Research on Intraband Photodetectors

1.  Mid-Infrared Intraband Photodetector via High Carrier Mobility HgSe Colloidal Quantum Dots

Chen, M.; Hao, Q.; Luo, Y.; Tang, X., ACS Nano 2022, 16 (7), 11027-11035.

      In this work, a room-temperature mixed-phase ligand exchange method is developed to obtain a relatively high carrier mobility (∼1 cm²/(Vs)) on HgSe intraband colloidal quantum dot solids without any observable trap state. What is more, the doping from 1Se to 1Pe state in the conduction band could be precisely controlled by additional salts during this method, proved by optical and transport experiments. The high mobility and controllable doping benefit the mid-infrared photodetector utilizing the 1Se to 1Pe transition, with a 1000-fold improvement in response speed, which is several μs, a 55-fold increase in responsivity, which is 77 mA/W, and a 10-fold increase in specific detectivity, which is above 1.7×10⁹ Jones at 80 K. The high-performance photodetector could serve as an intraband infrared camera for thermal imaging, as well as a CO₂ gas sensor with a range from 0.25 to 2000 ppm.

2. Mid-IR Intraband Photodetectors with Colloidal Quantum Dots

Zhao, X.; Mu, G.; Tang, X.; Chen, M., Coatings 2022, 12, (4), 467.

In this paper, we investigate an intraband mid-infrared photodetector based on HgSe colloidal quantum dots (CQDs). We study the size, absorption spectra, and carrier mobility of HgSe CQDs films. By regulating the time and temperature of the reaction during synthesis, we have achieved the regulation of CQDs size, and the number of electrons doped in conduction band. It is experimentally verified by the field effect transistor measurement that dark current is effectively reduced by a factor of 10 when the 1Se state is doped with two electrons compared with other doping densities. The HgSe CQDs film mobility is also measured as a function of temperature the HgSe CQDs thin film detector, which could be well fitted by Marcus Theory with a maximum of 0.046 ± 0.002 cm²/Vs at room temperature. Finally , we experimentally discuss the device performance such as photocurrent and responsivity. The responsivity reaches a maximum of 0.135 ± 0.012 A/W at liquid nitrogen temperature with a narrow band photocurrent spectrum.

3. Size Distribution Effects on Mobility and Intraband Gap of HgSe Quantum Dots 

Chen, M.; Shen, G.; Guyot-Sionnest, P., The Journal of Physical Chemistry C 2020, 124, (29), 16216-16221.

In this work, we investigate the effect of size distribution on the mobility, the conductivity gap, and the intraband photoconduction of HgSe colloidal quantum dots (CQDs). Using electrochemistry, we measure the mobility for a series of ethandithiol cross-linked n-doped HgSe quantum dot films with different size distribution but a similar average size. The results show that mobility is exponentially dependent on size dispersion. This is interpreted as the size dispersion causing an increase in the average activation energy for hopping transport and the effect is reproduced by a model and a simulation. Comparing with the interband HgTe where the optical gap is between the valence and the conduction band, the conductivity gap in n-doped HgSe between the 1Se and 1Pe states is more strongly softened by the size distribution. This harms the intraband photoconductive properties and it implies that improved size distribution will be needed when using intraband photodetectors.

4. State-Resolved Mobility of 1 cm² /(Vs) with HgSe Quantum Dot Films

Chen, M.; Shen, G.; Guyot-Sionnest, P., The Journal of Physical Chemistry Letters 2020, 11, (6), 2303-2307.

HgSe colloidal quantum dot films are made by using a hybrid ligand exchange (HgSe/hybrid) in polar inks and compared with the solid-state ligand exchange using ethanedithiol (HgSe/EDT). In both systems, the conductance shows a peak at one-electron filling of the 1Se state and a dip at 2 electrons before filling the 1Pe state. The HgSe/hybrid films show a ∼100-fold increased mobility, reaching up to ∼1cm²/Vs for 7.5 nm diameter particles. While field effect transistor and Hall measurements give similar carrier density and mobility, the temperature dependence of the mobility is consistent with hopping transport.

5. Reversible Electrochemistry of Mercury Chalcogenide Colloidal Quantum Dot Films

Chen, M.; Guyot-Sionnest, P., ACS Nano 2017, 11, (4), 4165-4173.

The absolute positions of the energy levels of colloidal quantum dots of Hg (S, Se, Te), which are of interest as mid-infrared materials, are determined by electrochemistry. The bulk valence bands are at -5.85, -5.50, and -4.77 eV (±0.05 eV) for zinc-blend HgS, HgSe, HgTe, respectively, in the same order as the anions p-orbital energies. The conduction bands are conversely at -5.20, -5.50, and -4.77 eV. The stable ambient n-doping of Hg (S, Se) quantum dots compared to HgTe arises because the conduction band is sufficiently lower than the measured environment Fermi level of -4.7 eV to allow for n-doping for HgS and HgSe quantum dots even with significant electron confinement. The position of the Fermi level and the quantum dots states are reported for a specific surface treatment with ethanedithiol and electrolyte environment. The positions are however sensitive to different surface treatments, providing an avenue to control doping. Electrochemical gating is further used to determine the carrier mobility in the films of the three different systems as a function of CQD size. HgSe and HgS show increasing mobility with increasing particle sizes while HgTe shows a nonmonotonous behavior, which is attributed to some degree of aggregation of HgTe QDs.

6. Plasmon resonance enhanced colloidal HgSe quantum dot filterless narrowband photodetectors for mid-wave infrared

Tang, X.; Wu, G. f.; Lai, K. W. C., Journal of Materials Chemistry C 2017, 5 (2), 362-369.

Narrowband detection of mid-wave infrared is of great importance for many applications including thermal imaging and scientific research. Detection of narrowband signals is typically achieved by using broadband photodetectors combined with interference filters or interferometric optics, which inevitably increase the architectural complexity and transmission loss. Here, we report an uncooled and filterless narrowband photodetector for the detection of mid-wave infrared with an enhanced photoresponse by plasmon resonance. Colloidal HgSe quantum-dot films were employed as sensing materials with narrowband optical absorption due to the intraband 1Se–1Pe transition. Furthermore, gold nanodisk arrays were designed, fabricated and integrated with the HgSe quantum-dot films. Based on the near-field resonance created by the nanodisk structure, the responsivity at the center wavelength of the HgSe quantum-dot film based narrowband photodetector can be increased. In our experiments, four narrowband photodetectors with tunable center wavelengths of 4.2 μm, 6.4 μm, 7.2 μm and 9.0 μm were fabricated. After the integration with the plasmonic nanodisk array, the responsivity at the center wavelength of the four fabricated narrowband photodetectors was enhanced by 517%@ 4.2 μm, 288%@ 6.4 μm, 257%@ 7.2 μm, and 208%@ 9.0 μm, reaching up to 145 mA W⁻¹, 92.3 mA W⁻¹, 88.6 mA W⁻¹, and 86 mA W⁻¹, respectively. The results demonstrated that the full-width at half-maxima of spectral responsivity was also decreased by 42.9% to 59.9% after the integration of the plasmonic nanodisk array. The full-width at half-maxima for four types of photodetectors were all below 2 μm (center wavelength- full-width at half-maxima: 4.2–1.05 μm, 6.4–1.15 μm, 7.2–1.25 μm, and 9.0–1.75 μm), indicating a very sharp spectral photoresponsivity.