof.

7. A composition as claimed in claim 6, wherein the semiconductor particles are Ti20s.

8. A composition as claimed in any preceding claim, wherein the metal nanoparticles are selected from Gold (Au), Silver (Ag), Platinum (Pt), Palladium (Pd), Ruthenium (Ru), Osmium (Os), Iridium (Ir), Indium (In), or combinations thereof.

9. A composition as claimed in claim 8, wherein the metal nanoparticles are gold nanoparticles.

10. A composition as claimed in claim 8, wherein the metal nanoparticles are indium nanoparticles.

11. A composition as claimed in any preceding claim, wherein the semiconductor particles have a band gap between 0.01 eV and 0.25 eV, preferably 0.05eV and 0.15 eV.

12. A composition as claimed in any preceding claim, wherein the composition comprises from 1 wt% to 65 wt% of the metal nanoparticles, preferably 10 wt% to 40 wt% of the metal nanoparticles, based on the total weight of the composition.

13. A composition as claimed in any preceding claim, wherein the weight ratio of the semiconductor particles to metal nanoparticles is of from 100:1 to 2:3.

14. A composition as claimed in any preceding claim, wherein the electrically conductive bridges comprise a plurality of metal nanoparticles or comprise metal nanoparticles and conductive polymers.

15. A composition as claimed in any preceding claim, wherein the composition is porous.

16. A layer of photoactive material comprising the composition as claimed in any of claims 1 to 15.

17. A photodetector, preferably a mid-infrared photodetector, comprising the composition as claimed in any of claims 1 to 15, or the layer of claim 16.

18. A method of manufacturing a composition as claimed in any one of claims 1 to 15, the method comprising:contacting metal nanoparticles with semiconductor particles, the semiconductor particles comprising a band gap of less than 1.5 eV, preferably less than 1 eV; and forming electrically conductive bridges comprising the metal nanoparticles to interconnect the semiconductor particles and thereby form an electrically conductive network.

19. A method according to claim 18, wherein the bridges are formed using capillary forces that control the motion of the metal nanoparticles; or wherein the bridges are formed by contacting semiconductor particles that are decorated with dewetted metal nanoparticles on their surface.

20. A method of manufacturing a layer of photoactive material as claimed in claim 16 comprising depositing a composition as claimed in any one of claims 1 to 15 onto a substrate.

21. A method as claimed in claim 20, comprising forming electrically conductive bridges to interconnect the semiconductor particles and thereby form an electrically conductive network, after or during depositing the composition onto the substrate.

22. A method according to any of claims 18 to 21 , wherein the bridges are formed from evaporation of a solvent, optionally wherein the evaporation of the solvent occurs during deposition of the composition onto a substrate.

23. A method as claimed in any one of claims 18 to 22, further comprising thermally annealing the electrically conductive network to fuse the metal nanoparticles to the semiconductor particles, optionally wherein thermally annealing comprises annealing in an oven, microwave heating, laser heating, and / or radio frequency heating.

24. A method according to any of claims 18 to 23, wherein contacting the metal nanoparticles with semiconductor particles comprises attaching the metal nanoparticles to the semiconductor particles with organic ligands, optionally wherein the method further comprises the step of exchanging the organic ligands with new organic ligands.

25. A method according to claim 24, wherein forming bridges comprises thermally treating the semiconductor particles to remove the organic ligands and induce dewetting of the metal nanoparticles on the surface of the semiconductor particles.

26. A method according to claim 25, further comprising chemically treating the semiconductor particles and dewetted nanoparticles to remove any residual organic ligands, optionally wherein chemical treatment comprises washing the metal nanoparticles and semiconductor particles with an acidic or alkaline solution.

27. A method according to claim 24, wherein forming bridges comprises chemically treating the semiconductor particles to remove the organic ligands and induce dewetting of the metal nanoparticles on the surface of the semiconductor particles, optionally wherein chemical treatment comprises washing the metal nanoparticles and semiconductor particles with an acidic or alkaline solution.

28. A method of manufacturing a photodetector, preferably a mid-IR photodetector comprising manufacturing a composition or layer of photoactive material according to any of claims 18 to 27 and incorporating said composition or layer as a photo-detecting layer in the photodetector.

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