HgTe Quantum Dots Research Articles

Ultrasmall HgTe Quantum Dots with Near-Unity Photoluminescent Quantum Yields in the Near and Shortwave Infrared

Ultrasmall HgTe Quantum Dots with Near-Unity Photoluminescent Quantum Yields in the Near and Shortwave Infrared

Slow Hot-Exciton Cooling and Enhanced Interparticle Excitonic Coupling in HgTe Quantum Dots.

Rapid hot-carrier/exciton cooling constitutes a major loss channel for photovoltaic efficiency. How to decelerate the hot-carrier/exciton relaxation remains a crux for achieving high-performance photovoltaic devices. Here, we demonstrate slow hot-exciton cooling that can be extended to hundreds of picoseconds in colloidal HgTe quantum dots (QDs). The energy loss rate is 1 order of magnitude smaller than bulk inorganic semiconductors, mediated by phonon bottleneck and interband biexciton Auger recombination (BAR) effects, which are both augmented at reduced QD sizes. The two effects are competitive with the emergence of multiple exciton generation. Intriguingly, BAR dominates even under low excitation fluences with a decrease in interparticle distance. Both experimental evidence and numerical evidence reveal that such efficient BAR derives from the tunneling-mediated interparticle excitonic coupling induced by wave function overlap between neighboring HgTe QDs in films. Thus, our study unveils the potential for realizing efficient hot-carrier/exciton solar cells based on HgTe QDs. Fundamentally, we reveal that the delocalized nature of quantum-confined wave function intensifies BAR. The interparticle excitonic coupling may cast light on the development of next-generation photoelectronic materials, which can retain the size-tunable confinement of colloidal semiconductor QDs while simultaneously maintaining high mobilities and conductivities typical for bulk semiconductor materials.

High Mobility HgTe Quantum Dot Films with Small Energy and Dynamic Disorder

High Mobility HgTe Quantum Dot Films with Small Energy and Dynamic Disorder

Thermally stable high carrier mobility nanocomposite infrared photodetector

Quantum dots (QDs) show excellent optical properties, such as a high extinction coefficient, tunable colors, and superior photostability. However, the transport properties of QDs, such as carrier mobility, are quite limited, which hinder optoelectronic applications. On the other hand, carbon nanotubes (CNTs) generally have high carrier mobility and thermal stability with a weak optical response. These features inspire us to couple QDs with CNTs to achieve improved optoelectronics. We take infrared HgTe QDs and multi-walled CNTs as examples. With appropriate coupling between QD and CNT matrices, carrier mobility could reach 34.6–54.1 cm2/Vs in the nanocomposite, a 1000-fold increase compared with the reference. The nanocomposite benefits external quantum efficiency up to 12 500% and detectivity 1012 Jones on the 2500 nm infrared photodetectors. The CNT matrix also helps relaxing thermally generated carriers, improving the photodetector thermal stability. We also demonstrate that the device maintains high detectivity at a high operating temperature.

Long-Range Hot-Carrier Transport in Topologically Connected HgTe Quantum Dots.

The utilization of hot carriers as a means to surpass the Shockley-Queasier limit represents a promising strategy for advancing highly efficient photovoltaic devices. Quantum dots, owing to their discrete energy states and limited multi-phonon cooling process, are regarded as one of the most promising materials. However, in practical implementations, the presence of numerous defects and discontinuities in colloidal quantum dot (CQD) films significantly curtails the transport distance of hot carriers. In this study, the harnessing of excess energies from hot-carriers is successfully demonstrated and a world-record carrier diffusion length of 15µm is observed for the first time in colloidal systems, surpassing existing hot-carrier materials by more than tenfold. The observed phenomenon is attributed to the specifically designed honeycomb-like topological structures in a HgTe CQD superlattice, with its long-range periodicity confirmed by High-Resolution Transmission Electron Microscopy(HR-TEM), Selected Area Electron Diffraction(SAED) patterns, and low-angle X-ray diffraction (XRD). In such a superlattice, nonlocal hot carrier transport is supported by three unique physical properties: the wavelength-independent responsivity, linear output characteristics and microsecond fast photoresponse. These findings underscore the potential of HgTe CQD superlattices as a feasible approach for efficient hot carrier collection, thereby paving the way for practical applications in highly sensitive photodetection and solar energy harvesting.

Obviating Ligand Exchange Preserves the Intact Surface of HgTe Colloidal Quantum Dots and Enhances Performance of Short Wavelength Infrared Photodetectors.

A large volume, scalable synthesis procedure of HgTe quantum dots (QDs) capped initially with short-chain conductive ligands ensures ligand exchange-free and simple device fabrication. An effective n- or p-type self-doping of HgTe QDs is achieved by varying cation-anion ratio, as well as shifting the Fermi level position by introducing single- or double-cyclic thiol ligands, that is, 2-furanmethanethiol (FMT) or 2,5-dimercapto-3,4-thiadiasole (DMTD) in the synthesis. This allows for preserving the intact surface of the HgTe QDs, thus ensuring a one order of magnitude reduced surface trap density compared with HgTe subjected to solid-state ligand exchange. The charge carrier diffusion length can be extended from 50 to 90nm when the device active area consists of a bi-layer of cation-rich HgTe QDs capped with DMTD and FMT, respectively. As a result, the responsivity under 1340nm illumination is boosted to 1 AW-1 at zero bias and up to 40 AW-1 under -1V bias at room temperature. Due to high noise current density, the specific detectivity of these photodetectors reaches up to 1010 Jones at room temperature and under an inert atmosphere. Meanwhile, high photoconductive gain ensures a rise in the external quantum efficiency of up to 1000% under reverse bias.

Laser‐Printed Plasmonic Metasurface Supporting Bound States in the Continuum Enhances and Shapes Infrared Spontaneous Emission of Coupled HgTe Quantum Dots

AbstractIn order to advance the development of quantum emitter‐based devices, it is essential to enhance light‐matter interactions through coupling between semiconductor quantum dots with high quality factor resonators. Here, efficient tuning of the emission properties of HgTe quantum dots in the infrared spectral region is demonstrated by coupling them to a plasmonic metasurface that supports bound states in the continuum. The plasmonic metasurface, composed of an array of gold nanobumps, is fabricated using single‐step direct laser printing, opening up new opportunities for creating exclusive 3D plasmonic nanostructures and advanced photonic devices in the infrared region. A 12‐fold enhancement of the photoluminescence in the 900–1700 nm range is observed under optimal coupling conditions. By tuning the geometry of the plasmonic arrays, controllable shaping of the emission spectra is achieved, selectively enhancing specific wavelength ranges across the emission spectrum. The observed enhancement and shaping of the emission are attributed to the Purcell effect, as corroborated by systematic measurements of radiative lifetimes and optical simulations based on the numerical solution of Maxwell's equations. Moreover, coupling of the HgTe photoluminescence to high quality factor modes of the metasurface improves emission directivity, concentrating output within an ≈20° angle.

Improving phototransistor performance with polymer-quantum dot hybrid technology

Herein, the infrared photodetection performance of the HgTe quantum dots embedded in a polymer matrix was studied using an FDTD solution. The role of P3HT polymer in the hybrid structure was investigated by focusing on optical enhancements. The simulation explored critical parameters such as photo absorption capabilities, electric field distribution, and electrical generation rate. Also, the effect of phase separation level on device performance was studied by changing the quantum dot cluster size. A phototransistor was fabricated using the hybrid structure with silver electrodes to examine the theoretical works. Incorporating P3HT polymer into the active medium of the device showed significant improvement in current density value at room temperature.

Rational development of new phosphine telluride precursors for the synthesis of telluride quantum dots

A series of phosphine tellurides with various electronic natures and steric hindrances were prepared. The solutions of these reagents were investigated by 31P/125Te NMR. Quantum chemical calculations were applied for the investigation of the stability of the monomers and complexes with a second phosphine using the DFT method. A good match between the reactivity in the nanocrystal synthesis and the NMR/DFT results was observed. Finally, we developed a tricyclohexylphosphine telluride reagent that is easier to handle and gives highly reproducible results in the synthesis of telluride nanocrystals than most currently applied tellurium solutions in trioctylphosphine (TOP-Te). The solvent effects, Hg:Te ratio and temperature dependences were studied for tricyclohexylphosphine telluride in the synthesis of HgTe quantum dots. The developed reagent was highly efficient in the synthesis of several telluride nanocrystals, namely CdTe, PbTe and ZnTe.

Si:HgTe Colloidal Quantum Dots Heterojunction-Based Infrared Photodiode

Integrated circuits and optoelectronics are currently dominated by silicon technology. However, silicon’s response wavelength is typically less than 1,100 nm, limiting the application of silicon in machine vision, autonomous vehicles, and night vision. For infrared photodetectors, HgTe colloidal quantum dots (CQDs) are promising materials. Because of the adjustable bandgap, it responds over a wide spectral range. However, the construction of a high-quality junction between Si and HgTe CQDs continues to be difficult, thus restricting the scope of its application. In this article, we describe the synthesis, characterization, and correlation of HgTe CQDs with reaction temperature and nanocrystal size. We then fabricated HgTe-CQDs/silicon infrared photodiodes and discussed how the silicon resistivity affected their performance. We found that the devices prepared from 9.1 nm HgTe quantum dots synthesized at 80°C and a silicon substrate with a resistivity of 20–50 Ω·cm has optimal performance parameters. This results in a responsivity of 0.2 mA/W for 1,550 nm incident light at room temperature. These results provide a direction for future silicon-compatible HgTe quantum dot infrared optoelectronics.

Short-wave infrared sensitive broadband photodetectors based on an HgTe quantum dot film

HgTe CQDs with tailored shapes (tetrapods, triangles, and distorted parallelograms) were successfully synthesized and employed to create vertical-structured broadband photodetectors.

Ultrafast Charge Carrier Dynamics and Transport Characteristics in HgTe Quantum Dots

Ultrafast Charge Carrier Dynamics and Transport Characteristics in HgTe Quantum Dots

Mid-Infrared HgTe Colloidal Quantum Dots In-Situ Passivated by Iodide

Today, colloidal quantum dots (CQDs) have received wide attention due to their properties of tunable infrared absorption. For example, HgTe colloidal quantum dots have shown excellent optical absorption (absorption coefficient α > 104 cm−1), spectral absorption tunability covering the entire infrared atmospheric window, and even the terahertz (THz). However, the efficient surface passivation of HgTe CQDs was limited by the highly sterically hindered long-chain organic ligands. Here, we demonstrate a new method to synthesize monodisperse mid-infrared HgTe CQDs, and the preparation process of the Hg precursor solution is optimized. With I− in-situ passivated on the surfaces, the spherical HgTe quantum dots are successfully synthesized with the tunability size from 8 to 15 nm. The noise current density of the photoconductive device is as low as 10−11 A·Hz−1/2 at 130 K with a frequency of 1 Hz.

Optical nonlinearities of mercury telluride quantum dots measured by nanosecond pulses

We synthesized the colloidal mercury telluride quantum dots (QDs) and analyzed their low-order nonlinear optical properties using 1064 nm and 532 nm, 10 ns pulses. HgTe QDs colloidal suspension showed the strong saturable absorption and negative nonlinear refraction (saturation intensity ISA = 5 ×108W cm−2, nonlinear absorption coefficients attributed to saturable absorption and reverse saturable absorption βSA = −4 ×10−9cm W−1 and βRSA = 1 ×10−8 cm W−1, nonlinear refractive index γ = −6 ×10−13 cm2 W−1) while using the 532 nm probe pulses. The nonlinear optical parameters of this suspension at 1064 nm were smaller (nonlinear absorption coefficient attributed to the two-photon absorption β2PA = 6 ×10−10 cm W−1, γ = −5 ×10−14 cm2 W−1). The negative nonlinear refraction of HgTe QDs was attributed to the thermal and Kerr effects in the case of 532 nm and 1064 nm probe pulses, respectively.

Magnetoresistance of high mobility HgTe quantum dot films with controlled charging

The magnetoresistance of HgTe quantum dot films, exhibiting a well-defined 1Se state charging and a relatively high mobility (1–10 cm2 V−1 s−1), is measured with controlled occupation of the first electronic state.

Photoconductive focal plane array based on HgTe quantum dots for fast and cost-effective short-wave infrared imaging

HgTe nanocrystals, thanks to quantum confinement, present a broadly tunable band gap all over the infrared spectral range. In addition, significant efforts have been dedicated to the design of infrared sensors with an absorbing layer made of nanocrystals. However, most efforts have been focused on single pixel sensors. Nanocrystals offer an appealing alternative to epitaxially grown semiconductors for infrared imaging by reducing the material growth cost and easing the coupling to the readout circuit. Here we propose a strategy to design an infrared focal plane array from a single fabrication step. The focal plane array (FPA) relies on a specifically designed readout circuit enabling in plane electric field application and operation in photoconductive mode. We demonstrate a VGA format focal plane array with a 15 μm pixel pitch presenting an external quantum efficiency of 4-5% (15% internal quantum efficiency) for a cut-off around 1.8 μm and operation using Peltier cooling only. The FPA is compatible with 200 fps imaging full frame and imaging up to 340 fps is demonstrated by driving a reduced area of the FPA. In the last part of the paper, we discuss the cost of such sensors and show that the latter is only driven by labor costs while we estimate the cost of the NC film to be in the 10-20 € range.

Nonlinear Absorption and Refraction of Picosecond and Femtosecond Pulses in HgTe Quantum Dot Films.

We report measurements of the saturated intensities, saturable absorption, and nonlinear refraction in 70-nm thick films containing 4 nm HgTe quantum dots. We demonstrate strong nonlinear refraction and saturable absorption in the thin films using tunable picosecond and femtosecond pulses. Studies were carried out using tunable laser pulses in the range of 400–1100 nm. A significant variation of the nonlinear refraction along this spectral range was demonstrated. The maximal values of the nonlinear absorption coefficients and nonlinear refractive indices determined within the studied wavelength range were −2.4 × 10−5 cm2 W−1 (in the case of 28 ps, 700 nm probe pulses) and −3 × 10−9 cm2 W−1 (in the case of 28 ps, 400 nm probe pulses), respectively. Our studies show that HgTe quantum dots can be used in different fields e.g., as efficient emitters of high-order harmonics of ultrashort laser pulses or as laser mode-lockers.

(Keynote) Electrochemistry Insights into the Development of Colloidal Quantum Dot Infrared Photodetectors

Colloidal quantum dots may provide a cheap and versatile approach to infrared detection and light sources. Indeed, advances in the past few years have led to commercial high definition shortwave infrared imaging with PbS quantum dots as well as academic demonstrations of solution processed mid-wave thermal imaging with HgTe quantum dots, dual band detection, flexible and hyperspectral sensors.This talk will start by describing how this progress derived from insights gained by electrochemical studies of colloidal quantum dots. Following a path previously traced for conducting polymers, solution phase electrochemical reduction and spectroscopy allowed the first observation of redox stability in quantum dot states of the colloidal quantum dots. This then led to electrochemical studies in films which confirmed ambipolar stability in the lead chalcogenides. The next key electrochemistry contribution was the development of the “solid-state ligand exchange” which increased 1000-fold the charge transfer rates across quantum dots. This led to the first observation of ohmic conduction in films of quantum dots and subsequent studies of hopping transport in these materials, before photoconductivity studies and translation to solid state gating. Electrochemistry is still routinely used to extract mobility in a state resolved manner, while that has only been recently realized with solid state gating. Electrochemistry is easily coupled with spectroscopy, and this allows much information on the states and their related interband and intraband transitions. Electrochemistry and photoluminescence have even been applied to single dots microscopy in the visible, which was used to study blinking and its elimination under reducing conditions. Electrochemistry also allowed to lower the laser threshold of quantum dot films under reducing conditions and this approach is now being translated to solid state devices by others. Finally, controlled doping is essential for devices and this is another area where electrochemistry provides much insights.This talk will then briefly describe the status of infrared photodetection beyond silicon with colloidal quantum dots, and emphasize the performance metrics and potential avenue for improvements.

Ultralow dark current infrared photodetector based on SnTe quantum dots beyond 2 μm at room temperature

Quantum dots (QDs) are promising materials used for room temperature mid-infrared (MIR) photodetector due to their solution processing, compatibility with silicon and tunability of band structure. Up to now, HgTe QDs is the most widely studied material for MIR detection. However, photodetectors assembled with HgTe QDs usually work under cryogenic cooling to improve photoelectric performance, greatly limiting their application at room temperature. Here, less-toxic SnTe QDs were controllably synthesized with high crystallinity and uniformity. Through proper ligand exchange and annealing treatment, the photoconductive device assembled with SnTe QDs demonstrated ultralow dark current and broadband photo-electric response from visible light to 2 μm at room temperature. In addition, the visible and near infrared photo-electric performance of the SnTe QDs device were well maintained even standing 15 d in air. This excellent performance was due to the effective protection of the ligand on surface of the QDs and the effective transport of photo-carriers between the SnTe interparticles. It would provide a new idea for environmentally friendly mid-IR photodetectors working at room temperature.

Synthesis of IR-emitting HgTe quantum dots using an ionic liquid-based tellurium precursor.

New scalable precursor chemistries for quantum dots are highly desirable and ionic liquids are viewed as an attractive alternative to existing solvents, as they are often considered green and recyclable. Here we report the synthesis of HgTe quantum dots with emission in the near-IR region using a phosphonium based ionic liquid, and without standard phosphine capping agents.