PbS Quantum Dot Films Research Articles

Wearable and sensitive heart-rate detectors based on PbS quantum dot and multiwalled carbon nanotube blend film

Wearable and sensitive photodetectors (PDs) have been demonstrated based on a blend film of PbS quantum dots (QDs) and QDs modified multiwalled carbon nanotubes (MWCNTs). Owing to the synergetic effect from high light sensitivity of PbS QDs and excellent conductive and mechanical properties of MWCNTs, the blend PDs show high sensitivity and flexibility performance: device responsivity and detectivity reach 583 mA/W and 3.25 × 1012 Jones, respectively, and could stand large number (at least 10 000 cycles) and wide angle (up to 80°) bending. Furthermore, the wearable and sensitive PDs have been applied to measure the heart rate in both red and near infrared (NIR) ranges. The presented PDs are expected to work as sensor candidates in integrated electronic skin.

Carrier Transport in PbS and PbSe QD Films Measured by Photoluminescence Quenching

The temperature-dependent quantum yield of photoluminescence (PL) has been measured in films of various sizes of PbS and PbSe quantum dots (QDs) capped with alkanedithiol ligands with lengths varying from 4 to 20 A. We demonstrate that PL within QD films can provide information about transport in a regime that is relevant to solar photoconversion. The ligand-length dependent PL quenching reveals behavior similar to that of ligand-length dependent carrier mobility determined from field-effect transistor (FET) measurements in the dark. The data are described by a model in which band tail luminescence is quenched upon thermal activation by charge separation and hopping followed by nonradiative recombination. We extract the tunneling parameter β and find values of 1.1 ± 0.2 A–1 except for a value of 0.7 for the smallest QD sample. Changes in the transport mechanism may be due to unique surface faceting or QD-ligand coupling that occurs in small QDs. Furthermore, we compare all-organic capped PbS QD films with...

Plasmonic Schottky Nanojunctions for Tailoring the Photogeneration Profile in Thin Film Solar Cells

Schottky nanojunctions are presented, fabricated from solution‐processed PbS quantum dot films and self‐assembled Ag nanoparticle arrays. If the Ag nanoparticle arrays are sufficiently dense to ensure the overlap of the individual depletion regions set up around the discrete Ag nanoparticles, open circuit voltages and shunt resistances similar to reference Schottky junctions with bulk Ag contacts can be achieved. Light is incident from the Ag nanoparticle side of the device and can excite localized surface plasmon resonances in the metal. These plasmonic resonances strongly concentrate the electromagnetic field within tens of nanometers of the nanoparticles. The resulting absorption profile is modified such that carriers are generated close to the metal–semiconductor interface, within the depletion region of the Schottky diodes. In this way, the generation profile is tailored across the device to achieve more efficient carrier collection. External quantum efficiency spectra exhibit clear spectral features characteristic of near‐field assisted absorption, as predicted by full field optical simulations. By comparing two different active layer thicknesses—resulting in one depleted reference device and one partially depleted—it is possible to differentiate between the optical (due to changes in absorption) and electrical (due to changes in carrier collection efficiency) effects of modifying the generation profile in Schottky diodes.

Determination of carrier lifetime and mobility in colloidal quantum dot films via impedance spectroscopy

Impedance Spectroscopy (IS) proves to be a powerful tool for the determination of carrier lifetime and majority carrier mobility in colloidal quantum dot films. We employ IS to determine the carrier lifetime in PbS quantum dot Schottky solar cells with Al and we verify the validity of the technique via transient photovoltage. We also present a simple approach based on an RC model that allows the determination of carrier mobility in PbS quantum dot films and we corroborate the results via comparison with space charge limited measurements. In summary, we demonstrate the potential of IS to characterize key-to-photovoltaics optoelectronic properties, carrier lifetime, and mobility, in a facile way.

Photophysics of PbS Quantum Dot Films Capped with Arsenic Sulfide Ligands

PbS quantum dots (QDs) of different sizes capped with short (NH4)3AsS3 inorganic ligands are produced via ligand exchange processes from oleate‐capped PbS QDs. The solid‐state photophysical properties of the control organic‐capped and the inorganic‐ligand‐capped QDs are investigated to determine their potential for optoelectronic applications. Ultrafast transient transmission shows that in the oleate‐capped QDs, carrier recombination at sub‐nanosecond scales occurs via Auger recombination, traps, and surface states. At longer times, intense signals associated with radiative recombination are obtained. After ligand exchange, the QDs become decorated with (NH4)3AsS3 complexes and relaxation is dominated by efficient carrier transfer to the ligand states on timescales as fast as ≈2 ps, which competes with carrier thermalization to the QD band edge states. Recombination channels present in the oleate‐capped QDs, such as radiative and Auger recombination, appear quenched in the inorganic‐capped QDs. Evidence of efficient carrier trapping at shallow ligand states, which appears more intense under excitation above the (NH4)3AsS3 gap, is provided. A detailed band diagram of the various relaxation and recombination processes is proposed that comprehensively describes the photophysics of the QD systems studied.

Quantum dot–layered double hydroxide composites for near-infrared emitting codes

Near-infrared (NIR)-emitting PbS-based quantum dots (QDs) and exfoliated layered double hydroxide (LDH) nanosheets were self-assembled to form photostable composites for NIR-emitting codes. PbS QDs prepared by a pyrolysis synthetic method or PbS–CdS (core–shell) QDs prepared by a cation exchange using PbS QDs were transferred to aqueous solutions by surface ligand exchange to dihydrolipoic acid (DHLA). The DHLA-decorated QDs and exfoliated LDH nanosheets were assembled by electrostatic interactions during vortexing. The QD–LDH composite films prepared by drop-casting exhibit more than 500% higher NIR PL intensities than the counterpart QD films (without LDH matrices) under identical conditions (i.e., QD concentration). Photostability tests under continuous light irradiation reveal that the PbS–CdS QD films are more photostable than the PbS QD films. Formation of the QD–LDH composites gives rise to further enhancement in the photostability of the film. The photostable PbS–CdS QD–LDH composite films were used to create novel NIR-emitting codes. NIR fluorescence images of a pattern of two kinds of PbS–CdS QD–LDH composite films were recorded with an InGaAs CCD camera equipped with optical filters.

Two strategies to enhance efficiency of PbS quantum dot solar cells: Removing surface organic ligands and configuring a bilayer heterojunction with a new conjugated polymer

We present two novel techniques for improving the efficiency of PbS quantum dot (QD) solar cells. First, plasma was applied to QD film with the aim of removing surface organic ligands, and then the chemical and optical properties of the QDs were investigated. Second, a thin layer of conjugated polymer was then deposited on top of the plasma-treated PbS QD film as a transportation layer for holes. The charge separation and subsequent transfer dynamics were examined, as were the resultant photovoltaic characteristics, according to the kind of polymer used. The developed device, which comprises a bilayer heterojunction of plasma-treated PbS QDs and poly[2,6-(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-alt-4,7(2,1,3-benzothiadiazole)] (PSBTBT), showed not only broad-range absorption of the solar spectrum, but also high charge transfer efficiency prior to recombination. This results in a largely increased power conversion efficiency (PCE) of 1.76%, compared to the 0.7% value of a PbS QD-only device not subjected to plasma treatment. This indicates that the proposed techniques are very useful for improving the efficiency of inorganic QD-based solar cells.

Stoichiometry Control in Quantum Dots: A Viable Analog to Impurity Doping of Bulk Materials

A growing body of research indicates that the stoichiometry of compound semiconductor quantum dots (QDs) may offer control over the materials' optoelectronic properties in ways that could be invaluable in electronic devices. Quantum dots have been characterized as having a stoichiometric bulk-like core with a highly reconstructed surface of a more flexible composition, consisting essentially of ligated, weakly bound ions. As such, many efforts toward stoichiometry-based control over material properties have focused on ligand manipulation. In this issue of ACS Nano, Murray and Kagan's groups instead demonstrate control of the conductive properties of QD arrays by altering the stoichiometry via atomic infusion using a thermal evaporation technique. In this work, PbSe and PbS QD films are made to show controlled n- or p-type behavior, which is key to developing optimized QD-based electronics. In this Perspective, we discuss recent developments and the future outlook in using stoichiometry as a tool to further manipulate QD material properties in this context.

Sharp exponential band tails in highly disordered lead sulfide quantum dot arrays

We employ temperature-dependent, illumination intensity modulated photocurrent spectroscopy (IMPS) to investigate the intra-band-gap density of states in films of PbS quantum dots (QDs). Using both coplanar electrode and stacked photovoltaic device configurations, IMPS measurements of PbS QD arrays show evidence of carrier trapping in exponential band tails extending from the band edges into the gap. The band tails have characteristic energies near 14 meV, similar to those found in other larger grain, polycrystalline bulk semiconductors, rather than the large Urbach energies normally associated with nanocrystals and porous/polycrystalline films. This result helps explain recent success in using QD solids in device applications and indicates potential for QD materials to compete with bulk materials in semiconductor applications.

Reversible Surface Electronic Traps in PbS Quantum Dot Solids Induced by an Order–Disorder Phase Transition in Capping Molecules

The electronic properties of semiconductor quantum dots (QDs) are critically dependent on the nature of the ligand molecules on their surfaces. Here we show the reversible formation of surface electronic trap states in the model system of solid thin films of PbS QDs capped with thiol molecules. As the temperature was increased from cryogenic to room temperature, we discovered a phase transition in the fluorescence spectra from excitonic emission to trap emission. The critical temperature (T(c)) of the phase transition scales with molecular length and in each case is close to the bulk melting temperature of the capping molecules. We conclude that an order-disorder transition in the molecular monolayer above T(c) introduces surface mobility and the formation of a disordered atomic lead layer at the QD/capping molecule interface, leading to electronic trap formation.

Bias-Stress Effect in 1,2-Ethanedithiol-Treated PbS Quantum Dot Field-Effect Transistors

We investigate the bias-stress effect in field-effect transistors (FETs) consisting of 1,2-ethanedithiol-treated PbS quantum dot (QD) films as charge transport layers in a top-gated configuration. The FETs exhibit ambipolar operation with typical mobilities on the order of μ(e) = 8 × 10(-3) cm(2) V(-1) s(-1) in n-channel operation and μ(h) = 1 × 10(-3) cm(2) V(-1) s(-1) in p-channel operation. When the FET is turned on in n-channel or p-channel mode, the established drain-source current rapidly decreases from its initial magnitude in a stretched exponential decay, manifesting the bias-stress effect. The choice of dielectric is found to have little effect on the characteristics of this bias-stress effect, leading us to conclude that the associated charge-trapping process originates within the QD film itself. Measurements of bias-stress-induced time-dependent decays in the drain-source current (I(DS)) are well fit to stretched exponential functions, and the time constants of these decays in n-channel and p-channel operation are found to follow thermally activated (Arrhenius) behavior. Measurements as a function of QD size reveal that the stressing process in n-channel operation is faster for QDs of a smaller diameter while stress in p-channel operation is found to be relatively invariant to QD size. Our results are consistent with a mechanism in which field-induced nanoscale morphological changes within the QD film result in screening of the applied gate field. This phenomenon is entirely recoverable, which allows us to repeatedly observe bias stress and recovery characteristics on the same device. This work elucidates aspects of charge transport in chemically treated lead chalcogenide QD films and is of relevance to ongoing investigations toward employing these films in optoelectronic devices.

Charge Trapping in Bright and Dark States of Coupled PbS Quantum Dot Films

Analysis of photoluminescence (PL) from chemically treated lead sulfide (PbS) quantum dot (QD) films versus temperature reveals the effects of QD size and ligand binding on the motion of carriers between bright and dark trap states. For strongly coupled QDs, the PL exhibits temperature-dependent quenching and shifting consistent with charges residing in a shallow exponential tail of quasi-localized states below the band gap. The depth of the tail varies from 15 to 40 meV, similar to or smaller than exponential band tail widths measured for polycrystalline Si. The trap state distribution can be manipulated with QD size and surface treatment, and its characterization should provide a clearer picture of charge separation and percolation in disordered QD films than what currently exists.

The Influence of Interlayer Molecules in Luminescence of Self-Assembled Bilayer PbS Quantum Dot Films

We synthesized organic solvent-soluble and water-soluble PbS quantum dots (QDs) with different sizes. The organic solvent-soluble PbS QDs dispersed in tetrachloroethylene were used to prepare bilayers structures of QDs bound by dithiol linkers on GaAs. The water-soluble PbS QDs were used to prepare bilayer structures of QDs on quartz based on alternating adsorption of polyelectrolyte. For bilayer films on GaAs, it was found that the stacking sequence of QDs affects the quantum yield and emission wavelength of the larger QDs. However, for bilayer films with different stacking sequences on quartz, the larger QDs show similar PL intensities and emission wavelength independent of the sequence. The probable mechanism for this difference observed is discussed in terms of charge transfer between QDs.

PbS Colloidal Quantum Dot Photodiodes for SWIR Detection

In this work, PbS colloidal quantum dot based photodiodes are realized compatible for the integration on ROIC's. Schottky photodiode architecture is selected for its fast response and moderate sensitivity. The device is formed from Indium tin oxide (ITO) anode, the photosensitive PbS layer and a schottky contact formed of titanium and gold. Pinhole-free uniform PbS quantum dots film achieved by optimized layer by layer spin coating process. Solid-state ligand change procedure applied during film coating using 3-mercaptopropionic acid (3-MPA) in order to remove long oleic acid ligand that degrades electron mobility in the film. Fabricated detectors achieved rectification ratios of 15 at +/- 1 V bias. Preliminary tests show a responsivity of 0.24 A/W is achieved. Specific detectivity D* is calculated as 3,54x1010 that is limited due to high dark currents.

The Photothermal Stability of PbS Quantum Dot Solids

We combine optical absorption spectroscopy, ex situ transmission electron microscopy (TEM) imaging, and variable-temperature measurements to study the effect of ultraviolet (UV) light and heat treatments on ethanedithiol-treated PbS quantum dot (QD) films as a function of ambient atmosphere, temperature, and QD size. Film aging occurs mainly by oxidation or ripening and sintering depending on QD size and the presence of oxygen. We can stop QD oxidation and greatly suppress ripening by infilling the films with amorphous Al(2)O(3) using room-temperature atomic layer deposition (ALD).

Variable wavelength photocurrent mapping on PbS quantum dot: fullerene thin films by conductive atomic force microscopy

Mixed films of PbS quantum dots and fullerene derivatives were investigated using spectrally resolved photoconductive atomic force microscopy (pcAFM) equipped with an external source measure unit to accurately measure small currents. Combining pcAFM with a tuneable laser light source allowed us to record local internal power conversion efficiency (IPCE) spectra and thus to determine the wavelength of highest photoactivity. Samples exhibited photoactivity in the near infrared around 900 nm with an IPCE of 10% at a single measurement point. Photocurrent images with a 5 nm pixel size were recorded showing inhomogeneous photocurrent distribution uncorrelated to the topography but reflecting the heterogeneous make-up of the film. The perpetual degradation of the photocurrent due to sample oxidation under ambient conditions was also monitored.

A p‐Type Quantum Dot/Organic Donor:Acceptor Solar‐Cell Structure for Extended Spectral Response

A coupled PbS quantum dot film and a PSBTBT:PCBM bulk heterojunction layer contribute comparable photocurrent in a new stacked solar-cell architecture with sensitivity in the near infrared and an efficiency >4%. With a focus on the energy level alignment between components, time-resolved microwave photoconductivity is used to elucidate the charge transport pathways for electrons and holes.

Correlation between Photooxidation and the Appearance of Raman Scattering Bands in Lead Chalcogenide Quantum Dots

In this report, we carefully study the effects of photooxidation on the Raman spectra of lead chalcogenide (PbX) quantum dots (QDs). Photoexcitation of PbS, PbSe, and PbTe QD films at 488 nm with power densities as low as 30 W/cm2 gives rise to several peaks related to both lead(II) oxide and the group VI chalcogenates (PbXO4). The amplitudes of these peaks are shown to increase with continuous laser illumination in air, but are completely absent for samples illuminated under rigorously air-free conditions. These results suggest that the ∼135 cm−1 Raman peak often assigned to an intrinsic PbX LO phonon is more likely an artifact arising from photooxidation. The myriad of potential photooxidation products formed quickly in laser-illuminated, air-exposed PbX QDs suggest that caution should be used in the assignment and interpretation of phonon spectra and phonon-mediated exciton relaxation pathways of these materials, unless the processing and experiments are conducted under air-free conditions.

AFM‐based photocurrent imaging of epitaxial and colloidal QDs

AbstractWe present photocurrent images and spectroscopic results of sub‐surface InAs quantum dots (QDs) and thin films of colloidal PbS QDs obtained at room temperature and under ambient conditions by means of photoconductive atomic force microscopy (pcAFM) under 916 nm and 900 nm illumination respectively.InAs QDs appear as dark areas in the photocurrent image, because imaging was performed at the wetting layer excitation energy. The PbS nanocrystal film imaged at the energy of the first excitonic transition displays local variations of the photoactivity. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Self-assembled colloidal PbS quantum dots on GaAs substrates

We report the fabrication and analysis of self-assembled monolayer and bilayer films of colloidal PbS quantum dots (QDs) on GaAs (001) substrates. 1,6-hexanedithiol is used as link molecule between QDs and GaAs substrates. Atomic force microscopy (AFM) and photoluminescence (PL) measurements confirm the formation of PbS QD film on GaAs. For the monolayer PbS QD film, the temperature-dependent PL shows a feature typical of close-packed film. For the bilayer PbS QD film fabricated from two different mean-sized PbS QDs, we find that the stacking sequence of QDs with different size affects the quantum yield and emission wavelength of the film.