GB2632395A - A photoactive material for absorbing infrared radiation, a device incorporating such photoactive material, and a method of making the photoactive material - Google Patents

Abstract

A photoactive material for absorbing infrared (IR) or near-infrared (SWIR) radiation is formed from a colloidal solution of nanorods of InAs, InAsP, InGaAs, TlInAs, InSb, InAsSb, InGaSb, InSbP and/or TlInSb. The nanorods have a longitudinal axis length of L < 100nm, cross-sectional width of W < 20 nm, and a length:width ratio of L/W >2. Image sensors 120A, 120B and photodetectors based on CSMOS ROIC wafers and utilising the photoactive material comprising nanorods (Fig. 2) in an active detection layer 225A, 225B avoid the toxicity problems of materials comprising heavy metals such as PbS and have fast response times in the nanosecond to picosecond range. Synthesis of InAs colloidal nanorods is by in situ complexation of readily available precursors such as complexation of In and As halides with lithium bis(trimethylsilyl)amide (LiN(Si(CH3)3)2) followed by reduction and subsequent growth of nanorods. Reaction conditions can be adjusted to tune the absorption spectrum of the nanorods deep into the SWIR range, surpassing a size barrier of conventional InAs QDs. The nanorods generally have a consistent shape and length, and a generally planar alignment. Clusters (Fig. 2: 22, 23) of nanorods are aligned adjacent one another and share the same polarity. The nanorods may taper along the length, one end broader than the other (Fig. 3).

Description

A PHOTOACTIVE MATERIAL FOR ABSORBING INFRARED RADIATION, A DEVICE INCORPORATING SUCH PHOTOACTIVE MATERIAL, AND A METHOD OF MAKING THE PHOTOACTIVE MATERIAL

Field

The present application relates to a photoactive material for absorbing infrared radiation, for example for use in fabricating an infrared image sensor. The present application further relates to a device (such as an image sensor or photodetector) incorporating such photoactive material, and a method of making the photoactive material.

Background

In general terms, "quantum dots" (QDs) are nanometer-scale semiconductor particles "having optical and electronic properties that differ from those of larger particles as a result of quantum mechanics" -see https://en.wikipedia.org/wiki/Quantum_dot for more details. Quantum dots are sometimes referred to as nanocrystals or nanoparticles.

It is known to use quantum dots for a sensor (detector) for infrared (IR) radiation, the portion of the electromagnetic spectrum spanning between visible light and radio waves. The incoming IR radiation is directly incident onto (and absorbed by) a layer of the quantum dots, which produces an electrical signal in response. This electrical signal is typically transferred to a silicon-based device and then converted into an image or other form of output.

Quantum dots can be formed from various materials which in turn affects their optical properties, such as the wavelengths at which they transmit or absorb light. Quantum dots are available which absorb (and hence can detect) light over a wide range of short wavelength IR (SWIR); this typically encompasses a wavelength range of 800-2500 nm.

Many existing quantum dot materials for SWIR detection are based on lead sulphide (PbS). However, lead is toxic and its use is subject to various regulations. Accordingly, there is interest in quantum dot sensors formed using other materials which avoid the toxicity problem of PbS (and other heavy metals). A further limitation of PbS quantum dots is a relatively slow response time, of the order of microseconds, whereas some potential applications for quantum dot IR detectors require a response time in the order of nanoseconds.

One existing material for use in SWIR detectors is indium gallium arsenide (InGaAs) which avoids the toxicity problem of PbS and other heavy metals. Current SWIR image sensors (1000-1700nm) include InGaAs thin films which are grown by molecular beam epitaxy. The integration of epitaxially grown InGaAs onto an image sensor involves a time-consuming process called hybridization.

As a consequence, the manufacturing costs for InGaAs thin film sensors are relatively high and so they are only suitable for niche applications which are less sensitive to pricing levels.

Another material for use in SWIR detectors is indium arsenide (I nAs) quantum dots, which again avoids the toxicity problems of PbS and other heavy metals and which also has a fast response time.

As described in "Fast Near-Infrared Photodetection Using III-V Colloidal Quantum Dots", by Bin Sun et al, Advanced Materials, Volume 34, Issue 33, August 2022 (see https://onfinelibrary.wiley.com/doVabs/10.1002/adma.202203039), such an InAs detector based on quantum dots having a pyramidal shape and a size of around 3 nm can absorb SWIR at a wavelength of 940 nm with an external quantum efficiency (EQE) of 30% and a 2 ns response time. The quantum dots in this paper have a pyramidal shape with a size of around 3nm.

One issue with InAs quantum dots is to grow larger sizes of quantum dots for better absorbance at longer wavelengths > 1200 nm. A SWIR sensor has been reported based on indium arsenide phosphide (In(As,P)) quantum dots, see "Colloidal III-V Quantum Dot Photodiodes for Short-Wave Infrared Photodetection" by Vladimir Pejovic et al, Advanced Science, Volume 9, Issue17, June 2022 (see https://onlinelibrary.wiley.com/doi/10.1002/advs.202200844). The disclosed particles are pyramidal in shape with size 7.4 nm. However, at 1400 nm this SWIR sensor provides a relatively low EQE (1%) and a relatively slow response time 0.6 -1.6 us. This lower quantum efficiency may lead to a decreased signal/noise ratio and greater power consumption.

Reference is further made to: "Synthesis and size-dependent properties of zinc-blende semiconductor quantum rods" by Kan et al, Nature Materials, Volume 2, pages 155-158, March 2003. This paper describes InAs nanorods which may be used with colloidal deposition as above. These quantum dots use gold nanocrystals as catalysts.

"Review on III-V Semiconductor Single Nanowire-Based Room Temperature Infrared Photodetectors" by Ziyuan Li et al, in Materials (Basle), March 2020 (see https://pubmed.ncbi.nlm.nih.gov/32204482/) describes the use of materials such as I nAs, InSb, InAsSb and HgCdTe for infrared photodetectors. Many nanostructures such as nanowires, nanotubes, nanopillars, nanorods and other low-dimensional materials have been developed to provide properties such as a small size (for small dark current), shorter response times, and larger bandwidths. The materials disclosed in this citation are generally prepared by physical methods such as epitaxial growth, rather than chemical methods such as colloidal deposition.

"GaAs-Based Nanoneedle Light Emitting Diode and Avalanche Photodiode Monolithically Integrated on a Silicon Substrate" by Linus Chuang, Nanoletters 2011, 11, 385-390, likewise involves a physical method of formation (monolithic integration).

"Short Wave Infrared Photodetectors and Imaging Sensors based on Lead Chalcogenide Colloidal Quantum Dots" by Zhixu Wu et al, in Advanced Optical Materials, Volume 11, Issue 1, October 2021, has a focus on SWIR imaging sensors, but these are based on lead chalcogenide and hence may be subject to one or more of the concerns identified above regarding the use of lead.

"Solution-Processable Infrared Photodetectors: Materials, Device Physics, and Applications", by Ning Li et al, in Materials Science and Engineering: R: Reports, Volume 146, October 2021, describes a wide range of infrared photon detectors.

US2012104325A1 discloses colloidal nanoparticles and inorganic capping agents bound to the surface of the nanoparticle for use in a large number of devices, including LEDs and imaging devices.

In summary, although there have been many positive developments relating to infrared photodetectors, there is ongoing interest in further enhancing infrared photodetectors to support technology and market opportunities.

Summary

The invention is defined in the appended claims.

As described herein, a photoactive material is provided for absorbing infrared radiation. The photoactive material is formed from a colloidal solution of nanorods of InAs, InAsP, InGaAs, TlInAs, InSb, InAsSb, InGaSb, InSbP and/or TlInSb. The nanorods have a longitudinal axis of length L < 100nm and a width, perpendicular to the longitudinal axis, of W < 20 nm, wherein the ratio of L/W > 2. In this context, the term nanorods should be understood to include any form of elongated quantum dots, such as elongated nanoparticles, elongated nanocrystals, nanosheets, and so on. An image sensor and a photodetector are also provided which utilise such photoactive material, as well as a method for making such a photoactive material.

Brief Description of the Figures

Various implementations of the claimed invention will now be described by way of example only with reference to the following drawings.

Figure 1 is a schematic diagram of an example of a SWIR image sensor as disclosed herein.

Figure 2 shows three images (a), (b), (c) acquired by a transmission electron microscope (TEM) showing examples of quantum dots (nanorods) as disclosed herein.

Figure 3 is an enlarged and annotated portion of image (c) from Figure 2.

Figure 4 is a further enlarged and annotated portion of image (c) from Figure 2.

Figure 5 shows seven images acquired by a transmission electron microscope (TEM) showing examples of quantum dots (nanorods) as disclosed herein having different first excitonic peaks.

Figure 6 is a graph showing infrared absorbance for examples of quantum dots (nanorods) such as shown in Figure 5 having different excitonic peaks.

Figure 7 comprises sections (a)-(e). Figure 7(a) is a schematic of the procedure followed to produce an InAs nanorod as described herein. Figure 7(b) is an X-ray diffraction plot supporting phase purity of the InAs nanorods. Figure 7(c) is somewhat similar to Figure 2 and shows a TEM image on the left and an HR TEM image on the right, with the growth direction of the In As nanorods indicated for the latter. Figure 7(d) shows spectra from X-ray photoelectron spectroscopy (XPS) which further support phase purity by the lack of oxide formation. Figure 7(e) provides Fourier transform infrared (FTIR) spectroscopy which indicates long-chain organic ligands that surface passivate the nanorods.

Figure 8 is an energy-dispersive X-ray spectroscopy (EDS) analysis of InAs nanorods showing a nearly 1:1 In to As atomic ratio.

Figure 9 provides TEM (top) and HRTEM (bottom) images showing the growth process in forming InAs nanorods which start as initial amorphous In nanoparticles, followed by an intermediate phase before reaching the final crystalline InAs nanorods. The particular sections of Figure 9 are as follows: (a) In nanoparticles; (b) intermediate In-InAs particles; (c) InAs nanorods; (d) a comparison of absorption spectra of the In nanoparticles, In-InAs intermediate particles, and InAs nanorods shown in Figure 9a-c; and (e) a schematic diagram showing the growth of crystalline InAs nanorods on top of the amorphous In nanoparticles.

Figure 10 depicts (a) Femtosecond transient absorption (fs-TA) spectra of 18.5 nm long InAs nanorods upon exciting with a pump at 1020 nm; (b) multiple excitation generation; and (c) annihilation dynamics of the InAs nanorods as a function of different pump excitations.

Figure 11 is the ground state bleach (GSB) recovery kinetics of 18.5 nm long InAs nanorods, upon pump excitation of 1020 nm.

Figure 12 comprises sections (a-d) showing fs-TA 2D color plots at different excitations of 18.5 nm long InAs nanorods.

Detailed Description

Current image sensor technology (based on silicon CMOS) operates primarily in the visible light range only, typically 300-800nm, analogous to the naked eye. Short wavelength infrared (SWIR) or visible-SWIR image sensors are able to capture images with detail far beyond the visible light range. As used herein, the (unqualified) term "light" should be understood as encompassing both optical (visible) electromagnetic radiation and also infrared electromagnetic radiation having a longer wavelength than visible light. As used herein, the term "SWIR" encompasses infrared electromagnetic radiation in the approximate wavelength range of 0.8 to 2.5 pm. A given SWIR device may operate across the entirety or just a portion of this range. In some cases, a SWIR device may also provide sensitivity outside the SWIR range, for example, a SWIR device may respond to the visible-SWIR range of 300-2500 nm.

In many industries, such as smartphones to automotive machinery and manufacturing to surveillance, sensor technology is fundamental to technological progress to provide devices with better information about their environment, thereby enabling more advanced operation and behaviour.

SWIR image sensor devices (also referred to as image detectors) enable image detection that goes significantly beyond human vision and produces images that are not achievable with conventional cameras. Devices including SWIR imaging capabilities have potential in many fields of technology, such as: -Smartphones: ToF (time-of-fight) sensors; enhancing security for biometric facial recognition, low light/dark conditions, 3D-photography, skin detection - Machine Vision: quality control; detecting defective products by seeing through packaging, plastic sorting, and so on - Automotive: safety of driver-assistance systems; improving vision at night/adverse weather However, for such SWIR image sensors to realise their full potential, there are still some significant limitations to address. Current SWIR image sensors, based on epitaxially grown InGaAs and primarily intended for imaging in the range 1000-1700nm, are complicated to manufacture and generally require a time-consuming process called hybridization. This manufacturing complexity has the effect of limiting production capacity for such sensors, whereby it is understood that the annual worldwide production of such image sensors is not more than 100,000 such imaging devices. The relatively low production numbers for these SWIR image devices leads to higher prices, typically $5,000 or more. This pricing level is only viable for niche applications, such as defence, space and medicine. The imaging devices are generally too expensive for more mainstream mass-adoption.

In addition to cost issues, existing SWIR imaging devices tend to be limited in resolution (and image size). For example, the size of InGaAs SWIR image sensors is typically limited to around 1.34 megapixels, which in turn restricts the resolution of images that can be obtained with such a device (see https://www.sony-semicon.com/en/products/is/industry/swir.html, https://www.sonysemicon.com/en/products/is/industry/swir.html).

SWIR image sensors based on colloidal quantum dot (QD) technology address such challenges. ODs are nanoparticles of semiconductor materials ranging in size (typically 1nm-20nm, but sometimes larger) that can be dispersed in colloidal liquid form using various solvents. This is a significant advantage, allowing the quantum dots to be deposited directly onto a silicon CMOS readout integrated circuit (ROIC) via technologies such as spin-coating/printing/photolithography to provide in effect a sensor chip for digital processing. This approach is convenient and affordable for accurately depositing the quantum dots onto the silicon substrate to satisfy any suitable form or dimensions, from small nanodroplets to larger device areas. Further, this approach is compatible with the mainstream manufacture and processing associated with large numbers of conventional (existing) silicon circuits -thereby avoiding more complicated and bespoke manufacturing processes.

The benefits of using colloidal QD technology include: -High manufacturing scalability e.g. up to 1 billion sensor units per/year; - Good affordability: depending on sensor volumes, a price reduction by a factor in the range 10-1000 would be expected, leading to a price typically in the range $5-500 per sensor device; and - Production of sensors with a large image size, for example in the range 2-10 megapixels, to support high resolution and/or a good field of view.

The following operational wavelengths are (without limitation) of particular interest for SWIR, at least for certain applications. In relation to eye tracking systems (for example), wavelength regions of interest include: a waveband centred on 940 nm; a waveband centred on 1200 nm; a waveband corresponding to or including 1300-1450 nm; and a waveband corresponding to or including 1750- 2500 nm. These wavebands represent regions of the infrared spectrum in which the earth's atmosphere has relatively high absorption. Over extended distances, such as corresponding to the depth of the atmosphere, there is significant attenuation of light at these wavelengths. As a result, comparatively little solar radiation in these wavebands passes through the atmosphere to ground level, and so there is little interference from background solar radiation. For machine vision applications, all wavelengths from 800 to 2500 nm are generally of interest. For automotive applications, wavelengths of (or in the range) 1200 nm and 1550 nm are of particular interest.

Figure 1 is a schematic diagram of an example of a SWIR image sensor as disclosed herein. In particular, Figure 1 depicts two different image sensors 120A, 120B, whereby image sensor 120B includes a pixellated array 201 B, but image sensor 120A does not have such a pixellated array.

The image detection devices 120A, 120B both comprise a layered (stack) structure which can be formed using standard CMOS lithographic techniques, including colloidal deposition. Each image detection device 120A, 120B has a silicon substrate which is used to provide a readout integrated circuit (ROIC) 260 for the image detection device. In effect, the silicon ROIC provides the image data to an external device for processing and analysis. The image detection devices 120A, 120B further comprise several thin layers deposited on top of the silicon ROIC 260. Progressing from the bottom up, the device structure includes a bottom electrode 240, a hole transport layer 230A, 230B, an optically sensitive (photoactive) layer of colloidal quantum dots 225A, 225B, an electron transport layer 220A, 220B, and a transparent top electrode 210A, 210B. Note that the top of the image detection devices 120A, 120B, as provided by the top electrode 210A, 210B, may also be regarded as the front end of the image sensing device, i.e. as the portion of the image sensing device 120A, 120B which directly receives incoming light (photons), with the silicon ROIC 260 then being regarded as the back end of the device.

Accordingly, the image detection devices 120A, 120B have bottom electrodes 240 formed on the top of the silicon ROIC 260, a hole transport layer 230A, 230B formed on top of the bottom electrodes 240, a layer of quantum dots 225A, 225B, formed on top of the hole transport layer 230A, 230B, and an electron transport layer 220A, 220B formed on top of the quantum dot layer 225A, 225B. The image detection devices 120A, 1208, are both further provided with a transparent top layer comprising a top electrode 210A, 2108 which is formed directly on the corresponding electron transport layer 220A, 220B.

It will be appreciated that the configurations shown in Figure 1 are provided as non-limiting examples, and other configurations may be used. For example, in other implementations, the hole transport layer and the electron transport layer may be swapped in position, and/or two or more hole transport layers and two or more electron transport layers may be used. Furthermore, in some implementations, glass or another material may be used to form the ROIC instead of silicon.

In operational terms, the top electrode 210A, 210B and the bottom electrode 240 are generally used to power the image detector 120A, 120B. The incoming electromagnetic radiation (light) passes through the transparent top electrode 210A, 210B and also through the electron transport layer to interact with and be absorbed by the quantum dots in layer 225A, 2258 to generate holes and electrons. The electrons formed (liberated) by the interactions between the incoming photons and the quantum dots are attracted towards the electron transport layer 220A, 220B; analogously, the holes formed (liberated) by the interactions between the incoming photons and the quantum dots are attracted towards the hole transport layer 230A, 230B. The holes and electrons are then transferred to the silicon ROIC 260 from the hole transport layer 230A, 2308 and the electron transport layer 220A, 220B respectively (via paths not shown in Figure 1) to provide the output image pixel data.

Since the quantum dots 225A, 225B provide the active detection layer, the layers above the quantum dots 225A, 225B are generally transparent to incoming radiation so that such radiation is able to progress to and hence be detected by the quantum dots in layers 225A, 225B. Note that this transparency applies in particular to the operational wavelengths used by the light source 110 and the light detector 120 to perform imaging; the layers above the quantum dots 225A, 225B may therefore be opaque (or only partially transparent) at wavelengths outside the set of wavelengths used by the image detectors 120A, 120B to perform imaging.

As mentioned above, the imaging device 120A has a different structure from the imaging device 120B, in that the latter incorporates a pixelated array 201B, in which all the layers stacked above the ROIC 260 have a pixelated structure. Thus the top electrode 210B, the electronic transport layer 220B, the quantum dot layer 225B, the hole transport layer 230B and the bottom electrode layer 240 share the same pixel structure, which therefore extends from top to bottom (front to back) across the imaging detector 120B. Typically, the spacings which extend vertically between adjacent pixels are produced using one or more etching techniques and may be filled with material that prevents cross-talk or interference between adjacent pixels, thereby helping to provide a high spatial resolution (low point spread function). Accordingly, the material between pixels is typically reflective or opaque, but generally not transparent for the light wavelengths of interest to the sensing device 1208.

In contrast, the imaging device 120A has a structure in which the bottom electrode 240 is pixelated (as for the imaging device 120B), but the higher layers in the device do not have a pixelated structure. Such a configuration for the imaging device 120A may be easier to manufacture than the pixelated array structure 201B of imaging device 120B, but the spatial resolution of the resulting image data may be slightly lower because the lack of isolation between different pixels could potentially lead to a higher level of cross-talk. In practice, the level of cross-talk in imaging device 120A may generally be maintained at an acceptable level because the voltage differential between the top and bottom electrodes 210A, 240 generally causes the electrons and holes to move primarily in a vertical direction (for the orientation shown in Figure 1). In addition, the layers shown in Figure 1 are also relatively thin, which further constrains the amount of cross-talk within device 120A.

The layer of quantum dots 225A, 2258 has a thickness typically in the range 50-500 nm, for example in the range 200-400 nm and serves as a photoactive material. A thinner layer of quantum dots (<50 nm) may allow certain photons to pass through the photoactive layer without being absorbed. A thicker layer of quantum dots (>400 nm) may make it harder for all interactions with incoming photons to be picked up by the electron and hole transport layers 220A, 220B, 230A, 230B. Accordingly, having a thickness in the range 50-400 nm can help to maximise the quantum efficiency of image sensors 120A, 120B.

In operation of the image sensors 120A, 120B, SVVIR light is incident upon the quantum dot layer 225A, 225B and generates electron-hole pairs (charges) in the quantum dot layer. As discussed above, these charges are collected in the corresponding transport layers: holes in hole transport layer 230A, 230B and electrons in the electron transport layer 220A, 220B. The greater the intensity of light received by the quantum dot layer 225A, 225B, the greater the generation of holes and electrons.

The electrons and holes in effect form a current for each pixel and the silicon ROIC 260 determines the output from this current for each individual pixel such that the output corresponds to the image intensity at that pixel. The image sensor 120A, 1208 outputs an image comprising an intensity value for each pixel in the set of pixels provided in the image sensor 120A, 120B. In many applications, the image sensor may be configured to provides a time series of images (or a video).

The quantum dots in layer 225A, 225B may be formed from various materials. By way of example (and without limitation) quantum dots active in the SWIR range may comprise InAs, InAsP, InGaAs, TlInAs, InSb, InAsSb, InGaSb, InSbP, TlInSb, Cu2S, Cu2Se Ag2S and/or Ag2Se, or core/shell quantum dots with any of the preceding constituents. Quantum dots based on indium, such as InAs, InAsP, InGaAs, TlInAs, InSb, InAsSb, InGaSb, InSbP, TlInSb, and also Cu2S, Cu2Se, Ag2S and Ag2Se quantum dots, do not contain heavy metals, and so are compliant with the regulatory requirements mentioned above. InAs quantum dots are a promising alternative to those containing PbS or other heavy metals to produce lead-free quantum dots for use in an image sensor for infrared wavelengths. In these wavelengths, InAs quantum dots generally have a lower EQE than PbS quantum dots, for example around 30% at a wavelength of 940nm and 5% at a wavelength of 1400 nm. InAs quantum dots do have a relatively fast response time which makes them attractive for applications in which this fast response time is beneficial, such as high-speed video and/or light detection and ranging (LIDAR).

The choice of material for the quantum dots in turn affects the optical properties of the quantum dots, including the wavelengths at which they transmit or absorb light. Quantum dots may be provided which absorb (and hence can detect) light over SWIR wavelengths spanning all or part of 0.8-2.5 pm. This is in contrast to existing image detection systems which use a silicon image detector and which are typically limited to use with optical wavelengths or wavelengths that extend a little into the infrared, for example up to 940 nm.

The energy levels, doping polarity, and charge carrier density in quantum dot semiconductors may be controlled by suitably chosen capping ligands, which donate charges to the nanocrystal core and introduce localized dipoles on their surface. Alternatively, the work function and other semiconducting characteristics may be controlled by chemical doping of the quantum dot core, for example with other material such as Zn, Al, Ga, TI etc. The capping ligands on the above quantum dots may comprise organic molecules or inorganic molecules, or a combination of both. Organic ligands may include, but are not limited to, aryl or alkyl thiols, such as 1,4-benzenethiol, 1,2-ethanedithiol, 3-mercaptopropionic acid, and so on. Organic ligands may further include N-heterocycles or amines, such as pyridine, 1,2-ethylenediamine, and so on. Inorganic ligands may include chalcogens (S, Se), pseudo halogens (SCN), or atomic halogens (I, Br, CI), chalcogens (S, Se), or more complex metal halides (Ink, InBr3, SnI2, SnBr2) or metal chalcogenides.

To enhance sensor performance, specialised layers, or charge carrier selective layers, may be introduced at the interface of quantum dot layers. The specialised layers decrease or increase the energy barrier for a certain charge carrier type. For example, an electron transport layer (ETL), or a hole blocking layer (HBL), favour electron transport. A hole transport layer (HTL), or an electron blocking layer (EBL), favour the transport of positively charged carriers, also known as holes. The specialised layers may be also called electron/hole transport material (ETM/HTM) or an electron/hole injection layer (EIL/HIL) , or electron/ hole injection material (EIM/ HIM) . The ETL layers may comprise inorganic materials such as ZnO, TiO2, Sn02, SrTiO3, Zn2SnO4, InAs, InSb, etc., or organic small molecules such as C60, TPBi, NPB, BCP, PCBM, PTCDA, BPhen, Alq3, etc. The HTL layers may comprise inorganic materials such as NiO, Mo03, Cul, Cu2O, CuSCN, InAs, InSb, etc., or organic small molecules such as 2TNATA, m-MTDATA, Spiro-OMeTAD, NPNPB, TPB, NPB, etc., or organic polymers such as PEDOT:PSS, P3HT, PTAA, Poly-TPD, MEH-PPV, PVK, etc. The carrier selective layer may be a conducting or semiconducting layer having a thickness in the range of 5-200 nm. In some implementations, the introduction of suitable HTL and ETL into a photodiode architecture may result in a decrease of dark leakage current from 1 mA/cm2 to 1 pA/cm2; in some implementations from 10 pA/cm2 to 10 nA/cm2. The introduction of suitable HTL and ETL layers may also increase the EQE figure of merit from 5% to 40%, in some implementations from 40% to 60%.

The bottom and top electrodes of an image sensor or photodiode may comprise a material selected from the group of metals: Au, Ni, Ag, Pd, Cu, Al, etc., or from the group of transparent conductive oxides: In -SnO2 (ITO), Al -ZnO (AZO), Al -SnO2 (ATO), F -SnO2 (FTO), and so on.

The work function of electrodes may be suitably chosen to favour the charge carrier transport through the interfaces.

The SWIR image sensors 120A, 120B based on colloidal quantum dots may be produced using known (widespread) CMOS technology that can be scaled up to a large number of units per year on large CMOS ROIC wafers with diameters 20 or 30 cm. Each individual wafer might contain from 100 to 50,000 sensors depending on the resolution of the sensors. This scaling is significant in reducing the cost of SWIR quantum dot sensors per unit which in turn makes it financially viable to utilise such a quantum dot SWIR sensor in many different types of device and application (in contrast to existing SWIR sensors which may be too expensive for use in some devices and applications).

As discussed above, PbS quantum dots currently dominate the market for SWIR image sensors. However, PbS quantum dots are prevented (or avoided) from use in significant consumer applications like smartphones, ARNR headsets and automotive applications because they contain lead, which is a "restricted element" according to restrictions of hazardous substances (RoHS regulations) -see https://en.wikipedia.org/wiki/Restriction_of Hazardous_Substances_Directive.

Furthermore, the response time of SWIR PbS sensors, namely the time between absorbing the light and converting it to an electrical signal, is typically of the order of microseconds. In contrast, for some significant applications such as smartphone, automobile, and ARNR, there is interest in a much faster response time, of the order of nanoseconds.

InAs colloidal quantum dots have emerged as a promising candidate for lead-and mercury-free solution-processed semiconductors for infrared technology due to their appropriate bulk bandgap, which can be tuned by quantum confinement, and promising charge-carrier transport properties. InAs quantum dots are able to overcome both of the above concerns -they do not contain lead or other heaving metals and their response time generally lies in the nanosecond-picosecond range. In addition, InAs quantum dots can be formed without the use or inclusion of high-cost materials, such as gold. For example, a SWIR photodiode with InAs quantum dots may have an EQE at 30% and a response time of 2 ns for operating at a wavelength of 940 nm (see "Fast Near-Infrared Photodetection Using III-V Colloidal Quantum Dots" as cited above). These quantum dots have a pyramidal shape with a size of around 3nm.

This method of growing of InAs quantum dots typically involves the use of such arsenic precursors as tris-trimethylsilyl arsine (TMS-As) and tris-trimethylgermyl arsine (TMGe-As). Methods using those precursors have generally been the most successful at producing monodisperse InAs QDs with sharp excitonic features and good SWIR sensor performance. Nevertheless, the use of TMS-As and TMGe-As presents difficulties as it is highly reactive, pyrophoric and commercially scarce. Mishandling TMS-As and TMGe-As can lead to the formation of arsine gas, which is lethal even at exposure levels as low as 10 ppm. Additionally, the high reactivity of TMS-As and TMGe-As makes it difficult to control the nucleation rate during the InAs QD synthesis. This results in the rapid consumption of molecular precursors, subsequently restricting the QDs to smaller sizes (<5 nm) that absorb below 1200 nm, as well as obscuring the reaction intermediates and mechanism.

An important issue for InAs quantum dots is to grow larger quantum dot sizes to help achieve longer absorbance profiles and consequently to fabricate a SWIR sensor with sensitivity > 1200 nm, particle size > 5 nm (which is of interest to many industries). As one example of recent work in this area, an SWIR detector has been disclosed which uses In(As,P) for sensing relatively long wavelengths with absorbance at 1400 nm (see "Colloidal III-V Quantum Dot Photodiodes for Short-Wave Infrared Photodetection" as cited above). These quantum dots are again pyramidal in shape with an approximate size of around 7.4nm. However, this SWIR sensor had an EQE down at 1% and a relatively slow response time of 0.6-1.6 microseconds. These properties may be due to non-optimal composition of In(As,P) quantum dots leading to poor performance of SWIR devices. The disclosed method of synthesis utilizes less reactive As precursors, including tris(dimethylamino)arsine and arsenic halides.

The approach described herein provides a controlled synthesis strategy using only readily available precursors to overcome the practical wavelength limitation of many existing InAs QDs, achieving monodisperse InAs nanorods with bandgaps tunable from -1200 to -1800 nm, thus extending deep into the short-wave infrared (SWIR) region. By controlling reactivity through in situ precursor complexation, it is possible isolate the reaction mechanism, producing InAs colloidal nanorods that display narrow excitonic features and efficient carrier multiplication. Notably, this strategy is a 'one-pot' synthesis that may solely utilize less reactive and readily available metal halide precursors to greatly simplify the fabrication of InAs nanorods. The synthesis involves the in situ complexation of In and As halides with lithium bis(trimethylsilyl)amide (LiN(Si(CH3)3)2), and proceeds via metal nanoparticle formation that directs the growth of InAs along a particular axis, leading to colloidal nanorod formation. The reaction conditions can be easily adjusted to tune the absorption spectrum of the nanorods from -1200 to -1800 nm, extending deep into the SWIR and surpassing the practical size barrier encountered by conventional InAs QDs. The resultant nanorods display sharp excitonic absorption features, having the narrowest half-width at half-maximum (HWHM) for the first excitonic feature within this wavelength regime, and exhibit carrier multiplication at low pump excitation fluences and energies -characteristics consistent with their high monodispersity and crystallinity. This methodology addresses both the size limitation and the precursor-related challenges of InAs QD syntheses, bringing these materials within the practical reach of a wide range of SWIR applications.

InAs QD synthesis normally involves the hot injection of highly reactive precursors or the use of strong reducing agents in the case of less reactive precursors. The high reactivity of the precursors or the use of strong reducing agents results in fast and uncontrollable nucleation, which limits the QDs to smaller sizes and also affects their size distribution. The present case provides an alternative method which helps to control the precursor reactivity for achieving larger size and monodisperse InAs QDs.

In metal nanoparticle synthesis, complexation of the metal ions with LiN(Si(CH3)3)2 prior to their reduction has been found to control their reactivity, leading to the formation highly monodisperse colloidal nanoparticles having a relatively large size. While a similar prior complexation was also found to produce the large size and monodisperse InSb ODs, the complexes of In and As with LiN(Si(CH,)3), are usually unstable and hence require storage at -40 °C in an inert atmosphere. To make the storage and handling of the In and As complexes practical and to control the reactivity, it was reasoned that the complexation should be carried out in situ, i.e., the In and As precursors are together complexed with LiN(Si(CH3)3)2 and subsequently the same precursor complex solution is reduced to get InAs QDs.

With this strategy, the synthesis of InAs nanorods using commonly available metal halide precursors was investigated. Equimolar amounts of InCI3 and AsI3 were dissolved in oleylamine in the presence of 6 molar equivalents (eq) of LiN(Si(CH,),), and degassed at 130 °C for 3 hours. The solution was then cooled down to room temperature, and variable amounts of the reducing agent, lithium triethylborohydride (Li(C2H5)3BH) in dioctyl ether, were injected. Due to the in situ complexation, no abrupt reduction occurred upon the injection of Li(C2H5)3BH, unlike for conventional synthesis methods. To controllably initiate the reduction process and subsequent growth of the InAs nanorods, the temperature of the solution was slowly raised. The reaction proceeded in a controlled manner, and once the desired temperature was reached, the reaction was quenched by blowing air on the reaction vessel. Dodecane thiol or oleic acid was then added to the nanorods solution to enhance its long-term colloidal stability.

Figure 2 shows three images acquired by a transmission electron microscope (TEM) showing examples of quantum dots forming part of a photoactive (photosensitive) InAs nanorods material as disclosed herein. In particular, Figure 2 comprises three TEM images namely a left image 2(a), a central image 2(b) and a right image 2(c). Images 2(b) and 2(c) are high-resolution (HR) TEM images.

Each image includes a bar (white) in the bottom left-hand corner representing the distance scale. For image 2(a), the bar has a length of 50nm, while for images 2(b) and 2(c) the bar represents a length of 1 Onm, with the 1 Onm bar for image 2(c) being somewhat longer than the 1 Onm bar for image 2(b). Accordingly, image 2(b) is magnified relative to image 2(a), and image 2(c) is further magnified relative to image 2(b).

Each image in Figure 2 can be considered as a 2-dimesional slice through a 3-dimensional structure comprising a quantum dot layer (such as layers 225A, 225B in Figure 1). However, the quantum dots shown in Figure 2, see in particular image 2(a), have a generally consistent length and elongated shape. To reflect this elongation, hereinafter we refer to the quantum dots in Figure 2 as nanoneedles or nanorods, or as elongated nanoparticles or elongated nanocrystals (or use any other similar terminology).

The apparently consistent shape and length of the nanorods 310 visible in Figure 2(a) (and also apparent in images 2(b) and 2(c) strongly indicate that (i) the nanorods 310 generally do have a consistent shape and length; and (ii) the nanorods generally have a planar arrangement, parallel to the plane of image 2(a), (b) and (c).

Regarding (i), if there was significant physical variation in the shape and/or sizing of the nanorods, we would expect such variation to be apparent in Figure 2. Instead, this indicates that the level of length variation (standard deviation) of the nanorods is relatively small compared to the typical (average) length of the nanorods. Likewise, if there were significant variation in the three-dimensional orientation of the nanorods as per (ii), i.e. nanorods lying at different angles to the plane of image 2(a), we would expect to see significant apparent variation in the sizing of the nanorods (reflecting their projection from three-dimensional space onto the image plane of Figure 2, for example with nanorods that lie close to perpendicular with respect to the image plane appearing greatly foreshortened).

The nanorods 310 in Figure 2 are primarily seen to have a planar arrangement, but there are some indications of depth in a direction perpendicular to the plane of the image. Thus nanorods 310K and 310L appear to overlap one another in region 21, the overlap being indicated by a darker tone. Nanorod 310K appears to further overlap nanorod 310N. The overall impression is that the depth direction is therefore sufficient for one nanorod to pass over or under another nanorod, but this does not detract from the general configuration in which the nanorods 310 share a coplanar orientation.

Figure 3 is an enlarged and annotated version of image (c) from Figure 2. Figure 3 includes a row of three nanorods, the left and centre nanorods being labelled 310, and the right nanorod being labelled 310A (this latter reference number being for ease of specific reference, not to indicate any physical difference between nanorod 310A and nanorods 310). It can be readily seen that nanorods 310, 310A are elongated in shape. Using the scale bar in Figure 3, and assuming that the nanorods 310 and 310A lie approximately in the plane of Figure 3 (for the reasons given above), we estimate that nanorod 310A has a length of approximately 30 nm while the left nanorod 310 has a length of approximately 23 nm.

The shape of the nanorod 310A (and also the other nanorods 310) is based on a longitudinal axis 301 which extends in a straight line from L1 to L2. A first end (head) 313 of the nanorod 310A is located adjacent L1 while a second end (tail 312) of the nanorod 310A is located adjacent L2. The similar shaping of multiple nanorods (including for example those shown in image 2(c) of Figure 2) indicates that the nanorods 310, 310A may have rotational symmetry about the longitudinal axis 301. The cross-section of a nanorod 310 in a plane perpendicular to the longitudinal axis may have various shapes, such as circular, elliptical, square or rectangular. The different shapes lead to different symmetries for the nanorod 310. For example, a circular cross-section has full rotational symmetry for any angle of rotation. A square has rotational symmetry of order 4, while an elliptical rectangular shape has a rotational symmetry of order 2. The cross-sectional shape may also possess one or more planes of mirror symmetry. For example, a rectangular cross-section has two mirror symmetries about 2 respective (perpendicular) planes each incorporating the longitudinal axis 301, with a first plane perpendicular to the smaller face of the rectangle, and with a second plane perpendicular to the larger face of the rectangle. An elliptical cross-section likewise has two planes of mirror symmetry, while a square has four planes of mirror symmetry (the same two planes as for the rectangle, plus two diagonal planes).

It can be readily seen in Figure 3 that the head 313 of the nanorod 310A is broader (more rounded) than the tail 312 of the nanorod 310A (which is more pointed). Accordingly, the diameter of the nanorods 310, 310A, as measured in a direction perpendicular to axis 301, narrows (tapers) along the direction from L1 to L2. Also, the radius of curvature at the head end 313 is greater (for a gentler curve) compared to the lower radius of curvature at the tail end 312 (which gives a sharper, more pointed curve). The shape of the nanorods 310, 310A therefore broadly resembles a carrot, in other words, a conical shape with a relatively narrow apex angle, a curved tip (apex) and likewise a curved base.

Figure 4 depicts a further enlarged and annotated version of nanorod 310A. Figure 4 shows a simple geometrical model of the nanorod 310A; although this model does not match the exact shape of nanorod 310A, nevertheless this model helps to parameterise certain aspects regarding the shape of nanorod 310A. In particular, the head portion 313 of nanorod 310A is modelled as a hemisphere of diameter D1, while the tail portion 312 of nanorod 310A is modelled as a hemisphere of diameter D2, with D1>D2. The head portion 313 is separated from the tail portion 312 by a body portion 314 which has a truncated (tapered) conical shape of length or height H. In other words, the body portion 314 extends the length of the truncated conical shape and tapers inwards when progressing from the head 313 to the tail 312. The head 313 and tail 312 are located at the respective ends of the nanorod 310A where the shape transitions from the truncated (tapered) conical shape to the hemispherical shape of the head and tail.

The size and shape of the nanorod 310A is therefore defined by these three components head 313, tail 312 and body 314, and their respective shapes and sizing D1, D2 and H, in conjunction with the previously mentioned rotational symmetry about longitudinal axis 301. In this model, we further define R1=D1/2 as the radius of the head hemisphere, R2=D2/2 as the radius of the tail hemisphere, and HT=H+R1+R2 as the total length of the nanorod 310A.

In various implementations, the total length of a nanorod 310 may lie in a range HT(1)-HT(2) in which HT(1) may be 10nm, 12mm, 15mm, 20mm, or 30 nm, and HT(2) may be 30nm, 40nm, 50nm, 60nm or 100nm (subject to HT(1) < HT(2). In some cases, the total length may correspond to HT as defined above for the model of Figure 4; in other cases, the total length may be determined on some other basis, for example, the longest straight line that can be contained within the nanorod 10, optionally wherein the longest straight line is constrained to pass through the centre of mass of the nanorod 310. Other plausible ways of defining and measuring the total length of the nanorod will be apparent to the skilled person.

In various implementations, the maximum width of a nanorod 310 may lie in a range W(1)-W(2), in which W(1) may be 3nm. 5nm or 7nm, and W(2) may be 8nm, 10nm, 12nm, 15nm or 20nm. In some cases, the maximum width may correspond to D1 as defined above for the model of Figure 4; in other cases, the maximum width may be determined on some other basis, for example, the longest straight line which is perpendicular to the longitudinal axis L1-L2 (see Figure 3) and contained within the nanorod 310, or the longest radial line which extends perpendicularly from the longitudinal and is contained within the nanorod (the radial distance would then be doubled to represent a diameter).

In various implementations, the ratio of the total length of the nanorod 310A to the maximum width may lie in the range E(1)-(E2), in which E(1) may be 1.5, 2.5 or 3.5, and E(2) may be 5, 6, 8 or 12. For these implementations, the total length and the maximum width may be defined, for example, as shown in Figure 4 or as discussed above, or in any other suitable manner. This ratio of the total length HT of the nanorod 310A to the maximum width reflects the elongation of the nanorods 310, 310A. In various implementations, the ratio of D1 at the head to D2 at the tail may lie in the range S(1)-S(2), where S(1) may be 1.2, 1.6, or 2, and S(2) may be 2.5, 4 or 6.

It will be appreciated that the above dimensions and ratios are provided by way of example, but without particular limitation. Furthermore, the model of Figure 4 is adopted to parameterise the shape of nanorod 310A more easily, but the model is only an approximation. Therefore the dimensions and ratios may be adjusted as appropriate to accommodate any inaccuracies in the shape of the model with respect to an actual nanorod 310.

One other aspect of nanorods 310, 310A is that the atomic structure within the nanorods is organised or patterned to have parallel lines (planes) or ridges -as indicated by reference number 315 in Figures 3 and 4. These lines typically extend across the whole width of the nanorods 310, 310A in a direction which is perpendicular to the longitudinal axis 301 of nanorod 310A. The spacing of these atomic planes is approximately 0.4 nm. These lines 315 may represent the growth of successive layers of the nanorods as they progress in a longitudinal direction. For example, in some implementations, the growth of a nanorod may commence with the head 313, then proceed in a longitudinal direction by adding successive layers 315, and then conclude (terminate) at the tail 312.

Returning to Figure 2, image (a) clearly shows that most of the nanorods 310 are in clusters of nanorods. Typically, at least 75%, and potentially at least 85%, of the nanorods 310 are in a cluster, where a cluster comprises two or more nanorods which are aligned with, and adjacent to, each other. For example, the cluster 23 comprises four nanorods aligned in a side-by-side arrangement, all extending diagonally from upper left down to lower right (according to the orientation of the page). Note that for cluster 23, the heads 313 are all to the upper left and the tails 312 are all down to the bottom right -this can be regarded as the nanorods 310 in cluster 23 all sharing the same polarity.

Image (a) also depicts some clusters that have a mixture of polarities. Thus cluster 22 comprises six nanorods likewise aligned with and adjacent to each other, with the nanorods again in a side-by-side arrangement extending diagonally from upper left to lower right. Counting from the left, the first, second, fourth and sixth nanorods have their respective heads 313 upper left (which we can label an "up" polarity), while the third and fifth nanorods in cluster 22 have their respective heads 313 lower right (which we can label as a "down" or opposing polarity).

Looking at image (a) as a whole, the different clusters adopt a wide spread of different directionality -in other words, the nanorods form multiple clusters in the photoactive material, the nanorods in any given cluster are aligned with one another, but the alignment is different from one cluster to another. In other words, although each cluster shows a clear directionality for the nanorods 310 therein, it is not apparent that the ensemble of clusters in image (a) have a (global) preferred or dominant directionality, but rather some randomness appears to exist at the level of clusters (rather than nanorods per se).

Depending of the synthesis conditions of the InAs nanorods, it is possible to change the resulting sizes of the nanorods and hence tune the absorption profile of the nanorods. In standard QD synthesis, usually the reaction temperature and growth time play crucial roles in determining the size of the particles. Surprisingly, both the reaction temperature and growth time had little or no effect on the size of the InAs nanorods for the approach described herein. Instead, it was found that the concentration of the reducing agent [Li(C2H5)3BH] primarily determined the size of the nanorods.

Consequently, by varying the amount of reducing agent, it was possible to tune the size of the InAs nanorods over a wide range.

Figure 5 shows seven images acquired by a transmission electron microscope (TEM) showing examples of quantum dots (nanorods) as disclosed herein having different excitonic peaks. More particularly, the images in the top row are conventional TEM images, while the images in the bottom row are corresponding HR-TEM images. The general features of these TEM and HR-TEM images have already been described above with reference to Figures 2, 3 and 4.

Figure 5 shows the TEM images of the InAs nanorods synthesized by varying the amount of reducing agent from 3 to 8 equivalents (eq) with respect to (w.r.t.) the In or As. The average length and base width of the InAs nanorods grew from 15 nm to 32.5 nm and 4 nm to 6.5 nm, respectively, upon increasing the amount of reducing agent from 3 to 8 eq. The first excitonic feature of the InAs nanorods changes from 1170 nm (for 3 eq) to 1650 nm (for 8 eq). The nanorod shape such as shown in Figure 5 may bring additional benefits to the semiconducting properties of thin films, such as higher charge carrier mobilities and thus higher device efficiencies and faster response time.

Each image in the top row of Figure 5 is labelled according to the first excitonic peak of the nanorods shown in that image. As discussed in more detail below, the absorption of light increases significantly at wavelengths shorter than the first excitonic peak wavelengths -i.e. on the higher frequency/energy side of the first excitonic peak. Increased absorption indicates a better optical detector, in that the incident light is transformed into a stronger electrical signal.

The lengths of the nanorods can be roughly estimated for each of the three HR-TEM images, whereby the length is 15 nm for nanorods having a first excitonic peak of 1170 nm, 20 nm for nanorods having a first excitonic peak of 1280nm, 23 nm for nanorods having a fist excitonic peak of 1380 nm, 27 nm for nanorods having a first excitonic peak of 1510nm, and 32.5 nm for nanorods having a first excitonic peak of 1650nm. Accordingly, a photoactive material can be configured to have an absorption peak which corresponds to an infrared waveband of interest by changing (tuning) the size (length) of the nanorods included in the material.

A similar effect is discernible from the top row of conventional TEM images. In these four images, the size of the 100 nm scale bar bottom left is fairly consistent across all four images, so that these four images have approximately the same scaling and therefore can be directly compared with one another. It is then clear that the nanorods in the first (left-most) image for excitonic peak 1170 nm are shorter than the nanorods in the fourth (right-most) image for excitonic peak 1650 nm, hence the nanorods shown in the fourth (right-most) image provide absorption extending out to longer wavelengths than the first (left-most image). Similarly, the second and third images have nanorods of intermediate length, and so provide absorption extending out to intermediate wavelengths.

Figure 6 is a graph showing curves of infrared absorbance for quantum dots (nanorods) such as shown in Figure 5 having different first excitonic peaks. In particular, the x-axis shows wavelength in nanometers, which extends from the lower (redder) part of the visible range across most of SWIR range. The y-axis corresponds to absorbance, but with a different offset (not marked) for each curve. In effect, these different offsets (artificially) spread out the curves along the y-axis to make the different curves easier to see. In practice, the red curve in Figure 6 typically has the smallest (or zero) offset along the y-axis. The pink curve has a slightly higher offset along the y-axis than the red line, and hence sits generally above the red curve. The purple curve has a still higher offset along the y-axis to sit above the red and pink curves. The blue and green curves are similarly offset upwards along the y axis to spread them from the red, pink and purple lines.

Each curve in Figure 6 corresponds to a different (respective) photoactive material. The different photoactive materials are distinguished from one another in that each material is customised to have a different first excitonic peak. In particular, the plot of Figure 6 shows 5 lines, namely red corresponding to a first excitonic peak of 1170nm, pink corresponding to a first excitonic peak of 1280nm, purple corresponding to a first excitonic peak of 1380nm, blue corresponding to a first excitonic peak of 1510nm, and green corresponding to a first excitonic peak of 1650nm. It will be appreciated that images of photoactive materials (nanorods) with these excitonic peaks are shown in Figure 5.

As discussed in relation to Figure 5, the length of the nanorods can be customised to control the first excitonic peak -having a longer length for the nanorods leads to a longer (higher) wavelength for the first excitonic peak. The length of the nanorods can be customised by altering the colloidal formation process.

The general shape of each absorption curve is that the absorbance is relatively low at the longer IR wavelengths. As the wavelength shortens, each absorption curve has an upward step, with an initial rise, followed by a flatter plateau. The absorption curve then begins to rise again from the step to the top left-hand portion of the plot, which shows high absorbances at shorter wavelengths (close to or in the visible range).

The location of the step for a given line corresponds approximately to the first excitonic peak for that line. In other words, it can be seen from Figure 6 that the longer the wavelength of the first excitonic peak, the further to the right of Figure 6 the step occurs, i.e. at longer wavelengths. For example, the step for the red line with first excitonic peak at 1170nm occurs at close to a wavelength of 1170nm; likewise for the step for the pink line with first excitonic peak at 1280nm which occurs at close to a wavelength of 1280nm; and likewise for the other three lines. Having the step change located close to the wavelength of the first excitonic peak can be regarded as a quantum effect, in that once a threshold wavelength has been reached in the direction of decreasing wavelength (i.e. of increasing energy), an incoming infrared photon has enough energy to activate the first excitonic peak -and so may be absorbed and detected, thereby causing the step increase in the absorption curve.

In contrast, photons with less energy (longer wavelength) than the first excitonic peak are unable to interact in this manner with the nanorods, and so have much lower absorbance.

The bandgap of the biggest-sized InAs nanorods having the first excitonic absorption peak at 1650 nm is as low as 0.68 eV, which corresponds to a wavelength of -1800 nm, which falls deep in the SWIR region. As far as we are aware, such a low bandgap with sharp excitonic features has not been previously disclosed for InAs QDs. Moreover, the nanorods exhibit sharp multiple excitonic features due to their high monodispersity, analogous to those displayed by II-VI semiconductor colloidal quantum dots (CQDs). The first excitonic feature of the largest InAs nanorods is red-shifted by almost 300 nm into the SWIR region, compared to the largest published monodisperse InAs ODs.

The HWHM of the first excitonic features in different spectral ranges was found to be between 33 and 63 meV -these are the lowest reported values within this wavelength regime. As discussed earlier, achieving the InAs QDs with a bandgap in the SWIR region is highly challenging but the approach described herein provides a direct synthesis of InAs nanorod CQDs having bandgaps deep in the SWIR region.

Figure 6 shows photoactive material having different lengths (and hence different first excitonic peaks) for different sets of nanorods. However, in some cases it may be desirable to combine two or more different sets of nanorods to obtain a composite broader range of SWIR absorption. For example, an imaging sensor 120A, 120B such as shown in Figure 1 may be provided with two layers of photoactive material -a first layer having nanorods of a first length (and hence first excitonic peak), and a second layer having nanorods of a second length (and hence second excitonic peak) to support absorbance across a wider range of wavelengths.

Figure 7, comprising sections 7(a)-7(e), illustrates various aspects of the formation and properties of the InAs nanorods described herein. Figure 7(a) shows the reaction scheme of the colloidally stable InAs nanorods which may be synthesized as described herein. Figure 7(b) is a graph showing an X-ray diffraction plot for nanorods such as shown in Figure 2. The x-axis represents diffraction angle and the y-axis represents the intensity of diffracted radiation. The blue lines show the reference data for indium arsenide based on the crystal structure for InAs, while the red lines show experimental results (an X-ray diffraction plot). The experimental results are subject to noise, but clearly show peaks in all the places corresponding to the peaks of the reference data. This indicates a high level of phase purity of the synthesized InAs nanorods, confirming that they are formed with the expected crystal structure, without contamination, distortion, and so on. Interestingly, the diffraction peak corresponding to the (111) plane appears to be sharper than the other peaks, potentially suggesting asymmetric growth of the particles along the [111] direction. Transmission electron microscopy (TEM) imaging has confirmed the synthesis of large-sized InAs nanorods, having an elongated cone or tapered rod shape with preferential growth along the [111] direction as shown in Figure 7(c). The high quality of the InAs nanorods is further confirmed by the X-ray photoelectron spectroscopy (XPS) analysis, which shows In 3d and As 3d core levels spectra from InAs without any oxide formation, see Figure 7(d). Moreover, the analysis reveals an almost 1:1 atomic ratio of In and As, which is further confirmed by the energy-dispersive X-ray spectroscopy (EDS) analysis of Figure 8 which has atomic percentages of 41.82 and 45.21 for In and As respectively. The nanorods are surface passivated by long-chain organic ligands, as shown by the presence of strong C-H stretching in their Fourier transform infrared (FTIR) spectrum of Figure 7(e).

We now provide some theoretical preliminary thoughts and theoretical considerations regarding the formation of InAs nanorods as described herein. It should be recognised that these thoughts are tentative in nature, and may be further developed or altered or corrected as more data is acquired and/or more analysis is performed. It is therefore emphasised that the approach described herein is not limited to or bound by any particular theory.

Mechanistic insights of InAs nanorod formation: A preliminary explanation of the mechanism of the synthesis reaction of InAs nanorods is provided below. Specially, an explanation is given to why the complexation of In and As with LiN(Si(CH3)3)2 leads to a rod formation rather than a low aspect ratio dot. As discussed earlier, the injection of Li(C2H5)3BH at room temperature does not immediately result in the reduction of the precursors; instead, the reduction process is initiated by increasing the temperature. As the temperature is slowly increased, the precursors start to undergo reduction, which is reflected in a slow and gradual colour change of the solution from transparent yellow at room temperature to black at 150 °C. With further increase in temperature, the colour of the solution gradually changes to dark brown at 180 °C, beyond which no further change in colour is observed. The changes in colour observed during the heating of the solution indicate the formation of some intermediate until 150 °C which subsequently reacts to yield InAs nanorods.

To capture this intermediate, various experiments were performed, some of which are presented in Figure 9 (sections (a)-(e). In these experiments, the reactions were quenched at different temperatures. Surprisingly, no InAs formation was observed till 150 °C, rather In nanoparticles were obtained (see Figure 9a). The composition of these nanoparticles was verified by EDS analysis. Unlike the In, the As remains in the solution in the complex form only. This was confirmed by reducing the As-complex in the absence of In, which did not result in any reduction. The In nanoparticles which were formed show high monodispersity but lack crystallinity, as seen from the HRTEM image in the bottom panel of Figure 9(a). In this image, no lattice fringes are observed. Also, these amorphous In nanoparticles show no features in their absorption spectrum (see Figure 9(d)). At 180 °C, only phase pure InAs nanorods are obtained, as shown in Figure 9(c). As expected, these InAs nanorods are crystalline (see the bottom panel of Figure 9(c)) and exhibit sharp excitonic features (see Figure 9(d)). The above experimental findings suggest that the amorphous In nanoparticles transform into crystalline InAs nanorods as the temperature increases from 150 °C to 180 °C.

To understand the mechanism behind this transformation, the reaction was quenched in-between the two phases. Interestingly, there were observations of an intermediate state in which crystalline InAs nanorods are growing on top of the amorphous In nanoparticles (see Figure 9(b)). This intermediate In-InAs phase has an absorbance in between that of In nanoparticles and InAs nanorods, as shown in Figure 9(d). According to the literature, such seeded growth is often used to grow InAs nanowires epitaxially. Consequently, in the present synthesis, the In nanoparticles act appear to as the seeds and direct the growth of InAs along the [111] direction. Furthermore, the same In nanoparticles act as the In source and are thus consumed during the reaction, as illustrated schematically in Figure 9. So, it is the In from the In nanoparticles and the As from the solution that react on the In nanoparticle seeds to initiate the InAs nanorod formation. The growth of the nanorod stops once either the In nanoparticles or As precursor is depleted.

Excited state properties: The aforementioned morphological and optical characterizations confirm the high quality of the colloidal InAs nanorods. From a device perspective, understanding the excited state properties is of particular importance. One powerful technique for studying excited state carrier dynamics is femtosecond transient absorption (ft-TA) spectroscopy, which has been utilized for the present case to investigate the excited state carrier dynamics and the many-body interactions in the colloidal InAs nanorods.

Figure 10 provides three plots, (a)-(c), relating to the formation and properties of the InAs nanorods described herein. Figure 10a illustrates an fs-TA plot of 18.5 nm long InAs nanorods at various delay times in response to a 1020 nm pump excitation. The ft-TA spectra at each delay have a negative absorption change around 1260 nm, which refers to the ground state bleach (GSB), consistent with the bandgap of the nanorods. The kinetic plot corresponding to the GSB is shown in Figure 11, which shows the excited state lifetime of 144 ± 12 ps due to repopulation of the ground state via carrier recombination. The III-V semiconductors, due to their smaller band gap, are expected to show efficient carrier multiplication along with a lower excitation threshold. However, the carrier multiplication in InAs ()Ds is highly debatable due to the presence of large surface defect concentrations, hence it was uncertain whether the highly crystalline InAs nanorods would show carrier multiplication. Accordingly, the InAs nanorods were excited with multiple pump excitations. The fs-TA spectra for different pump excitations are shown in Figure 12. As can be seen in Figure 12, upon increasing the excitation energy (from 1020 nm to 400 nm, i.e. 1.2 Eg to 3.1 Eg), a positive absorption immediately appears after excitation at -2.5 Eg (500 nm), thereby indicating the presence of carrier multiplication. The intensity of the positive absorption further increases at -3.1 Eg (400 nm) excitation. We note that the pump fluence at all excitations was kept the same at approximately 4.56 pJcm-2. The carrier multiplication and annihilation are illustrated in Figures 10(b) and (c), respectively. It can be seen that no carrier multiplication is observed for pump excitations with energies, e.g., 1.2 and 1.8 Eg. With increasing pump energy, a pump threshold of -2.5 Eg (see Figure 10(b)) is observed for carrier multiplication. Moreover, efficient carrier multiplication is further observed under -3.1 Eg pump excitation. A sub-10s ps multiple excitation annihilation is observed and compared in Figure 10(c).

Accordingly, the nanorods as disclosed herein provide a photoactive (photosensitive) material which can be used as the based for an infrared (especially SWIR) imaging sensor (photodetector) such as shown in Figure 1. Such an image sensor may be used in a wide variety of applications, including (without limitation) AR and VR, driver monitoring in automobiles for both day and night, and providing a human machine interface which is controlled by the user looking in a given direction. Many other uses of such photoactive material and imaging sensor will be apparent to the skilled person.

In conclusion, while various implementations and examples have been described herein, they are provided by way of illustration, and many potential modifications will be apparent to the skilled person having regard to the specifics of any given implementation. Accordingly, the scope of the present case should be determined from the appended claims and their equivalents. Furthermore, unless the context clearly indicates to the contrary, it is specifically disclosed herein that the features of any independent claim and/or its associated dependent claims may be combined with the features of any other independent claim and/or its associated dependent claims (irrespective of whether such a combination is explicitly claimed, since the claims are used to determine the scope of protection, not the overall disclosure of the application).

Claims (34)

1. Claims 1. A photoactive material for absorbing infrared radiation, the photoactive material being formed from a colloidal solution of nanorods of InAs, InAsP, InGaAs, TlInAs, InSb, InAsSb, InGaSb, InSbP and/or TllnSb, the nanorods having a longitudinal axis of length L < 100nm and a width, perpendicular to the longitudinal axis, of W <20nm, wherein the ratio of LNV > 2.
2. 2. The material of claim 1, wherein the nanorods are rotationally symmetric about the longitudinal axis and/or have mirror symmetry about one or more planes incorporating the longitudinal axis.
3. 3. The material of claim 1 or 2, wherein the longitudinal axis of each nanorod extends between a first end and a second end.
4. 4. The material of claim 3, wherein the first end is distinct from the second end.5. The material of claim 3 or 4, wherein the nanorods taper when travelling along the longitudinal axis from the first end to the second end.
5. 5. The material of any of claims 3 to 5, wherein the radius of curvature of the first end is greater than the radius of curvature of the second end.
6. 6. The material of any preceding claim, wherein the nanorods are arranged such that their longitudinal axes are substantially coplanar with one another.
7. 7. The material of any preceding claim, wherein the nanorods have a length in the range L1-L2, wherein L1 may be 10nm, 12mm, 15mm, 20mm, or 30 nm, and L2 may be 30nm, 40nm, 50nm, 60nm or 100nm.
8. 8. The material of any preceding claim, wherein the nanorods have a length L in the range 10-80nm, or preferably 15-55nm.
9. 9. The material of any preceding claim, wherein the nanorods have a ratio between (i) the length L of the nanorods, and (ii) the width W of the nanorods, such that L/W is in the range 1.5-12, preferably in the range 2-8,
10. 10. The material of any preceding claim, wherein the nanorods have a ratio R = La/Lp <0.2, preferably R <0.12, preferably R <0.08, where Lp is the mean value of the length of the nanorods in the photoactive material, and La is the standard deviation of the length of the nanorods in the photoactive material.
11. 11. The material of any preceding claim, wherein the nanorods comprises atomic layers perpendicular to the longitudinal axis, optionally with a spacing of 0.4 nm ±50%, preferably ±25%.
12. 12. The material of any preceding claim, wherein at least C% of the nanorods are incorporated into clusters, each cluster comprising two or more nanorods which are aligned with and adjacent to one another, and wherein C > 70%, preferably C > 85%.
13. 13. The material of any preceding claim, wherein at least some of the clusters include nanorods that are aligned with opposing directionality.
14. 14. The material of any preceding claim, wherein the first excitonic peak of the nanorods is configured to control the wavelength at which the infrared radiation is absorbed.
15. 15. The material of claim 14, wherein the nanorods in the photoactive material are customised in length to configure the first excitonic peak to absorb infrared radiation at a predetermined wavelength, wherein longer nanorods absorb infrared radiation at longer wavelengths.
16. 16. The material of any preceding claim, wherein the photoactive material is configured to absorb infrared radiation within at least a portion of the short-wave infrared (SWIR) wavelength range of 8002500 nm, optionally at a wavelength of > 1200 nm.
17. 17. The material of any preceding claim, wherein the nanorods form a photoactive layer having a thickness in the range from 50nm to 500nm
18. 18. The material of any preceding claim, wherein the nanorods form a direct bandgap semiconductor with an excitonic peak absorption coefficient above 102 cm-1.
19. 19. An infrared image sensor which includes a photoactive layer formed from the photoactive material of any preceding claim to convert incoming infrared light into an electrical signal representing an image.
20. 20. An infrared photodetector which includes a photoactive layer formed from the photoactive material of any preceding claim to convert incoming infrared light into an electrical signal representing a binary signal or spectrum.
21. 21. The infrared sensor of claim 19 or the infrared photodetector of claim 20, wherein the infrared sensor or photodetector includes a stack of layers comprising: a CMOS substrate, a bottom electrode, a hole transport layer, the photoactive layer, an electron transport layer, and a top electrode.
22. 22. The infrared sensor of claim 19 or 21 or the infrared photodetector of claim 20 or 21, further comprising first and second photoactive layers having respective nanorods of two different sizes.
23. 23. The infrared sensor of claim 19, 21 or 22 or the infrared photodetector of any of claims 20-22 having a response time below 100 ns, preferably below 10 ns, preferable below 1 ns.
24. 24. A device comprising machine vision camera, camera for robotics, virtual reality goggles, mixed reality goggles, a smartphone, an automotive camera, an eye interactive camera, or a set of contact lenses, wherein said device incorporates an infrared image sensor of any of claims 19 or 2123.
25. 25. A device comprising a smartphone or an eye interactive camera, wherein said device incorporates an infrared photodetector of any of claims 20 to 23.
26. 26. A method for synthesizing InAs nanorod comprising: (i) performing an in situ complexation of In and As metal ions; (H) adding a reducing agent to the produce of (i); (iii) heating the product of (ii) to initiate and complete a reduction to form colloidal InAs nanorods.
27. 27. The method of claim 26, wherein the In and As metal ions are provides as halides, optionally dissolved in in oleylamine.
28. 28. The method of claim 26 or 27, further including lithium bis(trimethylsilyl)amide (LiN(Si(CH3)3)2) to form the complexation the In and As metal ions.
29. 29. The method of any of claims 26 to 28, wherein concentration of the reducing agent in (iii) [Li(C2H5)3BH] is the primary determinant of the size of the nanorods, and optionally changing the concentration of the reducing agent may change the length of the nanorods by a factor of at least 2.
30. 30. The method of any of claims 26 to 29, wherein the reducing agent is lithium triethylborohydride (Li(C2H5)3BH) in dioctyl ether.
31. 31. The method of any of claims 26 to 30, wherein the heating of (iii) is performed in a first phase, optionally to temperature of around 150°, to form an intermediate material comprising non-crystalline In nanoparticles, and in a second phase optionally to a temperature of around 180°, to form the colloidal crystalline InAs nanorods.
32. 32. The method of any of claims 26-31, wherein the synthesis is performed in situ in a one-pot configuration.
33. 33. The photoactive material of any of claims 1-18, or the image sensor or photodetector of any of claims 19-23 containing such a photoactive material, wherein the photoactive material is synthesized by the method of any of claims 26-32.
34. 34. A photoactive material synthesized by the method of any of claims 26-32.