Colloidal Quantum Dots (CQD) made of inorganic heavy atoms with low frequency vibrations should facilitate long lived excited electronic states. This should allow CQDs to unlock the previously theorized great potential of quantum dot for infrared technologies. Using different starting semiconductor material, CQDs with interband or intraband transitions have been shown to cover the full infrared spectrum. Single element detectors have been realized from the near-IR to the LWIR. There is also a growing number of demonstrations of focal plane arrays with such materials, in the SWIR with PbS, and MWIR with HgTe. LED demonstrations cover the near-IR and are recently extending into the MWIR. This talk will describe the progress to date of CQD optoelectronic devices in the MWIR, as well the main target and challenges.
Colloidal quantum dots (CQDs) are a desirable platform for the development of next-generation infrared (IR) detectors thanks to their scalable synthesis, tunable optoelectronic properties, CMOS compatibility, and monolithic integration. However, CQD-based IR detectors typically have lower quantum efficiencies than epitaxial semiconductors and still require cryogenic cooling to achieve background-limited infrared photodetection. Developing CQD-based IR detectors that achieve state-of-the-art performance could bridge the gap between low-cost and high operating temperature detectors for IR sensing, especially for MWIR capabilities. Such a technology could significantly enable the advancement of compact, lightweight, and low-cost infrared systems for higher volume applications such as unmanned drone surveillance, driver-assisted vehicle navigation in low-visibility environments, and soldiermountable visual systems for advanced situational awareness. A systematic approach to materials development and detector design that relates material synthesis to detector optoelectronic properties will accelerate the development of CQD-based IR detector technologies. Such a system has not been explicitly established for CQD materials and their IR detectors. In this report, a process using a combination of empirical and numerical approaches has been described to guide and accelerate the development of CQD-based IR detectors. HgTe CQDs, one of the more mature IR CQD materials, was studied as a model system to provide useful feedback for establishing design rules and relationships between synthesis, material properties, and detector performance. Improvements to the performance of mid-wave infrared HgTe CQD photodetectors as an outcome of this study are demonstrated.
Flexible infrared detectors with multispectral imaging capability are attracting great interest with increasing demand for sensitive, low-cost and scalable devices that can distinguish coincident spectral information and achieve wide field of view, low aberrations, and simple imaging optics at the same time. However, the widespread use of such detectors is still limited by the high cost of epitaxial semiconductors like HgCdTe, InSb, and InGaAs. In contrast, the solution-processability, mechanical flexibility and wide spectral tunability of colloidal quantum dots (CQDs) have inspired various inexpensive, high-performance optoelectronic devices covering important atmospheric windows from short-wave infrared (SWIR, 1.5 – 2.5 μm) to mid-wave infrared (MWIR 3 – 5 μm). Here, a potential route leading to flexible infrared electronic eyes with multispectral imaging capability is demonstrated by exploring HgTe CQDs photovoltaic detectors. At room temperature, the HgTe CQDs detectors demonstrate detectivity D* up to 6 × 1010 Jones in SWIR and 6.5 × 108 Jones in MWIR. At cryogenic temperature, the MWIR D* becomes BLIP and increases to 1 × 1011 Jones. Besides high D* , the HgTe CQDs detector shows fast response with rise time below 300 ns. By stacking CQDs with different energy gaps or coupling CQDs with tunable optical filters, dual-band and multi-band infrared detection can be achieved in wide spectral ranges. Finally, infrared images are captured with flexible HgTe CQDs detectors at varying bending curvatures, showing a practical approach to sensitive infrared electronic eyes beyond the visible range.
Acoustic oscillations of metal nanoparticles can be used to study the properties of liquids at GHz frequencies and nanometer length scales. We use time-resolved spectroscopy to probe the dynamics of the metal nanoparticle oscillations utilizing a pump-probe technique. The incident pump laser pulse heats the nanoparticles leading to expansion and impulsive excitation of vibrations of the nanoparticles. The oscillations produce shifts in the plasmon resonance, which are monitored by measuring the change in absorption of a second weak broadband probe pulse. In our experiment, we immersed a sample of highly monodisperse gold bipyramids in water-glycerol mixtures from which we determined the damping resulting from the structure-liquid interactions. Performing these measurements over a range of temperatures provides a means to vary the fluid properties of a given water-glycerol mixture. Viscous damping could account for the measured results at low glycerol concentrations and sufficiently high temperatures but failed to describe the damping for high glycerol concentrations and sufficiently low temperatures. Accounting for the viscoelastic nature of the liquid mixtures mostly resolved the discrepancies, but consistently overestimated the degree of damping. Ultimately, allowing for a finite slip length produced good agreement with the measured damping rates. Our results show that standard assumptions in the fluid mechanics of simple liquids – a purely viscous response and the no-slip boundary condition – must be revisited at short length scales and fast time scales.
Infrared multispectral imaging with curved focal plane array (FPA) is attracting great interest with increasing demand for sensitive, low-cost and scalable devices that can distinguish coincident spectral information and achieve wide field of view, low aberrations, and simple imaging optics at the same time. However, the widespread use of such detectors is still limited by the high cost of epitaxial semiconductors like HgCdTe, InSb, and InGaAs. In contrast, the solution-processability, mechanical flexibility and wide spectral tunability of colloidal quantum dots (CQDs) have inspired various inexpensive, high-performance optoelectronic devices covering important atmospheric windows from short-wave infrared (SWIR, 1.5 – 2.5 μm) to mid-wave infrared (MWIR 3 – 5 μm). Here, a potential route leading to infrared electronic eyes with multispectral imaging capability is demonstrated by exploring HgTe CQDs photovoltaic detectors. At room temperature, the HgTe CQDs detectors demonstrate detectivity D* up to 6 × 1010 Jones in SWIR and 6.5 × 108 Jones in MWIR. At cryogenic temperature, the MWIR D* becomes BLIP and increases to 1 × 1011 Jones. Besides high D* , the HgTe CQDs detector shows fast response with rise time below 300 ns. By stacking CQDs with different energy gaps or coupling CQDs with tunable optical filters, dual-band and multi-band infrared detection can be achieved in wide spectral ranges. Finally, infrared images are captured with flexible HgTe CQDs detectors at varying bending curvatures, showing a practical approach to sensitive infrared electronic eyes beyond the visible range.
Devices based on small-gap mercury chalcogenide semiconductor nanocrystal inks have recently demonstrated increasingly high performance photodetection in the short and mid-wave infrared. These new colloidal inks are generating increasing interest because they could provide higher operating temperatures and vastly reduced costs compared to the current epitaxially-grown devices. However, in order to further increase detector operation temperatures and use these materials as infrared light sources, more detailed understandings of the carrier dynamics are required. Described here are picosecond mid-infrared absorption and emission studies of HgTe and HgSe colloidal quantum dots focusing on multicarrier nonradiative relaxation. Comparisons of the interband and intraband transitions in intrinsic and n-type systems reveal phenomena such as suppression of Auger relaxation in nanoparticles vs. bulk materials and slow or absent Auger relaxation in n-type quantum dots. Yet, the measured lifetimes are still limited by other nonradiative pathways that appear unique to small-gap nanomaterials. The deeper understanding of nonradiative relaxation in small-gap nanocrystals afforded by these experiments provides a path towards realizing high performance infrared photodetection near room temperature and robust mid-infrared light emission with colloidal quantum dots.
Colloidal quantum dots (CQD) attract much interest for optoelectronic applications, as potentially low-cost alternatives to epitaxial materials. In particular, in the mid-IR spectral range, CQD based on the zinc-blend mercury chalcogenides, Hg (S, Se, Te), lead efforts to create mid-IR technologies with solutions based materials. HgTe CQD with their tunable bandgap above 3 microns show promise as mid-IR detectors at a lower cost than existing HgCdTe (MCT) detectors. Progress towards the manufacture of mid-infrared cameras, improved sensitivity of PV devices, and new modalities, will be reviewed. HgS and HgSe CQDs also allow photodetection in the mid-IR because they are stably n-doped in ambient conditions, and they show an intense intraband transitions tunable in the mid-IR. Intraband CQDs is therefore another promising approach that broadens the types of materials considered. For both approaches, some of the challenges are similar, as one needs to develop tightly size-controlled colloidal quantum dots, and an interfacial chemistry that maximizes mobility and carrier lifetime, yet allows for controlling the doping.
II-VI colloidal quantum dots (CQDs) have made significant technological advances over the past several years, including the world’s first demonstration of MWIR imaging using CQD-based focal plane arrays. The ultra-low costs associated with synthesis and device fabrication, as well as compatibility with wafer-level focal plane array fabrication, make CQDs a very promising infrared sensing technology. In addition to the benefit of cost, CQD infrared imagers are photon detectors, capable of high performance and fast response at elevated operating temperatures. By adjusting the colloidal synthesis, II-VI CQD photodetectors have demonstrated photoresponse from SWIR through LWIR. We will discuss our recent progress in the development of low cost infrared focal plane arrays fabricated using II-VI CQDs.
This talk will discuss surface gating of the carrier density in colloidal quantum dots and the applications for infrared detection and emission. Doping of carriers in colloidal quantum dots (CQD) is important for a number of applications. While doping in bulk semiconductors is achieved with heterovalent impurities, CQDs can be doped by charge transfer from outside using reducing reagents or electrochemistry. In addition, the positions of the energy levels inside the particle and the outside Fermi level depend on the polarity of the interface, such that the surface composition can affect doping. This is the concept of surface gating. A striking demonstration is the filling and emptying of the quantum states of HgSe and HgS quantum dots by enrichment of the surface in metal or sulfur atoms. These are the first CQDs to exhibit carrier doping in ambient conditions. CQDs for infrared photodetection or emission is now extended to wider gap systems by using the intraband transitions of the doped quantum dots, first investigated nearly two decades ago.
Suspensions of HgTe colloidal quantum dots (CQD) are readily synthesized with infrared energy gaps between 3 and 12 microns. Infrared photodetection using dried films of these CQDs has been demonstrated up to a cutoff wavelength of 12 microns. The synthesis of CQDs and the fabrication of detector devices employ bench-top chemistry techniques, leading to the potential for the easy manufacture of infrared photon detecting imagers at low cost. Recent electrical and optical measurements of these CQD films are discussed. Recent successful prototypes of complete focal plane arrays from CQD films and commercially-available ROICs are also described.
Monodisperse suspensions of HgTe colloidal quantum dots (CQD) are readily synthesized with infrared
energy gaps between 3 and 12 microns. Infrared photodetection using dried films of these CQDs has
been demonstrated up to a wavelength of 12 microns, and HgTe CQD single-elemnet devices with 3.6
micron cutoff have bee nreported nad show ogod absorption <(10^4 cm^-1), response time and
detectivity (2*10^10 Jones) at at emperature of 175 K; with the potential fo uncooled imaging. The
synthesis of CQDs and fabrication of detector devices employ bench-top chemistry techniques, leading
to the potential for rapid, wafer-scale manufacture of MWIR imaging devices with low production costs
and overhead. The photoconductive, photovoltaic and optical properties of HgTe CQD films will be
discussed relative to infrared imaging, along with recent achievements in integrating CQD films with
readout integrated circuits to produce CQD-based MWIR focal plane arrays.
Colloidal quantum dots present an opportunity as infrared and liquid processed materials. Initial results in 2011 showed mid-infrared detection with HgTe colloidal quantum dots in the mi-IR range, 3-5 microns. This has been now extended to the long-wave IR, 8-12 microns. The infrared response from the HgTe colloidal quantum dots arises from the absorption of light across the gap created by the confinement. The large dots absorbing the LWIR are about 20 nm in size and the size dispersion will need improvements. While Interband absorption requires the material to be zero or small-gap semiconductors, intraband transitions have no such limitations. However, this requires doped colloidal quantum dots. Two colloidal quantum dot materials, the small gap (0.6 eV) b-HgS and the zero-gap HgSe turn out to be stably doped with electrons. This has led to the observation of Mid-IR intraband photoconduction in both systems and alternative materials for IR detection.
There are several basic challenges, besides fabrication and reliability. The proximity of the surface from the excitation leads to very short excited lifetimes due to nonradiative processes. Controlling the surface will be the avenue to lengthen the lifetime, while plasmonic coupling may lead to shorter radiative lifetime. Since the surface is easily chemically modified, it also leads to strong changes in the Fermi level and this will need to be controlled.
In this talk, I will describe my understanding of the potential and limitations of this material approach to infrared detection, while discussing aspects of transport, photoluminescence, doping and photovoltaic responses.
The majority of modern infrared photon imaging devices are based on epitaxially grown bulk
semiconductor materials. Colloidal quantum dot (CQD)-based infrared devices provide great
promise for significantly reducing cost as well as significantly increased operating temperatures
of infrared imaging systems. In addition, CQD-based infrared devices greatly benefit from band
gap tuning by controlling the CQD size rather than the composition. In this work, we investigate
the absorption coefficient of HgTe CQD films as a function of temperature and cutoff
wavelength. The optical absorption properties are predicted for defect-free HgTe films as well
as films which vary from ideal.
The transport and thermal properties of HgTe colloidal quantum dot films with cut-off
wavelengths in the mid-IR are investigated. The cut-off wavelength of this material can be tuned
over the 3-5 μm range, which makes it a promising alternative to existing high cost detectors. Post
deposition processes such as ligand exchange and atomic layer deposition are investigated as a way
to increase the carrier mobility.
HgTe colloidal quantum dot have been demonstrated for mid-infrared photoconduction. The potential and challenges
associated to the materials are discussed.
Quantum dots are nanometre-sized semiconductor particles exhibiting unique size-dependent electronic
properties. In order to passivate the nanocrystals surface and to protect them from oxidation, we grow a shell
composed of a second semiconductor with a larger bandgap on the core (for example a core / shell CdS / ZnS).
However, the lattice mismatch between the two materials (typically 7% between ZnS and CdS) induces
mechanical stress which can lead to dislocations. To better understand these mechanisms, it is important to be
able to measure the pressure induced on the semiconductor core. We used a nanocrystal doped with manganese
ions Mn2+, which provide a phosphorescence signal depending on the local pressure. A few dopant atoms per
nanoparticle were placed at controlled radial positions in a ZnS shell formed layer by layer. The experimental
pressure measurements are in very good agreement with a simple spherically symmetric elastic continuum
model[1]. Using manganese as a pressure gauge could be used to better understand some structural phenomena
observed in these nanocrystals, such as crystalline phases transition, or shell cracking.
We demonstrate three-dimensional optical trapping and orientation of individual Au nanorods, Au/Ag core/shell
nanorods, and Au bipyramids in solution, using the longitudinal surface-plasmon resonance to enhance optical forces.
Laser light that is detuned slightly to the long-wavelength side of the resonance traps individual and multiple particles
for up to 20 minutes; by contrast, light detuned to the short-wavelength side repels rods from the laser focus. Under
stable-trapping conditions, the trapping time of individual particles depends exponentially on laser power, in agreement
with a Kramers escape process. Trapped particles have their long axes aligned with the trapping-laser polarization, as
evidenced by a suppression of rotational diffusion about the short axis. When multiple particles are trapped
simultaneously, evidence of interparticle interactions is observed, including a nonlinearly increasing two-photon
fluorescence intensity, increasing fluorescence fluctuations, and changing fluorescence profiles as the trapped particle
number increases.
Colloidal growth of plasmonic nanostructures may present some advantages such as shape control at the nm
scale with atomic smoothness of the surfaces and possibly reduced damping. We show that the seed-mediated
growth of gold nanostructures is strongly dependent on the gold seed nanocrystal structure. Starting with
gold seed solutions prepared such that they are either single crystalline or multiply twinned, growth yields
either nanorods with good control over the aspect ratio (~10%) or elongated bipyramidal nanoparticles. The
nanorods are single crystalline while the gold bipyramids are penta-fold-twinned. The gold bipyramids are
also strikingly monodisperse in shape with the sharpest ensemble surface plasmon resonance reported so far.
Silver can be coated onto the gold nanostructures leading to a large blue-shift of the longitudinal plasmon
resonance. Surprisingly, even a thin silver layer introduces much additional damping explained as scattering at the
Au/Ag interface. Silver can be converted to silver sulphide yielding a large red-shift. The metal-semiconductor
composite materials may present interesting nonlinear optical properties which are being currently investigated.
Finally, the nonlinear optical scattering from individual Au nanorods was measured under excitation by ultrafast
laser pulses on resonance with their longitudinal plasmon mode. Surprisingly, the ultrafast nonlinearity can be
attributed entirely to the heating of conduction electrons and does not exhibit any response associated with
coherent plasmon oscillation. This indicates an unanticipated damping of strongly driven plasmons.
We have measured nonlinear scattering from plasmons in individual Au nanorods and have correlated second-harmonic
activity of Ag nanoparticles and clusters to morphology. The measurements reveal novel ultrafast nonlinear phenomena
related to electron confinement. Surprisingly, the coherent plasmon response is suppressed relative to the hot electron
response indicating enhanced plasmon dephasing. In a parallel set of studies we demonstrate nanometer scale
localization of the nonlinear optical response of single nanoparticles and aggregates and correlate this with their
morphology. Position markers are fabricated on an optical and electron-transparent substrate (Si3N4 thin film) that
allows optical measurements and transmission electron microscopy (TEM) imaging of the identical nanoparticles or
aggregates. The second harmonic (SH) activity optical image of individual Ag nanostructures is registered with the
TEM image. Centroid localization of the optical signals allows correlation with better than 25 nm precision. This is
sufficient to determine the origin of optical "hot spots" within multi-particle aggregates.
We demonstrate three-dimensional optical trapping and orientation of individual Au nanorods in solution, taking advantage of the longitudinal surface-plasmon resonance to enhance optical forces. Stable trapping is achieved using laser light that is detuned slightly to the long-wavelength side of the resonance; by contrast, light detuned to the short-wavelength side repels rods from the laser focus. Under stable-trapping conditions, the trapping time depends exponentially on laser power, in agreement with a Kramers escape process. Trapped rods have their long axes aligned with the trapping-laser polarization, as evidenced by a suppression of rotational diffusion about the short axis. The ability to trap and orient individual metal nanoparticles may find important application in assembly of functional structures, sorting of nanoparticles according to their shape, and development of novel microscopy techniques.
Excitation of plasmons in a metal nanoparticle leads to localization of electromagnetic fields within the particle, which is expected to result in strong optical nonlinearities. We study ultrafast nonlinearities in optical scattering from single gold nanorods under resonant excitation at the plasmon frequency, and observe changes of as much as 20% in the scattering cross section over the 20-fs laser pulse duration. Unexpectedly, the magnitude of the ultrafast nonlinearity is the same as that due to heating of conduction electrons in the metal.
The Si(111)/H surface has provided a model system for the vibrational dynamics of phonon coupled surfaces. It has also been an ideal testing ground for the time-resolved surface nonlinear optical probe.
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