6 research outputs found
Probing Charge Carrier Dynamics to Unveil the Role of Surface Ligands in HgTe Narrow Band Gap Nanocrystals
Colloidal nanocrystals are an interesting
platform for the design
of low cost optoelectronic devices especially in the infrared range
of wavelengths. Mercury chalcogenides have reached high maturity to
address wavelengths above the telecom range (1.5 μm). However,
no screening of the surface chemistry influence has been conducted
yet. In this paper, we systematically probe the influence of a series
of ligands, Cl<sup>–</sup>, SCN<sup>–</sup>, 1,2-ethanedithiol,
1,4-benzenedithiol, 1-octanethiol, 1-butanethiol, As<sub>2</sub>S<sub>3</sub>, and S<sup>2–</sup>, on the photoconductive properties
of HgTe nanocrystal thin films. A high bandwidth, large dynamic transient
photocurrent setup is used to determine the photocarrier dynamics.
Two regimes are clearly identified. At the early stage (few nanoseconds)
a fast decay of the photocurrent is resulting from recombination and
trapping. Then transport enters in a multiple trapping regime where
carriers present a continuously decreasing effective value of their
mobility. The power law dependence of the conductance can be used
to estimate the trap carrier density and determine the value of the
Urbach energy (35–50 meV). We demonstrate that a proper choice
of ligand is necessary for a trade-off between the material performance
(μτ product) and the quality of the surface passivation
(to keep a low Urbach energy)
Band Edge Dynamics and Multiexciton Generation in Narrow Band Gap HgTe Nanocrystals
Mercury
chalcogenide nanocrystals and especially HgTe appear as an interesting
platform for the design of low cost mid-infrared (mid-IR) detectors.
Nevertheless, their electronic structure and transport properties
remain poorly understood, and some critical aspects such as the carrier
relaxation dynamics at the band edge have been pushed under the rug.
Some of the previous reports on dynamics are setup-limited, and all
of them have been obtained using photon energy far above the band
edge. These observations raise two main questions: (i) what are the
carrier dynamics at the band edge and (ii) should we expect some additional
effect (multiexciton generation (MEG)) as such narrow band gap materials
are excited far above the band edge? To answer these questions, we
developed a high-bandwidth setup that allows us to understand and
compare the carrier dynamics resonantly pumped at the band edge in
the mid-IR and far above the band edge. We demonstrate that fast (>50
MHz) photoresponse can be obtained even in the mid-IR and that MEG
is occurring in HgTe nanocrystal arrays with a threshold around 3
times the band edge energy. Furthermore, the photoresponse can be
effectively tuned in magnitude and sign using a phototransistor configuration
Coupled HgSe Colloidal Quantum Wells through a Tunable Barrier: A Strategy To Uncouple Optical and Transport Band Gap
Among
semiconductor nanocrystals (NCs), 2D nanoplatelets (NPLs)
are a special class of nanomaterials with well controlled optical
features. So far, most of the efforts have been focused on wide band
gap materials such as cadmium chalcogenide semiconductors. However,
optical absorption can be pushed toward the infrared (IR) range using
narrow band gap materials such as mercury chalcogenides. Here we demonstrate
the feasibility of a core/shell structure made of a CdSe core with
two HgSe external wells. We demonstrate that the optical spectrum
of the heterostructure is set by the HgSe wells and this, despite
the quasi type II band alignment, makes the band edge energy independent
of the inner core thickness. On the other hand, these core/shell NPLs
behave, from a transport point of view, as a wide band gap material.
We demonstrate that the introduction of a wide band gap CdSe core
makes the material less conductive and with a larger photoresponse.
Hence, the heterostructure presents an effective electric band gap
wider than the optical band gap. This strategy will be of utmost interest
to design infrared effective colloidal materials for which the reduction
of the carrier density and the associated dark current is a critical
property
Wave-Function Engineering in HgSe/HgTe Colloidal Heterostructures To Enhance Mid-infrared Photoconductive Properties
The
use of intraband transition is an interesting alternative path
for the design of optically active complex colloidal materials in
the mid-infrared range. However, so far, the performance obtained
for photodetection based on intraband transition remains much smaller
than the one relying on interband transition in narrow-band-gap materials
operating at the same wavelength. New strategies have to be developed
to make intraband materials more effective. Here, we propose growing
a heterostructure of HgSe/HgTe as a means of achieving enhanced intraband-based
photoconduction. We first tackle the synthetic challenge of growing
a heterostructure on soft (Hg-based) material. The electronic spectrum
of the grown heterostructure is then investigated using a combination
of numerical simulation, infrared spectroscopy, transport measurement,
and photoemission. We report a type-II band alignment with reduced
doping compared with a core-only object and boosted hole conduction.
Finally, we probe the photoconductive properties of the heterostructure
while resonantly exciting the intraband transition by using a high-power-density
quantum cascade laser. Compared to the previous generation of material
based on core-only HgSe, the heterostructures have a lower dark current,
stronger temperature dependence, faster photoresponse (with a time
response below 50 μs), and detectivity increased by a factor
of 30
Intraband Mid-Infrared Transitions in Ag<sub>2</sub>Se Nanocrystals: Potential and Limitations for Hg-Free Low-Cost Photodetection
Infrared photodetection based on
colloidal nanoparticles is a promising
path toward low-cost devices. However, mid-infrared absorption relies
on interband transitions in heavy metal-based materials, which is
a major flaw for the development toward mass market. In the quest
of mercury-free infrared active colloidal materials, we here investigate
Ag<sub>2</sub>Se nanoparticles presenting intraband transition between
3 and 15 μm. With photoemission and infrared spectroscopy, we
are able to propose an electronic spectrum of the material in the
absolute energy scale. We also investigate the origin of doping and
demonstrate that it results from a cation excess under the Ag<sup>+</sup> form. We demonstrate photoconduction into this material under
resonant excitation of the intraband transition. However, performances
are currently quite weak with (i) a slow photoresponse (several seconds)
and (ii) some electrochemical instabilities at room temperature
Short Wave Infrared Devices Based on HgTe Nanocrystals with Air Stable Performances
Colloidal
quantum dots (CQDs) are candidates of interest for the
design of low cost IR detector, especially in the short wave infrared
(SWIR; 0.8–3 μm), where the vicinity of the visible range
makes the high cost of available technologies even more striking.
HgTe nanocrystals are among the most promising candidates to address
SWIR since their spectrum can be tuned all over this range while demonstrating
photoconductive properties. However, several main issues have been
swept under the rug, which prevents further development of active
materials and devices. Here we address two central questions, which
are (i) the stability of the device under ambient air condition and
(ii) the reduction of dark current. Encapsulation of HgTe CQDs is
difficult because of their extreme sensitivity to annealing, we nevertheless
demonstrate an efficient encapsulation method based on a combination
of O<sub>2</sub> and H<sub>2</sub>O repellant layers leading to stability
over >100 days. Finally, we demonstrate that the dark current reduction
can be obtained by switching from a photoconductive geometry to a
photovoltaic (PV) device, which is fabricated using solution and low
temperature based approach. We demonstrate fast photoresponse (>10
kHz) and detectivity enhancement by 1 order of magnitude in the PV
configuration at room temperature. These results pave the way for
narrow bandgap CQD based cost-effective optoelectronic devices in
developing next generation SWIR photonic systems