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Synthetic Platform for Device-Agnostic Quantum Dot IR Photodectors


RT&L FOCUS AREA(S): Quantum Sciences, Microelectronics


TECHNOLOGY AREA(S): Electronics, Materials, Sensors


OBJECTIVE: Develop a ‘synthetic platform’ and design rules to produce device-agnostic semiconductor nanocrystal (quantum dot/QD) absorbers for infrared sensors.


DESCRIPTION: Current infrared (IR) sensors typically use epitaxial semiconductors or microbolometers as optoelectronic absorbers. Epitaxial semiconductor sensors achieve high speed and sensitivity, but are expensive and typically operate at cryogenic temperatures. Microbolometer cameras may operate uncooled, but operate only in the long-very long wave IR (LWIR-VLWIR) range with limited sensitivity and speed.


Quantum dots are already widely employed for photonic applications such as LEDs. Using QDs as IR absorbers requires control of the optoelectronic properties (e.g. charge transfer) of QD assemblies, which remain poorly understood [1-7]. Current approaches to QD-based sensors require developing QD materials in parallel with a sensor ‘scaffold’ (e.g. readout integrated circuit/ROIC). Only recently have advances in electronics and materials enabled a commercial IR sensor featuring a QD absorber [7,8].


Improved understanding and control of optoelectronic properties of QD assemblies would allow QDs to be optimized to a chosen sensor design, without the need to develop sensor scaffolds and QD absorbers in parallel. Such ‘device-agnostic’ QDs would be compatible with the variety of QD image sensor designs (e.g. photoconductive, photovoltaic, bulk or low dimensional absorbers…) already in development.


This STTR targets ‘leap-ahead’ innovation to mature QD IR absorber technology as an alternative to microbolometers or epitaxial semiconductors. QD-based IR sensors could enable low-cost, high-speed, lightweight uncooled infrared detectors across the short- to long-wave infrared (SWIR-LWIR) regions. Additionally, QD absorbers could enable uncooled mid-wave IR (MWIR) cameras, a current technology gap [2-5,9,10]. The advantages of QD-based IR detectors compared to current technologies (e.g. microbolometers or epitaxial semiconductors) are a result of quantum confinement phenomena in QDs and simple processing.


Due to quantum confinement effects, QD absorbers may bypass performance-limiting thermal noise issues encountered in bulk semiconductors and operate with little/no cooling. Wavelengths including SWIR-LWIR can be achieved by controlling QD size and composition. As photodetectors, QD devices are capable of high sensitivity and high response speeds compared to microbolometer-based systems [1-5,8].


QDs are prepared in simple liquid suspensions, which may be painted, printed, dropped or spun onto a sensor scaffold such as a ROIC. This processing enables rapid, low-cost synthesis of large volumes of QDs (>1m2 absorber/day for <$1K materials) [3,4,7,8].


A successful synthetic platform for applying QD absorbers in IR cameras will result in improved sensor performance and stability, lower costs and faster development times compared to current technologies. This will require:

- identification and understanding of the phenomena determining the final properties of QD-based absorbers,

- accurate screening methods to characterize the relevant properties of as-synthesized QDs,

- model(s) capable of predicting sensor/absorber performance from QD screening results, and

- synthetic protocols and processes to control the QD properties that determine final sensor performance.


The product of this STTR will be a platform to produce QDs designed and optimized based on specifications for a given image sensor. QDs would be compatible with the variety of QD infrared image sensor designs already in development. Further, a single sensor design could be optimized for various wavelengths by choosing the appropriate QD to apply to the basic scaffold.


PHASE I: Determine the feasibility of synthesizing device-agnostic QDs for IR absorption. The Phase I deliverable is a report. In the Phase I report the performer should:

A) Identify class(es) of QDs of interest (e.g. II-VI, III-V, perovskite…) and relevant QD properties for investigation (e.g. size distribution, surface chemistry, photoluminescence…). Identify synthetic approach(es) for QDs of interest. Identify existing methods and/or propose testing methods to measure the QD properties of interest.

B) Identify potential uncooled QD absorber sensor formats of interest and relevant sensor performance metrics for prediction/evaluation (e.g. detectivity, responsivity, stability, quantum efficiency…). Identify existing and/or propose test device(s) to characterize sensor performance metrics. For identified metrics, determine ranges of performance that would be competitive with current industry-standard technologies and allow comparison between QD devices and current uncooled technologies.

C) Propose a systematic approach to develop and demonstrate relations between properties identified in A) and performance metrics identified in B).


PHASE II: Execute systematic investigations into QD properties and sensor performance outlined in Phase I report. Validate QD characterization methods identified in Phase I. Synthesize and characterize QDs with variable properties of interest (as identified in Phase I). Verify synthetic control of QD properties of interest.


Fabricate and validate test device(s) for sensor characterization identified in Phase I. Apply synthesized QDs to test devices and identify correlations and/or causal relationships between QD properties and final sensor performance.


Based on experimental results, identify the primary QD properties governing final sensor performance. Construct predictive model(s) for relating QD synthesis/QD properties to sensor performance.


Successful Phase II results will clarify basic design rules for QD-absorbers and devices. Phase II deliverables should include design rule(s)/model(s) for predicting and optimizing at least one sensor metric to match or exceed the ‘industry competitive’ range identified in Phase I. Other successful outcomes could include demonstrating design-optimization models for multiple metrics in one sensor design, or one metric in multiple sensor designs. Rules/Models should be accompanied by a ‘library’ of synthetic protocols to control the relevant QD properties.


PHASE III DUAL USE APPLICATIONS: The ultimate product of the proposed STTR will be a synthetic platform comprising design rules for QD-based absorbers and a set of synthetic protocols for implementing said design rules. Recent and ongoing developments in sensor scaffolds for QDs have demonstrated and continue to optimize multiple sensor designs. A device-agnostic synthetic platform for QDs, as envisioned here, will enable broad application of QD-based sensors tailored to specific missions. Combining the appropriate sensor scaffold with the optimal QDs accesses a broad design envelope to provide interchangeable ‘plug-and-play’ IR sensors with optimized wavelength, resolution and size, weight, power and cost (SWaP-C) parameters.


Potential applications for QD-based IR imagers include wearable sensors with comparable or better SWaP-C than current microbolometers and improved speed and sensitivity. Such sensors are ideal for the Army Integrated Visual Augmentation System (IVAS), for example. Small drones and autonomous vehicles operating in degraded visual environments would also benefit from the low cost and tailorable performance of QD imagers.


Current applications for QD absorbers are based on the designs of conventional IR cameras. Another ‘leap-ahead’ enabled by determining design rules for QD-absorber fabrication is commercialization of novel camera designs using large-scale detectors and/or novel geometries (e.g. curved, flexible sensors) not possible with current technologies.



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KEYWORDS: Quantum Dot, Nanocrystal, Infrared Detector, Infrared Absorber, Long Wavelength Infrared (LWIR), Uncooled, Sensor, Low-dimensional materials

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