Recent Progresses in Mid Infrared Nanocrystal based Optoelectronics

Abstract

Over the past few years, colloidal nanoparticles have started to be investigated for their optical properties in the mid-infrared, past 3 microns. Research on detector application has led to background limited detection and fast video imaging at 5 microns. With further development, one could imagine that these new materials could vastly reduce the costs of infrared technology and this would lead to a trove of new applications for infrared imaging into our daily lives. This article reviews the progress regarding the optical, transport and photodetection properties of thin film based on these materials, and the three different ways by which infrared resonances have been realized with colloidal nanoparticles: interband absorption with small gap semiconductor quantum dots, intraband absorption in lightly doped quantum dots, and plasmonic resonances in heavily doped nanocrystals. Index Terms— Colloidal quantum dot, infrared, HgTe, HgSe, self-doping, photoresponse.

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Abstract— Over the past few years, colloidal nanoparticles
have started to be investigated for their optical properties in the
mid-infrared, past 3 microns. Research on detector application
has led to background limited detection and fast video imaging at
5 microns. With further development, one could imagine that
these new materials could vastly reduce the costs of infrared
technology and this would lead to a trove of new applications for
infrared imaging into our daily lives. This article reviews the
progress regarding the optical, transport and photodetection
properties of thin film based on these materials, and the three
different ways by which infrared resonances have been realized
with colloidal nanoparticles: interband absorption with small gap
semiconductor quantum dots, intraband absorption in lightly
doped quantum dots, and plasmonic resonances in heavily doped
nanocrystals.
Index Terms— Colloidal quantum dot, infrared, HgTe, HgSe,
self-doping, photoresponse.
I. INTRODUCTION
In the mid infrared (λ>3µm) the thermal emission of a room
temperature object typically prevails over the reflection of a
warm source. The mid-wave infrared (MWIR) between 3 and
5 m is an eye-safe region and an atmospheric transparency
window. The long-wave infrared (LWIR) between 8 and 12µm
is where the peak of the 300K blackbody radiation coincides
with another atmosphere transmission window. Both ranges
are of interest for long distance thermal imaging.
There are two main types of infrared detectors, quantum
detectors (sensitive to photon) and thermal detector (sensitive
to energy). Quantum detectors operate like visible
photodetectors but require small energy gaps and this is
challenging from a material perspective. The dominant narrow
band gap semiconductors are InSb for the MWIR, as well as
Hg1-xCdxTe (MCT) which is tunable from MWIR to LWIR by
adjusting the composition. To achieve high performance, the
materials need to be single crystals with minimal defects and
dislocations. On a commercial scale, this is achieved by
epitaxial growth on lattice matched substrates by molecular
beam epitaxy or chemical vapor deposition, and this leads to
high costs. Over the past twenty years, heterostructured wide
band gap semiconductors have been developed using
superlattices. The infrared absorption is created using
intersubband transitions within one of the wells of the
superlattice (GaAs/AlGaAs) and these are called Quantum
Well Infrared Photodetectors (QWIP) [
1
] or Type II
transitions across wells of the different materials, (InAs/GaSb),
called T2SL (Type II Super Lattice) or SLS (Strained Layer
Super-lattice). Research on Quantum Dot Infrared Detectors
(QDIP) based on self-assembled epitaxial quantum dots also
started about twenty years ago [
2
-
6
]. QDIPs, while initially
very promising, have suffered from limited size control and
the low density of the epitaxial quantum dots. Today the
dominant commercial quantum detectors are still based on
InSb and MCT [
7
]. The quantum detector technologies aim for
high performance applications with high detectivity and high
speed. However, the detectors are costly and, combined with
the requirement for cooling, their use is largely restricted to
defense and research such as high definition camera for
astronomy imaging and spectroscopy.
Bolometer detection is the leading room temperature
technology combining low NETD (30-50mK) with operation
at 300K. Bolometers are widely used for cheap thermal motion
sensors, and bolometer cameras are leading the market for
consumer IR imaging. Two disadvantages of bolometers are an
inherently slow time constant of a few ms, and an intrinsically
lower detectivity than quantum detectors, especially in the
MWIR. Figure 1 shows the specific detectivity achieved with
various detector technologies.
Figure 1 Detectivity as a function of wavelength for different detector
technologies. The circles highlight results obtained with HgTe and
HgSe CQD, adapted from ref [
8
]. PC and PV are respectively used
for photoconductor and photovoltaic.
Since the previous technologies are not likely to lead to a price
disruption, an alternative may come from solution processing.
Organic electronic which is often seen as the low cost
alternative to silicon electronic is ineffective in the mid
infrared but inorganic colloidal quantum dots (CQD) are a
potential alternative. For the last 30 years, CQDs attracted
scientific interest because of their captivating size tunable
optical features and because of the ease of fabrication. The
first mass market for these nanoparticles appeared a couple
year ago as they started to be used as phosphors for TV
display [
9
].
Recent Progresses in Mid Infrared Nanocrystal
Optoelectronics
Emmanuel Lhuillier, Philippe Guyot-Sionnest

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Most of the earlier efforts focused on wide band gap material
such as cadmium chalcogenides, Cd(S,Se,Te), with bright
visible fluorescence. The lead chalcogenides CQDs, primarily
Pb(S,Se), brought the materials to the near-IR with promising
developments as optical detector and solar photovoltaic. HgTe
CQDs opened the mid-infrared spectral range with
photoluminescence and photodetection [
10
-
12
].
One may also use the intraband transition of CQDs which
removes the need for a small bulk bandgap and widely
expands the range of possible IR CQDs. This strategy has so
far been demonstrated for HgS and HgSe. In parallel, there
has been much interest in semiconductor nanocrystal that
exhibit plasmonic resonances, tunable in the mid-IR with the
control of the carrier density. These might also become part of
future IR technology.
II. NANOCRYSTALS WITH INFRARED PROPERTIES
Figure 2a. band structure of bulk HgTe. b. discrete spectrum of HgTe
CQD calculated by a tight binding method, adapted from ref [13]. c.
Energy of the HgTe interband absorption edge as a function of the
nanoparticle size (diameter), adapted from ref [14].
A. HgTe interband CQDs
The leading material for mid-IR CQDs is currently HgTe. It is
a zero gap semiconductor, also called a semimetal, and any
absorption edge can a priori be obtained with quantum
confinement. There is a small number of other semimetals,
including C (graphene), Bi, Sb, and binary materials, HgSe,
Cd3As2. There are also very small gap semiconductors such as
SnTe, Bi2Te3 or Ag2Te which have been made as nanocrystals.
The ternary materials, HgCdTe and PbSnTe, show a zero gap
over a range of compositions and InAsSb has a small gap.
These materials could be potential alternatives to the HgTe.
Figure 2a shows the k.p band structure for HgTe. The Γ6 band
which has generally the symmetry of a conduction band (in
CdTe for example) is below the Γ8 bands which usually have
the valence band symmetry. This inversion of the band is
responsible for the “negative value” of the band gap reported
for HgTe and HgSe. When HgTe is intrinsic, the Fermi level
lies where the two Γ8 bands meet. The weakly dispersive
heavy-hole band with Γ8 symmetry plays the role of the
valence band, while the conduction band is the upper Γ8 band.
The lowest energy interband transition in CQDs occur between
the quantum confined states from these two Γ8 bands.
Calculation of the electronic structure of HgTe CQD using a
tight binding method (see Figure 2b) have shown a good
correlation with experimental results [
13
,
14
].
Figure 3 a. absorption spectra of HgTe quantum dots of two sizes,
adapted from [21]. b TEM image of the material associated with the
spectra of part a. This early synthesis was reaching the 3-5µm range,
but the band edge was poorly defined and the material was strongly
aggregated. c. Photocurrent spectra of HgTe quantum dots of
different sizes, adapted from ref [22]. d TEM image of the material
associated with the spectra of part b. This material presents shaper
band edge and reduced aggregation. e. Absorption spectra of HgTe
quantum dots of several sizes, adapted from ref [13]. f. TEM image
of the material associated with the spectra of part c..g Absorption
spectra of CdTe and HgTe nanoplatelets, adapted from ref [27]. h.
TEM image of the material associated with the spectrum of part g.
For the historical development of the HgTe nanocrystals, the
reader is referred to an extensive review of the synthesis of
small gap chalcogenide nanoparticles [10]. Briefly, the
colloidal synthesis of HgX compounds started with aqueous
precipitation [
15
-
17
] and evolved towards organometallic
methods [
18
-
20
] striving for increasingly better control of size
and optical properties. The initial small nanocrystals of
Hg(Cd)Te had optical absorption and bright luminescence in
the near infrared and the interest was to apply the materials for
near-IR biolabelling and optoelectronics around the telecom
wavelength. In 2011, Keuleyan et al [
21
] developed an HgTe
CQD synthesis to obtain optical absorption in the 3-5µm
range, specifically for mid-infrared detection. These HgTe
CQDs had an absorption tail into the mid-IR and allowed the
first photodetection of mid-infrared light with CQDs.
However, they showed no resolved excitonic feature, see
Figure 3a, and were highly aggregated, see Figure 3b.
Improvements of the synthesis led to better resolved features
[
22
] and improved detection properties. A typical synthesis
starts with HgCl2 in an oleylamine solvent. At a particular
temperature, a near stoichiometric amount of TOPTe is
injected. The growth temperature can be maintained for a few
minutes to hours, leading to slow growth of the particles.
Temperatures between 70°C and 120°C allow to reach a range
of sizes such that the band edge covers the range from 2
microns to 8 microns at room temperature. An optimized
procedure can lead to materials with multiple absorption
features as seen Figure 3e. This is indicative of a good size
dispersion reported at ≈ 5%. However, as seen in Figure 3d,
the particles are not spherical and do not appear particularly
uniform in the TEM image, showing the possibility of further
improvements.
Using cation exchange, it is possible to start from CdTe and
CdSe nanoplatelets [
23
-
26
] (NPL) with a perfectly controlled

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thickness to generate NPL of HgTe and HgSe [
27
] shown in
Figure 3g. These HgTe NPL present the advantages of the 2D
geometry already observed for the cadmium chalcogenide
compounds: (i) narrow optical feature (PL linewidth is below
60meV for an emission at 1.4eV) and fast emission (50ns PL
lifetime). The optical absorption edge remains currently in the
near infrared (800-1000nm), which correspond to 1.5eV of
confinement energy, due to the small thickness of the NPL.
The growth of thicker mercury chalcogenides NPL with sharp
optical absorption in the mid-infrared has not yet reported.
B. Intraband transitions with HgS and HgSe CQDs
The intraband transition between the first two quantum
confined states of CQDs is also an IR transition and many
experiments explored the transition between the lowest
conduction band states 1Se to the next 1Pe state. While the
infrared 1Se-1Pe intraband absorption was observed in CdSe
CQDs upon photoexcitation of carriers [
28
] or after charge
transfer doping [
29
,
30
] the carriers were only stable in inert
conditions. HgS [
31
] and HgSe [
32
,
33
] CQDs were found to
be naturally n-doped and to show the infrared 1Se-1Pe
intraband absorption stable in ambient conditions.
Figure 4c show spectra of HgSe nanocrystals. At high energy
the absorption is due to interband absorption. At smaller
energy, there is the intraband absorption. As discussed further
below, the relative intensity of the intraband absorption and
interband excitonic absorption can be tuned by the surface
chemistry. When extended to larger sizes as shown in Fig. 3b,
the intraband absorption can be tuned to 20 microns [33]. HgS
CQDs show similar optical properties. The zinc blend HgS is
often reported as a zero-gap semiconductor in contrast to the
more stable form of the red cinnabar which has a gap in the
visible range, however CQD spectroscopy indicates that zinc
blend HgS has a moderate gap of 0.65 eV [
34
].
Figure 4 a. TEM image of small HgSe CQD. The inset is a high
resolution image highlighting the lattice fringes. b.TEM image of
large HgSe CQD. The inset is a high resolution image which show
the polycristaline nature of the largest CQD. c absorption spectra of
two sizes of HgSe CQD. The absorption arises from interband
transitions at high energy and intraband transitions at low energy.
Adapted from ref [33]. d Energy of the HgSe intraband absorption
peak as a function of the nanoparticle size (diameter).The error bars
represent the full width at half maximum of the size distribution
obtained by TEM and the full width half maximum of the absorption
peak.
The synthesis of HgS and HgSe CQDs derives from that of
HgTe. TOPS and TOPSe being less reactive than TOPTe,
seleneourea and thiourea have been used as sources of selenide
and sulfide respectively. Another synthetic scheme used a
more reactive mercury salt based on mercury acetate, in oleic
acid and oleylamine, which allows reaction with TOPSe. This
led to CQD with size below 15nm, see Figure 4a. Phosphine
binds to Mercury and in an effort to obtain larger CQD, a
phosphine free synthesis was developed [
35
]. With SeS2 as a
selenium precursor, larger (up to 50nm) HgSe CQD were
reported [33], see Figure 4b.
Concerning the origin of doping in these two materials, the
following points have been identified. (i) The doping is
affected by surface modifications, see Figure 5a. Changing the
ligand can significantly affect the 1Se state occupation, see
Figure 5b. Similarly the exposure to metal ions adsorption
raises the doping level, while sulfide ions lower the doping
level, see Figure 5c. (ii) The ratio of the intraband absorption
and interband absorption [
36
] is shown for several different
ligand exchanges in Figure 5a. (iii) The doping of the larger
CQDs is far less affected by sulfides or ligand exchange than
for small CQDs. (iv) For the larger CQD, the intraband signal
gets narrower in spite of a worse size dispersion.
The stable doping of HgS and HgSe CQDs in ambient
conditions constrains the energy of the 1Se state to be close to
the ambient Fermi level. The latter is between the reduction of
H+ to H2, (-4.1 V vs vacuum at pH 7) and the reduction of
water to oxygen (-5.3V vs vacuum at pH 7). This is made a
priori possible by the large workfunction of the HgX CQD
[
37
] and their narrow bad gap.
The surface sensitivity of the doping has been attributed to the
energy shift of the CQD states due to a change of the
electrostatic potential of the CQDs, relative to the ambient
Fermi level [31]. A proposed mechanism is changes of the
dipole moments at the surface. This is similar to work-function
changes of metal surfaces due to the dipole layer set by
adsorbates, and with organic monolayers of different dipole
moment [
38
]. Adding a dipole layer shifts the energy of the
states with respect to the Fermi level of the environment,
according to Δ

= σd/εrε0 where σd is the surface dipole
density,
0

the vacuum permittivity and
r

the medium
dielectric constant. In narrow band gap materials, large effects
can be expected with reasonable dipole magnitude. There have
been several other studies on the suggested role of the dipole
moment on CQD surfaces in adjusting the redox potentials
[
39
] or in optimizing the injection barriers from dots to
polymers [
40
]. However, direct measurements of the Fermi
level and the dipole density have not yet been performed, and
several prior studies reported no correlation between the
dipole moment of the adsorbates and the energy level shifts of
CQDs [
41
,
42
]. Complex effects have been observed with

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HgSe/CdS (see Figure 5d) and HgS/CdS CQDs where the
growth of a CdS shell can lead to a disappearance of the
doping in a colloidal solution but recover in films. This
indicates that doping is rather sensitive to the environment,
which is both a challenge and an opportunity.
Figure 5a. Absorption spectra of small HgSe (5nm) CQD before and
after their surface modification with the indicated ligands. b. Relative
population of the conduction band 1Se state for 4.7 and 5.7 nm HgSe
CQD for different capping ligands, adapted from ref [36] c.
Absorption spectra of HgS core upon successive exposures to S2- and
Hg2+ ions, adapted from ref 31. d. Absorption spectra of HgSe core
and HgSe/CdS core shell CQD, adapted from ref. [
43
].
As the nanoparticles are made smaller, doping should be more
difficult since electrons in the 1Se state may become too
reducing for the environment, including the surface.
Conversely, as the nanoparticles are grown larger, the higher
energy quantum states may move below the environment
Fermi level and pick up electrons. Therefore many more
carriers may be present in the larger nanoparticle. At some
point the intraband resonances of all the electrons may be
better described as a collective surface plasmon resonance and
this has been discussed for n-ZnO [
44
] and n-HgS [34].
C. Surfaces plasmons in semiconductor nanoparticles
While the connection between the intraband transitions and the
surface plasmons is recent, surface plasmon resonances in
semiconductor nanoparticles have been previously reported for
several systems starting with Cu2-xS [
45
].The surface plasmon
resonance arises from the free carriers of the material. It
appears as an intense infrared absorption at energies below the
semiconductor gap, just like an intraband transition. Other
materials showing the surface plasmon resonance include Al:
ZnO [
46
], In:CdO [
47
], P:Si [
48
] and several other systems.
The reader is referred to recent reviews [
49
-
51
]. The surface
plasmon resonance can be quite narrow. For example, In:CdO
nanocrystals show a resonance as narrow as ≈ 700cm-1 in the
mid-IR [47]. Recent single particle linewidth for Al:ZnO
nanocrystals have been measured to be ≈ 600 cm-1 for a center
position of ≈ 2500 cm-1 [
52
]. Even narrower ensemble
linewidth of 300 cm-1 at a center position of 1000 cm-1 have
been observed for the larger HgS nanocrystals that are
proposed to be highly doped and for which the resonance is
also assigned to a plasmon [34]. While single electron
intraband transitions narrow to meV widths at low
temperatures [
53
], the plasmonic linewidths of doped
nanocrystals are expected to remain broad even at low
temperature because they are limited by carrier-carrier
scattering. The fluorescence should also be very weak and it
has not yet been reported for the semiconductor plasmonic
nanoparticles. The carrier-impurity scattering rate can be
minimized by using dopants with low scattering potentials and
this can be extracted from the bulk effective mass and
mobility. Using remote doping or charge transfer may also
improve the linewidth. Narrower linewidths have indeed been
observed with ITO nanoparticles with Sn segregated near the
surface [
54
].
The evolution from the few electrons (intraband) to the many
electrons (surface plasmon) has been discussed for metal
nanoparticles [
55
-
57
] as well as for charge-transfer doped ZnO
and HgS nanoparticles. The intraband-surface plasmon
evolution is likely a rather generic feature of all nanoparticle
semiconductors that can sustain many electrons. In the case of
larger (10 nm) HgSe [33] and Ag2Se nanoparticles [
58
], strong
resonances in the LWIR were observed and assigned to
quantum confinement but they may have a plasmonic character
as well. The strong IR resonance afforded by the surface
plasmon resonance of semiconductor nanoparticles could
become beneficial for bolometric detection with narrow
spectral response and it could be used to improve the optical
properties of mid-IR devices.
III. OPTICAL AND ELECTRICAL PROPERTIES OF HGTE, HGSE
AND HGS
For photodetection, the quantum efficiency of photocarrier
generation and the lifetime of the carriers are important
parameters. High efficiency requires that the photogenerated
exciton rapidly dissociate and that the charge carriers travel to
the collecting electrode without recombination. This calls for
maximizing the mobility of the charge carriers and minimizing
non-radiative recombination.
Eliminating non-radiative process is particularly challenging
for small gap excitations. At present the mid-IR
photoluminescence of CQDs is rather weak. For 2µm emission
the PL efficiency is around 1% and drops to 0.01% at 5µm
[
59
]. One difficulty is the presence of organic ligands on the
CQD surface. The latter are typically made of organic
molecules which also absorb in the mid-IR and the near field
coupling of the semiconductor with the surrounding organic
shell leads to fast energy transfer [
60
]. An example is shown in
Figure 6a where the PL efficiency drops significantly as it
overlaps with the C-H stretch vibration. A strategy is therefore
to push the integration of the CQD into a fully inorganic
matrix to prevent the charge transfer to the ligands [
61
].
The interband absorption of HgTe redshifts as the temperature
decrease [
62
], see Figure 6a, of the order of 200-400µeV.K-1
depending of the CQD size. MCT alloys show similar red
shifts with decreasing temperature. The temperature
dependence of the intraband transitions in HgSe is however
opposite [32] and this is not yet explained.

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Figure 6 a. PL of HgTe CQD at different temperatures ranging from
300K to 7K. As the PL peak overlaps the C-H bond absorption, the
PL efficiency drops, adapted from ref [59]. Time resolved
photoluminescence for HgTe CQD. The PL lifetime presents two
component, one around 50ps due to Auger recombination at higher
pump power (lower curve), and one around 400ps due to non-
radiative geminate recombination, adapted from ref [59].
The dynamic of carriers in narrow band gap CQDs materials
has not yet been much investigated mostly due to technical
limitations. An important issue with bulk IR detector materials
is Auger recombination. Auger leads to short carrier lifetimes
at room temperature and this strongly decreases the detectivity.
With quantum dots there is a possibility of reduced Auger
rates which would allow higher operation temperature. There
are limited measurements of the Auger rate in HgTe CQDs and
the biexciton Auger recombination is as fast as ≈ 50 ps. [59,
63
], see Figure 6b. The narrow band gap of the HgX CQD
have also attracted some attention for multiexciton generation
[
64
] in HgTe [
65
] and HgCdTe [
66
] CQD. Obviously more
needs to be done in this direction in the future.
As synthesized, the HgTe nanocrystals are rather intrinsic
semiconductors [
67
,
68
] and films exhibit ambipolar character
with similar electron and hole mobility, see Figure 7a-b, and
this is a good feature for fast photodetection. Devices can be
made in air and show good enough stability at the present
stage of the development. The measured mobility depends on
the nanocrystal synthesis procedure, likely by the difference in
aggregation, and it may depend on the measurement method,
using either electrochemical gating or solid state gating.
Ligand exchange of the more dispersed HgTe CQD [22] with
short organic ligand (such as ethanedithiol=EDT) leads to
electrochemically measured ambipolar mobility of ~10-2 cm2V-
1s-1 [68] Values of 10-4-10-3 cm2V-1s-1 are reported when the
same procedure is applied to HgSe nanocrystal [32].
Using an organic ligand may reduce the photocurrent
efficiency and the ideal capping matrix needs to combine a low
absorption in the mid infrared and a type I band alignment
with the nanocrystal. Some success was reported with
As2S3/propylamine as capping agent [
69
,
70
]. With HgTe CDQ
this raised the carrier mobility compared to EDT [70] as
shown in Fig.7b. In the case of HgSe, a mobility of 90 cm2V-
1s-1 has been reported [33] using a liquid phase ligand
exchange with As2S3 derived ligands. While HgTe is
ambipolar, HgSe and HgS show only n type behavior in
different gating configurations, see Figure 7a-b. This is
consistent with their n-doped character probed by optical
measurements, see Figure 4c, and it may also indicate a lack of
chemical stability as p-type materials. The presence of doping
has been confirmed even at the single particle level using on
chip tunnel spectroscopy [
71
]. The doping of the 1Se followed
by the 1Pe state has been reported by electrochemical gating
with a reference electrode, see Figure 6c. The conductance
displays a local minimum resulting from Pauli blockade once
the 1Se states is filled. The higher mobility from the 1Pe state
as well as near quantitative 3-fold increase in electron loading
was also reported. The transfer curve measured in a one
electrode ion-gel gating, see Figure 6d, also display a non-
monotonous gating that arises from filling the 1Se state.
Figure 7 a Transfer curve (drain current vs gate voltage) for a field
effect transistor with a channel made of HgTe CQD capped with
As2S3 ligands. b mobility as a function of temperature for HgTe CQD
with As2S3 or EDT, adapted from ref [70] c. Electrochemical gating
of HgSe CQDs cross-linked with ethanedithiol, showing the
electrochemical current (blue line), the conductance (red line), and
the extracted differential mobility black curve. The peaks are
assigned to the 1Se and 1Pe states. d. Transfer curve of a electrolytic
transistor with a made of HgSe CQD capped with As2S3 ligands.,
adapted from ref [33]. The curve presents a minimum, assigned to
filling of the 1Se state.
IV. PHOTODETECTION PROPERTIES OF HGTE, HGSE AND HGS
Photoconduction with HgTe nanocrystal was first investigated
[
72
] in the near-infrared for the telecom wavelengths
[20,
73
,
74
] and for solar cell [
75
,
76
]. Mid-infrared
photoconduction is at present the property of the HgX CQD
which drives the most interest in these materials [
77
,
78
].
In the mid-infrared, HgX devices have been operated in a
photoconductive and photovoltaic geometry [
79
].
Photoconductive devices are built by drop-casting materials on
(interdigitated) electrodes. In terms of performance, a
limitation is that complete light absorption requires rather
thick films. Another limitation is the 1/f noise [
80
] that is
present in biased systems. As shown in Figure 1, the specific
detectivity is a traditional measure of performance, and various
reported values of the D* of HgX CQDs are indicated. For
better comparisons, one would also need to report the response
temporal bandwidth as this allows to distinguish between
bolometers and quantum detectors operation. The best HgTe

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of Selected Topics in Quantum Electronics
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CQD detector properties reported to date at 5 microns are with
the photovoltaic geometry, see Figure 8a. With this geometry,
and with 0V bias, the detectivity was reported to be in the
regime of Background Limited Infrared Performance (BLIP)
where the noise is limited by the 295K background radiation
[79], along with response times of microseconds, internal
quantum efficiency of 40%, and specific detectivity of 4x1010
Jones at 5 microns.
At present, the detectivity of CQDs decreases very fast with
increasing temperature or for redder wavelengths. The reddest
detection with HgTe dots has been demonstrated up to 12
microns but with low detectivity of ≈ 107 Jones at 80K. When
dealing with the design of infrared detector material, the final
judge remains the formation of an image. By coupling a film
of HgTe CQD with a silicon read out circuit, the first mid-IR
imaging with CQD was recently reported [
81
].Figure 8c-d
shows an image taken at 120 frame per second to illustrate the
faster response than bolometer cameras. The reported noise
equivalent temperature difference (NETD) was 100mK for 5
microns imaging [
82
].
Figure 8 a. Current as a function of applied bias for a photovoltaic
device at 90K in the dark condition (black line), receiving ambient
295K radiation (blue line) and receiving 600ºC blackbody
illumination (red line). A scheme of the device is given as inset,
adapted from ref [79]. b. Spectral dependence of the detectivity of a
HgTe CQD film coupled or not to a plasmonic structure, adapted
from ref [83]. c. and d. are images of a focal plane array for which the
active layer is HgTe CQD, adapted from refs [80,82].
The mid-IR CQDs need significant improvements before they
can become a viable competition to existing technologies.
One advantage of solution processing is that it affords device
improvement strategies which may not be as easily applied to
the wafer materials. By coupling near-IR HgTe CQD with
gold nanorods, Chen et al [
83
] reported a three-fold
improvement in detectivity shown in Figure 8b. Coupling the
absorption of the MWIR CQD with plasmonic structures [
84
,
85
] allows to increase the absorption of thin layers of CQDs.
The pixel formation of Visible/mid-IR bicolor [
86
] and MWIR
Multispectral detectors is also be facilitated with solution
processing [
87
]. Finally, there is a need to develop less toxic
materials. Even if the CQDs lead to cheap and performant
cameras, the toxicity of Hg is a serious concern for consumer
electronics. Possible alternatives such as Cd3As2 and PbSnTe
should be investigated. The intraband approach should allow a
much wider range of materials. Although CQD intraband
photodetection has barely started to be investigated, there is
also the possibility of fundamental advantages such as better
control of Auger process and narrower spectral detection.
V. CONCLUSION
Over the past few years, HgTe CQDs have been the leading
materials for CQD based MWIR photodetection, having
demonstrated BLIP performance, fast response and integration
into camera imaging. The introduction of self-doped
nanocrystals of HgSe and HgS has also allowed to use
intraband transitions as an alternative route to mid-IR CQD
detectors. With these initial demonstrations of some degree of
feasibility within a relatively short time, one may expect a
significant growth in this field.
The research is partly driven by the end-goal of performant
and affordable uncooled infrared imaging. There are many
possible directions to follow in order to address the numerous
challenges. Higher mobility and longer carrier lifetimes need
to be achieved, possibly by inorganic encapsulation. As in bulk
materials, small changes of doping have large effects on device
performances and doping needs to become a controllable
property. Optical management, using various methods, from
dielectric stacks to plasmonics, will need to be investigated to
maximize the photon collection efficiency. There is a need and
an opportunity to develop new nanofabrication process which
preserve or enhance the CQD properties. Developing lower
toxicity compounds will be crucial for consumer electronic
applications and there will be a need to achieve long term
stability in devices. With such progress, CQDs could indeed
transform IR technologies.
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of Selected Topics in Quantum Electronics
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Emmanuel Lhuillier is an engineer from ESPCI. He did his PhD at Onera
under the supervision of Emmanuel Rosencher on the electronic transport of
superlattices. In 2010, he joined the Guyot-Sionnest group, developing the
photoconductive properties of HgTe nanocrystals. In 2012, he joined the
Dubertret’s group at ESPCI where he investigated the optoelectronic
properties of colloidal quantum wells. Since 2015, he is a CNRS research
scientist in the Institute of Nanoscience of Université Pierre and Marie Curie.
His group is dedicated to the optoelectronic studies of confined materials.
Philippe Guyot-Sionnest is a graduate from Ecole Polytechnique, France,
and obtained his physics PhD from UC Berkeley under the supervision of
Yuen Ron Shen, on the topic of surface nonlinear optics. His research group
at the University of Chicago started in 1991 and has contributed to CQD
research since, including the core/shell strategy for improved fluorescence,
charge transfer doping, conductivity, and the more recent mid-IR
applications.