Uncooled High Detectivity Mid-Infrared Photoconductor Using HgTe Quantum Dots and Nanoantennas

Using a metal/insulator/metal (MIM) structure with a gold nanoantenna array made by electron beam lithography, the responsivity of a HgTe colloidal quantum dot film is enhanced in the mid-infrared. Simulations indicate that the spatially averaged peak spectral absorption of an 80 nm film is 60%, enhanced 23-fold compared to that of the same film on a bare sapphire substrate. The field intensity enhancement is focused near the antenna tips, being 20-fold 100 nm away, which represents only 1% of the total area and up to 1000-fold at the tips. The simulated polarized absorption spectra are in good agreement with the experiments, with a strong resonance around 4 μm. A responsivity of 0.6 A/W is obtained at a 1 V bias. Noise measurements separate the 1/f noise from the generation–recombination white noise and give a spatially averaged photoconductive gain of 0.3 at 1 V bias. The spatially averaged peak detectivity is improved 15-fold compared to the same film on a sapphire substrate without an MIM structure. The experimental peak detectivity reaches 9 × 109 Jones at 2650 cm–1 and 80 kHz, decreasing at lower frequencies. The MIM structure also enhances the spatially averaged peak photoluminescence of the CQD film by 16-fold, which is a potential Purcell enhancement. The good agreement between simulations and measurements confirms the viability of lithographically designed nanoantenna structures for vastly improving the performance of mid-IR colloidal quantum dot photoconductors. Further improvements will be possible by matching the optically enhanced and current collection areas

Intraband Luminescence from HgSe/CdS Core/Shell Quantum Dots

HgSe/CdS core/shell CQD are synthesized, and the changes in the optical absorption and luminescence are investigated. While HgSe quantum dots are naturally n-doped after synthesis, both as colloidal solutions and as films, the HgSe/CdS core/shell dots in solution lose the n-doping, as seen from the optical absorption in solution. However, n-doping is regained in films, and the intraband luminescence of the films of HgSe/CdS is greater than that of the cores. The shell also vastly improves the stability of the quantum dots films against sintering at 200 °C. After annealing at that temperature, the HgSe/CdS films retain a narrow intraband emission and sustain a higher laser power leading to brighter emission at 5 μm

Evidence for the Role of Holes in Blinking: Negative and Oxidized CdSe/CdS Dots

Thin shell CdSe/CdS colloidal quantum dots with a small 3 nm core diameter exhibit typical blinking and a binary PL intensity distribution. Electrochemical charging with one electron suppresses the blinking. With a larger core of 5 nm, the blinking statistics of on and off states is identical to that of a smaller core but the dots also display a grey state with a finite duration time (∼6 ms) on glass. However, the grey state disappears on the electron-accepting ZnO nanocrystals film. In addition, the grey state PL lifetime on glass is similar to the trion lifetime measured from electrochemically charged dots. Therefore, the grey state is assigned to the photocharged negative dots. It is concluded that a grey state is always present as the dots get negatively photocharged even though it might not be observed due to the brightness of the trion and/or the duration time of the negative charge. With thick shell CdSe/CdS dots under electrochemical control, multiple charging, up to four electrons per dot, is observed as sequential changes in the photoluminescence lifetime which can be described by the Nernst equation. The small potential increment confirms the weak electron confinement with the thick CdS shell. Finally, the mechanism of hole-trapping and surface oxidation by the hole is proposed to account for the grey state and off state in the blinking

Superconductivity in Films of Pb/PbSe Core/Shell Nanocrystals

Superconductivity in films of electronically coupled colloidal lead nanocrystals is reported. The coupling between particles is <i>in situ</i> controlled through the conversion of the oxides present on the surface of the nanoparticles to chalcogenides. This transformation allows for a 10⁹-fold increase in the conductivity. The temperature of the onset of the superconductivity was found to depend upon the degree of coupling of the nanoparticles in the vicinity of the insulator–superconductor transition. The critical current density of the best sample of Pb/PbSe nanocrystals at zero magnetic field was determined to be 4 × 10³ A/cm². In turn, the critical field of the sample shows 50-fold enhancement compared to bulk Pb

Magnetoresistance of Manganese-Doped Colloidal Quantum Dot Films

The magnetoresistance of films of manganese-doped colloidal quantum dots of CdSe, ZnO, HgS, and ZnTe is investigated. At low concentration of manganese ions (1% or less), the hyperfine splitting of the Mn²⁺ electron spin resonance is resolved and similar to that of the bulk doped materials, indicating successful doping into the nanocrystals. At high Mn concentration (∼10%), the hyperfine splitting disappears because of interaction between the Mn²⁺ ions. Thin films of Mn:CdSe, Mn:ZnO, and Mn:HgS quantum dots are charged negative by applying an electrochemical potential, and the magnetoresistance is measured down to 2 K and up to 9 T. At low charging level, the magnetoresistance of thin films is positive, exhibits little effect of the manganese dopant, and is instead consistent with predictions from the variable range hopping model and the squeezing of the wave function of the quantum dots. At high charging level, the magnetoresistance becomes linear both for Mn:CdSe and Mn:ZnO, and this is not explained. At high Mn doping and low temperature, the positive magnetoresistance is greatly increased at low fields. This is proposed to be a signature of electron-magnetic polarons on the transport properties of the quantum dot films

Hot Electron Extraction From Colloidal Quantum Dots

Hot electrons are created in core/shell CdSe/ZnSe colloidal quantum dots by mid-infrared intraband (4 μm) excitation and are probed by time-resolved visible spectroscopy. The hot electron, in the first excited conduction state 1P_e of the CdSe core, is efficiently extracted by tunneling through the ZnSe shell. Electron extraction times are temperature-independent. They range from ∼100 ps for thick, ∼3 nm, uniform ZnSe shells to <4 ps for high-surface-area irregular ZnSe shells, and they compete favorably with intraband relaxation. The hot electron extraction leads to a quench of the visible photoluminescence. This is a first step toward infrared detection using the intraband transitions of colloidal quantum dots

HgS and HgS/CdS Colloidal Quantum Dots with Infrared Intraband Transitions and Emergence of a Surface Plasmon

HgS colloidal quantum dots (CQDs) are synthesized at room temperature using a dual-phase method. The HgS CQDs ranging from 3 to 15 nm exhibit air-stable n-doping and infrared intraband absorptions. For HgS CQDs of small sizes, the doping density is close to 2 electrons per dot, while for larger ones, their intraband absorption peaks shift to as far as 10 μm and exhibit Lorentzian line shapes. Under reducing potentials, these long-wavelength absorption peaks increase in strength and blue shift. This behavior can be explained through a classical model of the local field, showing how the degenerate single-electron transitions shift to a frequency that is the quadratic mean of the individual transition and a surface plasmon coming from a number of oscillators. This indicates that the intraband absorption of large, n-doped HgS CQDs is therefore becoming a surface plasmon. The same synthetic method works for HgS/CdS core/shells. Encapsulating HgS in a CdS shell removes the natural n-doping of the HgS cores, resulting in an interband photoluminescence at 1.5 μm with ∼5% quantum yield. The n-doping partially recovers upon film formation, and increases in strength after ligand exchange and annealing. The core/shell greatly improves the thermal stability of the HgS cores, allowing an annealing temperature as high as 200 °C

Gold Bipyramid Nanoparticle Dimers

An aqueous synthesis of gold bipyramid dimers is presented. The methodology, its selectivity, and the characterization of the resulting structures with optical dark-field and scanning electron microscopy are presented and discussed. In the bowtie orientation, the dimers exhibit a 20% red shift in their plasmon resonance as compared to the individual particles, with a weak dependence on the interparticle separation. From the analysis, it was found that the in situ absorption peaks that develop during the assembly can be assigned to specific dimer structures, which has not been shown previously. Last, the kinetics of the assembly are analyzed

Photoluminescence of Mid-Infrared HgTe Colloidal Quantum Dots

The photoluminescence quantum yield of HgTe colloidal quantum dots is measured from 1800 to 6500 cm^–1. There is a steep drop to low energy reminiscent of the generic gap law. However, direct evidence of energy transfer to the C–H stretch and overtone vibrations is apparent when temperature tunes the PL wavelength of a given sample through the vibrational resonances. Calculations based on the radiative rate and resonant energy transfer to the ligand vibrations appear to account for much of the quantum yield drop. Power-dependent photoluminescence lifetime measurements on 3.7 nm particles show fast, ∼50 ps, biexciton lifetime similar to other colloidal quantum dot systems of similar sizes