PbS Nanocrystal Quantum Dots Research Articles

Targeted Positioning of Quantum Dots Inside 3D Silicon Photonic Crystals Revealed by Synchrotron X-ray Fluorescence Tomography.

It is a major outstanding goal in nanotechnology to precisely position functional nanoparticles, such as quantum dots, inside a three-dimensional (3D) nanostructure in order to realize innovative functions. Once the 3D positioning is performed, the challenge arises how to nondestructively verify where the nanoparticles reside in the 3D nanostructure. Here, we study 3D photonic band gap crystals made of Si that are infiltrated with PbS nanocrystal quantum dots. The nanocrystals are covalently bonded to polymer brush layers that are grafted to the Si–air interfaces inside the 3D nanostructure using surface-initiated atom transfer radical polymerization (SI-ATRP). The functionalized 3D nanostructures are probed by synchrotron X-ray fluorescence (SXRF) tomography that is performed at 17 keV photon energy to obtain large penetration depths and efficient excitation of the elements of interest. Spatial projection maps were obtained followed by tomographic reconstruction to obtain the 3D atom density distribution with 50 nm voxel size for all chemical elements probed: Cl, Cr, Cu, Ga, Br, and Pb. The quantum dots are found to be positioned inside the 3D nanostructure, and their positions correlate with the positions of elements characteristic of the polymer brush layer and the ATRP initiator. We conclude that X-ray fluorescence tomography is very well suited to nondestructively characterize 3D nanomaterials with photonic and other functionalities.

Structural and electrical transport properties of PbS quantum dots under high pressure

The high-pressure investigation of the PbS nanocrystal quantum dots (NQDs) with large Bohr radius has been conducted by in situ high-pressure X-ray diffraction, high-resolution transmission electron microscopy and alternating current impedance spectroscopy experiments up to 41.0 GPa. PbS NQDs present a similar pressure-induced structure phase transition path to bulk and nanocrystal PbS. The electronic conduction and the ionic conduction coexist in PbS NQDs, and the resistivity is generally larger than bulk and nanocrystal PbS. Above 9.1 GPa, the deviatoric stress drives the fusion of the nanoparticles, which leads to the reduction or the disappearance of grain boundary effect as well as a great influence on the dielectric performance.

Photoluminescence lifetime and absorption spectrum of PbS nanocrystal quantum dots

We determine the photoluminescence (PL) lifetime of octadecylamine capped PbS nanocrystal quantum dots (QDs) with different particle sizes, temperatures, and background materials, by measuring their time-resolved PL spectra and fitting the observation with a double-exponential function. The observed size dependence of absorption-peak shift shows a linear increase with growing particle size. Both the quantum-size confinement and surface-defects induced transitions show impacts on the PL lifetime, where size-dependent lifetime appears as a negative exponential decrease between 2.57–0.71μs for 2.35–5.61-nm QDs dispersed in toluene at room temperature. The temperature-dependent lifetime shows an insensitive feature, with peak seen at 3.42μs at 200K and averaging 2.27±1.15μs within the temperature range 7–298K for 3.6-nm QDs doped in UV-gel. Organic solutions used as background materials have weak effect on the lifetime due to similar dielectric coefficients.

High infrared photoconductivity in films of arsenic-sulfide-encapsulated lead-sulfide nanocrystals.

Highly photoconductive thin films of inorganic-capped PbS nanocrystal quantum dots (QDs) are reported. Stable colloidal dispersions of (NH4)3AsS3-capped PbS QDs were processed by a conventional dip-coating technique into a thin homogeneous film of electronically coupled PbS QDs. Upon drying at 130 °C, (NH4)3AsS3 capping ligands were converted into a thin layer of As2S3, acting as an infrared-transparent semiconducting glue. Photodetectors obtained by depositing such films onto glass substrates with interdigitate electrode structures feature extremely high light responsivity and detectivity with values of more than 200 A/W and 1.2 × 1013 Jones, respectively, at infrared wavelengths up to 1400 nm. Importantly, these devices were fabricated and tested under ambient atmosphere. Using a set of time-resolved optoelectronic experiments, the important role played by the carrier trap states, presumably localized on the arsenic-sulfide surface coating, has been elucidated. Foremost, these traps enable a very high photoconductive gain of at least 200. The trap state density as a function of energy has been plotted from the frequency dependence of the photoinduced absorption (PIA), whereas the distribution of lifetimes of these traps was recovered from PIA and photoconductivity (PC) phase spectra. These trap states also have an important impact on carrier dynamics, which led us to propose a kinetic model for trap state filling that consistently describes the experimental photoconductivity transients at various intensities of excitation light. This model also provides realistic values for the photoconductive gain and thus may serve as a useful tool to describe photoconductivity in nanocrystal-based solids.

Size-Dependent Charge Transfer Yields in Conjugated Polymer/Quantum Dot Blends

We investigate the effect of quantum dot size on photocurrent and photoinduced charge transfer yields in blends of the conjugated polymer, poly((4,8-bis(octyloxy)benzo(1,2-b:4,5-b′)dithiophene-2,6-diyl)(2-((dodecyloxy)carbonyl)thieno(3,4-b)thiophenediyl)) (PTB1), with PbS nanocrystal quantum dots (QDs). These hybrid solar cells exhibit external quantum efficiencies of over 70% and power conversion efficiencies of up to 2.8%. We use photoinduced absorption (PIA) spectroscopy and device EQE measurements to probe long-lived charge transfer at the polymer/QD interface as a function of QD size. We observe that both the PIA signal associated with charge formation on the polymer, as well as the external quantum efficiency of the hybrid photovoltaic devices decrease in magnitude with increasing quantum dot size, despite the broader absorption spectrum of the larger dots. We interpret these results as evidence that PTB1/PbS blends behave at least partially as bulk heterojunction (BHJ) solar cells, and conclude that the long-lived charge transfer yield is diminished at larger dot sizes as the energy level offset at the polymer/quantum dot interface is changed through decreasing quantum confinement.

Optical properties of PbS nanocrystal quantum dots at ambient and elevated pressure

We investigated pressure-dependent changes in the optical properties of PbS nanocrystal quantum dots (NQD) by combining X-ray scattering and optical absorption spectroscopy in a diamond anvil cell. We discovered that the excitonic absorption peak vanishes as the NQD crystal structure reversibly undergoes the pressure-induced phase transition from rock-salt to orthorhombic structure. In the rock-salt phase, the pressure coefficient ∂E(g)/∂P of PbS NQD is negative and decreases in magnitude with decreasing NQD size. The basic theoretical model based on literature values of the PbS bulk modulus significantly overestimated the change in pressure coefficient with NQD size. We present a model that includes the size-dependence of both the pressure coefficient and bulk modulus to describe the experimentally observed optical and structural trends.

Hot Hole Transfer Increasing Polaron Yields in Hybrid Conjugated Polymer/PbS Blends

We use quasi-steady-state photoinduced absorption (PIA) to study charge generation in blends of poly(3-hexylthiophene-2,5-diyl) (P3HT) with PbS nanocrystal quantum dots as a function of excitation energy. We find that, per photon absorbed, the yield of photogenerated holes present on the conjugated polymer increases with pump energy, even at wavelengths where only the quantum dots absorb. We interpret this result as direct evidence for transfer of hot holes in these conjugated polymer/quantum dot blends. These results help understand the operation of hybrid organic/inorganic photovoltaics.

Increased open-circuit voltage in a Schottky device using PbS quantum dots with extreme confinement

We fabricated the PbS nanocrystal quantum dots (NQDs) based Schottky structure device (ITO/PbS/LiF/Al) with varying bandgap of NQDs from 0.8 to 2.2 eV. The open-circuit voltage increased monotonically with NQD's bandgap until 0.67 V, achieved using extremely confined, 1.5 nm sized-PbS NQDs. The power conversion efficiency reached the maximum value over 3% under AM 1.5 with NQD's bandgap of about 1.3 eV. Size-dependent photovoltaic evaluation in extreme confinement regime provides basis for efficient multi-junction solar cells composed of PbS NQDs of different sizes.

Measurement of electronic states of PbS nanocrystal quantum dots using scanning tunneling spectroscopy: the role of parity selection rules in optical absorption.

We study the structure of electronic states in individual PbS nanocrystal quantum dots by scanning tunneling spectroscopy (STS) using one-to-two monolayer nanocrystal films treated with 1, 2-ethanedithiols (EDT). Up to six individual valence and conduction band states are resolved for a range of quantum dot sizes. The measured states' energies are in good agreement with calculations using the k · p four-band envelope function formalism. A comparison of STS and optical absorption spectra indicates that some of the absorption features can only be explained by asymmetric transitions involving the states of different symmetries (e.g., S and P or P and D), which points towards the relaxation of the parity selection rules in these nanostructures. STS measurements also reveal a midgap feature, which is likely similar to one observed in previous charge transport studies of EDT-treated quantum dot films.

Hole transfer from PbS nanocrystal quantum dots to polymers and efficient hybrid solar cells utilizing infrared photons

Polymer/inorganic-nanocrystals bulk heterojunction solar cells, where inorganic semiconductor nanocrystals such as CdSe, CdS, CdTe, ZnO, TiO2, and silicon, replace the fullerene molecules as the electron acceptors, typically exhibit a power conversion efficiency (PCE) below 3% even after tremendous engineering efforts to optimize the nanocrystal size, shape, and nanoscale morphology. One promising feature of polymer hybrid solar cells is the ability to sensitize conjugated polymers, which on their own absorb only in the visible part of solar spectrum, into the infrared spectral range using infrared-active lead salt nanocrystal quantum dots (NQDs). Here we observed for the first time hole transfer from PbS NQDs to polymers as evidenced by the quenching of the PbS photoluminescence (PL), a sign of the presence of charge separating type II heterojunction. The type II band-offset at the NQD/polymer heterojunction enables efficient hole extraction from NQDs and leads to a record PCE of 3.80%, realized in a planar junction configuration under simulated air mass 1.5 global (AM 1.5G) irradiation of 100mW/cm2. The photocurrent has an extended spectral range spanning from the ultraviolet (UV) to the infrared (IR). Contributions from the polymer and PbS to the photocurrent were identified. Infrared photons (>700nm) contribute about 30% of the photocurrent and yield a high external quantum efficiency (EQE) of 20% at 1050nm.

High‐Sensitivity p–n Junction Photodiodes Based on PbS Nanocrystal Quantum Dots

AbstractChemically synthesized nanocrystal quantum dots (NQDs) are promising materials for applications in solution‐processable optoelectronic devices such as light emitting diodes, photodetectors, and solar cells. Here, we fabricate and study two types of p‐n junction photodiodes in which the photoactive p‐layer is made from PbS NQDs while the transparent n‐layer is fabricated from wide bandgap oxides (ZnO or TiO2). By using a p–n junction architecture we are able to significantly reduce the dark current compared to earlier Schottky junction devices without reducing external quantum efficiency (EQE), which reaches values of up to ∼80%. The use of this device architecture also allows us to significantly reduce noise and obtain high detectivity (>1012 cm Hz1/2 W−1) extending to the near infrared past 1 μm. We observe that the spectral shape of the photoresponse exhibits a significant dependence on applied bias, and specifically, the EQE sharply increases around 500–600 nm at reverse biases greater than 1 V. We attribute this behavior to a “turn‐on” of an additional contribution to the photocurrent due to electrons excited to the conduction band from the occupied mid‐gap states.

Spectral Dependence of Nanocrystal Photoionization Probability: The Role of Hot-Carrier Transfer

We conduct measurements of photocharging of PbSe and PbS nanocrystal quantum dots (NQDs) as a function of excitation energy (ℏω). We observe a rapid growth of the degree of photocharging with increasing ℏω, which indicates an important role of hot-carrier transfer in the photoionization process. The corresponding spectral dependence exhibits two thresholds that mark the onsets of weak and strong photocharging. Interestingly, both thresholds are linked to the NQD band gap energy (E(g)) and scale as ∼1.5E(g) and ∼3E(g), indicating that the onsets of photoionization are associated with specific nanocrystal states (tentatively, 1P and 2P, respectively) and are not significantly dependent on the energy of external acceptor sites. For all samples, the hot-electron transfer probability increases by nearly 2 orders of magnitude as photon energy increases from 1.5 to 3.5 eV, although at any given wavelength the photoionization probability shows significant sample-to-sample variations (∼10(-6) to 10(-3) for 1.5 eV and ∼10(-4) to 10(-1) for 3.5 eV). In addition to the effect of the NQD size, these variations are likely due to differences in the properties of the NQD surface and/or the number and identity of external acceptor trap sites. The charge-separated states produced by photoionization are characterized by extremely long lifetimes (20 to 85 s) that become longer with increasing NQD size.

Broadband Absorbing Bulk Heterojunction Photovoltaics Using Low-Bandgap Solution-Processed Quantum Dots

We describe bulk heterojunction (BHJ) solar cells containing blends of colloidal PbS nanocrystal quantum dots with several new donor-acceptor conjugated polymers. Using photoinduced absorption spectroscopy we found that blends of PbS quantum dots with one polymer, poly(2,3-didecyl-quinoxaline-5,8-diyl-alt-N-octyldithieno[3,2-b:2',3'-d]pyrrole) (PDTPQx), produce significantly more photoinduced charge than blends of PbS with the other donor-acceptor polymers or with traditionally studied polymers like [2-methoxy-5-(3',7'-dimethyloctyloxy)-para-phenylene vinylene] (MDMO-PPV) and poly(3-hexylthiophene) (P3HT). Photovoltaic devices made with PDTPQx/PbS blends exhibit power conversion efficiencies 10-100 times larger than previously reported BHJ blends made with IR-absorbing quantum dots.

PbS quantum dot electroabsorption modulation across the extended communications band 1200–1700nm

We report electric field-induced modulation of absorption in PbS nanocrystal quantum dots across the spectral region 600–2000nm, encompassing the entire telecommunications band. The maxima in the electroabsorption spectra correspond with the positions of the first excitonic peak, confirming the predominance of excitonic broadening as the basis for the observed effect. We estimate the change in dipole moment to be on the order of 20 D in 7nm diameter PbS nanocrystals compared to previously-reported ∼100D in CdSe.

Anti-Stokes Photoluminescence in Nanocrystal Quantum Dots

In this study the fusion method was used to synthesize PbS nanocrystal quantum dots (QDs) embedded in S-doped glass matrix (SiO2−Na2CO3−ZnO−Al2O3−PbO2−B2O3). Optical absorption, atomic force microscopy (AFM), and photoluminescence (PL) were used to investigate different samples. Experimental data indicate that the PbS QD-size is controlled by the different annealing times the S-doped glass matrix is submitted to. The size-dependence of the optical transitions in PbS QDs were obtained by numerical calculation and compared with the experimental data. A strong anti-Stokes photoluminescence (ASPL), from green (2.409 eV) to violet (2.978 eV), was found in all samples, at room temperature. A possible microscopic mechanism leading to the ASPL, which involves a two-photon absorption in two separate steps through a surface state, is proposed.