Ag2se and ag2s quantum dots (QDS) or nanocrystals to produce the high-speed photodetectors

Abstract

Method to synthesize the Ag2Se and Ag2S quantum dots and develop close to 200 % photogeneration efficiency solution-processed high-speed IR photodetectors, which is due to the multiple electron generation effect. In particular, disclosed herein are methods for synthesizing silver chalcogenide quantum dots, specifically Ag₂Se and Ag₂S, for application in high-speed photodetectors. The synthesis involves a solution-phase approach conducted at controlled temperatures to preserve desirable semiconductor phases and to achieve uniform quantum dot morphology and composition. These quantum dots are integrated into photoconductive device structures that exhibit enhanced photoresponse in the short-wavelength infrared (SWIR) region. The photodetectors demonstrate rapid response times and high efficiency, making them suitable for applications such as optical communication, medical imaging, remote sensing, and quantum optics. The disclosed methods offer an environmentally benign and scalable alternative to conventional high-speed photodetector fabrication techniques.

Description

[0001] Ag2Se AND Ag2S QUANTUM DOTS (QDs) OR NANOCRYSTALS TO PRODUCE THE HIGH-SPEED PHOTODETECTORS

[0002] FIELD

[0003] The present application relates to high-speed photodetectors using separate Ag2Se and Ag2S quantum dots (QDs) or nanocrystals. Each type is used separately in its own photodetector.

[0004] BACKGROUND

[0005] High-speed photodetectors have wide applications in self-driving by light / laser detection and ranging (LIDAR / LADAR), optical quantum, night vision, astronomy, medical imaging, and microscopy, (as demonstrated in Figure 1).

[0006] The state-of-the-art high-speed photodetectors have two categories. The first one is avalanche photodiodes (APDs) based on electron avalanche mechanism in a PN junction semiconductor devices such as Si or GaAs materials. Although these devices are chip size, which are to be integrated into most applications, the fabrication methods of chemical vapor deposition (CVD) or molecular-beam epitaxy (MBE) requires ultra-high vacuum, high temperature and slow growth rate, leading to expensive instrument cost. The second category is the photomultiplier tube (PMT), which is based on electron cascade effect under high electrical field. As a result, those large size vacuum tube devices are challenging to integrated into detection systems, in addition, a higher electrical field is required.

[0007] Developing a highly efficient, environmentally benign, and cost-effective high-speed photodetector remains a significant challenge, particularly within the optical communication bandwidth, such as the short-wavelengthinfrared (SWIR) range (A>1500nm). Beyond applications in quantum optical communication, infrared (IR) high-speed photodetectors have extensive utility in areas including medical imaging, Light / Laser Detection and Ranging (LIDAR / LADAR), and astronomy. Traditional materials, such as Hgi-xCdxTe (MCT) or PbSe / PbS photodetectors, can extend the mid-IR range to a few micrometers; however, they pose significant challenges due to the toxicity of heavy metals such as mercury and cadmium. Additionally, their operation requires cryogenic temperatures with liquid nitrogen cooling.

[0008] Although lll-VI semiconductor quantum well photodetectors provide a non-toxic alternative, their fabrication relies on costly and time-intensive molecular beam epitaxy (MBE) growth methods. Moreover, while superconducting nanowire photodetectors exhibit exceptional sensitivity, they necessitate operation at even lower cryogenic temperatures, typically requiring liquid helium. Consequently, there is a pressing need for a new generation of high-speed photodetectors that can operate in the SWIR range while being environmentally benign, highly efficient, and economically viable.

[0009] Recent advances in solution-processed quantum dot-based photodetectors have demonstrated significant potential in addressing these challenges. For example, solution-processed high-speed and high-quantum-efficiency quantum dot infrared photodetectors have been reported, showing promise in infrared detection applications (Jianbo Gao, Son C Nguyen, Noah D Bronstein, A Paul Alivisatos, “Solution Processed High Speed, High Efficiency Quantum Dot Infrared Photodetectors" , ACS Photonics, 3, 1217-1222 (2016)).

[0010] Additionally, carrier multiplication through transient photocurrent in device-grade films of quantum dots has been explored as a method forenhancing photocurrent response in nanocrystal arrays (Jianbo Gao, Andrew F Fidler, Victor I Klimov, “Observation of Carrier Multiplication through Transient Photocurrent in Conducting Films of Lead Selenide Quantum Dots", Nature Communications, 6, 8185 (2015)). Such mechanisms are crucial for improving the sensitivity and efficiency of next-generation photodetectors. Furthermore, the carrier transport dynamics in high-speed photodetectors have been studied in materials like black phosphorus, which highlights the importance of material selection in optimizing detector performance (Jianbo Gao, Apparao M Rao, Hongbo Li, Jianbing Zhang, Ou Chen, “Carrier Transport Dynamics in High Speed Black Phosphorus Photodetectors” , ACS Photonics, 5, 1412-1417(2018)).

[0011] Moreover, multiple electron generation mechanisms, such as Auger recombination, have been reported in nanocrystal arrays, further demonstrating the potential of quantum dot-based devices for high-speed photodetection applications (Nano Lett, 2019). These findings provide a strong foundation for the development of new photodetector technologies, including the Ag2Se and Ag2S quantum dot-based photodetectors disclosed herein.

[0012] Therefore, an innovative high-speed photodetection technology, which can be fabricated in a cost-effective way such as solution-process on lightweight substrates, would be very beneficial to the photodetection industrials. In addition, it meets the technologies performance metrics including spectral range, dead time, dark count rate, efficiency, timing jitter, and detection area. In term of scientific interest, a new device operation mechanism or new photo physics generation mechanism is needed to address above challenge in a fundamental way.SUMMARY

[0013] Disclosed herein are high-speed photodetectors using Ag2Se and Ag2S quantum dots. Each photodetector is fabricated separately using either Ag2Se or Ag2S, rather than a mixture or composite, forming a layered structure where a thin film of Ag2Se or Ag2S quantum dots is deposited onto a glass substrate and integrated with gold transmission lines to enhance performance. Ag2Se and Ag2S quantum dots were synthesized using a well-controlled hot injection nucleation method, ensuring uniform size uniformity and optimized infrared absorption in the 1000-1800 nm range. Ag2Se and Ag2S are ideal candidates for SWIR photodetectors due to their low bandgap, which corresponds to an absorption threshold extending to a few micrometer-wavelength. By integrating these quantum dots into a photoconductive device structure, a photogeneration quantum efficiency approaching 200% was observed, enabled by the novel multiple electron generation effect. Furthermore, the high-speed performance of these photodetectors using ultrafast photocurrent spectroscopy was evaluated which allowed for the ability to precisely characterize the detector's dead time. Experimental results confirm a dead time of approximately 100 ps, with response characterization conducted using a 40 GHz oscilloscope.

[0014] Thus, the present disclosure provides a method for synthesizing Ag2Se quantum dots, comprising:

[0015] preparing a silver precursor by dissolving a silver containing compound in a solvent and introducing a stabilizing agent and a reactive agent under inert conditions to form a stable precursor solution;

[0016] preparing selenium precursors by dissolving selenium and sulfur in acoordinating or reactive solvent system to obtain a reactive precursor solution; injecting the selenium and sulfur precursors into the silver precursor solution under heating in a preselected temperature range to initiate Ag2Se quantum dot formation;

[0017] controlling the growth of the Ag2Se quantum dots by maintaining reaction conditions for a predetermined duration of time;

[0018] halting quantum dot growth by cooling the reaction mixture to form Ag2Se quantum dots; and

[0019] purifying the Ag2Se quantum dots through any one or combination of a series of dispersion, precipitation, and separation steps to remove impurities and aggregates.

[0020] The preselected temperature range is from about 80°C to under 135°C. The present disclosure also provides a method for synthesizing Ag2S quantum dots, comprising:

[0021] preparing a silver precursor by dissolving a silver containing compound in a solvent and introducing a stabilizing agent and a reactive agent under inert conditions to form a stable precursor solution;

[0022] preparing selenium and sulfur precursors by dissolving selenium and sulfur in a coordinating or reactive solvent system to obtain a reactive precursor solution;

[0023] injecting the selenium and sulfur precursors into the silver precursor solution under heating in a preselected temperature range to initiate Ag2S quantum dot formation;

[0024] controlling the growth of the Ag2S quantum dots by maintaining reaction conditions for a predetermined duration of time;halting quantum dot growth by cooling the reaction mixture to form Ag2S quantum dots; and

[0025] purifying theAg2S quantum dots through any one or combination of a series of dispersion, precipitation, and separation steps to remove impurities and aggregates. In this aspect preselected temperature range is from about 120°C to about 200°C.

[0026] The silver precursor may be prepared using a silver salt dissolved in an organic solvent, with the silver salt being any one or combination of silver acetate, silver nitrate, silver carbonate and silver oxide, with a preferred silver salt being silver acetate.

[0027] The stabilizing agent may be any combination of 1 -octadecene (ODE) and 1 -dodecanethiol (DDT), oleylamine and trioctylphosphine oxide (TOPO). A preferred stabilizing agent is the combination of 1 -octadecene (ODE) and 1-dodecanethiol (DDT).

[0028] The coordinating or reactive agent may be a derivative of silylamide. Preferred reactive agents are one or both of Li[N(SiMe3)2] and Sn[N(SiM 63)2)2.

[0029] The present disclosure also provides a photodetector, comprising: a substrate having opposed first and second surfaces;

[0030] a photoactive layer comprised of the Ag2Se quantum dots that absorb light, creating electron-hole pairs located on the first surface; and

[0031] at least two spaced metallic contacts located on the photoactive layer that attract and collect the generated electron-hole pairs to produce a photocurrent measured by a detector to which the at least two spaced metallic contacts are connected to.The present disclosure also provides a photodetector, comprising:

[0032] a substrate having opposed first and second surfaces;

[0033] a photoactive layer comprised of the Ag2S quantum dots that absorb light, creating electron-hole pairs located on the first surface; and

[0034] at least two spaced metallic contacts located on the photoactive layer that attract and collect the generated electron-hole pairs to produce a photocurrent measured by a detector to which the at least two spaced metallic contacts are connected to.

[0035] These photodetectors may further comprise a metallic layer located on the second surface and electrically grounded.

[0036] The photodetector may further comprise a passivation layer on the photoactive layer to minimize surface defects on the photoactive layer; and an anti-reflection coating on the passivation layer to minimize light loss.

[0037] A further understanding of the functional and advantageous aspects of the invention can be realized by reference to the following detailed description and drawings.

[0038] BRIEF DESCRIPTION OF THE DRAWINGS Embodiments disclosed herein will be more fully understood from the following detailed description thereof taken in connection with the accompanying drawings, which form a part of this application, and in which:

[0039] Figure 1 shows a prototype solution-processed IR photodetector in UPQD group (center figure) and its various potential applications in many fields.Figure 2 pictorially shows the fabrication of the high-speed photodetector from the synthesized Ag2Se and Ag2S quantum dots.

[0040] Figure 3 is a plot of photocurrent (I) versus time (picoseconds) for an exemplary photodetector produced according to the present disclosure.

[0041] DETAILED DESCRIPTION

[0042] Without limitation, the majority of the systems described herein are directed to high-speed photodetectors using Ag2Se and Ag2S quantum dots (QDs) rather than composites or hybrid materials. Each photodetector is fabricated separately using either Ag2Se or Ag2S QDs as the active material, forming a layered structure. As required, embodiments of the present invention are disclosed herein. However, the disclosed embodiments are merely exemplary, and it should be understood that the invention may be embodied in many various and alternative forms.

[0043] The accompanying figures, which are not necessarily drawn to scale, and which are incorporated into and form a part of the instant specification, illustrate several aspects and embodiments of the present disclosure and, together with the description therein, serve to explain the principles of the process of producing multi band gap nanocrystal ensembles for solar-matched energy harvesting. The drawings are provided only for the purpose of illustrating select embodiments of the apparatus and as an aid to understanding and are not to be construed as a definition of the limits of the present disclosure.

[0044] As used herein, the terms, “comprises” and “comprising” are to be construed as being inclusive and open ended, and not exclusive. Specifically,when used in the specification and claims, the terms, “comprises” and “comprising” and variations thereof mean the specified features, steps or components are included. These terms are not to be interpreted to exclude the presence of other features, steps or components.

[0045] As used herein, the term “exemplary” means “serving as an example, instance, or illustration,” and should not be construed as preferred or advantageous over other configurations disclosed herein.

[0046] As used herein, the terms “about” and “approximately”, when used in conjunction with ranges of dimensions of particles, compositions of mixtures or other physical properties or characteristics, are meant to cover slight variations that may exist in the upper and lower limits of the ranges of dimensions so as to not exclude embodiments where on average most of the dimensions are satisfied but where statistically dimensions may exist outside this region. It is not the intention to exclude embodiments such as these from the present disclosure. Unless otherwise specified, the terms “about” and “approximately” mean plus or minus 25 percent or less.

[0047] It is to be understood that unless otherwise specified, any specified range or group is as a shorthand way of referring to each and every member of a range or group individually, as well as each and every possible sub-range or sub-group encompassed therein and similarly with respect to any sub-ranges or sub-groups therein. Unless otherwise specified, the present disclosure relates to and explicitly incorporates each and every specific member and combination of sub-ranges or sub-groups.

[0048] As used herein, the term "on the order of", when used in conjunction with a quantity or parameter, refers to a range spanning approximately one tenth toten times the stated quantity or parameter.

[0049] As used herein, the coordinating conjunction “and / or” is meant to be a selection between a logical disjunction and a logical conjunction of the adjacent words, phrases, or clauses. Specifically, the phrase “X and / or Y” is meant to be interpreted as “one or both of X and Y” wherein X and Y are any word, phrase, or clause.

[0050] Methods and Characterization

[0051] Materials and Synthesis of Ag2Se and Ag2S

[0052] Ag2Se and Ag2S quantum dots are not synthesized simultaneously; they are produced separately in independent synthesis processes to ensure distinct optical and electronic properties.

[0053] The most popular synthesis strategy used silver acetate (AgNOs) as Ag precursor to obtain a wide range size dots, the injection temperature is higher than 140°C. It is well-known that Ag2Se has a phase transition at 135 °C. The low temperature (<135°C) phase of orthorhombic p-phase demonstrates semiconductor property with band gap of 0.15 eV, while the high temperature cubic a-phase (>140°C) demonstrates a metallic property. To avoid this phase transition and maintain semiconductor properties, Ag2Se synthesis is conducted below 135°C.

[0054] Rather than using the AgNOs as is typical in the field silver acetate was used as the precursor, and a highly reactive agent including Li[N(SiMes)2] or Sn[N(SiMe3)2]. Without being bound by any theory, the inventors speculate that the N(SiMes)2 group is the key factor in quantum dot nucleation under low temperature during this synthesis process because a metathesis reaction between the silver salt and silylamide results in the generation of short-livedsilver silylamide complexes to facilitate the nanocrystal nucleation. As a result, right after the hot injection, it is possible to control the dot growth time under the low synthesis temperature of 100°C, which is much slower than the high temperature > 140°C.

[0055] Ag2Se and Ag2S are fabricated separately as individual quantum dot solutions and are not mixed during synthesis or in the final photodetector device. Each detector is based on a single material, either Ag2Se or Ag2S.

[0056] Silver acetate and 1 -dodecanethiol (DDT), 1 -octadecene (ODE) are mixed in a three necked flask to make the Ag precursor. For the selenium and sulfur precursors, the stock solution was prepared by stirring Se and S in tri-n-octylphosphine (TOP) and highly reactive agents such as Li[N(SiMes)2] or Sn[N(SiMe3)2] until all dissolved. Then diphenylphosphine (DPP) with an amount in the range from about 0.05 to about 0.2 mL were mixed and loaded in a syringe, which was then swiftly injected into the flask above 100°C, followed by an hour growth at a controlled temperature of either about 120°C or about 200°C for Ag2S or below 135°C (for Ag2Se) high temperature. Then the quantum dot growth was quenched by fast cooling using pressured air.

[0057] The purification was done by dispersing the crude product in toluene, followed by precipitation by adding anhydrous ethanol and centrifugation. After centrifugation in a range from about 5000 to about 10,000 rpm, the precipitated quantum dots were further purified. The quantum dots were dispersed in toluene and centrifuged at 5000 rpm to remove possible aggregated quantum dots formed in the middle of purification.

[0058] In summary, the synthesis process disclosed herein for producing quantum dots of Ag2Se and Ag2S involves a low temperature, preciselycontrolled hot injection nucleation method. Silver acetate is used as a precursor, in combination with highly reactive silylamide agents such as Sn[N(SiMe3)2]2 (IUPAC name: tin(ll) bis[bis(trimethylsilyl)amide]) and Li[N(SiMes)2] (IUPAC name: lithium bis(trimethylsilyl)amide) to ensure accurate nucleation and growth of the quantum dots. This approach avoids high temperatures (above 135°C for Ag2Se and 200°C for Ag2S) and employs a metathesis reaction to produce quantum dots with uniform size and enhanced optical properties, making them ideal for infrared photodetector applications.

[0059] There are several key considerations when synthesizing the dots. The first relates to temperature control. The synthesis temperature must remain below 135°C to prevent the phase transition of Ag2Se from the semiconducting p-phase to the metallic a-phase. The second is the need for the reactive agents as the N(SiMe3)2group in silylamide agents is key for facilitating nucleation via a metathesis reaction with silver salts. With respect to surface ligands Dodecanethiol (DDT) plays a dual role in stabilizing the quantum dots and controlling their surface properties. Finally, maintaining an inert environment (e.g., a nitrogen environment) throughout the synthesis is key to preventing oxidation of the precursors and quantum dots.

[0060] Summary of Materials and Synthesis of Ag2Se and Ag2S

[0061] The synthesis of Ag2Se and Ag2S quantum dots is performed separately, not simultaneously. Each material undergoes an independent synthesis process optimized for its properties, ensuring that they retain their distinct electronic and optical characteristics. The final photodetector devices are layered structures, where either Ag2Se or Ag2S is used as the activephotoactive material in separate detectors.

[0062] Ag2Se Synthesis:

[0063] • Precursor: Silver acetate

[0064] • Selenium-to-silver acetate molar ratio: 2.5:1

[0065] • Synthesis temperature: Below 135°C to prevent phase transition from semiconducting p-phase to metallic a-phase

[0066] • Solvent and stabilizing agents: 1 -octadecene (ODE) and dodecanethiol (DDT)

[0067] • Reactive agents: Li[N(SiMe3)2] and / or Sn[N(SiMe3)2]2.

[0068] Ag2S Synthesis:

[0069] • Precursor: Silver acetate

[0070] • Sulfur-to-silver acetate molar ratio: 2.5:1

[0071] • Synthesis temperature range: 120°C-200°C

[0072] • Solvent and stabilizing agents: 1 -octadecene (ODE) and dodecanethiol (DDT)

[0073] • Reactive agents: Li[N(SiMe3)2] and / or Sn[N(SiMe3)2]2.

[0074] It is noted that the method of produced Ag2Se can be applied to synthesis of Ag2S. The selenium and sulfur precursors are prepared using a molar ratio of selenium to sulfur to silver acetate of 2.5:1:1.

[0075] The inventors contemplate that alternative silver precursors, such as silver nitrate, silver carbonate, or silver oxide, could potentially be used.Similarly, using alternative reactive agents beyond Li[N(SiMe3)2] Sn[N(SiMe3)2]2 containing N(SiMe3)2are contemplated to be efficacious.

[0076] Quantum Dot Purification and Integration

[0077] After synthesis, the quantum dots are purified and dispersed in hexane. The purification process includes centrifugation in a range from about 5000 to about 10,000 rpm to remove impurities and aggregated quantum dots. Solvent exchange and precipitation using toluene and anhydrous ethanol to isolate pure QDs. The resulting quantum dots are then deposited as a thin film onto a glass substrate via spin coating to fabricate high-speed photodetectors. Each photodetector is based solely on either Ag2Se or Ag2S, and they are not mixed or used in composite form.

[0078] Device Fabrication

[0079] Referring to Figure 2, after the synthesis of Ag2Se and Ag2S using the procedure described above, a layer of material consisting of either Ag2Se or Ag2S quantum dots (not a mixture or composite of both) is deposited onto a glass substrate by spin coating to form a thin film. The subsequent thin film is then integrated with coplanar gold (Au) transmission lines. The transmission lines are separated by a 25 pm gap by thermal evaporation through a shadow mask. The transmission line structure greatly reduce device RC (resistance capacitance) time constant (TRC) by reduced device area and the electrode As

[0080] spacing since C =

[0081]

[0082] where A, d, and £ is the area of the active device (< 5X1 O'5cm-2), d the electrode spacing (25 pm), and permittivity of quartz (4.5EO, S0« 9 x 10-12C2N-1m-2), respectively, see Jianbo Gao, Lyran Kidon, Eran Rabani, A Paul Alivisatos, “Ultrahigh Hot Carrier Transient Photocurrent in Nanocrystal Arrays by Auger Recombinatio”, Nano letters, 19, 4804-4810(2019). Thefollowing is the schematic device top view, cross-section view, and prototype photodetector.

[0083] ULTRAFAST PHOTOCURRENT SPECTROSCOPY CHARACTERIZATION

[0084] The subsequent thin film is then integrated with coplanar gold (Au) transmission lines. The transmission lines are separated by a 25 pm gap by thermal evaporation through a shadow mask. Photocurrent measurement is done by integrating the above devices within a custom-built sample holder which is put under a vacuum environment via a cryostat. The cryostat provides a sealed environment; in which, the experimental temperature can be varied while keeping the system under a vacuum. This setup prevents ionization reactions with air and electrical arcing within the device at high electric fields. The required electric field is generated via a source meter, and the photocurrent is collected by a 40 GHz sampling oscilloscope. An ultrafast response time of 25 ps was achieved. The limiting factors for this fast time resolution were the bandwidths of photoconductive devices, the 40 GHz sampling oscilloscope, cables, and connectors.

[0085] Figure 3 shows the ultrafast photocurrent response from the high-speed photodetectors. The time-resolved photocurrent can be generally characterized by a fast rise with a sub-20 ps time resolution followed by a rapid decay up to 150 ps, then a slow tail extending beyond ~ ns range. The fast decay is due to the trap filling and the slow decay is due to the multiple-trapping-and-release carrier transport along those defect states. Thus, the full width at half maximum of the photocurrent decay, i.e. the deadtime is approximately 100 ps. The photogeneration efficiency can be calculated by the following by the following equation,I

[0086]

[0087] peak(.ty T Fe) ■>

[0088] where Ipeak t) is photocurrent peak (linked to efficiency), ih and peare the hole and electron mobilities, respectively, is the quantum yield which is the product of quantum efficiency of carrier photogeneration, carrier collection, and exciton dissociation efficiency, N is the effective photon number, E is the electric field, and L is the electrode spacing.

[0089] With the laser power 5 uW at 1 KHz repetition rate, we can extract the photogeneration efficiency is approximately to 180 %. The photogeneration efficiency also can be confirmed by the integral ratio of pulsed photon number and the total charge collected, which is also consistent with the 180 % result. This was validated through charge collection analysis, confirming that multiple electron generation contributes to enhanced device performance.

[0090] Mechanism Discussion

[0091] Without being bound by any theory, the inventors believe that the photogeneration efficiency is approximately to 180 % is due to the multiple electron generation effect. This novel effect is facilitated by utilizing excess photon energy to generate multiple electrons, as show in the following figure. For instance, when the photon energy exceeds three times the band gap (Eg) of the photodetector, the excess energy enables the generation of two electrons, which are extracted as current in the photodetector circuit. Specifically, for the absorption threshold of 4.5 urn Ag2Se quantum dots, the 1500 nm wavelength photoexcitation generates two electrons. Additionally, the carrier lifetimes are limited by 100 ps (picoseconds), driven by auger recombination effects, where one electron interacts with another electron to promote higher excited state. As a result, the short lifetime of multiple electrons enables a desirable faster deadtime, further enhancing device performance. These effects have been demonstrated in prior studies on carrier transport dynamics and multiple carrier generation in high-speed quantum dot photodetectors, see Jianbo Gao, Son C Nguyen, Noah D Bronstein, A Paul Alivisatos, “Solution Processed High Speed, High Efficiency Quantum Dot Infrared Photodetectors" , ACS Photonics, 3, 1217-1222 (2016), Jianbo Gao, Andrew F Fidler, Victor I Klimov, “Observation of Carrier Multiplication through Transient Photocurrent in Conducting Films of Lead Selenide Quantum Dots", Nature Communications, 6, 8185 (2015)). The two types of photodetectors made from (Ag2Se) and silver sulfide (Ag2S) photodetectors have similar performance, due to the similar narrow band gap threshold.

[0092] In summary, high-speed photodetectors having been made using silver selenide (Ag2Se) and silver sulfide (Ag2S) quantum dots. These photo-detectors deliver superior light-to-electrical signal conversion and is optimized for highspeed applications. In addition, the photodetectors operate at count rates exceeding 10 GHz, and exhibit near 200% quantum efficiency for exceptional performance. The have been produced on lightweight and flexible substrates ensure cost-effective production and have a broad spectral range from about 1000 to about 1800 nm, ideal for infrared (IR) applications.

[0093] Specific applications for these high-speed photodetectors includes, but is not limited to 1) autonomous vehicles providing improved imaging for obstacle detection in low-light environments; 2) optical communications due to accelerated data transfer in fiber optic networks; 3) LiDAR systems with enhanced accuracy and range for mapping and detection; 4) medical imaging with optimized diagnostic techniques such as OCT (optical coherencetomography) and NIR (near-infrared) imaging; 5) industrial quality control enabling precise flaw detection in manufacturing processes; and 6) quantum technologies such as secure quantum communication through high-speed photo detection.

[0094] Embodiments

[0095] In an embodiment there is provided a method for synthesizing Ag2Se quantum dots, comprising:

[0096] preparing a silver precursor by dissolving a silver containing compound in a solvent and introducing a stabilizing agent and a reactive agent under inert conditions to form a stable precursor solution;

[0097] preparing selenium precursors by dissolving selenium and sulfur in a coordinating or reactive solvent system to obtain a reactive precursor solution;

[0098] injecting the selenium and sulfur precursors into the silver precursor solution under heating in a preselected temperature range to initiate Ag2Se quantum dot formation;

[0099] controlling the growth of the Ag2Se quantum dots by maintaining reaction conditions for a predetermined duration of time;

[0100] halting quantum dot growth by cooling the reaction mixture to form Ag2Se quantum dots; and

[0101] purifying the Ag2Se quantum dots through any one or combination of a series of dispersion, precipitation, and separation steps to remove impurities and aggregates.

[0102] In an embodiment the preselected temperature range is from about 80°C to under 135°C.In an embodiment the silver precursor is prepared using a silver salt dissolved in an organic solvent.

[0103] In an embodiment the silver salt is any one or combination of silver acetate, silver nitrate, silver carbonate and silver oxide. In an embodiment the silver salt is silver acetate.

[0104] In an embodiment the stabilizing agent is any combination of 1-octadecene (ODE) and 1 -dodecanethiol (DDT), oleylamine and trioctylphosphine oxide (TOPO).

[0105] In an embodiment the stabilizing agent is 1-octadecene (ODE) and 1-dodecanethiol (DDT).

[0106] In an embodiment the coordinating or reactive agent is a derivative of silylamide.

[0107] In an embodiment the reactive silylamide derivative is one or both of Li[N(SiMe3)2] and Sn[N(SiMe3)2]2.

[0108] In an embodiment the reaction mixtures cooled to a temperature in a range from about 20°C to about 30°C to form the quantum dots, with cooling performed rapidly within about 60 seconds to prevent excessive crystal growth.

[0109] In an embodiment the predetermined duration of time is in a range from about 30 minutes to about 90 minutes.

[0110] In an embodiment purifying of the quantum dots is carried out by a combination of solvent exchange and centrifugation.

[0111] In an embodiment the quantum dots having a mean diameter in a range from about 2nm to about 10 nanometers (nm).

[0112] In an embodiment purifying the quantum dots is performed by dispersing the quantum in toluene, precipitating with anhydrous ethanol, centrifuging themixture in a range from about 5000 to about 10,000, repeating the dispersion and centrifugation steps to remove impurities and aggregated quantum dots, and dissolving the purified quantum dots in hexane to produce a solution suitable for device fabrication.

[0113] In an embodiment the final purified quantum dots are dissolved in hexane at a concentration from about 5 to about 20 mg / mL for subsequent device fabrication.

[0114] In an embodiment the silver precursor is heated at to a temperature in a range from about 120°C to about 200°C under an inert atmosphere of nitrogen or argon.

[0115] In an embodiment the quantum dots have a tunable absorption range of 1000 nm to 1800 nm, with enhanced sensitivity in the shortwave infrared (SWIR) region suitable for infrared or visible light applications.

[0116] In an embodiment a photodetector is provided comprising:

[0117] a substrate having opposed first and second surfaces;

[0118] a photoactive layer comprised of Ag2Se quantum dots, produced by the methods of claims 1 to 17, that absorb light, creating electron-hole pairs located on the first surface; and

[0119] at least two spaced metallic contacts located on the photoactive layer that attract and collect the generated electron-hole pairs to produce a photocurrent measured by a detector to which the at least two spaced metallic contacts are connected to.

[0120] In an embodiment the photodetector further comprises a metallic layer located on the second surface and electrically grounded.In an embodiment the photodetector further comprises a passivation layer to minimize surface defects on the photoactive layer; and / or

[0121] an anti-reflection coating to minimize light loss.

[0122] In an embodiment there is provided a method for synthesizing Ag2S quantum dots, comprising:

[0123] preparing a silver precursor by dissolving a silver containing compound in a solvent and introducing a stabilizing agent and a reactive agent under inert conditions to form a stable precursor solution;

[0124] preparing selenium and sulfur precursors by dissolving selenium and sulfur in a coordinating or reactive solvent system to obtain a reactive precursor solution;

[0125] injecting the selenium and sulfur precursors into the silver precursor solution under heating in a preselected temperature range to initiate Ag2S quantum dot formation;

[0126] controlling the growth of the Ag2S quantum dots by maintaining reaction conditions for a predetermined duration of time;

[0127] halting quantum dot growth by cooling the reaction mixture to form Ag2S quantum dots; and

[0128] purifying theAg2S quantum dots through any one or combination of a series of dispersion, precipitation, and separation steps to remove impurities and aggregates.

[0129] In an embodiment the preselected temperature range is from about 120°C to about 200°C.In an embodiment the silver precursor is prepared using a silver salt dissolved in an organic solvent. In an embodiment the silver salt is any one or combination of silver acetate, silver nitrate, silver carbonate and silver oxide.

[0130] In an embodiment the stabilizing agent is any combination of 1-octadecene (ODE) and 1 -dodecanethiol (DDT), oleylamine and trioctylphosphine oxide (TOPO).

[0131] In an embodiment the stabilizing agent is 1-octadecene (ODE) and 1-dodecanethiol (DDT).

[0132] In an embodiment the coordinating or reactive agent is a derivative of silylamide. In an embodiment the reactive silylamide derivative is one or both of Li[N(SiMe3)2] and Sn[N(SiMe3)2]2.

[0133] In an embodiment the reaction mixtures cooled to a temperature in a range from about 20°C to about 30°C to form the quantum dots, with cooling performed rapidly within about 60 seconds to prevent excessive crystal growth.

[0134] In an embodiment the predetermined duration of time is in a range from about 30 minutes to about 90 minutes.

[0135] In an embodiment purifying of the quantum dots is carried out by a combination of solvent exchange and centrifugation.

[0136] In an embodiment the quantum dots having a mean diameter in a range from about 2nm to about 10 nanometers (nm).

[0137] In an embodiment purifying the quantum dots is performed by dispersing the quantum in toluene, precipitating with anhydrous ethanol, centrifuging the mixture in a range from about 5000 to about 10,000, repeating the dispersion and centrifugation steps to remove impurities and aggregated quantum dots,and dissolving the purified quantum dots in hexane to produce a solution suitable for device fabrication.

[0138] In an embodiment the final purified quantum dots are dissolved in hexane at a concentration from about 5 to about 20 mg / mL for subsequent device fabrication.

[0139] In an embodiment the silver precursor is heated at to a temperature in a range from about 120°C to about 200°C under an inert atmosphere of nitrogen or argon.

[0140] In an embodiment the quantum dots have a tunable absorption range of 1000 nm to 1800 nm, with enhanced sensitivity in the shortwave infrared (SWIR) region suitable for infrared or visible light applications.

[0141] In an embodiment there is provided photodetector, comprising:

[0142] a substrate having opposed first and second surfaces;

[0143] a photoactive layer comprised of Ag2S quantum dots, produced by the methods of claims 21 to 37, that absorb light, creating electron-hole pairs located on the first surface; and

[0144] at least two spaced metallic contacts located on the photoactive layer that attract and collect the generated electron-hole pairs to produce a photocurrent measured by a detector to which the at least two spaced metallic contacts are connected to.

[0145] In an embodiment the photodetector further comprises a metallic layer located on the second surface and electrically grounded.

[0146] In an embodiment the photodetector further comprises a passivation layer on the photoactive layer to minimize surface defects on the photoactive layer; andan anti-reflection coating on the passivation layer to minimize light loss.References

[0147] 1. Photocurrent in Nanocrystal Arrays by Auger Recombination. Nano Lett 19, 48044810 (2019).

[0148] 2. J. B. Gao, A. M. Rao, H. B. Li, J. B. Zhang, O. Chen, Carrier Transport Dynamics in High Speed Black Phosphorus Photodetectors. Acs Photonics5, 1412-1417 (2018).

[0149] 3. J. B. Gao, S. C. Nguyen, N. D. Bronstein, A. P. Alivisatos, Solution- Processed, High-Speed, and High-Quantum-Efficiency Quantum Dot Infrared Photodetectors. Acs Photonics 3, 1217-1222 (2016).

[0150] 4. J. B. Gao, A. F. Fidler, V. I. Klimov, Carrier multiplication detected through transient photocurrent in device-grade films of lead selenide quantum dots. Nat Common 6, (2015).

Claims

CLAIMS1. A method for synthesizing Ag2Se quantum dots, comprising:preparing a silver precursor by dissolving a silver containing compound in a solvent and introducing a stabilizing agent and a reactive agent under inert conditions to form a stable precursor solution;preparing selenium precursors by dissolving selenium and sulfur in a coordinating or reactive solvent system to obtain a reactive precursor solution;injecting the selenium and sulfur precursors into the silver precursor solution under heating in a preselected temperature range to initiate Ag2Se quantum dot formation;controlling the growth of the Ag2Se quantum dots by maintaining reaction conditions for a predetermined duration of time;halting quantum dot growth by cooling the reaction mixture to form Ag2Se quantum dots; andpurifying the Ag2Se quantum dots through any one or combination of a series of dispersion, precipitation, and separation steps to remove impurities and aggregates.

2. The method according to claim 1 , wherein the preselected temperature range is from about 80°C to under 135°C.

3. The method according to any one of claims 1 or 2, wherein the silver precursor is prepared using a silver salt dissolved in an organic solvent.

4. The method according to claim 3, wherein the silver salt is any one or combination of silver acetate, silver nitrate, silver carbonate and silver oxide.

5. The method according to claim 3, wherein the silver salt is silver acetate.

6. The method according to any one of claims 1 to 5, wherein the stabilizing agent is any combination of 1 -octadecene (ODE) and 1 -dodecanethiol (DDT), oleylamine and trioctylphosphine oxide (TOPO).

7. The method according to any one of claims 1 to 5, wherein the stabilizing agent is 1 -octadecene (ODE) and 1 -dodecanethiol (DDT).

8. The method of any one of claims 1 to 8, wherein the coordinating or reactive agent is a derivative of silylamide.

9. The method according to claim 8, wherein the reactive silylamide derivative is one or both of Li[N(SiMe3)2] and Sn[N(SiMe3)2]2.

10. The method according to any one of claims 1 to 9, wherein the reaction mixtures cooled to a temperature in a range from about 20°C to about 30°C to form the quantum dots, with cooling performed rapidly within about 60 seconds to prevent excessive crystal growth.

11. The method according to any one of claims 1 to 10, wherein thepredetermined duration of time is in a range from about 30 minutes to about 90 minutes.

12. The method according to any one of claims 1 to 11 , wherein purifying of the quantum dots is carried out by a combination of solvent exchange and centrifugation.

13. The method according to any one of claims 1 to 12, wherein the quantum dots having a mean diameter in a range from about 2nm to about 10 nanometers (nm).

14. The method according to any one of claims 1 to 13, wherein purifying the quantum dots is performed by dispersing the quantum in toluene, precipitating with anhydrous ethanol, centrifuging the mixture in a range from about 5000 to about 10,000, repeating the dispersion and centrifugation steps to remove impurities and aggregated quantum dots, and dissolving the purified quantum dots in hexane to produce a solution suitable for device fabrication.

15. The method according to any one of claims 1 to 14, wherein the final purified quantum dots are dissolved in hexane at a concentration from about 5 to about 20 mg / mL for subsequent device fabrication.

16. The method according to any one of claims 1 to 15, wherein the silver precursor is heated at to a temperature in a range from about 120°C to about 200°C under an inert atmosphere of nitrogen or argon.

17. The method according to any one of claims 1 to 16, wherein the quantum dots have a tunable absorption range of 1000 nm to 1800 nm, with enhanced sensitivity in the shortwave infrared (SWIR) region suitable for infrared or visible light applications.

18. A photodetector, comprising:a substrate having opposed first and second surfaces;a photoactive layer comprised of Ag2Se quantum dots, produced by the methods of claims 1 to 17, that absorb light, creating electron-hole pairs located on the first surface; andat least two spaced metallic contacts located on the photoactive layer that attract and collect the generated electron-hole pairs to produce a photocurrent measured by a detector to which the at least two spaced metallic contacts are connected to.

19. The photodetector according to claim 18, further comprising a metallic layer located on the second surface and electrically grounded.

20. The photodetector according to claims 17 or 18, further comprising: a passivation layer to minimize surface defects on the photoactive layer; and / or an anti-reflection coating to minimize light loss.

21. A method for synthesizing Ag2S quantum dots, comprising:preparing a silver precursor by dissolving a silver containing compound in a solvent and introducing a stabilizing agent and a reactive agent under inertconditions to form a stable precursor solution;preparing selenium and sulfur precursors by dissolving selenium and sulfur in a coordinating or reactive solvent system to obtain a reactive precursor solution;injecting the selenium and sulfur precursors into the silver precursor solution under heating in a preselected temperature range to initiate Ag2S quantum dot formation;controlling the growth of the Ag2S quantum dots by maintaining reaction conditions for a predetermined duration of time;halting quantum dot growth by cooling the reaction mixture to form Ag2S quantum dots; andpurifying theAg2S quantum dots through any one or combination of a series of dispersion, precipitation, and separation steps to remove impurities and aggregates.

22. The method according to claim 21, wherein the preselected temperature range is from about 120°C to about 200°C.

23. The method according to claims 21 or 22, wherein the silver precursor is prepared using a silver salt dissolved in an organic solvent.

24. The method according to claim 23, wherein the silver salt is any one or combination of silver acetate, silver nitrate, silver carbonate and silver oxide.

25. The method according to claim 23, wherein the silver salt is silver acetate.

26. The method according to any one of claims 21 to 25, wherein the stabilizing agent is any combination of 1 -octadecene (ODE) and 1-dodecanethiol (DDT), oleylamine and trioctylphosphine oxide (TOPO).

27. The method according to any one of claims 21 to 25, wherein the stabilizing agent is 1 -octadecene (ODE) and 1 -dodecanethiol (DDT).

28. The method of any one of claims 21 to 27, wherein the coordinating or reactive agent is a derivative of silylamide.

29. The method according to claim 28, wherein the reactive silylamide derivative is one or both of Li[N(SiMe3)2] and Sn[N(SiMe3)2]2.

30. The method according to any one of claims 21 to 29, wherein the reaction mixtures cooled to a temperature in a range from about 20°C to about 30°C to form the quantum dots, with cooling performed rapidly within about 60 seconds to prevent excessive crystal growth.

31. The method according to any one of claims 21 to 30, wherein the predetermined duration of time is in a range from about 30 minutes to about 90 minutes.

32. The method according to any one of claims 21 to 31 , wherein purifying of the quantum dots is carried out by a combination of solvent exchange and centrifugation.

33. The method according to any one of claims 21 to 32, wherein the quantum dots having a mean diameter in a range from about 2nm to about 10 nanometers (nm).

34. The method according to any one of claims 21 to 33, wherein purifying the quantum dots is performed by dispersing the quantum in toluene, precipitating with anhydrous ethanol, centrifuging the mixture in a range from about 5000 to about 10,000, repeating the dispersion and centrifugation steps to remove impurities and aggregated quantum dots, and dissolving the purified quantum dots in hexane to produce a solution suitable for device fabrication.

35. The method according to any one of claims 21 to 34, wherein the final purified quantum dots are dissolved in hexane at a concentration from about 5 to about 20 mg / mL for subsequent device fabrication.

36. The method according to any one of claims 21 to 35, wherein the silver precursor is heated at to a temperature in a range from about 120°C to about 200°C under an inert atmosphere of nitrogen or argon.

37. The method according to any one of claims 21 to 36, wherein the quantum dots have a tunable absorption range of 1000 nm to 1800 nm, withenhanced sensitivity in the shortwave infrared (SWIR) region suitable for infrared or visible light applications.

38. A photodetector, comprising:a substrate having opposed first and second surfaces;a photoactive layer comprised of Ag2S quantum dots, produced by the methods of claims 21 to 37, that absorb light, creating electron-hole pairs located on the first surface; andat least two spaced metallic contacts located on the photoactive layer that attract and collect the generated electron-hole pairs to produce a photocurrent measured by a detector to which the at least two spaced metallic contacts are connected to.

39. The photodetector according to claim 38, further comprising a metallic layer located on the second surface and electrically grounded.

40. The photodetector according to claims 38 or 39, further comprising: a passivation layer on the photoactive layer to minimize surface defects on the photoactive layer; andan anti-reflection coating on the passivation layer to minimize light loss.

Images