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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.

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 ∼1 cm2/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.

Spherical and tetrahedral HgTe colloidal quantum dots (CQDs) are synthesized, and their doping is tuned electrochemically. Compared to spherical dots of a similar volume, the tetrahedral CQDs show a decrease in confinement energy as well as a sharper band edge absorption. The intraband spectra of the tetrahedral CQDs also display a smaller splitting from spin-orbit coupling. The shape-controlled synthesis with an improved size distribution and sharper optical features could find applications in optoelectronic devices.

Mid-infrared core/shell quantum dots are synthesized starting with HgTe and HgSe cores which emit around 5 μm with interband and intraband transitions, respectively. HgTe/CdTe and HgSe/CdX (X = S, Se, and Te) are grown by colloidal atomic layer deposition at room temperature. HgSe/CdSe is also grown by hot-injection. For films of the core/shells, even a two-monolayer shell leads to a vastly improved stability against annealing, with no observable changes up to the alloying temperatures of ∼180 °C for HgTe/CdTe and ∼250 °C for HgSe/CdX. The improvement in the photoluminescence remains however small for all systems. In this study across interband and intraband emissions, the brightest emitter at 5 μm is the intraband transition of HgSe/CdSe with a quantum yield of ∼10–3.

HgTe-based colloidal quantum dots (CQDs) fabricated between 10 nm and 20 nm in size readily lead to infrared cutoff wavelengths between 3 μm and 12 μm, due to their quantum confinement. In previous work, infrared photodetection using these films has been demonstrated to detect radiation out to a wavelength of 12 μm, and imaging in the mid-wave infrared region. In this work, a complete focal plane array and imager was fabricated and its performance measured for detecting radiation out to 12 μm. The photoconductive and optical properties of these HgTe CQD films are described, along with recent advancements in CQD detector technology. Anticipated improvements in the CQD synthesis and film deposition chemistries and techniques can raise the specific detectivity of these CQD films, bringing them closer to room-temperature operation.

International audience; AbstractHgTe colloidal quantum dots (QDs) are of interest because quantum confinement of semimetallic bulk HgTe allows one to synthetically control the bandgap throughout the infrared. Here, we synthesize highly monodisperse HgTe QDs and tune their doping both chemically and electrochemically. The monodispersity of the QDs was evaluated using small-angle X-ray scattering (SAXS) and suggests a diameter distribution of similar to 10% across multiple batches of different sizes. Electron-doped HgTe QDs display an intraband absorbance and bleaching of the first two excitonic features. We see splitting of the intraband peaks corresponding to electronic transitions from the occupied 1Se state to a series of nondegenerate 1Pe states. Spectroelectrochemical studies reveal that the degree of splitting and relative intensity of the intraband features remain constant across doping levels up to two electrons per QD. Theoretical modeling suggests that the splitting of the 1Pe level arises from spin-orbit coupling and reduced QD symmetry. As a result, the fine structure of the intraband transitions is observed in the ensemble studies due to the size uniformity of the as-synthesized QDs and strong spin-orbit coupling inherent to HgTe.

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. A...

Photoconductivity is demonstrated with monodispersed HgSe colloidal quantum dots that are illuminated with radiation resonant with 1S(e)-1P(e) intraband electronic absorption, between 3 and 5 μm. A doping of two electrons per dot gives the lowest dark current, and a detectivity of 8.5 × 10(8) Jones is obtained at 80 K. Photoluminescence of the intraband transition is also observed. The detector properties are discussed in terms of the measured photoluminescence quantum yield, the electron mobility in the 1P(e) state, and the responsivity. The intraband photoresponse allows to fully harness the quantum confined states in colloidal nanostructures, extending the prior limited use of interband transition.

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.

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.

International audience; Colloidal quantum dots presenting optoelectronic properties into the mid-infrared are reviewed with an emphasis on HgTe. Interband transitions with narrow band gap material and intraband transitions with wide band gap semiconductors can both address the infrared range of wavelengths. Semimetals are particularly promising since, by controlling the particle size, a gap can in principle be opened and tuned through the full infrared spectrum. HgTe has been quite successful recently, and colloidal synthesis has allowed the tuning of the absorption edge from 1 and 5 µm at room temperature. The cutoff wavelength and absorption coefficient of these materials have been discussed. Electrochemistry experiments have demonstrated that both n and p nanoparticles are stable. Films of the HgTe colloidal quantum dots have shown room temperature photoconduction when excited in the mid-IR. With novel processing leading to improved detectivity, these materials are becoming of significant interest as an alternative to the much more expensive epitaxial detector technologies.

Superconductivity in films of electronically coupled colloidal lead nanocrystals is reported. The coupling between particles is in situ controlled through the conversion of the oxides present on the surface of the nanoparticles to chalcogenides. This transformation allows for a 10(9)-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 × 10(3) A/cm(2). In turn, the critical field of the sample shows 50-fold enhancement compared to bulk Pb.

In nanocrystal solids, the small density of states of quantum dots makes it difficult to achieve metallic conductivity without band-like transport. However, to achieve band-like transport, the energy scale of the disorder should be smaller than the coupling energy. This is unlikely with the present systems due to the size polydispersivity. Transport by hopping may nevertheless lead to an increased mobility with decreasing temperature for some temperature range, and such behavior at finite temperature is not proof of band-like conduction. To date, at low temperature, variable range hopping in semiconductor or weakly coupled metal nanocrystal solids dominates transport, as in disordered semiconductors.

Films of colloidal quantum dots of HgTe can be charged n- and p-type by electrochemistry. Infrared spectroscopy of the charged dots reveals interband bleach and intraband absorption. Similar mobilities, up to 0.1 cm2/(V s), are observed for both carriers, but p-type films show larger photocurrent while n-type films show larger magnetoresistance.

The model of variable range hopping conductivity predicts a crossover between Mott and Efros-Shklovskii as a function of temperature and density of states. This is observed using monodispersed CdSe colloidal quantum dot 3D solids where the density of states at the Fermi level is varied by electrochemistry. At low density of states, both below the lowest state (

Studies of acoustic vibrations in nanometre-scale particles can provide fundamental insights into the mechanical properties of materials because it is possible to precisely characterize and control the crystallinity and geometry of such nanostructures. Metal nanoparticles are of particular interest because they allow the use of ultrafast laser pulses to generate and probe high-frequency acoustic vibrations, which have the potential to be used in a variety of sensing applications. So far, the decay of these vibrations has been dominated by dephasing due to variations in nanoparticle size. Such inhomogeneities can be eliminated by performing measurements on single nanoparticles deposited on a substrate, but unknown interactions between the nanoparticles and the substrate make it difficult to interpret the results of such experiments. Here, we show that the effects of inhomogeneous damping can be reduced by using bipyramidal gold nanoparticles with highly uniform sizes. The inferred homogeneous damping is due to the combination of damping intrinsic to the nanoparticles and the surrounding solvent; the latter is quantitatively described by a parameter-free model.

The applications and physical properties of colloidal quantum dots are briefly reviewed and contrasted with those of Stransky–Krastanov grown quantum dots. To cite this article: P. Guyot-Sionnest, C. R. Physique 9 (2008).

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.

PbSe nanocrystal colloids exhibit a well-defined excitonic structure with the lowest energy exciton tuning from 0.5 to 1 eV, as a function of size. Band-edge fluorescence is observed from 1.2 to 2 μm with a small Stokes shift, sub-μs lifetime, and near-unity quantum yield. Upon pumping at 1.064 μm, the first exciton decay is consistent with radiative relaxation at low pump intensity and with Auger recombination at higher pump intensities. Optically induced absorption is observed at approximately midgap. These transitions exhibit strengths similar to the interband exciton and are size-tunable. They are assigned to the 1Se,h−1Pe,h and 1Pe,h−1De,h intraband excitations. Intraband pump−probe measurements of the 1Se,h−1Pe,h transition reveal a short lifetime and the absence of the phonon bottleneck.

Modern semiconductor technology has been enabled by the ability to control the number of carriers (electrons and holes) that are available in the semiconductor crystal. This control has been achieved primarily with two methods: doping, which entails the introduction of impurity atoms that contribute additional carriers into the crystal lattice; and charging, which involves the use of applied electric fields to manipulate carrier densities near an interface or junction. By controlling the carriers with these methods, the electrical properties of the semiconductor can be precisely tailored for a particular application. Accordingly, doping and charging play a major role in most modern semiconductor devices.

The adsorption and laser-induced desorption behavior of CO on W(110) has been studied using sum frequency generation (SFG) and thermal desorption spectroscopy (TDS). The temperature and coverage dependence of the CO-stretch SFG-signal was measured. Laser irradiation of CO on the surface showed CO desorption and almost no dissociation of CO by visible laser pulses of 15–45 mJ/cm 2 without damaging the surface. Simulations of the surface reactions are in agreement with these measurements. Furthermore, it was observed that the non-resonant SFG-intensity change caused by CO or O adsorption is correlated to the work function change.

We measure the dielectric dispersion of CdSe nanocrystal colloids and show the existence of large dipole moments of 25 and 47 debye for 34 and 46 \AA{} diameter nanocrystals, respectively. The magnitude is consistent with the expected spontaneous polarization of the bulk wurtzite CdSe lattice and implies a potential drop of $\ensuremath{\approx}0.25\mathrm{V}$ across the nanocrystal. This effect, which is intrinsic to the wurtzite structure but has been largely overlooked, should be incorporated in the description of the quantum confined electronic states.

We report direct spectroscopic evidence for parity-forbidden electronic transitions in CdSe nanocrystals and relate it through perturbation theory to an internal electric field. This field is consistent with an estimated spontaneous polarization of Ps=−0.6 μC/cm2 for CdSe. The observed mixing of odd and even band edge hole states by this symmetry breaking internal electric field indicates that current theories of nanocrystal electronic structure, which assume inversion symmetry, require substantial revision.

The vibrational dynamics of CO at two electrochemical interfaces is studied as a function of the electrode potential, for COPt(100) in 0.1M aqueous H2SO4 and COPt(110) in 0.05M C16H36ClNO4 acetonitrile electrolyte. The measured lifetime of ∼1.7 ps is in line with those previously determined for dry CO-metal interfaces. The lifetime appears to be independent of the electrolyte composition, and unaffected by an electrode-potential variation as large as 2 V achieved for the non-aqueous electrolyte experiment. These measurements suggest that the 2π∗ CO acceptor orbital involved in the substrate/adsorbate charge transfer process is much broader than 0.8 eV.

We review our recent nonlinear spectroscopies of nanocrystals and synthetic efforts to improve their luminescence properties. A two-photon spectroscopic study of CdSe nanocrystals as a function of size is presented and compared with predictions from the effective mass model with spherical confinement. We also detail our efforts at improving the luminescence properties of nanocrystals which have culminated in a 50% fluorescence quantum yield for inorganic capping of some sizes of CdSe nanocrystals. Finally, we present the application of two-photon microscopy to resolve fluorescence from single nanocrystals at room temperature and cryogenic temperatures.

International audience; Colloidal quantum dots (CQDs) with mid-infrared (IR) tunable bandgaps present a new paradigm for mid-IR materials, but advances in charge transport and collection are needed for practical use in electronic applications. By replacing the organic ligands in films of HgTe CQDs with As 2 S 3 , a problem of oxidation in air is avoided and charge carrier mobilities are improved 100 fold compared to the standard organic ligand exchange. The composite inorganic HgTe/As 2 S 3 material allows for ambipolar field effect transistors with on-off ratios up to 10 7 , and photodetectors with high sensitivity, reaching 3x10 10 Jones at 230 K with a 3.5 micron cutoff wavelength.

Self-assembly of decanethiol on Ag(111) is studied using ultrahigh impedance scanning tunneling microscopy. The surface frequently shows monatomic silver islands covering about 20% of the surface. Molecular domains with an average size of ∼70 A as well as the molecules themselves are also imaged. From these images, three important quantities regarding the molecular structure are determined : (1) the lattice constant is 4.61 ± 0.15 A ; (2) two sets of 6-fold lattice orientations separated by 20.7 ± 2.3° are observed ; (3) far from domain boundaries, height variations of nearest neighbors are less than 0.1 A. A model is proposed to explain how such a structure could have arisen.

Electrochemical effects on self-assembled monolayers ofalkanethiols, C n H 2n+1 SH (n = 9, 10, 18), on Au, Ag(111), and Pt(111) are investigated using sum-frequency generation. Layers on Au and Ag(111) show no noticeable effects. Layers on Pt(111) show gauche transformations that are reversibly eliminated at negative or positive potentials.

International audience; Thermal deflection spectroscopy allows to measure very small absorption and uncovers absorption tails extending well below the bulk bandgap energy for CdSe quantum dots films after ligand exchange by sulfide. In this monodispersed system, the redshift, the broadening, and the absorption tails cannot be solely attributed to electronic coupling between the dots. Instead, mixing of hole states from the quantum dot and surface is proposed to dominate the changes of the interband spectra at the absorption edge.

International audience; The use of colloidal material offers an interesting alternative to top down approaches for the realization of low cost infrared detectors. We demonstrate photoconduction in thin films of a colloidal material in the mid-infrared (up to 7 μm), using HgTe colloidal quantum dots. Thin films of the colloidal quantum dots have a large absorption coefficient (>104 cm−1), and the photoconductive response is dramatically improved by encapsulating the nanoparticle into an inorganic matrix of As2S3. Such devices show fast response and large detectivity (>1010 jones) at temperatures above 200 K.

International audience; We report on HgTe colloidal-based quantum-dot mid-infrared (IR) photode-tectors. We demonstrate tuning the cutoff wavelength of this detector from the near IR up to 7 lm. Responsivity values of films of HgTe nanoparticles a few hundred nanometers thick are in the 100 mA W À1 range even at room temperature. The detectivity of this film is limited by large 1/f noise. Time responses as short as 200 ns are achieved.

We investigated the effectiveness of low temperatures in reducing plasmon damping by measuring the homogeneous linewidth of plasmon resonances $(\ensuremath{\sim}1.5\text{ }\text{eV})$ in nanoscale gold bipyramids at temperatures from 293 to 6 K. The linewidth drops linearly with temperature and approaches a constant value at approximately 50 K. Measurements were performed on monodisperse ensembles as well as on single particles. The 30% decrease in the homogeneous linewidth with decreasing temperature is well accounted for by the reduced electron-phonon scattering. The other relaxation mechanisms---electron-electron scattering, electron-surface scattering, and radiative relaxation---do not change significantly with temperature.

Near- and far-field optical properties of several types of gold nanoparticle are investigated with rigorous three-dimensional computational electrodynamics. The primary focus is on the results obtained with the finite-difference time-domain method, although some results obtained with the discrete dipole approximation are also given. We first consider spheres and prolate spheroids, where analytical solutions are available for comparison. The spectra of gold nanorods and pentagonal bipyramids are then investigated and excellent agreement with recent experimental optical spectra is found. The local field enhancement $(\ensuremath{\mid}\mathbf{E}\ensuremath{\mid}∕\ensuremath{\mid}{\mathbf{E}}_{0}\ensuremath{\mid})$ is studied at the longitudinal plasmon resonance. Sharper structural features produce more significant enhancement and the largest enhancement of more than a factor of 200 is seen around the poles of the bipyramid. The fields within the nanoparticles are also studied. Whereas the field magnitude is nearly uniform within small spheres and spheroids, it can be nonuniform for nanorods and bipyramids. The field magnitude decreases from the center toward the poles in the case of nanorods, but increases rapidly in the case of bipyramids. A large internal field enhancement by more than a factor of 30 is seen for the bipyramids, which suggests that these particles will exhibit strong optical nonlinearities.

In colloidal quantum dots, Energy dissipation by intraband, interband, and 'Auger' relaxation have very different mechanisms and rates which are being probed by transient optical measurements. Surface vibrations, phonons and spectator carrier play important roles.

Intraband spectra of CdSe nanocrystal colloids are observed. The photoexcited nanocrystals, of average diameters 31.5, 38, and 43 A, exhibit an isolated and strong infrared-induced absorption with peak energy between 0.5 and 0.3 eV. The energies and cross sections of the resonance are consistent with a one-electron transition between the delocalized 1Se and 1Pe states of the strongly confined quantum dots.

Conduction in thin films of PbSe nanocrystals doped by electrochemical gating has been studied. Charging the film, with either electrons or holes, increases the conductance by orders of magnitude. The electrons in the 1S(e) state of nanocrystals in these films have a mobility as high as 5.0 x 10(-3) cm(2) V(-1) s(-1). Electrons in the 1P(e) state were found to have a differential mobility up to 3-5 times greater than the electrons in the 1S(e) state, and a mobility minima was found corresponding to the complete filling of the 1S(e) state. The temperature and electric field dependence of conductance in the film, measured between 4.3 and 135 K, were both well described by a variable range hopping model.

We measure nonlinear optical scattering from individual Au nanorods excited by ultrafast laser pulses on resonance with their longitudinal plasmon mode. Isolating single rods removes inhomogeneous broadening and allows the measurement of a large nonlinearity, much greater than that of nanorod ensembles. Surprisingly, the ultrafast nonlinearity can be attributed entirely to heating of conduction electrons and does not exhibit any response associated with coherent plasmon oscillation. This indicates a previously unobserved damping of strongly driven plasmons., Comment: Revised text

The temperature and electrical field dependent conductivity of n-type CdSe nanocrystal thin films is investigated. In the low electrical field regime, the conductivity follows G ~ exp(-(T*/T)^0.5) in the temperature range 10K, Comment: Submitted to Physical Review Letters on Dec. 22, 2003

A Comment on the Letter by A. L. Roest, Phys. Rev. Lett. 89, 036801 (2002. The authors of the Letter offer a Reply.

International audience; Electrical 1/f noise is measured in thin films of CdSe, CdSe/CdS, ZnO, HgTe quantum dots and Au nanocrystals. The 1/f noise, normalized per nanoparticle, shows no systematic dependence on the nanoparticle material and the coupling material. However, over 10 orders of magnitude, it correlates well with the nearest neighbor conductance suggesting some universal magnitude of the 1/f noise in these granular conductors. In the hopping regime, the main mechanism of 1/f noise is determined to be mobility fluctuated. In the metallic regime obtained with gold nanoparticle films, the noise drops to a similar level as bulk gold films and with a similar temperature dependence.

CdSe, InP, and ZnO colloidal quantum dots of 1.5-2.5 nm radius show

In bulk semiconductors, the absorption of one photon of energy greater than the bandgap leads to one exciton. In quantum dots, however, it could be possible to create multiple excitons from a single high-energy photon, a finding with dramatic implications for the efficiencies of devices such as solar cells.

Electrochemical methods are used to inject charge into films of colloidal semiconductor nanocrystals of PbSe. The injection of electrons and holes into quantum confined states is confirmed by monitoring changes in the IR absorption spectrum. Holes are injected into the 1Sh state, causing a bleach of the 1Sh-1Se and 1Sh-1Pe interband transitions and inducing a 1Sh-1Ph intraband absorption. Electrons can be sequentially injected into the 1Se and 1Pe states, first bleaching the 1Sh-1Se and 1Ph-1Se interband transitions and inducing a 1Se-1Pe intraband absorption, and then bleaching the 1Sh-1Pe transition and inducing a 1Pe-1De intraband absorption.

International audience; Room temperature photodetection with HgTe colloidal quantum films is reported between 2 and 5 µm for particles of sizes between ~5 and ~12 nm diameter, and photodetection extends to 7 µm at 80 K. The size-tuning of the absorption of HgTe colloidal quantum dots, their optical cross section and the infrared absorption depth of films are measured. The tuning with radius is empirically given by ${\lambda }_{\mathrm{BE}}^{\mathrm{QD}}=\frac{30.5}{\sqrt{1+(43/{R}_{\mathrm{QD}})^{2}}-1}$, where R is in nm. The optical cross section of the colloidal dots at 415 nm is approximately proportional to their volume and given by ${\sigma }_{\mathrm{Hg}}^{415}=2.6\pm 0.4\times 1{0}^{-17}~{\mathrm{cm}}^{2}/\mathrm{mercury}$ atom. The size-dependent optical cross section at the band edge ~1.5 × 10−15 cm2 is consistent with the expected oscillator strength of the quantum dots. The absorption depth of HgTe colloidal dot films is short, about 1–2 µm, which is an advantage for thin film devices. These properties agree rather well with the expectation from the k ⋅ p model. HgTe colloidal quantum dot thin films show a strong tuning with temperature with a large positive thermal shift between 0.4 and 0.2 meV K−1, decreasing with decreasing size within the size range studied and this is attributed primarily to electron–phonon effects.

Films of n-type CdSe/CdS colloidal quantum dots show positive and negative magnetoresistance (MR) with magnetic field lower than 0.4 T. The MR sign and magnitude vary with the occupancy of the lowest state, the temperature, and the electrical field. Low occupancy leads to +20% MR at 10 K, while high occupancy leads to ∼−5% MR. The MR reaches +35% in mixed samples of large and small dots. Intrinsic films are photoconductive and display positive MR.

The conduction of thin films of n-type CdSe colloidal quantum dots is studied at low temperature and under magnetic field. At medium and high magnetic fields (10 T), the films exhibit positive magnetoresistance consistent with the variable range hopping model. At low magnetic field(

Solid films of CdSe colloidal quantum dots are conductive when reduced by electrochemistry. The dark conductivity and the photoconductivity of these n-type CdSe nanocrystalline solids have been investigated as a function of film treatment. When the films are treated with NaOH, a carrier mobility of ∼10−2cm2∕Vs and a charging/discharging time of ∼10ms are obtained. Under an excitation intensity of ∼0.75mW∕cm2, a significant photoconductivity of ∼10−3Scm−1 has been observed. The high mobility allowed demonstrating the operation of a field effect transistor.